DigitalVideoAndHDTVAlgorithmsAndInterfaces.pdf

Palmer, James M., “Getting Intense
on Intensity,” in Metrologia 30 (4):
371–372 (1993).
dA
Normal
θ
Φ
[x, y, z]
dω
Figure B.1 Geometry associated
with the definition of
radiance. The quantity dA
represents unit area; the
quantity dω represents unit
solid angle. Projected area
falls off as cos θ.
James M. Palmer points out that the term intensity is
widely misused, for no good reason, because it is one of
the seven base units of the SI system! Beware so-called
intensity expressed in units other than watts per steradian,
W·sr -1. Some authors in thermal engineering,
and some authors in the computer graphics field of
radiosity, use the term radiant intensity for what I call
radiance, which I will now describe.
Radiant flux density in a specified direction is radiance
(symbol L). Formally, radiance is radiant flux differentiated
with respect to both solid angle and projected
area; the geometry of this definition is depicted in
Figure B.1. Radiance is expressed in units of watts per
steradian per meter squared, W·sr -1 ·m -2 (or
W/sr/m 2). For a large, non-point source, radiance is
independent of distance.
Radiance can be considered to be the fundamental
quantity of radiometry: All other radiometric quantities
can be computed from it. You might find it intuitive to
start with radiance, and then consider the following:
• Radiant intensity is radiance integrated across an area.
• Irradiance is radiance integrated through solid angle,
that is, integrated across all directions in a hemisphere.
• Flux is irradiance integrated across area, or equivalently,
radiant intensity integrated through solid angle.
All of these radiometric terms relate to a broad spectrum
of wavelengths. Any of these terms may be limited
to a narrow spectrum by prepending spectral to the
term, subscripting the letter symbol with λ, and
appending per nanometer (·nm -1 ) to the units.
Photometry
So far, I have discussed the physical quantities of radiometry.
Photometry is entirely analogous, except that
the spectral composition of each quantity is weighted
by the spectral sensitivity of human vision, standardized
as the luminous efficiency of the CIE Standard
Observer (graphed in Figure 20.1 on page 205).
APPENDIX B INTRODUCTION TO RADIOMETRY AND PHOTOMETRY 603