A method of detecting an intrusion into a volume defined relative to a path across an opening along which path a closure member for the opening can be controllably displaced to define a gap in the opening, the gap being bounded on one side by the closure member and on the other side to the one side by a boundary member; the volume including at least one threshold region on at least one side of the path; the volume having as a base the threshold region and rising above the threshold region; a first group of at least two emitters (T, E) of electromagnetic signals located on either the closure member or the boundary (15A, 16A) and directed into the volume and a second group of at least two receivers for electromagnetic signals located on the opposite side of the gap to the emitters (T, E); emissions from the emitters (T, E) being directed into the volume but not directly towards the receivers; the emitters (T, E) and receivers forming part of a network including processing means which form a compound sensor for use in relation to operation of the closure member and for defining sensitivity to an intrusion into a region or regions of the volume; each emitter (T, E) being adapted to direct a primary beam of e.m. radiation into the volume; each receiver being adapted to receive a secondary beam generated by the reflection of a primary beam from a target intruding into the volume; characterised by the steps of: causing at least one of the emitters (T, E) to emit a primary beam along a first axis (T') into the volume; allowing for at least one of the receivers to receive a secondary beam of radiation along a second axis (R') from the volume; the secondary beam being generated by reflection of a primary beam from a target intruding into the volume; the secondary beam being generated in the volume by way of, but not constituted by, the or any primary beam from an emitter (T, E); and enabling one or more of the receivers to provide an output signal into the network of the compound receiver characteristic of the, or each, secondary beam detected by at least one of the receivers in the second group; and providing that where the compound sensor serves to establish: that when an excess gain ratio is relatively small in an output signal from a single receiver caused by reception by the receiver of a secondary beam and the size of the gap is relatively large then the compound sensor functions so as to increase sensitivity of detection existing within a given predetermined region of the volume by comparison with sensitivity in another region or regions in the volume; that when an excess gain ratio is relatively large in an output signal from a single receiver caused by reception by the receiver of a secondary beam and the size of the gap is reducing or small then the sensor functions so as to increase sensitivity of detection within more than one region of the volume by comparison with sensitivity in another region or regions in the volume. causing at least one of the emitters (T, E) to emit a primary beam along a first axis (T') into the volume; allowing for at least one of the receivers to receive a secondary beam of radiation along a second axis (R') from the volume; the secondary beam being generated by reflection of a primary beam from a target intruding into the volume; the secondary beam being generated in the volume by way of, but not constituted by, the or any primary beam from an emitter (T, E); and enabling one or more of the receivers to provide an output signal into the network of the compound receiver characteristic of the, or each, secondary beam detected by at least one of the receivers in the second group; and providing that where the compound sensor serves to establish: that when an excess gain ratio is relatively small in an output signal from a single receiver caused by reception by the receiver of a secondary beam and the size of the gap is relatively large then the compound sensor functions so as to increase sensitivity of detection existing within a given predetermined region of the volume by comparison with sensitivity in another region or regions in the volume; that when an excess gain ratio is relatively large in an output signal from a single receiver caused by reception by the receiver of a secondary beam and the size of the gap is reducing or small then the sensor functions so as to increase sensitivity of detection within more than one region of the volume by comparison with sensitivity in another region or regions in the volume. that when an excess gain ratio is relatively small in an output signal from a single receiver caused by reception by the receiver of a secondary beam and the size of the gap is relatively large then the compound sensor functions so as to increase sensitivity of detection existing within a given predetermined region of the volume by comparison with sensitivity in another region or regions in the volume; that when an excess gain ratio is relatively large in an output signal from a single receiver caused by reception by the receiver of a secondary beam and the size of the gap is reducing or small then the sensor functions so as to increase sensitivity of detection within more than one region of the volume by comparison with sensitivity in another region or regions in the volume.