> 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!

(Your meter *will* use the reference white of D50 to calculate things like 
dominant wavelength, L*a*b* colour, saturation, etc).

> There is a strong need to take the reference white and the whitepoint of 
> any device dependent target color system (and e.g. RGB or CMY or YCC are 
> always device dependent) into account.

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

It is completely incorrect to do any chromatic adaption before plugging the 
XYZ values into the calculation.

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.

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

Taken from: http://en.wikipedia.org/wiki/Lab_color_space

"Lab values do not define absolute colors unless the white point is also 
specified."

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