Characteristics of Visual Perception: Colorimetry

The Sense of Color:

The color of an incandescent ("luminous with intense heat") body gives us our most visceral sense of both color and temperature.

The relationship between color and temperature is quantified in the radiative law of so called "blackbody" emission.  The key fact is that the maximum radiative output is given by Wien's displacement law -- i.e.

l_maxT = (dominant wavelength)x(temperature) = 0.3 mm -^oK

Additive Color Mixing

A perceptual fact:

In additive (multi-source) color mixing processes, any color may be obtained by summing three primaries.
 
 

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A second perceptual fact:

Humans do not have a uniform perceptual response across the visible spectrum.  If, for example, a human subject is asked to judge the subjective "brightness" of two sources which are emitting an equal amount of radiant energy, the subject will judge a green source brighter than a red or blue source.

This non-uniform perceptual response has measure for countless observers and is summarized in the so called relative luminosity curves.
 

The quantification of color or "colorimetry" must take into amount these two perceptual facts.  Thus, one particular definition of "white" or "achromatic" light -- i.e.the definition of so called "equal energy white" -- is given by: (Color references: the commonly used RGB phosphors)

1 lumen of W = 0.30 lumens of R + 0.59 lumens of G + 0.11 lumens of B

which simplifies when expressed in terms of "trichronic units" -- viz.

1 lumen of W = 1 T(R) + 1 T(G) + 1 T(B)

where 1 T unit of red equals 0.30 lumens of red, 1 T unit of green equals 0.59 lumens of green (approximately 0.87 milliwatts) and 1 T unit of blue equals 0.11 lumens of blue.

Thus, to represent any color in terms of trichromic units, we write

 1 T(C) = x T(R) + y T(G) + z T(B)

where {x, y, z} are the trichromatic coordinates (coefficients) of the color C.  These coordinates obey Grassman's law  -- i.e.x + y + z = 1 --  so that only two coordinates are independent.

To see more detail look at the EBU (PAL/SECAM) trichromic representation of colors.

The lumen and the lux
Light Measurement Handbook,  Alex Ryer
Lighting Terms and Units
Color Theory Lecture Notes
The Color Bars Signal -- Why and How