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Video Cameras
(Chapter 2   Video Surveillance, Continued)

Lighting requirements and nighttime applications

Most schools generally will not attempt to use exterior CCTV cameras during the nighttime because of the high light levels that are required.

For exterior nighttime CCTV applications, proper lighting is very important. A number of lighting types are available. These types include incandescent, fluorescent, and high-intensity discharge. Incandescent lighting is the most expensive to operate and includes the flood or quartz lights that are commonly used for exterior home security applications. Most fluorescent lighting is used indoors for office and work area lighting. High-intensity discharge lighting is the least expensive to operate (more light is produced with less power consumption) and is the most common for commercial exterior lighting applications. It includes high-pressure sodium and low-pressure sodium lighting. A disadvantage of high-intensity discharge lighting is the restrike time. If a momentary power outage occurs, these lights will go out and can take up to several minutes to return to full brightness. The advantages of high- and low-pressure sodium lighting, however, outweigh this disadvantage for CCTV applications.

Low-pressure sodium lighting is the most desirable choice for exterior CCTV applications because it is somewhat more efficient to operate than high-pressure sodium, and the types of light fixtures available provide a fairly uniform light pattern. A disadvantage to low-pressure sodium is the monochromatic yellow light it produces, which some people find objectionable.

Important items to consider for nighttime camera lighting are illumination level, camera sensitivity, lens type, light-to-dark ratio, area of illumination in the camera field of view, and lighting position. Note: These are not simple issues to be addressed by a neophyte. Be certain that you discuss lighting issues with your local power company or lighting expert.

Illumination level, camera sensitivity, and lens type. Lighting levels must be high enough for the camera to produce a useable image. The light level required will depend on camera sensitivity and lens type and quality. Black-and-white cameras generally have more light sensitivity than color cameras and are recommended for most nighttime applications. A minimum illumination level of 1.5 foot-candles, as measured on a horizontal plane 1 foot off the ground, is recommended for a black-and-white camera with a sensitivity specification of 0.007 foot- candles faceplate illumination. This assumes the camera has a good-quality, F/1.4 fixed focal lens. A color camera or a camera with a zoom lens will require a higher light level in order to get equivalent brightness and contrast.

Light-to-dark ratio. A recommended maximum light-to-dark lighting ratio is 6 to 1 (as measured on a horizontal plane 1 foot off the ground). This maximum applies to the entire area of interest that the camera is viewing. It is also recommended to design the lighting for a 4-to-1 ratio to allow for some degradation over time. A 6-to-1 light-to-dark ratio will prevent areas that are so dark or so bright that a person or object would be obscured.

Area of illumination in the camera field of view. A minimum illumination of 70 percent of the camera field of view is recommended. A camera is an averaging device. If too little of the field of view is illuminated, the camera will average between the illuminated areas and the nonilluminated areas, resulting in blooming and loss of picture detail in the illuminated area.

Lighting position. The position of lighting in relation to the camera field of view is also important. As much as possible, light sources must be kept out of the camera's field of view. Lights that are illuminating a camera scene should be mounted higher than the cameras. When determining a location and field of view for a camera, extraneous light sources, such as building-mounted lighting for pedestrians that will be in the camera view, must be considered. Extraneous light sources can cause blooming and streaking in a camera, rendering portions of the field of view unusable. Distant light sources that are relatively dim are usually not a problem.

Other lighting. Another type of lighting is known as infrared (IR) or near infrared. The spectrum for this lighting is just below red and is not visible to the human eye. Most black-and-white cameras have sensitivity into the infrared. A black-and-white camera can be used with this type of lighting to observe areas at night without having lighting that is visible to humans. To make use of IR lighting, the camera must not have an IR cut filter. Cameras can be ordered without IR cut filters; be sure to specify no IR cut filter when ordering.

Commercial IR light sources include incandescent and the light emitting diode (LED). The incandescent type typically use a 300- to 500-watt lamp and a visible light cut filter. These are expensive to purchase ($800-$1,200) and expensive to operate and maintain (2,000 hours is a nominal life expectancy of the incandescent lamp). The LED type emits light in the IR and is also expensive to purchase (around $1,200) but uses less power and has a much longer life expectancy. The incandescent type will provide more illumination than the LED type. With either type of IR light, more light fixtures will be required to illuminate an area than with visible lighting. While IR lighting has the advantage of not being visible to humans, it is fairly expensive.

Alternatives to lighting. There are two camera technologies that can see at night without the use of artificial lighting. These technologies are intensified cameras and thermal cameras, though they are probably both cost-prohibitive for most schools. Intensified cameras use a photomultiplier (light intensifying) tube in front of the camera imaging device. Depending on the generation of the photomultiplier tube, these cameras can produce a picture in conditions ranging from moonlight to starlight. Disadvantages of these cameras include initial costs, maintenance costs, and lower resolution. Costs for an intensified camera can begin around $8,000. The photomultiplier tube has a life expectancy in the range of 8,000 to 10,000 hours, requiring replacement every 1-2 years depending on the amount of use. In terms of horizontal TV lines, intensified cameras have lower resolution than a good-quality surveillance camera.

Thermal cameras are sensitive to thermal energy radiated by objects. The low-end and minimum-performance thermal cameras start around $7,000. The high-performing thermal cameras range up to $30,000 and require equipment for cooling the thermal imaging device. This cooling equipment can be maintenance intensive. Resolution is also lower than in general CCTV surveillance cameras. Uncooled cameras are currently coming down in price and may offer a better alternative in the future.

 



Research Report:   The Appropriate and Effective Use of Security Technologies in U.S. Schools