Device for detecting the properties of a web of material transported in the longitudinal direction

A method for detecting properties of a web of material, said web of material having a surface and a longitudinal direction and being conveyed in said longitudinal direction using a device comprising in combination:

• a crossbar, said crossbar extending across the web of material;
• an infrared spectrometer, said infrared spectrometer having an input side and an output side;
• a holographic grating which is arranged in the infrared spectrometer;
• infrared detectors, where said infrared detectors are arranged at the output side of the infrared spectrometer and are formed by a detector matrix having n lines and m rows of infrared sensitive individual sensors;
• and
• a plurality of optical waveguides, each waveguide having an entrance area and an exit area, said entrance area being located in vicinity of the surface of the web of material, being oriented towards said surface and being fastened to said crossbar, said exit areas of the optical waveguides being connected to the input side,
• wherein the optical waveguides are arranged side by side in one line at the input side of the spectrometer, infrared spectra inputted into the entrance areas of the individual optical waveguides appearing in rows side by side at the output side of the spectrometer, and the spectra of up to m optical waveguides are distributed and detected in up to n spectral areas,
• wherein the detector matrix is interrogated from time to time and at each interrogation only a maximum of 80% of the n×m individual sensors are interrogated.