Rupert Weber ( wrote:
> D50 vs D65
> ==========
> Another question during transformation to Lab is, which illuminant or 
> reference white to use.
> That part, I don't quite understand yet. Does that depend on the source 
> data, or simply on how the monitor is calibrated?

Well, Lab as a color space is not fully defined. Without further
information it is not possible to have a Lab color calculated back to
e.g. sRGB.

The problem is, that the "white" in Lab (100%, 0, 0) has no absolute
color coordinates. You don't know if this is a reddish white of a light
bulb or the blueish white of a fluorescent tube. In that sense Lab is
not an absolute color space, unless you define what you actually mean by

So if you have a sRGB image (where sRGB defines D65 as the white point)
you can easily transform this into an Lab image, but you need to attach
the information "the "white" is actually D65" to the result, to be able
to meaningfully be able to transform the image back.

Maybe this helps a bit,
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