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Chapter 2:
Radiometric Quantities, the Language
Radiative transfer describes the ways that real bodies radiate power or energy to and from each other. This radiation is described in terms of energy, power, or certain geometric characteristics of power—its areal density (power per unit area), for example. An understanding of this transfer can only be obtained if the language is understood. So, at this point, several definitions are in order. 2.1 Angle and Solid Angle The angle, more precisely the linear angle, is defined as the length of arc of a circle divided by the radius of the circle. An alternative and completely equivalent definition is the length of arc of a unit circle. The units of angle are degrees and radians and their subdivisions are arcminutes, arcseconds, milliradians, and microradians. It was the ancient Babylonians who first defined the degree as being 1/360th of a full circle. The natural measure, however, is the radian. It is a length of arc divided by the radius, so that there are 2π radians in a full circle. But an angle is a subtense, a linear dimension divided by another linear dimension. As Figure 2-1 shows, a straight line and even a curved line can subtend the same angle as an arc on the circle. So, the definition should read that a linear angle is the projection of a line on a unit circle, and the line need not be straight. The angle is really determined by the endpoints of the line.
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