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Seperation Prism Technical Data |
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Separation Prism Technology
For best image quality and
ease of use, separation prisms should have a few simple
characteristics:
- All output images should be
oriented in the same direction as the input image
- All channels must have the
same optical path length
- The prism transmission
should handle all polarizations with good uniformity
- All coatings should be
protected from the environment
- Ample space should be
available for mounting of filters and sensors
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| To meet these
requirements, Optec designs and builds prisms according to the
techniques developed by DeLang and others at Philips Research in
the early 1960s for use in television cameras based on geometric
techniques known since the late 1800’s.
The basic RGB “Philips prism”
consists of three prism segments with angles carefully designed
to minimize polarization of incoming light and to produce double
reflections so that all output images have the same orientation
as the input. The top illustration shown is not to scale.
In an RGB prism, light from a
lens enters from the bottom through a surface perpendicular to
the lens optical axis, often passing through a band-limiting
filter first. The light then strikes a longpass filter that
allows the red and green light through but reflects blue.
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This filter must be backed up by
an air gap, typically only 10 microns wide, with a sealed edge
to produce total internal reflection in the next segment. The
blue light is reflected a second time and exits perpendicular to
the blue face where it may be shaped by an external filter
before striking the sensor.
The second filter is a shortpass type that reflects red and
passes green. This filter is deposited on one prism face but
does not require an air gap so the segments are glued together.
The red is reflected a second time and then exits. The green
passes straight through and exits. The positions of the output
faces are placed so that all three optical path lengths are
identical. The bottom illustration shows the ray traces in a
typical prism.
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More or Fewer Channels
Extending the Philips layout to more (or fewer) channels is
straightforward but requires extreme care is design to avoid
imbalance in optical path lengths or variations in the size of
the ray bundle each channel can accommodate.
For two channels, the center
(red) angled segment is omitted because no air gap is then
required. Either a longpass or shortpass or even a bandpass
filter can be provided at the junction between the segments.
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2-channel prism |
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| For four
channels, a three channel prism is stacked on a two-channel
prism. The top (nominally green) channels is lost on the bottom
section because it serves only as a conduit to the top section.
The bottom section must be larger than the top section to
provide equal optical paths and be rotated 90 degrees to provide
room to mount all of the sensors. As in a two-channel prism, the
first channel (purple) can handle either the shortest or longest
wavelengths in the band. |
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4-channel prism |
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For five channels, a three
channel prism is stacked on another three-channel prism. The top
(nominally green) channels is lost on the bottom section as in
the four-channel prism. Similarly, the bottom section must be
larger than the top section and be rotated 90 degrees. As in a
three-channel prism, the first channel must be backed up by an
air gap. |
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5-channel prism |
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Neutral Separation
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All of the color
separation prism layouts may also be used for neutral
separation. In neutral prisms, the color separation
coatings are replace with metal or dichroic neutral
films so that the same spectral content is directed to
two or more ports. This is useful where the spectral
bands are not known in advance, where sensors are to be
optically butted or where images from two non-matched
sensors are needed simultaneously. While usually the
coatings are designed to provide the same energy on all
ports, as in the illustrations, other divisions are also
possible. |
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