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Let's say we could travel anywhere in the solar system, at what distance would it be safe for a human to stare at the sun (without eye protection) and not get eye/retina damage?
(Not sure where this question is good for, here, or perhaps Biology.SE, so let me know.)
If the sun is visible as a disc, then it is certainly can cause damage. As you move further from the sun the disc appears smaller, but the surface brightness remains the same. The eye can resolve a disc of about 1 arcminute, and the sun is about 30 arcminutes when viewed from Earth. So you would need to be at least 30 times further from the Sun than the Earth is, which puts you almost exactly at Neptune.
From that distance, the sun would be a magnitude -19 star. If you stared at it, it could probably still damage your eyes, however, your eyes are constantly jumping (saccades) which might move the image of the sun around the retina enough to prevent damage. There is, of course, no atmosphere to protect you from UV, so let's hope that's not a problem.
Beyond Neptune the sun gets dimmer, as it can't appear to get any smaller, and there is probably little risk by the time you reach the main part of the Kuiper belt.
How to Stare at the Sun (Without Frying Your Retinas)
Staring at the sun through unfiltered cameras will record only a glowing splotch. But anyone can view the sun in photographs&ndashwith the right kind of camera equipment.
Alan Friedman, a greeting card printer by day, is an avid astrophotographer by night. With a pretty simple setup, he's produced images that rival Hubble shots in terms of beauty.
Friedman starts with a standard telescope and a diagonal&mdashan attachment that, with the use of a prism to reflect light at a specific angle, allows viewers to look perpendicular to the telescope, giving them views they'd otherwise have to crouch below the telescope to see. He then uses a high-speed, high-resolution webcam and a hydrogen alpha filter. The camera allows him to scan many frames and select the crispest one as his final photograph, and the filter allows him to look at the sun safely, as well as take better images of it. The filter has many layers of material that interfere with various wavelengths of light, leaving an image that only includes certain levels of light emitted by burning hydrogen. This wavelength is both safe for the human eye and perfect for taking detailed photos of the sun&mdasha far cry from the white blob an unfiltered camera would record.
Hobbies such as astrophotography are pretty addicting, and Friedman has progressed to much more elaborate equipment. But you can get started right now with a homemade telescope, a good filter, and a webcam.
Scientists have been staring at the sun for centuries. Before the advent of advanced imaging techniques that let people look at the sun without hurting their eyes, people such as Galileo observed sunspots and other solar phenomena by gazing at the sun when it was on the horizon or covered by clouds. Today space telescopes not only keep a constant watch on the sun, but do so without having to account for any atmospheric interference.
The Solar Dynamics Observatory, launched in February 2011 and currently in orbit at 22,000 miles above Earth, is NASA's main heliocentric project. SDO contains three instruments, which together take photographs of the sun every 10 seconds. With 10 times the resolution of an HD television, the probe takes every shot in eight wavelengths of light. Other solar observation satellites, such as the Solar Terrestrial Relations Observatory and the Solar and Heliospheric Observatory, also take pictures of the sun in eight wavelengths simultaneously.
The sun emits light in all visible colors, but since our eyes combine them all for processing, we see the sun as being white or yellow. Since each wavelength of light represents a different temperature of burning gas, focusing on one instead of another can allow researchers to capture images of different layers of the sun. That's how they know what's going on inside. Focusing on yellow-green light shows the surface of the sun, which is about 10,000 degrees Fahrenheit, but looking at extreme ultraviolet light reveals atoms at around 11 million F&mdasha temperature many solar flares reach, allowing NASA researchers to see and document them.
There's one occasion that turns even the most casual of astronomers into a regular Icarus: the solar eclipse.
On a normal day the sun's rays are so bright that evolutionary failsafes kick in if we try to stare too long. Blinking, eye watering, and pupil dilation keep your sensitive peepers safe. But during an eclipse, just enough visible light is blocked to trick your reflexes, and you're able to keep your eyes open. The harmful UV rays are still present, though, so you'll still get the sunburned corneas. And without the pain and blinking that unblocked sunlight produce, you're more likely to stare long enough to sustain permanent retina damage. That's why we're all warned not to look directly at a solar eclipse.
The easy fix is a pair of specially made sunglasses (most standard glasses won't block enough UV rays to be of any help) branded either for eclipses or welding, which work by blocking enough of the strongest wavelengths of light to save your eyeballs. You can also make a pinhole projector, which projects a shadow of the eclipse onto a piece of cardboard or the ground. It produces a reversed image when light passes through a hole and hits a flat surface. Pinhole projectors are useful for any sun-blocking phenomenon, such as last year's transit of Venus. On a normal day a pinhole projector will just show you a round sun, but if anything is passing in front of it, you'll get a great view&ndashwithout roasting your eyes.
Somewhere around 5000-5400 K (as opposed to the Sun's effective surface temperature of
5700K) would probably do it. Such a star would be straddling the dividing line between G-class "yellow dwarfs" (they aren't yellow) and K-class "orange dwarfs", and would probably hover somewhere around 60% or 70% of the Sun's luminosity (= light output, or close enough for government work) depending on a variety of factors. However, if you just replaced the Sun with an orange dwarf the Earth would freeze, so there's that.
It's also worth pointing out that staring at the Sun is dangerous because our ancestors never had to evolve structures that would allow them to do so. This being worldbuilding and not astronomy, it's perfectly conceivable for a species to have developed the ability to stare at a much, much brighter star than the Sun, even, if they had a reason to.
Be a Citizen Scientist
Want to truly capture something special during the solar eclipse? Choose one (or more) of these experiments that you can contribute to or do on your own.
Explore Dramatic Temperature Changes
Totality is cool – literally. The temperature drops during a total solar eclipse, in some cases by as much as 15 degrees Fahrenheit. Using a thermometer, you can measure the temperature in the hour before the moon’s shadow passes over and then again when it engulfs you. Some thermometers should be able to catch the quick change that happens during those two minutes.
Give Back: By recording that data you can contribute to a citizen science database using the , which is sponsored by NASA.
Totality is more than just what you see, it’s also what you hear. When everything goes dark, the sounds swirling around you change. Many noisy animals go silent like they do at night, while others begin to make a ruckus. The birds quit singing, the frogs start croaking, and the crickets chirp-chirp-chirp away. Take a moment during totality to stop looking at the sun and to listen.
Give Back: You can sign up in advance to help record the symphony of totality as a part of a project called the .
While listening to the animals, you can also observe how they react to the phenomenon. There are a number of anecdotal reports of animals acting strange during totality like reef sharks swimming out from their daytime cover or spiders .
Give Back: Using an app like the app by the California Academy of Sciences you can help scientists track the activities of certain animals and plants during the eclipse. You can record what they are doing before totality and then during totality and contribute your responses to scientists’ project.
You’ll probably be taking pictures during the eclipse, so why not contribute some of those images to science in addition to your Instagram feed?
Give Back: The is a wide-scale citizen science project that aims to gather images of the coming eclipse from across the country and stitch them together to provide a continuous view of the event as it travels from the West Coast to the East Coast. Their goal is to get a long look at the sun’s mysterious corona, which is its white and wispy outermost layer. The corona is only visible from Earth during a total solar eclipse when it leaks out from behind the moon and creates a ring of faint streamers around it. Usually during eclipses scientists only get a few minutes to study the corona before it disappears. But because this eclipse is going across the United States, it presents an opportunity for people along the line of the eclipse to contribute their images during totality to the Eclipse Megamovie app. It will essentially extend totality from being about two and a half minutes to about 90 minutes.
You can also use the eclipse to help scientists better understand Earth’s ionosphere by collecting radio signals. The ionosphere is a layer of Earth’s atmosphere that is about 50 to 600 miles above the planet’s surface. It is teeming with ions and free electrons. There, radio waves get refracted.
Give Back: A country-wide project called aims to use radio receivers to study the eclipse’s effects on the ionosphere. Their plan is to send off radio signals before, during and after the eclipse from a location in Colorado and another in California. Their volunteers will use custom-built radio receivers connected to smartphones or laptops to pick up those signals from across America so they can understand how sunlight affects the ionosphere. This experiment is similar to one that was conducted by scientists during a solar eclipse in 1912.
Lunar Shadow Speed
How fast does the moon’s shadow race across America during a total solar eclipse? If you’re watching totality from a location where you can see a lot of land features, you may be able to figure that out.
Try This: Because the Earth is a sphere that rotates on its axis, the speed of the moon’s shadow during the total solar eclipse varies in different locations.
According to , the steps to take to roughly calculate how fast the moon’s shadow is moving by you are:
- Record the time when the totality began and everything was plunged into darkness.
- Record the time when totality ended and everything became light again.
- You can figure out how long in minutes it took the moon’s shadow to pass over you by taking the difference between these times.
- Then you can convert that time into decimal hours using an online calculator.
- Since the moon’s shadow has an average diameter of about 110 kilometers, or about 68 miles, you can then divide the time you got in hours into 110 kilometers and receive the speed in kilometers per hour from your location. You can then convert that to miles per hour.
Opportunities to observe total solar eclipses are rare and well worth looking at with eyes wide open - but ONLY at the appropriate moment. so advises Dr Karl.
You get a solar eclipse when the Moon gets between the Earth and the Sun. In a total eclipse, the Moon blocks all of the light, creating an eerie deep twilight, and suddenly, in the middle of the day, you can now see the stars. But many people, when given the chance, never enjoy the free cosmic thrill of a total solar eclipse, because they believe the myth that looking at an eclipse, or even being outdoors, will make you go blind.
In fact, a total eclipse of the Sun is pretty harmless, but I'll get to that soon. A partial eclipse is far more dangerous. In a partial eclipse, the Moon blocks some of the light, and you might notice that the sunlight is not as bright, which you would probably put down to a cloud covering it. But even if 99% of the Sun is covered, that tiny crescent remaining is truly bright enough to blind you, if you stare at it for anything longer than the briefest glimpse.
The first thing to realise is that as the Sun slides into eclipse, it does not begin to emit new and strange forms of damaging radiation - it just squirts out what it always has. You can certainly damage your eyes by staring at the Sun when it is partially covered by the Moon - in exactly the same way as you can damage your eyes by looking at the full Sun.
The second thing to understand is the mechanism of how the Sun can damage your eyes. The Sun gives out light, but it also gives out heat energy. This heat energy is focused and concentrated onto the central part of your retina, which deals with fine vision. If you stare at the Sun for long enough, you will burn out that central part of the retina. Your peripheral vision will be fine, so you'll still notice something approaching from the corner of your eye. But if you try to read any fine print, it'll be fuzzy, as if you were looking through a metre of seawater, or a glass smeared with petroleum jelly.
