Color Workflow

Color workflow is a general term applied to the management of color information for imaging documents and devices. It involves such diverse areas as human color perception, the physical properties of light, the measurement of color (spectroscopy), and the capture, storage, and display of images.

Theory

The physical properties of light and the response of the human visual system are the key theoretical areas of color workflow. Engineering and management of digitizing devices, color displays, and color dyes and pigments for printing depend on the mathematical representations of color modeled by various “color spaces.”

A color space or color model provides a numerical representation of color values. The most well-known color space is probably the CIE 1931 XYZ Color Space, one of the first mathematical models of color. Numerical values for representing a color typically consist of tuplets of numbers (XYZ, RGB, CMYK, Lab, YUV) rather than frequency values of light. Frequency values are inadequate because very different spectra can produce exactly the same color sensation for a human observer. The color spaces used for color workflow all depend on the response of the human visual system.

The human visual system responds to three different gamuts of wavelengths, that correspond to three different types of cone cells in the human eye: short, long, and middle wavelengths with different response curves. Color spaces attempt to “normalize” the response curves in terms of standard sets of primary colors or other values. The XYZ values in the CIE models correspond roughly to red, green, and blue primary colors, which in turn correspond (very roughly) the three spectral gamuts to which the human eye is sensitive.

 

CIE Color Matchng Functions (graph)

The CIE standard observer color matching functions.
Source: Wikimedia Commons.
License: GNU Free Documentation License.
Reference: CIE 1931 Color Space (Wikipedia article)

Once we know the gamut of colors that different devices are capable of capturing or displaying, we can provide a consistent means of mapping color from one device to another in the color workflow chain from image capture, to display and editing, to printing and reproduction. Various models, called “rendering intents,” are used for mapping. Relative colorimetric and perceptual intents are the ones most commonly used in graphic arts.

Practice

Every device involved in the color workflow chain, from capture, to display to output, is capable of representing only a portion of the entire range of chromatic values represented by the CIE color space. In addition, devices may produce color through additive color, where primary colors sum to white, as in color monitors, or by subtractive color, where primary colors sum to black, as in printers.

 

Comparison of color spaces

Comparison of standard color spaces. ProPhoto, the widest, created for photography, was developed by Kodak. Adobe RGB was designed to capture most of the colors reproducible on CMYK printers. sRGB, created by HP and Microsoft, is intended for use on monitors, printers, and the internet.
Source: Wikimedia Commons.
License: Creative Commons Attribution.
Author: Cpesacreta. Reference: Color Space (Wikipedia article)

A display device such as a color monitor, which uses three primary colors for additive color, can effectively represent all those colors that fall within a triangle on the CIE color space, where the vertices of the triangle are the XYZ representation of the monitor’s primary colors. A printer with six different color pigments can typically represent all the colors within a hexagon on the CIE diagram. Every monitor and every printer has slightly different color response, though manufacturing standards promise a certain uniformity. The color response of a particular device is called its color profile.

 

Comparison of RGB and CMYK color gamuts

Comparison of RGB and CMYK color gamuts. The RGB color space typical of computer monitors can represent more colors than a four color printer with cyan, magenta, yellow and black inks.
Source: Wikimedia Commons.
License: Public domain.
Reference: Color Space (Wikipedia article)

As we move color information along the color workflow chain, we would like to have both a uniform representation of the information in the document, and some way of knowing how each device will change the visual presentation of the underlying data. This is where color profiles come into play. Color profiles describe the color response of a device–the set of colors which it is capable of representing. Documents are typically captured as RGB values: color profiles map RGB values onto the colors a device can actually display.

The capturing device has a particular set of colors it can actually capture, its color profile. Devices may have certain biases, which can be corrected, particularly if a standard color chart is include in the scan. A digital photograph, for example, is affected by the light that falls on it, by the lens used to capture it, and by in-camera processing during the capture. The RGB values in a scan are usually regarded as a starting point and manipulated as the operator sees fit, but a device profile can be used for an initial adjustment. By assigning the original file a wide color space such as Adobe RGB the gamut of color can be preserved as it moves along the color workflow chain. ProPhoto, a huge space used for photographic reproduction, is also used in printing, though less commonly. sRGB, the smallest commonly used standard color space, is generally best used for images on the internet, perhaps assigned at the end of the color workflow chain.

