A method of determining the rate of change of optical thickness of a thin-film during deposition comprising the steps of illuminating the thin-film (5) with electromagnetic radiation (3) having a range of wavelengths, measuring the transmission spectrum (6,7) of the thin-film (5) at least twice during the deposition process to determine the wavelength .lamda..sub.t or turning points in the transmission spectrum, and using the measurements to determine the rate to change of optical thickness of the thin-film as a function of time. The method further comprises the steps of predicting a time T in the growth process at which the wavelength .lamda..sub.t of the turning point in the transmission spectrum of the thin-film will be substantially equal to the wavelength .lamda..sub.d of the turning point in the transmission spectrum of thin-film at its optical design thickness, and interrupting the growth process such that growth ceases at time T.