[Radiance-general] irradiance vs. radiance / luminance vs. illuminance

Nick Doylend radiance at ndoylend.fastmail.fm
Mon Sep 8 03:52:16 PDT 2008


Problem solved, I think.  The confusion arose from me jumping to
conclusions based my comparison of luminance/illuminance images.  I saw
the bright floor plane outside and assumed it was a mistake.  It
actually turned out that the reflectance of the local ground plane was
set to 0.25 while gensky defaults to 0.2.

Perhaps 0.25 is a bit too high for asphalt but reasonable for concrete? 
BRE [1] list a table of surface reflectances with asphalt = 0.07 and
concrete = 0.2 ~ 0.3.  Anyone know of a more detailed reference for
surface reflectances?

Nick

[1] BRE Report 380 2000 - Environmental site layout planning: solar
access, microclimate and passive cooling in urban areas

On Fri, 05 Sep 2008 12:12:33 -0400, "Jack de Valpine" <jedev at visarc.com>
said:
> Hi Nick,
> 
> In order to provide help on this one, it would be good to provide
> information on the parameters being used as a starting point. So output
> from rad -n <rif file> would be good. Another thing that might be
> helpful is to get the material definition for the ground plane object.
> 
> -Jack
> 
> Nick Doylend wrote:
> > Hi again,
> >
> > I've been puzzling over an extremely bright ground plane in one of my
> > images.  I think I've tracked it down the the use of rpict's -i switch
> > for calculating irradiance rather than radiance.  Can anyone help
> > clarify whether I'm on the right track?
> >
> > If I want a luminance image, i.e. with levels corresponding to what one
> > might actually see (and can compare cd/m2 values at different points) I
> > omit the -i switch (and calculate radiance).  If I include the -i switch
> > (and calculate irradiance), then I obtain illuminance (lm/m2 levels). 
> > In both cases the radiometric units and turned into photometric units
> > (using Radiance's 179lm/W conversion factor) by the image viewer
> > software (I'm using the Ecotect RadianceIV viewer).
> >
> > Radiance is W/m2.sr while irradiance is W/m2.  Luminance is lm/m2.sr
> > (cd/m2) while illuminance is lm/m2 (lux).  I think this means radiance
> > and luminance are properties of a (solid angle) source while irradiance
> > and luminance are properties of a (flat) surface?
> >
> > So my extremely bright ground plane is actually showing its lux level
> > (the amount of light falling on it, rather than the amount of light it's
> > reflecting).  If I want to illustrate the relative brightness of
> > different surfaces I guess I need to go back and calculate luminance
> > rather than illuminance.
> >
> > I did notice that if I clicked on the bit of 'ground' beyond the ground
> > plane I get a much smaller value (in fact the same as the value in the
> > luminance image - because I'm clicking on the ground as defined in the
> > sky file - a glow source).  Why is this?  Does the concept of
> > illuminance on a direct light source not make sense?
> >
> > Sorry for the long winded email equivalent of thinking out loud, and I
> > appreciate that my understanding of photometry is pretty sketchy.
> >
> > Nick
> >
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> >
> >   
> 
> -- 
> # Jack de Valpine
> # president
> #
> # visarc incorporated
> # http://www.visarc.com
> #
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> 



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