🚀 go-pugleaf

     In a time long long ago, in an industry (photoengraving) far, far
away...

     Macbeth Daylighting, of Newburgh, New York, used to sell a device
called a ChromoCritic, mnaufactured by Macbeth Arc Lamp Company (of
Philadelphia PA, now Cambridge MD, which is where I came in).  The
sole purpose of this device was to allow the backlighting of a color
transparency to be changed to match what the customer wanted on the paper.
Macbeth Daylighting also sold glare free lighting fixtures with a
constant (around 5400 degrees K) North sky daylight for examining the
paper.

    My granfather not only found out that the preferences of the
customers changed with the time of day (in a controlled lighting
environment).  But that the changes were consistant from day to day
and user to user.  In other words the way a person percieves backlit
(or CRT) color compared to reflective (or paper) color changes with
the time of day (actually internal biological clock) all other thing
being held invariant.

    The SMPTE (Society of Motion Picture and Television Engineers) did
a lot of work on this and other problems during the late fifties and
early sixties when trying to make color television something other
than a novelty, and finally came up with some not too impossible rules
for color correction that work.  But to get some reaction other than,
"I think these are the same picture," you are fighting a real uphill
battle.

     Incidently, if you think I'm kidding you, drive down the street
at night.  You can instantly tell which rooms in houses contain a
television because of the color of the light which seems very blue and
although varying in intensity it seems to vary little in hue.  But to
those watching the picture, it seems to show all colors of the
rainbow.  What gives?  Well, the television image is deficient in
yellow relative to an "normal" light source, and the percieved
contrast in hue is principally due to the red/blue ratio.  (See the
work of Ewdin Land on color perception for more information.  He
showed that by using just two colors as little as 10 angstroms apart
he could create an illusion of full color.  However, the fullest
saturation of full color from two images came when one was shown with
red light the other with white.)

      So, if you read Land, and work at it, you can show a beautiful
"Land"scape, with green grass and aquamarine seas on your screen.  But
none of the colors in the image will have an (RGB) G value higher than
zero.  Now try to print that on your printer....

--

					Robert I. Eachus

     When the dictators are ready to make war upon us, they will not
wait for an act of war on our part." - Franklin D. Roosevelt