A Short Course Book
Curtin's Guide to Digital Cameras
And Other Photographic Equipment

Image Sensors—Types

 
This photo shows the pixels on an image sensor greatly enlarged. Courtesy of IBM.
 
 
A silicon wafer used to make image sensors. Courtesy of IBM.
 
 
Click to see where the name "charge-coupled device" comes from.
When using a film camera you can insert any kind of film you want. It's the film you choose that gives photographs distinctive colors, tones, and grain. If you think one film gives images that are too blue or red, you can change to another film. With digital cameras, the "film" is permanently part of the camera so buying a digital camera is in part like selecting a film to use. Like film, different image sensors render colors differently, have different amounts of "grain," different sensitivities to light, and so on. The only ways to evaluate these aspects are to examine some sample photographs from the camera or read reviews written by people you trust.

Initially, charge-coupled devices (CCDs) were the only image sensors used in digital cameras. They have been well developed through their use in astronomical telescopes, scanners, and video camcorders. However, there is now a well-established alternative, the CMOS image sensor. Both CCD and CMOS image sensors capture light using a grid of small photosites on their surfaces. It's how they process the image and how they are manufactured where they differ from one another.

  • CCD image sensors. A charge-coupled device (CCD) gets its name from the way the charges on its pixels are read after an exposure. The charges on the first row are transferred to a place on the sensor called the read out register. From there, the signals are fed to an amplifier and then on to an analog-to-digital converter. Once the row has been read, its charges on the readout register row are deleted, the next row enters, and all of the rows above march down one row. Each row is "coupled" to the row above so when it moves down, the one above moves down to fill its old space. In this way, each row of pixels can be read—one row at a time.
  • CMOS image sensors. Image sensors are manufactured in factories called wafer foundries or fabs where the tiny circuits and devices are etched onto silicon chips. The biggest problem with CCDs is that they are created in foundries using specialized and expensive processes that can only be used to make other CCDs. Meanwhile, larger foundries use a different process called Complementary Metal Oxide Semiconductor (CMOS) to make millions of chips for computer processors and memory. CMOS is by far the most common and highest yielding chip-making process in the world. Using this same process and the same equipment to manufacturer CMOS image sensors cuts costs dramatically because the fixed costs of the plant are spread over a much larger number of devices. As a result of these economies of scale, the cost of fabricating a CMOS wafer is significantly less than the cost of fabricating a similar wafer using a specialized CCD process. Costs are lowered even farther because CMOS image sensors can have processing circuits created on the same chip. When CCDs are used, these processing circuits must be on separate chips.
Despite their differences, both types of sensors are capable of giving very good results and both types are used by major camera companies. Canon even uses CMOS sensors in their high-end digital SLRs.


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