Optical measuring instruments for measuring, monitoring and controlling the covering processes, in particular during thin layer photovoltaic processes for manufacturing solar cells, which are based on copper, indium, gallium and sulphur, amorphous silicon, cadmium telluride, cadmium sulphide, organic or inorganic and other materials, namely measuring instruments based on specular spectral reflectometry with material aligned optical spectral range and with a single or multiple measuring heads integrated in the covering process (in-line application); apparatus for measuring, monitoring and controlling the layer thickness and refraction indices, the absorption coefficient, roughness, texture, conductivity, texture parameters and related optical parameters of the layers during the manufacturing of thin layer solar cells, namely optical spectral reflectometers for measuring, monitoring and controlling the physical parameters during manufacturing of thin layer solar cells