We report on the use of optical interferometry employing fringes of equal chromatic order (FECO) in a surface force apparatus (SFA) to determine film thicknesses and refractive indices of confined media for a wide range of separations. In particular, we show how to calculate the surface separation (film thickness) based on two fringes whose contact position was not measured. We discuss the measurement accuracy, and though the theoretical accuracy is I Angstrom for all separations, we show that in practice, for large separations, it is very hard to get to this accuracy. (C) 2003 Elsevier Inc. All rights reserved.