The light beams (12) vertically cyclically scan the protection field (13). The light transmitters (11) and the light receivers (14) are connected to an electronic control and evaluation unit (15), having a transmission pulse generator (21). The light transmitters (11) are switched on for a determined time interval. Depending on one or more events in the protection field (13), taking place at predetermined positions, which produce an interruption of a light beam (12), which hereby effects the absence of the electric output signal of the relevant light receiver (14), is not used for the activation of a warning or switch off signal.
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