Abstract: According to the inventive method for determining the thickness of at least one layer provided on a substrate, the measurement can be constructed simply and reliably and a reliable measuring result can be obtained by measuring reflection—and/or transmission light intensity values of zero order in dependence on the wavelength and calculating said reflection—and/or transmission light intensity values using an iteration model which is dependent on the individual layer parameters. The layer parameters are altered in order to introduce a consistency between the measured values and the calculated values and the substrates have geometrical structures whose geometrical dimensions are used as further parameters of the iteration model. The inventive method also provides a means of determining the geometry of structures in the substrate, for example the depth, width and the repetition interval of grooves in a blank for geometrical storage media, such as CDs.