Design and Performance of a Planar Array Infrared Spectrograph that Operates in the 3400 to 2000 cm-1 Region

A prototype, no-moving-parts, plane array infrared spectrograph (PA-IR) capable of routine spectral acquisition in the 3400 to 2000 cm21 region has been constructed. The instrument includes a continuous source, Czerney–Turner type monochromator system and an infrared camera that incorporates a 320 3 256 pixel InSb focal plane array detector cooled with liquid nitrogen. PA-IR spectra (;3400 to 2550 cm21) of polystyrene (PS) and poly(ethylene naphthalate) (PEN) Ž lms have been obtained at a resolution of ;8 cm21 with excellent signal-to-noise ratios (SNR). Peak-to-peak noise levels of ;1 3 1023 absorbance units are observed for single acquisition spectra with 1.5 ms integration times and 17 ms total acquisition times. Integration times as low as 10 ms are possible (with good SNR); however, data acquisition is limited by the frame rate (60 frames/s) of the software acquisition package currently used. In this work, an apertured image of the source is displayed over ;20 rows of the array and the expected square root improvement in SNR is observed when multiple rows and/or frames are averaged. We have also shown that a square root improvement in SNR continues to occur with further signal averaging, providing noise levels as low as 1.5 3 1025 absorbance units. Several additional advantages and options associated with the PA-IR method are discussed, including time-resolved spectroscopy, real-time monitoring, and spectroscopic mapping.