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Prism Cameras

Prism-based multichannel cameras

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Why Prisms?

An optical prism is a block of glass with some of its outer faces polished so they can serve some optical function, generally as windows or mirrors.  Prisms can be simple cubes or rectangular solids perhaps with some internal structure or they can be complex geometric shapes designed to redirect incoming light in a desired way.  In multispectral cameras, prisms are used to split the light in an image into two or more channels that can be separately detected by image sensors.  Several types of prism can perform this function but those generally selected provide control over polarization and image orientation as well as spectral content. 

In a prism properly designed for use in image splitting, the output images are perfectly aligned with each other so that when sensors are placed in the image paths, the data collected will also be aligned.  This permits calculations based on the relative intensities received by the sensors without the need to geometrically align the data by electronic means.  Errors common in multi-camera designs due to parallax, magnification mismatch, axial chromatic aberration, misalignment, and lens variation are all minimized or eliminated when all of the sensors are secured to one prism assembly.

How Many Channels?

The channel number boundary between multispectral and hyperspectral cameras is indistinct because it somewhat depends on the definition of those two terms.  For our purposes, "Multispectral" denotes a camera in which simultaneous two-dimensional image of several spectral bands are provided while "Hyperspectral" denotes a camera in which close sampling without gaps of a wide spectral band is the goal.  Quest cameras additionally allow for detection of other optical properties such as polarization, so Quest denotes their cameras more generally as "Multichannel".

Three Channels

The most common prism cameras, including most of the Quest models, use the Philips prism, so-called because it was patented by two engineers from Philips for use in broadcast cameras having three Plumbicon camera tubes.  The Philips prism has three output channels, originally used for Red-Green-Blue color separation. The Philips prism has an odd geometry designed to minimize polarization while supplying three images in the same orientation and with the same optical path length inside the prism.  Another prism that looks useful for three channels is the the X-cube but this prism has a seam down the center that causes irregularities in the image. The X-cube is also strongly polarizing and produces one channel that is flipped from the other two.

Two Channels

A simplified version of the Philips prism is also used for two-channel imaging.  Other prisms that provide two channels include a pentaprism with external compensation prisms to provide two erect images at 90 degrees to each other having the same in-glass path length. A right-angle beamsplitter can be used but it requires careful polarization control and produces a pair of mirror images.

Four Channels

Several prism combinations can produce four images but these require special design to assure equal path lengths and can become quite large when larger image sensors are needed.  Custom optics with long flange focal distance are almost always required.

When the four channels required are RGB and one other, then it is possible to put a Bayer array sensor on one port of a two-channel prism and a monochrome version of the same sensor on the other port.  The color image has some impairment in image quality but this type of assembly is simple and compact.

Five channels

Five channel assemblies have been built by stacking one Philips prism on top of another larger one.  This works, but appropriate lenses are difficult to find.  They are also costly due to the difficulties in aligning the sensors in five planes. 

A more common five channel implementation uses a sensor with a Bayer filter array on one Philips prism port to provide color while the other two ports have custom bands assigned.

More Channels

Because it is now possible to obtain image sensor with custom matrix filters aligned at the pixel level, the possible number of channels rises to 20 and beyond.  In addition, matrix filters that detect polarization are also available so multispectral polarization cameras are feasible, if complex.

Links to More

Below are links to additional information about prisms, multispectral application and related technology.

Prism Information

The Philips Prism (PDF)