Infrared open path emission spectrometry is a nonintrusive remote sensing method of determining the composition and temperature of exhaust gases of various origin (industrial combustion processes, aircraft engines, etc.). It has many advantagesover conventional methods, which rely on extraction of gas samples from the exhaust. The measurement set-up is simple and, by definition there are no parts of the instrumentation within the exhaust gas itself Thus measurements affect in no way the gasesunder investigation. Being an optical method it furthermore allows the investigation of otherwise unaccessible objects (e.g. hazardous fires). Especially the Fourier transform infrared (FTIR) technique offers fast simultaneous measurements of manycomponents. In addition field measurements are possible and have been demonstrated by various research groups. The derivation of quantitative gas parameters from the measured spectra requires a rather complex procedure: first the raw spectra have to beconverted into spectra of radiometrical units (radiance) by means of a radiometric calibration of the spectrometer. Then from these spectra quantitative gas parameters (temperature, concentration) can be derived. In general this is performed byapplication of a least squares fit procedure which itself requires concentration/temperature calibrated gas parameters establishing the relation to the gas radiance. Accuracy and precision of the derived parameters depend on various influences within theevaluation procedure. One of it, the radiance calibration, has been investigated regarding its typical systematical errors. These errors propagate throughout the analysis process and may become the main source of error. This leads to requirements for theradiometric calibration.
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