One definite case of a partial solar eclipse causing blindness happened on February 4, 1962, with military personnel in Hawaii. The partial eclipse did not cover all the Sun, and there was still enough energy in the exposed sliver of Sun to burn out the centres of their retinas. Over the next few days, many of the soldiers had trouble in shooting accurately on the rifle range. Their vision had dropped from 20/20 to 20/200, which is 10 times worse. Most of them recovered, but some had permanent loss of visual sharpness.
So when the next total eclipse of the Sun rolls around, just remember a few rules:
1. It's OK to look at the totally-eclipsed Sun with the naked eye - but only when the Sun is totally covered by the Moon, so you have to pick the right moment
2. Never look at the partially-eclipsed Sun with the naked eye. Even a slim crescent has enough energy to blind you
3. It's safe to look at the fully-exposed or partially-exposed Sun with approved filters, such as professional Solar Viewing Mylar filters. But never look directly at the Sun with smoked glass, exposed photographic films, or Mylar food packaging.
But so long as you don't look directly at the Sun, it is perfectly safe to be outdoors as the Sun drifts in or out of the eclipse, while you go about your normal business. It is also perfectly safe to look at the totally-eclipsed Sun. This is because the Moon, which is 400 times smaller than the Sun, is also 400 times closer - so it can totally obscure all direct light from the bright part of the Sun.
Now it's safe to briefly abandon the pinhole camera, and gaze up in awe at the Total Eclipse of the Sun, complete with the shimmering corona, and all its associated splendors.
At what distance would it be safe for humans to stare at the sun? - Astronomy
I've read that you can translate a telescope array to the distance the telescopes are apart. For instance, say you use 10 telescopes in tandem, coast to coast in the US, you essentially get a "United States" sized telescope.
How does this work? What would adding even more telescopes do? By that I mean, if I increased the amount of telescopes to 20, but they were all still in the US, would that increase resolution or some other statistic? What does increasing the distances effective increase (say now they're spread over all of Earth. )?
Thank you for your time. (Ithaca represent!)
This map shows all of the telescopes that were linked to create the Event Horizon Telescope--the telescope that captured the first picture of a black hole. Image courtesy of the EHT Collaboration. This is a very good question, and relevant to the recent release of the first-ever picture of gas near a black hole's event horizon! What you are describing is a technique called "interferometry". You've described it very well--telescopes that are far apart can be linked by this technique to create a "virtual telescope" with a resolution equal to a single telescope with a size equal to the distance between the linked telescopes. First, I'll answer what would happen if you (1) spread the telescopes farther apart, and (2) added more telescopes, then I'll explain why.
If you add more-distant telescopes to the array, you can see smaller details in the resulting image. That's why they used telescopes from all parts of the world to take the picture of the black hole--the black hole is very small and far away and appears extremely tiny, so we need telescopes that are very far from each other.
If you add more telescopes, but don't increase the farthest distance between any pair, you'll acomplish two things. The first is that you will have to stare at the source you're observing for less time, because more telescopes means more light-collecting area, so you collect light faster. The objects we observe with these techniques are often quite dim, meaning the exposure time for the image has to be quite high to collect enough light. The second is that the quality of the image goes up, but in an unintuitive way. The size of the smallest details that you can make out doesn't improve, but the amount of detail you can see goes up. I think I can explain best with a demo.
This is what an image of a single dot would look like if you took an image with just 2 linked telescopes. It doesn't look like a dot at all! This striped pattern is the result of the same physics that cause a pattern of dots when you shine a laser through a double-slit. These images are courtesy of Andrea Isella.
With three linked telescopes, you start seeing all kinds of dots, even though we're only taking a picture of one dot.
With four linked telescopes, there are less of these artificial dots.
With eight linked telescopes you can start to see that the dot in the center (the real dot) is brighter than all the artificial dots.
There are other ways to get rid of the artificial dots too. When you take long exsposures, the Earth will rotate while you are observing your target, which helps a lot. The image below includes the rotation of the Earth over like 6 hours.
A diagram of delay time between telescopes. The green lines connect photons that were emitted at the same time, illustrating the fact that light arrives at the telescope on the right first. The length "L" is directly related to the delay time between telescopes. Image courtesy of Bob Emery. The physics behind this are kind of complicated. The short version is that these arrays of telescopes measure the delay of the arrival of light between pairs of telescopes. If there is no delay, i.e. the light arrives at both telescopes at the same time, that means the line to the source has to be exactly perpindicular to the line between the two telescopes. If the delay time equals the distance between the two telescopes divided by the speed of light, that means that the source must lie on the line that connects the two telescopes. Every angle to the source corresponds to a delay time, so if you can measure this delay time, you can measure the angle to the source. If you use telescopes that are farther away from each other, the delay times are larger and easier to measure, resulting in more precise angles to the source. Combining more than two telescopes requires the use of fancy math, namely Fourier Transforms and Correlation Functions to produce images like the ones I showed above or the first images of the black hole.
About the Author
Christopher Rooney is a fourth-year grad student at Cornell and was editor-in-chief of Curious from 2018-2020 (meaning that anything wrong on the website could very likely be his fault). Christopher studies galaxies far, far away trying to find the galaxy where Star Wars took place trying to characterize star-formation at a time in the history of the Universe when stars were being formed extremely quickly. He also works on the detectors used to measure the light from these galaxies.
When you think of a word to describe Astronomy, "Dangerous" doesn't normally come to mind. Its not.
There are however, dangerous things you can do while observing.
A telescope is a light amplifying device. Pointing it at a bright object such as the sun and looking through the eyepiece can severely damage your eye.
As /u/Orangelantern experienced, even having your telescope pointed NEAR the sun without a proper filter attached to the aperture can cause catastrophe.
Fortunately for /u/Orangelantern, He was not looking through the telescope when the sun burned through his eyepiece
"It was really really clear outside today, so i decided to put my solar filter on my telescope and take a look at the Sun. I made the mistake of not putting the glass filter on before i aligned the scope near the sun. After about 3 or 4 seconds I smelled burning plastic. I made a joke to my Dad who was making pancakes about how he was burning the pan when i noticed it was actually my own fault."
Imagine that being your eye. Solar observing is nothing to joke about. Before each observation you must first go over a mini checklist in your head.
Is the filter properly attached to the telescope?
Does it have any cracks or holes?
Is the Glass/Mylar firmly attached to the inner perimeter of the holder?
Is my Finderscope attached or filtered as well?
Am I in an environment conducive to the possibility of the filter falling off? ( Sunday football game with rowdy drunk people about is not a very safe time to stare at an object capable of burning a hole through your skull)
Venturing out of the safety of your backyard poses some risks to astrophotographers. Because of the nature of light pollution, dark sites tend to be very remote, and don't usually have cell reception. If something bad were to happen while you were far out in the middle of nowhere you could be in trouble. Always be wary of the indigenous population. An angry bear is usually not the most welcome guest at your camp. Also be careful of snakes if you live in a climate where they live. They can easily ruin your day.
One of the biggest problems people face when heading out into the desert is that the desert can actually get freezing cold at night. A place that can be 90 degrees Fahrenheit (32 Celsius) during the day could drop to below freezing at night. Packing T-Shirts and Shorts wont help you if you are huddled up inside your tent while its 30 degrees Fahrenheit (-2 Celsius) outside. Also, ALWAYS bring extra water. Dehydration is especially dangerous because people tend to try to conserve water while out in the desert. Rangers have found people dead from dehydration with water still left in their canteens. If you are thirsty, drink. A good way to check if you are dehydrated is to observe the color of your urine. If it's too brown, drink some water. /r/HydroHomies
Radios and white lights are also useful at a dark site. While preserving your night vision may help with the star gazing, having a bright light around in case of an emergency is a must.
Try to not get your car stuck in the mud or snow! If you cant get out by yourself, have no cell service, and are somewhere so remote that there aren't any other people around, you might be in trouble. That's why you should always pack extra water and some food.
Finally, if at all possible, camp with a buddy, that way you can eat them when you starve. Or, better yet, tell someone you're going out somewhere remote, and give a time you should reasonably be back home to check-in.
What is a parsec?Illustration of the shift in perspective that happens as an observer moves between the 2 positions, in this case, with respect to a tree and distant mountains. From each position, the observer sees the same tree. But, from one position to the other, the tree appears to move with respect to the mountains. This apparent shift is called parallax. Image via Las Cumbres Observatory.
If you ever heard professional astronomers talking among themselves, you wouldn’t hear much talk of light-years. The concept of a light-year – the distance light travels in a single earthly year, or about 6 trillion miles (nearly 10 trillion km) – is a great way to think about distance scales in the universe. But light-years aren’t as useful as parsecs when it comes to measuring those distances. A parsec – a unit of distance equal to about 19 trillion miles (more than 30 trillion km) – is more closely related to how astronomers go about the business of figuring out the size of the universe.
To find the distance to a nearby star, astronomers use triangulation. You can try it for yourself, right now. Hold your finger in front of your face, focus on something in the distance, and close first one eye, then the other eye. As you alternate eyes, you’ll notice your finger appears to dance back and forth in front of your face. The motion is, of course, an illusion. Your finger isn’t moving. Each eye sees your finger from a slightly different angle. So the finger’s location, relative to stuff in the background, looks different. This apparent shift is called parallax, from a Greek word meaning alternation.
If you measure the angle over which your finger appears to move, you can figure out how far your finger is from your face. Likewise, astronomers measure angles to find the distances to stars. Rather than blink their eyes, however, astronomers move the Earth.
Or rather, we use the fact that Earth moves around the sun.
Ready for a definition of parsec? Here it is. One parsec is the distance to an object whose parallax angle is one arcsecond. Don’t be thrown by the terms parallax angle, and arcsecond. Keep reading, and we’ll explain.
The video below from Las Cumbres Observatory does a good job explaining what a parallax angle is.
Or look at the diagram below. It illustrates the definition of parallax angle, and also of the word parsec:
One parsec is the distance to an object whose parallax angle is one arcsecond. The radius of the Earth’s orbit equals one astronomical unit (AU), so an object that is one parsec distant is 206,265 AU (or 3.26 light-years) away.
Let’s see how these illustrations actually work, in astronomy. If, for example, we observe a star in December, and then look at it again in June, the Earth will have gone halfway around its orbit. We’re looking at the star from two locations around 186 million miles (300 million km) apart. If the star is reasonably close, then – from one side of Earth’s orbit to the other – it will appear to move ever so slightly.
Add some trigonometry, and the parallax angle, combined with the size of Earth’s orbit, lets astronomers calculate the distance to the star.
