Wasn't it John Pye who wrote:
>Hi all
>
>I have an application for POVRAY that seems a little different from what
>most users are doing, and it doesn't seem to be something that's directly
>addressed in the documentation, so I wonder if anyone could offer any
>suggestions.
>
>I'm trying to assess the imaging properties of a number of reflectors. This
>means that I want to create some geometry for a set of mirrors including
>scattering,  geometric surface imperfections, etc, then I want to create a
>light source in the form of a 'sun shape', then I want to see what the
>resulting light intensity at my target plane becomes.
>
>The normal use of POVRAY seems to revolve around the idea of a 'camera'
>which is a single point 'observer'; Raytracing in this case means tracing
>*backwards* from the eye of the beholder to the different spots in the
>scene.
>
>My use seems a bit different: I want to have an imaging plane and I want to
>'catch' rays over that plane. The overall result should be data about the
>intensity of light striking various points on my imaging (target) plane.

That sounds very much like casting photons, which POV-Ray can do. It's
typically used for getting correct shadows cast by curved refractive
objects.


>
>Obviously I can approximate this analysis by creating an imaging plane, and
>then creating a camera that looks at that imaging plane. This won't always
>be practical however, as sometimes the act of creating a visible barrier at
>the imaging plane will cause the optics of the problem to change, eg by
>creating repeated reflections when none are desired.

You can switch off those reflections by specifying no_reflection for
your image plane object. The plane will be visible to the camera but it
won't be reflected.

You might also want to also specify no_shadow if there's any possibility
of the image plane interfering with the optics by casting shadows onto
the geometry.

>Finally, is there any way that POVRAY can generate output that shows the
>location of rays in 3-D space? For example if I create a ray-trace any only
>wish to trace a couple of hundred rays, is there any way I can view the
>paths taken by those rays through my scene?

There's no built in facility for doing that, but some of the concepts
described in the SDL Raytracer tutorial in the documentation. [In the
3.6 documentation it's "2.3.10  SDL tutorial: A raytracer"]. Some of the
SDL ray tracing code might be what you need to calculate the paths of
some rays, then you could indicate the paths with thin cylinders.

-- 
Mike Williams
Gentleman of Leisure

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