Rupert Weber wrote:
> D50 vs D65
> ==========
> Another question during transformation to Lab is, which illuminant or 
> reference white to use.

The whole point of L*a*b* as a color adjustment space, is that 100,0,0
is the white that the observer is adapted to, ie. RGB = 100%, 100%, 100%.
That's why you feed the white point into the function that converts XYZ to 

D65 is the actual white point of sRGB, but since ICC PCS (Profile
Connection Space) has a D50 white, an sRGB ICC profile throws in
a chromatic adaptation transform from D65 to D50.

So there are two different ways you might choose to convert
from sRGB to L*a*b* :

1) Convert from sRGB to XYZ using the sRGB space primaries, and
    feed D65 in as the L*a*b* white point.

2) Convert from sRGB to D50 adapted XYZ using an sRGB ICC profile, and
    feed D50 in as the L*a*b* white point.

[Note that, just to add some complexity, a properly ICC based color managed
  system  would allow the RGB to L*a*b* conversion to choose the conversion
  intent, and might declare the "Lab space" to be ICC PCS L*a*b*, which will 
  a fixed D50 white reference.]

Graeme Gill.
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