An electronic system based on long-wavelength infrared (LWIR) sensors is being developed to detect aircraft flying near an upward-aimed, high-power laser beam. In the intended application, the system would alert a human laser operator and/or generate a signal to turn off the laser whenever an aircraft came within a specified exclusion zone - an approximately conical zone, surrounding the laser beam, from which the aircraft must be excluded to protect the human occupants of the aircraft against the laser beam. There could be a potential market for systems like this one in the laser-light-show, free-space laser communication, and lidar industries.

The use of LWIR (in contradistinction to shorter wavelengths) makes it possible to detect the blackbody emission from the skin of an aircraft; it is not necessary to rely on emission from hot engine parts. The system includes two LWIR cameras: a wide-field camera of 18-mm focal length and a narrow-field camera of 75-mm focal length. Both cameras have focal-length/aperture-diameter ratios ("f numbers") of about 1.

The performance of the system was analyzed in tests on observation of several airplanes ranging from a single-engine propeller to full-size commercial multiengine jet transport. In all cases, the system was found capable of detecting and providing alerts for airplanes flying at altitudes between 500 and 11,000 ft (approximately between 0.2 and 3.4 km). In each case, the system provided an alert at least 0.5 second before the aircraft intercepted the laser beam. Inasmuch as the time to close the laser shutter was 0.1 second, the system thus provided a margin of 0.4 second.

This work was done by Keith Wilson, Vachik Garkanian, and Tom Roberts of Caltech and Brian Smithgall of Image Labs International for NASA's Jet Propulsion Laboratory.