>>   sqrt(pi * A) / pi

>A is the emitter area, right?

Correct.

>Can you tell me about the origin of this formula?

I solve it myself and it is based on basic geometry formulas. I also create
Cornell box scene to test it. Instrutions to Cornell Box scene are here:

  http://www.graphics.cornell.edu/online/box/data.html

It works. I can't explain better because my notes and files are home and I
have to use net from cafe until I get own Internet connection.

>> 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.

>Ehmm... can you explain more about this?

Photographers use gray surface which reflects 18% light back to make
adjustments because it looks half bright.

  0.18^x = 0.5

  x = ln(0.5)/ln(0.18)

I'm not sure about this gamma. I'm still looking more information about
simulating camera and human eye dynamic range.

Matti

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