This invention concerns a method and an instrument for optical measuring of the thickness of materials in form of transparent or semitransparent film which comprises in combination optical lighting components, signal receiving components, control and supervisor components, all integrated and contained in a sole body. The measuring instrument is as a whole applicable to a production line of the material that has to be measured, both in fixed and dynamic position, is interfaced with remote display interface and could be interfaced and interact with a possible control system of the operating of the production line it is connected with.