The most accurate method would be to fill the room with scattering media, and
shine a cylindrical light source through it. However this will take long time to
render.
You can 'fake' it by using fog (or absorbing and emitting media) for the
atmosphere, and a long cylinder filled with emitting media for the laser beam.
Generally this setup should look good enough and render pretty fast.

-- 
Margus Ramst

Personal e-mail: mar### [at] peakeduee
TAG (Team Assistance Group) e-mail: mar### [at] tagpovrayorg

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