Spectral and Non-Spectral Colors
Updated: May 24, 2022
The term spectral color is used to dene color judgments that are evoked
from a single narrow band of the VLS. These could be categorized as Blue, Cyan, Green, Yellow, Orange, and Red–as well as all hues, interpolated between a neighboring hue.
Humans have visual perceptions in addition to spectral colors, referred to as non-spectral or extra-spectral. For example, a rainbow represents an incomplete range of color perceptions, it is merely displaying the VLS; the full-color palette is in the mind. Anything made from more than one wavelength of light is considered nonspectral, but some of the main visual sensations are:
Magenta and Purple
Magenta and Purple are sensations made from photons from both ends of
the spectrum, a mixture of blue correlated wavelengths and red ones. Magenta tends to have a brighter connotation and lean towards a red perception, while purple leans more towards a blue perception and has a dimmer connotation.
Gray, when viewed under photopic vision (well-lit conditions), is
representative of a reasonably equal distribution of wavelength intensities, enough to excite each cone equally. Grays|meaning anything along the gradient black to white–contain no specific hue and they have a minimum saturation value.
White is the sensation of a maximum tone; photons are exciting all three retinal cone types enough that they are ring o nerve impulses at maximum capacity. This means there is a high number of photons present. White also has no specific hue value, but its tone is at maximum. Its saturation is at a minimum because the definition of saturation is a narrow bandwidth, and white requires a broad power distribution within the human gamut. When one says an object is "white" they mean "a very light hue-less gray." White happens whenever there are enough photons to excite cones to their limit, and this can be caused by a variety of SPDs so long as they are powerful enough. If a white SPD pattern was reduced to a human's comfortable intensity range then one could better determine whether or not it was gray, or had a dominant hue.
Black pushes the idea of what it means to be extra-spectral because it is
theoretically not made of any photons at all, but it is certainly not a color
perception along the spectral locus so it is technically extra-spectral. A black sensation occurs when no photons are exciting any cell receptors. This could be because there is no light in the area, or because whatever object appears black is absorbing the light that is present. Black only has a tone component, and that tone is zero. The hue and saturation could be any value because black does not have a specific hue or saturation. When one says an object is "black" they mean "close to theoretical black;" most objects are going to reflect a little bit of light. Actual total black is very hard to achieve, only a few materials are said to absorb all light.
Brown. This can refer to many color sensations but is usually a darker and/or desaturated yellow or orange[4, 43]. Browns have a less equal distribution of wavelengths than gray skewing towards the red-yellow spectrum.