Advanced Imaging


Advanced Imaging Magazine

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

Broad-Spectrum Performance Via VisGaAs



Broad-Spectrum Performance Via VisGaAs

New processing techniques have enabled both visible and near-IR imaging on a single detector.

by Ted Hoelter

September 2003

September cover image

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A single photodetector that captures images in both the visible and near-infrared allows users to perform tasks that heretofore required two detectors. Similarly, the ability to design such detectors into a camera brings flexibility to applications in the 850 to 1700 nm range that previously dictated the use of two cameras.

FPAs (Focal-Plane Arrays) based on indium gallium-arsenide (InGaAs) are used in a variety of commercial and military tasks, including various types of laser imaging, including characterization of the beam and imaging with NIR laser illumination. Others include imaging through paint for forensic analysis, outdoor imaging using night glow and incorporation into spectroscopic instruments.

Figure 1: Reflectance (performance model).While other technologies span most of the NIR spectrum covered by standard InGaAs FPAs (900 to 1700 nm), most other technologies have drawbacks. For example, indium antimonide (InSb) can cover the same range with a high-performance sensor, but the material requires cryogenic cooling, resulting in a higher-cost, lower-reliability camera because of the sterling cooler. Short wavelength mercury cadmium tellurium, (HgCdTe), like InGaAs, does not require cryogenic cooling, but is more expensive because of limited availability of the detector material. Pyrometers can be used in many NIR applications, but they have significantly reduced sensitivity compared to photon detectors.

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