In
Figure 7,
is the ``cone'' angle and is the
``azimuthal'' angle.
Emission occurs over the hemisphere defined from to and
to .
While angles are measured in * radians*, solid angles are measured in
* steradians*.
A hemisphere contains steradians, as follows:

The probability of emission of a particle at angle should properly be
weighted by .
As particles originate inside the solid, their probability of escaping from
the solid and being emitted is inversely proportional to the path length
**s** they must travel through the material to escape.
In
Figure 8,
we depict this conceptually for particles originating a
depth **d** beneath the surface.
Thus, emission at angle is proportional to or .
The emission from many real surfaces is well modelled in this fashion.
Such surfaces are termed Lambertian or ``diffuse'' surfaces.

Figure 8: Emission from Within a Solid.