To add to the other answers and Flippiefanus's answer that both invisible IR and UV light can permanently damage the eyes without any sensation of damage or injury being felt. Invisible IR damage causes damage through thermal loading: the retina absorbs heat from incident light faster than its vascular network can draw the heat away. High power UV can do the same, but for shorter visible and UV wavelengths a second mechanism is photochemical toxicity - where photons of energy comparable to organic molecule bond energies beget chemical changes in the retina or even nuclear in the biological sense DNA changes with the attendant neoplasia risk - is a much more dangerous factor because it is damaging at far lower levels than are needed for thermal damage to happen.
Long term, chronic exposure to UV is even more of a problem. Cataracts arising from photochemical damage to proteins are the foremost cause of human blindness on the planet. Very long term, chronic exposure to low levels of UVB such as one encounters on normal, sunlit days, especially at lower, tropical latitudes or snowy environments, are an overlooked hazard. One should generally encourage children to wear sunglasses, conforming to a sound eye protection standard, as the eye's lens is particularly transparent to UV under the age of Lastly, there is even some evidence that high peak powered IR pulses can give rise to severe photochemical damage and that the laser safety standards are inadequate in the way they deal with it.
See for example:. Glickman R. But it does sound thoroughly reasonable from a physicist's standpoint. On interaction with the complicated organic molecules in the eye, high peak power pulses yield much shorter wavelength light through nonlinear processes. Significant production of even UV can result, hence the risk of photochemical damage. The problem here is that the safety standards including ISO blithely assume that the thermal loading on the retina is the only problem.
Therefore, they are too forgiving of pulsed lasers with small duty cycles: the standards will accept a level of laser power as intrinsically safe if its average power is small. As I have discussed, very low levels of UV can be a problem, and this is even more so when the light enters the eye as IR as it is then deeply penetrating. Conversion to shorter wavelengths can happen beneath the retina's layer of shielding melanin, so the retina is particularly vulnerable to this kind of exposure.
Therefore, until the standards are updated to account for this factor, I have been assuming that IR light is a mixture of IR together with one tenth of its power at each of one half and one third of its nominal wavelength, and applying the safety standards both to the IR and the assumed second and third harmonic UV if I am called on. This is obviously highly conservative, especially if the IR light isn't pulsed, but until someone convinces me that the standards are taking these things into account, that's what I am going to do.
In response to the edited question which went after the idea of being "dazzled" by the light, it would not work the way you would like. We show the spectrum "darken" as we leave the visible region, because we are showing a roughly constant intensity of light from one side of the spectrum or the other.
When you start talking about making the light brighter, we have to be a bit more formal:. Graphs like the one above show the frequency response of our cones. There's a few variants on the y-axis some use a unit system where the blue is more sensitive , but it doesn't matter for this answer.
We can see that as we move past nm, into IR, the red light response goes down. But it doesn't go to zero right away. So if you had a very bright light just outside this region, it would stimulate the red cones in your eye. But here's the trick: for all pain and dazzling responses, we rely on the signals that we receive from the rods and cones.
There are no pain neurons in the retina, so these are all the signals our brain gets and obviously it has to become a signal to the brain before it could dazzle us. So what we would see with your very bright IR light is an ultra-pure red -- bothersomely pure because it's so far away from the peak of the green cones.
But you would not see it as "dim," because by definition, you're stimulating the cones to cause the dazzling effect. It would have to appear very bright, just like any other dazzling light. Of course, the limit to this is when you get to the IR region where your sensitivity gets low enough that second order effects like thermal heating become important.
That's where the other answers pick up. But below that point, the light would be dazzling because it's bright, or it would not be dazzling because it's dim.
You can't have dazzling and dim from a signal processing perspective. Yes, and indeed this is a very important issue. And the real serious part is - unlike what you've said, it may not, and perhaps even will not, be "painful". And the reason for that is that the perception of any sort of discomfort or pain, "over-stimulation", etc. And by definition, these rays do not stimulate the eye, so it cannot react with pain.
And this kind of intensity is very often encountered with lasers, and it is a significant safety hazard. In particular, if the laser is intense but at a wavelength the eye does not readily perceive, it may notice little to no light, and feel little to no pain , so none of the usual defensive reflexes will be triggered including the all-important pupil constriction response Now of course, not all wavelengths will affect the eye in the same way, as the eye's materials, like anything else, are not necessarily transparent to or respond to all wavelengths in the same way.
In particular, if the radiation frequency is sufficiently low, so the wavelength sufficiently long and thus far into the infrared, it will not be able to reach the retina, but can still reach the cornea the part of the eye that is on the outside of the lens and cause damage there, which may not be immediately apparent but puts one at risk for cataracts.
Indeed this can also happen with intense diffuse sources of such longer-wave infrared light as well with prolonged viewing, e. This can be seen from a graph of the Planck curve at suitable temperature - usually about K - although of course glass is not a great blackbody radiator given its transmissivity in the visible, but nonetheless it is especially opaque to far or long wave infrared hence why that thermal infrared cameras need to be made with germanium lenses instead of glass lenses and thus will be a better blackbody there.
A special infrared-blocking goggle is thus standard protective wear for this purpose, if one is going to glass-blow professionally and thus be exposed to this radiation on a long-term basis. The treatment requires high-intensity light, but instead of lasers, NASA has developed powerful light-emitting diodes for the job. Lasers tend to damage cells, whereas LEDs can deliver light in a way that is less harmful to tissue New Scientist magazine, 25 September , p In a study that will appear in Proceedings of the National Academy of Sciences , Whelan blinded rats by giving them high doses of methanol, or wood alcohol.
This is converted by the body into formic acid, a toxic chemical that inhibits the activity of mitochondria. But if the rats were treated with LED light with a wavelength of nanometres for seconds at 5, 25 and 50 hours after being dosed with methanol, they recovered 95 per cent of their sight.
Remarkably, the retinas of these rats looked indistinguishable from those of normal rats. The results have raised the hope that the LED technique could be used to treat people for a range of eye diseases known to be caused by mitochondrial problems.
Whelan also thinks it will help treat laser injuries to the retina, apart from areas where cells have been completely destroyed. Whelan has already tested the LEDs on 30 children suffering from mucositis, a painful side effect of cancer chemotherapy. If a person is walking a rocky path, operating machinery, a vehicle or aircraft, this temporary loss of vision could cause injury or disaster.
At night, when the pupil is most open, the effects would bemagnified. Some basic rules with lasers: Never direct a beam onto another person, especially their face. Do not shine it onto a mirror or mirror-like surface. Do not look at the beam through binoculars or a microscope. One last thing -- some government entities have banned or restricted laser pointers. Some states and some cities have or have proposed age limits on the purchase or use of pointers. The United Kingdom bans the use of class 3A pointers.
Laser pointers are high-tech tools, not toys. Newsletter Get smart. Sign up for our email newsletter. Already a subscriber? Sign in. Thanks for reading Scientific American.
Create your free account or Sign in to continue. See Subscription Options. Go Paperless with Digital. This answer comes from Douglas A. He is also adjunct lecturer in the nuclear engineering department.
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