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CMOS Vs. CCD

Introduction:
CMOS (complementary metal oxide semiconductor) and CCD (charge coupled device) imaging sensors have been around since the late 1970's. Both are a type of technology used in digital cameras of all types to capture digital images. They can be used to capture still images or video. CCD and CMOS sensors each have their own advantages and disadvantages. In the last several years (2004-2007) both have improved significantly in their individual weaknesses.
 
CCD:
Both of these digital sensors convert light into an electric charge and create an electronic signal. In a CCD sensor, each pixel's charge is transferred over the surface of the chip to a collecting output node. Most CCD's use just one node, though some chips have more. This output node then converts this charge to voltage, buffers it, and sends it off as an analog signal. Because all of the pixels charges are processed by one node, the image uniformity is high (this ensures that image quality remains high).
 
CMOS:
In a CMOS sensor, each pixel has its own charge converter. The sensor also includes noise correction, amplifiers, and circuits to digitize the signal. All of these extra circuits end up reducing the area available for light capture resulting in lower image quality. Also, because each pixel does its own conversion, there will be some variance between identical charge conversions.
 
Comparisons:
Traditionally CCD's have been used as a benchmark in performance and image quality. This is mainly because the available manufacturing processes in the past were unable to create CMOS sensors with enough uniformity and small enough size to be used for high quality images. With the advance of lithography CMOS sensors are becoming more and more common due to a variety of factors.

CMOS sensors are able to integrate more functions on the chip and use less power, but at the cost of image quality and manufacturing cost. CCD sensors offer high image quality at the cost of a fairly high power consumption. The drawbacks of each type of sensor have been getting better as time goes on and we will no doubt see them producing identical images in the near future.

At this point in time there seems to be no clear winner. CCD's produce slightly better image quality with less noise, but the cost is power consumption. CMOS cameras can run for days and are generally cheaper provided the imaging sensor is the same size. It cost a lot of money to produce a CMOS sensor that gives high quality images hence high end digital cameras with CMOS chips usually cost more than an identical camera with a CCD chip.

Below is a table of the differences between CMOS and CCD imaging sensors.
 
  CCD CMOS
Pixel Signal Electron packet Voltage
Chip Signal Voltage (analog) Bits (digital)
Uniformity Very High Low to Moderate
Noise Low Moderate
Complexity Low High
Cost Low High
Camera Cost Depends on design Depends on design
Dynamic Range High Moderate
Speed Moderate to high High
Power Consumption High Low to Moderate
 
The Choice:
Choosing a camera based solely on its digital sensor is not a good idea. These days the performance of each type is nearly identical. CCD's will produce slightly higher quality images with less noise at the cost of power. CMOS sensors can produce high quality images but require many more circuits to do so hence their higher cost in professional cameras. You should be choosing a camera based on its features and ease of use rather than a sensor type. Generally the differences between high quality CMOS and CCD chips is not even noticeable so why worry about it? Just get out there and enjoy your camera.

Cheers

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