A new optical method for determining film thickness is described in which a set of measured transmittance values for a sample of unknown thickness is correlated with sets of transmittance values predicted for different known thicknesses. A reiterative calculation is used to determine the thickness for which the predicted and measured values yield an optimum correlation. The method is particularly suited to on-line use in a film production process, where the sample is moving and cannot be contacted. The predicted transmittance values may be based on a model involving either absorption or interference. A simple, robust optical system can be used, even when using an interference-based model capable of X/100 accuracy. Criteria for selection of measurement wavelengths and bandwidths are outlined, and strategies for the reiterative calculation sequence are explained.