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1 Designer’s Guide – Video Motion Detection This guide describes some of the as

1 Designer’s Guide – Video Motion Detection This guide describes some of the aspects which should be considered when designing a system incorporating one or more Digital Video Motion Detection systems (VMD). 2.1 Site Design for Video Motion Detection (VMD) By employing sophisticated detection algorithms, advanced VMDs have significantly reduced the number of nuisance alarms found with older technology VMD systems in outdoor environments. By following some simple site design techniques, the VMD will provide optimum performance, with high probability of target detection and low nuisance alarm rates. Section 2.1.1 to Section 2.1.3 describe some these techniques. These guidelines also apply to the design of any high quality Closed Circuit Television (CCTV) system and are not unique to VMD systems. 2.1.1 Lens Selection and Camera Distance When deciding on the lens required for a particular camera position, it is first necessary to decide; 1) The type of movement to be detected (e.g. walking people, crawling targets or cars) 2) The area to be covered (i.e. the field of view) 3) The level of security required A general rule for a high probability of detection under varying environmental conditions is that the target of interest should be at least 3 detection zones high or wide for VMDs that process relationships between detection zones. For VMDs that analyze on the pixel level then only 1 detection zone is required. Do not be confused by the number of programmable detection zones on the screen for units that have 1000 then 3 or more are required, units that have 280 require only on zone. When describing a camera lens system, the horizontal field of view is normally restricted to a maximum value. The maximum horizontal field of view determines the minimum likely size of a target and the maximum distance between the camera and the target for a particular lens. 2 Figure 2-1 The effect of increasing the lens focal length with a constant horizontal field of view. Figure 2-1 shows the effect of limiting the horizontal field of view to 25 meters (80 feet) on the maximum target distance for two different lenses. For a constant horizontal field of view, increasing the lens focal length increases both the maximum distance between the target and the camera, and the dead zone beneath the camera. A typical VMD camera lens design is normally based on a maximum horizontal field of view of 25 meters (80 feet) in good lighting conditions. At this distance, a standing person is approximately 3 to 4 detection zones in height (1000 zones VMD) or 1 detection zone height (280 zone VMD). Camera Format 1/3” Camera Height 3.5 meters (11.5 feet) Lens Focal Length 16mm Lens Focal Length 8 mm Horizontal Field of View 25 meters (80 feet) Horizontal Field of View 25 meters (80 feet) Maximum Distance between the target & camera 83 meters (273 feet) Maximum Distance between the target & camera 42 meters (137 feet) Dead-zone beneath the camera 114 meters (45 feet) Dead-zone beneath the camera 6 meters (20 feet) 3 If a larger field of view is used, the average light change within a detection zone is smaller and the video movement detection will be poorer. Consequently the sensitivity of the VMD would need to be increased, which could adversely affect the number of nuisance alarms, particularly during poor environmental conditions. The horizontal field of view should therefore be restricted to a maximum of 30 meters (100 feet). For medium to high security applications which require the detection of rolling or crawling target, the maximum horizontal field of view should be reduced to 20 meters, dependent on the environmental conditions of the application. After deciding on the maximum horizontal field of view for reliable detection, the maximum distance between the camera and the target is then calculated for different focal lengths. The formula is the following; Distance (Lens focal length) X (Maximum horizontal field of view) Between = (Camera format width) Camera And Target Where: Distance between the camera and target is in meters (or feet) Maximum horizontal field of view (using meters or feet as above) Lens focal length is in millimeters Camera format = 12.8 mm for a 1” camera Width (mm) = 8.8 mm for a 2/3” camera = 6.4 mm for a ½” camera = 4.4 mm for a 1/3” camera Table 2-1, Table 2-2 and Table 2-3 use this formula to show