System for gap scanning

A gap scanning system wherein a plurality of emitters 38 of beams of electromagnetic radiation, such as of infra red, are directed from one side of a gap 33 of variable size to the other side where a beam from one emitter in the plurality can be received by a complementary receiver or detector in a plurality 36 thereof; the gap providing for access through the gap; characterised in that the number of emitters in the plurality thereof differs from the number of receivers in the plurality thereof. Typically the number of emitters in the plurality thereof is greater than the number of receivers in the plurality thereof. The invention further comprises a lift or elevator installation with a lift door system incorporating a gap scanning system as hereinbefore described. The gap is scanned to detect whether something is intruding into or there is an obstruction in the gap.