CCD (charge coupled device) and CMOS (complementary metal oxide semiconductor) image sensors are two different technologies for capturing images digitally. While they are often seen as rivals, CCDs and CMOS imagers have unique strengths and weaknesses that make them appropriate to different applications. Neither is categorically superior to the other, although vendors selling only one technology often claim otherwise. The choice depends far more on the application...and the vendor.

Both types of imagers convert light into electric charge and process it into electronic signals. In a CCD sensor, every pixel's charge is transferred through a very limited number (often one) of output nodes to be converted to voltage, buffered, and sent off-chip as an analog signal. All of the pixel can be devoted to light capture, and the output's uniformity (a key factor in image quality) is high. In a CMOS sensor, each pixel has its own charge-to-voltage conversion, and the sensor often also includes digitization circuits, so that the chip outputs digital bits. These other functions reduce the area available for light capture, and with each pixel doing its own conversion, uniformity is lower. But the chip requires less off-chip circuitry for basic operation.

CCDs have been the dominant solid-state imagers since the 1970s, primaridly because CCDs gave far superior images with the fabrication technology available. CMOS image sensors required more uniformity and smaller features than silicon wafer foundries could deliver at the time. Only recently has semiconductor fabrication advanced to the point that CMOS image sensors can be useful and cost-effective in some mid-performance imaging applications.

CCDs offer superior image performance (as measured in quantum efficiency and noise), and flexibility at the expense of system size. They continue to rule in the applications that demand the highest image quality, such as most industrial, scientific, and medical applications.

CMOS imagers offer more integration (more functions on the chip), lower power dissipation (at the chip level), and smaller system size. They are well-suited to high-volume, space-constrained applications, such as security cameras, PC peripherals, fax machines, and some automotive applications.

Costs are similar at the chip level. Early CMOS proponents claimed CMOS imagers would be much cheaper because they could be produced on the same high-volume wafer processing lines as mainstream logic or memory chips. This has not been the case. The accommodations required for good imaging perfomance have limited CMOS imagers to specialized, lower-volume mixed-signal fabrication processes. CMOS imagers also require more silicon per pixel. CMOS cameras may require fewer components and less power, but they may also require post-processing circuits to create higher image quality.

The money and attention concentrated on CMOS imagers means that their performance will continue to improve, eventually blurring the line between CCD and CMOS image quality. But for the forseeable future, CCDs and CMOS will remain complementary. Each can provide benefits that the other cannot. AmexSecurity's approach is "technology-neutral": we are one of the few vendors able to offer real solutions with both CCDs and CMOS.