> A cylindrical light source could be better for a laser.

That's what I said, cylindrical =)

Ooh, you misunderstood my "cylinder lights are really point light sources
with their effect constrained by a cylinder." What I meant by this, is that
internally, POV-Ray takes two steps to see if a point is lit by a
cylindrical light source:

1. check to see if the point is contained within the cylinder that you've
defined for the cylindrical light source.
2. if it is, check to see if there's a straight line from the point to the
light source's origin.

The problem is that all of the rays aren't parallel. Points that are very
close to the light source's position, but still inside the cylinder, might
be lit up unexpectedly because they're being lit *from the center of the
cylinder*, not parallel to the cylinder.

A cylinder light with an infinitely large cylinder behaves identically to a
regular point light source.

This works well enough normally. But when there are photons, I'm not sure
how it behaves, since photons may be reflected. It's no longer able to
perform step number one since the photons are likely to be out of the
cylinder but still legitimate, so if you put a reflective object close to
the light source, it might behave strangely. It's hard to explain what I
mean.

Here's what I mean: if you put a cylindrical light source with radius one at
the origin and shine it at a perfectly transparent plane at z=3 (with no
refraction or reflection), and make it shoot photons at that plane, I think
the other side of the plane will look like there was a point light source at
the origin, and the plane had a dark, shadow-casting pigment everywhere
except for a transparent hole with radius one at x=y=0.

Try that same image you just posted with a cylinder light source and no
actual cylinder object, and see if it still works. It might; I'm not sure.
Move the light real close to the photon reflecting object to see.

 - Slime
[ http://www.slimeland.com/ ]

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