Path: utzoo!mnetor!uunet!husc6!think!ames!pasteur!ucbvax!decwrl!jumbo!murray
From: murray@jumbo.dec.com (Hal Murray)
Newsgroups: sci.electronics
Subject: Seeing IR
Message-ID: <2871@jumbo.dec.com>
Date: 2 May 88 00:15:58 GMT
Distribution: na
Organization: DEC Systems Research Center, Palo Alto
Lines: 50
Keywords: visible, IR, LED, laser

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.