Abstract: A mixture of dyes each having an optical property (e.g. fluorescence intensity) which is sensitive to an external factor is caused to interact with a system that is to be monitored, such that the system provides the external factor and therefore influences the optical properties of the dyes. The optical properties of the individual dyes are individually measurable (e.g. being spectral intensities at different wavelengths). Their values are subjected to pattern analysis, e.g. using an artificial neural network analysis, leading to an output that characterises the system or its state.