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

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On Fri, 05 Sep 2008 12:12:33 -0400, "Jack de Valpine" <[email protected]> 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
> 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).
>
> (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
>
> _______________________________________________