Occupancy sensors are indoor lighting controls that detect activities within certain area. They provide convenience by turning lights on automatically when someone enters a room and reduce lighting energy use by turning lights off soon after the last occupant has left the room. This technology also incorporates sensing of natural light to adjust the level of artificial lighting, based on the adequacy of the available natural light. Some types of photo-sensitive controls operate by turning the controlled lights on or off based on natural light levels, while others operate by continuously dimming the controlled lights. A wide variety of technologies are involved to detect occupants and send appropriate signals to area lighting.

Lighting controls play a critical role in electric lighting systems, providing the function of turning the lights on and off using a switch or adjusting light output up and down using a dimmer. Properly specified and installed, lighting controls help managers achieve an organization’s goals for lower energy costs, improved worker satisfaction and greater flexibility.

The energy consumption of lighting can reach 20-60% of total electric consumption in any office building or at any private or public places. On national scale the reduction of this energy consumption can be vital. One of the most effective energy savings opportunities not typically employed by conventional, discrete component control systems is some form of occupancy control strategy. Energy-efficient lighting control of buildings includes infrared/ultrasonic occupancy devices.

Dimming increases lamp life and saves energy - a light that is dimmed by 25% uses about 20% less energy while lamp life is increased fourfold. Automatic lighting controls can save energy and improve safety. They offer convenience to homeowners and can be a high-tech portion of a whole-house automation system.

Today, nearly 60% of an average business’s energy usage occurs in lighting its facilities. Next to heating and air conditioning systems, lighting systems account for the greatest energy consumption and costs. These significant costs can be managed more effectively through the use of lighting controls. Over the past several years, the use of lighting controls has increased throughout the country and organizations that have implemented lighting controls have realized a wide range of benefits. Some of the benefits Include:

Energy savings: Perhaps the primary benefit is that of energy savings. Lighting controls can result in energy savings of more than 30%, reducing building operating costs by 10% or more.

Convenience: When selected and implemented correctly, lighting controls operate transparently in the background, enabling users to reduce their energy usage conveniently and without disruption for building occupants.

Flexibility: Many lighting control systems are designed for maximum flexibility, to accommodate changes in workspace configuration, schedules, and activities.

Information: Some lighting control systems can provide a great deal of information about lighting energy usage, identifying when and how much energy is being used by a specific department or facility space. This information facilitates the ability to divide overhead costs among departments accurately.

Productivity: One less tangible benefit of significant economic value to businesses is the increase in employee productivity that can result from optimal lighting 

This is an obvious choice for areas with access to natural light, such as atriums, perimeter offices, hall ways, and other types of areas with skylights. Most exterior lighting will be ideally suited for light level control as well, such as parking areas, customer service areas (i.e., drive-through facilities), and building entranceways.

Occupancy based lighting can be efficiently used in

• Offices 
• Factories 
• Workshops 
• Schools and libraries 
• Hospitals 
• Canteens 
• Staff rooms 
• Residential homes 
• Corridors and stairwells 
• Military accommodation 
• Student accommodation 
• Rest rooms 
• Shopping malls and in many other places 

Determining the timing of automatically switching off the lights after the occupied space is vacated is quite challenging when trade-off between occupant's comfort and energy saving is concerned. For instance, traded off of the lights when the occupancy occurs may cause annoyance. Use of occupancy sensors to control lighting, heating and cooling according to motion detected within an intermittently occupied area can solve this problem. Occupancy sensors can save up to 80% of the lighting and HVAC energy when properly applied.

There are three basic sensor technologies involved: infrared, ultrasonic, and acoustic.

Infrared (IR) technology senses body heat. IR requires a straight “line-of-sight” in order to operate properly. IR is ideal for small offices and other regularly shaped rooms as well as high spaces (auditoriums, open classrooms, large open offices with low or no partitions, factories and other large work facilities).  

Ultrasonic (US) technology emits a high-frequency sound that reflects off room surfaces. US sensors have good sensitivity and range where small motions must be detected. They suit irregularly shaped spaces and room obstructions such as medium to high partitions, large furniture or structural columns. Due to the high sensitivity associated with US, air currents or other small movements produced by the ventilation system or motion in adjacent spaces may trigger false-on conditions, requiring attentive calibration. Ultrasonic sensors are available in a variety of frequencies; be sure to note any other ultrasonic emitting equipment on the job and specify a distinct frequency for each controller. Check that sensor frequency does not overlap hearing-aid frequency.

