Filmetrics F-20 is a device that enables instant thickness and refractive index measurements of thin films. Physical phenomena is based on analysis of selected wavelength range that is reflected from the examined (multi)layer. Computer programme fits a mathematical model to obtained measurement results producing a thickness value. Our reflectometer works in a spectral range 380 nm – 1050 nm and can measure layer as thin as 15 nm. With this device real-time thickness measurements are possible to perform in various experiments including sequential infiltration synthesis and solvent vapor annealing.