Introduction
Originally invented in 1969, the CCD (Charge Coupled Device) is one of the most mature forms of image sensors still on the market. While not as flexible as more complex technologies (such as
CMOS), that simplicity brings with it a minimalist design that provides a number of inherent advantages. CCDs are often extremely fast and can natively provide extremely high sensitivity. It is for these reasons that CCDs still remain one of the most popular sensor designs on the market despite the emergence of several newer competitors.
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Readout Mechanism 
Fig. 1 - An animation showing the operation of a conventional three-phase CCD imager. By varying the state of a set of transistors, electrical charges are forced into adjacent photosites and read out of the sensor in a process similar to a bucket briggade. The simplicity of this mechanism allows CCDs to maintain extremely high
fill rates.
One of the major advantages that CCD sensors have over competing technologies is the simplicity of the readout mechanism. Using a system similar to a bucket brigade, these sensors use a set of transistors in each pixel to 'shift' electrical charges through adjacent photosites (see
Fig. 1). This process continues until the entire row has been read out.
Fig. 2 - Using the mechanism shown in
fig. 1, the rows are fed into a dedicated readout column where they are then shifted out of the CCD. The electrical charge captured by each photosite is then measured by an
ADC, processed by the camera and stored in an file.
In digital image sensors, each row is typically shifted into a dedicated readout column (see
Fig.2). Once this column is loaded, it is slowly fed out of the sensor using the same mechanism. After each shift, the camera's
ADC measures the electrical charge stored in the pixel and uses that information to generate a two-dimentional image file.
This process is repeated until all of the pixels have been read out of the sensor and processed by the camera. This mechanism allows images to be read out of the CCD in an efficient and orderly manner, and requires relatively simple designs to achieve.
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Multi-Channel Designs
Some camera manufacturers require high readout speeds that the above system isn't able to achieve. In order to work around this, many vendors have made use of multi-channel readout schemes to allow several pixels to be measured in parallel.
The above examples are a simple single-channel design - every pixel is shifted to a single output, where it is measured and processed. Multi-channel CCDs typically split the frame into zones and read them out in parallel. This multiplies the rate at which images can be read out of the sensor, providing a sizable increase in performance.
In the Canon
EOS 1D, for instance, the frame was split down the middle to provide dual-channel readout. Pixels on the left side of the frame were fed to the left and pixels on the right side were fed to the right - each side had a seperate readout column, and two ADCs read data from both sides in parallel with one another.
As each channel only had to worry about half of the pixels on the sensor, the readout process could be completed in half the time. This allowed extremely high frame rates without a need to change the underlying sensor technology.
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Problems
The main problem with multi-channel CCD designs is that they are very susceptable to production variances between the various readout mechanisms. As the pixels travelling through each channel are taking a different path, any differences in the components used on either side of the sensor (amplifiers, wiring,
ADCs, etc.) will become visible in the final image.
As such, vendors using this technique have to be very careful to minimize production variances and calibrate their cameras to compensate for what remains. This is not a trivial task, and has been the reason for a number of problems that users have seen in their cameras.
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Variants
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Full-Frame Transfer
The mechanism covered above is the conventional full-frame approach to CCD design. In these sensors nearly all of the area of a photosite can be used to capture light, providing sensitivity that is difficult to match with other technologies. Unfortunately, as these sensors are continuously sensitive to light they must be in the dark when reading the image out of the sensor. As such, a mechanical shutter is required when using cameras based on these sensors.
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Interline Transfer
In this more common design, masks are placed over every second row so that they aren't exposed to light. Once an image has been captured, it can quickly be transfered to the masked rows. The above mechanism can then be used on these darkened pixels even if light is still falling on the sensor itself. In addition, the unmasked cells can be used to capture a second image while the first is still being read out of the sensor. This allows interline transfer CCDs to operate at higher speeds than their full-frame bretheren.
The process of shifting image data to the masked cells provides an inherent
electronic shutter. As such, these sensors can begin and end an exposure with a simple electronic signal and don't necessarilly require mechanical shutter assemblies. Due to this capability, many cameras with Interline CCDs allow flash synchronization at nearly all shutter speeds.
Unfortunately, as half of each photosite must be covered by a mask these imagers are not as sensivitive as full-frame CCDs.
Microlenses can help to offset this issue, however they are not capable of completely offseting the reduced
fill rate of this design.
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See Also - CMOS - A newer type of imager that allows engineers more flexibility in designing the sensor than the older CCD design.
- LBCAST - Similar in design to CMOS sensors but using a different type of transistor in their underlying design to help decrease noise levels.
- Bayer Filter Array - Alows inherently monochromatic image sensors to capture and interpret colour information.
- Anti-Aliasing Filter - A special filter that adds a small amount of blur to captured images in order to prevent aliasing artifacts.
- Electronic Shutter - Used in some types of image sensors, electronic shutters eliminate some of the limitations imposed by mechanical shutter assemblies.
Linear Mode
