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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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