> What actually happens is it shows up before it strikes the block, but then
> does not show up again at the other side.  I have been looking at the
> source code of other scenes that do something similar (particularly the
> advanced/optics.pov scene that comes with povray) but I can't really tell
> what I am doing wrong.

Neither can I, but actually you are not getting any photons at all.
The beam of light you see before the block is not related to photons,
it's the normal interaction of the spotlight with scattering media.
Note that no photons are stored during the render, which probably
implies that the target is blocked. Adding photons {pass_through}
to the box containing the fog at least causes some photons to be
shot and stored, although I'm not sure if this yields the correct
effect and why it should be necessary.

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