Fresnel Sapphire Split Prism Spectrometer for Engine Emission Diagnostics

OPTRA, Inc., is proposing a low-resolution ( 15 nm) Fresnel sapphire split-prism spectrometer specifically tailored to the identification of molecular species present in the emissions from an engine source. The proposed approach will examine the 3-5.5 ¿m spectral region, allowing for the extraction of concentration information of CO2, CO, NO, HCs, and particulate matter. Sapphire was chosen because of its very high dispersion in this spectral region. The split-prism configuration is used to counter the effects of the birefringence of sapphire, which otherwise would produce separate overlapping spectra for the s and p polarizations. The Fresnel realization of the prism allows for a more compact system and potentially higher throughput. The spectral bandwidth for each detector channel is selected to maximize signal-to-noise while retaining adequate spectral resolution for the identification of molecular species. A room-temperature amorphous silicon microbolometer array will be used as the detector, allowing coverage of the full 3-5.5 ¿m spectral region. Using reflective optics and containing no moving parts, this spectrometer will be compact, rugged, and ideally suited for its purpose.