Digital BW, The Print

--- In DigitalBlackandWhiteThePrint@yahoogroups.com, "carlislematthew 
<carlislematthew@h...>" <carlislematthew@h...> wrote:

> When people have asked WHY not using the yellow ink prevents 
> metamerism we start to throw scientic terms around and nobody wants 
> to admit that they don't know what's going on.  Apparently, the 
> yellow ink has a "peaky spectral response".  Lovely.  Everyone nods 
> their heads and mutters "yes, peaky spectral response - that makes 
> sense".  Does it though? 

Peaky spectra **IS** what causes metamerism, regardless of whether 
you're talking about inks or paint chips or geology minerals! The 
physics and perceptual science of metamerism is not in question.

The cones in your retina have three very broad and overlapping 
sensitivity curves, but all three centered in the middle (430, 530, 
560 nm)  However real-world pigments and light sources often have 
sharp peaks and valleys and their peaks are often higher or lower 
than the cones' peak sensitivity.  If the reflectance spectrum of a 
pigment and the emission spectrum of light source both have, say 
sharp peaks at 680 nm and 540 nm the resulting color will look 
yellow.  Shift the peak of the emission spectrum to, say, 500 nm and 
the light will look only a LITTLE different to the eye, but the 
pigment will look red.

> Does this really mean that the yellow ink goes magenta in
> tungsten light and green in daylight?  

No one is proposing that the yellow ink changes color, and if you're 
a "scientist by nature" you wouldn't ask such a question.    The 
color you see is a result of the COMBINATION of wavelengths hitting 
your retina.   So If you have equal amounts of red, green, and blue 
hitting your retina you see white (or grey).    Remove (or reduce) 
the yellow because its reflectance peak(s) don't line up with the 
emission spectrum and the result is that cones with the middle 
spectral sensitivity (530) nm don't receive as much stimulation so 
your eye sees a different color.

The inkjet makers make their job more difficult by using 7 inks.   
The eye only has 3 photopigments for color perception.  Getting all 7 
to play nice together is like herding cats.   

None of this proves the yellow ink theory - as I've said before here, 
anyone with a decent hi-res scanner that can show the individual dots 
should be able to post sample images of IP and Epson grayscale images 
so we can SEE what inks it's using and count the relative number of 
dots of different colors.   I posted such images wrt a Black-Only 
discussion but I don't have IP.