A nulling, self-calibrating infrared radiometer is being developed for use in noncontact measurement of temperature in any of a variety of industrial and scientific applications. This instrument is expected to be especially well-suited to measurement of ambient or near-ambient temperature and, even more specifically, for measuring the surface temperature of a natural body of water. Although this radiometer would utilize the long-wavelength infrared (LWIR) portion of the spectrum (wavelengths of 8 to 12 µm), its basic principle of operation could also be applied to other spectral bands (corresponding to other temperature ranges) in which the atmosphere is transparent and in which design requirements for sensitivity and temperature-measurement accuracy could be satisfied.
The underlying principle of nulling and self-calibration is the same as that of a typical microwave radiometer, but because of differences between the characteristics of signals in the infrared and microwave spectral regions, the principle must be implemented in a different way. The instrument (see figure) would include an infrared photodetector equipped with focusing input optics [e.g., lens(es) and/or mirrors] and an input LWIR band-pass filter. An optomechanical device would periodically chop the input to the photodetector between a reference gray body (ideally, a blackbody) and a test body (the temperature of which one seeks to measure). The reference body would be mounted on or in a heater, a thermoelectric cooler, or other temperature-control device suited to the particular application. If needed, a band-pass LWIR filter could be placed in front of the photodetector.
The AC component of the photodetector output would be fed to a lock-in amplifier. The reference or synchronization signal for the lock-in amplifier would be derived from a device that monitored the motion of the chopper. The output of the lock-in amplifier would be a rectified signal approximately proportional to the difference between the radiances of the test body and the reference body. This signal would be used as an error signal in a feedback control loop, which would adjust the power supplied to the temperature-control device and thereby adjust the temperature of the reference body in an effort to reduce the error to zero. The temperature of the reference body would be measured by any of a variety of commercially available contact temperature sensors, which can routinely afford accuracy and long-term stability within 0.1 K. Hence, as long as the error voltage remained at zero and assuming that the emissivities and other radiant properties of the test and reference bodies were sufficiently similar, it could be assumed that the measured reference-body temperature was a close approximation of the test-body temperature.
Initial development efforts have been concentrated on the chopper, which is a major innovative feature of the instrument design. The desired characteristics of the chopper include pure chopping, low power consumption (a few milliwatts), high reliability (millions of cycles), and a chopping frequency of several hertz. "Pure chopping" signifies that at any given instant of time, the infrared radiation incident on the photodetector would be that from the test body and/or the reference body and not from the chopper itself and that, moreover, during one designated portion of the chopping cycle, the photodetector would "see" only the test body while during another designated portion of the cycle it would "see" only the reference body.
The chopper design under consideration at the time of reporting the information for this article calls for an electromechanical resonator comprising two permanent magnets and a reflective prism mounted on leaf springs. Each permanent magnet would interact with one of two electromagnet coils. One electromagnet coil would be driven by amplifier to excite vibrations. The other electromagnet coil would serve as a pickup coil, providing feedback to the amplifier to set up oscillations at the mechanical resonance frequency.
This work was done by Robert Ryan of Lockheed Martin Corp. for Stennis Space Center.
This invention is owned by NASA, and a patent application has been filed. Inquiries concerning nonexclusive or exclusive license for its commercial development should be addressed to the Intellectual Property Manager, Stennis Space Center; (228) 688-1929. Refer to SSC-00124.