| 摘要 |
Remote sensing of the temperature of a greybody or blackbody radiator is effected by passing its radiation ( 24 ) through a modulated infrared filter spectrometer. The infrared filter comprises, in sequence, a band pass filter ( 20 ), a first polariser ( 21 ) which polarises the radiation, an electro-optical element ( 22 ) which splits the polarised radiation into two orthogonally polarised components, and a second polariser ( 23 ). A lens ( 28 ) images the radiation leaving the second polariser onto a detector ( 27 ). The electrical signal from the detector ( 27 ) is input to a numerical analyser. The electro-optical element ( 22 ), typically comprising a birefringent crystal assembly ( 25 ) and a birefringent trim plate ( 26 ), is configured so that the net optical delay of the orthogonally polarised components passed through it is such that the recombined components are at or near a peak or trough in their interferogram. A sinusoidally varying voltage is applied to the electro-optical element to modulate the net delay of the components passed through the electro-optical element. The numerical analyser is programmed to compute the harmonic amplitude ratio (the ratio of signal amplitudes at the fundamental and second harmonic of the frequency of the modulating voltage) of the signal that it receives from the detector ( 27 ). The harmonic amplitude ratio is a function of the temperature of the radiator, which can be estimated by reference to a calibration look-up table.
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