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.
Nick Doylend wrote:
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.
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