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1.5 AREA ARRAY AND LINEAR IMAGE SENSORS | ||
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Hand a group of camera designers a theory and a box of components and you'll see fireworks. They will explore every possible combination to see which works best. Since there is usually no perfect solution to a given problem, a number of alternatives arise; each with its own unique pros and cons. At the moment, designers have two types of components to play with: area array and linear sensors.
Most cameras use area-array sensors with photosites arranged in a grid. This allows the sensor to cover the entire image area and capture an entire image all at once, just like a film camera.
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Area array image sensors have their photosites (pixels) arranged in a grid so they can instantly capture a full image. Courtesy of VISION. |
Area array sensors are incorporated into cameras in a variety of ways.
- One-chip, one-shot cameras use different color filters over each photosite to capture all three colors with a single exposure. This is the most common form of image sensor used in consumer-level digital cameras.
- One chip, three shot cameras take three separate exposures: one each for red, green, and blue. A different colored filter is placed in front of the image sensor for each of the colors. These cameras cannot photograph moving objects in color (although they can in black & white) and are usually used for studio photography.
- Two-chip cameras capture chromonance using one sensor (usually equipped with filters for red light and blue light) and luminance with a second sensor (usually the one capturing green light). Two-chip cameras require less interpolation to render true colors.
- Three-chip cameras use three full frame image sensors; each coated with a filter to make it red-, green- or blue-sensitive. A beam splitter inside the camera divides incoming images into three copies; one aimed at each of the sensors. This design delivers high-resolution images with excellent color rendering. However, three-chip cameras tend to be both costly and bulky.
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Bob Caspe at Sound Vision has a great paper on the way image sensors are used in cameras. For more on this topic, click the More Info button. |
Almost all scanners and a few high-end professional cameras, use image sensors with photosites arranged in one or more rows. To capture a picture, these linear sensors scan the image focused by the lens and build the image up a row at a time. The number of pixels captured in one direction is determined by the physical number of photosites on a row of the sensor. The number in the other direction is determined by how far the sensor is moved or "stepped" between rows. Cameras with these sensors are useful only for motionless subjects and studio photography. They are widely used for catalog photography and product shots..
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As a linear sensor scans an image a line at a time it gradually builds up a full image. |
Linear sensors are used in two ways:
- Linear sensors contain only a single row of photosites. To capture colors, a color filter is placed over the sensor for three separate exposures—one each to capture red, blue or green; or, cyan, yellow, and magenta.
- Tri-linear sensors use three rows of photosites—each with a red, green, or blue filter (or cyan, yellow, and magenta). Since each pixel has its own photosite, colors are captured very accurately in a single exposure.