Wednesday, May 30, 2007
All About Image Sensors
At the heart of every digital camera is an electronic marvel
Most, if not all, photographic image sensors rely on microlenticular arrays, or microlenses, to improve their functionality and image quality. While photographic film is made up of millions of silver-halide crystals distributed evenly over two dimensions of a substrate, electronic image sensors aren't completely covered with photosites and their surface isn't perfectly flat.
Films react to light hitting from any angle, but because of the bucket-like shape of digital photosites, sensors require light to strike them with minimal deviation from perpendicular. In order to redirect light entering at a greater angle—say toward the edges of a full-frame sensor—sensor manufacturers employ tiny domed lenses over every pixel. Some sensors also employ microlens shifting, whereby the lenses near the edges of the sensor are moved inward slightly to better deal with the angle of incident light.
More than straightening out light at the edges, though, microlenses also help sensors fill in the gaps between photosites. At the microscopic level, a sensor would look like a checkerboard with space between each square. Microlenses placed over those gaps redirect otherwise wasted light into adjacent pixels, bringing the light-sensitive area, or fill factor, of a sensor from as little as 50 percent up to a sensitivity of 90 percent or more.
Foveon sensors are based on CMOS technology and behave in many ways more like film than traditional imaging sensors. The company's X3 chip utilizes a proprietary three-layer design to capture red, green and blue wavelengths at every pixel. Because silicon absorbs different wavelengths at different depths, Foveon embedded three light sensors stacked together in the top 1/10,000th of an inch of the chip. In this sense, the X3 sensor resembles Kodachrome's color-sensitive layers. And like Kodachrome did, Foveon sells the technology on finer detail and more accurate colors, as well as minimizing problems with moiré and artifacts. The color transformation matrix is aggressive, however, so it also amplifies noise. Price per chip is greater too, but the manufacturer considers it has three image pixels at every photosite, meaning not only more resolution, but also a lower price per pixel.
Fujifilm is also working to capture all colors of light at every pixel. Its organic CMOS sensor currently in development utilizes photoelectric conversion films that have the added benefit of providing a flatter surface, thereby eliminating the need for microlenses and ultimately leading to a sensor with potentially lower cost and more film-like performance. Whether Foveon's and Fujifilm's technologies will overcome the head start of CCD and CMOS is unclear, but each at least provides a glimpse of the possibilities that lay ahead for camera sensors.
Citing new technology from South Korean company Planet 82, Canon's Westfall says camera sensors could conceivably someday deliver image quality at ISO 6400 that approaches today's ISO 100. Foveon's Turner isn't convinced that ISOs will jump that dramatically, but he does expect incremental improvements. And like Westfall, he suggests keeping an eye on image-processing developments from DxO, DBlur Technology and OmniVision Technologies that are already allowing for modifications to focus and depth of field post-exposure.
For the foreseeable future, it appears that CCD and CMOS technology will coexist and that the rapid improvement of each will continue. As Olympus' Pelkowski says, “There is still going to be a tremendous amount of evolution in chip technology. It's very difficult to tell which way it's going to go. You never know. There could be new technologies that come out that are going to completely blow away what now exists.”
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