Charged Coupled Device(CCD); You have probably heard about CCDs, they are a type of lightsensor used in scanners, fax machines, digital still- and video cameras.

The CCD is made of silicon, the most used semiconducting element today. A CCD sensor is built up off up to several millions photosensitive cells. When light hits the one of these cells,a release of free electrons in the silicon will occur. By applying a negative charge to the connected electrodes, the free electrons in the silicon will be repeled and the part of the substrate that is closest to the electrodes gets a positive charge. This charge is used to find out how much light hit the cell, the higher the charge, the more light hit. By combining the measurements of all the cells you'll get a grayscale image of the light that hit the array.

A colour CCD works in the same way, but uses colour filters to seperate the individual color components. There are two ways to do this:

3-pass scanning:
3-pass scanning uses three individual colour filters(One that lets red light through, one for green, and one for blue). The image wil be read once trough each filter. Now we have three images, one representing the intencity of each colour. These are then combined by the software in to a colour image. 3 pass scanning was used in some scanners but is not used much any more since it's takes three times as long to scan a complete picture.

One pass devices also uses filters but here they are thin lines fixed to the CCD so they cover one vertical line of CCD cells each. This means that the first coloum of cells might have a filter that red can pass through, the next has a filter that lets green trough, and number three have a filter that lets blue light pass. This is then repeated the full length off the array. As in three pass scanning you get three images, that are mixed by software in to one. This metod only needs to be read once, and is called one-pass. The drawback of this type of array is that you only get 1/3 of the horisontal resolusion on the finished picture, but since the cells in modern CCDs is so small, there is no problems in compensating for this. This is the way modern CCD scanners and cameras uses.

This is the most used image sensor at this time but it has been replaced in cheap digital cameras by CMOS based sensors. CMOS Image sensors are cheaper and uses less power, but have lower resolution and is less sensitive to light, so CCD is still used in more expensive, high resolution cameras.

Addition :
Some color imaging devices uses mirrors that are transparent to one colour wavelength(dichroic mirrors) or prisms to split up the colours, and uses one CCD for each colour.

Basic working principle

                Green CCD
          Green  ./....| Blue CCD
      t.mirror   /:    |
  _______\.......:/....| red CCD
  light  /       /  R  | 
                 Red transparent

(The prism types works in a similar fashion, by using dichroic surface reflections in the glass.)

This means that you can get a higher resolution and sharper image, but the added CCDs and optics makes the device more expencive.

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