> I have to ask about this. Many sources say that radiance is analogous to
> luminance.

OK sorry I didn't understand the radiance/irradiance thing (I usually work 
with luminance and luminous intensity etc) - so I looked them up.

Illuminance is the same as irradiance, and describes how much power per unit 
area hits a surface from all directions.  If you were dealing with infra-red 
light, you could use this to work out how hot something got.

Luminance is the same as radiance, it describes how bright something seems 
to be from a particular direction.  It tells you how much power is coming in 
a certain direction in a certain area, it is directly related to how many 
photons would hit a pixel in a camera or a rod in your eye (or anything else 
with finite area).

Luminous intensity is the same as radiant intensity, this describes how much 
power is emitted in a certain direction, but it says nothing about the area 
that the energy is spread over.  So the same luminous intensity could be 
coming from two planes, but because one is tilted differently from the other 
the luminance would appear to be different.

For a lambertian surface, the luminance is constant from all viewing angles, 
but the luminous intensity of each point depends on cos(x).

In your calculations, you can either use luminous intensity and factor in 
for the geometry, or use the luminance directly and not worry about the 
geometry.  The 1st will simpplify to the 2nd in most cases anyway, but I 
don't know, maybe it is easier to understand the 1st case, or better for 
later when you have non-lambertian surfaces, I don't know.

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