The present invention provides improved methods and apparatuses for accurate measurements using interferometers. A functional relationship between the optical path difference and time is determined from a reference signal from an interferometer. The times at which the interferometer had specific optical path differences can be determined from the functional relationship. Those times can then be used to select times at which the spectroscopic signal from the interferometer was produced at the specific optical path differences. The invention can be applied, as examples, to maintain instrument calibration, and to transfer or compare calibrations or measurements across different interferometers.