Ch. 1 - Electronic Imaging Theory and the SPOT Camera
Color Images
User’s Guide to the SPOT Cooled Color Digital Camera, 6/9/98
6
Color Images
Because a CCD chip is inherently monochromatic (i.e., black and white), color filters
must be used to extract the color information from the image. There are three basic
design methodologies:
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Single CCD design using color masked pixels
- Digital cameras designed in this
manner have one CCD chip with color filters physically bonded to each pixel on the
chip. Some pixels get red filters, some pixels get green filters and some pixels get
blue filters. Since each pixel is only able to measure the intensity of one color, the
intensity of the missing two colors for this pixel must be estimated, based on nearby
pixels that have precise measurements of the missing colors. For example, a pixel
that has a green filter will have an exact measurement of the green color value, but
the red and blue values must be estimated, based on the nearest red and blue pixels.
This estimation is known as
interpolation
.
Pros and Cons:
A digital camera designed in this way is inexpensive and can freeze
moving images with a single exposure. However, because they interpolate brightness
values, such cameras tend to produce images where the fine details are smeared; they
cannot provide the high resolution that is available from a non-filtered CCD chip.
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Three CCD design using color beamsplitter
- This design uses three CCD chips.
The incoming image first goes through a color beam splitter which directs the red
light to one chip, the green light to a second chip and the blue light to a third chip.
The chips are very accurately aligned so as to achieve near perfect registration
between the three chips. The red, green, and blue values for each point on an image
are measured by the corresponding pixel on each of the three chips.
Pros and Cons:
This design produces an image that retains the high resolution of the
individual chips. This type of camera can also freeze moving images with a single
exposure. However, because most of the cost in a high resolution CCD camera is in
the CCD chip itself, three chip cameras are far more costly than their single chip
counterparts.
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Single CCD design that takes three pictures (three pass method)
– Cameras
designed in this way expose a single CCD chip three times, once to red light, once to
green light, and once to blue light. One variant in this type of camera is how the chip
is exposed to the different colors of light. Some cameras switch individual red, green
and blue glass filters in front of the chip for each exposure. Others use a liquid
crystal filter that changes from red to green to blue as different voltages are applied to
it. By exposing the chip three times, each cell on the CCD is able to measure all three
color values.
Pros and Cons:
This technique attains the high resolution of the three CCD design
without the high cost associated with using three chips. However, because these
cameras require three exposures, they cannot freeze moving images.
