The Strangeness of the Internal Lightroom Color Profile
Photoshop color management is complicated, but color management in Adobe Lightroom just works. So well in fact that most users have never really thought about how it works. Here's the scoop.
By way of comparison, Photoshop has been around a lot longer than Lightroom. Lightroom was built from the ground with the presumption that most of the images photographers work on would be color managed. Not so with Photoshop. In fact, Photoshop has been around so long that the first release came out several years before the International Color Consortium (the ICC of color management fame) was formed. Color management was a later add-on to Photoshop and has always been somewhat clumsy in its implementation. Adobe tried to retain support for longtime users who had established workflows, while adding new features to keep up with industry trends such as color management. Unfortunately, it's hard not to judge that by attempting to keep everyone happy they've ended up confusing everyone. Users who didn't want to mess with color management often inadvertently messed things up by setting their preferences incorrectly by accident or by misunderstanding. Users who did want to benefit from color management had to master a number of esoteric technical details to do it right. But with Lightroom, most of the technical details of color management are hidden under the covers so well many users never give a passing thought to the fact that they're there at all.
Although Adobe Lightroom can work with JPEG and other file formats, it transparently supports most raw image file formats, and I'd venture to say that most Lightroom users are working with raw files. Yet regardless of file type, Lightroom manages them all the same. On the Photoshop side of things, raw images and non-raw are kept completely separate. Photoshop itself can't even open a raw image. It has to be converted first in Adobe Camera Raw or a third party raw converter. Adobe RGB or ProPhoto RGB images over here, linear gamma raw files over there. But in Lightroom, everything gets treated the same.
There is no choice of working space color profile in Adobe Lightroom. Internally, Lightroom doesn't use any of the well-known color spaces. Adobe invented a new color space just for Lightroom. It shares a lot in common with ProPhoto RGB, but uses a linear 1.0 gamma rather than the standard 1.8 gamma that ProPhoto normally uses. The color coordinates and gamut of Lightroom match that of ProPhoto, but the gamma curve differs radically. This allows raw images to retain their native linear gamma during all edits potentially resulting in smoother gradients and better results for certain edits than would be the case if they were converted to a more traditional working space like Adobe RGB (2.2 gamma) or ProPhoto RGB (1.8 gamma).
Even CMYK and grayscale images get converted to this special Lightroom color space. So too do non-raw RGB images. If images do have an embedded profile, that profile gets retained for the Library preview image, but once opened in the Develop module all edits are done in the Lightroom linear color space. If an image has no embedded profile it is assumed to be sRGB. Library thumbnails for raw images are generated using Adobe RGB. The same holds true for display images in the Book and Slideshow modules.
Lightroom calculated everything using either 16-bits per channel or in some cases 32-bits per channel. Even if you import an 8-bit jpeg image the Develop module will convert it to 16-bits per channel. This allows the program to retain maximum quality when editing in such a huge color space.
This whole linear gamma thing is very different from the way the human eye sees things. We see doubling or halving of brightness as being uniform increases or decreases. That's where this whole thing of doubling and halving for exposure theory comes from. But that's not how a digital camera sensor sees the world. It basically just counts the number of light photons striking each photosite (pixel) sensor while the shutter is open. That's what linear gamma is all about.
Since Lightroom edits everything in this same linear gamma world you might wonder why the Exposure and other Develop module sliders don't react strangely. It seems as if moving the Exposure control to the right some distance increased brightness by one stop, it would be necessary to then move it twice as far again to bump exposure up by another stop, and so on. In order to avoid this non-intuitive user experience, Adobe applies a sRGB conversion to the data behind the sliders. So while retaining the linear gamma for the actual image, the sliders behave as if the image had the sRGB gamma of 2.2 that matches more closely to the way our eyes perceive brightness.
Thankfully, Lightroom automatically does all this for us. This way, we can pay attention to working with images, not color management settings.