nospam
from "color management for photographers: hands on techniques for photoshop users" by andrew rodney, page 53:

Editing in LAB: I have nothing against the LAB color model. However, there are a group of people who feel that editing in LAB is the only way to accomplish specific corrections, making it sound like a macho editing space. It is true, there are a few correction techniques that rely on a document being in LAB color space. The question becomes whether its worth taking the time or worse, producing image degradation to convert from a working space to LAB and back. Every time a conversion to LAB is produced, the rounding errors and severe gamut mismatch between the two spaces can account for data loss, known as quantization errors. The amount of data loss depends on the original gamut size and gamma of the working space.

...

Some users are under the impression that Photoshop does all its conversions to and from LAB, converting on-the-fly. this is untrue as it would greatly slow down performance. Instead, Photoshop uses LAB as a reference when conducting many operations. Photoshop is not actually converting pixel data between color spaces unless you, the user, actually ask for this. None of these issues should be interpreted as implying that a conversion from working space to LAB is bad. Just be aware of the issues involved with this kind of conversion and whenever possible, try to use similar techniques that can be conducted in the RGB working space.

Eric Stevens
I don't really understand what he is saying. He seems to be thinking of using a colour engine which works differently from what is described in most of the books I have read.

What he says is also contradictory. For example:

"Some users are under the impression that Photoshop does all its conversions to and from LAB, converting on-the-fly. this is untrue as it would greatly slow down performance. Instead, Photoshop uses LAB as a reference when conducting many operations. Photoshop is not actually converting pixel data between color spaces unless you, the user, actually ask for this."

Photoshop must be converting pixel data unless the image source, your monitor all have identical profiles.

You can get access to this 2005 publication at http://tinyurl.com/m3u98fb .

His description of working spaces doesn't sound at all right and certainly doesn't include device independent colour spaces.

On page 54 he says:

"The RGB working spaces provided by Photoshop are synthetic mathematical constructions. In fact, for a time, I used to call these RGB working spaces 'Quasi-devide-independent' until my technical editor, a true color scientist asked me to refrain from the practice."

Rodney's criticism of the burden imposed on the CPU by the conversion to Lab space has probably been rendered irrelevant by the advent of fast multicore processors and the GPU processors which can help with intensive general processing.

But that was back in 2005 or earlier. It may well be that at that time Adobe was using their own synthetic RGB working space.

The best clue I can get as to what Adobe might be doing now is on pages 15 and 16 of http://tinyurl.com/jwskdc6 (Color Image Processing, Methods and Applications - Edited by Rastislav Lukac, Konstantinos N. Plataniotis - 2007) where the general discussion is of the Adobe Color Space and the file tags required to convert images to Lab. But 2007 is still some time in the past and I would like to see something more uptodate.