scott wrote:
>> The CIE has defined D50 as reference white for the device independent 
>> xyz color space and e.g. every spectrophotometer including my own one 
>> is calibrated to D50 for exactly this reason.
> 
> Sure, but your meter does not use the reference white to calculate xyY 
> values, these are calculated directly from the measured spectral 
> response using only the colour matching functions - a "reference white" 
> does not appear in these calculations at all!
>
Err, my spectrophotometer has to illuminate the surface to be measured 
to get the reflected spectral data (otherwise everything would obviously 
be just pitch black) and therefor it is calibrated to D50. I can also 
measure spectral emissions (e.g. to calibrate a monitor) but thats a 
different story.


> Of course, but showing an exactly specified colour (like XYZ, xyY, Yuv 
> etc) on an sRGB monitor is a simple process.  You can see the 
> calculation here:
> 
> http://en.wikipedia.org/wiki/SRGB#Specification_of_the_transformation
> 
Thanks for the link but I'm already quite familiar with such things, 
they are part of my per day pay-job.


> It is completely incorrect to do any chromatic adaption before plugging 
> the XYZ values into the calculation.
> 
Err, no.


> FWIW, if you do attempt a chromatic adaption, eg from D50 to D65, what 
> you are in affect doing is saying "ok these colours come from a 
> reflective object lit with D50, now tell me how they will appear if I 
> had instead lit it with D65".  I don't understand why you would want to 
> do that if you simply want to show a particular xyY colour on a monitor.
>
Your main misconception seems to me that you are assuming xyY values are
some given values already there by definition, but within a real world 
work-flow they are almost always the result from measurement of a 
reflected spectrum by a spectrophotometer (like e.g. the Munsell xyY 
values or the ones for the GretagMacbeth Colour Chart or databases for 
real world materials or Pantone colors and so on...). And measurement 
indeed implies that objects are illuminated by some device.

And as you insist on your "showing on a monitor" phrase, this is in fact 
quite complicated and a different beast as using s(c)RGB just as a 
working color space e.g. within POV-Ray (and it seems to become the de 
facto standard there).
sRGB on your monitor also assumes a dominant lighting condition of about 

to take the chromatic adaption of the human visual system into account. 

sounds familiar to you, remember the D50 reference white, even if you 
still seem to thing this doesn't matter.

In other words sRGB as an output device color space requires a viewing 

Therefor *in this case* there has no (mathematical) chromatic adaption 
to be applied as the chromatic adaption is assumed to be done by *your* 
eye/brain system.
How many working places of computer users do you think will match this 
condition? Does yours?

Anyway, for professional environments - where the work flow implies full 
color-management and the closest possible visual color match on various 
devices is needed - there are much more evolved concepts around (defined 
by the CIE and ICC) for taking the environment lighting condition into 
account.

But again, all this is not relevant when using s(c)RGB as a working 
color space and there has something like the Bradford chromatic adaption 
to be applied for given xyY values to make e.g. POV-Ray calculate with 
"good" RGB values.


>> You can only ignore refrence white for e.g. converting from CIE xyz to 
>> CIE L*a*b 'cause L*a*b is also a device independent color space.
> 
> Err no, CIE L*a*b* colour space *needs* a reference white.

Very true. And it just happens that CIE L*a*b is just another 
mathematical representation of the CIE xyz color space where the same 
*need* for reference white exists.


> xyY (or XYZ, Yuv etc) does not.

Wrong. See above.

-Ive

P.S. sorry if I may sound rude when it comes to this subject but not a 
so long time ago I was forced to explain such things more than once a 
day to people who had not the faintest idea what color management is 
about but still did insist in having s strong opinion how such a thing 
should work - and I do not mean you ;). I guess it has something to do 
with the fact that color vision is such a natural thing to humans that 
*everybody* seems to think he has also a basic understanding what color 
science is about...

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