This might be of interest to people that need accurate WP handling.
Turns out Photoshop clips the "b" channel in the negative direction (toward blue) when the media white point is used during printer (LUT based) profile processing. It clips the at just over -5. which can affect high OB content profiles. For instance, Epson Ultra Premium Glossy has a white point of Lab=97, 2, 7. If you do a reverse convert of (255,255,255) in Photoshop that profile will give you a value of 97,2,5. Same with a white point of 97,2,10. It also clips the "b" channel at 5. In the other direction there is no clipping up until at least b=40 which is quite a yellowish media.
This occurs using the Adobe engine. The clipping range is similar in the Microsoft Engine though it introduces slightly more error - around .1 to .2 dE.
This primarily affects proofing, especially hard proofing, but also soft proofing with "show paper white" checked since most rendering, PI and RI, ignores the media white point.
EtoA: This effect is unrelated to profile accuracy. It's purely how the White Point is interpreted by Adobe.
