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CCD Camera Helps Analyze Print Head Problems

Photonics Spectra
Aug 1997
Ruth A. Mendonsa

Charge-coupled-device (CCD) technology plays a leading role in allowing visualization of ultrafast-motion events. A high-resolution, high-speed CCD camera from The Cooke Corp. in Tonawanda, N.Y., is making life a lot easier for one manufacturer of inkjet printers.

A fiber optic bundle with a collimating lens backlights the droplet stream. The FlashCam is at left.

The print heads on these devices eject tiny cometlike drops of ink that can break up on their way to the paper, spinning off tails of trailing bodies called satellite drops. Microseconds behind the main drops, satellite drops occasionally land outside the narrow target areas, resulting in problems with sharpness of printed characters and graphic elements. The trajectory of the main drops also can affect the print quality of inkjet printers, so print heads must eject drops that come out straight and on target from the firing chamber.

Catching it in the act

Inkjet print heads that emit well-formed drops free of satellites and trajectory problems require arduous, sophisticated testing and analysis techniques. One producer of inkjet printers, Lexington, Ky.-based Lexmark International, wanted to upgrade its evaluation techniques. Its stroboscopic testing system, which comprises a microscopic lens focused on the print head, a strobe light and a video camera, was effective enough for the repetitive part of the testing, but not for nonrepetitive events.

According to Jack Morris, a development engineer at Lexmark, the strobe works as long as the drops, firing out at 5000 Hz or greater, form the same way each time. But if there is a nonrepetitive action, where something happens irregularly or periodically, it is not effective enough. The exposure time of the video camera with a strobe is insufficient to capture single events such as when the jet misdirects or the satellites are not forming uniformly.

Meeting the challenge

The two challenges to visualizing and analyzing print head problems are size and speed. "Small 50-µm drops travel through the area of interest in about 100 µs and then they're gone," said Morris. The FlashCam from Cooke Corp. provided a closer look at the inkjet process than had ever been possible.

Lexmark chose the FlashCam because it was the least expensive CCD camera with a 1-µs shutter speed, and because it could capture multiple images in a single frame. Its 1-µs shutter speed allows Lexmark to capture detailed images of the ultrafast-moving ink drops and their submicron satellite droplets. By visualizing these events, Lexmark engineers can learn how to put the highest-quality dots on the page, improving print quality of the 300- and 600-dots-per-inch inkjet printers and speeding up the printing process as well.

"CCD technology helps us produce a variety of high-quality, versatile inkjet printers, many of which can be purchased for $300 or less," said Morris. "This combination of inkjet affordability and laser-quality performance enables just about anyone to produce anything from high-impact color presentation materials to sparkling birthday cards."

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