The invention relates to a device for selectively determining the position of zones with different emission behaviors within and around the zone of interaction of a thermal process that is produced by a laser beam (12), wherein energy is supplied to a work piece (10) in a locally limited manner. A position-sensitive optical sensor device (56) is provided that determines the position of the center of the respective emission zone. An optical filter device (54) is mounted upstream of said sensor device. The sensor device (56) is provided with at least one diode element that is linked with a data processing unit (58). The inventive device allows a distance control between the site of the locally limited energy supply on the work piece (10) and the treatment head and/or a process control.
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