In my U.S. Pat. No. 4,479,056 issued Oct. 23, 1984 and based upon a German patent document - open application DE-OS No. 31 19 720, I have described a motion detector for space surveillance which comprises a concave mirror forming an optical system for focusing infrared radiation collected from the viewing fields of horizontal and vertical planes in the space under surveillance onto an infrared sensor or detector, the sensor or detector being provided at one end of a connecting element in which the rays of radiation are multiply reflected enroute to the mirror and from the mirror to the sensor.
In this construction, a window is provided in one wall of the connecting element or box, at one end of which the concave mirror is mounted and at the other end of which the radiation sensor is provided.
With this system, utilizing a spherically concave mirror, it is possible to increase the activation reliability of the alarm utilizing the motion detector because a number of viewing fields can be monitored as a result of the fact that the foci of the radiation from the various fields lie along the detector axes which are inclined to the optical axes of the optics formed by the mirror as a result in part of the reflection upon the inner surfaces of the boxlike structure.
With this technique, while a number of viewing fields in the vertical plane can be monitored with a comparatively small unit, azimuthal range is somewhat limited because at distances of say 200 m from the box, the spacing of the fields to which the device can respond becomes sufficient to allow for entry of an intrusion and the presence or movement of an intruder without interruption or detection by one of the viewing fields.
In many applications, moreover, the reliability is not satisfactory because of the comparatively small number of horizontal viewing fields.
In that arrangement, moreover, the walls of the box serving for internal reflection, generally converge toward the end of the box provided with the sensors. This has been found to be a disadvantage in some cases because of the comparatively high cost of fabrication of the device.
Naturally one can enlarge the window of the box and/or increase the length and/or number of sensors and thereby increase the number of viewing fields in both planes. This does not increase reliability necessarily and represents a comparatively expensive and even impractical solution to the problem since it almost invariably is associated with an increase in size of the motion detector and hence the ability of an intruder to discover it and, by blocking the detector, avoid the monitoring action.