There's a lot more to getting and keeping accurate monitor color than meets the eye. New calibration technologies work with color management software to offer unparalleled color matching.
Bill Hilliard
Color monitors present the illusion that color output is possible. But there's a considerable difference between the color you see on-screen and what you get for output. Although some programs let you pick colors that match Pantone, Trumatch, or other color reference systems, the colors displayed on-screen never match the colors printed. With few exceptions, everyone gets surprised at print time.
For most users, the CRT monitor is the color reference that counts. These users aren't desktop publishing professionals. They don't have color scanners, and they don't try to match reference colors. They're just n
ormal users of office suites, presentations or charting programs, or other applications that display color. For them, the monitor is the device in the computing chain that defines their color palette. They want printed copies or transparencies to match the colors they see on-screen.
What We Want, or What's White?
Besides color the printer can match, users have other expectations. The monitor's overall color balance should be consistent from day to day and from unit to unit of a particular brand and model. This is especially important for work that other computer users will look at. Also, colors should be uniformly pure across the entire monitor, even in the corners.
The monitor should also provide a pleasing look on-screen. Research suggests that North Americans and Europeans prefer a warm "paper white" tone on-screen, while Asians, on average, prefer the cooler, bluish-white tone most monitors default to. Because most monitors are made in Asia, they come with a bluish cas
t.
What are cool white and warm white? A monitor's natural color balance is called its white-point color temperature, referring to the color displayed when all three phosphors--red, green, and blue--are fully illuminated. The temperature terminology is based on a visual equivalence between the monitor's white and the temperature of a so-called "black body" that emits light when heated, such as in an electric stove's heating element. At room temperature, the element appears black. As its temperature rises, the element appears first deep red (thus the term red hot) and then yellow. If you could increase the temperature of the element further, without it melting or burning up, it would look white (white hot) and then bluish white.
At every point, you can measure the temperature of the element in degrees Kelvin (absolute) and associate the color appearance of the black body with a certain white-point color temperature. For example, place a monitor (showing a white screen) next to a heating element.
As the heating element's temperature goes up, it will approach the color of the monitor's white screen. At the instant they match, the temperature of the heating element, in degrees Kelvin, is the white-point color temperature of the computer screen. The bluish white of most monitors is 9300 Kelvin. A 5000 white point (what some call paper white) is nearly the same as direct sunlight; this is the graphic arts industry's standard light for evaluating color.
In the Eye of the Beholder
Color is hard to quantify because of imprecise perception and subjective interpretation. If you say "candy-apple red" and "cherry red" to two people, each individual will imagine different colors, because their color sensitivity and past experiences will be different.
Differences in surrounding light affect color perception. An apple that looks so delicious outdoors under sunlight looks quite different under greenish fluorescent lights at home (see the figure
"How Red Is the App
le?"
). A picture of that same apple on your computer monitor will look different at a 5000 paper-white balance than at the more typical bluish white.
Also, the same apple displayed in front of a bright window will appear duller than when it's in front of a darker surround. This is known as contrast effect, and it's undesirable for accurately judging color. Reflections off the monitor face can skew on-screen colors; morning light can shift screen colors in a reddish direction, and mid-afternoon lighting will cause a yellow cast. Individual color sensitivity varies; even people with "normal" color vision may be biased toward red or blue.
Altered States
In 1991, NEC (Wood Dale, IL) introduced on its higher-end monitors the first widely available method (called AccuColor) for adjusting the CRT to get WYSIWYP (what you see is what you print) color. Because of NEC's influence on the market, most monitor makers offer similar controls on their 15-inch or larger monitors.
AccuColor and its competitors let you manually adjust the monitor's RGB gain controls. In essence, you can adjust the monitor's tint. If your image of an apple prints fine but looks pale on-screen, you might bump up the monitor's red gun relative to the others. But while this helps you match the apple's red on-screen, the overall white background of your windows or desktop also takes on a reddish tint. You've matched one color at the expense of 16 million others. This is the crudest adjustment for monitor color balance.
