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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
To meet these requirements, color separation prisms are designed and built 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.
 

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.
 
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.

2-channel prism

 
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.
 

4-channel prism

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.

5-channel prism

 

Neutral Separation
 
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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