Nephelometers polar

An instrument for angular light scattering measurements is often called a nephelometer, from the Greek work nephele for cloud. To be more precise, the instrument shown schematically in Fig. 13.5 is a polar nephelometer, so named because of its angular detection capability. Its essential elements are a col-... 

Figure 13.5 Schematic diagram of a polar nephelometer for measuring angular scattering. FI and F2 are possible polarizing filters.

The simplest, and probably most obvious, way to measure scattering matrix elements is with a conventional nephelometer (Fig. 13.5) and various optical elements fore and aft of the scattering medium. Recall that we introduced Stokes parameters in Section 2.11 by way of a series of conceptual measurements of differences between irradiances with different polarizers in the beam. Although we did not specify the origin of the beam, it could be light scattered in any direction. Combinations of scattering matrix elements can therefore be extracted from these kinds of measurements. There are now, however, two beams—incident and scattered—and many possible pairs of optical elements these are discussed below. 

The heart of the polarization-modulated nephelometer is a photoelastic modulator, developed by Kemp (1969) and by Jasperson and Schnatterly (1969). The latter used their instrument for ellipsometry of light reflected by solid surfaces (the application described here could be considered as ellipsometry of scattered light). Kemp first used the modulation technique in laboratory studies but soon found a fertile field of application in astrophysics the modulator, coupled with a telescope, allowed circular polarization from astronomical objects to be detected at much lower levels than previously possible. 

A photoelastic modulator mated to a nephelometer is shown schematically in Fig. 13.12 (Hunt and Huffman, 1973, 1975 Perry et al., 1978). The transmission axis of the linear polarizer P is fixed at 45° relative to the stress... 

The nephelometer measures the scattered intensity in Wm sr as a fimction of the polar angle 0. Then, the scattering phase function can be obtained based on the analysis derived by Privoznik et al. (1978) and leading to... 

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