Re: LED eye damage
- From: don@xxxxxxxxxxxxxx (Don Klipstein)
- Date: Mon, 22 May 2006 06:08:08 +0000 (UTC)
In article <252h0owgfs7i$.o1749mghva5c$.dlg@xxxxxxxxxx>, dalai lamah wrote:
Un bel giorno Dirk Bruere digitò:
At what point, given the rapidly increasing power of visible light
output from LEDs, will the possibility of eye damage have to be taken
seriously? I know that looking into my 1.5W LED torch it's at the point
where a part of the retina will be washed out for minutes. And that's
after the external lens which provides a fairly wide beam.
In theory, today's brighter LED could already damage the eye: the SiC LED
from Cree have an optical power of 250 mW, and if you collimate the beam so
that all the power enters the eye, you will have similar effects of a class
IIIb LASER (definitive injury of the eye for medium-short exposures).
There is an alternative limit in terms of joules per square centimeter
per steradian, which I have calculated (I hope correctly) equates to 63
joules per square centimeter of "lambertian" emitting surface during
viewing period that could be 10,000 seconds for "Class I" exposure. And
if I did not screw this up, surface emitting 63 joules per square
centimeter (all or an adequately signidficant portion visible) in 1 to
9,999 seconds is short of Class IIIb even if total output power is over 5
mW.
However, in normal situations you won't go over class I or class II. To
make an example, let's use some optics so that the beam of our 250 mW LED
will have a width of 30 degrees (e.g. a flashlight).
I would suggest 10 degrees.
If you consider a
distance from the LED of 1 m and a pupil area of 10 mm^2,
When pupil diameter is a factor with visible light, the regulation
allows 7 mm pupil diameter or basically 40 mm^2 where it states relevance
and where it applies.
21 CFR 1040.10
Although in practice a pupil will usually shrink to less than 10 mm^2,
maybe as little as 3 mm^2 if you arev looking at "hazardously bright
light".
you will have a
total beam area of 0.214 sr and a pupil area of 1e-5 sr. Therefore, the
optical power that will enter into your eye would be 0.25*1e-5/0.214: less
than 12 microwatt, i.e. like a class I LASER (totally harmless).
Although this example is largely harmless, I do see the need to point
out:
1. The upper limit of "Class I" is .4 microwatt and non-inclusive (.4
microwatt is Class II while anything less and guaranteed less is Class I
no matter how little below .4 microwatt).
2. An LED flashlight even with a high power LED can achieve a beam area
about an order of magnitude smaller - so make that 110-120 microwatts.
3. If you need to CYA, then you need to consider a 40 square millimeter
pupil, and you could then be up against .4-.5 milliwatt.
The "acid test worst case" is a really big dark-adapted pupil or a
pupil dilated by the pupil-dilating eyedrops administered sometimes by
ophthalmologists, and that is 10 mm diameter, or about 79 square
millimeters. But where 21 CFR 1040.10 specifies a pupil diameter where
pupil diameter is relevant, it says 7 mm diameter which is about 40
square millimeters.
When it comes to LEDs and LED flashlights, you may end up relying on
power per unit area of emitting surface. For example, a more intense
white "Luxeon" could emit .4 watt from a chip surface magnified to roughly
(my eyeball estuimate) a 1.6 mm square, and that is roughly 16 watts per
square centimeter - which means you hit maximum safe exposure in about 4
seconds if you are close enough for 1/4 milliwatt or more to pass through
your pupil. This is about 1/4 the upper limit of Class II.
However, some time ago a Cree distributor showed me one of their LED in
action, and before lighting it he warned me not to look directly at it. And
he was right, these LED produce a light that you can't stand, even from
several meters.
I hear enough cautions and warnings related to looking into enough
LEDs... I would consider in general high brightness LEDs to be equivalent
to Class II lasers, and laser class II is a wide range of 2500 to 1.
At least 21 CFR 1040.10 and .11 do not have legal regulatory force on
LEDs, incandescent lamps, halogen lamps, other non-laser light sources...
Although I suspect your lawyer if competant in such area might advise to
use that as a guideline for non-laser light sources.
NOTE - I am talking about visible light here. IR and UV gets worse,
notably for at least some instances of Class II being changed to Class
IIIb if the wavelength changes from an "officially visible" one to an
"officially infrared" one. At least that does not apply to white or other
high visible brightness LEDs!
- Don Klipstein (don@xxxxxxxxx)
.
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