The invention is directed to a safety device for a machine that includes a first transceiver unit for emitting a first light beam in a first time slot and for receiving a second light beam. The safety device includes a second transceiver unit arranged for emitting the second light beam in a second time slot that is different from the first one. The safety device also includes an evaluation unit for determining a reflecting object between the first transceiver unit and the second transceiver unit, only if the first light beam is received by the first transceiver unit in the first time slot or the second light beam is received by the second transceiver unit in the second time slot. Additionally, a non-reflecting object may be detected, if by neither of the two transceiver units in both time slots one of the two light beams is received.