In this image, the line from the star to Earth, at top, and the line from that star to the sun, below, can be said to represent a radius measure, with the star marking the center of this (not drawn in) circle. One radian (radius as measured along the circle’s circumference) equals 57.2958 degrees or 206,265 arcseconds, so a star with a parallax of one arcsecond must be 206,265 times the Earth-sun distance away. Image via astronomy.stackexchange.com.
These angles are miniscule. They’re too small for degrees to be a practical unit of measurement. That’s why parallax angles are typically measured in arcseconds – a unit of measurement equivalent to the width of an average human hair seen from 65 feet (20 meters) away – not degrees. There are 3,600 arcseconds in one degree.
And here’s how we arrive at parsecs as a unit of distance: one parsec is the distance to an object whose parallax angle is one arcsecond.
The term parsec is just over 100 years old. It first appeared in a 1913 paper by English astronomer Sir Frank Watson Dyson, and the term stuck. If you see a star with 1/2 arcsecond of parallax, it is two parsecs away. At 1/3 arcsecond, it is three parsecs away. And so on.
Basically, astronomers liked it because it made the math easier!
This image shows the closest stellar system to the sun, the bright double star Alpha Centauri A and B, and their distant and faint companion Proxima Centauri. At just over 1 parsec away – and likely bound to the other 2 stars of Alpha Centauri – Proxima is the closest star to our sun.
One parsec is approximately 19 trillion miles (30 trillion km). That’s a bit over three light-years. The Voyager 1 probe, launched in 1977, is the most distant manmade object from Earth. It is a mere six ten-thousandths of a parsec away. The nearest star to the sun, a small red dwarf named Proxima Centauri, is just over one parsec from us.
That is actually fairly typical in our neck of the galaxy – one star for every cubic parsec – but it’s not typical everywhere. In the cores of globular clusters, the density can reach well over a hundred stars per cubic parsec!
The center of the galaxy lies just over 8,000 parsecs from us in the direction of the constellation Sagittarius.
The Andromeda Galaxy, the closest spiral galaxy to our own, is nearly 800 kiloparsecs away. A kiloparsec is one thousand parsecs.
At larger scales, astronomers start to talk of megaparsecs and even gigaparsecs. That’s one million and one billion parsecs, respectively. These are generally reserved for the largest structures in existence. The Virgo Cluster, a conglomeration of thousands of galaxies towards which our own Local Group is falling, lies 16 megaparsecs from home. It would take 54 million years to reach it traveling at the speed of light.
The Virgo Cluster, containing over 1,000 galaxies, sits 17 megaparsecs away. That’s about 54 million light-years. The black circles in this image are actually nearby stars that have been removed from the image, which is via Chris Mihos/ESO/Wikimedia Commons.
Bottom line: One parsec is defined as the distance to a star that shifts by one arcsecond from one side of Earth’s orbit to the other. One parsec is about 30 trillion kilometers, or just over three light-years.
General interest questions
Below is more information about laser pointer safety in general. There is a separate FAQ for "doubters" -- people who think concern over laser pointers is overblown. Many aviation-related questions are on the FAQ for doubters page.
If you have a question not answered by either FAQ, please contact us using the link at the very bottom of this page.
- LASER POINTER HAZARDS
- What are the biggest problems with laser pointer misuse?
- What other laser pointer problems are there?
- Have laser pointers ever caused a vehicle or aircraft accident?
- When does a laser pointer get powerful enough to be dangerous?
- How do I know if I have a too-powerful laser pointer?
- On a CSI:Miami episode, a laser pointer brought down a plane by injuring the pilots' eyes when they were 1/4 mile (1320 feet) in the air. Is this possible?
- A child aimed the beam from a laser level at my toddler son's eyes. Will he be OK?
- A teacher aimed red, green and purple handheld laser pointers at my 4-year-old child.
- Can a laser that lights a match cause problems to eyes by reflection?
- STORE LASER SCANNERS
- Can the laser scanner at a checkout injure my eyes?
- LASER USE WHEN HUNTING
- When hunting before dawn or during dusk, I want to know where my friend is. Can he aim a laser pointer at me as I wait in a tree stand?
- Is it safe to view the green laser dot from a laser gunsight, as I look through the reticle?
- LASER HAIR LOSS COMB
- I have a Lasergain XL laser comb for hair loss. Is it hazardous to look at the laser beams? I had them reflect off my glasses a couple of times.
- LASER STAR PROJECTOR
- I have a galaxy starry sky projector laser -- is this safe to use?
- REFLECTED LASER LIGHT
- I am a calligrapher and use a laser level so I can write on straight lines. I am looking for an hour or two at lines of red Class IIIa laser light on paper. Could this harm my eyes over time?
- I used a laser pico projector when tracing images for up to 6 hours at a time. Now my right eye is having problems. Could this be from over-exposure to the laser pico projector light?
- SAFE TO WATCH VIDEOS OF LASERS?
- I was watching a video of a laser beam that was aimed right at the camera. Can laser light hurt my eyes through a TV or phone screen?
- SAFE AND LEGAL USE IN THE NIGHT SKY
- Aircraft can look like stars. What is the best way to point out stars in the night sky?
- In the U.S. it is illegal to aim at the flight path of an aircraft. Given that just about anywhere in the sky there could be a flight path, is this a problem for legal laser use?
- Can I aim a laser pointer at a drone?
- SELLING LASER POINTERS
- I have a product kit which includes laser pointer(100mW-400mW). B2B. Can I sell them in Europe and the rest of the world?
- LIMITING A LASER POINTER DIODE
- I want to limit a 200 mW laser diode so it looks dim, and then look directly at it. Is this safe?
- LASER POINTER TECHNOLOGY
- How is a laser pointer different from other lasers?
- What is the maximum allowed power?
- What laser pointers are legal?
- What should I do if I have an "illegal" pointer or high-powered laser?
- Are high-power laser pointers required to have specific features?
- What is the maximum power needed for laser pointing?
- What laser color is best?
- I want to make my own laser to burn things. Is this dangerous?
- What is a "military grade" laser?
- What is a "commercial grade" or "industrial grade" laser?
- CONTROL MEASURES
- Is a laser pointer ban effective?
- How does laser misuse compare with knife misuse?
- I am upset and want to pass a law against lasers. Where do I start?
- What is the SAE G10T and why should I care?
- WEBSITE SPONSORSHIP
- How can I support LaserPointerSafety.com?
- Why is ILDA helping sponsor this website?
There are two main areas of concern.
One is the problem of distracting or interfering with pilots' vision when laser pointers are aimed towards aircraft. People have been arrested and even jailed for shining lasers towards planes and helicopters. (See the aviation incident news page for many articles about aircraft/laser incidents, and the Sentences page for fines and jail terms.) So don't do it!
The other problem is eye injuries caused by a person aiming a more powerful handheld laser in their own eyes or at others who are close by. This can cause temporary or even permanent eye injuries. This problem is discussed in more detail on the Consumer laser eye injury info page.
- Laser pointers have been aimed at cars, busses, trains, boats, barges, and ferries. Just as with aircraft, this can distract or temporarily blind a motorist or driver &mdash this is obviously unsafe.
- During riots or civil disturbances, some protesters have aimed lasers in the eyes of police. Where this is prevalent, police now have eye protection available.
- At sporting events, spectators have aimed laser pointers at players such as football goalies. This is unsportsmanlike (to say the least!) as well as a potential eye hazard for the player.
- At concerts and movie theaters, sometimes an audience member will think it is funny to wave the laser dot around on the stage or screen.
Lasers have been misused by aiming at vehicles or aircraft for decades. This website&rsquos author is aware of vehicle-aiming incidents as early as 1981. Regarding aircraft, from 2004 when the FAA began requiring pilots to report laser illuminations, through December 2018 there have been almost 50,000 incidents in the U.S. where lasers were aimed at pilots.
In the discussion below, &ldquoaccident&rdquo is defined as an incident that results in actual damage to the vehicle, aircraft or property or that results in a bodily injury (e.g., anything beyond a claimed laser light injury to the eyes). In contrast, &ldquoincident&rsquo is something potentially hazardous or dangerous, which does not result in property damage or bodily injury.
For example, laser light in a pilot&rsquos eyes may have caused a missed approach and a subsequent go-around. While this incident is cause for serious concern, it did not result in an aircraft accident.
The following information is current as of March 1 2019.
Lasers causing vehicle accidents
- On October 25 2016, a person shining a green laser at another driver caused a three-car crash which resulted in body damage to the vehicles. There were no reported injuries due to the crash or due to the laser light. The incident occurred on Interstate 5 in Oregon.
- The website author is aware of one other documented accident caused by a laser pointer. This comes from a 1999 Springfield, Missouri laser pointer ordinance that references a local accident: &ldquoa three-car collision, where a young man pointed a laser light into the car ahead of him and startled the driver, causing him to slam on his brakes and create a pileup.&rdquo
- In 1998, a man going nearly 100 mph caused a five-vehicle crash that killed four teens in Morgan Hill, California. Prior to the crash, the man was aiming a laser pointer at other cars. According to the Associated Press, &ldquoLaw enforcement officials partially blamed the accident on the laser pointer&rdquo, although a SF Gate story filed at the same time was less certain: &ldquo[I]investigators were trying to find out what role, if any, the laser pointer may have played in the crash.&rdquo
- The author has heard informally of five vehicle accidents in France, around 2014, caused by laser visual interference but has not been able to find documentation.
- As of March 1 2019, there have been no documented cases of the light from a laser causing aircraft accidents (e.g., a crash or injury-producing incident).
There is no specific threshold between a "safe" laser beam, a potentially hazardous one, and a clearly dangerous beam. The following are some guidelines.
Even a "legal" (in the U.S.) 5 milliwatt laser pointer can be a potential hazard if the light distracts or temporarily flashblinds a person such as a pilot. This is why you NEVER aim a laser pointer at an aircraft, or the driver of a vehicle.
For direct damage to the eye, the exact severity will be due to many factors: beam power, exposure time, beam/eye relative motion, distance from the laser, retinal injury location, and a person&rsquos physiological/genetic susceptibility to eye injury (some people are more sensitive than others).
- If a person deliberately stares into a laser, even a small 1 milliwatt beam could cause a spot on the retina.
- Safety standards are based on a person blinking and/or turning away from a bright light within 1/4 second. Taking this into account, an accidental exposure to a 5 milliwatt beam is considered tolerable, as long as the person is not overriding their blink reflex. A 1998Lancetarticle by Mensah, Vafidis and Marshall states &ldquoA 5 mW laser with high retinal irradiance is too weak to cause retinal damage, even if shone in the eye for several seconds.&rdquo
- After some point, even blinking and moving isn't fast enough to prevent injury. As a very rough approximation for laser pointer use, above 10 milliwatts the potential hazard from general use outweighs the benefit of a brighter beam. This does not mean that an injury will occur just that there starts to be a potential hazard.
