naive question about printing in color

[libordeath.com]

Here goes: why couldn't we print in color with an RGB plus black ink set? When transparent CMY inks (the subtractive primaries) overprint, they produce R, G, or B (the additive primaries) depending on what combination is used. Wouldn't the same thing happen, but in reverse, with overprints of transparent inks in R, G, and B? What am I not understanding here?

 

[Deleted member 16349]

Here goes: why couldn't we print in color with an RGB plus black ink set? When transparent CMY inks (the subtractive primaries) overprint, they produce R, G, or B (the additive primaries) depending on what combination is used. Wouldn't the same thing happen, but in reverse, with overprints of transparent inks in R, G, and B? What am I not understanding here?

Over prints of any two combinations of R, G or B will give black. Well maybe not black but a muddy dark brownish colour. Even if you printed the RGB without overprinting in a similar geometry to what a monitor would have the RGB points of the pixel, you would get a dark colour because the RGB inks would absorb too much light.

If you look at the spectal curves for CMY and RGB it is much easier to understand what happens. If you look at a spectral curve for R ink, one will see that it is high in the red region and low in the green and low in the blue regions. A G ink curve would be high in the green region but low in the red and blue regions. (for CMY inks, Y only absorbs blue, M only absorbs green, C only absorbs red)

Inks are filters so if you put one filter over another it is quite easy to understand what parts of the spectrum are getting filtered out and therefore will not reflect back to the eye. R filters everything but red and if it is overprinted with G then the green filters out the remaining red resulting in all wavelengths being filtered out and therefore what is visible is just a dark muddy colour. Since no inks are ideal, the wavelengths in the visual spectrum are not filtered uniformly and therefore a nice pure darl black is not obtainable.

Even though I commented on the red region of the spectrum or the blue region, the colour is not in the spectrum of the light. Colour does not exist in Nature. Colour is only in your mind. Light is real but colour is just a perception. Thinking about these issues in terms of what happens to light makes the analysis easier because it is physics. Find out how the light is affected and then think about how that will affect the perception of colour.

I hope that helped.

 

[meddington]

Here goes: why couldn't we print in color with an RGB plus black ink set? When transparent CMY inks (the subtractive primaries) overprint, they produce R, G, or B (the additive primaries) depending on what combination is used. Wouldn't the same thing happen, but in reverse, with overprints of transparent inks in R, G, and B? What am I not understanding here?

In the case of using RGB inks, these are still subtractive due to the process at hand...pigments/dyes on paper. Only with light are the RGB primaries truly additive, and thus create white when combined. Its a difference of absorption of light versus emission of light. Per your example, how would you conceivably create yellow (or cyan or magenta for that matter)? Printing is always a subtractive process...uh, unless your printing OLEDs.

 

[libordeath.com]

These explanations are great–they really clarify the situation for me. Thanks!