Friday, July 6, 2012


FPA - focal-plane array, an array of light detectors placed in the focal plane of a lens or optical system to record images, such as the sensor chip that takes the place of film on a camera. Often refers to infrared detector arrays, but also can be applied to arrays which respond to other parts of the spectrum, particularly visible and radio bands.

The concept of a focal-plane array is a natural one in the age of digital photography, but a Google Book search finds 10 references dating from the 1960s, mostly referring to infrared detectors for military or astronomical applications. It may have originated in the military; one reference was to a 1963 government document on the defense budget. Extending the search to the start of 1980 found another 450 publications that used the term, too many to examine in detail.

FPA wasn't a common term even then. It's not listed in the index of my 1978 edition of The Infrared Handbook. The only sub-entries under "array" are "silicon diode vidicon," "staring CCDE signal processing," and "in scanners." But those entries do show how imaging technology was evolving toward the FPA concept.

The vidicon was an imaging tube used in analog television, which recorded images as charges on a photoconductor and read them out with an electron beam. By 1978, arrays of silicon photodiodes were available, and had begun to be used for electronic imaging, leading to odd hybrid terms to describe how silicon diode arrays had come to replace television-like cameras in the near infrared.

Scanning and staring refer to different ways of recording images with a limited number of detectors. Today pixels are cheap, so it's natural to put a chip containing a million or more light-detecting elements into the focal plane, where it stares at the scene being imaged. In the 1970s detectors were much more expensive, especially in the infrared, so electronic images often were recorded by scanning a linear array of detectors across the image plane (or by scanning the image across the detector array).

Both technologies live on today. Scanning arrays are standard in flatbed scanners, which drag a linear sensing array across a page. Staring arrays have gone much farther. They are standard in cameras for applications including high-resolution scientific imaging in the near infrared. A gigapixel FPA with response in three bands stretching from 250 to 1100 nm will be launched next year on the European Space Agency's Gaia satellite.

FPA comprised of Geiger-mode avalanche photodiodes was packaged in a ceramic case with microlenslet array, readout circuit, and thermoelectric cooler by Princeton Lightwave (Cranbury, NJ).

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