POV-Ray : Documentation : Cylindrical Lights
  POV-Ray 3.6 Documentation Online View Spotlights Parallel Lights Cylindrical Lights

The cylinder keyword specifies a cylindrical light source that is great for simulating laser beams. Cylindrical light sources work pretty much like spotlights except that the light rays are constrained by a cylinder and not a cone. The syntax is:

        <Location>, COLOR cylinder
    radius Radius | falloff Falloff | tightness Tightness |
    point_at <Spot>

Default values:

  radius:     0.75 degrees
  falloff:    1    degrees
  tightness:  0

The point_at, radius, falloff and tightness keywords control the same features as with the spotlight. See "Spotlights" for details.

You should keep in mind that the cylindrical light source is still a point light source. The rays are emitted from one point and are only constraint by a cylinder. The light rays are not parallel. Spotlights Parallel Lights

Copyright 2003-2021 Persistence of Vision Raytracer Pty. Ltd.