Color, pigment, and light question

Factual Questions
1

Lately I’ve been trying to get my mind around the rough concepts of colored light and how they relate to pigment. It seems simple enough, but when I think about it, there is a problem that I can’t resolve. This is my understanding of the basic principles:

The three primary colors of light are red, blue and green.

Red light plus blue light equals magenta light.
Red light plus green light equals yellow light.
Green light plus blue light equals cyan light.
All three together make white light.

This is additive mixing, mixing colors of light directly. This is what television and computer screens must use in order to create the appearance of many shades, since screens give off light.

The subtractive method of producing light is a bit different. Instead of combining red, green and blue light, you start out with white light and filter out the light you do not need, and the light that is not blocked out will produce the color you want. It is easiest to understand when explained in terms of filters.

A magenta filter blocks green light.
A yellow filter blocks blue light.
A cyan filter blocks red light.

You can also mix the subtractions, so to speak. White light filtered through both magenta and cyan filters should appear totally blue, and so on. I think I get that.

Now, I presume that the subtractive mixing method is used for pigment, which is what I’m having trouble with. A pigment that appears red actually absorbs blue and green light and reflects the red light back to the eye. So if I mix equal quantities of, say, blue and yellow together, what should I get according to the rules of light?

Blue absorbs red and green, and reflects blue.
Yellow absorbs blue, and reflects red and green.

What I come up with is an awful behemoth of a substance that absorbs half of all green light, half of all blue light, and half of all red light. (I suspect this is not exactly right, but I’m not sure how to account for the fact that every substance molecule reflects a different color.) According to my computer settings, that should make a grey exactly halfway between white and black. But everybody knows that yellow and blue don’t make grey- they make green. What gives?

I do note, however, that light filtered through a yellow filter and then a cyan filter (of the three subtractive primary colors, cyan is closest to blue) will indeed appear green, as the filters do away with blue and red respectively. However, pigment mixing is not really quite the same as filter adding, is it?

Now, is it that our primary paint colors from kindergarten, red, yellow and blue, are merely simplified analogues of the true subtractive primary colors, magenta, yellow and cyan? I don’t understand.

I would be delighted if somebody would illuminate the subject a bit for me. Thanks.

2

Why does yellow reflect red and green? Wouldn’t it absorb everything but yellow?

Well, I’m honestly now sure why with subtractive color mixture that yellow and blue make green either, but I can tell you that they do make grey in the world of additive color mixture, which maybe part of your confusion. (I could tell you why too, if you are curious: basically you use the same sensory neurons to detect both, one in the positive direction (more active neurons), the other in the negative direction (less active). Together in the right ratio their sum total would be neutral.)

Don’t know too much about subtractive color mixture though.

3

Sure, it’s the same thing. You describe pigments in terms of which primary additive colors of light that they absorb. Yellow absorbs blue, but lets red and green light pass unimpeded. And remember, red and green combine to make yellow, which is what you see.

At least, I think so.

Part of the problem here is that blue subtracts two primary additives, red and green, leaving only the remaining primary color, whereas yellow subtracts only one primary additive color, blue, leaving red and green which combine to make a secondary color, which is of course also a primary subtractive color.
So you’re mixing a secondary with a primary no matter what color mixing method you use.

4

Blue paint isn’t a “narrowband” filter. It actually reflects green, blue, and violet. In other words, it’s akin to cyan.

Mixing real-world yellow paint with real-world blue paint gives deep green.

5

Well, again I don’t really know how subtractive color mixing works, but something is confusing here. Red and green light, like blue and yellow, will produce grey in the world of additive color mixture, (and I’m quite sure that mixing red and green paint (i.e. subtractive CM) won’t give you yellow).

So if yellow pigment reflected red and green light (and absorbed blue) as you say, we’d be talking about ACM. (i.e. the red and green light is mixed by the eyes).

So, I think you might be misunderstanding how SCM works. I don’t know either, but once you are talking about reflecting various colors of light which are mixed by the eye, it’s ACM.

I’m also unsure of why:

I’ve always been under the impression that the reflected color is simply the reflected color (and thus not described in terms of a particular combination of the 3 primary colors). IOW, yellow pigment reflects yellow light.

6

It’s possible to design a yellow dye and a blue dye which, when mixed together DOESN’T make green. You just need a Yellow that doesn’t pass any blue or even greenish-blue light. Combine that with a Blue which doesn’t pass any green or even greenish blue. The combination gives black (or at least dark grey.) One filter eats blue and violet, the other one eats red yellow and green. Most paints don’t act light that.

I’ve played around with those colored transparent filter-books you get from Lee or from Rosco theatrical supply. There are some colors of yellow and blue filter where one color blocks ALL the light that the other filter passes, and those filters look black when stacked together.

