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A National Measurement System for Radiometry, Photometry, and Pyrometry Based upon Absolute Detectors

IV.  Filter Detector Systems and Use

c)  Pyrometry

The accurate radiometric determination of the gold point was the first step in establishing the temperature unit based upon absolute detectors [42]. The procedures outlined in Ref. [44] and in the solution of eq. (21) in the previous section define the temperature unit based upon absolute radiometry. The increasing availability of high quality infrared and near infrared detectors afford the opportunity to extend this methodology to even lower temperatures. It is important to characterize the quality of the blackbody sources used in these calibration strategies in order to ensure uniformity, high emissivity, stability, and account for any diffraction effects. Efforts are underway at NIST to develop control techniques for blackbody sources that have a radiometric sensor for the input to the feedback system and thus stabilize the system for the parameters of concern. The Special Publication [9] describing pyrometry at NIST is being rewritten and should be available in the near future.

The discussions in these sections have focussed on the use of FR systems using interference filters or other bandpass limiting mechanisms. The concepts developed do not depend on a particular technology for providing the bandpass limiting mechanism and in fact could include well characterized dispersive instruments using gratings or prisms. As the technology develops it may be possible to consider dispersive instruments with array detectors that capture a portion of the spectra and can define temperature directly as well as provide increased speed in determining spectrum distributions. Fourier transform spectroscopic devices offer some hope for application in this area but will require further development of the procedures to characterize these instruments for full spectroradiometric use.

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