- At around 100 milliwatts, an accidental exposure at close range may cause a change to the retina which can be defined as an eye injury. The victim may or may not notice it depending on where the spot is on the retina. The injury may heal after a few days or weeks if the exposure is not too severe. According to the 1998 Lancet article, &ldquoBetween 100 and 500 mW of diode energy is required to produce a clinically retinal burn.&rdquo
At around 150 milliwatts, the beam from a laser can be felt on the skin, depending on the beam focus, skin color (absorption), etc. At roughly 500 milliwatts, the laser's beam begins to be a skin burn hazard if the person is within a few meters of the beam.
Incidentally, even powerful industrial lasers cannot cause deep burns, severed limbs, gun-type injuries or other effects seen in science fiction movies. While multi-watt laser beams are definitely serious eye hazards, they are ineffective at causing incapacitating body injuries.
Some additional information is in an article from Scientific American, &ldquoCan a pocket laser damage the eye?&rdquo
According to the Food and Drug Administration, which regulates lasers, about 60 percent of lasers they tested in 2018 were over the power listed on the label &mdash or the label did not list a power level. Lasers called &ldquopointers&rdquo or sold for pointing, are required to be less than 5 milliwatts in the U.S., and less than 1 milliwatt in countries such as the U.K. and Australia.
These are tips from FDA on how to tell the strength of a handheld laser:
- If the pointer is small and runs on button batteries, its output probably is less than 5 milliwatts.
- If it's pen-sized and runs on AA or AAA batteries, it's likely to be more powerful and may exceed 5 milliwatts.
- If it's flashlight-sized and runs on a cluster of AA or AAA batteries or runs on lithium batteries, it likely exceeds 5 milliwatts.
- Pointers sold with battery chargers probably drain their batteries quickly and are likely to be overpowered.
- Some pointers are sold with a removable cap that spreads the beam into a pattern. If used without the cap, the beam becomes a single beam that could exceed 5 milliwatts.
- Look for keywords that sellers might use to indicate a pointer is highly powered without saying that it's over 5 milliwatts: powerful, bright, ultra, super, military, military grade, super bright, high power, ultra bright, strong, balloon pop, burn, burning, adjustable focus, lithium battery, lithium powered.
- Look for videos or photos that show the laser burning, melting, balloon popping or show a bright, well-defined beam of light
- Look for purchaser comments on websites that tout the brightness or power of the product.
(The episode is "Money Plane", first aired March 7, 2005.) The CSI:Miami laser scenario as presented was not plausible.
A legal, off-the-shelf laser pointer like the one on the show has a maximum power of 5 mW. At night, a beam from this laser could cause glare out to about 1200 feet. It would prevent a pilot from seeing past the light, until the light was removed. Already, at 1320 feet, the glare level would be very low &mdash distracting but manageable at night.
To make it even less plausible, the laser exposure on the show happened on a clear, sunlit morning. In such a case, the pilot would see a green flash but a pointer would not cause glare or flashblindness. The reason is that the pupil is constricted in bright light, so less light can enter the eye. That means the laser no longer is the brightest, most obscuring light source.
CSI:Miami took another dramatic liberty. The pilot was said to have &ldquocorneal scarring&rdquo. However, visible light from a laser goes through the clear cornea and is absorbed by the retina. A laser pointer could not cause corneal scarring (though pilots exposed to visible laser light have subsequently rubbed their eyes so hard that they scratched their corneas &mdash a painful and fortunately temporary condition).
Just for reference, a 5 mW laser is an eye hazard up to about 50 feet from the laser. For a pilot who is 1320 feet in the air, the laser light would be far too weak to cause any eye injury.
Despite the flaws in the CSI:Miami episode, it is good to inform the public about the general idea that laser pointers can potentially be hazardous. This is why you should never aim a laser at or near an aircraft.
According to your letter, the laser level is Class 3R with an output less than 5 mW. This would not cause eye injury assuming your 2-year-old son blinked, turned his head or otherwise did not stare for seconds into the beam.
Even if the laser is slightly more powerful than 5 mW &mdash for example, if it actually was 10 or 15 mW &mdash any possible injury would be minor and would heal.
I understand that you have been to two ophthalmologists. All of them could not see your son&rsquos entire retina, but they did not see any injury or abnormality in the central retina. This is to be expected. Should there be &ldquohidden&rdquo damage in the outer part of the retina, even this would not adversely affect useful vision.
If this case involved laser pointers, I would be more cautious. Often laser pointers and handheld lasers that only emit beams are mislabeled. They may say &ldquo (Thanks to JP, who asked this question April 20 2018)
At a power level of 5 milliwatts, there is no damage that can be caused by quick glances into the eyes.
That said, the teacher showed EXTREMELY poor judgment and set a bad example for the children. No one should ever deliberately aim a laser pointer at someone's face or into their eyes.
Five milliwatts is the maximum allowed in the U.S. for a laser to be sold as a "pointer". This low power level would not cause eye injury for a momentary exposure, where the laser is being waved around or flashed across eyes. However, if a person was to deliberately stare into a 5 mW laser pointer for many seconds, then heat can build up in the eye and cause retinal damage. But that was not the case in this situation, so the children are OK.
I originally had concerns over the purple laser pointer, since it was probably well over 5 mW. (This is because human eyes do not pick up blues, purples and reds as well as green and yellow light. If a green 5 mW laser looks a given brightness, a red 5 mW laser will appear only about 25% as bright, and a purple laser would appear only about 3% as bright. Said another way, the purple laser would have to be roughly 30 times more powerful, around 180 mW, to appear as bright to the eye as a green 5 mW laser.)
Fortunately, I understand from a phone conversation that the teacher aimed the purple pointer only at the ceiling, not at the children. This is good. However, the other two lasers also should have been aimed well above or away from the children as well. I am glad to hear that you spoke with the teacher and he will not be doing this again.
(Thanks to JM, who wrote December 6 2018)
I did not see the item at eBay when I just looked. Can you send a photo of the laser? Can you tell me what type of battery it uses?
Generally, if a handheld laser uses button batteries, one AA, or one AA then it is probably in the safe Class 2/Class 3R range of being below 5 mW.
If it uses more than one AA or a battery larger such as 18650 then it is probably Class 3B which is not considered safe.
If the website says the power is 0.5 mW (1/2 milliwatt) this is probably completely untrue, as the Class 2 limit is 1.0 mW and 0.5 would be even dimmer.
Certainly if the laser can light a match and can char or burn dark materials it is Class 3B (above 5 mW).
The power of the laser can cause a problem to the eye by reflection. Specifically, it is a "diffuse reflectance hazard" meaning that even looking at the dot on a close-up surface could cause eye injury. A special problem is that a person might be staring at the dot, for example to keep it on a match head while trying to light it, and so any damage would be in the central part of the vision of both eyes.
I would never look directly at the laser dot in such a case. If for some reason I had to burn a match head etc., I would use safety glasses, or if not available, dark sunglasses and not look directly at the dot, only off-axis.
I would also only allow a 16-year old to use a Class 3B laser when they are supervised by a responsible adult.
(Thanks to BT, who wrote Feb. 6 2020)
The very short answer is &ldquono&rdquo. Here are more details:
In stores you often see a hand-held scanner which is aimed at the Universal Product Code &ldquostripes&rdquo on a package. Or you see a window in the checkout counter, over which the UPC code is passed.
Sometimes LEDs are used as the light source, but often lasers are used. You can tell if it is a laser because there will be a pattern or geometric shape made up of thin lines this is one example:
Image via Wikipedia, by Alessio Damato
Having the scanned laser light go into your eyes is not hazardous. In the U.S., the laser power for a checkout scanner must be below 5 milliwatts. This is the same as the power limit for a laser sold as a pointer.
It is difficult enough for a 5 mW laser pointer to cause damage to a person&rsquos eyes. You pretty much have to stare at the beam at very close range, making a deliberate exposure to the single &ldquodot&rdquo of the pointer. A 1998 Lancet article by Mensah, Vafidis and Marshall states &ldquoA 5 mW laser with high retinal irradiance is too weak to cause retinal damage, even if shone in the eye for several seconds.&rdquo
For a store&rsquos laser scanner, the power is further spread out by being scanned rapidly over an area. This means that the &ldquodot&rdquo of laser light cannot remain on the same area of the retina and build up heat.
While having a laser scanned pattern in your eyes can be annoying (and rude if deliberately done by a cashier), there is no cause for concern.
A very quick test is to close your eyes and see if you have any afterimage from the exposure (similar to the afterimage caused by a camera flash, or the sun glinting off a reflective object.) Normally you should not have an afterimage, or it should fade in less than a minute. If you do have a longer-lasting afterimage, or any new spots in your visual field, you may wish to have an eye exam by an ophthalmologist or a retinal specialist. Even then, this is suggested only for an extraordinary exposure such as deliberately staring into the scanner, or if the scanner is suspected to be malfunctioning and is brighter than normal.
It would not be economical to require laser scanners to have additional safety features, such as eye detection (to turn off if they see an eye) or a direction detector (to turn off unless the laser is facing downwards). Between the low power of the laser itself, plus the added safety of a constantly-moving beam, an accidental or unwanted exposure is not hazardous.
(Thanks to L.B., who asked this question February 10 2016)
As I understand your request, you want your friend to aim a laser pointer at you so that you will see the light and then know his location. This will happen outdoors in dim conditions: pre-dawn and dusk. You'll be in a stand with a window so you can look out and see his location.
Yes, your plan is feasible and safe with a low-powered Class 2 laser pointer (less than 1 milliwatt). You can get these online or at a pet or office supply store. The cost should be just a few dollars.
Either red or green should be fine. There is no difference between them in terms of eye hazard (color does not affect the hazard). The green will be more visible, but then you are at a relatively short distance so you should see the red just fine. And red is usually less expensive.
Just to be clear, the technique is that your friend will aim the laser pointer towards your location. You'll see a flash of light, as if he had aimed a red flashlight at you. One difference is that the beam is narrow enough so that only persons (or animals) looking straight back at the laser will see the flash.
You will NOT see the beam in mid-air except perhaps in unusual conditions such as rain, mist or fog.
Don't worry about eye injury. The Nominal Ocular Hazard Distance for a 1 mW laser is 24 feet. Plus you will not be staring into a steady light instead, your friend will be aiming with his hand which is difficult to hold steady on a target. So the beam would be in your eye for only brief instants.
