An active spectrometer

This article describes a new type of correlation spectrometer, dubbed an "a ctive spectrometer.'' It utilizes an oscillating mirror in a 1/4-m-Rowland circle spectrometer configuration to displace the spectrum periodically in the dispersion direction relative to fixed exit slits. The wavelength of th e light passing through each slit is modulated sinusoidally at the mechanic al oscillation frequency and its intensity is measured with a spot detector such as a photomultiplier or photodiode. With a PC-based control/data acqu isition system, the signal from each detector is sampled digitally at a fre quency constrained to be an integral multiple of the mirror drive frequency . From the resultant phase locking between the signal sampling and the mirr or oscillation, it follows that the correlation function between the signal and mirror phase angle is obtained simply by averaging the data from succe ssive mirror scans. Such a scan-averaged spectrum with low noise may then b e compared with digitally stored reference spectra obtained through the sam e instrument. ac coupling of detector and amplifier results in rejection of continuum light and, when photodiodes are used as detectors, the maximizat ion of preamplifier gain. (C) 1999 American Institute of Physics. [S0034-67 48(99)01202-2].