[Radiance-general] Luminance to RGB

[Lars O. Grobe]

Hi Chantal!

> The data I have is only luminance (cd/m2), from walls, ceiling and floor
> of a real room. The intention is to simulate
> that room in Radiance  (...)

Please note that Radiance expects reflectance. Luminance is not a 
"material property" (except for light sources), but the result of a 
certrain amount of light leaving a surface of given 
reflectance/transmittance at a certain direction. The approach with grey 
charts relies on the assumption that both the patch of known reflectance 
and the sample are lid by the same source under very similar directions, 
and that both samples are also seen under almost identical directions. 
So if you know luminance for a patch of 0.4 reflectance and luminance 
for a patch of 0.2 reflectance, you can make an estimate on the 
reflectance of a patch that appears with a luminance between the two. 
Both the camera and your eye just record that luminance, so you must 
"see" a known reference at the same time to make guesses on the reflectance.

Cheers, Lars.