The invention relates to a method for determining optical properties by measuring intensities on a thin layer, wherein light is irradiated onto a carrier that has said thin layer and that is at least partially transparent. Interferences on the at least one thin layer are measured as the relative intensity of at least one superposition wave, optionally using filter arrangements provided for this purpose, whereupon the reflection coefficient(s) and/or the transmission coefficient(s) from the reflection and/or the transmission on the thin layer are determined. Preferably, the intensity of at least two superposition waves is measured. The light may be irradiated directly onto the carrier. The invention also relates to a device for determining optical properties by measuring intensities on a thin layer, said device comprising an analysis unit which stores at least one lookup table. The method and the device are preferably used in the area of homeland security.