Sensor increases sensitivity and dynamic range

Says Abbas El Gamal, a professor in the department of electrical engineering at Stanford University (Stanford, CA), "The ultimate goal of digital camera development is to replace silver halide film as the main image-capture medium. This is a challenging goal because film has very high resolution and a dynamic range of more than 12 bits." Achieving such high resolution and dynamic range at low cost is extremely challenging for solid-state image sensors.

Whereas larger numbers of pixels have led to higher image resolution in CCDs, any further pixel increase is known to adversely affect their sensitivity, signal-to-noise ratio, and dynamic range, because the size of each pixel becomes increasingly smaller. To overcome these problems, Fuji Photo Film (Greenwood, SC) and FujiFilm Microdevices have jointly developed a CCD with an octagonal pixel shape and built-in signal-processing algorithms.

RIGHT. Unlike conventional CCD devices (left), the octagonal shape of Fuji's new Super CCD (right) offers CCDs higher sensitivity and signal-to-noise ratio and wider dynamic range.

Because of the octagonally shaped photodiode, the device is claimed to offer higher sensitivity and signal-to-noise ratio and wider dynamic range than conventional CCDs. Already, both companies have demonstrated a 2-Mpixel, 1/2-in. CCD with a photodiode area 1.6 times as large as the area offered by conventional CCDs with 2 million pixels. This larger area proportionately improves the sensitivity, signal-to-noise ratio, and dynamic range of the device.

"The honeycomb structure, borrowed from the anatomy of the human eye, achieves higher pixel density than the more traditional interline or progressive-scan square-grid structure at lower manufacturing cost," says El Gamal. And, the device's honeycomb pixel arrangement better suits the distribution of spatial frequencies of image data in nature and the idiosyncrasies inherent in human vision.

Compared with conventional CCDs, the honeycomb pixel arrangement, combined with oversampling, permits digital zooming with little image quality deterioration, according to Fuji. The honeycomb pixel arrangement also allows the skipped readout of image data without sacrificing image quality, therefore offering full-motion video output. Unlike a conventional CCD with progressive scanning capabilities that requires a complex structure, the structurally simpler Fuji device can be adapted to function as a digital camera equipped only with an electronic shutter.

The honeycomb structure is not, however, without its drawbacks. Says El Gamal, "The main argument against using this structure is that it requires more signal processing than the traditional square grid." However, with advances in the computational power and reductions in the cost of DSPs, the additional signal processing needed is fast becoming economical.