>> Fade_power should be 1 to be physicaly correct. A value of 2 is perfect
> for a
>> light source, double the distance and get 1/4 the lighting, not for fading
>> trough some substance. Light absorbtion trough a substance is linearly
>> proportional to the thicknessm, double the thickness and the effect
> double, it
>> don't quadruple.
>> Any way, for water, fade_distance is relatively large.
>>
> I don't agree about fade power in water :
> daylight rays have to get twice (down and back up) through water before
> reaching your eye so double the depth quadruple the thickness

That's still linear.

If it was quadratic, 3x the depth would be 9x the darkening. By your 
reasoning, it would be 6x.

Note also that POV-Ray will see that the light passes through the water 
twice and darken it twice anyway, so you only need fade_power=1...

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