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Abstract

Apparatus for adjusting the position of an edge between two surface porti of a moving belt with different spectral reflectivities where required in connection with work or treatment needed along the edge of one of the two surface portions while the belt is moving. It is a pre-requisite condition that the mutually adjacent surfaces of the belt are sufficiently significantly optically detectable which is impaired, for example, in the presence of surface contamination by dirt and the like. In order to obtain a significant signal in such circumstances, the edge region is illuminated and an imaging or focussing lens associated with a beam splitter is arranged in the focus beam. Following the beam splitter the two part-beams pass through one filter each to impinge on in each case a photodiode receiver, each filter blocking passage in that wavelength range in which the other filter is transmissive. The diode potentials which are produced by the two part-beams then are applied in the form of a quotient to control or steer a device which effects transverse displacement of the belt. A sufficiently significant control signal is obtained even if the reflectivity of the surfaces is reduced by scratches, fingerprints or other dirt contamination.

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