[Radiance-general] Luminance variation with distance

[BOWEN, CHARLES K. (JSC-SF5) (JEC)]

I am a na�ve user of Radiance involved in modeling various lighting
conditions encountered during spacecraft docking/berthing operations.
Recently I encountered some unexpected luminance values when modeling
distant views of docking lights.  

As the separation between the viewpoint and a light source is increased, the
luminance of the source should remain constant.  Sources I have modeled
using Radiance behave in this way until they occupy a solid angle relative
to the observer of less than 4.9e-06 steradian or so (for example, a
two-inch radius disk source at 134 feet).  Beyond this distance, however,
the modeled luminance of the source decreases in inverse proportion to the
square of the separation distance.  This behavior is only exhibited by light
sources.  In contrast, models of small reflective surfaces illuminated by a
fixed local source exhibit constant luminance as the surface viewing
distance is increased, finally disappearing from view altogether when they
occupy a solid viewing angle of approximately 2.2e-06 steradian.  Can anyone
explain the light sources'departure from ideal constancy of luminance with
distance?

I'll gladly provide additional information about my modeling attempts to
anyone interested.

Charles K. Bowen, Ph.D.
Graphics Research and Analysis Facility (GRAF)
Johnson Space Center, NASA