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A hue is the color characteristic of a pigment, a particular chemical molecule, when illuminated by sunlight.

Of course such a molecule will not emit a single frequency of light, but will emit a spectrum of light at different frequencies and intensities. This depends on the absorption characteristics of the molecule and its emission characteristics. In some cases, for example, light energy can be absorbed at one frequency and re-emitted at different frequency. In any case, what results is a spectrum of color.

The eye perceives this spectrum as being of a single color, called the pigment's hue. https://physics.stackexchange.com/questions/ask My question is whether this hue is truly indistinguishable from energy emitted purely at that frequency. For example, the pigment vermillion, mercuric sulfide, has a nominal wavelength of 607nm. If we create a pure light source of that frequency only, is it indistinguishable from the light of the pigment to the eye?

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  • $\begingroup$ related: chemistry.stackexchange.com/a/102492/23561 $\endgroup$
    – A.K.
    Oct 5 '18 at 20:51
  • $\begingroup$ Yes, and no... The gist is that the hue of a pigment depends on the light source which is shining on the pigment. So with different light sources a pigment can have different hues. $\endgroup$
    – MaxW
    Oct 5 '18 at 21:02
  • $\begingroup$ @MaxW As it says in the question, the light source is assumed to be the midday sun. $\endgroup$
    – Shaka Boom
    Oct 6 '18 at 3:57
  • $\begingroup$ The colour of most objects does not come from their fluorescence or phosphorescence, which is often weak or absent, but from light reflected off them by sunlight. The reflected light and that observed by the eye are those frequencies that the object does not absorb, i.e. made up of many wavelengths. How we perceive this is another matter and is how of brain interprets what the eye detects. The 'colour wheel' describes what absorbing wavelengths produce the colours we perceive. $\endgroup$
    – porphyrin
    Oct 6 '18 at 8:50
  • $\begingroup$ Can you please provide references for "The eye perceives this spectrum as being of a single color"? Since human beings have three type of cone cells en.wikipedia.org/wiki/Cone_cell , it is not evident that it perceives a spectrum as being of a single color. $\endgroup$
    – toliveira
    Oct 6 '18 at 10:59
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Certainly not

The problem is not, strictly, one of physics but one of perception. The eye doesn't detect individual wavelengths of light but the relative amount of red, green and blue light (but the sensors in the eye cover a relatively wide and overlapping range of wavelengths centred on what we might measure as "pure" blue, green and red. If a "pure" yellow light (say a wavelength of around 580nm) is shone into the eye it will excite both the green and red sensors and the brain will interpret the result as yellow. So the actual colours we perceive will match a pure light of that colour in that case. But we could get the same effect by shining two lights into the eye of pure green and pure red. The brain can't tell the difference and you would still perceive yellow.

But this system is no perfect and we can perceive colours that don't appear anywhere on the spectrum of "pure" colours. An example of this is magenta which is colour that does not appear in an spectrum of pure, single wavelength, light (or on the rainbow). The brain manufactures this colour when red and blue sensors are activated but green sensors are not. A very simple illustration and explanation of this is given in this short Royal Institution video.

This simple examples shows that colour perception does not relate to or match the perceived colour of a "pure" wavelength of light. Perceived colour is more complex that simply matching the wavelength in a light source and there are colours that we perceive that simply don't have a matching wavelength.

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    $\begingroup$ Continuing the physiology, the green receptor comes in two wavelength flavours and some women are heterozygous for it. Unfortunately the wavelengths are rather close and as far as I know, experiments to test for tetrachromatic vision have been inconclusive. Because the two kinds of green receptors produce such very similar signals in every natural case, the brain doesn’t seem to bother to wire itself up to distinguish between them. $\endgroup$
    – Martin Kochanski
    Jun 5 '19 at 7:12
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The colour we perceive for any compound, such as vermillion is not associated with a single wavelength, but rather with all of the wavelengths reflected by the compound that are in the visible portion of the spectrum.

The reflectance spectra of vermillion under different conditions is show below. Source reflectance spectra of vermillion

We can see that most visible light with wavelengths below ~600 nm is absorbed by the compound. A laser beam with a wavelength of 607 nm would have this colour (HEX: #FFC800, RGB: 255, 200, 0).Source

FFC800

Vermillion is a brilliant red/scarlet colour (HEX: #e34234, RGB: 89, 26, 20). Source

e34234

The particular hue that we perceive for vermillion is due to intensity of the all the wavelengths of light reflected by the compound.

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It seems that the premise "the eye perceives this spectrum as being of a single color, called the pigment's hue" is wrong.

Consider, for example the CIE-1976 colorspace. Its chromaticity diagram is a diagram in two dimensions. Just the boundary of it is the result of a pure frequency light source. The colours in the middle of the diagram are the result of at least two frequencies.

[Edited after Shaka Boom's comments.]

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  • $\begingroup$ First of all, this is a comment, not an answer. Second of all, the hue is the angle on such charts. $\endgroup$
    – Shaka Boom
    Oct 6 '18 at 14:34
  • $\begingroup$ It may be a wrong answer, if you think so, but it is an answer. It says that a hue does not have necessarily an equivalent frequency of light because some hues require more than one frequency. $\endgroup$
    – toliveira
    Oct 6 '18 at 15:08
  • $\begingroup$ Isn't the purple at (u',v') = (0.3,0.1) one hue and the bluish green at (u',v') = (0.1,0.5) another hue? $\endgroup$
    – toliveira
    Oct 6 '18 at 15:11
  • $\begingroup$ Indeed, you are right, the hue is the angle on such charts. So two different colours can have the same hue. $\endgroup$
    – toliveira
    Oct 6 '18 at 15:16

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