Colin wrote:

> So, say for example I put a point
> light source above a flat surface. If I render it, I see that it's brightest
> directly below the source. At a point some distance away from this centerline,
> it is dimmer. What I would like is a metric for how much dimmer it is, as a
> function of position. I assume this would be trivial from within the
> ray-tracing algorithm; I'm just curious if it's possible from the user end.

I think the only way from the user end is to go via the brightness.

You can position an orthographic camera to look top down on the
plane you are interested in. Using a homogenous texture and suitable 
light intensity should yield a reasonable intensity image.

You need to make sure that the surface is lighted by photons only.
This should be the case if the light source is completely blocked
by some transparent object which is a photon target.

You will wish to adapt the gamma settings to get a linear
response curve, use 16-bit output for higher precision, and
possibly try HDR output (this is in MegaPOV, there is also
HDR support in 3.7b but I'm not sure its also for output?)

This should already give you the relative intensities.
Converting the pixel intensity to a physical unit may be
tricky. For reference, you'd probably need some test lens
which directs all photons onto the visible area.

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