Introduction
A family of digital image sensors used by a variety of cameras currently on the market. The term CMOS actually refers to a method of producing semiconductors and does not directly describe a specific sensor technology. As such, this term can be used to refer to a wide range of different sensor types.
With that said, most digital cameras advertising their use of a
CMOS imager are referring to a specific design called an
active pixel sensor. The details of this design, as well as the advantages and disadvantages that it brings with it, are covered in the following section.
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Active Pixel Sensor 
Fig 1 - An animation illustrating the readout process used by common CMOS imager designs. Cameras manually address each pixel on the sensor and read their values out one-by-one. As this process is managed entirely by the camera, this design provides designers with additional degrees of freedom.
While CMOS imagers can be produced in a wide variety of different ways, most current implementations are based on the
active pixel sensor design. In this design, pixels are connected to a number of readout rows and columns that can be switched on and off by external signals.
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Readout Process
Durring the capture phase, all of these connections are left open and each photosite captures light falling on the sensor. As this design does not provide
electronic shutter capability, the exposure is controlled by a mechanical shutter. Once the shutter is closed, the camera is then free to begin reading the image out of the sensor.
The readout process is handled by addressing each pixel, one-by-one, and reading the resulting data out of the sensor. The camera activates the switches in a single row and a single column thus providing single pixel with a link to the output line (see the animation in
figure 1). Once that is done, the resulting signal can be amplified, run through an
Analog-to-digital converter (ADC) and the pixel value stored into an image file. This process is repeated until every pixel has been read out of the imager and stored in an image file. Once complete the sensor is reset and can then be used to capture the next image.
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Advantages
As the readout process is generally controlled manually by the camera (rather than the inherent technique used in
CCDs), this provides designers with the freedom to capture only portions of the frame. For instance, the Nikon
D2Xs provides a special
high-speed crop mode that only reads out the central portion of the image. By skipping the peripheral areas of the image, the readout process can be completed in a shorter time and the camera can operate at higher frame rates.
In addition, as it is produced in the same manner as conventional ICs this allows additional logic to be incorporated directly within the sensor. This flexibility provides engineers with a wide range of tools to improve the quality of images produced by the camera. For instance, companies like Canon have used this facility to incoporate powerful noise reduction routines within the imager itself.
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Disadvantages
The traditional disadvantage of CMOS imagers has been that their additional complexity often reduces the percentage of each pixel actually used to collect light (the
fill rate). This, in turn, reduces the inherent sensitivity of those photosites when compared to simpler implementations. Fortunately, the ability to incorporate noise reduction technology on the sensor and the use of
microlenses have allowed modern designs to largely overcome this problem.
Additionally, as
active pixel sensors incorporate an amplifier in each photosite on the sensor they can be sensitive to another form of noise. Small production variances can cause the gain of each amplifiers to be slightly different, which can cause visible noise patturns in the resulting image. Luckilly, this patturn is relatively predictable so many modern cameras incorporate electronics that can automatically remove this
fixed patturn noise durring the readout phase. This process also provides the benefit of removing
dark current noise, making sensors with this technology well suited to long exposures.
When compared to
CCD-based imagers, the additional complexity in the readout mechanism used in CMOS sensors often mean that it is more difficult to maintain high frame rates. As such, many high-end CMOS imagers have taken to using a large number of parallel readout
channels to compensate for this disadvantage. For instance, the Canon
EOS 1D Mk.II uses a total of
eight different channels to achieve its 8.5fps frame rate. While this does restore performance, it can add significant expense to the overall camera design.
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Other Designs
As mentioned above, the term CMOS imager can be used to describe a wide range of different technologies. While most CMOS-based cameras use the
active pixel sensor design described above, it is not the only possible arangement. The following sections briefly describe a number of other implementations currently in use.
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Foveon X3
One of the better known alternative CMOS designs,
Foveon X3 imagers provide the unique capability of actually measuring all three colours at each and every photosite. Using a patented process, this technology layers three sensors on top of one another to measure three distinct groups of wavelengths. This avoids the pitfals generated by
Bayer CFAs, providing higher levels of sharpness at a given resolution. Please read
this article for further information on this topic.
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Similar Technologies
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LBCAST
A technology used by Nikon on the
D2H and
D2Hs,
LBCAST incorporates a design similar to CMOS. While CMOS-based sensors use MOSFET transistors, however, this technology makes use of JFET transistors in an attempt to minimize noise levels. While the low-level implementation is different, the overall design of these sensors is very similar to what is described above. As such, they share many of the characteristics described here. Please read
the dedicated article for further information on this technology.
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See Also - CCD - One of the oldest designs, charge-coupled devices are still one of the leading types of image sensors.
- Foveon X3 - Another CMOS implementation that is capable of natively capturing colour images.
- LBCAST - A similar technology using JFET transistors rather than the MOSFET transistors used on CMOS imagers.
Linear Mode
