Using Monitors on their Side
Another way of looking at monitors
The aspect ratio of a standard CCTV picture is 4 units wide by 3 units high, typically complicated by being based on the diagonal measurement of the tube or sensor. A 12" monitor would have dimensions of about 220mm wide by 165mm high. Camera lenses have the vertical and horizontal angles of view in the same proportions.
It is standard practice to set up cameras and monitors to view in this normal plane, but is this always the best way to look at a scene?
Many systems protecting a perimeter are looking along a long narrow field of view as illustrated in the diagram 1. This shows the view on a monitor in its normal orientation and with the camera mounted conventionally. The field of view will be determined by the vertical angle of the lens. It can be seen that there is a great deal of the screen showing areas not necessarily important in relation to the scene being monitored.
If the camera is turned through 90° and also the monitor, as in diagram 2, the part of the scene being monitored is now represented in greater detail. This is because the orientation of the required scene is in a better relationship with that of the monitor.
(Believe it or not, the two screens shown are the same size and the corect ratio)
It is not just a question of rotating the camera and monitor, because the field of view will now be determined by the horizontal angle instead of the vertical angle of the lens. As previously stated this is in the ratio of 4:3, therefore a lens with a longer focal length will be required. This is quite straightforward. Having calculated the required lens using the vertical angle, simply find a lens that has the same horizontal angle of view. For instance a 12.5mm lens has horizontal and vertical angles of view of28.4° and 21.3° respectively. A 16mm lens has horizontal and vertical angles of 22.3° and 16.8° respectively. Therefore the horizontal angle of the 16mm lens is nearly equal to the vertical angle of the 12.5mm lens.
The longer focal length lens, though, produces a larger image on the screen for the same scene content. This is illustrated in diagram 3. The angle of view is the same in both diagrams.
It can be seen therefore that more of the important part of the scene is displayed when the camera and monitor are rotated through 90°.
This is obviously a somewhat controversial point of view and in reality must take into consideration factors of other cameras to be viewed which may require the conventional arrangement. However there may be occasions when some lateral thinking may pay dividends.
This chapter is supplied by Mike Constant and was originally published in CCTV Today. Mike is the author of 'The Principles & Practice of CCTV' which is generally accepted as the benchmark for CCTV installation in the UK.