Lighting Research Center Lighting Research Center
    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.

Appendix: Detailed Methodology

Occupancy Sensors
The performance of a PIR sensor can largely be captured through two characterizations: the spatial sensitivity pattern and the absolute sensitivity of the infrared detector. The testing used a Peltier cooler device, which had a 24°C temperature difference between its front and back sides; flipping it from one side to the other simulated human movement in a room-temperature environment. For this test all sensors were set to their maximum sensitivity settings. As shown in Figure 5, NLPIP tested spatial sensitivity to only 30° from nadir, but the sensors are able to detect motion across a wider field.

Figure 5. NLPIP tested occupancy sensors to 30° from nadir. The sensors are able to detect motion outside of the tested sectors.

The spatial and absolute sensitivity testing was conducted by mounting the sensor on a bar goniophotometer and oriented so that the nadir direction was aligned along the length of the bar. A 5 cm × 5 cm Peltier cooler device was mounted 2.5 m away from the sensor. The hot side of the Peltier cooler device was approximately 37°C and the cool side was approximately 24°C, which corresponds to the temperatures of a human body and room-temperature surroundings, respectively. A stepper motor flipped the Peltier device by 180° (taking ~300 ms to do so), then paused for 0.5 seconds, and then flipped it back. During the flip and for 2 seconds after the flip the state of the PIR signal was monitored for a detection response.  If the sensor registered motion, it was allowed to reset before the Peltier device was flipped again. This test setup allowed NLPIP to make sensitivity measurements more cost effectively than by following the National Electrical Manufacturers Association (NEMA) Occupancy Motion Sensors Standard NEMA WD 7- 2011. In order to measure the spatial sensitivity pattern, the goniometer was set to scan elevation angles from 0 to 30° in 1° increments and azimuthal angles from 0 to 355° in 5° increments.

To measure the 50% sensitivity threshold, the Peltier cooler device was moved farther from the occupancy sensor until only half of the flips were detected. Since detection of low-level signals is determined probabilistically, three to six trials were conducted at each distance to determine the detection percentage. The threshold distance corresponding to a 50% detection rate was calculated. Threshold distance was calculated for two angles of view: nadir, and at the approximate center of one of the sensitivity sectors next in elevation angle from nadir (typically 16° elevation angle). For this test, all products were set to their maximum sensitivity settings.

To compare the performance of wired and wireless photosensor systems, NLPIP purchased Lutron and Leviton wired and wireless photosensors that were appropriate for controlling the lighting in one room. Three of the systems (excluding the Leviton wireless photosensor system) could control multiple lighting zones separately, but NLPIP did not test this capability. While WattStopper occupancy sensors were investigated, their photosensor products were not tested because they did not offer a wireless photosensor at the time of the study, and the goal of the testing was to compare wireless photosensors with wired photosensors from the same brand.

The Lutron wired controller was connected to a Lutron ECO10 0-10V dimming ballast. The Lutron wireless controller was connected to a Lutron digital Ecosystem H-Series dimming ballast, one of the Lutron products it was designed to work with. Both the wired and wireless Leviton systems were connected to a Universal Triad dimming ballast.

The Leviton photosensor can be powered by either its integrated PV module or a replaceable battery, and NLPIP installed a battery for this test. The Lutron photosensor can be powered with only a replaceable battery.

The photosensors were tested in the Lighting Research Center (LRC) Daylighting Controls Simulator per the methods described in the NLPIP Scale Model Bench Test (NLPIP 2007). This device is a box with two openings that simulates an empty room that has a rectangular floor plan and a window. A high-power white LED is used to simulate daylight entering through the opening. It can provide over 6600 lux at the opening. This apparatus has a controller that measures the analog voltage (0-10V) from the photosensor, ballast power, illuminance in the simulator and relative light output in the LED box outside the opening. A custom software program changed the LED light output to mimic changing daylight levels over one day. The lighting from pre-dawn to post-dusk was simulated over a two-hour period. The LRC modified the simulator from that described in the NLPIP Specifier Reports (NLPIP 2007) in the following way:

  • A shielded high-power LED was used to simulate daylight in the sun box. It provided 0 to >6600 lux at the window aperture between the sun box and the scale model. The LED was controlled by a custom software program that allowed the LED current to be changed to mimic the time course of daylight over one day. The rate of increasing daylight provided by the LED was approximately 100 lux per minute to allow for time delays and response times. (NLPIP did not attempt to replicate the spectrum of daylight with either the metal halide or LED light sources. The LED allowed better control over light levels than the MH.)
Wireless Communications
NLPIP tested the maximum distance the sensors and controllers could be separated and still communicate reliably. The sensors were powered with new batteries and moved progressively further from the controllers. The testing was done in an office environment. Line-of-sight was maintained between the sensors and controllers.

