Digital BW, The Print

--- In DigitalBlackandWhiteThePrint@yahoogroups.com, "Anthony 
Atkielski" <anthony@a...> wrote:


> If RGB were sufficient for transformations from any 
> curve to any other curve, then it would be impossible
> for us to see yellow by mixing red and green. 

I'm sorry, but this is a FASCINATING statement!   Could you 
elaborate? 

We see yellow because the photopigments for red and green in the 
retinal cones both happen to overlap their sensitivity in the range 
of wavelengths we call 'yellow', between about 540 and 560 nm.  


> Since, however, we _do_ see yellow by mixing red and green, clearly
> there is information lost in the conversion to RGB. 

Clearly there is information lost in your understanding of the 
physiology of color perception.


> Once a yellow surface is photographed in RGB, we can never
> know if it was really yellow, or just a blend or red and
> green. 

If a blend of red and green produces the perception of yellow it IS 
yellow.   If you take a photo of a scene with an RGB monitor in it 
with a yellow screen you know that the yellow is created from red 
and green phosphors.  Are you saying that black and white film will 
record this differently from an identical yellow created by say 
mixing cadmium yellow paint? 

The only way what you're saying could occur is if the sensing 
elements (whether film dyes, photopigments, or CCD filters) were 
NONoverlapping.   Imagine an RGB sensor that ONLY saw 640 nm, 500 nm 
and 440 nm.   If you had something in your scene at 560 nm you would 
see yellow, amd TMax film would record a shade of gray, but the RGB 
sensor would see black.

What you're probably thinking of is metamerism.  That happens when 
you have a light source illuminating a pigment or dye whose 
reflectance spectrum has peaks or valleys with respect to the 
emission spectrum of the light source.    But that's not a function 
of the recording medium.  Using the above numbers, if I have LEDs of 
440, 500, and 650 nm illuminating a color patch that only reflected 
at 560 nm it would look black.  It would also photograph black with 
all film and with all digicams.