Occupancy Sensors Deliver Lighting on Demand

Sensors are wonderful things. Many of us have one by the front door, which turns lights on automatically when we get home late at night. This reassures us that no-one is lurking in the shadows, and gives us enough light so we can see to unlock the door. In the home, sensors provide benefits including convenience, security and cost savings (through having lights on only when required). In commercial buildings, these same benefits are multiplied many times over.

Lighting can account for up to 40% of the energy used in commercial buildings, and the cost of that energy is creeping up year by year. One of the easiest ways to reduce energy use and to cut costs (and emissions) is to turn off lights when they are not required. Manual light switches exist so that humans can turn lights on and off. Most of us are good at turning lights on, but we often forget to turn them off when leaving a room.

That’s where occupancy sensors come in. Originally designed for use with security systems, occupancy sensors have been refined and enhanced to control lighting and HVAC in commercial and residential spaces. These sensors detect activity within a specified area, and provide convenience by turning lights on automatically when someone enters. They also reduce costs and energy use by turning lights off soon after the last occupant has left.

Using occupancy sensors to turn lights off when areas are unoccupied helps to reduce energy waste and costs by between 35% and 45% (according to the California Energy Commission).

Most sensors are configurable, and can be adjusted for the required levels of sensitivity and accuracy. This helps to avoid false triggering, which can be caused by things like air movements from HVAC vents and the movement of warm air in front of a sunny window. Some sensors also allow you to set time delays between the sensor detecting a lack of occupancy and turning the lights off (usually between 10 and 15 minutes).

Occupancy sensors are best suited to areas where people spend variable amounts of time and often forget to turn lights off when leaving, such as meeting rooms and private offices. There are two main types of occupancy sensors used with lighting and building automation systems: Passive Infrared (PIR) and Ultrasonic.

Passive Infrared (PIR) sensors detect occupancy by passively measuring the infrared radiation being emitted from the objects in their view. Motion is detected when an infrared source (such as a person) passes in front of another infrared source with a different temperature (such as a wall). The PIR sensors react to the changes in heat patterns created by the moving person and turn lights on and off accordingly.

A curved faceted lens defines the field of view as a fan-shaped series of vertical and horizontal “cones” of detection projected from the sensor. The farther an occupant is from the sensor, the wider the gaps between these cones, and the larger a motion needs to be to trigger the device.

PIR sensors are highly resistant to false triggering, but are strictly line-of-sight and cannot “see” around objects or over partitions. These sensors are ideally suited to areas with little or no obstruction, such as small offices and meeting rooms.

Ultrasonic sensors emit an inaudible high-frequency (25-40 kHz) sound wave, which bounces off objects, surfaces and people. When the waves bounce back to the sensor, their frequency is measured. These sensors can “see” around objects and surfaces as long as the surfaces are hard enough to bounce back the sound waves for detection.

Ultrasonic sensors are sensitive to all types of motion and generally have zero coverage gaps (being able to detect movements not within line of sight). However, they are more expensive than PIR sensors, are more prone to false triggering, and may interfere with other ultrasonic sensors or hearing aids.

These sensors are best suited for indoor use, in areas that are large, contain obstructions, or are unusually shaped such as open offices, large conference rooms, and restrooms. Dual technology/Hybrid sensors combine both PIR and ultrasonic technology to deliver maximum reliability and coverage with a minimum of false triggers.

These sensors allow for wide coverage and are suitable for a wide range of applications. However, they are more expensive than PIR or ultrasonic sensors, and typically require more adjustments. Hybrid sensors are a good choice for large open areas, and for areas with unusual occupancy patters or work requirements.

Occupancy sensors can be mounted on the ceiling or on the wall (like a light switch). Careful planning is required to ensure sensors are located where they will detect occupancy and occupant activity in all parts of the room. In a small area, such as a private office, a single sensor will usually provide sufficient coverage. In larger spaces, multiple sensors will likely be needed to obtain full coverage. (Be aware also that coverage and range can vary between sensor manufacturers.)

It is important to get the sensitivity setting correct, which determines the amount of movement required to trigger lights to turn on, stay on, or turn off. If the sensitivity is too high, the sensor might turn lights on even though the area is unoccupied. Setting it too low might leave your occupants in the dark!

Time delay settings specify the amount of time the sensor waits between perceiving the room is empty and turning the lights off. Shorter time delays produce higher energy savings, but may shorten lamp life due to more frequent switching. Longer delays avoid continual on-off cycles in areas where occupants enter and leave frequently. They also help to overcome brief periods when an occupant is moving very little. Manufacturers often recommend a minimum time delay of 15 minutes.

You also need to ensure that the lamps you are using are suitable for occupancy sensing. For example, HID lamps require long warm-up times, so are not suited to being switched on and off by occupancy sensors. CFLs can also be sensitive to rapid on/off cycling in situations where only brief illumination is required. Better results for these lamps may be obtained by using scheduling to switch between low power and full power (dimming).

Many of today’s commercial lighting and building automation systems use occupancy sensors to turn lights on and off based on whether or not an area is occupied. Not only does this reduce energy consumption and save money, it also helps buildings to comply with the “automatic shut-off of building lighting” requirements of energy codes such as ASHRAE 90.1 and California Title 24.

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