Manufacturing Color

For color printing, the ability to produce a large number of apparent colors is just as important as high resolution for producing realistic color images and smoothly shaded graphics. The difficulty in producing high-quality color with a laser printer is that the imaging process is basically binary. Where the laser strikes the organic photoconductor (OPC) drum or belt (or where it doesn't, depending on the design), toner is attracted to form the image.

Unmodified, this binary process forms the basis for bilevel color printing. The image is built from four applications of toner (i.e., cyan, magenta, yellow, and black), where each raster dot in the image has a spot of one or more of the toners or it doesn't. A given dot in the final image can then have one of 16 colors, depending on which combinations of color toner it contains. To get more than 16 colors, a printer must re sort to dithering, using clusters of differently colored raster dots to represent larger dots of more varied color. You gain a greater number of apparent colors in exchange for lower resolution and loss of detail.

A more sophisticated approach to getting more colors is to treat the laser-imaging process as partly analog. If you vary the laser pulse duration just right, you can get toner dots of different size and increase the number of colors severalfold. It's called multilevel or contone (not true continuous tone) color printing. As with dithering, you can increase color range by dropping resolution (this time as lines per inch) so that you can increase the range of dot sizes. Compared to dithering, you sacrifice less resolution to get a given number of colors.

Multilevel color printing is tricky, however. There is only a small window of pulse durations in which to adjust dot si ze, and small changes in pulse width bring big changes in dot size. Humidity, temperature, the age of consumable components, and the conductivity of the paper also affect the consistency of results. For these reasons, Tektronix chose bilevel dithering at a higher resolution (1200 dpi) for its Phaser 550. The company believes it gets color quality comparable to contone at 600 dpi, but with more consistency.

Printers using the Canon color-laser engine did better in our color tests than the Phaser 550 using contone printing at 600 dpi. To ensure consistency, the Canon engine calibrates itself every 100 pages or anytime you replace a toner cartridge. An optical sensor spot-checks toner density on the drum and adjusts laser pulse width accordingly. Consumables have a recommended life and yield, which also helps maintain color consistency.

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Different Approaches to Color

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