What They Are

A device color gamut, or simply gamut, is an indication of the range of colors which may be produced on an imaging system. The gamuts of different systems will be different. For example, most printing systems cannot produce colors as dark as those attainable on CRT displays, while most CRT displays cannot produce Cyans and Yellows as pure as those available with most CYMK inksets.

To further complicate matters, not all CMYK printing gamuts will be alike, nor will all CRT gamuts be alike. High-speed, high-volume magazine printing generally produces smaller gamuts than does commercial printing, so gamut depends upon market. Gamut also depends on other factors, such as colorant set, so using different inks in printing or different phosphors in monitors will influence the gamut. (In printing, the order in which the inks are printed will change the gamut, too!)

What They Are Not

Color, as we know, is perceived three-dimensionally. A single two-dimensional plot of a gamut, by itself, does not convey sufficient information about a gamut.

A rectangular projection of a gamut onto the (a* b*) plane does have its place as one of several tools for evaluating gamuts. Far less useful are "gamuts" plotted in the CIE Chromaticity Diagram, which is visually non-uniform, and grossly magnifies the importance of darker chromatic colors -- the darker such colors get, the more distorted they become. (This is because chromaticity diagrams are distance-distorting affine projections of color space.) Such diagrams are useful only for specialized applications, such as illumination and video.

What They Are Good For

Gamuts are useful for answering many questions, particularly:

Is a given color producible on a given imaging system?

If you have colors featured in your company logo ("trade-dress" colors), it is natural to want to know whether a certain imaging system, such as a color printer, can produce those colors. If it can, you have the option of making the colors on every page printed on that printer.

Does increasing the amount of colorant result in a larger gamut?

For reasons beyond the scope of this webpage, increasing the amount of colorant does not automatically result in an increase in gamut size. We have seen instances where a double-hit of ink actually made a gamut considerably smaller.

What halftoning will produce a larger gamut?

Changing conditions, such as halftone type (clustered, diffuse), screen ruling (frequency), or overlap (dot-on-dot, dot-by-dot, angled screening) will affect the gamut. Knowing which will produce a larger gamut can be useful in selecting a set of halftoning conditions.

Which paper will produce a larger gamut?

Even when using the same printer, inks, and settings, changing the paper or print media can change the size of the gamut. Sometimes certain important colors are attainable using one paper but not the other. Know which paper to use by evaluating the gamut produced on each.

Can the same sized gamut be obtained with one paper using less ink than on another?

This is just one of many questions we answered for a client, allowing them to show how their premium paper was actually cheaper to use than a lesser-quality product, even though it cost more.

What colors are common to the gamuts of two different imaging systems? Which are in only one of the two?

If an image which has been originally prepared for one imaging system, such as a CRT, is to be rendered on a second (such as a printer), knowing which colors will be outside the gamut of the second imaging system can avoid problems.

How much bigger will the gamut become if a fifth (sixth, seventh) ink is added?

Not only do additional inks have the potential to increase the overall size of a gamut, but they also frequently increase the number of spot colors one can simulate with process color printing. Knowing the specifics here can guide a printer in the decision to use custom-mixed spot colors or to simulate them with process color, potentially saving money.

How Gamut Information is Represented

Gamut information may be represented in a number of ways, including:

Numerical Gamut Representations

A collection of numbers, often hundreds or thousands, can represent gamut information at the lowest level. This type of gamut representation is difficult to understand, though it often serves as the basis for other, more easily worked with, representations.

Gamut Volume

This is a single-valued summary of a gamut, approximating how many visually distinct colors may be produced by an imaging system. Like all singled-valued summaries, in should be used in conjunction with other representations to provide a more complete picture. By itself, it is often useful in comparing gamuts from very similar systems, such as those produced by one piece of equipment using different papers, or different inks, or different settings.

Proportion of Selected Colors In-Gamut

Another way in which the size of a gamut may be quantified is the proportion of the colors in a palette, such as those in a popular spot-color specification system, are contained within the gamut. This is useful, because if a color is in-gamut, it usually means that it can be produced with the system to which the gamut pertains.

Two-Dimensional Projections

Projections of a gamut, for example onto the (a*, b*) plane in the CIELAB color space, tell only part of the story: they neglect the essential aspect of lightness, so they should be used in conjunction with other representations. CIE Chromaticity diagrams (with or without the horseshoe-shaped spectrum locus) are visually non-uniform, and seriously distort the importance of most colors.

Another thing to be wary of here are the number of samples from the gamut actually used to make the plot. All too frequently, such projection plots are made using only the colors Red, Green, Blue, Cyan, Magenta, and Yellow. Such hastily made plots, which are easily recognized because they look like irregular hexagons (line segments for sides), are nearly devoid of information. A well-made (a*, b*) projection plot will be produced using data from many gamut-limit colors, and will have curves between most, if not all, of the primaries.

Two-Dimensional Cross Sections

Gamuts may be "sliced" and the resulting cross-section examined. There are two types of cross sections customarily used:

• Vertical Slices, which show the gamut boundary for a particular hue (and its complement); and
• Horizontal Slices, which show the gamut boundary for a particular level of lightness (L*).

Naturally, several slices in each orientation are necessary to get a reasonably complete feel for a gamut. You may wish to have hue slices for every 15 or 20 degrees, and lightness slices for every 5 or 10 units of L*.

Armed with a collection of these slices for two sets of conditions (same printer and paper, with different ink sets, for example), it is useful to overlay one on top of the other for comparison purposes.

Three-Dimensional Gamut Solid

This is a valuable way to represent a gamut. It is a three-dimensional gamut representation, so all perceptual attributes are present. Although it is often impractical to produce an actual, physical three-dimensional gamut solid, one may be modeled and viewed on a computer. This is an especially useful representation for comparing two gamuts, such as from two imaging systems or from a single system using different colorants.

What We Do

We offer a standard service of gamut measurement, analysis, and evaluation. It is currently available for hard-copy devices, such as printers. As few as one or as many as a hundred (or more) combinations of devices, settings, inks, and papers may be measured, analyzed, and evaluated.

We provide you with a test target in digital form. The test target contains over 150 differently colored patches. You print the target under your choice of conditions, and send the printouts to us. We measure them and perform an analysis using our own state-of-the-art software. We return to you a report which specifies the volume of the gamut in CIELAB space, the proportion of colors in-gamut from a popular spot color formulation system, a 2-D projection of the gamut onto the (a*, b*) plane, a collection of hue and lightness slices, and a virtual gamut which you can view interactively.

Write us at gamuts@acolyte-color.com for pricing.

Make Your Own Demo Gamut

In order to get a feel for interactively exploring a 3-D virtual gamut solid, we have a free demo gamut page. Although the gamut solid it generates is nowhere near as detailed as from our pay service, it can help give you an idea of one of the more abstract elements of color gamut. You enter the CIELAB coordinates of the device White, Black, Red, Green, Blue, Cyan, Magenta, and Yellow, and a virtual gamut solid is returned to your browser.

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