DOE Study Examines Sensors in Tunable Lighting Systems

The U.S. Department of Energy has released a report on the initial performance and reliability of chromaticity sensors used for tunable LED lighting systems. The performance of LED lighting is known to change over time because of lumen depreciation, shifts in chromaticity, and changes in the drivers. Sensor-driven lighting systems can compensate for these aging-related changes by adjusting the amount of light and color quality to maintain a consistent lighting environment. They can also adjust to changes brought on by daylighting in the space. However, the impacts of changes in the control sensors over time on the reliability of tunable LED systems has not previously been examined.

The study, conducted by RTI International through a contract with KeyLogic Systems, looked at the initial performance and aging characteristics of a sensor consisting of a series of six photodiodes that respond to different wavelengths to adjust illuminance, correlated color temperature (CCT), and chromaticity. The sensor devices were exposed to a 5,000-hour accelerated stress test: one group at a continuous room temperature operating life, a second at a continuous but elevated ambient temperature of 75°C, and a third in a temperature-humidity environment of 75°C and 75% relative humidity (also known as a 7575 environment). After testing, the performance of the devices was measured using the light from a parabolic aluminized reflector (PAR) lamp on an open optical bench in a dark room.

This study found that a chromaticity sensor used in the control system of a tunable lighting installation has high reliability—even under relatively harsh test conditions. No abrupt failure of the sensor was found after 5,000 hours of testing, even in the relatively harsh 7575 environment. One parametric failure after 5,000 hours of 7575 testing was observed. Such a failure would produce a change in a tunable device that would be noticeable to an observer. Light from the tunable fixture would likely be blue-shifted (relative to the true value) to compensate for the yellow-shifted readings of the chromaticity sensor. However, minimal changes in illuminance would occur because the accuracy of this sensor reading appears to be less affected by the 7575 environment.

This finding underscores that moving from traditional lighting measurements involving mainly surface and vertical illuminance to dynamic lighting systems controlling illuminance, chromaticity, and CCT will require greater understanding of the long-term performance of the sensors controlling such systems and the changes in their reliability caused by aging and environmental exposure.

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