In the last several years, many CMOS APS photodiode image sensors have been developed for consumer and commercial use. These sensors typically rely on illumination levels on the order of 100 Lux (a well-lit room) for optimal operation at video frame rates. While these sensors produce high-quality images under these conditions, there is significant image degradation as the light level is reduced, due to a combination of small pixel size, small pixel conversion gain and high read noise (greater than 25 e–, resulting from the non CDS nature of photodiode operation).
There are, of course, several applications that require much higher sensitivity than the CMOS APS photodiode can typically offer. Automotive applications, security cameras, and camcorders, among others, would all benefit from APS sensors which could provide high quality images at video rates in night or darkened indoor lighting conditions. In addition, the necessary optics could be simplified by the ability to operate the sensor over a continuous range of lighting up to full sunlit conditions with a fixed iris setting. These goals, however, are opposed to each other necessitating a pixel with adjustable sensitivity. CCD sensors also have problems operating at both of these extremes, with the typically small pixel size limiting the low-light sensitivity and image smear interfering with bright light operation.