As many have pointed out, it is very difficult to
reproduce colors because of the cones broad response,
but the situation is actually worse that. Since each
person's response curve varies, he or she will have a
different perception of a color generated by mixing.
The Exploratorium has two experiments that
dramatically demonstrate this situation. Neither
should be too hard to duplicate in the lab.
The first is called "Disagreeing about Color."
<http://www.exploratorium.edu/cmp/exnet/exhibits/group7/disagree/index.html>
In it, a central dot of color is surrounded by a ring
of 11 dots of color. The central dot is created by
filtering an incandescent light source. The 11
surrounding dots are created by mixing red and green
LED's. The exhibit asks viewers to try to match which
surrounding dot matches the central dot in color. When
doing this experiment with a large group of my
students, none of whom fail the traditional color
blindness tests, they have considerable disagreement
about which colors match. Although two dots were more
popular than others, six different dots were selected.
By the way, between the frequency response of the
camera that took the picture on the website above and
my monitor, none of the dots look like they match, but
in person everyone agrees that some dot matches.
The second is called "Seeing Yellow," and it takes the
same idea even farther.
<http://www.exploratorium.edu/cmp/exnet/exhibits/group7/seeing_yellow/index.html>
Two light sources, one from the right and one from the
left, hit two pieces of translucent plastic that are
nearly touching. The left light source is a simple
incandescent film strip projector, gelled to pass only
yellow. On the other hand, the right light source is
combination of red and green lights generated by
LED's. Rheostats let the viewer control the
intensities of the LED's. The object is to match the
color of the light generated by the film strip
projector with the LED's. It is hard to do, but
usually you can get something that is satisfying. The
surprising thing is that the people around you will
rarely agree that you have it just right. They will
change the settings so that it looks right for them.
Now it won't look right for you.
My conclusion? Students used to ask me how I knew that
the yellow I saw was the same as the yellow they saw.
I used to say that we were trained to say yellow for a
particular sensory impression.
I now know that they are right. We tend to see colors
differently. My yellow probably isn't the same as
yours.
Marc "Zeke" Kossover
The Jewish Community High School of the Bay
San Francisco, CA, home of the Exploratorium