Lightness and Brightness and Other Confusions

saturation, taken together, and is expressive of the quality of color sensation
as distinguished from its intensity.” (Optical Society of America 1973, p. 66).
Chromaticness, defined in another way, is one of the variables of colour in the
NCS system. Colours are judged according to their relative similarity with the
six elementary colours yellow (Y), red (R), blue (B), green (G), black (S) and
white (W), where the first four are called chromatic. Chromaticness is derived
from the perceived proportion of a chromatic colour of a given hue (C) in
relation to its whiteness plus blackness. The sum of all three is always 100%.
Chromaticness (c) can also be given as the equation c = (y or b) + (r or g). In
terms of whiteness and blackness, the equation is 100 – (w+s) = c. In the NCS
literature chromaticness is defined as “... a scale between the achromatic
greys and the most chromatic colour of a specific hue”. (Hård et al. 1996; see
also Fridell Anter 2000, pp 26–27).
Saturation and purity
Saturation is a generic word that one nevertheless seldom comes across in
casual speech about colour. It is widely used in professional language of art,
design and science, though, and there it has got several parallel definitions. In
Hermann von Helmholtz’s words saturation, or Sättigung, is the proportional
mixture of "white" and pure monochromatic light of equal brightness. In the
context of a colorimetric colour space the saturation of a colour can be understood
as its proximity to its fully chromatic outer limit 83 .
In the language of colorimetry saturation and chroma are separate concepts.
According to Rolf Kuehni (Kuehni 1983, p 39) varying the brightness of a
coloured lamp in total darkness is equivalent to varying its chromaticness.
The brighter a red light shines, the higher its chromaticness. If one were to
mix the red light of a lamp with the “white” light of another lamp of equal
brightness (luminance) in varying proportions, one would create varying
saturations of the red colour. Kuehni concludes: “Chromaticness is an absolute
measure of chromatic content of a color regardless of its brightness,
while saturation is a measure of the chromatic content of colors of equal
brightness.” (Kuehni 1983, p 40).
The CIE 1931 Yxy colour space was designed so that it would include all possible
colours visible to man, including monochromatic lights of the highest purity. David
MacAdam defined the limits of chromaticity for surface colours in relation to the CIE
Yxy space in 1935. These Mac Adam limits define the maximum purity of surface
colours viewed under standard illuminant C. These limits vary in relation to the theoretical
maximum outer limit of the CIE 1935 xy chromaticity diagram. The limits decrease
in size as the luminance factor Y increases. (See Billmayer & Saltzman 1981, p
50).
88