A particle is emitted from a solid surface into the ``space'' or
* hemisphere* above the surface.
Locations on the hemisphere are characterized by * solid angle*.
We introduce the concept by analogy to linear dimensions.
Recall the definition of geometrical quantities as follows:

Table 4: Analogy of Angles to Lengths.

Referring to the Table 4 and Figure 7, a line is generated by sweeping a point linearly through space, and an area is generated by sweeping a line through space. Similarly as shown in Figure 7, an angle is generated by sweeping a line through an arc. A solid angle is generated by sweeping two ``orthogonal'' angles ( and ) through arcs.

Figure 7: Generating an Angle and a Solid Angle.