Calibrating the Monitor Display
CMSes (color management systems) are software applications designed to adjust all colors--on-screen or the printed output--to produce WYSIWYP color. (For a complete look at CMSes and how they work, see "Consistent Color".)
In addition to CMSes working directly with applications software, such as Adobe Photoshop or QuarkXPress, a limited degree of color management is being built into operating systems, notably Apple's Mac S
ystem 7.x (ColorSync), Microsoft's forthcoming Windows 95 (using Kodak's Precision Color Management System, or KPCMS), and future versions of SunSoft's Solaris.
Sonnetech produces KPCMS-based software for Windows 3.1, which is distributed with the newest monitors from Nanao, EIZO, Mitsubishi, Nokia, and Idek/Iiyama. NEC recently added Colorific as a no-charge, or bundled, option for every AccuColor-compatible monitor in its line.
A CMS is a filter or equalizer that converts a monitor's original color definitions into those the printer needs to produce matching output. How well this works depends on the accuracy of the conversion formulas and the device profiles used to define the monitor's and printer's individual color-rendering characteristics.
ColorSync and Windows 95 provide default, or generic, "monitor profile" data files to describe the colors on various monitors. They don't provide data on the individual values of the specific monitor you are using. Needless to say, WYSIWYG color
matching suffers.
Monitor makers get around this limitation by providing optional or bundled devices called display calibrators. Printer makers, too, are beginning to supply calibration routines in their drivers, enabling you to reset your printer so that you can work better with KPCMS, ColorSync, or other color management systems.
Display calibrator devices maintain colors by measuring--and in some cases, adjusting--your monitor's color characteristics. They also create a "profile" that accurately describes how your monitor shows colors.
Hardware-display calibrators work with the computer's video circuitry, using a suction-mounted photo sensor and a serial connection. They set up a feedback measurement loop between the video signals and the colors that the sensor can see. Hardware calibrators for the Macintosh are available from Radius (Santa Clara, CA), RasterOps (Santa Clara, CA), and Daystar (Flowery Branch, GA). Units for PCs running Windows are available from Radius and Nanao (Torra
nce, CA).
Other calibrators are software-based working at the operating environment level. Typically, you visually compare on-screen color with supplied color samples. Often these systems use optical illusions that help eliminate the need for special knowledge. With a mouse, you feed information back to the CMSto create an accurate color profile for that monitor in that location. Some systems are tied to specific hardware, such as Radius' ColorComposer, for use with Radius or SuperMac monitors and graphics boards.
In the future, if a monitor complies with the VESA's (Video Electronics Standards Association's) new DDC (Display Data Channel) standard, you can adjust its internal color balance and color response curve (gamma) without knowing anything about the video card, other than that it is DDC-compliant. DDC defines a two-way communications channel that lets the host PC adjust the monitor's color-rendering characteristics to a known state without sacrificing brightness, contrast, or color gamut
. It does this by changing electronic gain controls inside the monitor, which gives better dynamic range (i.e., greater on-screen color gamut) than would adjusting (thus limiting) digital values in the graphics board's DACs (D/A converters).
Windows 95's Plug and Play architecture will support DDC, and NEC, Nanao, and EIZO are already shipping DDC-compliant monitors.
Seeing Beyond the Surface
How well does a monitor suppress glare and ambient light? Reflections from the CRT's phosphors themselves need to be reduced, lest they reduce blackness and image quality. To cut or diffuse external reflections, makers treat the CRT's surface by chemical etching, mechanical etching, or applying silica. However, all these methods defocus the screen image a bit.
Another approach is to apply a thin-film AR (antireflective) coating directly on the CRT face (the best and most expensive method), bond AR panels to the CRT, or supply external slide-on panels. A few manufacturers, such
as Matsushita, combine thin-film AR coatings with fine-grain silica particles. This provides a more cost-effective method for reducing specular reflections and glossiness without the adverse yield and production issues associated with more costly AR coatings.
