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Foveon
X3® Technology - The Revolution in Color Imaging |
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In
monochrome image sensors, each pixel generates a signal representative
of the light arriving there. Thus, the sensor output is simply
a sequence of individual pixel measurements of intensity easily
assembled into a matrix for analysis or display. Color imaging
has, until now, been more spatially and optically complex than
monochrome imaging requiring some type of applied filter to partition
the incoming light into three bands and then carefully controlled
alignment to preserve or intensive calculation to reconstruct
the color data at each pixel.
In the least complex color cameras, a single monochrome sensor
is sequentially covered by three color filters, producing data
that is accurately registered but separated in time. Such cameras
cannot image motion and use light inefficiently.
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To maintain the pixel alignment but allow for motion, three monochrome
sensors can be mounted on a color separation prism. This arrangement
also makes good use of the light because the separation filters
only absorb a small fraction. Prism assemblies, though, are expensive
to build because they require three sensors and precise alignment.
Their long optical path length also limits the selection of optics.
With its three stacked layers of photosensors, the Foveon X3 direct
technology alone combines the optical simplicity of monochrome
imaging, the geometric accuracy of sequential color and the efficiency
of the prism assembly in a single CMOS device.
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Color Filter Array
Image Sensors |
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The majority
of color cameras now use color matrix sensors because they are
inexpensive and simple to use with most optics. In these sensors,
each pixel has an individual color filter, most commonly in the
Bayer pattern shown in this illustration.
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Color filter
arrays lead to two important problems. First, the filters absorb
most of the light, passing only that part desired to sense a specific
color band. In addition, since each color band is detected in
a geometrically separate location, reconstruction of the color
for each pixel requires estimations and assumptions that can only
approximate the actual color information received at the center
of the pixel group. The resulting color errors appear as color
aliasing that is impossible to remove. Aliasing can be reduced
by blurring the incoming image but only at the expense of overall
sharpness. Finally, because reconstructing the image requires
calculating the two missing color values for each pixel, either
the offset in the data must be ignored or a very large number
of computations must be carried out. Neither approach accurately
represents the incoming image.
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Foveon
X3 Direct Image Sensor |
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In the Foveon
X3 direct image sensor, three photodiodes are formed in every
pixel, stacked like the three layers in color film. This arrangement
utilizes the wavelength-dependent light absorption property of
silicon to produce natural filters that use the incoming light
to greatest advantage.
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When every
pixel senses all the color, artifacts from color filter offsets
are eliminated. In the illustration, the color banding is gone
but a close examination shows that aliasing just like that from
an unfiltered monochrome sensor is still present. Because no optical
blurring is needed to control color artifacts the full resolution
of the sensor is always available. The color resolution of Foveon
X3 image sensors is identical to their monochrome resolution so
there is no need to reconstruct missing color data by complex
computation.
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