DigitalVideoAndHDTVAlgorithmsAndInterfaces.pdf

SMPTE RP 71, Setting Chromaticity
and Luminance of White for Color
Television Monitors Using Shadow-
Mask Picture Tubes.
luminance is proportional to physical power; in that
sense it is like intensity. However, its spectral composition
is intimately related to the lightness sensitivity of
human vision. Luminance is expressed in units of
cd·m -2 (“nits”). Relative luminance, which I will
describe in a moment, is a pure number without units.
The luminous efficiency function is also known as the
Y(λ) color-matching function (CMF). Luminance, Y, is
one of three distinguished tristimulus values. The other
two distinguished tristimulus values, X and Z, and
various R, G, and B tristimulus values, will be introduced
in Color science for video, on page 233.
You might intuitively associate pure luminance with
gray, but a spectral power distribution having the shape
of Figure 20.1 would not appear neutral gray! In fact, an
SPD of that shape would appear distinctly green. As
I will detail in The CIE system of colorimetry, on
page 211, it is very important to distinguish analysis
functions – called color-matching functions, or CMFs –
from spectral power distributions. The luminous efficiency
function takes the role of an analysis function,
not an SPD.
Relative luminance
In image reproduction – including photography, cinema,
video, and print – we rarely, if ever, reproduce the absolute
luminance of the original scene. Instead, we reproduce
luminance approximately proportional to scene
luminance, up to the maximum luminance available in
the reproduction medium. We process or record an
approximation to relative luminance. To use the unqualified
term luminance would suggest that we are
processing or recording absolute luminance.
In image reproduction, luminance is usually normalized
to 1 or 100 units relative to a specified or implied reference
white; we assume that the viewer will adapt to
white in his or her ambient environment. SMPTE has
standardized studio video monitors to have a reference
white luminance of 103 cd·m -2, and a reference white
chromaticity of CIE D 65 . (I will introduce CIE D 65 on
page 224.)
206 DIGITAL VIDEO AND HDTV ALGORITHMS AND INTERFACES