The most effective way to save lighting energy is to make sure that lights are off in unoccupied space. Occupancy sensors are designed to perform this task automatically.
Occupancy sensors can produce tremendous energy savings in commercial facilities. Open offices areas typically realize a 10% savings in lighting energy while conference rooms, restrooms, and other infrequently used spaces can realize savings up to 50%. The reduction in lighting energy will also reduce the overall cooling load in facility. Air conditioning savings during the hotter months can amount to an additional 20% savings.
There are three types of technologies used in occupancy sensors: infrared, ultrasonic, and microwave. Infrared sensors are passive devices, receivers which simply detect the heat given off by the human body. Ultrasonic and microwave sensors are active devices which transmit a signal into the space and respond to changes in the signal as it returns to the unit. Occupancy sensors can be configured for remote or wall mounting.
Occupancy sensors can be used in a variety of ways in an assortment of spaces. They are generally most useful in spaces where lights are typically left on but occupancy is intermittent or infrequent, like these:
- Private Offices
- Open Offices
- Store Rooms
- Conference rooms
- Airport Gate areas
- Computer rooms
Energy savings will depend on the type of space. For example, an open office may only experience a 10% energy savings while a restroom may see 50 to 75 percent savings.
Wall mounted occupancy sensors are most commonly used in smaller spaces such as individual offices. These units are particularly useful in retrofit applications because they quickly installed in place of existing wall switches. Wall mounted units provide adequate sensitivity for hand motion up to 300 square feet of and larger motions up to 750 square feet.
The sensor's field of view are adjustable on many wall mounted sensors. This is helpful because there are many variations in office layouts. Adjusting the field of view helps to eliminate the problem of sensing activity outside the office. For example, if the wall mounted unit is improperly placed and calibrated, it could detect motion in the corridor just outside the office. In this type of case the lighting would turn on unnecessarily when people walk down the hall.
The performance of an occupancy sensor depends largely on its placement. This should be considered during the design phase of a lighting project. Manufacturers usually provide information regarding the sensor's coverage pattern and sensitivity. Obviously, this information is essential in determining the number of units to install and where to install them.
In an open office, unobstructed by walls, bookshelves, or cubicles. Either passive infrared or ultrasonic devices will work in this case. However, where a portion of the space is shielded from the line of sight of the sensor, ultrasonic devices can deal with this obstacle while infrared sensors cannot.
This diagram shows how placement and sensitivity are important. An ultrasonic sensor installed near an air-conditioning vent may detect air motion and trigger the device unnecessarily. In this example, the sensor should either be moved to another location or replaced with a passive infrared device sensor.
Coverage, of course, depends on sensor type and location. This diagram shows an appropriate placement for an occupancy sensor. All of the space is covered with little or no wasted coverage.
This diagram, on the other hand, shows a sensor with a coverage pattern smaller than the space requires. Movement in space on the right will not be detected.
Sometimes a space will require multiple sensors. In this diagram, coverage of the two sensors fills the space with some overlap in the middle of the room. Overlapping coverage is redundant but in most cases unavoidable to insure complete coverage.
When the space is smaller than the sensors
coverage pattern there is excess coverage that can affect the effectiveness of the system. In this diagram, the sensor is placed so that the coverage spills over into the space outside the door. So even activity outside the room will cause the lights to go on. In this example, moving the sensor to the opposite wall would solve the problem.
Technology types (resource)
Ultrasonic sensors generate an inaudible signal at 20 to 40 kHz which is broadcast into a space. The signal bounces off surfaces and returns to a receiver unit within the sensor. When there is motion in the space the signal returns to the unit at a slightly different frequency which triggers the sensor to open the lighting circuit. This shift in frequency is known as the Doppler effect.
Ultrasonic sensors are not as limited to line-of-sight motion detection as infrared sensors. They have limited sensitivity around obstacles such as walls or cubicles. These sensors are best specified for open offices, restrooms, or store rooms where the many obstacles make infrared sensors useless. Ultrasonic sensors are also more sensitive to line-of-sight movement than infrared sensors. Small hand motions can be detected up to 20 feet away compared to about 10 feet for the infrared sensor.
Ultrasonic devices are generally more expensive than infrared devices, but they provide greater coverage for larger spaces. These units are more sensitive to motion than infrared units but are also more prone to false triggering. False triggering occurs when the sensor detects non-occupant movements: This might include air movement from the ventilation system or even a breeze through an open window. Proper calibration can solve this problem.
Passive infrared occupancy sensors
Infrared occupancy sensors respond to occupancy by detecting changes in object surface temperature and movement. They're designed to detect the peak wavelengths of heat that are emitted by humans, not other heat-generating objects such as computers and copiers.
A passive infrared sensor detects heat only within its "line of sight." It cannot detect the heat through walls or other obstacles. They detect motion within a fan shaped pattern. The sensor's ability to detect motion also depends on the distance between the sensor and the location of the motion. At short distances, within 10 feet, the sensor can detect very small motions such as hand movement. As the distance between movement and the sensor increases, larger motions are required to trigger the sensor. When a person is 30 to 40 feet away from the sensor, it will only detect large movements such as a person walking around.
Occupancy sensor features
Many occupancy sensors offer features that allow customization. Both wall mounted and remote mounted sensors can operate under "Always off", "Always on", or "Automatic modes." "Always off" overrides the sensor control and simply turns the lights off until some one physically switches the device to a different mode. "Always on" keeps the lights on until the mode is switched. "Automatic" determines when to turn lights off and on based on occupancy.
The sensitivity of occupancy sensors is adjustable. This optimizes the performance of a unit. Sensitivity can be reduced where HVAC air movement triggers the sensor. The sensitivity can also be increased to allow the unit to detect smaller movements.
Sensors have a slight time delay between the time movement stops and the time lights are actually turned off. Most devices allow this time delay to be adjusted from 30 seconds to 30 minutes.
Other features such as indicator lights, audible alarms, fail-on functions, and masking labels are also available. Refer to equipment spec sheets for details on these and other occupancy sensor features.