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This brings us rather neatly to an explanation of the Foveon X3 sensor, which has some fundamental differences to conventional CCD or CMOS image sensors. Although it still relies on the photoelectric properties of semiconductors, in this case each photocell actually has three semiconductor receptors, buried at different and precisely calculated depths in the silicon wafer.

As it turns out, when light passes through silicon, different frequencies penetrate to different depths. By having thee separate sensors set at the right depths, each photocell site can detect red, blue and green light simultaneously, so there is no need for the Bayer filter and the demosaicing and interpolation required to produce a final image. In theory, the Foveon sensor can produce much sharper, more detailed pictures with more accurate colour reproduction than conventional Bayer-type sensors. The technology behind the Foveon sensor is still relatively new compared to CCD and CMOS Bayer sensors, but as the successful Sigma SD series of digital SLRs has demonstrated, it is already capable of producing very impressive results.

Microlenses

Unlike the light-sensitive material in photographic film, the actual semiconductor photocells are not located directly on the surface of the sensor chip. Instead, they are located in tiny pits in the surface. This helps to eliminate cross-talk and charge leakage between photocells, which improves image quality, but it does lead to a problem. Like sunlight shining down a well, only light that enters the pit from a relatively narrow angle will strike the sensor. As the angle of incidence decreases, the amount of light hitting the sensor decreases, which causes vignetting problems with some extremely wide-angle lenses, especially those designed to work with film cameras. The edges and corners of the image will be noticeably darker than the centre.

To counteract this, a minute lens is fitted over each photocell, directly under the Bayer filter, to direct light from lower angles into the pit and onto the sensor. These microlenses can clearly be seen in the microscope photograph on a previous page.

This isn’t a perfect solution, and vignetting with wide-angle lenses can still be a problem on some cameras, but in-camera processing or special computer software can be used to correct it.

Sony’s Super HAD CCD uses a larger and more complex microlens system to collect light more efficiently, which potentially gives them better light-gathering capabilities than conventional microlens CCDs, although the advantage is relatively small.

The World of Tomorrow

So what does the future hold? Well, development of the innovative Foveon sensor will continue no doubt, but the main camera and electronics manufacturers have a lot of money invested in CCD and CMOS technology and will certainly continue to develop it. The race for ever more megapixels shows little sign of slowing despite the diminishing benefit of such sensors in compact cameras. Sharp has recently announced a 12-megapixel CCD for compact cameras, which will probably start appearing in production models within the next few months.

Meanwhile improvements in microlens technology, new thinner LCD screens and paper-thin batteries could potentially lead to digital cameras that are literally the size of a credit card, with sensor, lens and monitor all mounted directly onto a single ultra-thin board.

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