[Radiance-general] Luminance variation with distance

Thu, 12 Dec 2002 17:42:03 +0100

we know didly squat!

> -----Original Message-----
> From:	BOWEN, CHARLES K. (JSC-SF5) (JEC)
> [SMTP:[email protected]]
> Sent:	10 December 2002 20:55
> To:	'[email protected]'
> Subject:	[Radiance-general] Luminance variation with distance
> 
> 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
> 
> 
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