ABSTRACT
The terms spectral radiant sterance and spectral radiance have identical meanings. Spectral radiance is the conventional term. Because of its importance, we shall devote substantial space to its definition, and we shall consider several ways of defining it; several ways of measuring it; and its production, properties, dimensions, and units. The asymmetrical definition of the spectral radiant sterance is the one most commonly presented, but it hides the basic symmetry of the concept of sterance. Sources of known spectral radiant sterance are important for calibration of a radiometer, because it is usually possible to produce a known spectral radiant sterance more accurately than one can measure separately the physical dimensions of the radiometer and the responsivity of the detector. The spectral radiant intensity of a source in a given direction is defined as the integral over the projected area of the spectral radiant sterance in that direction.
