POV-Ray : Documentation : Attenuation
  POV-Ray 3.6 Documentation Online View Dispersion Simulated Caustics Attenuation

Light attenuation is used to model the decrease in light intensity as the light travels through a transparent object. The keywords fade_power, fade_distance and fade_color are specified in the interior statement.

The fade_distance value determines the distance the light has to travel to reach half intensity while the fade_power value determines how fast the light will fall off. fade_color colorizes the attenuation. For realistic effects a fade power of 1 to 2 should be used. Default values for fade_power and fade_distance is 0.0 which turns this feature off. Default for fade_color is <0,0,0>, if fade_color is <1,1,1> there is no attenuation. The actual colors give colored attenuation. <1,0,0> looks red, not cyan as in media.

The attenuation is calculated by a formula similar to that used for light source attenuation.

If you set fade_power in the interior of an object at 1000 or above, a realistic exponential attenuation function will be used:

   Attenuation = exp(-depth/fade_dist)

The fade_power and fade_distance keywords were originally specified in finish but are now properly specified in interior. They are accepted in finish for backward compatibility and generate a warning message. Dispersion Simulated Caustics

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