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This ratio (not the front aperture/sample distance) determines the solid angle of this receiver configuration. The FOV is determined by the clear aperture of the lens, which must be kept larger than the illuminated spot on the sample. The Fig. 3b design is unsuitable for near specular measurement because the relatively broad collimated specular beam will scatter from the receiver lens for several degrees from specular. It is also limited in measuring large incident angle situations where the elongated spot may exceed the FOV.

There are three obvious choices for making wavelength scans. Filters (variable or discrete) can be employed at the source or receiver. 20 Details of these techniques are beyond the scope of this discussion; however, a couple of generalities will be mentioned. Even though these measurements often involve relatively large bandwidths at a given wavelength (compared to a laser), the NEBSDF is often larger by a few orders because of the smaller incident power. Further, because the bandwidths change differently between the various source types given above, meaningful measurement comparisons between instruments are often difficult to make.

The geometry is shown in Fig. 1. The value qs is the polar angle in the scatter direction measured from reflector normal and Ω is the differential solid angle (in steradians) through which dPs (watts) scatters when Pi (watts) is incident on the reflector. The cosine comes from the definition of radiance and may be viewed as a correction from the actual size of the scatter source to the apparent size (or projected area) as the viewer rotates away from surface normal. The details of the derivation do not impact scatter instrumentation, but the initial assumptions and the form of the result do.