Broadband optical monitoring

A method of in situ monitoring a thin film deposition process on a substrate, the method including a) defining a desired spectrum, the desired spectrum being a transmission or a reflection spectrum; b1) illuminating the substrate by means of a light source emitting light within the desired spectrum, b2) receiving light reflected from the substrate or transmitted through the substrate, b3) determining a transmission or a reflection spectrum out of the received light; c) defining a spectrum or a combination of spectra in dependency of the spectrum determined in step b3) to be a current spectrum; d) determining a weight spectrum as a function of the current spectrum; e) calculating a real number as a function of the current spectrum, of the desired spectrum and of the weight spectrum; f) exploiting the real number as indication for a deviation of the current spectrum from the desired spectrum.