Photodiode-based system controls color

Think of a certain brand of camera film, cola, or even lawn tractor, and as likely as not a particular color will come to mind.

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Think of a certain brand of camera film, cola, or even lawn tractor, and as likely as not a particular color will come to mind. But manufacturers can calibrate color until they turn (an uncalibrated) blue in the face and still not achieve consistency if the printer or painting equipment producing the colored surface is not itself consistent.

The best route to stability in a manufacturing process is often the implementation of closed-loop control. Taking this approach, and combining it with the development of rugged and miniaturized equipment, a team of engineers at Xerox (Webster, NY, USA; www.xerox.com) has created a spectrophotometer-based color-control system that can fit inside a printer and provide color information in microseconds, allowing page-by-page control of color.

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Color-sensing system contains eight LEDs and six photosensors in a metal cap mounted to a circuit board (inset). Light from the LEDs exits the central lens. Lalit Mestha of Xerox holds the device with all its LEDs on (top). LEDs spectra cover the entire visible spectrum, a requirement for accurate determination of color (bottom).
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A 78-mm-diameter hockey-puck-shaped assembly contains the color-capturing optoelectronics, which include eight light-emitting diodes (LEDs) that cover the visible spectrum and six photodetectors that measure different portions of the visible spectrum (see figure). The eight LED dies are assembled into an emitter a few millimeters in size. The LED light strikes the printing surface at normal incidence; the detectors are mounted in a ring around that point and oriented at 45° to eliminate effects from specular reflection. The assembly is fixed to a microprocessor-filled circuit board; the system also includes software for control, timing, and color calculation.

In use, the LEDs are turned on sequentially with all six photodiodes taking readings, resulting in a matrix of values, explains Lalit Mestha, the team's lead scientist. The signal is captured within 15 µs. The software converts this matrix into a full-spectrum-equivalent color reading. This set of color values is compared to a predetermined ideal, resulting in an error value; the printer's color parameters are adjusted to bring the error to zero. Sometimes, if the color is far off, more than one iteration is required.

This fully automated procedure is in contrast to the conventional approach, in which large and expensive spectrophotometers using tungsten-halogen light sources produce measurements "long after the fact" in a semi-automated manner. Further, some customers don't have access to such costly instruments, so they take the old-time route—eyeing the printer's output and then manually tweaking its color adjustments. "It is very hard to get precise adjustments that way," says Mestha.

Currently, the Xerox device looks at printed color patches bearing the color of interest, but in development are system configurations that look at points within images. Because the customer images will often vary in color across the area, the most uniform relevant spot of color on the image will be examined. The company is now working with potential licensees. Mestha notes that other uses for the color-control sensor include examination of textiles, paint, plastics, cardboard, and wallpaper.

John Wallace
Senior Editor
Laser Focus World

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