DigitalVideoAndHDTVAlgorithmsAndInterfaces.pdf

Michael Brill and Bob Hunt agree
that R, G, and B tristimulus values
have no units. See Hunt, R.W.G.,
“The Heights of the CIE Colour-
Matching Functions,” in Color
Research and Application, 22 (5):
337 (Oct. 1997).
6
10
2 2
25. 4 ⋅12
≈
2
ft
10. 764
2
m
Image science
In image science, luminance is usually normalized to
a range of 1 (or occasionally 100) with respect to
a specified or implied white reference, and expressed
without units. The term luminance is often used as
shorthand to refer to this unitless quantity; however, it
is properly called relative luminance. The term luminance
factor should be avoided for this quantity, since
that term refers to a property of a surface or material:
Luminance factor is the ratio of luminance of a surface,
under specified conditions of light source, incidence,
and observation, to the luminance of a perfectly diffusing
(“Lambertian”) surface, under the same conditions.
In color science, luminance (Y) is one of three distinguished
tristimulus values standardized by the CIE; the
other two distinguished tristimulus values are X and Z.
Other tristimulus values such as [R, G, B] are related to
CIE [X, Y, Z] values by a 3×3 linear matrix product. As
I mentioned above, absolute luminance has units of
cd·m -2, or nits. Relative luminance, and other tristimulus
values such as X, Z, R, G, or B, are pure numbers.
Units
Many bizarre units have been used for illuminance and
luminance. I urge you to abandon these, and to adopt
the standard SI units.
To convert illuminance into lux (lx), use Table B.2:
To obtain lm·m –2 (lx),
multiply by numerically
lm·cm –2 , phot, ph 10 4
10000
lm·ft –2 , footcandle, fc 10.764 10.764
metercandle 1 1
Table B.2 Conversion of illuminance into lux
606 DIGITAL VIDEO AND HDTV ALGORITHMS AND INTERFACES