gs on the screen. Scanners also use the RGB model by measuring the amount of reflected white light that the devices' RGB filters allow to pass through.
But the same RGB color signal on two different devices normally creates two different color impressions, because RGB colors vary according to the characteristics of the hardware. For example, open the same document on a desktop machine with a CRT monitor and on a notebook with an LCD screen, and you'll see an obvious difference. Another complication is that the RGB color model doesn't cover the visible color spectrum completely.
Color printing usually employs another color scheme, called CMYK (short for cyan, magenta, yellow, and black; the letter
K
is used to stand for black rather than the letter
B
so that it won't be confused with blue). A
s tiny dots interspersed on paper, these four inks combine to convey the desired color tone.
CMYK colors suffer the same drawbacks as RGB colors. They vary with printer, ink, and paper characteristics. In addition, the CMYK color space is smaller than the RGB spectrum, making it even more difficult to visualize on paper what you see on the screen -- let alone the richness of the color tones present in the real world.
Scanning, manipulating, and printing RGB images are all part of the routine work flow of every graphic designer working with DTP systems. Therefore, RGB values have to be matched to CMYK values so that predictable results can be obtained. Because the RGB-to-CYMK conversion results in four separate films, this process is called
separation
.
Since neither RGB nor CMYK color values are absolute but instead depend on the specific hardware used, it's impossible to define a color exactly. Identical images, processed identically but originating from two different scanners, can very l
ikely yield slightly different color tones in print. Therefore, unless designers can refer to a hardware-independent color space, any combination of scanner and printer needs its own transformation model (known as
calibration
) based on the device's characteristics (called
profiles
).
Traditionally, color management was all about calibrating the relevant input and output devices (e.g., monitors, scanners, proof printers, and printing presses) against each other or against a reference device and then converting colors according to this calibration. This is a time-consuming and costly process that often involves trial-and-error methods. It works reasonably well as long as the devices are used in the same configuration, but today's ubiquitous use of DTP requires a more flexible solution.
This is why the ICC, an industry consortium that includes such companies as Adobe, Agfa, Apple, Kodak, Microsoft, Silicon Graphics, and Sun Microsystems, standardized a cross-platform profile format for c
olor devices in 1993. One of the ICC's most important objectives was to incorporate color-space transformation as a standard interface in the OS -- the same way that, for example, standard graphics interfaces, such as Graphical Device Interface (GDI) and QuickDraw, work.
To achieve this goal, the ICC defined a Color Management Framework that standardizes the format of all types of input, output, and display-device profiles. It provides an API for different color management methods, such as Apple's ColorSync and Microsoft Windows' Image Color Matching (ICM) feature.
Device-Independent Color Spaces
In contrast to the traditional calibration method with respect to a reference device, the ICC approach is based on a device-independent color space, which is known as LAB colors. LAB colors were defined by the International Lighting Commission (CIE). This color description is very close to the color perception of the human eye. In the LAB model, an L-value stands for brightness (on a scal
e of 0 to 100), and the
A
and
B
coordinates define the distance of a color from a reference white point.
It's important to realize that the LAB color space is not only hardware independent but also includes all visible colors so that no colors are lost during conversion to the LAB model. Of course, some LAB colors cannot be printed due to limitations of the CMYK spectrum. Most color management solutions therefore use
gamut mapping
(see the sidebar "Color Talk") to ensure that any deviations between the original colors and the CMYK colors are as small as possible.
ICC profiles consist of a table of contents followed by tagged data. They can be embedded in TIFF, PICT, or Encapsulated PostScript (EPS) files. Profile types include input, display, and output devices, as well as profiles for color-space conversion.
Profile generation starts with colorimetric measurements of a set of colors generated from the display or printer. A scanner is usually gauged by scanning a target
image with an even distribution of device-independent colors and then comparing it to a reference file. For a printer, on the other hand, a typical test contains approximately 210 CMYK or CMY color patches; each is read with a colorimeter and, again, compared to a set of reference color values.
