Recent advances in the development of a multiwavelength imaging pyrometer

A multiwavelength imaging pyrometer (MWIP) is described that permits real-time remote sensing of temperature profiles of targets with unknown emissivity by measuring the spectral radiance of a target at several distinct wavelengths using a 320×122-element PtSi ir CCD imager with an assembly of seven narrowband ir filters in the range from 1790 to 4536 nm. Based on these measurements, the temperature and model parameters of the target emissivity are determined simultaneously from the least-squares fit of the theoretical model of the ir camera output signal to the experimental data. The real-time least-squares minimization is accomplished by combination of Levenberg-Marquardt and simulated-annealing algorithms. The experimental MWIP system also includes a least-squares-based calibration algorithm for evaluation of effective values of peak filter transmissions and center wavelengths based on the detection of radiation emitted by the precalibrated blackbody source over a wide range of temperatures. To achieve high radiometric accuracy, the ir CCD camera was operated with black-level and background subtraction and with compensation for dark-current charge as a function of the detected signal level. To minimize the effect of the response nonlinearity on the accuracy of real-time MWIP temperature estimation, we have developed an algorithm that provides for imager operation at fixed preselected signal level for each spectral channel by adaptively changing the duration of the optical integration time of the imager. Initial testing demonstrated an accuracy of ±1.0°C for real-time temperature measurements of the center of the blackbody aperture in the range from 500 to 1000°C. Temperature resolution of ±3°C was demonstrated for the blackbody source viewed through a double-side polished silicon wafer with unknown spectral transmissivity in the temperature range from 500 to 900°C.