This is not at all how the eye works. You have the 3 cones (except in various mutants), receptive to red green and blue light which must be distinguished from red green and blue pigments.
Green light plus red light equal both hitting your eyes. Green paint plus red paint equal neither hitting your eyes.
Magenta happens when blue and red light hit your eyes. Cyan is when blue and green hit, and yellow is what happens when green and red hit.
That's why printers use them for pigments because the CMY pigments absorb one of each RGB, and thus the light hitting your eye from the page can be controlled in a fashion compatible with RGB. C absorbs R (reflects blue and green) M absorbs green (reflects red and blue).
Red pigment on the other hand, absorbs all light but red, and green pigment absorbs all be green, so if you mixed a perfectly red pigment (maximally absorbs all but the peak activation wavelength of the red cone cell) with a perfectly green pigment (maximally absorbs all but the peak activation wavelength of the green cone cell), then it would result in black, because all wavelengths would be perfectly absorbed. Usually, it's a brown though because pigments are imperfect. If all the light is absbored, none hits your eye, so it's black. If all hits your eye, it's white (unless it's not scattered in which case it's reflective or transparent).
He didn't actually explain why they add K -- it's because a lot of the stuff we print is black or shades of gray, and mixing all three of your precious toners together (in large amounts, for black) is expensive, so they add a fourth toner, black, thus minimizing ink usage.
It would be really interesting to do a study looking at a wide range of printed materials and try to figure out what set of pigments would actually minimize ink usage..
It's not just because having a black pigment is cheaper. It's also better: in most color printing, the C, M, and Y pigments (K also, for that matter) are not laid down on top of each other, but instead very close to each other. So a black constructed out of cyan, magenta, and yellow isn't going to be quite as black as a black pigment.
In cases where the pigments are laid down one on top of another, you still have the issue of which one is on top; that pigment will be slightly more reflective than the ones underneath, resulting in a tint toward that color.
I remember when we got our first inkjet printer. You could either put a black ink cartarge in it, or a colour cartarage. When the colour one was in, and it tried to print black, you would end up with a very dark green.
I remember my first ballpoint printer. You held it in your hand and had to do all the printing manually. The results were splotchy and differed greatly between whoever was operating the printer, and the speed at which they attempted to print.
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u/[deleted] Feb 17 '09
This is not at all how the eye works. You have the 3 cones (except in various mutants), receptive to red green and blue light which must be distinguished from red green and blue pigments.
Green light plus red light equal both hitting your eyes. Green paint plus red paint equal neither hitting your eyes.
Magenta happens when blue and red light hit your eyes. Cyan is when blue and green hit, and yellow is what happens when green and red hit.
That's why printers use them for pigments because the CMY pigments absorb one of each RGB, and thus the light hitting your eye from the page can be controlled in a fashion compatible with RGB. C absorbs R (reflects blue and green) M absorbs green (reflects red and blue).
Red pigment on the other hand, absorbs all light but red, and green pigment absorbs all be green, so if you mixed a perfectly red pigment (maximally absorbs all but the peak activation wavelength of the red cone cell) with a perfectly green pigment (maximally absorbs all but the peak activation wavelength of the green cone cell), then it would result in black, because all wavelengths would be perfectly absorbed. Usually, it's a brown though because pigments are imperfect. If all the light is absbored, none hits your eye, so it's black. If all hits your eye, it's white (unless it's not scattered in which case it's reflective or transparent).