The 3CCD colour
advantage: Let the benefits speak for themselves
to Multi-sensor Cameras
Do you need high-precision
colour, enhanced resolution and less noise?
All sensors, whether single/multiple CCD or CMOS types, read physical
data (light) and translate them into digital information (pixel data).
what makes a multi-sensor array better at capturing and translating
colour than a single sensor imager?
How does a 3-CCD sensor work
3-CCD (sometimes called "three-chip") cameras employ,
as the name suggests, three sensors, each one capturing a separate
image of red, green or blue light.
Light after being transmitted through the lens passes through a
assembly (see illustration
which splits light into 3 paths, and each path is
filtered into one of the 3 component colours and then transmitted
onto the sensor. The sensors are accruately positioned and aligned
so that a ray of light passing therough the lens and prism hits
the same corresponding pixel on each of the 3 sensors.Therefore
every pixel in the image measures each of the 3 colours truly. This
separate capturing of colour channels means that the camera has
more colour information and control over the individual red, green
and blue planes. It produces a truer, deeper colour image with better
precision than a single-CCD.
By comparison, single-CCD cameras use a Bayer filter which can only
have a single colour filter on each pixel. They capture only 1/3rd
of the incoming colour information and so must reconstruct the remaining
2/3rds by estimation using interpolation, a value for the "missing"
information. For example a pixel with a green filter only measures
green. The create a red value for this pixel the camera uses surrounding
pixels with a red filter and so extimates a red value for the green
pixel. The same is done for its ble colour. This interpolation of
data results in a lower effective resolution.
Benefits of multi-sensor CCD technology for colour imaging
Superior colour quality without interpolation
Compared to cameras with a single CCD, three-CCD cameras generally
provide superior image quality through enhanced resolution and lower
noise. By taking advantage of the separate values for red, green
and blue values for each pixel, multi-sensor cameras achieve much
better accuracy in the colour output than do their single-CCD counterparts.
|Special prism coating increases
The spectral curves resulting from the hard dichroic prism coatings
are much steeper than the curves from the soft polymer dyes used in
single-CCD sensors. This enables the 3-CCD cameras to produce exceptionally
accurate colour data without the uncertainty that comes from the overlap
regions. (See diagrams on left)
alignment gives greater precision
New 3-CCD cameras feature precise sensor alignment of less than 1/4
-pixel tolerance (roughly 1 µm accuracy), to provide better
spatial precision than single-CCD cameras whose Bayer filters result
in uncertain outputs for edge positions.
spectral curves from a 3-CCD prism camera
spectral overlap from 1-CCD Bayer mosaic filter
|High Dynamic range
In a single-CCD colour sensor camera, in addition to reducing colour
precision, the overlap in the colour filter response also results
in part of each pixel’s well capacity filling with photons from
the wrong colour (cross talk), and so decreasing the available well
capacity. With multi-sensor CCD cameras less colour overlap from the
dichroic coatings enable each channel to use the full well capacity
of the pixel, allowing the maximum possible dynamic range.
The two images (right) compare the results from a 5-megapixel
Bayer colour camera on the left with the 2-megapixel 3_CCD (the
JAI AT-200) on the right. Despite having 2.5X the resolution, the
5-megapixel camera’s soft polymer dye colour filters and the
required Bayer interpolation process results in significant color
contamination, less differentiation between similar colours, and
reduced sharpness of the image.
With their high resolution and high colour fidelity, 3_CCD sensors
are ideal for a wide range of applications where colour is critical.
** print inspection
** paint matching
** medical diagnostics
** flat panel quality verifications
** semiconductor production
** food sorting
A dichroic prism is coated with dichroic filter material that works on
the basis of thin-film interference. Light hitting the top surface of
the filters reinforces or interferes with light hitting the bottom surface
of the filter. It splits the visible light beam into distinct wavelength
bands and transmits the passing band so that this colour can be separately
detected on one of three CCD arrays.
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