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

[Nick Doylend]

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