I see the term "Color Profile" a lot and I have this vague idea that it helps you adjust color values based on the unique differences of both cameras, displays and printers from some ideal device. Where can I find a good guide to what I need to know about color profiles and when and how to use them to get good results and fine grained control of my workflow and prints?
Answer
Color Management is a scientific process by which various devices used in an image processing workflow can be used despite differences in their supported color. Every device is only approximating some of the total range of colors humans can see, and this limited range is called its "color gamut". Each device has limitations, but those limitations differ from device to device. Different methods of rendering color also introduce additional differences, so the RGB "additive" model of computer screens and digital sensors is radically different, and perceived quite differently, than the CMYK (or variations thereof) "subtractive" model of print.
Color Spaces & Profiles
A Color Profile is a file that specifically defines the color parameters of a device, and when used in conjunction with systems and software that support color management, allows "safe" conversion of image data to and from various devices. This is important, as your camera, computer screen, and printer will all have different color gamuts, and proper conversion of image data between each is critical to ensuring accurate representation of the original scene you may have photographed.
A simple example would be taking a photograph of a vibrantly colored scene that contains bright reds, greens, violets, etc. By default, most cameras save images in JPEG using the sRGB color profile. A high-powered graphics workstation is likely to have a computer screen capable of rendering a greater range of colors, usually the Adobe RGB color space. While both the sRGB and Adobe RGB color spaces cover much of the same gamut, Adobe RGB supports more shades of green and orange, but covers a little less than some parts of sRGB in the vibrant reds and violets. Any colors captured by your camera in the areas that the Adobe RGB color space does not cover will need to be approximated. The same issue arises when you print your images, as printers generally support smaller gamuts than either a camera or a computer screen. Colors supported by Adobe RGB or sRGB will need to be mapped and approximated in the color space supported by a printer.
Color Management
The ICC Color Management system, supported by most modern operating systems like Windows and MacOS, as well as by professional image and photograph editing tools like Photoshop, Lightroom, and Aperture, take care of these conversions for you. You just need to make sure you choose the appropriate color profile for each of your devices. Generally speaking, most DSLR cameras support the standard sRGB and Adobe RGB profiles when saving to JPEG. When using RAW, the full color range supported by the camera sensor will be available, which in modern digital cameras can greatly exceed the sRGB or Adobe RGB gamut. Film can support a considerable gamut as well, and a high quality drum scan of a 4x5 transparency may also cover a wider gamut than either sRGB or Adobe RGB. Computer screens generally support sRGB, however professional graphics screens usually support Adobe RGB, while the upper echelon of screens may even support a broader gamut. Newer screens may support the very wide gamut ProPhoto RGB, which covers a much wider range of green, violet, and red/orange tones that the human eye can see (as well as some which the human eye can't see) which are generally excluded by most other gamuts. For a computer screen, it is important to calibrate the screen, and generate a custom profile that will make the best use of the gamut the screen supports. Regardless of what gamut a computer screen supports, it is best to avoid actually selecting sRGB, Adobe RGB, or any other common color profile for it in your ICC management settings. Always calibrate for accurate color representation and tonal range distribution.
While color management system is capable of handling conversions between color spaces for you, it is important to realize the "intent" behind a conversion. If you have an image in the Adobe RGB space that makes use of the extended range of colors, and need to convert that image to sRGB, you are converting into a smaller color space that is not capable of directly representing all of the original colors. Your intent may be to "clip" the unsupported colors, mapping them to the nearest supported color in the destination space. While this will accurately represent all directly supported colors in the destination space by choosing the closest match, it will not accurately represent any out-of-gamut colors. This may result in hard edges, banding, and other undesirable artifacts. This is called Relative Colorimetric intent. Alternatively, you may choose to shift all of the colors from the source space to the destination space. This has the effect of compressing, rather than clipping, the colors represented in your image. The result is a shift in possibly all of the colors of your image, not just the ones that are out of gamut. This shift is done in such a way that the resulting image is still perceived the same way, and although the colors are no longer all exactly the same, it still appears the same. This is called Perceptual intent. If your original image does not make use of any (or many) colors supported by a larger gamut, you may choose to convert with relative intent to preserve accuracy. However, if your original image does make use of colors supported by a larger gamut, a perceptual shift may be more appropriate, although possibly less accurate.
Print Profiles and Management
More important than the color spaces used by the camera or computer screen, however, are the color profiles used for print. The colors you perceive when viewing a print are highly dependent upon a few key factors: the inks used, the paper used, and the intended viewing lighting. Papers come in a very wide variety of tones, from ultra bright stark white to much duller, warmer, earthier tones. The gloss or matt finish of a paper also affects how colors are perceived. The tone of the paper affects how colors look when printed, and require different mixtures to accurately represent any particular color. The inks used also determine what colors can be represented on paper. Normally its just Cyan, Magenta, Yellow, and Black, however modern ink jet printers and professional/industrial printers may use many more colors. The raw color pigments or dyes used also affect the gamut of a print. In addition to the color of paper and mixture of inks, the light a print is viewed in can have an effect on your perception of color.
