POV-Ray : Documentation : Atmospheric Attenuation
  POV-Ray 3.6 Documentation Online View Atmospheric Media Interaction   2.4.8 Light Groups Atmospheric Attenuation

Normally light coming from light sources is not influenced by fog or atmospheric media. This can be changed by turning the media_attenuation on for a given light source on. All light coming from this light source will now be diminished as it travels through the fog or media. This results in an distance-based, exponential intensity falloff ruled by the used fog or media. If there is no fog or media no change will be seen.

Note:in POV-Ray 3.0 this feature was turned off and on with the atmospheric_attenuation keyword. Atmospheric Media Interaction   2.4.8 Light Groups

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