[Radiance-general] Luminance variation with distance

Thu, 12 Dec 2002 16:52:58 -0000

I appologise for the last e-mail - it was not intended to trouble the group.

Regards

Andrew

> -----Original Message-----
> From:	Andrew Bissell [SMTP:[email protected]]
> Sent:	12 December 2002 16:42
> To:	'[email protected]'
> Subject:	RE: [Radiance-general] Luminance variation with distance
> 
> 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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> > http://www.radiance-online.org/mailman/listinfo/radiance-general
> 
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