To test the maximum communication distance, the following procedure was used:

  1. The receiver, with its load, was placed at one end of a hallway in a commercial building. 
  2. The background electromagnetic energy level at the receiver was measured.
  3. The transmitter was moved progressively farther from the receiver. At various distances, a signal was sent from the transmitter to turn the lamp on or off. The test ended when the system could not communicate or the longest possible line-of-sight distance was reached.
  4. The maximum distance at which the sensor could successfully communicate to the controller was measured.

To test EMI, the sensors (powered with batteries) were placed 15 ft (4.6 m) from the controllers. A signal generator (TPI Synthesizer Version 5.0; RF-Consultant, Austin, Texas) was used to generate electromagnetic energy. The generator was uncalibrated, but the signal frequency and strength was measured using a handheld spectrum analyzer (RF Explorer ISM Combo; Nuts About Nets, Bellevue, WA). The power of the emissions was increased in steps until the receiver no longer acted on signals sent by the transmitter. The signal generator was set to the same frequency as the wireless communication signal for each system, shown in Table 2. Testing was done in a laboratory and office environment, without protection from EMI. Background RF energy was measured at the transmission frequencies used by the three systems and found to be between -100 dBm and -94 dBm.

To measure the presence of electromagnetic energy, the RF Explorer spectrum analyzer was used. The frequency accuracy is ±10ppm and the absolute power accuracy is ±5dBm (RF Explorer 2015).

The signal generator (TPI Synthesizer) was also used to generate electromagnetic energy to test for interference with communication. The device is uncalibrated. However, the testing did not rely on the generator to determine the RF power. Instead, the electromagnetic power and frequency were measured with the RF Explorer spectrum analyzer.

To test EMI, the following procedure was used:

  1. Each controller was wired to an incandescent lamp and to line power. The controller is part of one circuit that also includes the load and line power.
  2. Each sensor/transmitter was powered with a battery. The Lutron sensor can be powered only by a replaceable battery. The WattStopper sensor is intended to operate using its integrated PV, but a battery is supplied for setup, and this was used during the testing. The Leviton sensor was powered with the optional battery specified in the instructions.
  3. The sensors and controllers were paired so they could communicate with one another.
  4. The sensor was set on a wood counter and the controller and load were set on a plastic cart 15 ft (4.6 m) away.
  5. The front of the controller and the side of the sensor with the PIR detector lens were faced directly toward one another. Under the assumption that the RF emitters are close to omnidirectional, this orientation should give similar results as an actual installation.
  6. The background electromagnetic energy level at the receiver was noted.
  7. The frequency at which the sensor sends its signal and the signal power at the receiver were measured with the spectrum analyzer.
  8. Electromagnetic emissions at the frequency or frequencies used by the transmitter were generated by the signal generator. The power of the emissions was increased in steps until the receiver no longer acted on signals sent by the transmitter.
Energy Generation and Storage
To calculate battery life, NLPIP made the following assumptions:
  • Self-discharge rates were 2% per year for lithium replaceable batteries (Jacobs 2013) and 2.4% per day for the internal super capacitors (electric double layer capacitors) built into each occupancy sensor (Panasonic 2012).
  • Occupancy was assumed to be 4 hours per day (50% of an 8-hour work day), 365 days per year. NLPIP found that occupancy rates have a small effect on battery life.
  • The working voltage range of the capacitor is from 4.5 (fully charged) to 2.7 volts.
  • The occupancy sensor will continue operating until the battery is fully discharged.
  • Energy from photovoltaic modules was not included, even if a module was built into the occupancy sensor. This worst-case scenario would be appropriate for dim or infrequently-lighted locations.

The initial capacity level of each sensor’s battery is as shown in Table 4.

The battery run time was calculated using the following discharge rate equation:

Equation 1

where Q is the charge (in coulombs) available in the battery or capacitor, k is the rate at which charge is used by the device (coulombs/day) and r is the self-discharge rate (%/day). The discharge rate due to the load consists of three parts: 1) the steady state, non-transmitting operating current (Iop), 2) transmitting charge/day during unoccupied time periods (Ivacant), and 3) transmitting charge/day during occupied time periods (Ioccupied). In equation form:

Equation 2
Equation 3
Equation 4

where η is the fraction of time the space is occupied. For these calculations occupancy was assumed to be 50% of an 8-hour work day: η = 0.50(8h/24h) = 0.165.

The solution to the first-order differential equation describing the remaining charge is:

Equation 5

Q0 is the initial battery or capacitor charge (in coulombs) at time = 0. Life is given by solving this equation for time when Q(t) = 0 (i.e., no charge left).

To measure PV electricity generation, the super capacitor voltage was set to 3.0 volts.


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