Why are some shades of blue invisible to photocopiers?  

For example http://www.business-supply.com/office_supplies_2013899_sanford-prisma-color-art-pencils.html
 has "Copy-Not, Non Photo" blue pencils.  I know some checks are
designed with anti-copy measures that say "This is void if the
background isn't blue".  Obviously color copiers work around this, so
I know it's a feature of basic copiers.

---

Clarification of Question by creon-ga on 23 Jun 2005 21:54 PDT

I gather that the shade in question is called "non-photo blue", but I
still can't figure out exactly how it works or precisely what shade
that is.

---

Request for Question Clarification by czh-ga on 23 Jun 2005 23:17 PDT

Hello creon-ga,

http://teched.vt.edu/gcc/HTML/VirtualTextbook/PDFs/PrintersDigest.pdf
Printing Reference Guide

See page 48. Is the explanation given here sufficient?

~ czh ~

---

Clarification of Question by creon-ga on 24 Jun 2005 17:21 PDT

Pretty close.  "With the new color copiers, even the lightest blue
shades of color can be reproduced. Many of the new black and white
copiers have the same capability," implies that it is, in fact, a
technical limitation, but I still don't know where the limitation
comes from.

It also answers part of the question about precisely what color
"non-photo blue" is (any light blue; they recommend Pantone 304).

But the basic question, "why are some shades of blue invisible", is
still not answered, which is what I'm really after.

(I can't help but suspect that means that the answer will turn out to
be really boring, like "the plate just isn't sensitive enough to blue
light", but I'd like to know why that is.)

---

Request for Question Clarification by czh-ga on 25 Jun 2005 01:02 PDT

Hello creon-ga,

This has turned into a very frustrating question. Most of the sites
that refer to "non-photo blue" or "non-reproducible blue" simply
explain what it is and the history of its use but not the reason for
it not copying.

I decided to take another tack. I thought that if I understood how
photocopying works I might get the answer to why certain shades of
blue will not copy. Here are some links on the extremely complex
process of photocopying. I believe the answer as to why the blue
doesn't copy has to do with how certain colors get charged in the
copying process. Take a look at this article and let me know if this
is sufficient explanation for your question.

~ czh ~

http://electronics.howstuffworks.com/photocopier.htm/printable
How Photocopiers Work

http://www.sciam.com/askexpert_question.cfm?articleID=00058E5A-ACBD-1E84-85F7809EC588EEDF
How does a photocopier work? 

http://www.ohiohistory.org/resource/oahsm/notebook/mayjun1989.html
PLANNING NEWSLETTERS, PART TWO ? DESIGN
Producing Camera Ready Copy

Producing camera ready copy means making a paste-up of each newsletter
page to be duplicated. Paste-up involves taking the elements of the
newsletter, nameplate, headlines, text, and illustrations, and
arranging them on a sheet of paper or mounting board so that they are
composed ready to copy. "Non-reproducible" blue pens and pencils are
often used to make layout guidelines because the blue will not be
picked up by a camera or photocopier. Process cameras and most
photocopiers record visual signals as either black or white, nothing
between. Reds and oranges become black; light greens and blues become
white.

While some parts of spectrum are more difficult to read (scan) and produce
than other, the fact is that certain limitations are built into color copiers
on purpose, namely crime prevention:
" Color copiers are considered
'counterfeiting machines' in many circles in governments because they
are so good and are so easy that a 12-year old can press a green button
and have a decent duplicate. In the past, that 12-year old would have
needed about $100,000 worth of equipment as well as a ton of expertise
in the graphic arts to create that one little image. So this is a very
powerful technology that prevents color copying and triggers a black
output or barred image or on some color copier models a red output or a
blue output occurs. The point is that the copy is totally unusable. Just
a quick color copier horror story. A local ice cream company called us
because they printed 10,000 full color unprotected coupons for $1 off of
a gallon of their ice cream. They told us they received over 43,000
coupons back..."
http://www.twst.com/ceos/ABK617.html

Governments and industry cooperate to reduce the fraud:
...
"Recommendations:
...
Color detection - This is a key technology because humans are very
good at detecting differences in color. What this does is detect the
characteristic color of frequently counterfeited documents ("banknote
green"). When a user attempts to print a banknote using the correct
color green in the correct density for a bill, the printer modulates
the color somewhat to produce distinct, visible bands of color. The
change in the color won't be visible in other images that use lots of
green (photos of trees, for example), but are evident in bills..."
 http://www.cooltown.com/cooltown/mpulse/1003-counterfeiting.asp

SEARCH TERMS: Counterfeiting , color copier, shades of blue

It is a (intentionally)  complex issue 
 
Hedgie

As I said in my clarifications, I don't fully understand how a
photocopier charges the particles based on the exposure to the color
spectrum. Unfortunately, I also don't understand how hedgie-ga's
explanation that color copiers are used in counterfiting adds anything
to explaining what is "the technical limitation" or where "the
technical limitation comes from" that creon-ga is asking about.

~ czh ~

The reason older photocopiers don't copy blue is very simply that the
drum was activated by reflected light from the scanner. The chemical
process that creates the "semiconductor" on the drum surface is
sensitive to a fair proportion of the light spectrum, but frequently
lost sensitivity t the blue portion of the range.  Organic
photoconductors had a better response to blues but still cut off a
portion of the spectrum.  Newer Photocopiers (Digital) operate
differently and are dependent on the frequency range of the CCD in the
scanner, they are usually pretty good at the blues, and frequently
have settings that enable Text enhancement, enabling a setting of a
contrast point, anything darker than that point is registered as black
anything lighter as white.