The only thing to worry about is if you get a mislabeled pointer. The pointer label might state that it is Class 2, but for various reasons -- evasion of import restrictions, incompetence at the factory -- the actual light output might be significantly higher than the U.S. limit for laser pointers which is Class IIIa or 3R (less than 5 milliwatts).
My suggestion is to buy three laser pointers, made by different companies, from different sources. They cost so little that this is a small investment in safety. Check out their brightness and use the dimmest of the three. If they all look about the same, great. But if one or two are much brighter, then do not use these for your hunting application.
Finally, it should go without saying: DO NOT USE THE LASER SIGHT ON A FIREARM for this application. If you have a gun or rifle that uses a laser for targeting, NEVER aim that laser at a person (except if you are a police officer, or for self-defense when you intend to fire upon a person).
For the location application described above, use a handheld laser pointer, preferably in a color different than the color on your firearm. For example, if your hunting companion has a green laser sight, use a red pointer for locating purposes. If you see a red light aimed at you, fine. But if you see a green light you know that somehow your companion has mistaken you for a target and you need to take action.
(Thanks to Phil D. who asked this question October 5 2017)
I have some standard laser gunsights from Leapers now mounted on rifles with scopes. Both are very good, long distant ones with bright green laser dot, not cheapies.
If I look through my scope, close up or a far view setting, and have my green laser on, the bright green laser is now brightly centered in the scope's reticle.
My question is if this is safe to the eye's retina to view a green dot laser this way, being thus magnified in the scope?
Yes, it is safe to view the green laser dot at a distance through the scope's reticle.
Laser light is dangerous when the beam is directly aimed into your eye. This is because the eye's lens will focus the beam down about 10,000 times to concentrate it on your retina. Whether an injury occurs will depend on the laser concentration (irradiance which is power over a given area) and on how long the beam sits at the same point on the retina.
Using a reticle, binoculars, camera, etc. to look at a distant laser dot is fine.
It is possible to incur an eye injury by looking at just the dot up close. This is called a diffuse exposure. It happens with very powerful lasers, when you are staring for a number of seconds. For example, if someone is trying to burn a hole in paper or light a cigarette with a Class 4 laser, and they stare right at the laser dot as things are smoldering --not smart!
Coming back to your use. again, looking through the reticle in the way the laser gunsight is intended to be used is perfectly safe.
(Thanks to Steve T., who asked this question December 13 2018)
The Lasergain XL has 32 low-powered red lasers.
The idea is for balding persons to move the device over their scalp like a comb.
The Lasergain XL appears to be safe. According to the manufacturer, each of the lasers is Class 3R, meaning less than 5 milliwatts. This will not hurt your eyes for momentary exposure. In fact, if you wanted to injure your eyes you'd have to deliberately stare into one of the lasers.
Sometimes there are products where the laser is claimed to be lower power, perhaps to get around import or safety regulations, but really the laser is much higher power. This does not appear to be the case with the Lasergain XL, as far as I can tell from the website information. So each laser is likely to be Class 3R, and even if it is slightly higher in power it would not become hazardous for momentary exposure until it is at least 10 times Class 3R, or 50 mW. (I assume all of the lasers look about the same brightness. If one is much brighter, simply cover it with black electrical tape or similar.)
In addition to the low power, the laser light appears to be diffused. From the scalp photo, these do not look like 32 sharp dots from narrow beams. Instead, they are wider areas of light. This is spreading the light out, so that if you looked into the beam(s) all 5 mW from each laser would not be focused to a pinpoint on your retina. This diffusion is safer than the same amount of laser light focused in a sharp beam.
The reflection off your glasses also increases safety compared with directly looking into the laser light. Reflection off a typical glass surface will be about 4 to 8 percent of the original beam power -- so a reflection has less power than the direct beam.
Incidentally, having 32 lasers instead of one does not matter too much from a safety standpoint. It is not possible for all 32 lasers &mdash or even 2 lasers &mdash to be focused on the same area of the retina at the same time. If you were to stare into the device long enough you would get 32 smaller retinal injuries, not one large 32-times injury.
(Thanks to the person who asked this question May 16 2019.)
I have recently purchased a galaxy starry sky projector light, and my husband pointed out that some classes of laser are harmful even if reflected off a surface. I have since been anxiously searching to determine if my light is safe! I have not been to come to any conclusive answer. I'm hoping you might be able to help answer whether or not this product is safe to use?
The box says that it is a Class 3R laser with wavelength 532 nm. I have it set up on a table so that is projects upward onto the ceiling. Are there any hazards with staring at the ceiling for an hour, with the laser projecting off the white walls, or shinier painted trim, or ceiling fan?
Assuming the laser star projector is correctly labeled, it would not be potentially harmful as long as you do not stare directly into the beams, or into a sharp, mirror-like reflection of the beams.
I say "correctly labeled" because some laser pointers and laser light show projectors have Class 3R labels indicating they are safe and legal (in the U.S.), but the laser is much more powerful. So you should always treat any laser, especially from a questionable source or unknown brand, as if it is more powerful than it really is.
From the Amazon description, seller Hei Liang appears to be knowledgeable about safety laws and thus probably the device is Class 3R. A U.S. brand, Blisslights, makes a similar projector. They originated star projectors. If you want to get a star projector guaranteed to comply with U.S. laws &mdash perhaps because you want to use the projector around children &mdash then check into the Blisslights version .)
Here is the laser star projector you purchased:
Inside the star projector is a relatively powerful laser. The beam goes through a holographic diffraction grating and is split up into hundreds of smaller "beamlets". For a legal-to-sell Class 3R laser projector, the maximum power of the strongest beamlet cannot exceed 5 milliwatts.
Don't worry about beamlets somehow combining. You cannot simultaneously get two beamlets in your eye at the same time AND have them focus to the same spot on the retina.
This means the maximum power that would be on your retina in any one "dot" area would be 5 mW. This power is recognized to be safe as long as a person does not deliberately look into the laser beam, or at the sharp reflection of a laser beam (e.g., reflected from a mirror in which case the beam is about 96% of its full strength).
It is OK if you accidentally get beamlets in your eyes for example, if you are walking through the starfield. If the exposure is short -- if you are not staring -- you'll be fine. Even if you should stare for a couple of seconds you'll be fine.
The safety issue with lasers is that the coherent light can be focused by the eye down onto a very small spot on the retina. It is kind of like using a magnifying glass in the sun to burn a leaf. The leaf is fine in the regular non-magnified sun, but if you hold the focused sun dot on the leaf long enough, it starts to smoke. With the low power of a Class 3R laser we are not talking about smoke of course. But the general principle applies that you don't want a laser dot to be on your retina in the same spot, building up heat. That's why if you are moving through the beamlets, and/or if you blink and turn away, the retinal dots don't stay in the same exact spot on the retina. They can't build up heat. Again, that's why we say "Don't stare into the beam".
I would not recommend putting a laser star projector where a child could intercept the beamlets. They may stare into the beams since they don't know any better. It would be fine in a child's room on the ceiling. If you have a curious child I would keep it out of their reach, or only have it on when you are in the room as well.
You don't need to be afraid of the device -- even around children -- but you do need to take some common sense steps. It sounds like you already have, by researching any potential issues with the laser.
(Thanks to Rebekah C. who asked this question August 2 2020)
First, the laser power already is low. You have a Class IIIa (also called 3R) laser which has a maximum output of 5 milliwatts. This is considered safe for momentary (less than 0.25 seconds) unintentional viewing of the direct beam going into your eyes. As long as you do not override your blink reflex or aversion response, and look directly into the laser beam for many seconds, you will be fine.
Second, the laser&rsquos already low power is being spread out in two ways: 1) By being made into a line instead of a dot and 2) By hitting the paper and thus diffusing in many directions. Note that you can see the laser line from many different angles and positions. This indicates the beam power is spreading throughout the room. Your pupil is intercepting just a small part of all that diffused light.
Even though you are looking at the diffused line for an hour or two, this does not &ldquobuild up&rdquo damage.
For visible-light lasers, the primary eye injury mechanism is thermal. Visible light goes through the clear lens where it is absorbed on the retina. If the power is too strong, and the light stays in one area long enough, heat cannot be carried off by blood vessels, and the retina will start to burn.
Thermal damage does not accumulate over time. It is like being in a house for many hours which is at a comfortable 72° F (22.2° C). This does not &ldquobuild up&rdquo so you are overheated or start to burn &mdash you remain comfortable.
(Note that blue light can cause photochemical damage which would require a separate analysis. In this case, the laser level light is red so the only damage mechanism is thermal.)
What power would it take to be a potential hazard? A 499 milliwatt laser &mdash the most powerful Class 3B laser &mdash is a diffuse reflection hazard if you aim the visible-light laser &ldquodot&rdquo at a piece of white paper, and your eye is within 5 inches (12 cm) of the dot and you stare at it for more than 10 minutes. Keep in mind the laser beam is not going directly in your eye. The light is bouncing off a piece of paper or other non-reflective surface that spreads out the light in all directions.
It is primarily Class IV (4) lasers &mdash 500 milliwatts or more &mdash that can realistically be diffuse reflection hazards. For example, if you look at the dot from a 1000 milliwatt (1 watt) visible laser, and your eye is within 1.5 feet (44 cm) of the paper, and you stare at it for more than a minute, this could potentially cause a retinal burn. If you look at the dot from a 10,000 mW (10 W) visible laser within 1.8 feet (60 cm) for more than 10 seconds, this could potentially cause a retinal burn.
In summary, looking at a diffuse line of red light from a Class IIIa (3R) laser, even for a number of hours, will not cause any eye injury or damage.
(Thanks to Eugene from Ukraine, who asked this question February 7 2017)
According to three experts consulted by LaserPointerSafety.com, the exposure was not potentially hazardous.
Although the person looked at the light at close range &mdash the projector was less than a meter from the tracing paper, and the person&rsquos eye was less than 1/2 meter from the paper &mdash and for a long time (up to 6 hours), the reflected laser light from the paper was not strong enough to cause eye damage.
The exposure was insufficient to cause photochemical damage, where ultraviolet or blue light exposure causes &ldquosunburn&rdquo of the cornea and lens. And it definitely could not cause thermal damage, where concentrated laser light like from a beam goes through the transparent cornea and lens, and is absorbed by the retina.
The experts suggested other causes, particularly age-related macular degeneration which can occur over a short period of time.
More information is on the Laser pico projectors page scroll down to the &ldquoCase study&rdquo section.
(Thanks to B.H. who asked this question August 28 2017)
No, if you see laser light as you watch TV or videos on a screen, your eyes cannot be hurt. There is no actual laser light emitted by the screen &mdash just a video picture of what the laser light looked like to the camera sensor.
