- OVERVIEW OF COLOR MANAGEMENT -

"Color management" is a process where the color characteristics for every device in the imaging chain is known precisely and utilized to better predict and control color reproduction. For digital photography, this imaging chain usually starts with the camera and concludes with the final print, and may include a display device in between.

Swans Subject
Camera LCD Printer
Camera --> Display Device --> Printer

Many other imaging chains exist, but in general, any device which attempts to reproduce color from another device can benefit from color management. As a photographer, it is often critical that others see your work how it is intended to be seen. Color management cannot guarantee identical color reproduction, as this is rarely possible, but it can at least give you more control over any changes which may occur.

CONCEPT: THE NEED FOR REFERENCE COLORS

Color reproduction has a fundamental problem: different color numbers do not necessarily produce the same color in all devices. We use an example of spiciness to convey both why this creates a problem, and how it is overcome.

Let's say that you're at a restaurant with a friend and are about to order a spicy dish. Although you enjoy spiciness, your threshold for it is limited, and so you also wish to specify a pleasurable amount. The dilemma is this: a "mild" degree of spiciness may represent one level of spice in Thailand, and a completely different level in England. Restaurants could standardize this by establishing that one pepper equals "mild," two equals "medium," and so on, however this would not be universal. Spice varies not just with the number of peppers included in the dish, but also depends on how sensitive the taster is to each pepper. "Mild" would have a different meaning for you and your friend, in addition to meaning something different at other restaurants.

calibration table

To solve your spiciness dilemma, you could undergo a one-time taste test where you eat a series of dishes, with each containing slightly more peppers (shown above). You could then create a personalized table to carry with you at restaurants which specifies that 3 equals "mild," 5 equals "medium," and so on (assuming that all peppers are the same).

Computers color manage using a similar principle. Color management requires a personalized table, or "color profile," for every device which associates each number with a measured color. This way, when a computer tries to communicate colors with another device, it does not merely send numbers, but also specifies how those numbers are intended to appear. Color-managed software can then take this profile into account and adjust the numbers sent to the device accordingly. The table below is an example similar to the personalized spiciness table you and your friend created, which compares the input number with an output color.

Input Number (Green)   Output Color
Device 1 Device 2
200 —>
150 —>
100 —>
50 —>

Real-world color profiles include all three colors, more data, and are often more sophisticated than in the above table. In order for these profiles to be useful, they have to be presented in a standardized way which can be read by all programs.

COLOR MANAGEMENT OVERVIEW

The International Color Consortium (ICC) was established in 1993 to create an open, standardized color management system which is now used in most computers. This system involves three key concepts: color profiles, color spaces, and translation between color spaces. A color space relates numbers to actual colors and contains all realizable color combinations. When trying to reproduce color on another device, color spaces can show whether you will be able to retain shadow/highlight detail, color saturation, and by how much either will be compromised. The following diagram shows these concepts for conversion between two typical devices: a monitor and printer.

LCD   Printer
Input Device   Profile Connection Space   Output Device
Additive RGB Colors
RGB Profile
(RGB Space)
Subtractive CMYK Colors
CMYK Profile
(CMYK Space)

The color profile keeps track of what colors are produced for a particular device's RGB or CMYK numbers, and maps these colors as a subset of the "profile connection space" (PCS). The PCS is a color space which is independent of any device's particular color reproduction methods, and so it serves as a universal translator. The PCS is usually the set of all visible colors defined by the Commission International de l'éclairage (CIE) and used by the ICC. The thin trapezoidal region drawn within the PCS is what is called a "working space." The working space is used in image editing programs (such as Adobe Photoshop) and defines the set of colors available to work with when performing any image editing.

Each step in the above chain specifies the available colors, and thereby defines a color space. If one device has a larger gamut of colors than another device can produce, some of that device's colors will be outside the other's color space. These "out-of-gamut colors" occur with nearly every conversion and are called a "gamut mismatch." A color management module (CMM) performs all calculations needed to translate from one space into another, and is the workhorse of color management. A gamut mismatch requires the CMM to make key approximations that are specified by a "rendering intent." The rendering intent is often specified manually and includes several options for how to deal with out-of-gamut colors.

This all may seem a bit confusing at first, so for a more in-depth explanation of color spaces, profiles, and rendering intent, please visit:

Color Management, Part 2:Color Spaces
Part 3:Color Space Conversion

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