Tuesday, January 30, 2007
Understanding Image Sensors
The sensor is the soul of your digital camera and knowing how it works will help you to compose better images
Labels: Learning Center
As far as sampling the color information, with most CMOS image sensors, a similar technique to the CCD is used to filter the photosites so that they receive the appropriate colors of light. One manufacturer, Foveon, uses the properties of silicon itself to filter out the various colors of light. This process allows each photosite to capture red, green and blue rather than just one color. (The Foveon image sensor is currently only available on Sigma digital SLRs, but we expect to see other camera manufacturers employing the sensor in the future.)
Comparing Design Differences
So how do these design differences compare? Let's break it down into a few specific topics.
Power Consumption. It turns out that in order to read out all of that data from the photosites in a row-by-row and then pixel-by-pixel fashion in a CCD, there are all sorts of sophisticated timing signals that need to be generated. And while it's technically possible to put all of those timing generators and other necessary processing functions on the CCD, it's not very economical to do so.
With a CMOS image sensor, many of the processing functions can be built into the chip during manufacture much like Pentium CPUs have onboard memory built right into the chip. So a CCD requires additional integrated circuit chips compared to a CMOS image sensor in order to process the data. Anytime you add circuitry, you add power consumption. Therefore, a CCD will consume more power than a CMOS image sensor.
Speed. By having to shift charges around the CCD and parse out the data one row at a time, a CCD can take a long time to create an entire image. With the CMOS sensor having individual charge converters and amplifiers in each photosite, the whole image can be read out much faster.
Yet another speed increase is found with the CMOS sensor if you use special capture functions that utilize only a part of the sensor. Let's say that you're capturing an image using only one half of the sensor. Since the CMOS chip can access only the pixels it needs, the frame rate of the image capture is increased. With a CCD, even if you're only using the middle part of the sensor, the chip still has to shift the data from all the photosite rows, one by one, until it gets the data it needs.
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