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Smoke and Mirrors: Ultra-Rapid-Scan FT-IR Spectrometry

As the scan time is decreased, the SNR, and hence the dynamic range of the interferogram are also reduced. The amplitude of the optical signal remains constant - independent of scan velocity. As the detector bandwidth is opened to accommodate the broader range of frequencies more of the thermal background noise is passed. The net result is that the SNR per scan drops as the square root of the scan time, while in principle, the SNR per measurement time remains constant.

This plot shows the dynamic range of the interferogram as a function of modulation frequency. The exact offset of the green diamonds is a function of experimental variables such as source temperature, throughput, detector d*, etc. The important point is that flash A/D converters, represented by the pink squares, are available with sufficient resolution to record interferograms to 100 MHz laser modulation frequencies and beyond, were it possible to scan a mirror that rapidly.

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