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3.2.1 Local Spherical System--Directional Distribution     continued...

We now turn our attention to emission from a general surface. In a specific time interval, the rate of emission of particles, , emitted over the hemisphere from differential area dA may be related to the rate of particles emitted per unit solid angle per unit projected surface area, , as follows:

If is constant, i.e., independent of and (as it is for Lambertian emission), then

Therefore, we obtain exactly half as many particles by weighting the solid angle with the projected area () for emission (recall that there are steradians in a hemisphere). In practice, we define a ``perfect'' emitter, which emits the maximum rate of particles over the hemisphere at each steradian. The ratio of the rates of particle emission from an actual (real) surface to that from a ``perfect'' emitter is termed the emittance, , which typically depends upon .