However, a direct beam into the camera lens might damage the camera sensor .
(Thanks to Carla F. who asked this question December 3 2019.)
The U.S. law signed by President Obama in Feb. 2012 makes it illegal to knowingly aim laser pointer beams at an aircraft, or at the flight path of such an aircraft.
Fortunately for amateur astronomers or other legitimate outdoor users, there is little chance of having the flight path clause invoked by prosecutors, for the following reasons:
- The cases that are brought for trial are ones where a person deliberately aimed at an aircraft. Someone on the aircraft saw beams coming near or at the aircraft. They then either called police, or they were the police.
- In most prosecuted cases, there are multiple beam illuminations involved -- e.g., a laser is tracking the aircraft and illuminates it multiple times. It is rare for any single-illumination incidents to be identified or prosecuted.
- Usually the person prosecuted has some sort of antisocial characteristic such as a criminal record, being on probation or in a gang, being hostile with arresting officers, possessing drugs at the time of arrest, etc.
In an abstract sense, any laser beam in the sky is probably touching some aircraft's flight path. But this has not been the type of case that worries safety experts, or the type of case that prosecutors bring to trial.
Not in the United States, because drones (unmanned aircraft systems) are considered aircraft by the Federal Aviation Administration. Since it is illegal to aim a laser pointer at an aircraft, or the flight path of an aircraft, in U.S. airspace, then it is also illegal to aim at a drone.
We do not know whether it is illegal in other countries to aim a laser pointer at a drone. We suspect it might be, for two reasons. First, countries outside the U.S. often adopt aviation policies similar to the FAA. Second, the laser light can be hazardous to the drone operator's view.
The light from a laser pointer could block the view of the operator's camera. It could also possibly damage the camera sensor &mdash especially since camera sensors can be more sensitive to laser damage than the human eye.
This answer will be for all lasers and pointers. In the U.S. there is not a distinction between selling to businesses or to consumers. I do not know if other countries make such a distinction.
In the U.S., the Food and Drug Administration (FDA) defines a "laser pointer" as a laser that the manufacturer or seller calls a "pointer" or which is sold for pointing purposes. Such a laser is required to be emit less than 5 mW. It is possible to sell higher-powered handheld lasers, but they cannot be "pointers" or for pointing, and they must have all the safety features of their class (3B or 4) such as keyswitches or similar, emission indicators, etc. FDA tries to discourage these so you may have to work to get such a laser approved.
I am not as familiar with European laws. Some countries are very strict such as the U.K. and Sweden. They may restrict your product if the laser inside is a pointer (small, handheld, a momentary on/off button, etc.).
I'm sure there are countries outside the U.S. and Europe which have no laser regulations -- or no effective enforcement -- where your product could be legally sold. Whether it is a good idea to sell kits with 100-400 mW lasers is a different situation.
In the U.K. and all European countries except Switzerland, I believe the limit on laser pointers is 1 mW (not 5 mW like in the U.S.). In Switzerland the limit is Class 1 or 0.39 mW.
If you do want U.S. certification, there are companies that can help such as Laser Compliance Inc., Phoenix Laser Safety, and Rockwell Laser Industries. All can be found via Google.
One final note about your kit. In the U.S. the laser importation form, FDA Form 2877 &ldquoDeclaration for Imported Electronic Products Subject to Radiation Control Standards,&rdquo allows for unfinished laser products to be imported. But FDA recognizes a difference between "true" unfinished lasers (for example, lasers that will go into an OEM product) and people trying to get around the law by importing everything but the power cord and labels. Keep this in mind as you design your kit.
(Thanks to E.A. who asked this question December 12 2019)
Hello. I wonder if it is safe to stare directly into a low power laser pointer beam.
I have a red laser pointer with max power 200 mW, but I have reduced its power (by connecting it to a MasTech HY3002 power supply and limiting both voltage (to 1.8 V, normally driven by 3V) and current (below power supply least significant digit) so the spot is barely visible on a white paper. The output power is likely below 1 mW (measured by a thermocouple-based power meter, designed for CO2 lasers). I wonder if it is safe to stare directly into such an downpowered beam. The visible laser beam is used to properly align a high-power (70 W) CO2 laser beam.
From the start I want to clearly define your phrase "stare directly into". I assume you mean that the beam will go directly from the laser into a person's eye.
I mention this because you said the purpose is to align a 70W CO2 laser. I personally would not stare at a device where there is a 70W laser at the other end, even if not energized. Maybe if the AC power plug is out, but then again how does the low-power laser beam get energized?
If you mean "stare directly at " the dot from a laser beam then that is safe. Looking down on a work surface at the dot from a less-than-1 mW laser is safe. I don't know exactly how long that is safe for, but I'm pretty sure it would be safe for an 8-hour exposure. That's because it is not the direct beam into the eye, but a diffuse exposure where the dot is on a surface that diffuses the light more or less uniformly.
For staring directly into a laser beam so it goes directly into a person's eye, there are a couple of considerations. First, how long is the longest "stare" into the laser beam? A few seconds, or minutes, or hours?
Below is a table of selected Maximum Permissible Exposure values for various times. Note that the MPE levels listed are in milliwatts per square centimeter . This is power over an area (irradiance). It is NOT the power in milliwatts coming out of the laser.
For example, the Class 2 limit for a laser is just under 1 milliwatt. This is safe for momentary exposure in the human eye. That time is taken to be 0.25 seconds or less, meaning that a person exposed to a Class 2 laser should blink, turn away, etc. within 0.25 seconds and there is not expected to be any visually detectable injury. From the table, the MPE for a 0.25 second exposure is 2.54 milliwatts per square centimeter.
I can't tell you right off what the power of a laser that is safe for a 1 second or a 10 second exposure is. However, consider the Class 2 laser. The power is 1 mW, the MPE is 2.54 mW/cm². Assume the ratio of 1:2.54. That would mean for a 1 second exposure, the laser output power would be 0.71 mW (1.80 / 2.54) for a 10 second exposure it would be 0.40 mW (1.01 / 2.54).
If you had a laser pointer diode with a maximum output power of 1 mW, and you limited the voltage and current so the light became significantly dimmer, I might be comfortable with allowing a person to look directly into the beam for a few seconds. That's because if anything goes wrong with the circuit, the maximum power output is 1 mW. If the light suddenly gets brighter, the person can blink, turn away, etc.
I'm not at all comfortable with your setup. You are starting with a 200 mW diode. If anything goes wrong, a person could get 200 mW in their eyes which may cause injury before they can react to blink, turn away, etc.
Now, you could put the 200 mW diode through a neutral density filter of OD 2 (100 times attenuation) so the output is around 2 mW, THEN limit the voltage and current. Or simply use an OD 3 filter (1000 times attenuation) so the beam is 0.2 mW. I do not know if the dot from that laser would be visible on white paper in a normally lit indoor room.
Also, a thermocouple-based power meter is NOT valid for such low powers. These work by measuring the heat generated by the laser beam. A beam with power this low does not generate any appreciable heat. It is hard to tell the laser heat from normal atmospheric fluctuations.
You need a silicon-based power meter. Coherent sells a handheld wand-style power meter called the Laser Check , for about $400. I have seen less expensive ones on eBay. They may be fine for hobbyist purposes, but I personally would not trust them with my vision.
(Thanks to an anonymous person who asked this question September 18 2020)
Surprisingly, there is no generally accepted definition of a laser "pointer".
In the U.S., the federal FDA/CDRH indicates that pointers are "hand-held lasers that are promoted for pointing out objects or locations" with output power less than 5 milliwatts. According to FDA, promotion of lasers above 5 milliwatts "for pointing and amusement" violates FDA requirements and U.S. law.
(Some may consider this to be a loophole. If a hand-held laser is not promoted for pointing or amusement purposes, then it can legally be sold.)
Starting in 2010, FDA/CDRH appeared to be closing the loophole by defining handheld portable lasers as "surveying, leveling and alignment" (SLA) lasers. Since FDA/CDRH has authority over SLA lasers, the agency may use this new regulatory interpretation to limit the sale of handheld portable lasers over 5 milliwatts. For more information, see the page FDA authority over laser pointers and handheld lasers.
In New South Wales (Australia), a pointer is a Schedule 1 Prohibited Weapon: "A laser pointer, or any other similar article, consists of a hand-held, battery-operated device with a power output of more than 1 milliwatt, designed or adapted to emit a laser beam and that may be used for the purposes of aiming, targeting or pointing."
In Victoria (Australia), a pointer is also a prohibited weapon. It is defined as: "A hand-held, battery-operated article designed or adapted to emit a laser beam with an accessible emission limit of greater than 1 mW."
If one wants to own a laser with greater power, it is easy enough to do so. There is the inconvenience of having to run off of mains (AC) power, but then again AC outlets are everywhere, including automobiles (using a low-cost inverter like the one shown below).
Also, if an evil person wanted to do harm with a laser beam, it would be easy for them to use a regular laser. A ban or restriction on pointers would have no effect on them.
More information on existing and suggested definitions of &ldquolaser pointer&rdquo is on the page &ldquoIf you are writing a laser law&hellip&rdquo.
There is no "maximum" power in the U.S. and many other countries. A person can buy a laser of whatever power they want, even tens of watts.
For use by the general public as a laser "pointer", the maximum is supposed to be 5 milliwatts (U.S.) or 1 milliwatt (U.K.). Obviously, much more powerful handheld lasers are available. As long as they are not advertised for pointing or beam-display purposes, sale of lasers above 5 mW is legal in the U.S. Also, it can be difficult or low priority for law enforcement to track down illegally-marked or distributed lasers.
For more information, see the Rules for U.S. consumers page.
The short answer is that laser pointers under 5 milliwatts (U.S.) or under 1 milliwatt (U.K.) are legal for sale. Details are below.
Manufacturing and sales: Lasers sold to the public as "pointers" or for pointing purposes must be less than 5 milliwatts (5/1000 of a watt). This power is high enough so the laser "dot" is sufficiently visible for pointing out things, and is low enough to not be an eye hazard under conditions of accidental exposure. It is not intended or legal to sell lasers for pointing that are 5 milliwatts or more. Beginning in 2010, the FDA/CDRH is classifying handheld portable lasers as "surveying, leveling and alignment" (SLA) lasers, and may be trying to further restrict sales of lasers above 5 milliwatts based on this new rules interpretation. For more information, see the pages Rules for U.S. sellers and FDA authority.
