# Colorimetry: Violet

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Ok, so...

Violet is a color of the visible light at the highest end of the spectrum with a wavelength between 380nm-450nm and freq between 668Thz-789Thz (traveling through air I assume). It is considered one of the monochromatic colors (meaning, it is only defined by its wavelength/frequency, and not the combination with others).

Now, our retina can only perceive Red, Green and Blue... each of the three types of cells are more sensitive to certain wavelength/frequency close to red, green and blue... and depending on the combination of the intensity of these three the brain interpolates it in order for us to see for example Yellow (somewhere between Green & Red).

If we look at any light spectrum chart, we can see more or less where Red, Green and Blue colors are located and in the middle between them the colors that would come by mixing them.

However, if I look at the RGB values of the Violet color, its a Mix of Red and Blue... which are at the opposite side of the visible spectrum!

If Violet is really only dependent on being an electromagnetic wave with length between 380nm-450nm/668Thz-789Thz... and our retina can only perceive color by mixing RGB... then:

How come a monochromatic color ranging to the highest end of the spectrum (where the blues are) be perceived by our eyes as a mix with red when "red cells" are only sensitive to the wavelengths/frequency of the lowest side of the spectrum???

Thanks!

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Remember that human photoreceptor being monochromatic is a simplification. Actually a green light will activate red photoreceptor and a red light will activate green photoreceptor.

If you know XYZ CIE-diagram, the "monochromatic reference line" is never touching the 0,0 (which means Z = 1, pure blue) 0, 1 (which means Y = 1, pure green) or 1, 0 (pure red) points.

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This is also why the RGB output of monitors APPEAR to reproduce colors representing frequencies that they don't actually emit.

A.K.A. This is not yellow, it's a trick that makes us interpret it as yellow :D

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> red when "red cells" are only sensitive to the lowest side
This is plain wrong.

I like frequencies (and you using them)! But I think it is a capital H. Like in most units named after persons.

When you use a smooth spectrum (maybe for every ray in your simulation you choose a random frequency between 400 and 800 THz) you would have to assume some average sensitivity curve for the receptors which will be (by definition) not exactly yours.

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Also keep in mind that color is a sensation, a perceptual response to electromagnetic radiation.  It is not a physical property.

Thus you can't say certain wavelengths are violet; you can only say we see these wavelengths as "violet".

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Also keep in mind that color is a sensation, a perceptual response to electromagnetic radiation.  It is not a physical property.

Thus you can't say certain wavelengths are violet; you can only say we see these wavelengths as "violet".

That's true, but wavelength of emitted light is a physical measurable property, and violet in particular can be defined as a wavelength band - as well as being defined as the sensation of perceiving that wavelength band :)

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Thanks everyone!

I didn't knew cone cells were still receiving to some degree other frequencies as well. Now that makes much more sense explaining why violet's RGB include's red...

however, I still have a little doubt. If we look at the frequency response graph:

https://en.wikipedia.org/wiki/File:1416_Color_Sensitivity.jpg

at around the ~400nm (where violete is) we can see both green and red cones absorbance almost the same (around 35-40 photons), while blue cones almost double the absorvance (around ~90)....  (I corroborated this graph's info with the original research paper here to make sure it was accurate)...

Wouldn't that mean that in RGB to see violet we would need values of R40%, G40%, B90% ?

(while in reality violete is more like R40%, G0%, B60%)

Sorry, just trying to see the correlation between physical cone sensivity and RGB values.

Thanks!

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I did not remember that there was so much overlap.
You could take the spectrum of your monitors sub-pixels and calculate how much they excite each type of cone. You will notice that green will almost always be excited. But at night, red appears as black.

I checked back to make sure that at least intensity is not based on humans, but gamma curve is mathematically precise. I still wait for SI to kick Candela out of the group of fundamental units.

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You will notice that green will almost always be excited. But at night, red appears as black.

IIRC, the rod cell's sensitivity curve peaks around teal and is similar to the "blue cone" curve. Some military flashlights are red, as this lets you temporarily use them to see using your "red cones", while not interfering with your rod cell activation (they basically don't perceive the flashlight).

Edited by Hodgman

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