Abstract: The invention relates to methods and systems for measuring and/or monitoring the chemical composition of a sample (e.g., a process stream), and/or detecting specific substances or compounds in a sample, using light spectroscopy such as absorption, emission and fluorescence spectroscopy. In certain embodiments, the invention relates to spectrometers with rotating narrow-band interference optical filter(s) to measure light intensity as a function of wavelength. More specifically, in certain embodiments, the invention relates to a spectrometer system with a rotatable filter assembly with a position detector rigidly attached thereto, and, in certain embodiments, the further use of various oversampling methods and techniques described herein, made particularly useful in conjunction with the rotatable filter assembly.