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Facts and Fiction

Choosing an imager means considering not only the chip, but also its manufacturer and how your application will evolve.

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uch has been made in the past five years of the potential for CMOS imagers and of the impending demise of the incumbent image-sensing technology, CCDs. Strong claims by the proponents of a resurgent CMOS technology have been countered by equally forceful claims by CCD defenders. In a pattern typical of battling technologies (both with significant merits but also lacking maturity in some regards), users have become leery of performance representations made by both camps. Overly aggressive promotion of both technologies has led to considerable fear, uncertainty and doubt.

Imager basics

For the foreseeable future, there will be a significant role for both types of sensor in imaging. The most successful users of advanced image capture technology will be those who consider not only the base technology, but also the sustainability, adaptability and support. They will perform the best long term in a dynamic technology environment that the battle between CCDs and CMOS promises to deliver. Both image sensors are pixelated metal oxide semiconductors. They accumulate signal charge in each pixel proportional to the local illumination intensity, serving a spatial

sampling function. When exposure is complete, a CCD (Figure 1) transfers each pixel’s charge packet sequentially to a common output structure, which converts the charge to a voltage, buffers it and sends it off-chip. In a CMOS imager (Figure 2), the charge-to-voltage conversion takes place in each pixel. This difference in readout techniques has significant implications for sensor architecture, capabilities and limitations. Eight attributes characterize imagesensor performance: • Responsivity, the amount of signal the sensor delivers per unit of input optical energy. CMOS imagers are marginally superior to CCDs, in general, because gain elements are easier to place...