PS, “yellow” is weird. If you stack a red filter on a yellow filter you get… red! Not orange. This is because most yellow filters don’t just pass pure yellow frequencies. They also pass lots of red and lots of green. Human eyes can’t tell the difference, since human eyes only have red-sensors and green-sensors. If you shine yellow light in your eyes, it triggers both the red and the green sensors, so you see yellow. But if you shine only pure red and pure green light in your eyes (no yellow light whatsoever), you’ll still see yellow-colored light.

I have seen “true” yellow filters which pass only yellow frequencies. That kind of filter does look orange when combined with a red filter. It also looks much darker yellow than the usual yellow filters, since the usual yellow filters are passing all that red and green light.

7

Human eyes have red/green sensors, blue/yellow sensors, and light/dark sensors. If you shine yellow light into your eyes it will trigger the yellow/blue sensors in the yellow direction. This is why color blindness involves the loss of colors in pairs (most commonly red and green).

Pure red and pure green light into your eyes will be perceived as grey.

8

Note that on the physical level there isn’t any color. Adding two or more light sources together is a simple matter of adding up their spectral intensities. Dividing the continuous spectrum of visible light into ranges of color depends on the observer.

No. Not sensors, but processing channels maybe. (I’m sure bbeaty meant there aren’t special yellow-sensors in addition to the R & G ones, not that there aren’t any other.) There have been shown to be four types of photoreceptors in our eyes. Rods for colorless night vision, and three cone types responsible for color perception at normal light levels.

According to their respective range of wavelengths which they react most to, they’re called red, green, and blue cones. Or more accurately, since there’s quite a bit of overlap, (L)ong, (M)iddle, and (S)hort (-wavelength) cones. Any spectrum of light that reaches your eyes will be weighted by their three different response curves into three different stimulus values.

You’ll see yellow. The subsequent neuronal processing has nothing to do with the physical perception of light. The opponent channels can’t be accessed individually anyway, one channel can’t be balanced by any combination of light without affecting both the other two. Blue + yellow light appears white (or bright gray if you wish) because all cones are stimulated equally in the first place, rather than yellow/blue-stimuli cancelling each other out.

Red-looking light (either consisting of a single wavelength or some spectral range) mixed with green-looking light will appear yellow, because it stimulates the red- and green-sensors in your eye similar to how a single spectral peak in the yellow range would stimulate both.

That’s why, the other way around, if you see a color, you can’t deduce the exact shape of its spectral curve using only your eyes. To resolve it into more than three channels, you’d need a spectrometer. It’s inaccurate to say “this yellow filter passes yellow light” because it doesn’t necessarily have to, to appear yellow to us. Without having a clue about the absorption spectra it’s impossible to predict the resulting color if you combine them. Luckily, real-world colors aren’t too far off from what you’d expect.

Ideal theoretical blue pigment mixed with yellow pigment would be black. What isn’t absorbed by the one is by the other. There isn’t any light reflected in subtractive theory, only absorbed or not. But as you can see, you won’t get far with the strict theory in the real world. Ideal pigments simply don’t exist, you seldom know a spectrum, you can’t get darker than black, translucent dyes get covered by opaque pigments. The artist’s red-yellow-blue system works well enough if you work with your eyes and real materials. But I wish it was called an art then, not a color theory.

9

Is it possible to have two sources of light (one pure and one mixed) and a coloured filter that will entirely (or nearly so) pass the pure wavelength and entirely block the wavelengths in the mixed beam?

10

Oops, that’s two sources of light that appear the same colour to the average human eye.

11

(or vice versa on the pass/block thing)

12

Sure. Shine a red and a green LED on a diffuser screen, then adjust the brightnesses to produce a nice bright yellow. The yellow is illusory, and since it’s made of red and green light it won’t pass through a narrowband yellow filter. But the light from a yellow LED will get through the filter, since that light is actually yellow in frequency, not a combination of red and green.

Actually I think LEDs might be too broadband to show off the effect well. (But red and green LEDS certainly do make light that looks bright yellow when combined!)

Better to get three diode lasers, or three line-spectrum tubes as used in physics class: a neon tube that puts out a pure red, a mercury tube for pure green, and a sodium tube for pure yellow. If you buy a narrowband “sodium filter” it will pass the yellow light from the sodium tube but will block the light from the others. Yet if you mix the red and green light from the other two tubes, it will look bright pure yellow even though there’s no “yellow frequencies” in it at all.

I think “sodium block filters” also exist: filters which absorb just the yellow frequency of sodium lamps. That kind of filter would pass the “yellow” which is made from a mixture of red and green, while blocking the genuine yellow from the sodium bulb.