To accurately reproduce colors in print, a unique color profile that is specific to the inks being mixed and the paper being printed on is required. For maximum accuracy, a profile that includes the kind of light the print will be viewed in may also be used. If you do your own printing, and use a variety of papers, you could quite likely end up with dozens of different color profiles, one for each combination of paper type, printer/inks, and lighting conditions.
Print vs. Screen: Gamut's and Reality
Many professional ink jet printers and ink systems support a larger gamut than either sRGB or Adobe RGB. In the case of Canon ipF5100/6100 printers, which make use of a 12-color LUCIA pigment ink system, they can represent a considerably larger gamut than sRGB, and it even exceeds Adobe RGB in some areas. Epson's UltraChrome pigment ink system is similar, offering less green coverage but extended magenta, violet and possibly orange coverage. This fact is generally only applicable to the most modern professional ink jet systems, such as Canon PIXMA & ProGraph w/ LUCIA or Epson StylusPro w/ UltraChrome 3.
Professional ink jet printers are generally capable of a greater range of colors than your average commercial printing (offset printing or similar processes), by almost two-fold. Professional ink jet's have a gamut that covers about 800,000 colors, while offset printing covers about 400,000-480,000 (See "Wide Gamut approaching Full Gamut" for reference). In comparison, Adobe RGB covers about 1.3 million colors, while ProPhoto RGB covers about 2.9 million colors. There are different schools of thought on how many distinct colors may be perceived by the human eye, however based on scientific studies it ranges from 2-3 million up through ten million or so.
Given the differences between various methods of printing, it is important to use the correct color space that can preserve the colors that may be reproduced with your chosen printing method. In the case of professional ink jet printing, ProPhoto RGB (even though it can not be fully and accurately represented on screen yet) will generally support better results with ink jet reproduction. Commercial printing supports considerably fewer distinct colors than professional ink jets, however those colors still cover the area of a fairly wide gamut that nearly spans Adobe RGB. If you ship your work off to a lab, in general, they tend to require sRGB, so your likely stuck with it. I honestly can't say what the equipment in a photo lab is capable of, however you are likely losing some color range regardless.
A New Color Space
For all the popularity of sRGB and Adobe RGB, they are not really ideal color spaces for much photographic work. The ProPhoto RGB color space covers a much wider range of colors, however it can not be fully reproduced on either a computer screen or in print. It also supports colors that are outside of the range of human vision, which poses its own problems. A newer color space has been developed that is designed to better approximate the colors that we commonly work with, support the tools we commonly do that work in, and cover the range of colors commonly supported by the devices we use. This new color space, called Beta RGB, developed by Bruce Lindbloom, has some intriguing implications for both photographic workflow as well as printing. The Beta RGB color space has been meticulously designed to support the needs of photographers while attempting to minimize the problems with sRGB and Adobe RGB. While it is not possible to use the Beta RGB space when working with JPEG, it is possible to process a RAW file into TIFF or DNG and use the Beta RGB space.
Notes on # of Colors
A quick note on the numbers of colors quoted above. When discussing the colors supported by a gamut, or the human eye, the numbers generally refer to the total number of full intensity colors, from white to fully saturated distinct colors (chromaticity, or the measure of a color's purity irregardless of its intensity.) These numbers do not account for the entire range of color shades, or colors with differing degrees of brightness (intensity). The grand total range color in all shades visible to the human eye is tremendous...likely ranging in the trillions. The total colors and shades covered by any given color space is also higher than the baseline numbers quoted above. While Adobe RGB may cover 1.3 million distinct colors from white to saturation, Adobe RGB covers a greater range of shades of those colors.
It should also be noted that if you are working with 8-bit per channel (24bit) images, you always have 16.7 million "colors" (read that as distinct colors and their shades), regardless of the gamut. Those colors simply map to different distinct colors, and a wider gamut allows for a broader distribution of distinct colors and shades. If you are working with 16-bit per channel (48bit) images, you have some 270 billion "colors". This does not really translate to more distinct colors, however it does translate to a greater range of tones, or a more finely grained distinction in various levels of brightness. The same goes for 32-bit per channel HDR (96bit float) images. The total distinct colors of an HDR image would still be around 1-2 million...however you gain orders of magnitude more range on the brightness scale, and are able to represent a near-infinite range of brightness levels (or a near-infinite range of tones).
The measurement of "color" is a rather complex endeavor, and simply put, can not be effectively measured in only two dimensions. A full exploration of the total range of chromaticity (hue & colorfulness) and intensity, or hue, saturation, and brightness/lightness (or any other common model) requires an examination in three dimensions that takes the intensity of a color into account.
Reference
Here are some links to useful sites that cover Color Management:
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