| Sensors are wonderful things. Many of us have one | | | | (25-40 kHz) sound wave, which bounces off objects, |
| by the front door, which turns lights on automatically | | | | surfaces and people. When the waves bounce back |
| when we get home late at night. This reassures us | | | | to the sensor, their frequency is measured. These |
| that no-one is lurking in the shadows, and gives us | | | | sensors can "see" around objects and surfaces as |
| enough light so we can see to unlock the door. In | | | | long as the surfaces are hard enough to bounce back |
| the home, sensors provide benefits including | | | | the sound waves for detection. |
| convenience, security and cost savings (through | | | | Ultrasonic sensors are sensitive to all types of motion |
| having lights on only when required). In commercial | | | | and generally have zero coverage gaps (being able to |
| buildings, these same benefits are multiplied many | | | | detect movements not within line of sight). However, |
| times over. | | | | they are more expensive than PIR sensors, are more |
| Lighting can account for up to 40% of the energy | | | | prone to false triggering, and may interfere with |
| used in commercial buildings, and the cost of that | | | | other ultrasonic sensors or hearing aids. |
| energy is creeping up year by year. One of the | | | | These sensors are best suited for indoor use, in |
| easiest ways to reduce energy use and to cut costs | | | | areas that are large, contain obstructions, or are |
| (and emissions) is to turn off lights when they are | | | | unusually shaped such as open offices, large |
| not required. Manual light switches exist so that | | | | conference rooms, and restrooms. Dual technology |
| humans can turn lights on and off. Most of us are | | | | Hybrid sensors combine both PIR and ultrasonic |
| good at turning lights on, but we often forget to turn | | | | technology to deliver maximum reliability and |
| them off when leaving a room. | | | | coverage with a minimum of false triggers. |
| That's where occupancy sensors come in. Originally | | | | These sensors allow for wide coverage and are |
| designed for use with security systems, occupancy | | | | suitable for a wide range of applications. However, |
| sensors have been refined and enhanced to control | | | | they are more expensive than PIR or ultrasonic |
| lighting and HVAC in commercial and residential | | | | sensors, and typically require more adjustments. |
| spaces. These sensors detect activity within a | | | | Hybrid sensors are a good choice for large open |
| specified area, and provide convenience by turning | | | | areas, and for areas with unusual occupancy patters |
| lights on automatically when someone enters. They | | | | or work requirements. |
| also reduce costs and energy use by turning lights | | | | Occupancy sensors can be mounted on the ceiling or |
| off soon after the last occupant has left. | | | | on the wall (like a light switch). Careful planning is |
| Using occupancy sensors to turn lights off when | | | | required to ensure sensors are located where they |
| areas are unoccupied helps to reduce energy waste | | | | will detect occupancy and occupant activity in all |
| and costs by between 35% and 45% (according to | | | | parts of the room. In a small area, such as a private |
| the California Energy Commission). | | | | office, a single sensor will usually provide sufficient |
| Most sensors are configurable, and can be adjusted | | | | coverage. In larger spaces, multiple sensors will likely |
| for the required levels of sensitivity and accuracy. | | | | be needed to obtain full coverage. (Be aware also |
| This helps to avoid false triggering, which can be | | | | that coverage and range can vary between sensor |
| caused by things like air movements from HVAC | | | | manufacturers.) |
| vents and the movement of warm air in front of a | | | | It is important to get the sensitivity setting correct, |
| sunny window. Some sensors also allow you to set | | | | which determines the amount of movement required |
| time delays between the sensor detecting a lack of | | | | to trigger lights to turn on, stay on, or turn off. If |
| occupancy and turning the lights off (usually between | | | | the sensitivity is too high, the sensor might turn lights |
| 10 and 15 minutes). | | | | on even though the area is unoccupied. Setting it too |
| Occupancy sensors are best suited to areas where | | | | low might leave your occupants in the dark! |
| people spend variable amounts of time and often | | | | Time delay settings specify the amount of time the |
| forget to turn lights off when leaving, such as | | | | sensor waits between perceiving the room is empty |
| meeting rooms and private offices. There are two | | | | and turning the lights off. Shorter time delays |
| main types of occupancy sensors used with lighting | | | | produce higher energy savings, but may shorten lamp |
| and building automation systems: Passive Infrared | | | | life due to more frequent switching. Longer delays |
| (PIR) and Ultrasonic. | | | | avoid continual on-off cycles in areas where |
| Passive Infrared (PIR) sensors detect occupancy by | | | | occupants enter and leave frequently. They also help |
| passively measuring the infrared radiation being | | | | to overcome brief periods when an occupant is |
| emitted from the objects in their view. Motion is | | | | moving very little. Manufacturers often recommend a |
| detected when an infrared source (such as a person) | | | | minimum time delay of 15 minutes. |
| passes in front of another infrared source with a | | | | You also need to ensure that the lamps you are |
| different temperature (such as a wall). The PIR | | | | using are suitable for occupancy sensing. For example, |
| sensors react to the changes in heat patterns | | | | HID lamps require long warm-up times, so are not |
| created by the moving person and turn lights on and | | | | suited to being switched on and off by occupancy |
| off accordingly. | | | | sensors. CFLs can also be sensitive to rapid on/off |
| A curved faceted lens defines the field of view as a | | | | cycling in situations where only brief illumination is |
| fan-shaped series of vertical and horizontal "cones" of | | | | required. Better results for these lamps may be |
| detection projected from the sensor. The farther an | | | | obtained by using scheduling to switch between low |
| occupant is from the sensor, the wider the gaps | | | | power and full power (dimming). |
| between these cones, and the larger a motion needs | | | | Many of today's commercial lighting and building |
| to be to trigger the device. | | | | automation systems use occupancy sensors to turn |
| PIR sensors are highly resistant to false triggering, | | | | lights on and off based on whether or not an area is |
| but are strictly line-of-sight and cannot "see" around | | | | occupied. Not only does this reduce energy |
| objects or over partitions. These sensors are ideally | | | | consumption and save money, it also helps buildings |
| suited to areas with little or no obstruction, such as | | | | to comply with the "automatic shut-off of building |
| small offices and meeting rooms. | | | | lighting" requirements of energy codes such as |
| Ultrasonic sensors emit an inaudible high-frequency | | | | ASHRAE 90.1 and California Title 24. |