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### 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_Power and `fade_distance` Fade_Distance keywords 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. For realistic effects a fade power of 1 to 2 should be used. Default values for both keywords is 0.0 which turns this feature off.

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

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

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