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Tuesday, February 5, 2008

3CCD Camera Intro

Three-CCD or 3CCD is a term used to describe an imaging system employed by some still cameras, video cameras, telecine and camcorders. Three-CCD cameras have three separate charge-coupled devices (CCDs), each one taking a separate measurement of red, green, and blue light. Light coming into the lens is split by a trichroic prism assembly, which directs the appropriate wavelength ranges of light to their respective CCDs. Three-CCD cameras are generally regarded to provide superior image quality to cameras with only one CCD. By taking a separate reading of red, green, and blue values for each pixel, three-CCD cameras achieve much better precision than single-CCD cameras. Almost all single-CCD cameras use a bayer filter, which allows them to detect only one-third of the color information for each pixel. The other two-thirds must be interpolated with a demosaicing algorithm to 'fill in the gaps'.

The combination of the three sensors can be done in the following ways:

  • Composite sampling, where the three sensors are perfectly aligned to avoid any color artifact when recombining the information from the three color planes
  • Pixel shifting, where the three sensors are shifted by a fraction of a pixel. After recombining the information from the three sensors, higher spatial resolution can be achieved.[citation needed] Pixel shifting can be horizontal only to provide higher horizontal resolution in standard resolution camera, or horizontal and vertical to provide high resolution image using standard resolution imager for example. The alignment of the three sensors can be achieved by micro mechanical movements of the sensors relative to each other.
  • Arbitrary alignment, where the random alignment errors due to the optics are comparable to or larger than the pixel size.

Three-CCD cameras are generally more expensive than single-CCD cameras because they require three times as many elements to form the image detector, and because they require a precision color-separation beam-splitter optical assembly.

The concept of cameras using three image pickups, one for each primary color, was first developed for color photography on three glass plates in the late nineteenth century, and in the 1960s through 1980s was the dominant method to record color images in television, as other possibilities to record more than one color on the video camera tube were difficult.

Three-CCD cameras are often referred to as "three-chip" cameras; this term is actually more descriptive and inclusive, since it includes cameras that use CMOS active pixel sensors instead of CCDs.

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