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LED vs laser power comparison question

whiteskulleton

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Dec 9, 2022
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In terms of eye safety, is it wrong to compare irradiance from LED flashlight to a laser? In a published journal article titled "Optically Improved Mitochondrial Function Redeems Aged Human Visual Decline", they tried to improve eye function by shining a light into people's eyes for 3 minutes every day. The light was 40 mW/cm^2 at the cornea and they used "simple commercial DC torches" to do it. If the pupil of an eye is between 3 mm to 6 mm in diameter that would be 2.8-11.3 mW entering the eyes for 3 minutes. Would this be the equivalent effect of shining a 670-nm laser at 5 mW in the eye? That is the equivalent of shining a class 3B laser in your eye which is frowned upon which seems to indicate that there is a variable that I am missing when comparing LED lights to lasers.
I'm not trying to repeat the results with a laser or anything. I am trying to understand how dangerous it is to point a LED flashlight in your eye from a scientific standpoint. Is 10 mW from a light through the pupil of an eye via LED just as damaging as 10 mW from a laser? Since eye damage from lasers is more well documented, it is it correct to use laser safety standards for LED flashlights?
 



Eidetical

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Danger from any light source depends on the physical diameter of the source of the light you see. The sun is a large diameter source of light. It has a large "extent". The extent of a source determines how small the spot will be when focused by a lens. Smaller extent, smaller spot. If all the light goes into a smaller spot, the energy density (intensity) goes higher.

LEDs are small diameter extended sources, usually provided in a lens-tipped package and/or surrounded by a reflector. Both of those things make the LED appear to be larger than it really is. A laser appears to come from the smallest source possible. Essentially a single point. All the light that enters an eye from an LED will be spread out into an image of that source on the retina. Like looking at the sun, after exposure you will probably see temporary "after-images". With laser light, all the light is focused into a single point on the retina. That's not good. Permanent damage can be done before your body's "blink response" can work to protect the eye.
 

whiteskulleton

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Thank you for the reply. However, I'm still not convinced about the explanation. You can stick an LED flashlight in your eye and then the light source is no longer small. If the same amount of power enters the eye at the same wavelengths in both cases, I assume that would mean that the same amount of light and thus the same amount of photons enter the eye as well. In one case, the study seems to indicate that it could improve eye function and in the other it would cause permanent damage. This is counter intuitive and unexpected. If this is the case, then it seems that the energy--and thus amount of light*--entering the eye is not what damages eye but the quality of the light. Correct me if I am wrong but a laser works by creating a single uniform light beam. I hypothesize that since light behaves like a wave, that laser light beam would be a single wave with a high amplitude vs the LED which would be a bunch of smaller waves moving in different directions that might interfere with each other rather than adding together to damage the eye.

Also, is it correct to assume that shining a 10 mW laser at 670-nm into the eye for 3 minutes would cause permanent damage?
 

Eidetical

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Things you're not considering:
1) "Simple commercial DC torches" use a lens and/or reflector to make the apparent extent of the source enormous.
2) Light from an LED is broadband, so includes a continuum of wavelengths.
3) Light waves from an LED do not " interfere with each other rather than adding together to damage the eye".
4) It's the amount AND "quality" (by that I mean spot size) of the light on the retina that causes damage.
5) Lasers work by amplifying light, and some have outputs that are neither a single beam nor uniform.
6) Do NOT stare into 10mW of a 670nm laser beam for 3 minutes! It's fuckin' stupid. Just sayin', as would everyone else.
 




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