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

[Rob Guglielmetti]

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)?

Yes.

> 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...