Figure 1: Since on-screen display is the fastest and easiest way to view an otherwise invisible file, optimization, calibration, and profiling are very important for making the display an accurate and distortion-free “window” onto files. Students at Ryerson’s Graphic Communicatons Management program evaluate a printed proof in a standard viewing booth by comparing it to a soft proof on the Kodak Virtual Matchprint system.

Like party guests who don’t know each others’ names, plug-and-play digital devices do not “know” what color from different files ought to look like—unless standardized “nametags,” International Color Consortium (ICC) profiles—are applied.

The last column in this series (Tech Talk, Nov. 2009 S&DG, p. 78) described how standard working spaces are used to store digital color data, and how their ICC profiles can be embedded into the images. The next logical workflow step with a digital file is to display it on a color monitor.

A few years ago I was answering technical support questions for one of the world’s largest color instrument manufacturers, and I saw that many users were literally obsessed with accurate monitor calibration and profiling—a form of monitor madness, if you will. But their obsession was actually logical and justified because the monitor provides the fastest and most convenient “window” onto an unseen digital file of bits and bytes. Once monitor color is right, everything else can fall into place.

WHAT’S A MONITOR PROFILE?

An ICC monitor profile accurately characterizes the color of a monitor, enabling it to show the true color of a file (assuming the file has a known standard working space). When a characterization is performed, profiling software displays different colors on the monitor, which are measured with a color instrument, an emissive colorimeter or spectrophotometer. Emissive means that the instrument can read color radiated by the screen, as opposed to reflected color from a print. A colorimeter reads color through red, green, blue, and visual (clear) filters, while a spectrophotometer reads the entire visible spectrum. Both instruments convert the values into a mathematical model of human color vision known as CIELAB.

WHAT’S CIELAB?

CIELAB has nothing to do with laboratories, although the concept is rather technical. In 1931, the Vienna-based International Commission on Illumination (in French, Commission Internationale d’Eclairage, or CIE) developed a model of human vision, which was similar in concept to today’s investigations into mapping the human genome, or genetic structure. Color management assumes that printed and displayed colors are to be viewed by humans, who thus form the greatest common denominator of color.

Colors characterized in CIELAB are independent of any printing or display device, so are called device-independent color. CIELAB colors include L* (lightness), a* (red-green), and b* (blue-yellow). Popular image-editing programs like Photoshop can specify color in CIELAB. Although based on human vision, the color space is not very intuitive, so it is seldom used to specify color.

ICC MONITOR PROFILES

When a color instrument measures colors on a display (see Figure 1), it converts them from the display’s values, which depend on the device and are called device-dependent, to the device-independent CIELAB space. When working with a color-profiled monitor, your computer software converts files from device-dependent standard working spaces to device-independent CIELAB and then to the device-dependent monitor color.

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Figure 2: Color profiling software for monitors calibrates and profiles the display at the same time. Before calibrating, you select the contrast (gamma, usually 1.80 or 2.20) and color balance (color temperature, usually 5,000 or 6,500 K). If a monitor has hardware controls for brightness, contrast, and color temperature, then you would want to optimize these in advance for the most accurate and longest-lasting calibration.

After calibrating and profiling, X-Rite’s i1 Match shows a quality-control screen with target and actual values. The graph shows the linearization curves required to calibrate the monitor to the specified color temperature and contrast. The curves should look relatively smooth and even. Bumps in the curve could indicate a problem with monitor circuitry. After calibrating, i1 Match also shows a before-and-after screen where you can compare the effects of calibration on the screen.

LAST-MINUTE PREPPING

The ability to get a very accurate characterization of color led accuracy-obsessed users and system vendors to two important questions:

1.) How can I get the most accurate profile?

2.) How can I be sure the profile will be accurate over time?

Calibrating a monitor is a little like getting your driver’s license photo. Any motor vehicle office that has empathy for citizens will have a mirror where you can comb your hair before getting your picture taken. In the same vein, a color management system will have a calibration routine that “combs the hair” of the monitor. To a monitor, having no hair out-of-place means that the display becomes proportionally brighter with increasing color values. That is, there are no sudden jumps or dips as brightness increases. The display is said to be linear.

To linearize the display, the calibration routine shows the instrument increasing color values. This instrument measures these values, then creates a curve to ensure that the progression is linear. Like the prospective driver with freshly combed hair, the calibrated monitor makes a better profile.

Calibration specifications include the brightness curve, known as gamma, and the color balance, known as color temperature (see Figure 2 and Table 1).

ADVANCE PREP

How many people have gone for a driver’s license photo, only to realize they should have dressed up a little more, or not worn a bright white shirt that blanches out the photo? The same question could be asked about monitor profiling. If calibration is like combing your hair, then optimizing the monitor is like dressing for a photo you’ll be proud of.

Monitors often have hardware controls for brightness (sun icon), contrast (half-moon icon), and color temperature. Brightness actually sets the highlight, or lightest point, of the screen. Contrast sets the shadow, or darkest point. Having a properly optimized highlight and shadow gives you the widest range of color achievable.

Color temperature affects the balance of red, green, and blue to achieve neutral gray. Color operators want images that are free from color casts, in which

Luminance pop-up

the whole image shifts toward a particular color, like yellow. Good examples are film that has yellowed with age, or a yellowish digital photo that was taken indoors with the “Outdoor” setting. The easiest way to detect a color shift is by looking at neutral, or gray, colors.

Likewise, red, green, and blue monitor colors need to be balanced to produce neutral gray. Neutrality is expressed on a scale of color temperature, in Kelvins (K). Neutral gray is around 5,000–6,500 K. Lower values are more yellowish, and higher values are more bluish.

If a monitor has hardware controls for color temperature, you’ll get a more accurate and longer-lasting profile if you set those controls to same value you want to calibrate to in your profiling software.

WHAT’S NEXT?

Obviously, the great need for accurately viewing otherwise invisible color files has inspired considerable thought and analysis. Everyone wants to know how to obtain the best and longest-lasting monitor profile. The basic steps are summarized in Table 2.

SAVING YOUR PROFILE

Fortunately the same vendors who developed the logic for monitor calibration have also simplified it in easy-to-use, step-by-step interfaces. The most confusing part could be simply saving your ICC profile. Try to name the profile something that will help you identify it and remember how it was created. For example, a profile of an Apple Display, calibrated to a gamma of 1.80 and color temperature of 6,500 K on Jan.2, 2010 (you’re not going to calibrate on New Year’s day, are you?) could be called: “AppleDisplay_G180_D65_010210.icc.”

USING THE PROFILE

Ironically, in spite of all the thought involved in monitor profiling, using the profile is almost seamless. Most software automatically reads your system monitor profile. The video card converts color on the fly from standard working space to monitor color, providing the most accurate view of the file. However, an accurately profiled screen does not guarantee that any print will look like the screen. For print matching, you have to work back from the print to the monitor, as we’ll discuss in a future column.