A spectral interferometry apparatus and method are disclosed, that can be used to monitor or measure an unknown length by following a characteristic of an indicating signal. The measurement is performed by adjusting an optical path difference (OPD) in an interferometer until sound or light or both are obtained with the desired strength and pitch. Embodiments are presented where the unknown length is the eye length. Sound of different pitches are produced by scanning the channeled spectrum output of an interferometer with the object returning at least one of the interferometer optical signals. The scanning is performed by reading the signal of an analogue photodetector array driven by a nonlinear clock or by tuning a low cost swept source using a distorted driving signal.
