A commonly used method for the improvement of the signal-to-noise ratio of analytical signals is ensemble averaging of repetitive waveforms. In spectroscopy this technique is ordinarily implemented by scanning a spectrum repetitively at a constant scan rate and acquiring intensity data at a constant rate. The relationship between the scan rate and the data acquisition rate defines the wavelength calibration of the averaged signal. We have used this technique to acquire the absorption spectra of atoms and molecules in flame and electrothermal atomizers by continuum-source atomic absorption spectrometry using a refractor-plate wavelength modulator to accomplish repetitive wavelength scanning at frequencies of several 10s of Hz. A triangular modulation waveform is utilized in order to achieve a linear wavelength scan in both directions. However, we have encountered three problems with this technique: (1) there is a significant amount of hysteresis between the forward and reverse scans; (2) the phase of the modulator position lags the input waveform; and (3) the modulator position waveform is rounded off at the extremes of its excursion. Because of these problems we have in the past accumulated only the spectra measured during the forward-scan half-cycles and have discarded the reverse half-cycles. Moreover, we have used only the center section of the spectral region, in order to avoid the distortion at the ends of the scanned range.
T. C. O'Haver and J. M. Kindervater, "Closed-Loop Feedback Controlled Data Acquisition System for Ensemble Averaging Repetitively Scanned Spectra with Wavelength Modulation," Appl. Spectrosc. 42, 183-186 (1988)