White light is directed vertically onto a multiple-layer system of a SiO, a high-refractive TaO and an additional SiO layer (this additional layer can be chemically modified). The partial beams of the white light are reflected at each phase boundary and then refracted (transmitted). These reflected partial beams superimpose which results in an interference spectrum that is detected using a diode array spectrometer.<br>
Through chemical modification the upper SiO layer is changed in a way to allow interaction with target molecules. This interaction causes a change in the thickness of the physical layer d and the refractive index n within this layer. The product of both defines the optical thickness of the layer: n • d.<br>
A change in the optical thickness results in a modulation of the interference spectrum. Monitoring this change over time allows to observe the binding behaviour of the target molecules.