,
incident from all directions onto a small sphere divided by the cross-sectional area of that sphere. SI unit is
.
.
If the radiant power is constant over the area
,
.
Equivalent definition:
,
with
the solid angle of each beam passing through the given point on the surface and
the radiance of the beam at that point.
, for a parallel and perpendicularly incident beam not scattered or reflected by the target or its surroundings.