chapter 3, Radiometry Review

Excerpt

At least two issues are involved in the understanding of radiometry. One of these is physics and geometry, while the other is almost entirely semantics. They both are important. We start with some semantics and definitions so that we can speak the language.

3.1 Definitions of Important Radiometric Quantities

The fundamental terms in radiometry are radiance, radiant exitance, incidance, and radiant intensity. Radiance, usually written L, is the flux (in watts or photon rate) per unit projected area and per unit solid angle. It is the most fundamental of the radiometric terms. Radiant exitance M is the flux density radiated into a hemisphere. It can be obtained by integrating the radiance over the hemisphere. Nothing is implied about its directional quantities; it is the total flux in the hemisphere, no matter where it goes. Incidance E is the reverse quantity, the flux per unit area received from the overlying hemisphere, irrespective of its directionality. It too can be obtained by integrating the radiance over the hemisphere. Radiant intensity I is the flux per unit solid angle. It can be obtained by integrating the radiance over the area of the source. It is often used for unresolved (point) sources.

Each of these quantities can be a spectral quantity, the flux in a band, or a total quantity, that is, the flux in a band from zero to infinity. It can also be a quantity that is weighted by the response of the eye, in which case it is a photometric quantity. It can, on the other hand, be a photonic quantity. The power, for instance, is the energy per unit time, while the photon rate is the (average) number of photons per unit time. Usually we provide no subscript if the basic quantity is energy, a “q” (or “p”) if it is a photonic quantity, and a “v” if it is photometric (visible). Some authors use a “u” for the energetic quantities.

One author has taken advantage of the identical nature of the geometry of all of these, and dubbed them sterance (generalized radiance), incidance (generalized irradiance), exitance (generalized emitted flux density), and intensity with an adjective like radiant or luminous. Some authors also insist that emittance is a form of emissivity. That will not be done here.