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 wavelength, 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 sections will discuss radiation in terms of watts; the reader should remember that the discussion is equally valid for photometry,...