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> 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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