AbstractOptical thickness determination of very thin layers is based on measuring the reflectance and evaluating with a fit algorithm. Any fit algorithm however needs initial values for the parameters that need not to be far away from the expected final parameter values. This causes serious problems in the case of samples with spatially distributed coatings. We successfully circumvented the problem by combining the thickness determination with a color evaluation. The color evaluation proved to be helpful to set the proper initial values for each measuring position.