Last month, we began this series talking about closed circuit TV monitoring. In this month's article, we will begin covering the components of video systems. We won't get through all of them in one article, but we will be able to cover the most basic component — cameras — fairly well.
You don't have to understand every minute detail of these components, but it is important that you understand what these items are, how they operate and how they interact with all the other components.
The principal elements of a CCTV system are:
Video cameras do not “see” things in the same way that the human eye sees. The human eye adjusts to various focal lengths and light conditions automatically and usually faster than can be noticed. Video cameras cannot do so. They are limited to fixed fields of view, focal lengths and light sensitivities. Any applications of video cameras must be made with these facts in mind. In fact, in designing a system, it is best to go to the job site with a camera in hand to verify that the cameras actually pick up the desired images and fields of view.
Cameras currently come in four basic varieties. They are as follows:
Vidicon cameras.
SIT (silicon intensified target) cameras.
CCD (charged coupled device) cameras.
MOS (metal oxide semiconductor) cameras.
Vidicon cameras used to be the most commonly used type. They are relatively inexpensive and are well suited to most indoor uses. With normal indoor lighting, vidicon tubes provide an acceptable quality of signal. They are available in ⅔-in. and 1-in. formats to accommodate higher and lower levels of indoor light. (The format defines the size of the photosensitive area of the pickup device.) The larger format sensors have greater sensitivity and resolution. The vidicon is susceptible to damage from very bright lights and therefore cannot be used in outdoor locations.
SIT cameras are suitable for areas with light levels varying from low levels to bright sunlight. They are often used for outdoor parking lots, underground parking garages and similar areas. For special applications with extremely low levels of lighting, an intensified version of the SIT cameras, called the ISIT, is available.
SIT cameras respond much better to blue light than they do to a yellowish tint. This means that they will work well in areas with mercury or metal-halide lighting but will not work nearly as well in areas with high-pressure sodium lighting. (They should not be considered at all for areas with low-pressure sodium lighting, which is purely yellow.)
CCD cameras are an entirely solid-state type of camera. They are now the most popular type because their prices have dropped dramatically in the past few years. They have excellent operating characteristics. (They are now so good that they are sometimes used for TV studio work.) These cameras exhibit no geometric distortion, no lag and no image retention.
MOS cameras are also solid-state devices, and come in only the ½-in. format. MOS cameras do not do not perform quite as well as the CCD cameras, but they were often used in the past because they are less expensive than CCDs.
Solid-state cameras are quite durable, being able to cope with vibrations and even magnetic fields with very little difficulty.
Video lenses come in ½-in., ⅔-in., and 1-in. sizes to match video cameras. However, it is possible to use a lens that is larger than the format of the camera. In other words, you can use a 1-in. lens on a ⅔-in. camera. You cannot, however, use a ⅔-in. lens on a 1-in. camera.
The most important factor in choosing lenses is that they must be matched to the area you are trying to monitor. The proper focal length (the distance at which the camera is properly focused) and field of view must be coordinated to get the desired results.
Table 1 shows the calculations required for determining several key dimensions for ½-in. and ⅔-in. lenses.
It is recommended that all lenses be equipped with an automatic iris, except for those on vidicon cameras (which are used in indoor locations with a more or less constant light level). The iris controls the amount of light entering the camera. This keeps the light level reaching the sensor within acceptable limits. Sensing automatically brings the signal up or down to the proper level.
Most auto-iris lenses include a spot filter, which increases the range of light levels to which the camera will respond. This is usually mandatory for outdoor cameras due to the wide range of light conditions encountered.
It should go without saying that surveillance cameras should be firmly mounted. In addition to standard wall-mounting brackets, cameras may also be mounted in vandal-resistant cases, which are recommended in trouble-prone areas. Mountings are also available with built-in panning and tilting mechanisms. Obviously, these will cost a bit more than the standard mountings but will provide an additional benefit. However, they cannot be used in all installations, particularly not with video motion detector devices.
Outdoor cameras must (obviously) be protected from the elements. Many types of outdoor housings exist, primarily designed to shield the camera from rain, wind and direct sunlight.
In outdoor locations, however, wind and rain are not the only hazards. Vandalism is also a serious issue. To counteract this, a number of specialty housings have been developed, usually designated as “vandal-proof.”
This year, Iowa State University has completely overhauled its electrical courses to meet state requirements. Eleven courses are now being offered and a Code Changes course will be added soon. Usually, one or two of these will meet the requirements in your state. The courses are:
Of course, you can take any of these courses at any time for your own improvement; but to meet your renewal requirements, you will have to take the courses that are required by your state.
Since the requirements vary for each state, a public Web site has been set up at www.ElectricRenewal.com. All registration information, course descriptions and state requirements can be found there. You can also call the University office at (800)262-0015 or fax the office at (515)294-6223.
| Dimensions | ½-in. lens | ⅔-in. lens |
|---|---|---|
| Focal length, determined by height | 4.4 times distance/height | 6.6 times distance/height |
| Focal length, determined by width | 5.9 times distance/width | 8.8 times distance/width |
| Field height, determined by focal length | 4.4 times distance/focal length | 6.6 times distance/focal length |
| Field width, determined by focal length | 5.9 times distance/focal length | 8.8 times distance/focal length |