Optical wavelength is scanned linearly with time, and an optical field produced by a thin film on a detection plane is detected with a sinusoidal phase-modulating interferometer. Scanned wavelength is detected with another interferometer. The detected multiple-optical fields are backpropagated along the optical axis in a computer by use of the detected wavelengths. An optical field is reconstructed by summing the backpropagated fields over the multiple wavelengths. The intensity and phase distributions of the reconstructed optical field provide the positions of the thin film surfaces with an accuracy of a few nanometers. (C) 2015 Elsevier B.V. All rights reserved.