Chromatic adaptation with PDF Lab

I have a PDF target job containing a few hundred patches specified in the Lab colour space. I can import this into Photoshop specifying the 'Lab Color' mode and when I look at the Lab values of each patch they are exactly as the PDF says.

The problem comes when I choose to convert the image to a profile (any profile) because Photoshop appears to do no chromatic adaptation during the conversion. It is as though D50 is assumed, since that is the ICC standard. I can change X and Z values of the white point in the PDF to any numbers and the conversion is always unchanged. If the Y value is changed to any number other than 1, the Lab elements are not rendered in Photoshop.

Is there any way I persuade Photoshop to honour the chromatic adaptation implied by non-D50 white points? I am using CS2 on Windows.

Thankyou for any help.

John Jefferies

LAB in this case is D50 white point.
No, you can't do chromatic adapation after you have it in LAB - because the LAB numbers are referenced to D50.

Also, what are you trying to do? If we knew what you were trying to accomplish, we might be able to offer more help.

Thankyou, that has answered my question. I can't do what I want right now.

In PDF, Lab can refer to any reasonable white point since the XYZ of must be specified in the colour space. It is not restricted to D50 or any other standard, but of course you know that.

What I'm trying to do is verify the colour conversions of a PDF rip against Photoshop with the aim of getting an acceptable match. In particular, what should happen when Lab, or CalRGB, colour spaces with non-D50 white points in the PDF are combined with ICC output profiles with non-D50 media white points. With and without black point compensation.

This isn't a new problem since it is faced by most PDF rips with varying degrees of success. While I can probably achieve the same ends of getting a useful comparison with Distiller and other applications, Photoshop was the first tool of choice.