The solution in the end was to increase the number of photons to 100000 (I
was using 10000 before).  Also, it was necessary (but not sufficient) to
either make the box with the fog in it a target or make the photons
pass_through (thanks, Christian), although making it a target didn't really
make that much difference and only slowed things down.

I have a few other questions about this.  It seems that the number of
photons had to be increased because the beam of light was quite narrow.  Is
this actually the case and if so, why?

For the scattering media, I used in the end 'intervals 3 samples 40' because
this seemed to produce acceptable results, but what is the best advice for
choosing samples and intervals?  I read somewhere that increasing intervals
was slower than increasing samples.

Thanks for everyone's help so far.

Regards,
Oliver

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