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Occupancy Sensors

Occupancy sensors are the most common lighting control used in buildings today. Two technologies dominate: infrared and ultrasonic. Infrared sensors detect temperature changes in a room, and work well where the entire room is within the sensor's field of view. Ultrasonic sensors use high frequency sound, much like bats do, to detect motion (even around corners). Dual-technology sensors use both methods, increasing accuracy and flexibility, but at a higher price. Even though lamp running life may be somewhat shortened by increased switching due to occupancy sensors, the overall chronological life of lamps is usually extended by the reduced daily burn hours.

Time Scheduling

Large open office areas work well with simple time scheduling - automatic switching at fixed hours of the day. Overrides allow users to turn on the lights after hours (using wall switches or telephone dial-up codes). Time scheduling can be accomplished with simple time clocks or more sophisticated computer controls. To save more energy, time scheduling systems can be designed so that lights are turned on manually rather than automatically at the beginning of the day, but are turned off automatically at 1- or 2-hour intervals after close of business.

Bi-Level Switching

Some people prefer lower overhead lighting levels (especially if daylight is available). Lower light levels are often preferred for computer use, meetings or tasks that are not visually demanding. Bi-level switching can provide simple manual control. For example, in a typical 3-lamp fluorescent fixture, the outer lamps are switched separately from the middle lamp, allowing the user to switch on one, two, or all three lamps. This low-cost measure is a minimum control requirement in some state energy codes, and can provide a simple means of load-shedding during peak hours if the bi-level lighting circuits are remotely controllable.

Manual Dimming

In rooms where different light levels are needed at different times, such as conference rooms and some private offices, the use of manually-operated dimming controls is a common solution. These controls can be either wall-mounted or, for convenience, use wireless remote controls (like the controls for a TV or VCR).

Automatic Daylight Dimming

Automatic daylight dimming, or "daylighting," uses a light sensor to measure the amount of illumination in a space. Then, light output from a dimming ballast is adjusted to maintain the desired level of illumination. The combination of daylight dimming with appropriate task lighting is often very effective.

Corridors and open cubicles near windows, particularly those with task lighting, are good candidates for daylighting controls. Private offices with windows can also be equipped with individual daylight sensors. Initial commissioning and calibration of light sensors and controls is critical for effective daylighting, however; poorly calibrated daylight sensors can result in little or no savings, and may annoy occupants.

Demand Limiting

During peak demand periods utilities often charge significantly higher prices for electricity. Remote operation of dimming ballasts or bi-level switching helps operators to respond to price signals or utility requests to shed load to help avoid power outages.

How to Select Lighting Controls: Where and Why

Lighting controls can save energy and reduce peak demand in offices and other facilities. Controls save money while providing user convenience and an improved lighting environment. There are several different kinds of controls. The choice of control type should be based on lighting usage patterns and the type of space served (see Lighting Control Applications).

Areas with intermittent occupancy are well-suited to occupancy sensors. In large, open office areas with many occupants, scheduled switching ("time scheduling") is often an effective energy-saving strategy. In daylit offices, properly adjusted daylight sensors with dimming ballasts make sense. Because some workers prefer lower lighting levels, bi-level manual switching is another option. Advanced lighting controls can be used for demand limiting to allow building managers to reduce lighting loads when electricity demand costs are high.

Some types of lighting are not well suited to certain controls. For example, daylight dimming and occupancy sensing are not generally appropriate for high intensity discharge (HID) lighting (which requires a delayed re-start), whereas time scheduling is usually a good match for HIDs.

The Defense Logistics Agency (DLA) offers occupancy sensors, dimmable ballasts, and other lighting controls equipment through its Energy Efficient Lighting catalog, available on its Web site.


Appropriate Illumination Levels

Proper illumination levels depend on the type of work being performed, and on occupant preference. Recommended illuminance levels for offices range from 30 to 60 footcandles*, but the quality of the visual environment can have a substantial impact on the "appropriate" amount of illumination. In well-designed office spaces, with light-colored surfaces, appropriate task lighting, and careful placement of lights and furniture to avoid glare and shadows, much lower illuminance levels are acceptable, and usually even preferred.

Installation and Maintenance

Proper placement and orientation of both daylight and occupancy sensors is essential. Placement of controls should take into account furniture placement as much as possible. Occupancy sensors must be able to sense all occupants to avoid turning off lights while the space is occupied. At the same time, "false-on" incidents can be triggered by an automatic on/off sensor that is exposed to passersby in an adjoining hallway. Daylight sensors that are placed where they are exposed to an amount of daylight not proportionate to the daylight at the desktops being served will not properly control lighting levels (and will likely result in dissatisfied users who may attempt to disable the control system).

Set time scheduling controls so that the switching times and intervals make sense for the occupants and usage pattern of the space. Occupants need to know how to override the schedule easily when needed.

Choose daylight sensors that can be calibrated quickly and easily, and take the time to calibrate them correctly. The dimming adjustment should be easily accessible to the installer and provide an acceptable range of dimming.

Commissioning and calibration of lighting controls are essential if energy savings are to be achieved and maintained. Occupancy sensors with sensitivity set too high can fail to save energy, but occupancy sensors with too low a sensitivity or too short a delay time can be annoying to occupants. Similarly, improperly adjusted daylighting controls can dim the lights too low, causing occupants to override them (e.g., by taping over the sensor), or can fail to dim the lights at all.

Automatic on/off sensors switch on when they sense an occupant and off when they don't. Better energy savings are usually attained from sensors that are manual-on/automatic-off. These avoid unneeded turn-ons.


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