the maximum distance between the camera and the target for reliable detection, assuming a maximum horizontal field of view of 20, 25 and 30 meters. (66, 80, 100 feet) respectively, for some of the more commonly used lenses and 1/3” camera formats. The dead zone beneath the camera is also shown, for a camera mounted 3.5 meters (11 feet) above ground level. Most security applications would use the values given in Table 2-2, while high security applications may use Table 2-1. An example of a lens design is shown in Figure 2-2. The maximum horizontal field of view is selected to be 25 meters (80 feet) and the camera format is ½”. Note that the dead zone of one camera is covered by an adjacent camera. 4 1/3” Camera Format Lens Focal Length (mm) Maximum Distance Between Camera and Target (meters / feet) Dead Zone Beneath Camera (meters / feet) 4.8 6.0 8.0 12.0 16.0 25.0 35.0 50.0 20/66 25/82 33/108 50/164 67/220 104/341 (Note 1) 146/479 (Note 1) 208/682 (Note 1) 3/11 5/15 6/21 10/33 14/44 21/70 30/98 43/140 Table 2-1 – Lens Selection Chart High Security Applications 20 meter (66 feet) maximum horizontal field of view Camera height = 3.5 meters (11.5 feet) 1/3” Camera Format Lens Focal Length (mm) Maximum Distance Between Camera and Target (meters / feet) Dead Zone Beneath Camera (meters / feet) 4.8 6.0 8.0 12.0 16.0 25.0 35.0 50.0 25/82 31/103 42/137 63/205 83/273 130/427 (Note 1) 182/598 (Note 1) 260/854 (Note 1) 3/11 5/15 6/20 10/34 14/45 22/72 31/100 44/144 Table 2-2 – Lens Selection Chart Typical Security Applications 25 meter (80 feet) maximum horizontal field of view Camera height = 3.5 meters (11.5 feet) 5 1/3” Camera Format Lens Focal Length (mm) Maximum Distance Between Camera and Target (meters / feet) Dead Zone Beneath Camera (meters / feet) 4.8 6.0 8.0 12.0 16.0 25.0 35.0 50.0 30/98 38/125 50/164 75/246 100/328 156/512 (Note 1) 219/189 (Note 1) 313/1027 (Note 1) 4/13 5/16 7/22 11/36 14/46 23/75 32/104 45/146 Table 2-3 – Lens Selection Chart Absolute maximum values 30 meter (maximum) horizontal field of view Camera height = 3.5 meters (11.5 feet) Notes Ranges greater than 100 meters (325 feet) are not normally recommended due to the possibility of reduced visibility in poor weather conditions. Some sites may be restricted to lower ranges. Also, a longer focal length lens tends to ‘amplify’ the effects of camera movement in the image. A camera cannot be panned, tilted or zoomed without the risk of causing an alarm, unless video movement detection is inhibited while the view is being changed. This may be achieved with the external Detection Inhibit input available on most quality VMDs. 6 Figure 2-2 – Lens Selection Example Warehouse Office Building Storage 25mm 25mm 25mm 25mm 12mm 12mm 180 meters 590 feet 45 meters 150 feet Legend Camera Format 1/3” Camera Height 3.5 meters (11.5 feet) Maximum Horizontal Field of Vie w 25 meters (82 feet) Site Perimeter Camera Position & Lens Size Camera Field of View Camera Dead Zone below lens Opaque barrier along perimeter to reduce nuisance alarms 7 2.1.2 Camera Position and Mounting To minimize nuisance alarms, avoid mounting cameras; 1) Near lights (particularly infra-red illuminators), which could attract insects; 2) So that they point into lights, windows, the sun or in areas which have a large number of reflections or shadows; 3) So that they point at trees or plants which may move in the wind or drop leaves (note that tree shadows may also move in the wind); 4) So that they point into areas of vehicle headlight activity at night; 5) So that they point over water (consult the factory) 6) On poles or mounts which may flex or move in windy conditions The ‘dead zone’ under the camera also needs to be considered. Make sure that cameras are positioned (refer to Figure 2-2) so that this area is adequately covered by another cameras field of view. The area just in front of the dead zone can be vulnerable to very fast moving targets. It is a good policy to make sure that the field of view of the covering camera takes care of the dead zone plus another 10-15%. Also this may be corrected by VMDs that have perspective settings so different areas can be defined at different lengths from the camera. Important Note Vehicle head-lights sweeping the field of view at night should be avoided at all times. The VMD can detect and filter some of the resultant scene activity however, this activity can cause numerous shadows, shapes and sizes of various uploads/Geographie/ vmd-designers-guide.pdf