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Chapter
12
Principles of Radiometry
and Photometry
12.1 Introduction
In concept, both radiometry and photometry are quite straightforward;
however, both have been cursed with a jungle of changing and often
bewildering terminology. Radiometry deals with radiant energy (i.e.,
electromagnetic radiation) of any wavelength. Photometry is restricted
to radiation in the visible region of the spectrum. The basic unit of
power (i.e., rate of transfer of energy) in radiometry is the watt; in
photometry, the corresponding unit is the lumen, which is simply radiant
power as modified by the relative spectral sensitivity of the eye (Fig. 8.8
and 8.9 in Chap. 8) per Eq. 12.18. Note that watts and lumens have
the same dimensions, namely energy per time.
All radiometry must take into account the variation of characteristics
with wavelength. Examples are the spectral variation of emission, the
variation of transmission of the atmosphere and optics with wavelength,
and the differences in detector and film response with wavelength.
A convenient way to deal with this is to multiply, wavelength by wave-
length, all such factors together so as to arrive at one unified spectral
weighting function. Thus, all radiometry is spectrally weighted and it
should be apparent that photometry is simply one particular spectral
weighting. See Sec. 12.9.
The principles of radiometry and photometry are readily understood
when one thinks in terms of the basic units involved, rather than the
special terminology which is conventionally used. The next five sec-
tions will discuss radiation in terms of watts; the reader should
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