Jim Brodrick on the Performance of Flicker Meters
Republication of Postings from the U.S. Department of Energy (DOE) Solid-State Lighting Program
by Jim Brodrick, SSL Program Manager, U.S. Department of Energy
All conventional light sources modulate luminous flux and intensity — i.e., they flicker — to some degree. Flicker is garnering increasing attention from lighting designers and specifiers, the standards and specification community, and, consequently, lighting manufacturers. 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. But at this time, there’s no standardized test procedure for measuring photometric flicker from light sources, and manufacturers rarely report flicker characteristics.
To help specifiers determine the flicker behavior of lighting products, and to accelerate the development of standard test and measurement procedures, DOE has published a new report on the performance of three commercially available flicker meters. 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 the reference photoelectric characterization system developed by the Pacific Northwest National Laboratory.
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.
Based on the results of this study, DOE offers the following recommendations:
• Lighting manufacturers and testing laboratories should start characterizing lighting products for flicker. There are commercially available meters that enable the relatively straightforward characterization of a number of flicker metrics, including Percent Flicker, Flicker Index, and Fundamental Frequency, and manufacturers should report these metrics on lighting-product data sheets.
• When characterizing flicker, take measurements at full as well as one or more dimmed light levels, since dimming can not only increase the flicker seen at full output, but can also introduce different frequency content into the light-intensity waveform for certain lighting products. Be careful to ensure that the dimmed light levels are within the meter operating range, and note that uncertainties for most measurement systems can be greater at extreme measurement conditions; consult the meter specifications to understand expected performance. Additionally, consider the effects that sampling frequency and measurement duration have on the use of various techniques used to generate flicker metrics, as certain commercial flicker meters have fixed sampling frequencies, or limits on the total number of measurement samples that can be collected, regardless of measurement duration, and thus the waveform may not be an accurate representation of performance.
• Lighting designers and specifiers might consider purchasing a handheld meter and starting to characterize the flicker produced by specific products in the real world. When attempting to do so, watch out for ambient light and other conditions that might result in the handheld meter not yielding as accurate of a result as it does when used in a laboratory environment. For a flicker meter to accurately capture data, there can be no stray light from windows or other luminaires that might affect the light-intensity waveform. On the other hand, it should be recognized that the flicker affecting a person is the waveform that reaches the eye, which may be a composite of light from several sources.
• Follow the Illuminating Engineering Society, International Commission on Illumination, and National Institute of Standards and Technology developments for flicker terminology, flicker-characterization system requirements (including calibration procedures), standardized test and measurement procedures, and new or refined metrics — especially those that consider aperiodic waveform content or allow weighting factors to be applied to specific frequencies of interest for specific applications, as research becomes available. Note that even currently defined terminology is not used consistently. For example, while many in the lighting industry equate flicker with light modulation, it makes sense to some to differentiate between light modulation and its effects — and to further differentiate between those effects as flicker, stroboscopic effects, and phantom-array effects. Look for flicker meters that incorporate the latest terminology, metrics, requirements, and guidance, and/or are firmware-upgradeable.
While the results of the new report may be of interest to many lighting-industry stakeholders, the intended audience includes lighting and meter manufacturers, test laboratories, and standards and specification bodies. It’s hoped that this report furthers interest in measuring and reporting flicker, thereby enabling the use of flicker characteristics to mitigate the potential effects of flicker in lighting installations and accelerating the development of standard test and measurement procedures. The commercial availability of flicker meters should make it easier for designers and specifiers to minimize the risk of flicker-induced problems for their clients in the near future.
To see the full report, visit the DOE SSL website.