(Issue: November 2007 - Author: Rich Adams)

In Part 1 of this two-part series (Digital Graphics, Oct. 2007) we looked at the basics of color density. That article showed how the quality and repeatability of digital prints can be expressed as a single number — and how that number represents density. Describing colors in terms of numbers is the science of colorimetry. The article also showed how color density data can be gathered using an instrument called a densitometer.

However, a more advanced instrument, the spectrophotometer, can collect even more useful color information. The information gathered with this tool correlates better with human vision. Once the exclusive domain of research labs, ink manufacturers and large printing plants, today’s spectrophotometers are readily available, technologically more advanced and less expensive than ever before.

In fact, if you own a color management system, you may already have one in the shop. Since spectrophotometry and colorimetry are now part of the mainstream graphic arts lexicon, it may be beneficial to learn more about it — before your customers find out!

WHAT IT MEASURES
As we learned in Part 1, densitometers measure color through three filters: red, green, and blue. These measurements are essentially three bands of light that cover the entire visible spectrum (see Figure 1, at left).

Color geeks know that the visible spectrum of light is actually a small band of electromagnetic frequencies (see Figure 2 above). Wavelengths of these frequencies can be measured in units called nanometers (nm), which are one billionth of a meter. A human hair is about 50,000 nm. The visible spectrum ranges from about 380 nm (violet) to 720 nm (red).

A spectrophotometer measures fine bands of light, usually every 10 nm. Thus, a spectrophotometer may collect 35 samples in a single reading. This would include the reflectance at 380, 390, 400... 720 nm. The result is a spectral reflectance curve (see Figure 3 at left ).

RELATING IT TO HUMAN VISION
The spectral reflectance curve represents the most information that can be collected about a color. The real question is, ‘of what practical use is it?’ What’s this information good for? It would be nice to have a way of relating it to human vision. This is done with colorimetry.

In 1932, the International Commission on Illumination (the French called it the Commission Internationale de l’Eclairage, or CIE) developed a mathematical model of human vision that became the basis of colorimetry. Developing this model was a little bit like today’s hot news topic of mapping the human genome — though not nearly as complex.

The CIE started by noting that to see something there must be an observer, an object, and a light source. To characterize the color of an object, the observer and the light source must first be standardized. The object should be the only variable.

First, the CIE characterized some different light sources, such as incandescent, fluorescent and daylight, then lettered them from A to D. Most users are probably familiar with standard viewing conditions for the graphic arts, which specifies lighting of D50, or of calibrating the monitor to D65. Some people believe that the “D” in these illuminants refers to “daylight,” but it actually refers to the fourth type of light characterized by the CIE (A, B, C, D).

Secondly, the CIE characterized human vision. They used a setup something like a video game. They sat a group of volunteers in front of a white screen. One half of the screen was projected with a spectral light. For example, a 550-nm light would be green. On the other half of the screen they projected a mixture of red, green, and blue lights. The goal was for the subject to try and match the green spectral light by adjusting dimmers on the red, green, and blue lights. After the subject finished adjusting the lights, the researchers noted the positions of the dimmer knobs and graphed them. This was a lot easier than mapping the human genome!

CIE CHROMATICITY DIAGRAM
After standardizing the light source and the observer, the CIE used these to develop some different measurements and ways of graphing them. The simplest measurement was to plot red and green on a graph. This is known as the CIE chromaticity diagram, or xyY plot. This graph is commonly used today to show the color gamut of ink on paper, or how intense of a color a given ink can print on a certain media. It was also used to characterize ink in the first versions of Photoshop.

The CIE chromaticity diagram provided a handy way of graphing a color measurement so that it related to human vision. However, one problem was that the colors were not uniform across the graph. (Note that green occupies more space than red.) This lack of perceptual uniformity made it difficult to compare two colors. For example, if a customer complained that the color of his print didn’t match the original, the color difference in the CIE xyY space could be measured. However, the distance between two greens would not look the same as the distance between two reds. Enter, the CIELAB color space . . . .

