Wasn't it Alain who wrote:
>Sylvain Dubé nous apporta ses lumieres en ce 2008/03/12 10:26:
>> Hi,
>> I’m using POV-Ray to simulate a laser beam pointed to a screen. 
>>I’m also using
>> photons in my programming and I would like to know if it’s possible 
>>to detect
>> photons on my screen. Does there exist any function for this? I’m 
>>looking for a
>> command like ? trace ? to know where are my impact. I cannot know 
>>which approach
>> take. Is there somebody have an idea?
>>  Thank you
>>  Sylvain
>>
>The trace function return the intersection point between a line shoot 
>from an arbitrary point toward a previously deffined object. It only 
>return the first intersection point and, optionaly, the normal at the 
>found point. It ignore any reflectivity. It also ignore any transparency.
>It ignore any object exept it's target object.

If you know the reflection, transparency and IOR of the object you've 
hit, you can use that information to trace() new reflected and refracted 
photon paths onwards from that intersection point.

You don't know which object those reflected photons are going to hit 
next, so you have to use trace() against every candidate object and 
select the one that has the nearest intersection point.

-- 
Mike Williams
Gentleman of Leisure

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