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Chapter 15:
Polarization Effects
Every radiometer must be considered a polarizer to some extent, at least until the polarization properties have been shown to be insignificant. If it tends to induce polarization, it will also affect the polarization properties of any incoming polarized light. Then, the flux measured may not be representative of the flux emitted, because some has been rejected or altered by polarization. It is also a problem if the polarization properties of the calibration source are different from those of the unknown and the radiometers operate on these differences. This chapter describes some of the polarization properties of calibration sources and receivers and of radiometers and provides methods for calculating the polarization properties of an entire radiometer. 15.1 Descriptions of Polarization Light, that very special electromagnetic wave, can be polarized in different ways. The electric field may vibrate strictly in the vertical direction or strictly in the horizontal one, or in some other arbitrary plane. If it does, then it is called linear polarization in the direction of vibration, often specified as x and y or as s and p, standing for perpendicular (senkrecht in German) and parallel to the plane of incidence, respectively. The field vector, on the other hand, can follow the locus of a circle, and then the two possible components of polarization are left and right handed (circulation). The more modern descriptions are in terms of Mueller and Jones matrices. To some extent these two methods are complementary, but a full discussion of them is beyond the limits of this text. The Mueller method is described briefly and applied to some representative examples. A column matrix is used to describe the beam of light in which the elements are {IMCS}. In this text, for ease of typing, the column vector is represented by curly brackets {} and a row vector by square brackets []. The terms are the field intensity I with units of W m−2 (it seems useful to keep the symbols used by the writers in spite of the radiometric inconsistency); the degree of horizontal polarization, M; degree of +45° polarization, C; and degree of right-circular polarization, S. For examples, {1 0 0 0} is an unpolarized beam of unit intensity; {1−1 0 0} is a unit intensity vertically polarized beam. Table 15-1 provides information on most of the types and their Mueller representations.
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