Lightroom (classic) and ACR color spaces

This is a really arcane question, but I'm guessing that at least one person here knows the answer. I've searched in vain for an explanation from Adobe.

As I understand it, LR maps raw data to a proprietary variant of ProPhoto called Melissa. The image stays in that color space until one exports it, moves it into Photoshop or another external editor, prints it, or whatever.

ACR has a menu that allows converting to other color spaces, including Lab. LR lacks this.

So, here are the disagreements:

1. Does that ACR menu have any bearing on the internal color working space, or does it use the same working space as LR, only mapping to another space, such as lab, when moving the image out of the application into photoshop?

2. If you want to work in Lab in Photoshop, choosing the Lab option in ACR would open the image as a base layer in Lab in Photoshop. Coming from LR, one would have to open the image in Photoshop in whatever working space one selects, let's assume ProPhoto to keep things simple, and then convert the base layer or a copy to Lab. Would that make any practical difference?

Thanks for any help with this.

MelissaRGB is ProPhoto RGB primaries (gamut) with a 2.2 TRC like sRGB and is only used for the Histogram.

Internal processing color space is ProPhoto RGB primaries, 1.0 TRC and has no name. It is what it is, always for processing. Outside of soft proofing where you pick a specific profile, then the Histogram updates to reflect this.

You could make a custom RGB Working Space for Photoshop of either of the two ProPhoto RGB based color spaces above (we can go there if you wish). And you'd need this just to convert from it, to Lab.

Reported Lab values in Photoshop and ACR vs. LR differ which pisses me off. In PS/ACR, it truncates to +/- 128 but not in Lightroom Classic which is IMHO better. Indeed, LR and ACR do provide Lab readouts.

Make a green patch in Photoshop in ProPhoto RGB (G255/B0/R0), import into LR, read the Lab values. Do the same in Photoshop. At least LR lets you know, this isn't a real color ("Imaginary Color") based on the numbers.

For question #2, I don't know why you'd want to do any of this. :) For a number of reasons but a good one is, the two don't jive fully. Repeat fully.

Andrew,

Thanks very much

dan