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There are two types of image sensors: CCD (Charged Coupled Device) and CMOS (Complementary metal oxide semiconductor)
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Image Sensors
There are two types of image sensors: CCD (Charged Coupled Device) and CMOS (Complementary metal oxide semiconductor).

CCD Technology
The acronym CCD stands for Charge Coupled Device and is a technology that has been used for a long time when building cameras. Most high quality cameras feature a CCD sensor.

The benefits of a CCD sensor include:
  • Better light sensitivity: It can produce a fairly good image even in low light.
  • Better colours and sharper image: In recent years some new CMOS sensors have appeared - which have lowered the technological gap between the two technologies - but still CCD is superior and will be the better selection for the next few years.
  • High and even quality level: Typically the deviation between two CCDs of the same model is quite minimal.
  • Low background noise: A CCD produces less background noise than a CMOS sensor.

Some drawbacks with CCD technology include:
  • More expensive to produce: The CCD sensor is made in a non-standard process; hence, the unit has a higher price.
  • More expensive and complex to incorporate: It is more complex to build a CCD-based camera and it requires additional components.
  • When there is a very bright object in the scene (e.g. a lamp or direct sunlight), the CCD may bleed causing vertical stripes below and above the object. This phenomenon is called blooming and smear.

CMOS technology
The CMOS technology is the standard technology in which almost all chips are made, including memory chips, microprocessors and more. This makes the technology much more affordable than the CCD.

The benefits of CMOS include:

  • Lower cost as it is a standard process and there is no need for special components.
  • It is easier to design a camera based on CMOS than CCD. A CMOS sensor makes it possible to produce a smaller sized camera.

Some drawbacks with CMOS technology include:
  • Low light sensitivity: To be able to operate properly, the CMOS sensor requires a fairly bright environment.
  • In low light environments, a fixed-pattern noise appears. This can be seen as small dots or noisy lines in the image.
  • Higher signal to noise ratio than CCD's.

Image Sensors
Image sensors built on CMOS sensor technology give much better quality than previously seen on other sensors. Colours are nicer, and light sensitivity is almost as good as on a CCD.

A product-specific disadvantage of many megapixel cameras is that the scaling is done by simple line and pixel dropping. This means that while the full size image (VGA, 640x480) will be sharp, image information may be lost when the image is scaled down to a lower resolution.

In later products, scaling is performed by interploating nearby pixels to create a sharper image. This is possible because of a more advanced compression chip.

The recommendation of Axis Communications is to use a CCD-based camera in all situations where image quality or light sensitivity is of the highest concern. For applications where cost or size is critical, a CMOS-based camera is most probably the best solution.
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