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

[Greg Ward]

Hi Nick,

You generally seem to know what's going on, so let me just verify and  
clarify a few points:

> 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?

That is correct.  Bear in mind that certain surfaces do not report  
irradiance, such as glass, which you see through as usual with the -i  
option.

> 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.

Right.

> 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?

Right again -- sources don't report light landing on them  
(irradiance), since they are considered to be emitters, and thus  
aren't affected by the -i option.

-Greg