Am 02.06.2011 00:41, schrieb Crippso:

> I'd like to place a flat disc in front of my FPA that is acting as a lambertian
> emitter. The FPA will see part of this emitter through the aperture in the
> cylinder. Is it possible to include such surfaces in povray by setting their
> ambient value to a number representing the radiance of the surface?

As of POV-Ray 3.7, Lambertian emission is officially supported via the 
"emission" finish term. As to be expected, a value of 1.0 corresponds to 
the same brightness as an ideal Lambertian reflector illuminated 
vertically by a light source of unity brightness.

With POV-Ray 3.6x or earlier, the same effect can indeed be emulated via 
the "ambient" finish term; however, the value must be divided by 
whatever the global value "ambient_light" is set to (which defaults to 
1.0). Obviously, you'll need a non-zero ambient_light value for this to 
work.

> Much advice on radiosity suggests ambient should always be set to zero so I
> wasn't sure if using ambient was a good way of generating a lambertian emitter
> when using radiosity.

My suggestion in that respect is to use POV-Ray 3.7 and use the 
"emission" finish term.

If for some reason you are stuck with POV-Ray 3.6x or earlier, a 
feasible workaround is to set ambient_light to some very low value (such 
as 0.001), and increase the ambient finish term accordingly (e.g. 
1000.0) to compensate.

> Ultimately I would like to be able to use the pixel values I obtain when looking
> at the diffuse slab that represents the FPA and convert these to Watts (or
> photons per second) based on the amount of light emission from the flat disc.
> Any advice on the best way to acheive this would be very helpful too (I have
> already set gamma to 1.0)

POV-Ray doesn't prescribe a particular unit of measurement for 
brightness, so you could define an "emission" finish setting of 1.0 to 
correspond to a Lambertian emission of 1.0 Watts per square unit. In 
that case, a 100% white pixel value would correspond to a light input to 
the sensor of 1.0 Watts per square unit (provided it doesn't happen to 
be clipped of course).

As for gamma, be sure to set both the assumed_gamma global setting in 
the scene file as well as File_Gamma in the INI file to 1.0 for your 
purposes. I recommend PNG output (as it can still be viewed accurately 
with contemporary sotware, despite the linear gamma settings) or, even 
better yet, HDR or OpenEXR (as they prevent clipping and are linear 
anyway). The latter two require POV-Ray 3.7.

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