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Advanced Imaging Magazine

Updated: July 8th, 2008 05:26 PM CDT

Machine Vision in the Shortwave Infrared

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FEATURE ARTICLE

Figure 1 (above): InGaAs typically refers to lattice-matched In.53Ga.47As (blue curve) detecting where Si (purple curve) loses responsivity. The composition of InxGa1-xAs can be varied to adjust its spectral detection range to cover the entire shortwave infrared spectrum as indicated by the blue, green and red curves.

Machine Vision in the Shortwave Infrared

by Marshall J. Cohen and Martin H. Ettenberg

Cameras operating in the shortwave infrared ( SWIR , typically 1mm to 3mm) were first introduced more than ten years ago. For many years, they were technical curiosities, as they were large and difficult to use, and they had low resolution. In fact, it was unclear why, with the possible exception of astronomy, one would want to image in the SWIR band in the first place.

In recent years, there have been substantial advances in the performance of SWIR -sensitive indium gallium arsenide (InGaAs) focal plane arrays (FPAs) and cameras, as well as a much greater understanding of a wide variety of useful SWIR -band phenomenology. The original InGaAs FPAs were limited to 128x128 60mm pixels and were operated in large-size cameras (10cm x 10cm x 25cm) that required equally large power supplies. In contrast, the current state-of-the-art InGaAs FPAs include 1024x1024 element arrays with 18mm pixels flying in space, 1024 element linear arrays for spectroscopy and line-scan imaging, and 640x512x 25mm arrays used in a wide variety of commercial applications. At present, the smallest camera that operates in an infrared wavelength band includes a 320x256x25mm InGaAs FPA and is roughly the size of a 9V battery.

InGaAs FPAs and cameras are used in a wide variety of commercial, industrial and scientific applications. For our purposes, we will focus on their use in machine vision applications in which a camera is used to control industrial processes.

CONSIDERING The Whys and Wherefores of InGaAs

InGaAs is a member of the indium-gallium-arsenide-phosphide family. By mixing the four binary alloys (GaAs, InP, InAs, GaP) into ternary and quaternary alloys, it is possible to customize both the semiconductor energy bandgap that determines the optical properties, and also the crystal lattice structure.

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