A color space is a system for specifying colors; the
color value
is the unique combination of components that define a particular color. For example, the RGB color space uses three components (red, green, and blue), while CMYK uses four. RGB forms colors by adding light sources, whereas CMYK subtracts light from an illuminating source.
A scanner can usually acquire -- and a monitor can usually display -- more colors than the printer can reproduce. As a result, color-space conversion often involves a decision on what output colors to use to give an approximation of the input colors.
These decisions are typically based on one of two approaches.
Appearance matching
considers the subjective im
pression of a human viewer, and the so-called memory colors (e.g., flesh, sky, and foliage) must look acceptable to the human eye.
Colorimetric matching
, on the other hand, aims at reproducing as many colors as exactly as possible. However, compromises are inevitable. The result often does not look right to the human eye because subtle relationships between colors in an image change.
More Flexibility
It's important to understand that this type of
color management
takes place for display or output only. The image from the scanner remains unchanged, but profile information is made available for the scanner. This procedure enables an ICC-compliant color management system to adjust the same image for different output media (e.g., video, film, and paper). It also allows images from several different input devices to be processed in the same document. Each image is color-adjusted separately. To achieve this flexibility, many systems embed as much as 400 KB of
profile information in each image.
A new product from German software developer Helios Software, ColorSync 2 XT, avoids such overhead by managing profile information centrally. This extension works within QuarkXPress or on an Open Prepress Interface (OPI) server. It transforms colors from RGB, CMYK, and other models into device-independent LAB values and vice versa.
The tool also allows users to define LAB colors in QuarkXPress and use them for graphical and text elements. To define colors, users measure LAB values from a sample and then enter them into QuarkXPress. If the
gamut
-- the range of colors that a device can produce -- of the selected printer doesn't cover a specific color, the system informs the user. ColorSync 2 XT supports several color formats, including CMYK, LAB, LAB(LH), RGB, XYZ, Ycbcr, and YCC, as well as several image-file formats, including DCS 1 and 2, EPSF, Photoshop, Scitex CT, and TIFF.
Helios Software cooperates with Linotype-Hell, a leading prepress and publish
ing-systems company, and it uses Linotype-Hell's profile-generation software suite, which consists of the ScanOpen, PrintOpen, and ViewOpen programs. An advantage of the Helios product is that it uses a color model with 32,000 reference points, whereas many others use only 512.
As a QuarkXPress plug-in, ColorSync 2 XT makes color management an integral part of a popular DTP application, allowing the matching of scanned pictures and complete documents for a specific printer or monitor. It can also simulate a printing press or even a particular kind of paper on a proof printer. The program does all the color separation, gamut mapping, and black generation. As Helios president Helmut Tschemernjak puts it, "With ColorSync 2 XT, it's no longer necessary to adjust the scanner to the printer."
ColorSync 2 XT uses the Apple ColorSync 2 engine, which is integrated into the Mac OS. With its EtherShare OPI 2.0 networking product, Helios offers the same engine on an OPI server, allowing consistent color managemen
t for the Macintosh, Windows, and Unix. According to Tschemernjak, "This is where color management really belongs."
Florian Suessl of CitySatz, a lithography service bureau in Berlin, has reviewed several color-management products and concludes that "ColorSync 2 XT is currently the most elegant approach to color management; the original images remain untouched."
But the quality of a color printout depends on how well the profiles map the devices' characteristics. And the continuous adjustment necessary when, for example, a new batch of paper is started can still be costly. This is why printing experts engage in heated discussions about just how accurate color matching needs to be.
How much color difference can be discerned by a casual viewer? Studies indicate that a typical reader glances at a page for less than 3 seconds before deciding whether to examine it more closely.
Apparently, color quality is not a major influence during such browsing, where the eye remains on a number of locations on
the page for only about 200 milliseconds at a time. But as soon as the reader takes a closer look, even tiny color-tone differences might decide whether he or she likes or dislikes the advertisement.
Where to Find
Helios Software GmbH
Hannover, Germany
Phone: +49 511 364820
Fax: +49 511 3648269
E-mail:
info@helios.de
Internet:
http://www.helios.de
Truthful Color
