>I mean the simulation with the radiometry (or photometric) value of
>a lamp (power, light intensity, or Photometric Solid, etc...).


Best way to use POV-Ray light simulation is to multiply light source color
with real light luminance (cm/m^2). Then set fade_distance using following
formula:

  sqrt(pi * A) / pi

Real world fade_power is 2. Because POV-Ray doesn't have dynamic range
compression, this isn't enough. I recommend you to define two adjusting
variables "Gamma" and "Max_Luminance" to make dynamic range compression.
Gamma is the gamma used to compress image dynamic range and Max_Luminance
is the luminance (cd/m^2) where pixel luminance at range 0..1 is "1".
Max_Luminance controls image brightness. When using these variables you can
calculate light source color multiplier using formula:

  pow(light_source_luminance / Max_Luminance, Gamma)

And global Fade_Power to *all* light sources by using formula:

  (ln(-exp(-ln(2) * Gamma) + 2) + ln(2) * Gamma) * 2 / ln(3)

To simulate photograph dynamic range compression I think that Gamma should
be ln(0.5)/ln(0.18) because gray card which reflects 18% of light looks
half bright.
Light source color luminance (without multiplier) must be one. Good starting
value for Max_Luminance is 50. Light source colours luminances without
multiplier must be one. Example for white light, light color is <1, 1, 1>.

Because you probably want to make other light sources than omnidircetional,
you need to simulate light distribution using spotlights. If light source
is flat, use spotlight radius -90 falloff 90 tightness 1 for cosine
falloff.

I don't recommend to use Jaime's lightsys because it doesn't use physically
accurate lightning. (Sorry Jaime).

Matti

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