A method of in-line determination of thickness, optical properties and quality of thin films, the method comprising the steps of: (a) printing and/or coating and/or patterning monolayer or multilayer films of organic, hybrid organic/inorganic and inorganic materials or their combination in a roll-to-roll (R2R) or sheet-to-sheet (S2S) process on polymeric and other flexible substrates for the production of organic and printed electronics; (b) using a fast multiwavelength spectroscopic ellipsometer (FMWE) unit to collect experimental data simultaneously in a set of different wavelengths between 190 - 830 nm (or 1,5 - 6,5 eV), to acquire dielectric function spectra simultaneously and in short time, of at least 90 ms; (c) parameterizing and analysing the measured pseudo-dielectric function <ε(E)>=<ε 1 (E)> + i <ε 2 (E)> using a geometrical model consisting of the air/thin film(s)/polymeric substrate in which the determination of the optical properties of each phase has been realized with the modified Tauc-Lorentz (TL) model for determining the thickness and the quality of printed organic(s) or blended organic materials and/or hybrid and/or inorganic film(s); (d) moving the FMWE unit in both the lateral and longitudinal direction in relation to the direction of the web to determine thickness, optical properties and quality of said thin film(s) at several locations, ensuring the quality control of the printed material throughout the web and independently of the complexity of the printed pattern applied; (e) calculating energy gaps that are related to the properties and quality of thin films and to their suitability for the required applications. (a) printing and/or coating and/or patterning monolayer or multilayer films of organic, hybrid organic/inorganic and inorganic materials or their combination in a roll-to-roll (R2R) or sheet-to-sheet (S2S) process on polymeric and other flexible substrates for the production of organic and printed electronics; (b) using a fast multiwavelength spectroscopic ellipsometer (FMWE) unit to collect experimental data simultaneously in a set of different wavelengths between 190 - 830 nm (or 1,5 - 6,5 eV), to acquire dielectric function spectra simultaneously and in short time, of at least 90 ms; (c) parameterizing and analysing the measured pseudo-dielectric function <ε(E)>=<ε 1 (E)> + i <ε 2 (E)> using a geometrical model consisting of the air/thin film(s)/polymeric substrate in which the determination of the optical properties of each phase has been realized with the modified Tauc-Lorentz (TL) model for determining the thickness and the quality of printed organic(s) or blended organic materials and/or hybrid and/or inorganic film(s); (d) moving the FMWE unit in both the lateral and longitudinal direction in relation to the direction of the web to determine thickness, optical properties and quality of said thin film(s) at several locations, ensuring the quality control of the printed material throughout the web and independently of the complexity of the printed pattern applied; (e) calculating energy gaps that are related to the properties and quality of thin films and to their suitability for the required applications. <EOS>