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CCDs: Performance That Can’t Be Beat

March 26, 2009
Charge-coupled device (CCD) imagers have lost a bit of ground to CMOS counterparts, but they still deliver much greater sensitivity than CMOS types.

When it comes to high-performance imaging in applications like professional photography, machine-vision inspection, high-definition TV, wireless security, scientific, and military/ aerospace applications, charge-coupled device (CCD) imagers are the only choice. They can deliver sensitivity levels far higher than those offered by CMOS imagers. But then again, comparing CCD with CMOS imagers is like comparing apples to oranges. It all depends on the application. Both have excellent characteristics for specific applications. Some companies like Dalsa make both types to satisfy different needs.

Although we’ve seen numerous CMOS-imager technology developments, CCD developments haven’t been idle. Quantum-efficiency (QE) levels of CCD imagers are steadily increasing. QE is a measure of how well a specific sensor responds to different wavelengths of light. The higher the QE, the more sensitive a CCD is at a particular wavelength. Also, CCD imager signal-handling capabilities continue to improve. In addition, there’s been a steady decrease in CCD dark-current levels, pixel sizes, and operating voltages (lower power dissipation).

CCDs feature much higher fill factors that make it possible to produce imaging chips with tens of millions of pixels. Witness the Eastman Kodak KAF-50100 sensor array for high-end digital single-lens reflex (SLR) cameras. The 50-Mpixel device has a resolution of 8176 by 6132 pixels, with pixel sizes of 6 µm. This device provides the highest resolution available in the popular 48- by 36-mm optical format used in professional photography. It was produced using the company’s Truesense full-frame CCD technology.

In high-quality photographic films, CCDs are proving to be very useful. For years, Fujifilm has developed its high-quality film technology using its Super CCD Sensor platform. The Super CCD platform combines the simultaneous benefits of high resolution, high sensitivity, and wide dynamic range from a single sensor.

Fujitsu’s latest development, the Super CCD EXR, is aimed at satisfying the large demand for high image quality in the digital camera market. This sensor provides a new arrangement of the mosaic color filter, uses a new pixel binning technique, and incorporates a completely revised electronic control circuit.

CCD imagers are proving to be potent devices for medical research. Researchers at the Institute for Biological and Medical Imaging in Munich, Germany, and the Technical University of Munich devised a new and powerful technique that allows concurrent image capture of application-defined spectral bands using standard CCD imagers. The method is aimed at serving the needs of medical diagnostic imaging. It’s initially focused on the measurement of fluorescent probes and tissue blood oxygenation.

There’s no question that CMOS image sensors have taken over some applications once served largely by CCD sensors. But CCD imagers will always find certain niche applications where only they can deliver the goods.

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