Color Management

The Denver Public Library has recently instituted a color management system (CMS). The increase in the digitizing of  "color" items and the resultant inconsistencies were unacceptable to the curator of photographs for an institution that has often set digital imaging standards.

The color management process at a universal level is very new and is still emerging. Through research and testing the Denver Public Library has decided on hardware, software and a format for handling the unique color artifacts in it's collection.

Color Management

The challenge of digitizing items in color is to capture an original artifact (photographs, artwork, maps) in such a way as to closely resemble the color values of the original and to keep those values as consistent as possible between various hardware devices. The difficulty is that once an item is digitally captured, the devices that represent this artifact have progressively diminishing  color capabilities. The most costly and finely engineered scanners and digital cameras can only see a fraction of the colors discernible to the human eye. The finest high resolution monitors can see only a fraction of the colors those capture devices can see. And printers can reproduce even fewer of the colors a monitor can display.

Color management systems in general create a color environment that all of these devices can operate in. Specifically, the CMS that the Denver Public Library has adopted is a set of hardware and software tools designed to reconcile the different color representation capabilities of our scanners, monitors, and our digital copy camera.

The Need for Color Management

Devices such as monitors, scanners and printers see color differently. To further exacerbate the problem, color is not even consistent between the same devices! A Sony monitor displays color differently than an NEC monitor, Agfa scanners capture color differently than CreoScitex scanners, and Canon printers yield color differently than Epson printers. A classic example of the breakdown of color consistency is that an Agfa scanner captures an item in color and displays it on a Sony monitor. After color correction the file is saved as a satisfactory representation of the original. That file is then outsourced to a digital lab for printing, where the file is opened on an NEC monitor and printed on an Epson printer. When the print is returned to the customer it looks nothing like the original digital capture. Anyone who has done digital imaging has run into this problem and until only a few years ago this was regarded as an acceptable flaw. As digital imaging has become more sophisticated and standards have become more rigid this is no longer the case. Current technology won't yield an exact replica of an original artifact, but color management brings us closer and as it becomes more sophisticated it will bring us closer still.

Color management is necessary for another reason. Humans see color relatively similarly, but the hues and intensities of colors perceived by humans varies. The difference between a color's actual hue and intensity and the way it is perceived by a person is measured  using a scale called Delta E. This same scale is used to calculate differences in color perception by digital imaging  hardware. For example, a print of a standard set of colors may be measured against the known values of those colors. The difference is the Delta E rating. A Delta E rating of one is a barely perceptible visual difference. The average viewer would have trouble distinguishing colors with a Delta E rating of three or four. The test labs of the publication "Popular Photography" rate a printer or a scanner that averages a Delta E rating of eight or less as "excellent". Since everyone is different, everyone has a different sensitivity to color--a different Delta E.

Men are also prone to seeing color "incorrectly". Approximately one in eight men (about 12 percent) have a significant color deficiency. On the other hand only about one in 250 women have a significant color deficiency. Adding the inconsistency that humans introduce to the perception of color is fodder for the argument that a digital imaging project that captures artifacts in color needs a CMS.

The ICC Model

In 1993, in response to the booming proliferation of desktop publishing systems, the International Color Consortium  (ICC) was created. This consortium created standards to help keep color reliable in an image file consistent throughout the reproduction process. In this case reproduction can be as simple as opening and viewing an image file on a monitor.

There are three major components to an ICC color management system:

1. A color management module that interprets the unique color characteristics of the various hardware devices and gives instructions to the individual devices to make them see color similarly.

2. Profiles that define color characteristics of hardware involved in the digital imaging process--scanners, digital camera, monitors and printers. 

3. A device independent color space.

Colorsync is the color management module used by DPL to interpret these ICC profiles and give instructions to coordinate color consistency between imaging devices.

DPL researched various hardware and software packages that would define color characteristics of it's digital imaging hardware and determined that the hardware/software package best suited to it's needs was Profilemaker Pro 4.x software and the EyeOne spectrophotometer. Both of these are Gretag Macbeth products. The software creates ICC profiles of the  hardware in DPL's digital imaging lab.

The ICC chose the Commision Internationale de l'Eclairage (CIE) color space as the device independent color space. This color space represents all of the colors a scanner or camera can capture, a monitor can display or a printer can print. 

This graph depicts the CIE color space. The triangles represent the colors in that color space that the four DPL lab monitors are capable of displaying. This graph shows how profiling the monitors has brought them into nearly perfect alignment after color calibration and profiling. With calibration and profiling, the same file opened on all four monitors will display nearly identically. This is extremely useful in printing of digital files. An original item captured on one workstation can be opened on the separate print workstation and the image would display as it was originally captured. Profiling monitors is the first step in the calibration and profiling process. A major component of profiling imaging hardware is to align the scanner, digital camera and a digital print to the monitor. The acronym WYSIWYG is applicable here--What You See (on the monitor) Is What You Get (from the scanner, digital camera or printer). Monitors, then, must be calibrated and profiled before scanners, the digital camera or the printers in the DPL lab are calibrated and profiled.

This graph depicts the color characteristics of the lab scanners and the digital camera. Note that the devices most in alignment are the scanners, which are identical Agfa Horizon Ultra flatbed scanners. The digital camera, by a completely different manufacturer (Betterlight), is in proximity to the scanners but will capture color in subtly different ways. The digital camera sees more shades of green and blue than the scanners, but slightly less shades of red. Color management isn't perfect, but also note the intersection of the vast majority of colors common to all three devices. Without color management, the scanners and the digital camera would see the same number of colors, but the colors they would see would not be in alignment with the other devices. This alignment of the devices provides more consistent scanning--the same color original would be captured very similarly by all three devices.

The last graph depicts the color rendering capabilities of our two lab printers. The first is an Epson 9500 inkjet printer. It uses pigment based ink that is applied directly to various types of  papers. The second printer is a Fuji Pictrography 4000 printer. This printer uses a polaroid process, exposing light sensitive donor negative paper and then uses distilled water to apply the image to a positive receiver paper. While both print processes are very different, note that with profiling and calibration we have been able to make both devices print similarly.