A Magnetic Personality
Because a monitor uses magnets to aim its electron beams, it is sensitive to any changes in either external or internal magnetic fields. The larger the monitor, the greater its susceptibility to environmental variances. Clustered around the neck of the CRT is critical support circuitry, particularly the yoke with its two precisely wound electromagnets. All monitor manufacturers fine-tune color purity and convergence--the beam's ability to strike the screen in the proper place--with magnets that are glued onto the tube at the factory. But during shipping, these magnets can fall off, or the yoke can slip. Thus, it's always best to purchase a monitor from a reputable dealer or maker who will offer servic
e access or no-questions-asked returns.
All monitors automatically degauss (i.e., clear themselves of any stray magnetic fields) when they're powered up. In use, however, the monitor develops a relationship with other strong magnetic fields in its environment, including the Earth's magnetic fields, which are probably the strongest forces. In fact, to achieve the best viewing, position your monitor with the flow of the Earth's magnetic field: face the monitor west in the northern hemisphere or east in the southern hemisphere. Makers such as Nanao and EIZO calibrate their high-quality monitors differently according to their intended destination.
Many monitors allow manual degaussing to remove any stray magnetic effects during operation. These could be caused by poorly shielded power supplies, other monitors nearby, and unshielded loudspeakers. If your monitor sits in one place and never moves, manual degaussing may never be needed. But move your monitor after it's been on awhile, even just swivel
it 90 degrees, it will be out of sync with its new magnetic environment. Corners may twist, and colors may slip or smear. It's a good idea to turn your monitor off, degauss it manually at least once a week.
Smaller Boxes, Better Color, Lower Cost
In the future, we can expect shallower monitors based on CRTs with 110-degree beam deflection; today's monitors all use 90-degree tubes.
Also, expect improved convergence, color purity, and brightness uniformity across the face of the monitor. Philips (Nashville, TN) has introduced its Brilliance 21A monitor using a digital technology called CyberScreen to control the beam within tight tolerances.
Typically, a monitor's color purity, white point, and brightness uniformity vary by as much as 25 percent between the center of the display and the corners. To compensate for this error, CyberScreen monitors treat the monitor face as if it were composed of many separate square areas, or tiles. These tubes are preprogrammed at the
factory with unique correction factors for each tile--perhaps more correction in the corners, less correction at the center of the screen. Further, the unit's circuits automatically monitor an internal sensor to detect changes in external magnetic fields and maintain picture quality.
The specifications for the CyberScreen monitors are excellent. Misconvergence may be as low as .14 mm versus as much as .4 mm in corners for competitive monitors. And variations in brightness are held to less than 10 percent. This greater uniformity yields improved color matching. You can expect similar technology from Philips' competitors over the next year.
Other new technology in the works will enable image and geometry corrections across the face of the monitor on a scan-line-by-scan-line basis. This will produce big improvements in color purity, brightness, and misconvergence, and other corrections are eliminated in the factory almost altogether--even on 110-degree tubes.
Finally, bigger monitors will co
ntinue to come down in price, as more people rely on Windows and other GUI-based operating systems and need displays with more available real estate. By 1996, according to an estimate by Dataquest, 17-inch monitors should be selling in equal numbers to 14- and 15-inch units.
In today's business, academic, and technical settings, color is an integral part of the computing environment; color is an available and increasingly important tool in communicating information and making sense out of data. Almost everyone uses a color monitor these days, something that was not true a few years ago. The net result of increased color requirements, technical improvements, compatible color management software, and market trends will be that tomorrow's crop of color monitors will be better and cheaper than ever. They will provide users with consistent and predictable color that can be tailored to their specific needs and other equipment. And they will help put an end to all those unpleasant surprises at output time.
illustration_link
Even though both halves of the apple are the same shade of red, differences in surrounding light affect your color perception.
Bill Hilliard is a founder of Sonnetech, Ltd., a San Francisco-based developer of software and hardware to improve color fidelity. You can reach him on the Internet at
sonnetech@aol.com
or on BIX c/o "editors.'
On-Screen Color
