while all that is correct, it has nothing to do with eye damage from a flashlight.
even a short glare into direct sunlight has no permanent effect, so what should a flashlight do harm?
That's a good point, as the Sun is pretty bright, and if it doesn't cause permanent eye damage with brief exposure, then a flashlight wouldn't. For reference, the Sun's noontime luminance (surface brightness for those who are less
nitpicky
) is about 1.6 billion candela/m², which is more or less comparable to that of xenon arc lamps, I think, and many times brighter than anything commonly used in hand-held flashlights. As for the total amount of luminous flux entering the eye, the illuminance of noontime sunlight on a clear day is about 130,000 lux, and with a pupil diameter of say 2 mm, about half a lumen would actually make it into each eye. A particularly powerful xenon short-arc searchlight may be able to match this at short range and at its narrowest focus setting, but no LED-based flashlights currently.
I'm not sure what the threshold is for permanent damage to occur, but based on anecdotal evidence (due to accidents--nobody I know deliberately experiments with eye damage :duh2
I bet that looking directly at the Sun through a typical binocular would do it. The luminance of the Sun's magnified image would be the same at the most, but its area would be about 50 times greater, meaning anywhere from 50 to about 8 times the luminous flux could enter the eye, depending on the diameter of the pupil (likely closer to the latter). That's right, 4 puny lumens may be enough to fry your retinas to a certain degree, but it's hard to get anywhere near that much through your pupils all at once--no hand-held flashlight is going to do that.
By the way, I know a guy who lost most of the sight in his right eye as a child by looking at the Sun through a small telescope. :shakehead Back in the "good old days" many department store telescopes came with cheap solar filters that screwed onto the eyepiece. Potentially questionable filtering aside, the problem with these was that sometimes so much heat built up in the filters, which absorbed most of the concentrated light from the Sun as they were designed to do, that the filters would shatter, instantly exposing the eye to a greatly magnified image of the Sun. :duck: Depending on the aperture of the telescope and the focal length and exit pupil of the eyepiece, we may be talking about hundreds of times more luminous flux than our eyes are able to withstand, which is...very bad.... I wouldn't worry too much about LED flashlights, though.
