An optical system that combines radiation from four solid angles about an emission source into a single beam was used in conjunction with a detector-noise-limited, flame infrared emission (FIRE) radiometer to increase the optical throughput of the <i>f</i>/4 polychromator dispersion system. Equations for the optical layout of the four-angle, pre-dispersion optics were derived, and the performance of the system was compared with a traditional, single solid-angle lens system. The four-angle collection optics were found to increase the signal-to-noise ratio of HCI emission at 3.77 μm by a factor of 3.8, in agreement with the theoretically expected value. The signal-to-noise performance expected for detector-noise-limited conditions, background-shot-noise-limited conditions, and background-flicker-noise-limited conditions is discussed.

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