The incorporation of wavelength-modulated light into optical standing-wave interferometry is demonstrated for measuring small out-of-plane displacements. The optical standing-wave is generated by the superposition of the forward- and backward-propagating light beams, partially scattered by a single-layer nano-sphere scattering plate. The time-domain phase quadrature technique is used to detect and analyze the phase variation of the light scattered from the scattering plate for detecting small out-of-plane displacements. The experiments demonstrate the ability of the proposed method to measure out-of-plane displacements of several tens of micrometers with nanometric resolution. We also investigate periodic nonlinearity errors caused by changes in the modulation of the angular frequency and imperfect signals.