A three-CCD camera is a camera whose imaging system uses three separate charge-coupled devices (CCDs), each one taking a separate measurement of the primary colors, red, green, or 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. The system is employed by still cameras, telecine systems, professional video cameras and some prosumer video cameras.
Compared to cameras with only one CCD, three-CCD cameras generally provide superior image quality through enhanced resolution and lower noise. By taking separate readings of red, green, and blue values for each pixel, three-CCD cameras achieve much better precision than single-CCD cameras. By contrast, 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', resulting in a much lower effective resolution.
The combination of the three sensors can be done in the following ways:
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.
Some design goals for a prism assembly are:
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. Camcorders with three chips were called "3CCD" earlier and some are still called "3MOS" (derived from 3xCMOS, Panasonic) today.
Dielectric mirrors can be produced as low-pass, high-pass, band-pass, or band-stop filters. In the example shown, a red and a blue mirror reflect the respective bands back, somewhat off axis. The angles are kept as small as practical to minimize polarization-dependent color effects. To reduce unwanted reflections, air-glass interfaces are minimized; the image sensors may be attached to the exit faces with an index-matched optical epoxy, sometimes with an intervening color trim filter. The Philips type prism includes an air gap with total internal reflection in one light path, while the other prism shown above does not. A typical Bayer filter single-chip image sensor absorbs at least two-thirds of the visible light with its filters, while in a three-CCD sensor the filters absorb only stray light and invisible light, and possibly a little more for color tuning, so that the three-chip sensor has better low light capabilities.