clipka wrote:
> Wouldn't it be more logical in the case of reflective surfaces to apply 
> chromatic adaption to the /light source/ in the scene?
> 
Err, no. Let me put it this way: we use chromatic adaption for the 
reflective surface because we want to rule out the influence the used 
hardware did have for measuring its reflectivity. From this point of 
view raytracing is about having some objective surface color that is 
illuminated by some freely chosen colored light source and we are after
the reflected color that is the same as if the real world thing would be 
illuminated by our light source. We are definitely no longer interested 
in e.g. what kind of lamp was used to *measure* reflectivity and so 
(again) in this case chromatic adaption for the given spectral data or 
xyY value or whatever has to be used.


>>> 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...
> 
I was referring to the linearity within human color perception as stated 
by Grassmann and one of the major problems within the CIE rgb color 
space that e.g. various vectors but of *equal* length within the CIE xy 
diagram would represent *different* human perception of color difference 
(or DeltaE as this is called).
But I might get you completely wrong in what you are after here.


>> 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.
> 
AFAIR: the test colors have been also produced by metal vapor lamps with 
known wavelength but those where easier to build at this time as they 
had not to be made adjustable in brightness. I have a book at work with 
exact description of the Wright/Guild experiments and in case I'm wrong 
here I will correct myself in the next week ;)


> 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?

There was not a *single* color matching experiment, it was performed 
during almost 10 years with different people, different lamps and even 
slightly different setups. The resulting data was then assembled to give 

about 'brightness' issues, guessing they were quite happy to make the 
lamps work anyhow.

-Ive

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