Eric Stevens
But doesn't the Adobe color engine work in Lab anyway?

nospam
internally, but that's not the same as making a conversion of the image twice.

Eric Stevens
What are the two conversions?

nospam
rgb->lab->rgb

Eric Stevens
But isn't that exactly what the colour engine has to do?

It gets fed an RGB file from outside, then converts it to Lab for it's own processing and then converts it back to RGB for output to the monitor. It may also convert it to CMYK for output to a printer or saving it to a disc. http://www.eizo.com/global/library/management/cms/02e.jpg is the kind of diagram commonly used to explain this.

nospam
it doesn't.

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
Dan Margulis discusses this in detail. It is his opinion that, based on trials, whatever color distortions there may be are generally imperceptible.

Fraser, Murphy and Bunting (Color Management) regard Lab as the work horse of color management systems.

Wikipedia http://en.wikipedia.org/wiki/Lab_color_space says

"Because Lab space is much larger than the gamut of computer displays, printers, or even human vision, a bitmap image represented as Lab requires more data per pixel to obtain the same precision as an RGB or CMYK bitmap. In the 1990s, when computer hardware and software were limited to storing and manipulating mostly 8-bit/channel bitmaps, converting an RGB image to Lab and back was a very lossy operation. With 16-bit/channel support now common, the loss due to quantization is negligible."

But none of this explains why you think Lab requires extra color conversions.