The thickness change of a film is measured optically using self-interference of a single laser beam incident at the edge of the film. Theory suggests that when a half-plane phase shift is applied to a Gaussian laser beam, interference fringes appear in the near and far field, in which position varies with the amount of phase shift. By measuring fringe pattern displacement, we demonstrate detection of thickness changes in chitosan films induced by temperature rises of a few degrees centigrade. With a laser at 543 nm, the minimum detectable thickness change is 0.8 nm in ideal conditions (quarter wave films), corresponding with a phase shift of 0.02 rad, and the minimum detectable film thickness is ∼30 nm. Potential use for surface temperature measurements is discussed.