Color Management Part 1

Using knowledge of how humans see color, along with data from instruments such as densitometers and spectrophotometers, color can be mapped just like DNA.

THE CONCEPT OF color management is fascinating, and more than a little controversial. To those of you that took exception to the suggestion in last month's story (Proofing By the Numbers, August 2003) that color management was easy enough for a college student, I humbly concede. There is a wider body of knowledge, and more tools available for controlling color than ever before. Even so, that does not make the theory of color any easier to understand. Color management can be very challenging.

It's also an opportunity to seriously raise the bar on the science of printing as well as a conceivable competitive edge for companies that do get their color under control. Who are those companies? According to a TWGA report released in May, 63 percent of graphic arts firms in their survey said they used color management technology of some kind. That shouldn't be surprising, considering the applicable scope of electronic color control.

In its biggest sense, a color-managed workflow is one in which every color input and output device employed in the print production chain has been a) calibrated to its optimal performance under a given set of repeatable conditions; and b) has been profiled, and is capable of employing industry-standard ICC color profiling data. That means we have a color map for each monitor—if it is used for any approvals or color decisions, scanners, proofers, and presses.

In a smaller sense, we're probably only controlling color between two points in the production chain—for example, between a digital halftone contract proof and a narrow-web flexo printing press. In any case, the science is fundamentally the same, and based principally on the properties of light. If we break it down to this point, it begins to get much clearer. So that's where we'll begin this two-part mini refresher course on color.

The fundamentals

Color management starts with the light source. Any color we can see is contained in the light by which we are viewing an object, i.e. the sun, a table lamp, etc. As a form of energy, this light travels through space in wavelengths. The wavelengths we're concerned about are in the visible spectrum—wavelengths that are between about 400 nanometers (blue) and 700 nanometers (red) long. Below 400 nm would be ultraviolet, and above 700 nm would be infrared.

The second component of color management comes from the characteristics of the object reflecting the light source. As our light hits a press sheet, the pigment on its surface is absorbing some of the light waves and reflecting others. By measuring and recording those reflectance values, we begin our mastery over color.

Each ink color (yellow, magenta, and cyan) absorbs, or subtracts about a third of the visible light spectrum. By printing various combinations of the three inks (and adding even more inks) in different screen percentages, we can reproduce more and more colors in the visible spectrum. The maximum number and range of colors we can reproduce, given a particular ink set and substrate, is considered the gamut, and we can represent it on a three dimensional model. But first, we have to measure and plot it.

Instrumentation: Densitometers and spectrophotometers

To do that, we use an instrument—which instrument depends on what light properties we're interested in. A densitometer measures the amount of light reflected from a surface. It expresses the values along a logarithmic scale where 0.00 means all the light was reflected (the surface was white); 1.0 means 1/10 of the light was reflected; 2.0 means 1/100 was reflected (and the surface would appear black), and so on.

The densitometer has three filters (RGB), each of which can isolate roughly a third of the visible spectrum. Measuring the reflectance values through each of the red, green, and blue filters tells us the ink film densities for the cyan, magenta, and yellow, respectively. It tells us the "amount" of ink (or pigment) we have on the press sheet—which is what the pressman can directly control at that point. But it doesn't specify what color the ink is. In fact, we could take readings from two dark reds that were clearly different colors, but they could conceivably have the same density reading. Invaluable as the densitometer is press-side, it doesn't tell us what we need to know to build that 3D map.

The instrument that will allow us to do that is the spectrophotometer. Unlike the densitometer, the spectrophotometer has many filters—the exact number depends on the manufacturer/model. All those filters—let's say ours has 24—are used to break down the visible spectrum into much smaller regions. We then take measurements from our press sheet of the reflectance values within those regions and plot them along the width of the visible spectrum (400 nm-700 nm) to produce what is called a spectral curve.

This spectral curve represents precisely how the object—in our case, the press sheet—is reflecting light. The raw data in the curve is not dependent on the light source, but we know the source will affect the perception of color. So, we multiply the spectral data by an illuminant curve that represents the energy output of our light source. We use one of several standard curves to approximate conditions such as daylight or indoor fluorescent lighting.

We then add one more piece of information to the color data we've collected: our knowledge about how humans see color—as opposed to how the instrument sees color. Some very smart people at the CIE conducted Standard Observer Studies that resulted in three more curves: red, green, and blue.

Now, if we multiply our spectral curve data by the illuminant curve and by each of the three Standard Observer curves, we have what is known as xyz data—the DNA of color. How we use that DNA is the subject for our next installment, which will run in the October issue of packagePRINTING.

Acknowledgements

The author would like the thank the following for their invaluable contribution to this article: Dan Gillespie, president, Color Geek; Ralph Besnoy, manager of advanced color technology, Pantone; Ken Niepokoy, VP manufacturing and color technology, Pantone; and Bob Binder, senior training specialist, X-Rite.

by: Terri McConnell