irradiance vs. radiance / luminance vs. illuminance

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.


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/ while irradiance is W/m2. Luminance is lm/
(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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Marija Velickovic wrote:

What does this exactly mean when values on the image are traced in ximage?
For example if I have a window made of some glass material or some curtain on it made of trans material. On irradiance image when I try to trace value of some pixel on the window surface, what do I get? Is it irradiance of the objects behind the window (outside ground, buildings etc)?


Does window/curtain transmittance has an influence on reported value?

Depends. To be clear (pun!), let's say you have a view from the outside of the building looking in, and you trace a pixel that shows floor behind the curtain wall. In this case, the irradiance value returned is the irradiance on the floor, and it is of course modified (reduced) by the reflectance and absorptance of the glass. Now, let's say you have a view from inside a building, and you trace a pixel on the ground outside. In this case, the irradiance is not influenced at all by the glass, even though you are tracing a ray through the glass.

What happens for surfaces made of transparent BRTfunc materials (like double glazing)? Are they ignored too, or their irradiance value is reported?

That's one for Greg...