Volume 13 Issue 1
July 2015    
ballast - A device required by electric-discharge light sources such as fluorescent or HID lamps to regulate voltage and current supplied to the lamp during start and throughout operation. compatible ballasts - An abbreviated list of common ballasts that will provide the necessary circuitry for a photosensor to operate correctly. Other ballasts may also be compatible; contact the photosensor manufacturer for details. continuous dimming - Control of a light source's intensity to practically any value within a given operating range. capacitor - A device used in electric circuitry to temporarily store electrical charge in the form of an electrostatic field. In lighting, a capacitor is used to smooth out alternating current from the power supply. time delay range - For motion sensors, the range of time that may be set for the interval between the last detected motion and the turning off of the lamps. lamp - A radiant light source. luminaire - A complete lighting unit consisting of a lamp or lamps and the parts designed to distribute the light, to position and protect the lamp(s), and to connect the lamp(s) to the power supply. (Also referred to as fixture.) frequency - The number of cycles completed by a periodic wave in a given unit of time. Frequency is commonly reported in cycles per second, or hertz (Hz). electromagnetic interference (EMI) - The interference of unwanted electromagnetic signals with desirable signals. Electromagnetic interference may be transmitted in two ways: radiated through space or conducted by wiring. The Federal Communications Commission (FCC) sets electromagnetic interference limits on radio frequency (RF) lighting devices in FCC Part 18. electronic ballast - A ballast that uses electronic components instead of a magnetic core and coil to operate fluorescent lamps. Electronic ballasts operate lamps at 20 to 60 kHz, which results in reduced flicker and noise and increased efficacy compared with ballasts that operate lamps at 60 Hz. illuminance - The amount of light (luminous flux) incident on a surface area. Illuminance is measured in footcandles (lumens/square foot) or lux (lumens/square meter). One footcandle equals 10.76 lux, although for convenience 10 lux commonly is used as the equivalent. dimming ballast - A device that provides the ability to adjust light levels by reducing the lamp current. Most dimming ballasts are electronic. power - The power used by a device to produce useful work (also called input power or active power). In lighting, it is the system input power for a lamp and ballast or driver combination. Power is typically reported in the SI units of watts. photosensor - A device used to integrate an electric lighting system with a daylighting system so lights operate only when daylighting is insufficient. lux (lx) - A measure of illuminance in lumens per square meter. One lux equals 0.093 footcandle. nadir - In the lighting discipline, nadir is the angle pointing directly downward from the luminaire, or 0. Nadir is opposite the zenith. driver - For light emitting diodes, a device that regulates the voltage and current powering the source. photovoltaic (PV) - Photovoltaic (PV) cells produce electric current from light energy (photons). PV cells are joined to make PV panels. hysteresis - The dependence of the output of a system not only on its current input, but also on its history of past inputs. The electric light level set by a photosensor with hysteresis, for a certain photocell input signal, depends on whether that photocell signal is increasing or decreasing. Hysteresis provides stable operation in switching photosensors but is undesirable in dimming photosensors.

How robust are the wireless communications?

NLPIP investigated two aspects of wireless communication of occupancy sensor systems: EMI and the maximum communication distance between the sensor and the controller.2 Detailed test procedures can be found in Appendix: Detailed Methodology.

As shown in Table 2, NLPIP found that the unobstructed transmission distance was 92 ft (28 m) for the Leviton system and at least 110 ft (34 m) for the Lutron and WattStopper systems (the maximum indoor distance available for testing). These transmission distances are adequate for typical installations in commercial buildings and are within the manufacturers’ specifications.

Likewise, NLPIP found that EMI did not present a problem in the commercial building where testing was conducted. However, there may be some rare cases of EMI in other commercial buildings, and potential sources of RF at each system’s transmission frequency are noted in Table 2. This table also shows the interfering signal strength that was required to prevent the controller from receiving the signal from the occupancy sensor. The greater the interfering signal power that can be tolerated by the system, the more resistant the wireless communication is to EMI. (The RF generator that NLPIP used allows the radiated power to be set only in discrete steps, so only a range of interfering signal power could be determined.) As shown, the Lutron system was able to overcome a higher-power interfering signal than the Leviton and WattStopper systems. Even though NLPIP was able to intentionally jam the signal of each occupancy sensor system, and NLPIP did detect background RF energy at the transmission frequencies, the background RF was so weak that it did not interfere with the communication of the wireless systems. The background RF was typically between -100 decibel-milliwatts (dBm) and -94 dBm, which is at least an order of magnitude less than the power of EMI needed to jam the communication.

Table 2. Wireless communication results. When determining the power of signal that prevented communication, the distance between the sensor and controller was 15 ft (4.6 m). The power of electromagnetic energy is in the units of decibel-milliwatts (dBm). The greater the value, the greater the power. For example, -40 dBm is a higher power than -50 dBm. The accuracy of the power measurements is ±5 dBm.

Brand Transmission Frequency Maximum
Potential Sources
of EMI at
Power of Signal
at Receiver that
Leviton 303.4-
315.2 MHz
92 ft (28 m) garage door openers,
security systems,
car remote keyless entry,
military aviation
Between -73.5 dBm and -68 dBm
Lutron 433.6 MHz Undetermined,
but greater than
110 ft (34 m)
hobby transceivers,
wireless alarm systems,
wireless presentation remotes, wireless home weather stations, military radar, RFID
Between -50 dBm
and -46 dBm
WattStopper 902.8-
902.9 MHz
but greater than
110 ft (34 m)
cordless phones,
baby monitors,
walkie talkies
Between -77 dBm
and -74 dBm

The consequences of the loss of wireless communication depend on the control scheme in use and whether the EMI is intermittent or continuous. For example, in vacancy sensor mode (manual on, automatic off), a lack of communication will result in a reduction in energy savings. In occupancy sensor mode, it is unlikely but conceivable that an occupant could be left in the dark if a manual switch is not accessible.

If an installed lighting control system’s wireless communications are not reliable, potential issues can be explored by:

  • Using a handheld spectrum analyzer to determine if there is EMI (RF energy at the transmission frequency).
  • Making sure that the sensor is sufficiently charged; NLPIP found that the Leviton occupancy sensor was able to transmit a signal only half the distance shown in Table 2 when it was not fully charged (no battery was installed and the integrated PV received inadequate illuminance). Installing a battery will overcome this problem.
  • Noting if any objects obstruct the line between the sensor and controller.
  • Checking that the distance between the sensor and controller is less than the maximum distance specified by the manufacturer.


2 A third potential issue with wireless communication, interference from building materials, was not studied because it is not likely to be encountered within a single room, the focus of this study. The control manufacturers provide guidance on the amount the communication range is decreased based on the presence of building materials.


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