Ownership: There is no federal law against owning a laser, of any power. (Some states and localities may have their own laws.) Therefore, at the federal level, an "illegal laser pointer" is illegal only from the manufacturer's or seller's standpoint. An "illegal" laser is too powerful to be sold or promoted for pointing purposes, or it may be lacking required safety features. If such a laser is sold to end users, the manufacturer may be required to do a recall, repair, replacement or refund. It is then up to the end user whether they wish to comply with the recall, repair, replacement or refund notice. [NOTE: This analysis is based on LaserPointerSafety.com's research. If you need specific legal advice, consult a lawyer with experience in this area.] For more information, see the page Rules for U.S. consumers.
From a safety standpoint, what you should do depends on the laser's power. There is no need for a laser over 5 milliwatts for most indoor pointing purposes. For outdoor daytime use, a higher power such as 50 milliwatts would be necessary so the laser &ldquodot&rdquo is visible on a surface. For astronomy pointing purposes, you can see the beam of a green lasers in the 4 to 20 mW range.
Even for most experimenters and enthusiasts, there is usually no need for above 50 mW (exceptions: popping/burning experiments or home laser shows).
If you have a laser in this "extra caution range" of roughly 5 to 20-50 mW, you can discard the laser if you want, or use it with added care. Be especially careful not to annoy or injure bystanders. It is one thing if you are hurt, it is another thing to involve someone else.
If the laser is above 20-50 milliwatts, hazards are increased. Except for mature and careful experimenters, we recommend that you safely discard the laser.
LaserPointerSafety.com does not recommend that the general public own or use Class 4 lasers, which are 500 milliwatts and above (above 1/2 watt). If you must have a high-powered laser for some reason, be sure to read and always follow the safety warnings.
There is good information on the page Don't aim at head and eyes be sure to download and read the appropriate PDF flyer for your laser's power level. There is also information about the hazards and safe usage tips for Class 2 (up to 1 mW), Class 3R or IIIa (1-5 mW), Class 3B (5-500 mw) and Class 4 (500 mW and up) visible lasers.
In the United States, lasers above 5 mW (Classes 3B and 4) must have proper labeling, an emission indicator, and an interlock with a key or pin that prevents emission if the pin/key is removed. Note that this means the laser can remain continuously on as long as the pin/key is inserted and the switch or button is turned on (there does not have to be a momentary pushbutton that turns off when pressure is released). Also, lasers above 5 mW cannot be marketed as "laser pointers" or for purposes of surveying, alignment or pointing.
There may be some confusion between the original U.S. laser laws, 21 CFR 1040.10 and 1040.11, and CDRH's Laser Notice #50, which was first issued in 2001 and was updated in 2007. Laser Notice #50 allows U.S. marketing of laser products certified using international standard IEC 60825-1. This removes the requirement for a shutter, and for an emission delay circuit. Also, warning labels can follow IEC instead of CDRH, if desired. This harmonizes U.S. law with international standards.
50 milliwatts is probably the maximum needed power for almost any laser pointing use.
For seeing the laser "dot" on a wall or surface indoors or in dim light, 5 milliwatts of green is fine. The most demanding general-use pointing application is for pointing out objects in bright sunlight such as a daytime city architecture tour, and for pointing out stars at night when it is necessary to see the beam in mid-air. For these uses, 5-25 mW should be fine, with a maximum of 50 mW for tough situations (high ambient light brightness, showing stars to a large group).
The Norwegian Radiation Protection Authority states that 20 mW is the limit: &ldquoIt is not known that laser pointers that are stronger than approx. 20 milliwatts can be used for anything useful. The effect of laser pointers used to point out constellations and like at night, should not exceed a maximum of 20 milliwatts. The reason is that the beam can destroy the night vision of the spectators so that they can no longer perceive weak starlight.&rdquo (For Norway&rsquos regulations, see the International laws page, then click on the item &ldquoNORWAY: Possession and use regulated.&rdquo)
A 2010 study, &ldquoGreen Laser Pointers for Visual Astronomy: How Much Power Is Enough?&rdquo, had 23 observers adjust the power of a 532 nm green laser beam &ldquopropagating skyward through the atmosphere in a heavily light-polluted urban setting.&rdquo The lowest power where the beam could clearly be seen was between 1.4 and 5.6 milliwatts. The average of all powers chosen was 2.4 milliwatts. The authors concluded that &ldquoGreen laser pointers with output powers below 5 mW (laser classes American National Standards Institute 3a or International Electrotechnical Commission 3R) appear to be sufficient for use in educational nighttime outdoors activities, providing enough bright beams at reasonable safety levels.&rdquo
If you like to pop balloons, ignite matches, or put the laser through textured glass for a private light show in your home, you may want a more powerful laser. But this is no longer a POINTER application.
A green laser is the most visible. The eye sees green better (more efficiently) than other colors. A 5 mW green laser will appear much brighter than a 5 mW red or blue laser.
Note that in terms of eye injury hazards, the color does not matter. More milliwatts means a greater potential eye hazard, no matter what the beam color. (This is for visible lasers for infrared or ultraviolet lasers, the primary injury area is the cornea and not the retina.)
For more information on the apparent visibility of different colors, see the page Basic principles of hazards, item #5, about green lasers being more of a visual hazard than an equivalent red or blue laser.
In some news stories, sometimes an authority such as police or military will state that a &ldquomilitary grade&rdquo or &ldquomilitary strength&rdquo laser was used.
This is an imprecise term with no established meaning. The military does not grade or rate lasers except by the standard Class 1, Class 2 etc. classification system used for all types of lasers.
The authority is probably trying to say simply that the laser was stronger than a laser pointer. In the U.S., lasers sold as pointers cannot be over 5 mW in many other countries the limit for consumer lasers is 1 mW.
Lasers more powerful than this are readily available to consumers. Usually they are obtained from small-scale Internet sellers, or are purchased when on an overseas vacation.
The imprecise term &ldquomilitary grade&rdquo does NOT mean the laser was used by, or obtained from the military. It does NOT mean the laser has any special capability that &ldquocivilian lasers&rdquo wouldn&rsquot also have.
In rough terms, it may mean a laser at the higher end of Class 3B (a few hundred milliwatts) or Class 4 (anything above 500 milliwatts, which is the same as 1/2 watt). These lasers can cause eye injury for short exposures, and can be a skin burn hazard at relatively close ranges. It is important to note that laser light spreads out, so a &ldquomilitary grade&rdquo laser beam that is a significant eye hazard at close range, could be relatively safe or totally safe hundreds or thousands of feet away.
Finally, in cases where the laser is not found, it would be difficult to know how powerful the laser is. While some rough estimates can be done, it would not be wise to state with certainty that a particular power or Class of laser was used &mdash at least not without having the laser or having some additional information such as measurements of the beam power or irradiance.
Thanks to G.L. for asking this question, August 13 2018.
&ldquoCommercial grade&rdquo or &ldquoindustrial grade&rdquo are imprecise terms, probably intended to mean lasers with more power than laser pointers (1-5 mW).
There is no widespread use of these terms. There is no generally accepted definition.
See the answer above about &ldquomilitary grade&rdquo. Everything said about &ldquomilitary grade&rdquo also applies to the term &ldquocommercial grade&rdquo or &ldquoindustrial grade&rdquo.
Banning or severely restricting laser pointers seems like a simple, attractive solution to misuse such as pointing at aircraft. However, there are a number of problems:
- It is hard to effectively define laser pointers. To give one example, if "battery powered" lasers are banned, it still is relatively easy to find AC outlets in public spaces, or to use a low-cost inverter to run a laser off a car's 12-volt power socket.
- It is hard to enforce. In a world with Internet sales by mail, and easy world travel, it becomes difficult to check every package or person at Customs to see if they have a laser pointer.
- It does not stop someone who really wants a laser. It is easy to get new or used lasers, either by themselves or built into equipment. DVD and Blu-Ray players, and some video projectors, contain powerful laser diodes. If hobbyists can get these, so can anyone with evil intent. Said another way, "When laser pointers are outlawed, only outlaws will have laser pointers."
- It stops legitimate use of laser pointers by teachers, business people, astronomy educators and others who find a laser ideal for pointing out objects.
- "It is like banning the kitchen knife because we have people using the knives incorrectly," according to Professor Hans Bachor, president of the Australian Optical Society, as quoted by the Australian Broadcasting Corporation.
Anyone who wants to deliberately use a laser for bad purposes can easily do so, ban or no ban. For example, on a per capita basis, Australia's rate is 2.8 times the U.S. rate despite the ban. While there may be other factors, this is an indication that bans may not work. (We have a list of aviation-related laser pointer incidents in Australia.)
In 2013, it was reported that Australia's bans had the effect of making online pointers more unsafe. Sellers illegally understated the power of lasers, so they could be imported. 95% of pointers tested were above the Australian limit of 1 mW, and 78% of those tested were also above the US limit of 5 mW. Persons interested in whether bans work should read the article "Ban on laser pointers has been a 'detriment' to safety.
We have additional information on the page Tax handheld lasers and pointers?
For an interesting perspective, see this online debate about banning laser pointers. Note that there are some inaccuracies or misconceptions in the material so do not rely completely on the arguments and data in this online debate.
The Economist magazine printed an article "The Case for the Defence" on October 30 2013, stating "But no aeroplane accident has ever been convincingly attributed to a laser pointer, and numerous fail-safes make such an accident highly unlikely. Also, high-powered laser pointers are fun and useful&mdashespecially for stargazers. It would be a shame to see them banned because of a few foolish people. One hopes that politicians will see the value in these sorts of products. One does, at least: earlier this month the governor of New Jersey, Chris Christie, vetoed a bill that would have banned even low-powered laser pointers in his state."
Laser misuse pales next to knife misuse. In 2010 there were 130,000 assaults yearly with knives and cutting instruments. Compare this with with the 7,703 FAA-reported laser illuminations in 2016. Also, compare the over 2 million serious knife injuries each year with the handful of eye injuries reportedly caused by handheld lasers.
Certainly laser illuminations and injuries should be reduced as much as possible, especially with regard to aiming at aircraft. But the above data helps to give some perspective on the relative risk of these two handheld devices. Statistically, a person is much more likely to be injured by or assaulted with a knife or blade, than to be injured by a pointer or to be on an aircraft illuminated by a laser.
(For detailed statistics on deaths and injuries caused by knives, see the "Knives" section of the Risks of pointers and other items webpage .)
Any law restricting laser equipment or usage needs to be carefully considered. It should effectively address the problem without infringing on rights of legitimate users. This page has some suggestions. In addition, check out the list of selected international and U.S. laser laws. You can read both well-written statutes, and poorly-worded ones.
You may want to have SAE G-10T take a look at your proposed law, to help provide suggestions for improvement.
