Device for detecting characteristics of a moving paper web

A device for detecting properties of a moving web of paper, comprising:

• an infrared lighting device for illuminating the web of paper with infrared light, said infrared lighting device having an infrared light source and a light exit portion, a detector device for detecting infrared light reflected or transmitted by the web of paper said detector device having a light input portion, and a housing arranged in immediate proximity to the paper web and having a front face facing the web of paper, wherein at least one of the light exit portion and the light input portion is provided with optical fibers and is accommodated within the housing,
• wherein
• a plurality of n spherical lenses are arranged in the front face, said lenses forming exit and/or input windows,
• each of the spherical lenses is optically coupled within the housing to a respective one of the optical fibers by forming a coupling region therewith, said spherical lenses forming, together with the corresponding optical fibers, the at least one of the light exit portion and the light input portion,
• the spherical lenses are arranged in a sealed manner in the front face so that the housing is sealed and the coupling regions thus protected, and
• the spherical lenses are replaceably arranged in the front face so that spherical lenses of different diameters can be mounted into the front face.