I'm looking for a way to write or print something on a piece of paper
such that it is invisible, appearing blank to your eye.  But when you
photocopy it, the print appears.  Is this possible?  How would one
accomplish this?

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Request for Question Clarification by pinkfreud-ga on 03 Apr 2003 12:54 PST

Here you'll find a list of many security printing methods:

http://www.hitechprint.com/security_printing.html

If any of the methods listed on the site above are of interest, it
would help Researchers if you'd narrow the field by specifying exactly
which technique you want to know more about.

tej,

I think I've found every invisible ink under the sun, but nothing that
does what you want. There are inks that only appear under UV or black
light; Inks that change color with heat; Inks that appear when you dip
them in water; Inks that react to specific chemicals; Inks that appear
on beverage glasses when hot; Inks that only appear when exposed to
sunlight; Inks that change color when you try to erase them; and Inks
that glow in the dark.

But nothing that is invisible to the eye but can be "seen" and
reproduced by an ordinary photocopier.

The problem is not the ink technology (you can see there is plenty of
that!).  It's the the photocopier. A regular copier can only "see"
regular visible darks and lights, the same way we do. It isn't picking
up any "invisible" wavelengths that our eyes cannot see. And it can
only copy what it sees.

I suppose as a simple test, one could use the heat sensitive ink --
for example, the usual kid "secret message" recipe of lemon juice on
paper -- and then feed the message through the document feed of a
copier to see if the heat from the copier brings out the message.
Running the paper through a laser printer might even work better since
it would get hotter.

HOW INVISIBLE INK WORKS
http://www.kidzworld.com/site/p601.htm


That's the best I can do ...Sorry. 

Good luck!

--K~

That'a a toughie.
I doubt if any such process exists, for the reasons already provided. 
As to whether it is *possible*, well - maybe.
Photocopiers generally use quite a bright light source, certainly
brighter than ambient.  Often, it has a restricted spectral range too.
 I can conceive of a material, that when stimulated by bright light
(possibly of a particular wavelength), changes its reflective
characteristics.  However, it would be just as visible to a human
under bright lighting too.

Similar materials are available - those used in adaptive sunglasses
that become darker in light.

The paper is exposed to additional pressure when being photocopied --
as opposed to when one simply holds it. Maybe that could somehow
trigger something. Pressure-aware ink?