Chaos reigns?
I sent 2 replies to Nelson Blachman about detecting print, the first one
appears to have gone to "laalaa" land. so the second may be a bit obscure.
Just so as to assure you are all confused: here's a synopsis of my  first
reply and general summary of light probes.

There have been light probes around for years whose purpose is detecting the
presence of light.

The obvious purposes are for telling if you left the light on, and for telling
if you left an appliance on which has a pilot light.

Also, sometimes! they can be used to detect lights on some telephones.

The probe developed by Smith-Kettlewell RERC and manufactured by
John and Leslie Brabyn, has the advantage of having an infrared
"light" source in its nose as well as a photo transistor for
detecting the light.  The amount of light appearing at this
detector determines the pitch of a tone which you hear.

Thus, if you hold the probe about half an inch above a surface
which has dramatically differing colors, such as black/white
print, the pitch of the tone will vary widely as you move the
probe.

You can tell if there is print on a page, and where it is.  You
can not read with a light probe.

some well practiced people have learned to identify paper money
with one, depending on the various shapes of things on the
reverse of bills.

Other purposes include telling which of a pair of wires is black
and which red.  You can only detect differences in reflectance,
you can not detect actual colors.

There have been color detectors about for several years, but I
have rarely seen references to any particular device for long,
thus I must assume, never having tried one, that they don't do
well in the real world.  Why this may be true is complex and I
won't take the lists time with it.

Many light probes of varying sizes and utilities are available
from many suppliers of devices for the blind.

If you find a catalog which sells a variety of such devices and
doesn't carry light probes, they just ain't hip.

Confused yet?
If not, why not?
tom Fowle
Smith-Kettlewell RERC


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