Lightroom Color Spaces

by Michael Clark

In Lightroom, as most of us know there are no color spaces to choose until you export an image. I'd like to demystify exactly what is going on in Lightroom and how it deals with color spaces.

First, lets talk about color spaces in general. A color space by definition is a description of a range of colors that can be represented. The smallest color space is CMYK used for off set presses used by most of the printing industry. The sRGB color space is a little larger and used for the internet and this is also what most monitors display. Adobe RGB is quite a bit larger and is the color space that should be used in most digital cameras to get the best image quality. Lastly, ProPhoto RGB is one of the largest color spaces around and can incorporate all of the colors captured by a digital camera.

Anyone who has used Photoshop quickly realizes that color management is a big deal. You need to set up your color settings dialog box in Photoshop so that you know what color mode you are in. With Lightroom Adobe wanted to simplify the color management process. Lightroom was created to have a ProPhoto RGB color space with an sRGB tone curve. We'll get into what exactly that means in a moment. The reality is with Lightroom you are working up your images in a very large ProPhoto RGB color space. Exporting those images into an sRGB color space for example will lead to a certain amount of clipping in your histogram. Whether or not that clipping is important depends on the image.

Below is an image from a recent surfing shoot that I exported three different times into ProPhoto RGB, Adobe RGB and sRGB color spaces.

mclark_193_surf_laura.jpg

I went into Photoshop and took a screenshot of each histogram. Below are the histograms for the same image in ProPhoto RGB, Adobe RGB and sRGB color spaces and also the Lightroom histogram before exporting the image.

LR Histogram.png
Original Lightroom Histogram (ProPhoto RGB)

prophotorgb histogram.png
Photoshop ProPhoto RGB

Adobe RGB histogram.png
Photoshop Adobe RGB

srgb Histogram.png
Photoshop sRGB

Now as you can see the Lightroom histogram and the Photoshop ProPhoto RGB histogram are the same (as they should be - the Lightroom version is just stretched out a bit more than the Photoshop version). The Adobe RGB histogram has some clipping on the shadows side and since the ProPhoto RGB highlights had some headroom there is no clipping in the Adobe RGB highlights. Now the sRGB histogram has a big line on the left hand side showing significant clipping in the shadows and while there is no clipping in the highlights it is right up against the right side of the histogram. So, all this is to say that if you are not exporting into a ProPhoto RGB color space then you will most likely have some clipping in your exported images histogram.

Now, why would Adobe set up Lightroom to work with a ProPhoto RGB color space is the big question. The answer is ProPhoto RGB is the only color space that can contain all of the colors a digital SLR can produce. Most DSLR's produce colors that are outside of the standard Adobe RGB color space even though you have your camera set to Adobe RGB. Hence for the highest image quality in a raw processor one would use the ProPhoto RGB color space. I highly recommend that folks use the ProPhoto RGB color space when exporting images out of Lightroom and as their archival color mode - then in Photoshop one can convert the color space to whatever is needed for output and have a lot more control.

Now, I understand that my methods may not be yours. Hopefully this isn't super confusing. If you are shooting for the web and don't want to have your shadows and highlights clipped then you have a few options - leave a little headroom on either side of your histogram and tweak the levels in Photoshop or use Adobe Camera Raw in Photoshop where you can set the color space and adjust the histogram while looking at the color space you intend to output the image into.

So, here I have just explained a little about how the color space set up in Lightroom affects the image when it is exported. While this topic might seem like a small detail it is very important to note when you are working up your images - because how you adjust the exposure, recovery, fill light and blacks sliders in Lightroom affects the histogram - and in turn affects the clipping that will occur when you export your images if you are not exporting into the ProPhoto RGB color space.

That's it for this session. See you next week....

Adios, Michael Clark

9 Comments

Dave
2007-08-13 04:21:23
Interesting article Michael.


Which color space do you recommend when exporting images for printing? I've read conflicting opinions between using AdobeRGB and sRGB. Some people say most external print shops are set up for sRGB others say AdobeRGB is better because of the larger color space. What do you think?

