murray@jumbo.dec.com (Hal Murray) (05/02/88)
Since the cluster of messages a few weeks ago, I've been thinking, browsing, and asking my friends, trying to understand what it takes to "see" an IR source. I know it happens. I've seen one myself. The room was dim, but far from dark. It was my office with the lights out but a window open to an atrium. The light was comming from an IR LED at the other end of a fiber. I'd guess it was 100 meters away. Does anybody have a reference to good info about the spectral response of the eye? The best chart I found didn't label the vertical axis. It showed the traditional 3 bell shaped curves with a wide flare at 0. The red end of the graph ended at 700nM, about where the curve for the red cone hit 0. I found one chart of rod vs code sensitivity. At the red end, they are roughly the same. Mostly, I'm looking for graphs at low light levels, and/or ones that extend well into the IR region. Now, the electronics part of the question. What is the wavelength of a typical laser used for CR players? How bright are they? ... I have spec sheets for various lasers used for communications. In the communications game, lasers are much more expensive that LEDs. Thus they are only used for long and/or fast links. Lasers typically run at 1300nM, the min dispersion point for most fibers. (On long links, the limiting factor is not always power, but sometimes bit smearing due to the different wavelengths propagating at different speeds. This makes the min dispersion point very intersting.) Some lasers run at 1500nM, the normal min attenuation point for fibers. The idea is that the fiber wizards have managed to shift the min dispersion point to correspond to the min attenuation point so you can have the best of both worlds. It seems pretty unlikely to me that the eye would be able to see much at 1300nM, but maybe if it were real bright... In both cases the width of the spectrum is very narrow (ie 0 power at 1295nM) so there isn't likely to be much energy leaking all the way up to visible. I've also found several spec sheets for IR LEDs. 850nM is a common center for the spectral output, but the curve is bell shaped and the flare near 0 power is quite wide. It still seems like a long way over to 700nM. I did find a few spec sheets with the center wavelength much closer to visible, even one that had significant power in the visible region. Again, these are for communications, so they may not be typical for things like CD players. Anybody know anything about the chemistry/physics/magic that goes into makeing LEDs or lasers? I assume various recipes naturally prefer different wavelengths. Is there a common recipe for a CD laser that produces near visible IR, or are the ones we see just powerful enough to blast on through? Are lasers used in CDs bright enough to be dangerous? I don't remember seeing any laser warning signs, but I don't look at CD players very often.
awpaeth@watcgl.waterloo.edu (Alan W. Paeth) (05/02/88)
In article <2871@jumbo.dec.com> murray@jumbo.dec.com (Hal Murray) writes: >Since the cluster of messages a few weeks ago, I've been thinking, >browsing, and asking my friends, trying to understand what it takes to >"see" an IR source. I know it happens ... >...Does anybody have a reference to good info about the spectral response of >the eye? The best chart I found didn't label the vertical axis. It showed The photopic response of cones is good to nearly .9-1.0nm in the red, but decreases exponentially beyond about 7000A, much like the roll-off of a simple (one pole) filter. So with enough power, near I.R. is quite possible. You'd see this color as "red" -- the rods are out of the running; they become less sensitive to cones at about .68uM. I've stared straight into the beams of consumer (TV/VCR remote control) IR LEDs in a darkroom with my eye fully adapted and can just make out the light at the threshold, but not for all IR LEDS (I think that the straight GaAs versions are too long in wavelength, but some come much closer to visible). I saw some empirical charts on this stuff while at Xerox Electro-Optical (they have application in IR sniperscope illuminators and such -- the military is interested in detection of sources). We all felt that the data points were *presumably* from Rhesus monkeys and not prison convicts, because to validate the exponential model the beam powers stated were *VERY* high out near 1nm. Most tables (like CIE color charts) deal with con sensitivity as linear values, allowing color characterizations using linear models. I think I have a chart... Let's see, here is a table in "Modern Optical Engineering by Smith (McGraw Hill). Calling peak cone relative sensitivity "1" at .55u, we have 10^-3 at .72u, 10^-5 at .8, 10^-8 at .9, 10^-10 at 1.0, and 3*10^-12 at 1.1um (end of chart), all relative values. The light source in a typical CD is is a laser diode -- typically 0.4 mW at 30 nm wavelength. That makes it a relatively low power Class II laser, but then, when do you know to blink? /Alan Paeth Computer Graphics Laboratory University of Waterloo
doug-merritt@cup.portal.com (05/03/88)
And how about seeing UV? I know that the retina has some sensitivty
to UV, but that it is usually filtered out by the cornea. And that
people who have their corneas removed can see UV. Also I know that
the retina's sensitivity drops off gradually toward the IR and UV,
rather than sharply.
But what makes me curious is that every "black light" I've ever seen
has a peculiar quality to it that I never see in any other kind of
illumination. Hard to describe but it looks very specular, somewhat
like laser illumination of normal surfaces, and it always looks a little
fuzzy, possibly as if it were out of focus. When I ask other people if
they perceive anything like this, they look at me funny.
It strikes me that if I were perceiving UV, then the fuzziness would
make sense, given the chromatic abberation of the lens of the eye
(i.e. it's not going to focus UV as well as other wavelengths).
Is it possible that not all of the UV is being filtered out by my
cornea? Can some people with corneas see UV???
Doug Merritt ucbvax!sun.com!cup.portal.com!doug-merritt
or ucbvax!eris!doug (doug@eris.berkeley.edu)
or ucbvax!unisoft!certes!dougmcdonald@uxe.cso.uiuc.edu (05/04/88)
Diode lasers are available fromthe red (the 600 nm region) all the way out to the far infrared (20000 nm). The materials used vary with wavelength: aluminum, gallium and phosphorus in the red through indium and arsenic in the near IR to lead salts in the far IR. I have personally seen the output from one at 805 nm- it put out 10 milliwatt which was just visible when it hit a white card at 1 inch in a dark room.