MATERIAL TESTING AND REGULATING APPLIANCES AND INSTRUMENTS NAMELY OPTICAL FIBER TESTING AND REGULATING APPLIANCES AND INSTRUMENTS USED IN TELECOMMUNICATION, NETWORKS AND IN FIBER OPTIC NETWORKS; OPTICAL SPECTRUM ANALYZERS AND SPECTRUM ANALYZERS FOR FIBER-OPTIC NETWORKS, WAVELENGTH-DIVISION OPTICAL MULTIPLEXERS AND DEMULTIPLEXERS; OPTICAL SPECTRUM ANALYZERS AND SPECTRUM ANALYZERS FOR FIBER-OPTIC NETWORKS; OPTICAL REFLECTOMETERS AND POLARIMETERS MODAL DISTORTION IN A LIGHT BEAM MEASURING APPLIA...