Ive schrieb:

> Now, assuming we are using scRGB (sRGB primaries and whitepoint but 
> without gamma correction) as the POV-Ray internal RGB *working* color 
> space we have to apply chromatic adaption for these xyY values to make 
> them consistent with RGB values from other sources and especially with 
> the RGB values for light sources that are used within POV-Ray to 
> illuminate them.

Wouldn't it be more logical in the case of reflective surfaces to apply 
chromatic adaption to the /light source/ in the scene?


>> I'm pretty sure somehow that the /spectrum/ of the whitepoint should 
>> matter for such things as dispersion, but where and why adaption come 
>> into play still eludes me.
>>
> Well, in fact there is no chromatic adaption needed and I think that the 
> whitepoint of the POV-Ray internal RGB working color space shouldn't 
> matter at all for calculating dispersion samples (besides that it is 
> needed for the xyz->rgb conversion). I seem to remember in some quick 
> response I did state otherwise and in case this is true I'm sorry for 
> the confusion this might have caused.

Yes, you mentioned some chromatic adaptation. Apologies accepted, but I 
still seem to be confused :-)


>> Experiments were conducted to measure this per-wavelength response (a 
>> bit indirectly) in a manner that, to my understanding, only yielded 
>> /relative/ results: Conclusion could be drawn how much stronger a 
>> particular cone type is stimulated by wavelength A compared to some 
>> other wavelength B, but there was no way to infer how much stronger a 
>> particular wavelength stimulated cone type A as compared to cone type 
>> B. Thus, the immediate conclusions drawn from these experiments left 
>> open the question what "white" is (which comes as no surprise, given 
>> that it depends on the viewing conditions, i.e. the eye's "calibration").
>>
> There is such a thing as the Grassmann law. Google it for more details.

Hum... okay... so I googled it up. But I have no idea how it fits into 
the whole smash...


> No. They did use mercury (and other metals) vapor lamps to produce 3 
> different monochromatic light beams at three different wavelengths. Out 
> of my head theses have been around 435nm, 545nm and 700nm and as a side 
> note these values are not completely willingly chosen, they had also to 
> deal with the kind of vapor lamps that where available in 1931.

Yes, I read that.

> The "observer" could then adjust the intensity of the three beams until 
> the color that did result from the mixture did match a given one. So 
> there is no "hidden" whitepoint and no dealing with "what is white" at all.

Well, this is exactly the point I'm after here: This "given [color]" 
must have come from somewhere. From the experiment description, it was 
this very color (a spectral one, I presume) that they "measured" with 
the experiment.

This color must have had some intensity, and I guess the test persons 
did not just try to match the color, but the apparent brightness as 
well. I mean, after all, for instance both 700 nm and 740 nm are 
percieved as pretty much the same hue of red, except that one is 
percieved as brighter than the other.

So, was the intensity deliberately "normalized" to equal physical 
brightness?

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