Design Parameters for an Optimized Flame/Furnace Infrared Emission (FIRE) Radiometer

Several design aspects of an optimized flame/furnace infrared emission (FIRE) radiometer are discussed. For a source in thermal equilibrium, the optimum excitation temperature for CO₂ is predicted to be 2500 K, based on the stability of the molecule and the population of excited states as functions of temperature. The influence of spectral bandpass on the magnitude of flame background was studied for several wavelengths within the profile of the 4.4-µm emission band of CO₂. For the dispersive FIRE radiometer used in this study, the system was found to be detectornoise limited over the range of spectral bandwidths accessible by the instrument (0.01–0.08 µm). When a hydrogen/air flame was used as the excitation source, the maximum signal-to-background ratio was obtained at 4.40 µm with a spectral bandpass of 0.08 µm. For this same experimental arrangement, a mathematical model was used to predict that the maximum signal-to-noise ratio would occur at 4.40 µm with a spectral bandpass of about 0.25 µm. The influence of chopping frequency on detector noise was determined for two different types of wavelength isolation. When a room-temperature bandpass filter was employed, a modulation frequency of 600–1000 Hz was satisfactory for avoiding 1/f noise from the PbSe detector. However, when a monochromator was employed, chopping frequencies beyond 1000 Hz produced some additional reduction in 1/f noise. The specific detectivity for the PbSe detector (6.2 × 10⁸ cm Hz^0.5 W⁻¹), calculated from the measured noise at a modulation frequency of 600 Hz, is in good agreement with values reported in the literature for 4.40 µm under the same experimental conditions.