> Am 18.09.2015 um 20:57 schrieb MichaelJF:
>> clipka <ano### [at] anonymousorg> wrote:
>>
>>> Unless using an area_light with area_illumination on, fade_distance
>>> should typically be roughly equal to the hypothetical radius of whatever
>>> light source you are trying to simulate. So for instance, to simulate a
>>> classic frosted E27 incandescent light bulb, and presuming you're using
>>> a scale of 1m per POV-Ray unit for your scene, a fade_distance of 0.03
>>> would be about the right value.
>>
>>
>>
>> I wonder what part of the bulb determines this fade_distance: the frosted part,
>> the diameter of the socket (E27) or the incandescent part? I suppose the power
>> consumption given in Watt (Germany) with this kind of bulbs in former years and
>> now replaced by lumen with energy saving lamps usually influence the general
>> brightness only and not the fade_distance. The "incandescent" should influence
>> the color of the light source, so only the "frosted" remains. 3 cm is the
>> approximated radius of this bulbs and as they are frosted the point where their
>> full light intensity should arise. Is this the reason for this fade_distance?
>
> Exactly. As you get closer to the surface of the light bulb, the light
> source gets more and more non-point-like, and the inverse square law
> they teach us at school breaks down.
>

For an infinitely long linear light source, the illumination is 
effectively intencity/distance. When you get closer from a long light 
source, when the distance becomes small to very small relative to the 
lenght, you have a similar effect.

For an infinite plane, the illumination becomes independent from the 
distance. Here, also, when a planar light source is viewed from a small 
distance relative to it's extent, it can be approximated as an infinite 
plane.

I remember that I had to perform the demonstration for both cases using 
some intergral manipulations back in colege.



Alain

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