A method and system for transparently encrypting sensitive data stored in a directory is provided. The apparatus comprises a vacuum vessel, a reaction chamber with a reaction space into which the substrate can be transferred and which has infeed channels for feeding therein the reactants used in said thin film growth process, as well as outlet channels for discharging gaseous reaction products and excess reactants. Operating fluid is filled in the cavity. In addition, the material of the optical interference member are chosen to have a work function which is compatible with the highest occupied molecular orbital, or the lowest unoccupied molecular orbital of the electroluminescent layer, depending on the location of the optical interference member in relation to the anode, cathode and electroluminescent layer.