THE CIELAB COLOR SPACE
In an effort to get a more uniform color space, color geeks at the CIE did lots of number crunching to mathematically transform the horseshoe-shaped CIE chromaticity diagram into a spherical space known as CIELAB (see Figure 7 at right). “LAB” stands for L* (luminance), a* (red-green), and b* (blue-yellow). Although this might sound like a strange system, a color image can be converted in Photoshop to LAB color space, then viewed as separate grayscale, red-green, and blue-yellow channels.

CIELAB (also written CIE L*a*b*) is the basis of most of today’s color management systems. The LAB system is a model of human vision, the “greatest common denominator” of color. ICC color profiles relate the color of any device (e.g., digital camera, monitor, printer) to human vision. And human vision of color is where it’s at in the graphic arts world.

OK, WHAT DO I DO WITH IT?
Here are some of the ways to can use a spectrophotometer:

Spectral curves. A spectral reflectance curve is the “fingerprint” of a color, and can be used to characterize an ink or paper with great precision. For example, if two rolls of media have no label and it isn’t easy to tell whether they’re the same or not, use a spectrophotometer to read the spectral curve for each media. If the curves overlap precisely, the papers are the same.
The same can be done with inks. Assumimg the same situation with two inks, make a print of CMY with each ink, then use the spectrophotometer to read their spectral curves.

Optical brighteners. A second example is optical brighteners. These fluorescent dyes are added to media to make them “whiter than white” (see Figure 8 at left). The dye absorbs invisible UV radiation and re-emits it as blue light. This causes a hump in the spectral curve. UV brighteners can throw off ICC profiles, and can fade over time. Taking a spectral curve of the media measures the content of brighteners and whether they’re still effective or not.

Metamerism is a third example. Metamerism is an effect that makes two colors look the same under one lighting condition but not under another (see Figure 9 at right). When a customer complains, “Your print matches my corporate color in the shop, but at the tradeshow it doesn’t match.” Read a spectral curve of both colors. If the curves cross multiple times, it is metamerism! If there isn’t a mismatch, take a light reading at the tradeshow, then make an ICC profile customized to it.

CIE Chromaticity Diagram. This diagram has the advantage of presenting color data in visual form, but the color space is not uniform. The main advantage of using this space is to display color gamuts in an easy-to-read form (see Figure 10). The color gamut is the number of colors that an ink, media, and printer can reproduce. The bigger the gamut, the more the color will “jump off the page.”

CIELAB. Because of its perceptual uniformity, CIELAB is the preferred space for recording the color values of inks and papers. Large printers monitor incoming materials by reading the spectral curves and LAB values of inks to make sure one batch is the same as the next. The latest commercial printing specification, GRACoL 7, specifies the color values of inks in LAB.

A COOL TOOL
As the prices for spectrophotometers have come down, they have become more widely used for controlling print quality. Figure 11 illustrates how CIELAB measurements of two colors correlate more closely with human vision than do density measurements.

CIELAB is also useful for measuring the difference between two colors. The distance in CIELAB is known as delta-E (∆E). A ∆E of 0 means that two colors are the same. People can perceive a difference of 1–2 ∆E, while 6 or greater ∆E is considered noticeably different.

A practical use of ∆E is in establishing color tolerances. Example: a package printer prints 16-up medicine boxes on white boxboard. Color variation across the sheet makes some of the boxes lighter than others. When customers see this color variation on the shelf, they may assume some packages are older than others, or even that the concentration of medicine varies as much as the color of the packages! To establish a color tolerance, the printer asks the customer to sort packages into two piles: “acceptable” and “unacceptable” variation. After both piles are measured and graphed in CIELAB (see Figure 12), the “acceptable” pile falls within a circle of tolerance. Next time the customer visits, the printer will measure the prints beforehand, and only show the customer those that fall within the tolerance.

(Click here to read Part 1 of this article series.)