Daveed
2007-08-13 07:40:39
If I remember correctly, Lightroom actually uses a modified ProPhoto color map (I think it's referred to as Melissa RGB sometimes). Specifically, the map is linearized (gamma = 1.0). That would explain why the Lightroom histogram is stretched more than the the Photoshop histogram for the darker values.


Michael Clark
2007-08-13 08:15:45
Dave -


Printing in house on an ink jet printer, Adobe RGB or ProPhoto RGB is best. If you are sending out images to be printed then they are most likely printed in CMYK (if it is an offset press) and sRGB is better because it is a small color space and the color won't shift as much in the conversion.


The reality is there is no easy answer for this - talk with the printer. If they know their color management they have color profiles they can send you so you can convert your images to their cmyk profiles. But in the end it is always complicated. Color is one of the most complicated and difficult topics in photography period. Kodak and Fuji spent many millions on color issues back in the film days.

Dennis
2007-08-13 08:32:14
Interesting article. Just a few weeks ago I would like to export some photo's to have them printed by a photoservice. This photoservice provided me with their ICC profiles so I could export them with the correct color profile embedded.


I was unable to find a way to add these ICC profiles to Lightroom and I wonder if Lightroom even supports it.

Michael Clark
2007-08-13 08:37:39
Daveed -


Hello. You are correct. The Lightroom color space is internally called Melissa RGB (named after one of the QE managers at Adobe, Melissa Gaul). I've been told it has a gamma of 2.2 and that interpolations are linear. Don't quote me on that but either way it is a ProPhoto color space with an sRGB tone map and maybe some other stuff going on.

Michael Clark
2007-08-13 08:42:45
Dennis -


No, Lightroom doesn't allow users, at least as far as I know to add profiles to the export diualog. This is best done in Photoshop. On a Mac, you can add teh new profile to your Library > ColorSync > Profiles folder and then in Photoshop choose Edit > Convert to Profile and choose the printers profile. Now while that may sounds simple it is much more complex in reality, especially if your images are in ProPhoto RGB. Basiucally if you added saturation or even if you didn't trying to fit all of the ProPhoto RGB colors into a cmyk color space there are many colors that won't translate well so using the out of gammut dialog in photoshop becomes useful....it gets complcated fast so I won't go into it here. In the end Photoshop is where I would convert my color spaces....

Dennis
2007-08-13 13:30:29
Michael -


Thanks for your comment. Since Photoshop is a bit out of my league at the moment I stick with sending the files with the ProPhoto profile. The shop converts them and the prints look good. Thanks again for confirming my suspicions.

Daveed
2007-08-14 08:11:29
Here is a good overview of the Melissa RGB space:


http://www.ppmag.com/reviews/200701_rodneycm.pdf


My earlier memory was correct: The internal color space of Lightroom is a linear space (i.e., gamma = 1.0). That makes sense: It makes computations both simpler (i.e., efficient; multiplying with 1.0^n is a no-op) and accurate (avoidance of round-off noise accumulation). Here is a link to a more thorough discussion:


http://www.aim-dtp.net/aim/evaluation/gamma_error/index.htm


The "2.2 gamma" is part of a transformation done at the UI level: The histogram and the displayed RGB values are transformed from the linear space to something more in line with our perception. IOW, when Lightroom shows an RGB triplet of 25%, 75%, 50% (for example), that does not actually correspond to the internal data. Instead it's computed on-the-fly using sRGB-based curves, including a near-2.2-gamma curve (with apparently an exception for darker values).


My suggestion that the histogram was stretched because it reflected the linear quantization was therefore wrong (it would have been _much_ more stretched if that were the case).

Daveed
2007-08-14 08:15:12
Oh, and my impliciation that Melissa RGB is the name of the (linear) internal color space was mistaken. Melissa RGB is the space used for the UI (the one using ProPhoto base vectors combined with sRGB tone curves).