Saturday, February 21, 2009

Magenta Ain't A Colour

Magenta Ain't A Colour, article. Kewl.

Hangar said:
What a completely muddled load of nonsense. Of course the eye doesn't detect the wavelength of the light as such - it detects the responses of the three types of colour receptors (red, green and blue cones) and assigns a perceived colour based on those.

Magenta is a colour. It's just not a "pure" colour - one which can be represented by a single wavelength of light.

8 comments:

hangar said...

What a completely muddled load of nonsense. Of course the eye doesn't detect the wavelength of the light as such - it detects the responses of the three types of colour receptors (red, green and blue cones) and assigns a perceived colour based on those.

Magenta is a colour. It's just not a "pure" colour - one which can be represented by a single wavelength of light.

Paul Kierstead said...

Of course it can be represented by a single wave length of colour, it just isn't a wavelength that stimulates a single set of receptors in our eye (or on film or on a digital sensor, for that matter....).

eolake said...

"Of course it can be represented by a single wave length of colour"

I don't think so, it would be in the spectrum if that was so.

hangar said...

Light with a single wavelength (around 550 nanometres, I guess) somewhere between the peak sensitivity of the L (red) cones around 564 nm and that of the M (green) cones around 533 nm will be perceived as yellow. A mix of two wavelengths of light of roughly equal strength, one red at 564 nm and one green at 533 nm will also be perceived as yellow (give or take the small response of the blue receptors to this mix).

The point of the article as I understand it, though it is so dreadfully badly explained that I'm not absolutely sure, is that magenta is what we perceive when our L (red, 564 nm) and S (blue, 437 nm) cones are stimulated roughly equally but that the perceived colour is not, in this case, the same as the colour we'd see for light with a wavelength which is the average of those two wavelengths (around 500 nm) because that would be, more or less, green.

So yellow light can be a single wavelength but stimulates two types of cones. However, I think the article is right in that there's no single wavelength corresponding to magenta.

I don't think this is a very good description; diagrams and a bit of hand waving is required, I think. However, it's better than the original article as that's bound to be completely confusing without mention of the different types of receptors involved.

Sukiho said...

"I don't think so, it would be in the spectrum if that was so."

thats what I would have thought, I presume a spectrum, continues onwards but the colors arent visible (to humans anyway), if magenta were a single wave length we would see it at one end or the other, or am I missing something? Interesting anyway.

Pascal [P-04referent] said...

Hangar's perfectly right about color receptors and how their perception works. It's confirmed by biological science.
People who could see infrared or ultraviolet (it seems that a few rare women with a fourth type of mutant cones are the opposite of color-blind and see a bit of the UV spectrum) would need their own color names. Every time there's more than one wavelength perceived, you need a name for that combination.

[Why women? Because these genes are on the X chromosome, so it's mostly boys who have a mono-allelic defect and become daltonian. A defect usually carried by the mother, for instance I have partially inherited my grandfather's defective X chromosome. If I only have sons, the curse will be lifted... unless my wife is also a carrier! It's very unlikely for a girl to inherit two identical defective allels and have problems with colors.]

Orange, whether the single wavelength between yellow and red, or a combination of monochromatic yellow and red, or a combination of 75% red and 25% green, it's all the same to the retina.

I've actually demonstrated that the color brown is strictly equivalent to dark yellow. It is sort of confirmed by the icky metabolic by-products of bilirubin, by the way. But mine is a strictly RGB chromatic demonstration:
-In additive colors, as colored lightbulbs would create, yellow = red + green.
-Brown is made by mixing red and green pigments, using the substractive color palette.
-In substractive colors, red pigment absorbs green & blue, and green pigment absorbs red & blue. Usually imperfectly, otherwise any two mixed pigments of R/G/B would only give black. (The adequate pigments for the substractive color palette, as used in printing, are actually the complements of R/G/B, respectively cyan, magenta and yellow.)
-Therefore, absorbing wavelengths with R + G pigments means: "less G + less B" + "less R + less B". The final result is a little remaining G, a little remaining R, and practically no B left. Therefore, brown = a little remaining (R + G) = dark YELLOW.
You can verify it by playing with your computer's color palette, which in theory can render all colors.

As for magenta, well, I have my doubts. I mean, about magenta NOT being in the color spectrum. I get the feeling that it's most likely there, between blue and purple.
What really puzzles me, is how come purple, or more correctly violet, is seen by the additive perception of the human eye as "blue + red", and obtained by suppressing all green with R & B mixed pigments/paints. It's odd, given that red is at the opposite end of our spectrum, so any single wavelength perceived as "containing red" would normally be leaning towards it, not the other way.

About "purple vs violet", the thing is, in English they mean the same (or almost), while in French purple is a specific nuance of red. Something like dark pink, halfway between that sickening nuance on all girlie toy boxes and pure scarlet red.
Did you know I have a whole blasted collection of red nuances? It was part of the research for my book projects. Something to do with vampires, but that's all I'm saying. The details are for when I blog my whole bloody medical thesis on "the medical study of the «cursed epidemic disease» known as Vampirism". (Woo boy, have I got revelations for you. You'd almost believe it's real.)

Hangar,
What you say about equal stimulation of L and M is fantastically simple to explain:
If you mix equal parts of 564 nm and 437 nm, you are not stimulating the M, unlike what monochromatic yellow would do. So it IS, absolutely, a very different perception!

(N.B.: I'm assuming the letters and numbers are correct, I haven't checked them myself, they're very dry and technical details of my rather ancient Ophtalmology course.)

hangar said...

Pascal: I'm assuming the letters and numbers are correct,...

I got those numbers from the book Astronomy Hacks by Robert Bruce Thompson and Barbara Fritchman Thompson. Wikipedia concurs, more or less, and also explains that the 'L', 'M' and 'S' stand for "long", "medium" and "short" wavelengths. Ah, that's easier to remember!

Human tetrachromates is also interesting. My recollection (from some general interest biology book, I think, Dawkins, Gould, Jones?) was that this was better established than that article implies: that a small percentage of women were known to have two types of red receptors and that there was speculation of an evolutionary advantage in being able to better assess the health of a child with the better skin colour perception. Anyway, as I recall, tetrachromacy has nothing particularly to do with the UV end of the spectrum. There is, however, a condition where the front of the eye lets through more UV which is then perceptible by S cones which, in most people, are limited in range of wavelengths by the "filters" in front of them, rather than by their own sensitivity.

Pascal [P-04referent] said...

the 'L', 'M' and 'S' stand for "long", "medium" and "short" wavelengths.
That's what I thought. :-)
My Ophtalmology course didn't insist much on such details, more on the medically relevant parts.

And I had only heard about tetrachromacy through a small mainstream article, because obviously it's not related to medical problems a GP would have to treat.

So thanks for the interesting info. I for one always like to learn.