Once captured, a document is displayed on a color monitor. Every monitor has a set of colors it can display. A calibration device can determine the gamut of colors that the monitor is capable of displaying: its color profile. The color profile can be used to “tune” the monitor to a standard color display setting. The RGB values in the document are mapped to color values on the monitor. The color values are set by the video card as it maps RGB values to voltages applied to color phosphors on a CRT or to color filters on an LCD screen. Calibration can ensure that the displayed colors will be approximately identical on all monitors calibrated to the same standard settings.

The document on the monitor is produced with additive color and viewed through transmissive light. The document as printed is produced with additive color and viewed with reflective light. The differences in color representation are potentially huge. Color workflow for printing concentrates on three areas:

  • Providing a color profile for each inkset and paper used on the printer.
  • Using the printer’s color profiles to preview (“soft proof”) on a color-calibrated monitor how an image will appear when printed with a particular paper and inkset combination.
  • Providing standardized, uniform lighting in which to view the printed image and compare it to the original (if possible) and to the monitor image.
Equipment for Color Printing Workflow

A calibrated monitor is probably the most essential element in the printing color workflow. Color profiles for specific ink/paper/printer combinations can be obtained from manufacturers, usually for free, or from companies specialized in producing color profiles for printing. Standardized studio lighting and viewing booth can also help. All of these devices and technologies are designed to facilitate color workflow. They don’t make it a mathematically exact process. They enable the human operator to apply artisanal knowhow to manage and produce accurate color throughout the color workflow process, and most especially in printing accurate color reproductions.

Spectrometer: Used to calibrate the screen and measure the color profiles of inkset/paper combinations from the printer. Spectrometers can also measure spot color and ambient light, or the illumination provided by a color projector. These devices have become much less expensive than they used to be.

Viewing booth: A viewing device with standardized illumination and light temperature. Standardized illumination models define the range of light frequencies present in a light source. Light temperature is a global value for the overall color of the light, usually 5000K or 6500K. “5000K” and “6500K” refer to a model of color for “white” light used in photography and the lighting industry. These values run from cool at the low end (4000K) to warm at the high end (10000K). They should not be confused with the color temperature values used in physics and astronomy which run in exactly the opposite direction (bluer = hotter). 5000K is typically used in the printing industry. 6500K is the color of illumination of an LCD monitor. The lighting booth at Ignotus Editions uses the D50 standard illuminant and 5000K light.

Studio lighting: Studio space should be relatively dim when viewing images on the monitor, and otherwise uniformly illuminated by 5000K or 6500K lighting. The price for this sort of lighting varies, but there are relatively inexpensive ways of achieving the desirable illumination. Ignotus Editions currently uses 5000K fluorescent lamps to illuminate the viewing surface beside the printer. This is useful for reviewing large work.

Monitor: Aside from being calibrated, a uniform gray background is far preferable to a colorful background.

Etc: Color workflow seems to be a zone of at least as much obsession as, say, typeface design or the choice of Emacs or Vi. Some source advocate painting your studio a nice 18% gray, and wearing clothing to match. Bloodshot eyes from staring too long at a monitor while tweaking a stubborn color into exactly the right numeric value for output probably alter one’s perception, too.

Resources

CIE 1931 Color Space on Wikipedia. http://en.wikipedia.org/wiki/CIE_1931

Examples of various Color Spaces on Wikipedia. http://en.wikipedia.org/wiki/Color_space

Wikipedia article on Color Management. http://en.wikipedia.org/wiki/Color_management

Color Theory Applets at Brown University, an collection of interactive applets illustrating color theory (fun and useful). http://www.cs.brown.edu/exploratories/freeSoftware/catalogs/color_theory.html

Charles Poynton’s FAQs on color technology. A good source for some of the math behind the various color models, and discussion of how the models are used in color monitors and graphics cards. http://www.poynton.com/Poynton-color.html

The International Color Consortium, the folks who maintain the standard for color profiles. http://www.color.org/index.xalter