The system components are enclosed in a light-tight housing, with provision for the fiber to extend outside to collect the radiation. A computer algorithm is used to calculate the true temperature and emissivity of more » a target based on blackbody calibrations. The system includes a single optical fiber to collect radiation emitted by a target, a reflective rotating chopper to split the collected radiation into two or more paths while modulating the radiation for lock-in amplification (i.e., phase-sensitive detection), at least two detectors possibly of different spectral bandwidths with or without filters to limit the wavelength regions detected and optics to direct and focus the radiation onto the sensitive areas of the detectors. This invention is a fiber-based multi-color pyrometry set-up for real-time non-contact temperature and emissivity measurement. This paper analyzes performance and aberrations of this imaging diagnostic.
Different focal-length mirrors cannot be used to magnify the image without substantially sacrificing image quality. If one parabolic mirror is rotated 180 degrees about the optical axis connecting the pair of parabolic mirrors, the resulting image is tilted by 60 degrees. To eliminate image plane tilt, proper tip-to-tip orientation of the parabolic mirrors is required. Matched mirror pairs must be used so that aberrations cancel. This system incorporates 90-degree, off-axis parabolic mirrors, which can collect low f/# light over a broad spectral range, for high-speed imaging. Other wavelength bands of this image are split into high-speed detectors operating at 900–1700 nm, and at 1700–3000 nm for timeresolved pyrometry measurements. A special mask is inserted at the last intermediate image plane, to provide dynamic thermal background recording during the event. The 3000–5000-nm portion of this image is directed to an infrared camera which acquires a more » snapshot of the target with a minimum exposure time of 150 ns. Inexpensive, diamond-turned, parabolic mirrors relay an image of the shocked target to the exterior of the gas gun chamber through a sapphire vacuum port. We have designed a thermal-imaging system for studying shock temperatures produced inside a gas gun at Sandia National Laboratories. A further complication arises when optical elements near the experiment are destroyed. Shock-compressed materials undergo transient temperature changes that cannot be recorded with standard (greater than ms response time) infrared detectors. Thermal imaging is an important, though challenging, diagnostic for shockwave experiments.
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