Is a hue indistinguishable from its equivalent frequency of light?

Question

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?

Answer 1

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.

Answer 2

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

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

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

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

Answer 3

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.]