Acoustic or audible sensors rely on voices, machinery sounds, keyboard tapping and other typical daily noises. Background noise, such as a constant hum, and low-level noise are ignored. This technology works well in areas with high partitions or other obstructions, or high air movement within the space during unoccupied periods, such as kitchens and large washrooms.

Dual or triple technology sensors are available, as are intelligent sensors that self-adjust to occupancy data collected in a prescribed “learning period”. These sensors also reduce false-on and -off conditions. However, they must still be properly located, adjusted and calibrated in commissioning, and regularly maintained.  

Sensors have a field of view and take care in locating the mounting position to cover the occupied area of interest according to the manufacturer’s recommendations. The correct position will vary with the sensor’s coverage pattern. The filters present in the existing sensors generally have a low pass filter of around 1 second, and a large time constant. The term "one second" to describe the filter is used for the following reason. Usually human motion frequencies are in the range of 0.1-1 Hz. The filters are typically designed around this frequency and the filter attenuates DC components, which typically depend on temperature and vary greatly with the manufacturing processes, and higher frequencies which are generated not by human body motion but by other existing noise factors. Accordingly, the sensor looks at the change in the signal and not the signal itself. As such, any frequency information about the signal below this cutoff is lost. The frequency information that is lost is typically outside the frequency band of from about 0.25-1 Hz to from about 10-15 Hz or other suitable frequencies. The digital signal path occupancy sensor, on the other hand, can have access to the noise information related to the PIR signal as well as to frequency information that otherwise would have been filtered out by the low pass filter.

These should be adjusted after lamps and sensors are installed, room furnishings are in place, fluorescent lamps have burned-in for 100 hours, and HVAC systems are operating.

Important options available include:

A wide variety of technological devices and systems are available with us to control lighting systems. The most common of these are:

Caution:

Hours of fluorescent lamp life will be reduced (up to 40%), but calendar lamp life will be extended.

Features to be considered for the Selection of Occupancy Sensors:

To select the correct sensor technology determine whether passive-infrared (PIR), ultrasonic or dual-technology sensors are best matched to the application requirements.
PIR sensors sense the difference in heat emitted by humans in motion from that of the background space. These sensors detect motion within a field of view that requires a line of sight; they cannot "see" through obstacles and have limited sensitivity to minor (hand) movement at distances typically greater than 15 feet. The sensor is most sensitive to movement laterally across the sensor's field of view. The sensor's field of view can be adjusted.

PIR sensors are most suitable for smaller, enclosed spaces (wall switch sensors), spaces where the sensor has a view of the activity (ceiling and wall-mounted sensors), and outdoor areas and warehouse aisles. Incompatible application characteristics include low motion levels by occupants, obstacles blocking the sensor's view, mounting on sources of vibration, or mounting within six to eight feet of HVAC air diffusers.

Ultrasonic sensors utilize the Doppler principle to detect occupancy through emitting an ultrasonic high-frequency signal throughout a space, sense the frequency of the reflected signal, and interpret change in frequency as motion in the space. These sensors do not require a direct line of sight and instead can "see" around corners and objects, although they may need a direct line of sight if fabric partition walls are prevalent. In addition, ceiling-mounted sensor effective range declines proportionally to partition height. They are more effective for low motion activity, with high sensitivity to minor (hand) movement, typically up to 25 feet. The sensor is most sensitive to movement to and from the sensor. Ultrasonic sensors typically have a larger coverage area than PIR sensors. The sensor's view cannot be adjusted.

Ultrasonic sensors are most suitable for open spaces, spaces with obstacles, restrooms and spaces with hard surfaces. Incompatible application characteristics include high ceilings (more than 14 feet), high levels of vibration or air flow (which can cause nuisance switching), and open spaces that require selective coverage (such as control of individual warehouse aisles).

Dual-technology sensors employ both PIR and ultrasonic technologies, activating the lights only when both technologies detect the presence of people, which virtually eliminates the possibility of false-on, and requiring either one of the two technologies to hold the lights on, significantly reducing the possibility of false-off. Appropriate applications include classrooms, conference rooms and other spaces where a higher degree of detection may be desirable.
Regardless of sensor type, it should activate the lights as soon as the person enters the room, but should not monitor the area outside the door to avoid nuisance switching. The door swing should not obstruct the view of the sensor.