Band-pass- and polarization-filtered laser light exceeds polarization-suppressed blackbody light.
Marshall Space Flight Center, Alabama
An apparatus denoted a laser image contrast enhancement system (LICES) increases the contrast with which one can view a target glowing with blackbody radiation (a white-hot object) against a background of blackbody radiation in a furnace at a temperature as high as ≈1,500 °C. The apparatus utilizes a combination of narrowband illumination, along with band-pass filtering and polarization filtering to pass illumination reflected by the target while suppressing blackbody light from both the object and its background.
In a typical application, the target is about 1 cm in size and located as far as 30 in. (≈76 cm) into the furnace. In the absence of this or another contrast-enhancing apparatus, a white-hot target in a furnace is nearly or totally indistinguishable from the white-hot background. Unlike a prior contrast-enhancing apparatus that utilizes two intersecting optical axes for viewing and illumination of the target and requires a furnace opening as wide as 3 in. (≈8 cm) the LICES provides for both illumination and viewing of the target along the same path. Hence, the LICES makes it possible to utilize a narrower opening into the furnace: the LICES can function with an illumination/viewing tube only about half an inch (≈1.3 cm) wide.
The LICES (see figure) includes a laser aimed perpendicularly to the optical path to the target. (Optionally, another source of narrowband illumination could be used.) The laser light impinges on a polarizing beam splitter that turns the light onto the optical path to the target. The laser light passes through a quarter-wave retardation plate, which causes the light to become circularly polarized. The circularly polarized laser light passes undisturbed through a vertical polarizer, then travels along the optical axis to the target. Reflection from the target reverses the circular polarization. The reflected laser light passes again through the vertical polarizer and then through the quarter-wave retardation plate, which converts the reverse circular polarization to horizontal polarization.
The polarizing beam splitter passes the horizontally polarized reflected laser light, which then passes through a lens, a field aperture (which helps to increase contrast by blocking background light), a horizontal polarizer, and a filter having a 20-nm-wide wavelength pass band. The reflected laser light ultimately comes to focus in a charge-coupled-device (CCD) camera. At the same time, the crossed polarizers and the band-pass filter discriminate the wideband, randomly polarized blackbody light from both the target and the background.
Because the intensity of blackbody radiation is proportional to the fourth power of absolute temperature, it could be necessary to increase the laser power to maintain adequate contrast at higher temperatures. A prototype LICES has been found to yield high-contrast images at temperatures ≥1,500 °C.
This work was done by William K. Witherow and Richard R. Holmes of Marshall Space Flight Center and Robert L. Kurtz of Pace & Waite, Inc.