POV-Ray : Newsgroups : povray.general : How to get the light intenity that hits a surface? : Re: How to get the light intenity that hits a surface? Server Time22 Sep 2021 05:29:29 EDT (-0400)
 Re: How to get the light intenity that hits a surface?
 From: Bald Eagle Date: 16 Jul 2021 13:50:00 Message:
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"Grobi" <and### [at] boesmanncom> wrote:

> I would like to mis-use POVRAY (or any other tool) a bit to get to the light
> intensity that hits a surface of an object.
> The original problem comes from design questions of a photocatalyst, where the
> incident photons cause a chemical reaction.

> So my thinking goes like this: I can construct a scene with the geometry that
> holds my catalyst (usually some sort of 3D-grid, mostly cubic), edit the surface
> properties of the catalyst in terms of
> reflectivity, add a light source and let POVRAY calculate the scene.
> What I obviously get is an image of the scene as seen from the camera.
> Is there a way to get the intensity of light that hits surfaces that the camera
> does not see in such a scene? And then get it in such a way that it can be
> evaluated, lets say via ImageJ?
>
> Thanks for any hints!

What I would do as a good first approximation is model your scene and give
everything finish {diffuse 1}.  Then you can get 4 (or 6) different views - from
+/- each of the axes.  That will give you the renders with the light intensities
encoded in the pixel brightness values.

Then you can have a second scene that uses eval_pigment to scan the pixels and
read the rgb values.   Then you can sum all of the orthogonal views.

I would use a pigment of rgb 1, and an orthographic camera.

If you're doing photochemistry, then your catalyst will be in solution, and
absorbed photons won't be available to regions of solution farther away from the
light source, so likely something like Beer's Law will apply, and why
photochemists use things like falling film reactors.

You'll likely want to make sure your rendered pixels aren't saturated, and all
your pixels are between 0 and 1, but never AT 1.
See this thread as a way to check:
http://news.povray.org/povray.binaries.images/message/%3Cweb.6035d08b1995ddaf1f9dae300%40news.povray.org%3E/#%3Cweb.603
5d08b1995ddaf1f9dae300%40news.povray.org%3E
```