The ability to make thermal measurements and image through materials that confound more expensive thermal cameras is pushing shortwave infrared (SWIR) sensors into a growing number of industrial process monitoring and thermography applications, typically those of 100 °C or more.

SWIR sensors are sometimes overlooked in favor of midwave (MWIR) or long-wave IR (LWIR) sensors because SWIR radiation is absorbed by water in the atmosphere. However, the advantages of this technology are pushing SWIR’s application base, particularly for inspection of hot glass processes. SWIR cameras, based on indium-gallium-arsenide (InGaAs) technology, can image through glass, allowing the user to inspect for defects inside hot glass bottles or containers. Simultaneously, InGaAs-SWIR imaging can monitor the temperature uniformity during the cool-down process. Typically, LWIR or MWIR thermal cameras, utilizing microbolometer indium antimonide (InSb) or mercury cadmium telluride (HgCdTe) detectors, cannot see inside the glass container being inspected. Thus the imagery provided by these thermal cameras is only a temperature measurement of the outside surface of the hot glass container.

The graph shows relationships among the visible, SWIR, mid-wave, and long-wave infrared wavelengths. Also noted are the response curves for detectors in the visible and the shortwave infrared regions.
SWIR cameras easily can image through the standard safety glass or quartz window enclosures that are used to protect cameras and other equipment in harsh industrial environments. This allows InGaAs SWIR cameras to inspect furnaces and hot metal processes more easily than other technologies that use longer wavelength light. The use of long-wavelength imagers often requires exotic lenses or expensive window materials. These materials can be cost-prohibitive in monitoring hot processes, especially when the enclosures are subjected to hot liquid splatter, which can require periodic replacement of the protective window (see figure).

The other distinct advantage to SWIR monitoring of hot processes is the ability to image at high speeds (e.g. recovery of chemicals). Solid-state InGaAs cameras, with their high-frame-rate capability, high resolution, low lag, long lifetime, and superior signal-to-noise ratios, are now replacing commonly used tube technology. InGaAs area arrays and linear arrays can reach tens of thousands of frames or lines per second, enabling high-speed monitoring of fast-occurring phenomena.

The newly introduced high-speed, high-resolution windowing InGaAs camera from Sensors Unlimited, Goodrich Corporation, can image up to 15,000 frames per second with high linearity and excellent reliability. High-frame-rate, cryogenically-cooled InSb or HgCdTe cameras with windowing capabilities are 50 to 100 percent more expensive to purchase than comparably sized InGaAs cameras. The cryogenically-cooled cameras are also much larger in size, require more power to operate, and have shorter overall lifetimes. The uncooled microbolometer cameras do have lower overall costs than the cooled systems, but they are limited by their video field rate capabilities (e.g. 60 fields per second with a 320 x 256 pixel array).

Shortwave infrared imaging provides machine vision users with unique solutions for remote sensing, inspection, and process monitoring applications with an ease of integration and ownership that outpaces long-wave IR sensors. Modern, solid-state SWIR cameras based on In-GaAs technology are small, lightweight, and feature room-temperature operation with low power consumption. SWIR-In-GaAs real-time and/or high-speed imaging for hot process monitoring results in low cost of ownership and helps ensure success for the end user’s application.

This article was written by Martin H. Ettenberg, director of Imaging Products, and Douglas S. Malchow, applications engineer, at Sensors Unlimited, Goodrich Corporation. For more information, contact Mr. Ettenberg at 609-520-0610.

Imaging Technology Magazine

This article first appeared in the June, 2006 issue of Imaging Technology Magazine.

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