Note: This is a simplified, easy to understand explanation of what color management on the computer is all about. We have tried to minimize the jargon and technical explanations as
much as possible. Highly technical writings on color management can be found on the Internet if one wishes to pursue this subject further. Our intent is to give an explanation that can be
understood by the average person. A basic understanding of color management will help you, either directly or indirectly, make better digital prints. We will periodically add to, edit,
and update this writing. We welcome your feedback (email@example.com). If you find some errors, areas that are confusing, or need further explanation, please let us know.
Why We Need Color Management . . .
When importing, creating, editing, and viewing a digital photo it is critical we have consistent color all the way from the source (scanned image, digital camera image, etc.) to the final
print. A color management system can reconcile the color differences among these devices (digital cameras, scanners, monitors, printers) so we can be more certain the final print is what we
Color Space . . .
Each device operates within a specific range of colors. This is the devices color space or color gamut. No device whether it be a digital camera, monitor, or printer is capable of
producing the full range of colors seen by the human eye.
Color spaces are defined within two different color modes: RGB (red, green, blue) and CMYK (cyan, magenta, yellow, black). Scanners, monitors, and most digital image files function within
the RGB mode. Printers can function in either RGB or CMYK mode. This is one of the most confusing aspects of color management and requires further explanation.
Most of the printers we use, including inkjet printers use CMYK inks (or some variation). When using printer profiles the printer's color space is defined in terms of RGB and Lab color
values. When the printer profile is created the Lab color values are measured from the CYMK ink patches made by the printer's test chart. These color values are not saved in CMYK mode,
but in Lab color values. This allows the color space to be translated from Lab to RGB or RGB to Lab. So, our printers are actually functioning as RGB devices. This will be discussed
further near the end of this writing. More background information is needed here now to make this area of color management more clear.
Most RGB devices are capable of producing a greater range of colors than CMYK devices (our RGB printers!). Computer monitors, for example, can always display more colors than your printer is
capable of printing. Because of these differences in color spaces, colors can shift when transferring an image from one device to another. The function of a color management system is to
control and adjust for these differences and shifts in colors. The goal is for the colors in your final print to look the same as the colors on your monitor, and for these colors to look the
same as the colors in your original photo. Of course computer imaging gives you the ability to improve upon a less than perfect original photo (i.e. retouching the photo) and maybe create a
near perfect final digital print.
A Little History . . .
Color management has been around for a long time. They were known as prepress systems used to separate colors which were then sent to a four-color press machine for printing. The CEPS
installations were very expensive and proprietary in nature. That is, you had to go to one physical location for the entire process. There was no cross-platform compatibility. There
were no common standards between different CEPS installations.
Eventually the International Color Consortium (ICC) was formed around 1993 with the goal of creating, promoting, and encouraging the standardization of an open, cross-platform color management
system. In 1998 Photoshop 5.0 was the first application to implement the ICC standards. This first incarnation didn't work very well for consumers, so Adobe came out with an improved
version in Photoshop 6.0.
ICC Color Management Lets You Do Three Things . . .
1. Convert color images from one color space to another (e.g. RGB to Lab).
2. See colors on a monitor as they'll appear in print.
3. Create proof images on printers.
That's it. It doesn't fix bad photos. An average photo cannot be turned into a great photo using this system. It can only help give you some consistency when going from scanner or
digital camera to a monitor then to a printer.
The two most common color management systems in use now are Windows ICM and Apple's Colorsync. These are complex "color engines" that process mathematical data from the various devices and
digital images we are using. The CMS will also function as a feedback system. For example, information used to create a mathematical definition for the color space of a printed
image can be fed back to the monitor to simulate what the printed image will look like on the monitor (called a soft proof). Photoshop is capable of displaying a "soft proof" to simulate what
the print will look like before it is printed. One must be using a calibrated monitor in order for this to work well. Photoshop can also display "out-of-gamut" colors, or colors the
monitor can show but colors the printer cannot print. These will be the areas of your photo to look at carefully because they will be the areas where a color shift will occur. The color
shift may not be noticeable or it could be significant, but it will occur. There is usually very little one can do to avoid the color shifts. We just live within the limitations of this
technology. The latest most advanced printers are capable of increasing the range of printed colors but still fall short of what a monitor can display.
Now We Get To The Confusing Stuff . . .
When you need to convert colors from one color space to another color space, a means of handling "out-of-gamut" colors has to occur. The Color Management Module (CMM) is the engine of a CMS
that drives color conversions. The CMM uses look up tables (LUT) to perform these conversions. The CMM does little more than cross-reference a set of numbers from one color space to
another color space.
Before giving an example of a conversion process we need to talk about device-dependent color spaces and device-independent color spaces. Both are used by the CMM. RGB and CMYK are
device-dependent color spaces because they are usually associated with a device (scanner, monitor, printer, etc.). Lab Color and XYZ Color are theoretical color spaces that represent the
visible color spectrum. Lab and XYZ are device-independent color spaces that can represent any color in the spectrum. We need to use Lab or XYZ color spaces within color management
because we do not want to impose any unnecessary limitations on the conversion process by using a color space (like RGB or CMYK) with a limited range of colors.
