Reversing detection for an elevator car door

A light grid for mounting on a sliding door of an elevator car door, for object detection and for determining periodic elevator car door movements, comprising:

• at least one transmitter element on a transmitter strip, which emits radiation with a specific intensity,
• at least one receiver element on a receiver strip, which receives the radiation of an assigned transmitter element, and
• a control device for evaluating the at least one receiver element in regard to interruption of the radiation coming from an assigned transmitter element and for outputting a signal for object recognition upon interruption,
• wherein at least one receiver element is configured to output an intensity value for the intensity of the received radiation, and the control device is configured to evaluate the intensity value and to distinguish separate high, medium and low intensity value ranges with high, medium and low intensity values, with the transition between low and medium intensity ranges being defined by a predetermined first limit value and the transition between medium and high intensity ranges being defined by a predetermined second limit value, and to recognize an alternation phase with at least one alternating from the low via the medium to the low intensity value range and to output a signal in the event of an alternation phase having been recognized, and
• wherein the control device is configured to assign the high intensity value range to the range of low distance associated with a closed or slightly open elevator car door, to assign the medium intensity value range to the range of medium distance associated with a partly open elevator car door, and to assign the low intensity value range to the range of high distance associated with a wide open or fully open elevator car door, and the ranges of low, medium and high distance separately and directly succeed one another.