DigitalVideoAndHDTVAlgorithmsAndInterfaces.pdf

Figure 20.2 Luminance and lightness. The dependence of lightness (L*) or value (V) upon relative
luminance (Y) has been modeled by polynomials, power functions, and logarithms. In all of
these systems, 18% “mid-gray” has lightness about halfway up the perceptual scale. For details,
see Fig. 2 (6.3) in Wyszecki and Stiles, Color Science (cited on page 231).
The L* symbol is pronounced
EL-star.
Value (relative) or Lightness (L*)
100
80
60
40
20
0
0
0.2 0.4 0.6 0.8 1.0
Lightness (CIE L*)
Foss
Richter/DIN
CIE L*
Newhall (Munsell Value, “renotation”)
Priest
Luminance (Y), relative
In Contrast sensitivity, on page 198, I explained that
vision has a nonlinear perceptual response to luminance.
Vision scientists have proposed many functions
that relate relative luminance to perceived lightness;
several of these functions are graphed in Figure 20.2.
In 1976, the CIE standardized the L* function to
approximate the lightness response of human vision.
Other functions – such as Munsell Value – specify alternate
lightness scales, but the CIE L* function is widely
used and internationally standardized.
L* is a power function of relative luminance, modified
by the introduction of a linear segment near black:
⎧ Y Y
⎪903.
3 ; ≤ 0. 008856
Y Y
L*
⎪ n n
= ⎨ 1
⎪ ⎛ Y ⎞ 3
Y
⎪116
⎜ ; .
⎝ Y
⎟ − 16 0 008856 <
⎩⎪
n⎠Yn Eq 20.2
L* has a range of 0 to 100. Y is CIE luminance (proportional
to intensity). Y n is the luminance of reference
208 DIGITAL VIDEO AND HDTV ALGORITHMS AND INTERFACES