When the light in question is laser light, 21 CFR 1040.10 has an answer for that one!
Maximum safe exposure for a point source of visible or IRA wavelength is
1 millijoule in 2500 seconds.Maximum safe exposure for an area source translates to 63 joules emitted per square centimeter of emitting surface during time of viewing the emitting surface if the surface is an "ideal lambertian" emitting surface, (If I did not screw up the translation), even if divided into small segments and considering the worst of such segments small enough to be essentially unchanged by flattening if curved. Although the time period to consider there is 10,000 seconds.
And when considering which of the above 2 cases to consider if both are considerable, you are alowed to use whichever of these two gives the more favorable answer.
- Don Klipstein ( snipped-for-privacy@misty.com)