A hand-held instrument that contains two silicon-based charge-coupled-device (CCD) video cameras (see figure) has been developed for imaging hydrogen fires. This or a similar instrument is needed because the visible light emitted by a hydrogen fire is so dim that the fire cannot be seen by the unaided human eye — at least, not in bright daylight. Like some other CCD-camera-based instruments developed previously for the same purpose, this instrument is designed to operate at infrared wavelengths where hydrogen fires appear bright, relative to solar background light. One CCD camera is called the "cloudy" camera, while the other is called the "sunny" camera, to indicate the different lighting conditions under which the cameras are designed to operate. In front of the "cloudy" camera is a long-wavelength-pass filter with a cutoff wavelength of 800 nm; during overcast, this filter blocks enough background light to make a hydrogen flame appear bright against the background. In front of the "sunny" camera there is a long-wavelength-pass filter with a cutoff wavelength of 1,100 nm; this filter blocks the solar background in the presence of full sunshine, such that a hydrogen flame is brighter than the solar background. The infrared images in the cameras are converted electronically and displayed to the instrument operator as visible images on miniature cathode-ray tubes in electronic viewfinders. A switch enables the operator to select the camera depending on the current light conditions. Optionally, both cameras and their viewfinders can be used simultaneously for binocular viewing.
The instrument includes a nonimaging, InGaAs-based photodetector that has a field of view 40° wide. This photodetector is preceded by a band-pass filter with a nominal pass wavelength of 1,360 nm, which is the wavelength of a peak in the emission spectrum of a hydrogen flame. This photodetector provides additional spectral discrimination of a hydrogen flame; it can also be used to trigger an audible alarm and a visible flash by light-emitting diodes (LEDs) inside the viewfinders, thereby helping to prevent the operator from overlooking a small hydrogen flame.
This instrument can be used to view a hydrogen flame only 8 in. (20 cm) long from a distance of 50 ft (15 m) in full sunlight. It can also be used to image alcohol fires, typical hydrocarbon fires, and embers, which emit in the same spectral regions as do hydrogen fires. Because a hydrogen fire, an alcohol fire, a hydrocarbon fire, or an ember can be seen readily only through the instrument, the operator can readily distinguish between these phenomena and a bright artificial light or a solar reflection, which can be seen without the instrument.
This work was done by Heidi L. Barnes of Stennis Space Center and Harvey S. Smith of Lockheed Martin. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com/tsp under the Physical Sciences category.
This invention has been patented by NASA (U.S. Patent No. 5,726,632). Inquiries concerning nonexclusive or exclusive license for its commercial development should be addressed to
the Patent Counsel
Stennis Space Center; (228) 688-1929.
Refer to SSC-00040.