Our Example . . . .
We want to convert the RGB image we see on the monitor to CMYK for printing purposes. First, Photoshop sends the RGB values of a pixel in the image to the CMM. The CMM finds these values
in a LUT in the image's RGB source profile. The CMM then finds the same color values in Lab or XYZ. This is done for every pixel in our image. We now have an image defined in Lab or
XYZ color space. Next, CMM looks up the LUT in the printer profile for our CMYK print image. This is where our color shifts occur because some of the colors in our original RGB image, and
now Lab or XYZ color image, may not exist in our CMYK profile. So, CMM finds the next closest color values. The CMM sends these values back to Photoshop and we now have our image defined
within the constrains of CMYK. Remember our image is still residing in a Lab or XYZ color space. It has NOT been converted to CMYK color values. The printer profile we are using
contains CMYK color values (which define our range of printable colors), but these values reside within the printer profile in a Lab or XYZ color space. When our image is sent to print, the Lab
values convert to the RGB color space and finally our printer driver converts the RGB color values to CMYK color values for printing.
About Rendering Intent . . .
There are four different rendering intents available when converting your image from RGB to CMYK: Perceptual, relative colorimetric, absolute colorimetric, and saturation. Each
rendering intent tells the CMM how to convert colors for the desired effect. Most color-managed programs like Photoshop let you pick a rendering intent, but you can't in some programs, like
QuarkXPress 4.0. You'll need to look at your own application program to see if it offers the rendering intent options.
Perceptual Intent -- Perceptual intent compresses all colors in the image; overall saturation is decreased slightly, but the relationship between
all colors is preserved, meaning the image looks very similar to the original. The main negative of using this intent is that it compresses the color gamut, and it does this whether or not it
is needed for a given image.
Relative Colorimetric -- In relative colorimetric intent, out-of-gamut colors in the source image are converted to the closest available matches
in the destination image. This can cause some colors to be clipped. In-gamut colors are unchanged. The main negative here is that out-of-gamut colors will shift slightly, sometimes
causing a loss of detail or posterization in clipped areas. Images with a lot of detail in highlight areas may be affected by this intent.
Perceptual and relative colorimetric intent are the two most commonly used intents. Both remap the median white point of the original to that white point of the destination. There is no
hard and fast rule in choosing one over the other. If color quality is critical for you then its best to try both ways and see which you like the best.
Absolute Colorimetric -- Absolute colorimetric is the same as relative colorimetric except it does not remap the white point. This is
helpful when, for example, you want to ultimately print to newsprint but you are proofing on bright white paper. This intent will simulate on your bright white paper what your image will look
like on newsprint.
Saturation Intent -- Saturation intent will sacriface color accuracy for color saturation. You might use this intent for charts or graphs
or anywhere you need vivid colorful images. It does not work well for photographs.
What Are ICC Profiles . . .
ICC profiles are a set of numerical tables that define colors within a color space. ICC profiles can define color for a scanner, digital camera, monitor, printer, or a digital image. All
digital images are defined within a color space. The standard ICC profile for a RGB image will contain number tables for all of its RGB color values. The profile will also contain a set
of tables for Lab or XYZ color values. Using a LUT these color values can be translated back and forth, RGB to Lab, Lab to RGB.
Printer profiles are used by the CMM engine to produce accurate colors when printing and when soft proofing.
Custom Printer Profiles are individually made profiles that calibrate a specific printer to one type of paper using a specific inkset, and a specific set of printer driver settings. To
create the custom profile a set of color charts must first be printed on this printer with the appropriate printer driver settings. The color patches from these charts are then measured by a
colorimeter or spectrophotometer. Spectrophotometers are more advanced and accurate than colorimeters so are the preferred instrument for making quality profiles. The color values are
usually measured and recorded in Lab values, not RGB or even CMYK values. Despite the fact that most printers we use print in CMYK (or some variation), these printers function as RGB
printers. Printer profiles contain several sets of numbers defining RGB and Lab color values for all of the color patches that were measured by the spectrophotometer. They also contain
information about rendering intents. The printer profile can then translate color space back and forth. RGB to Lab and Lab to RGB using a specific rendering intent. Color values are
ONLY converted to CMYK by the printer driver when sending the image to the printer for printing. The conversion to CMYK values is basically a hidden process we have no control over.
How Does a Custom Printer Profile Help? Why Not Use Generic or Free Printer Profiles?
** A custom printer profile maintains the closest possible match between your original photo and the final print.
** Print output will be more predictable, reducing the need for more corrections to your photo.
** Greater predictability means fewer reprints, saving you money.
** Custom profiles may use less ink per print depending upon the printer used, saving ink costs.
** Custom printer profiles can usually print a wider range of colors than manufacturer's profiles.
** Grayscale values will be more accurate, making it possible to print quality B&W prints.
** You can create a custom printer profile for any type of paper so you are not tied to expensive manufacturer papers.
** Print onto a wide variety of third-party papers, canvas, and fabric media not normally profiled or available for your printer.
** A high quality custom printer profile often produces better shadow and highlight detail compared to generic profiles.