All conventional light sources modulate luminous flux and intensity to some degree, usually as a consequence of drawing power from AC mains sources. Many terms are used when referring to this time variation, including “flicker,” “flutter,” and “shimmer.” The Illuminating Engineering Society of North America (IES) Lighting Handbook defines flicker as “the rapid variation in light source intensity.” The periodic waveform that usually characterizes flicker can be principally described by four parameters: its amplitude modulation (i.e., the difference between its maximum and minimum levels over a periodic cycle), its average value over a periodic cycle (also called the DC component), its shape or duty cycle (the ratio between the pulse duration and the period of a rectangular waveform), and its periodic frequency (i.e., the number of recurring cycles per second).
Flicker is garnering increasing attention from lighting designers and specifiers, the standards and specification community, and, consequently, lighting manufacturers. An Institute of Electrical and Electronics Engineers group has developed a recommended practice for evaluating flicker risks, and ENERGY STAR® and California’s Title 20 are requiring the reporting of flicker performance and/or considering the adoption of flicker criteria. Some manufacturers appear to be giving flicker increased design priority, as evidenced by the improved performance of new product generations.
An understanding of why flicker matters and how much it varies across commercially available products is increasingly becoming essential to proper lighting design. Specifying the right product for a given application and risk sensitivity further requires the ability to quantitatively characterize flicker. At this time, however, there is no standardized test procedure for measuring photometric flicker from light sources, and manufacturers rarely report flicker characteristics.
Ideally, a test and measurement procedure would facilitate the capture of light-source intensity or luminance over time and potentially describe how to characterize periodic waveform characteristics (e.g., amplitude modulation, shape or duty cycle, frequency) using one or more metrics, and how to identify aperiodic characteristics. Both the IES Testing Procedures Committee and CIE Technical Committee 1-83: Visual Aspects of Time-Modulated Lighting Systems are considering the development of standardized test and measurement procedures for flicker.
THE PERFORMANCE OF FLICKER METERS
In February 2016, DOE published a report on the performance of three emerging flicker meters. The purpose of the study was to help specifiers determine the flicker behavior of lighting products, and to accelerate the development of standard test and measurement procedures. The results and analysis show that the commercial meters measured light-intensity waveforms and calculated essential flicker performance characteristics and metrics similarly, both to each other, and to a reference photoelectric characterization system. Some differences in performance were found, however, when measurements were taken of light-intensity waveforms with significant high-frequency content greater than the dominant 120 Hz found in many products at full output. If the meter was not appropriately configured, or if proper configuration was not possible given meter constraints, then the waveform characteristics were not accurately captured, often resulting in the calculation of flicker metrics that deviated significantly from the reference.