The SAE G10T Laser Hazards Subcommittee studies laser uses in airspace. Members include laser safety experts, pilots, military safety officers, and laser users for industry, military, research, and displays. They write reports such as ARP5293, &ldquoSafety Considerations for Lasers Projected in the Navigable Airspace.&rdquo Their recommendations are often adopted by aviation authorities such as the U.S. FAA.
The G10T subcommittee is one of the few groups monitoring laser/aircraft incidents. If they called for restrictions or a ban on laser pointers, their recommendations would carry great weight.
More information is on this website's page SAE G10T Laser Hazards Subcommittee.
The International Laser Display Association represents manufacturers of laser shows and projectors. While many ILDA members own and enjoy laser pointers, the pointers are not needed in creating shows.
ILDA does not have an official position on laser pointers, or on laser misuse. ILDA as a sponsor has provided some resources for this website, as a public service. One reason is that, if the general public sees pointers as dangerous, this could have an indirect negative impact on laser show productions.
Keep up-to-date on laser pointer incidents and issues by following @laserptrsafety on Twitter:
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Viewer's Guide to Hybrid Solar Eclipse April 8
Residents in parts of the United States will have a chance to watch the Moon partially eclipse the Sun on Friday, April 8. Within a very narrow corridor that extends for about 8,800 miles, the disks of the Sun and the Moon will appear to exactly coincide, setting up the most unusual type of eclipse known as a hybrid.
Solar eclipses are caused when Earth, the Moon and the Sun line up just right and the Moon casts a shadow on our planet.
On rare occasions, the Moon is at such a distance from the Earth that its pointed shadow is just long enough to touch Earth for only a short distance along its projected path. The eclipse is only total where the shadow actually intersects the Earth's surface at other points along the eclipse track, the Moon appears ever-so-slightly too small to obscure the Sun's face entirely.
From these places an annulus, or ring of the Sun's surface, remains to be seen, thus there is an annular eclipse. In essence, this is really nothing more than a fancy partial eclipse.
The effect is like a dark penny atop a shiny nickel. The Sun becomes a blazing ring of light at maximum effect.
The path of the central eclipse (where the phenomena of annularity-totality may be observed) first touches the Earth in the south Pacific Ocean at 18:54 GMT, just to the south and east of the South Island of New Zealand. The eclipse starts off as annular, with the Moon covering all but 8/10 of a percent of the Sun's disk, leaving only an exceedingly thin, and rapidly narrowing ring of sunlight shining at maximum effect.
Ten minutes later, at 19:04 GMT, the tip of the Moon's dark umbra strikes the Earth about 1,400 miles south of Tahiti, and the eclipse becomes total. No Pacific islands of any appreciable size falls within this narrow path of totality.
Traveling northeast across the vast expanse of the south Pacific, the duration of the total eclipse gradually increases, with the greatest eclipse, featuring all of 42 seconds of a glorious totality, occurring at 20:34 GMT, far out over inaccessible open ocean waters. The totality path will be less than 17 miles wide at this point. The magnitude of the eclipse is 1.007, which means the Moon completely covers the Sun and 0.007 of a Sun-width more.
The total phase then diminishes and the totality track narrows as it nears its end, the path then turning toward the east. At 22:00 GMT, about 500 miles due north of the Galapagos Islands, the tip of the Moon's shadow can no longer reach to the Earth's surface, so the Moon can no longer completely cover the Sun. Thus, it morphs back into an annular eclipse.
The thread of this annular eclipse path makes its first landfall in Central America, at the border of Costa Rica and Panama, over Panama's Azuero Peninsula, barely skimming the southern outskirts of the Panamanian City of David, as well as sweeping over the coastal town of Pedregal.
Interestingly, the silhouette of the Moon is not a perfect circle, but rather it is slightly prickly with mountains, which are relatively much higher than those on Earth.
So just before the transition from annular to total and later, just after the transition from total back to annular, the eclipse will become something neither annular nor total: it will be a broken annular. As lunar mountains protrude onto the hairline-thin ring of the Sun, it will be seen not as an unbroken ring but an irregular, changing, sparkling sequence of arcs, beads and diamonds very briefly encircling the Moon: a "diamond necklace" effect! This is a spectacle that viewers in the Panama and possibly Costa Rica might see.
The path then quickly slides across the base of the isthmus where Central joins to South America, then over the Gulf of Uraba and on into northern Colombia and central Venezuela before finally coming to an end at local sunset, at 22:18 GMT.
Parts of North America will see this as a partial solar eclipse. However, there will also be locations that will see nothing of this eclipse.
Partial eclipse: The Moon covers only part of the Sun.
Total eclipse: The Moon covers the entire disk of the Sun along a narrow path across the Earth.
Annular eclipse: The Moon is too far from Earth to completely cover the Sun. A thin ring of the Sun's disk surrounds the Moon.
If you have an atlas of the United States, draw a line starting from a point roughly from Imperial Beach, California and extend it northeast to Quincy, Illinois, and then east to Perth Amboy, New Jersey. All places above (or north) of this line will not have any view of the eclipse.
Meanwhile, those localities below (or south) of the line will be able to see at least a part of this eclipse near sunset, although for those places in the immediate vicinity of this line, the Moon's "bite" out of the lower edge of the Sun will be tantalizingly small.
For example, while the eclipse will not be visible from New York City, just 85 miles to the southwest, at Philadelphia, the edge of the Moon's dark silhouette will appear to encroach upon the Sun at 6:07 p.m. ET. Twelve minutes later, maximum eclipse will be attained, with the Moon only obscuring about 2 percent of the Sun's diameter (or just three-tenths of one percent of the total area of the Sun's disk). The "eclipse" -- if we can charitably call it that - will come to an end at 6:32 p.m. ET.
As one heads farther south, the eclipse will last longer and this slight dent will evolve into a more noticeable scallop out of the Sun's left rim.
From Washington, D.C., the eclipse will last 41 minutes, with just over 5 percent of the Sun's diameter covered at 6:19 p.m. ET. Continuing southward, from Raleigh, North Carolina, the eclipse will last 69 minutes from start to finish, the Moon covering a maximum of 15 percent of the Sun's diameter at 6:20 p.m. ET.
Along the Gulf Coast, the eclipse will last about 2? hours prospective observers will see anywhere from about 30 to 40 percent coverage, while for those in the Florida Keys, it will be a nearly three hour affair, with the Moon appearing to obscure about half of the Sun's disk.
From San Juan, Puerto Rico, nearly 68 percent of the Sun's diameter will be eclipsed, maximum eclipse coming at 6:22 p.m. AST. Eighteen minutes later, the Sun will drop down below the west-northwest horizon, making for a most unusual sunset!
For full prediction details for many cities are available from NASA.
In addition, NASA astronomer Fred Espenak has a website dedicated to the upcoming April 8 solar eclipse which contains maps, tables and additional prediction details.
Caution: Don't Look at the Sun
To look at the Sun without proper eye protection is dangerous.
Unlike a total eclipse of the Sun, concentrating its excitement into a few fleeting minutes, a partial eclipse can be watched in a relaxed manner from wherever one happens to be. Providing proper protection is employed, bservations can be made with or without telescopes or binoculars. However, looking at the Sun is harmful to your eyes at anytime, partial eclipse or no. Most people are under the mistaken impression that when a solar eclipse is in progress that there is something especially insidious about the Sun's light.
But the true danger that an eclipse poses is simply that it may induce people to stare at the Sun, something they wouldn't normally do. The result can be "eclipse blindness," a serious eye injury that has been recognized at least since the early 1900's. About half of the reported victims of eclipse blindness recover their precious quality of eyesight after a few days or weeks. The other half carries a permanent blurry or blind spot at the center of their vision for the rest of their lives.
Public warnings by news media have vastly reduced solar eye injuries at eclipses in the last few decades. After the solar eclipse that crossed the United States on March 7, 1970, no fewer than 245 cases of retinal injury were reported. Of these people, 55 percent suffered permanent impairment of vision. In contrast, after the solar eclipse of May 30, 1984, Sky & Telescope magazine was able to locate only three cases of eclipse blindness in the entire United States. During any direct observation of the eclipse, your eye or must be protected by dense filters from the intense light and heat of the focused solar rays.
By far, the safest way to view a solar eclipse is to construct a "pinhole camera." A pinhole or small opening is used to form an image of the Sun on a screen placed about three feet behind the opening. Binoculars or a small telescope mounted on a tripod can also be used to project a magnified image of the Sun onto a white card. Just be sure not to look through the binoculars or telescope when they are pointed toward the Sun!
A variation on the pinhole theme is the "pinhole mirror." Cover a pocket-mirror with a piece of paper that has a ?-inch hole punched in it. Open a Sun-facing window and place the covered mirror on the sunlit sill so it reflects a disk of light onto the far wall inside. The disk of light is an image of the Sun's face. The farther away from the wall is the better the image will be only one inch across for every 9 feet from the mirror. Modeling clay works well to hold the mirror in place. Experiment with different-sized holes in the paper. Again, a large hole makes the image bright, but fuzzy, and a small one makes it dim but sharp. Darken the room as much as possible. Be sure to try this out beforehand to make sure the mirror's optical quality is good enough to project a clean, round image. Of course, don't let anyone look at the Sun in the mirror.
Acceptable filters for unaided visual solar observations include aluminized Mylar. Some astronomy dealers carry Mylar filter material specially designed for solar observing. Also acceptable is shade 14 arc-welder's glass, available for just a few of dollars at welding supply shops. It also used to be widely advertised that two layers of fully exposed and developed black-and-white negative film was safe. This is still true but only if the film contains an emulsion of silver particles. But beware: some black-and-white films now use black dye, which is no longer safe. It is always a good idea to test your filters and/or observing techniques before eclipse day.
Unacceptable filters include sunglasses, color film negatives, black-and-white film that contains no silver, photographic neutral-density filters, and polarizing filters. Although these materials have very low visible-light transmittance levels, they transmit an unacceptably high level of near-infrared radiation that can cause a thermal retinal burn. The fact that the Sun appears dim, or that you feel no discomfort when looking at the Sun through the filter, is no guarantee that your eyes are safe.
The next solar eclipse will occur Oct. 3 this year. It will be an annular solar eclipse with a maximum duration of just over 4? minutes that will sweep across the Iberian Peninsula and stretches across the African Continent. Madrid, Spain finds itself directly in the center of the annular eclipse track and will see the mid-morning Sun turn into a blazing ring of fire for over four minutes.
But the next solar eclipse visible over a large swath of North America won't come until May 20, 2012, when the path of an annular solar eclipse passes across portions of eight southwestern states.