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 well do wireless photosensors perform compared with wired photosensors?

NLPIP last published a report on photosensors in 2007 (NLPIP 2007). The technologies used in the sensors tested for this study and their exhibited limitations have remained unchanged since the earlier report. Therefore, the reader is referred to that report for a more detailed understanding of these devices.

NLPIP used the procedure described in Appendix: Detailed Methodology to investigate the performance of wired and wireless photosensors from Leviton and Lutron by having them control the electric lighting in an empty scale-model room while exposing them to simulated daylight. (At the time of the study, WattStopper did not offer a wireless photosensor, so this brand was omitted from photosensor testing.)

NLPIP found that both the wired and wireless photosensors from Leviton use an “integral” control algorithm, which is designed to maintain a constant illuminance on the photocell rather than the work plane. As discussed in the 2007 photosensor report, integral control may not work well for spaces where the daylight enters through windows because it doesn’t allow for the changing task-to-ceiling illuminance ratio that occurs throughout the day. (For example, when only electric lighting is present, the ratio is typically 5:1, but when daylight comes in from the side it can be 1:1.) The use of this control algorithm contributed to the large 550 lux variation in work plane illuminance that the wired photosensor allowed as the simulated conditions transitioned from pre-dawn to noon, as shown in Figure 3. Another contributor to the large variation in work plane illuminance was the limited dimming capability of the controller-ballast combination that was used.

Figure 3. Leviton photosensor daylight simulation results. In this test, the work plane illuminance level in a scale model room is measured as the simulated daylight increases then decreases to mimic the diurnal pattern. The work plane illuminance includes the contribution from both daylight and electric light. The wired controller was set to "AutoCAL" mode for this test.

Figure 3

The wired and wireless Lutron photosensors, in contrast, use a “proportional response” control algorithm, whereby the control voltage sent to the luminaire is proportional to the sensor illuminance, which provides tighter control over the work plane illuminance than the integral algorithm. However, the Lutron photosensors exhibited another issue discussed in the 2007 photosensor report: a lack of independent control over the offset (the daylight level at which dimming starts) and the gain (the amount of dimming for a given amount of daylight), making it difficult to tune the photosensor to the room in which it is installed. The wired photosensor offers only one adjustment, called the “setpoint.” As shown in Figure 4, decreasing the setpoint decreases the work plane illuminance in the absence of daylight, but it also changes the gain, allowing the illuminance to dip below the desired level when low daylight levels are introduced. Having independent offset and gain settings would eliminate this issue. The wireless sensor system does have two independent adjustments, the photosensor “sensitivity” and a “high-end trim” adjustment on the controller for the maximum electric lighting level, but they are not as effective as independent offset and gain controls. As shown in Figure 4, changing the photosensor sensitivity only changes the size of the steps when it is dimmed. The target illuminance level on the work plane can be changed up to 50% by adjusting the high-end trim, which was not tested.

Figure 4. Lutron photosensor daylight simulation results. In this test, the work plane illuminance level in a scale model room is measured as the simulated daylight increases then decreases to mimic the diurnal pattern. The work plane illuminance includes the contribution from both daylight and electric light.

Figure 4b

Other notable testing results include:

  • Both the Leviton wired and wireless photosensor systems exhibited too much hysteresis. Some hysteresis is desirable to prevent excessive switching of electric lighting when daylight levels fluctuate, but the amount of hysteresis exhibited by these sensors left the simulated room with light levels lower than the Illuminating Engineering Society recommends (DiLaura et al. 2011) for a typical commercial environment when the daylight was decreasing at the end of the simulated day.  The hysteresis was more pronounced in the wired system, which allowed the work plane illuminance to fall to 100 lux before the lights switched back on.
  • The wireless Leviton photosensor has a mode that allows for dimming, but at the time of NLPIP’s testing, Leviton did not offer a dimming controller that works with this photosensor. Therefore, the photosensor system operates only as a switching system, which resulted in over 500 lux variation in work plane illuminance over the course of the simulated day.
  • The Lutron wireless photosensor system showed a step dimming response rather than a continuous dimming response, so it can provide only discrete electric light levels in steps of about 50-250 lux, and may not achieve the target illuminance level exactly. The step dimming occurred over a one-minute period, long enough that the change between the illuminance levels was not noticeable to NLPIP researchers.


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