DigitalVideoAndHDTVAlgorithmsAndInterfaces.pdf

Luminous efficiency, relative
1.0
0.5
0.0
V(λ),
Scotopic
I presented a brief introduction to
Lightness terminology on page 11.
Publication CIE 15.2, Colorimetry,
Second Edition (Vienna, Austria:
Commission Internationale de
L’Éclairage, 1986); reprinted with
corrections in 1996.
Y(λ),
Photopic
400 500 600 700
Wavelength, λ, nm
Figure 20.1 Luminous efficiency functions. The solid line indicates the luminance response of the
cone photoreceptors – that is, the CIE photopic response. A monochrome scanner or camera must
have this spectral response in order to correctly reproduce lightness. The peak occurs at about
555 nm, the wavelength of the brightest possible monochromatic 1 mW source. (The lightly
shaded curve shows the scotopic response of the rod cells – loosely, the response of night vision.
The increased relative luminance of blue wavelengths in scotopic vision is called the Purkinje shift.)
Until 2000, Y(λ) had the symbol
y _ , pronounced WYE-bar. The
luminous efficiency function has
also been denoted V(λ),
pronounced VEE-lambda.
Luminance
The Commission Internationale de L’Éclairage (CIE, or
International Commission on Illumination) is the international
body responsible for standards in the area of
color. The CIE defines brightness as the attribute of
a visual sensation according to which an area appears to
exhibit more or less light. Brightness is, by the CIE’s definition,
a subjective quantity: It cannot be measured.
The CIE has defined an objective quantity that is related
to brightness. Luminance is defined as radiance
weighted by the spectral sensitivity function – the sensitivity
to power at different wavelengths – that is characteristic
of vision. The luminous efficiency of the CIE
Standard Observer, denoted Y(λ), is graphed as the
black line of Figure 20.1 above. It is defined numerically,
is everywhere positive, and peaks at about
555 nm. When a spectral power distribution (SPD) is
integrated using this weighting function, the result is
luminance, denoted Y. In continuous terms, luminance
is an integral of spectral radiance across the spectrum.
In discrete terms, it is a dot product. The magnitude of
CHAPTER 20 LUMINANCE AND LIGHTNESS 205