- A feedback system for controlling properties of a single layer or multiple layer stack being applied on a blank or coated substrate by means of a vacuum coating process controlled by a plurality of process controlling means, the feedback system being characterized in that it comprises - At least one monitoring device for implementing at least two distinct measurement techniques for determining measurement signals at each of a plurality of locations spatially distributed over the coated substrate, whereby a first measurement technique is adapted for being applied simultaneously to more than one of the plurality of locations, and a second measurement technique is adapted for being applied to at least one of the plurality of locations, one of the first and second measurement techniques being a spectral transmission measurement, - At least one processing unit adapted for receiving the measurement signals, configured for determining, from the received measurement signals, actual values of a first layer property at the plurality of locations, and of a second layer property different from the first layer property at the at least one of the plurality of locations, and/or for determining deviations between the actual first and second layer property values and desired layer property values, - A controller for providing actuation signals for actuating the plurality of process controlling means, and for generating further control signals to further drive at least one of the plurality of process controlling means in the coating process based on the determined values and/or deviations between the actual layer property values and the desired layer property values and on knowledge from a layer model expressing layer parameters as a function of settings and/or variations of at least two of the plurality of process controlling means, such that the properties of the layer stack lie within predetermined tolerance values. - At least one monitoring device for implementing at least two distinct measurement techniques for determining measurement signals at each of a plurality of locations spatially distributed over the coated substrate, whereby a first measurement technique is adapted for being applied simultaneously to more than one of the plurality of locations, and a second measurement technique is adapted for being applied to at least one of the plurality of locations, one of the first and second measurement techniques being a spectral transmission measurement, - At least one processing unit adapted for receiving the measurement signals, configured for determining, from the received measurement signals, actual values of a first layer property at the plurality of locations, and of a second layer property different from the first layer property at the at least one of the plurality of locations, and/or for determining deviations between the actual first and second layer property values and desired layer property values, - A controller for providing actuation signals for actuating the plurality of process controlling means, and for generating further control signals to further drive at least one of the plurality of process controlling means in the coating process based on the determined values and/or deviations between the actual layer property values and the desired layer property values and on knowledge from a layer model expressing layer parameters as a function of settings and/or variations of at least two of the plurality of process controlling means, such that the properties of the layer stack lie within predetermined tolerance values.