In the field of physical security, security lighting is lighting that intended to deter or detect intrusions or other criminal activity on a piece of real property. It can also be used to increase a feeling of safety. Lighting is integral to crime prevention through environmental design.
Security lighting can be counter-productive. Turning off lights halved the number of thefts and burglary in Övertorneå Sweden. A test in West Sussex UK showed that adding all-night lighting in some areas made people there feel safer, although crime rates increased 55% in those areas compared to control areas and to the county as a whole.
While adequate lighting around a physical structure is deployed to reduce the risk of an intrusion, it is critical that the lighting be designed carefully as poorly arranged lighting can create glare which actually obstructs vision. Studies have shown that many criminals are aware of this effect and actively exploit it. The optimal design will also depend on whether the area will be watched directly by humans or by closed-circuit television, and on the location of the observers or cameras.
Security lighting may be subject to vandalism, possibly to reduce its effectiveness for a subsequent intrusion attempt. Thus security lights should either be mounted very high, or else protected by wire mesh or tough polycarbonate shields. Other lamps may be completely recessed from view and access, with the light directed out through a light pipe or reflected from a polished aluminium or stainless steel mirror. For similar reasons high security installations may provide a stand-by power supply for their security lighting.
Some typical considerations include:
- Reduce and prevent glare and situations mentioned above
- Shielded or full cut-off (FCO) lamp housings which conceal the bulb could be used, which should direct light onto the ground or target and away from observers. These lights should send no light above 80 degrees from the nadir. Lighting should be bright enough, and not "as bright as possible". In many cases a good rule of thumb is 0.5 watts per square metre (0.05 watts per square foot). This might need to be increased in very confused environments, but conversely can be reduced in very open environments. Multiple lamps of moderate power instead of a few powerful lamps will reduce glare, provide more even illumination with reduced pools of shadow, and provide some redundancy if one lamp's bulb blows out or develops a bad ballast.
- Prevent malicious tampering or interference. This means that besides the lamp itself, the entire circuit from the source (electric company or generator), through the wires, to the lamp and back should be protected.
- Luminaires should be accessible so that the maintainer can replace blown bulbs as quickly as possible and clean the luminaires periodically. However they should be protected or somehow made inaccessible to tampering.
- Ensure the electric meter box is locked or inaccessible, or else power the lights from a different line.
- Control and power lines, where outside or vulnerable, should be either buried well underground (in conduits preferably) or at a height of at least 8 metres (about 24 feet).
- Ideally multiple circuits should be used to prevent an accidental or malicious short or cut causing all illumination to fail.
Security lighting can be used in residential, commercial, industrial, institutional, and military settings. Some examples of security lighting include floodlights and low pressure sodium vapour lights. Most lights intended to be left on all night are high-intensity discharge lamps as these have good energy efficiency, thus reducing the cost of running a lamp for such long periods.
A disadvantage of low pressure sodium lamps is that the colour is pure yellow, so the illuminated scene is seen without any colour differentiation. Consequently, high pressure sodium vapour lamps (which are still yellowish, but closer to golden white) are also used, at the cost of greater running expenses and increased light pollution. High pressure sodium lamps also take slightly longer to restrike after a power interruption.
Other lights may be activated by sensors such as passive infrared sensors (PIRs), turning on only when a person (or other mammal) approaches. PIR activated lamps will usually be incandescent bulbs so that they can activate instantly; energy saving is less important since they will not be on all the time. PIR sensor activation can increase both the deterrent effect (since the intruder knows that he has been detected) and the detection effect (since a person will be attracted to the sudden increase in light). Some PIR units can be set up to sound a chime as well as turn on the light. Most modern units have a photocell so that they only turn on when it is dark.
An important limitation to the usefulness of security lighting is the simple fact that it is only useful at night. This is particularly significant for home owners because, contrary to a widespread myth, most household burglaries occur during the day, when the occupants are away at work or shopping.
As with any lighting, security lighting can reduce night vision, making it harder to see into areas that are unlit or are in shadow.
- "Outdoor Lighting and Crime, Part 2" by Barry Clark 2003, p. 18 and Figure 5
- "Svenska Dagbladet": "Halverad brottslighet i mörk kommun" (Halved crime in the dark city.) 2007 (The number of thefts and burglaries have halved in Övertorneå since the city was dark in the fall because of the nationwide family dispute with Ekfors Kraft. We thought it would be the opposite, says Sören Mukkavaara, police constable in Övertorneå.)
- "Light and Crime"
- Schneier on Security "Light and Crime"
- "The Dark Side: Making war on light pollution" by David Owen 2007
- "Sourcebook of criminal justice statistics Online": "Percent distribution of burglaries known to police. By place and time of occurrence, United States, 1976-2007"
- "Lighting & Crime" Archived 2009-06-03 at the Wayback Machine from the British Astronomical Association's Campaign for Dark Skies