> When rendering a test-scene,
> http://www.manganese-blue.com/povrandombug/test.pov ,
> containing a sphere with refraction-index so that the focal point is near
> the surface, the transmitted light appears speckled, no matter how many
> photons I use.
> The ready-rendered pics and the testfiles (since I may not upload anything
> here)
> are under
>    http://manganese-blue.com/index.php?main_page=page_2

In your scene with the 1.7 ior sphere, the area covered by photon
caustics is large, therefore you need many many more photons.
Calculating photons is processor intensive, it may take several
hours to properly render something like this to look right. That is
the reason for the photon settings, you can get an idea of what
your scene will look like before you do a final high quallity render
with a large count, or with a small spacing.

I also noticed that you used several identical light sources to
have increased brightness. You can accomplish this by
having a multiplier after your light color.
light_source {< 32, 11, -20> color LightGray*5}
Extra lights take extra rendering time, and more importantly in this
instance, the number of photons cast per light is lower because
of the ratio of total photon count to the number of lights.

> To me it seems like the random-generator were not so random, the effect
> beeing amplified by the close-to-the-surface focal-point of the sphere.
>
> I have examined the source, and it seems that the standard-c
> "rand()"-function is used; unfortunately on my debian/etch the source does
> not compile right (see 2 or 3 threads below), and I'm not a c-guru either.
> So I did _not_ succeed in compiling-in other random-generators for testing
> ....
>
> But I played around with the rand()-function, and it really seems to be 
> not
> equally distributed in some ways, but I'm still experimenting on that 
> right
> now.

It's my understanding that photons are cast in a spiral from a light source
in the direction of a target, not randomly. Identical lights are casting
photons which group in the same locations, making the caustics "spot"
a bit more than they would anyway. Fix the lights and use something like
spacing 0.001 and you'll probably have it look great after several hours.

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