Page 283 - Modern Optical Engineering The Design of Optical Systems

Principles of Radiometry and Photometry 263

irradiance is 42 percent greater than the cosine-fourth result. This is,
of course, a rather extreme case.)
It is now necessary to determine the radiance of the surface at C. The
diffuse surface at C reradiates 70 percent of the incident 1.96 W/cm 2
into a full hemisphere; the total power reradiated is thus 1.37 W/cm .
2
In Sec. 12.4 it was shown that a source of radiance N radiated
2
N W/cm into a hemisphere. Thus the radiance at point C is given by
RH 0.7 1.96 1.37
N 0.44 W ster 1 cm 2
C

The irradiance at E can now be determined from Eq. 12.11, noting that
the solid angle subtended by the aperture of the lens system is
2
1/(100) , or 10 4 ster, and substituting this for sin in Eq. 12.11,
2
2
H T N sin T N
E D C D C
0.8 0.44 10 4 0.35 10 4 W/cm 2
2
Since the photodetector at E has an area of 1 cm , the radiant power
falling on it is just 0.35 10 4 W, or 35 W.

12.7 Spectral Radiometry
In the preceding discussion, no mention has been made of the spectral
characteristics of the radiation. It is apparent that every radiant
source has some sort of spectral distribution of its radiation, in that it
will emit more radiation at certain wavelengths than at others.
For many purposes, it is necessary to treat intensity (J), irradiance
(H), radiance (N), etc. (in fact, all the quantities listed in Fig. 12.5) as

Figure 12.5 Radiometric terminology. The names, symbols, descriptions, and preferred
units for quantities in radiometric work.