Below is an article I wrote for the October issue of tED Magazine about industry efforts to tackle the challenge of LED flicker, with a focus on the recently published NEMA standard. Reprinted with permission.
Flicker is an old lighting issue diminished by the mass adoption of fluorescent electronic ballasts and returned with the advent of LED sources. Photometric flicker is the modulation of light source intensity or output over time.
Flicker may be external or internal to the lighting system. It may be visible (present in an immobile light source observed by an immobile observer) or stroboscopic (visible or invisible, and perceptible if the light source or observer is in motion). And its effects range from irritating to impairment, in some cases even if it is not perceptible by users. Studied have linked it to eyestrain, blurred vision and impaired performance. A small percentage of people is particularly susceptible and may suffer headaches and migraines. Flicker may also be problematic for videoconference applications, which use cameras.
The problem with LEDs is that, unlike traditional sources, they have no persistence. This means changes in forward current results in a nearly instant change in light output, potentially making flicker more pronounced.
Dimming LEDs is particularly concerning. Phase-control dimmers, which chop the AC waveform to produce dimming, may cause LEDs to rapidly cycle and produce flicker. If flicker is present, dimming may also make it more visibly pronounced, as flicker is more noticeable at lower light levels.
Generally, LED products featuring high-quality drivers that are properly paired with compatible controls will not produce objectionable flicker. These drivers are typically larger and more costly, however. In particular, digital controls generally do not induce flicker in the LED lighting system. So to minimize flicker, the electrical installation should minimize potential for electrical noise (external cause), feature LED products with high-quality drivers, and feature dimming controls are that are either digital or confirmed as compatible with the LED product. For maximum assurance, a test installation may be beneficial. Flicker can be measured in the field using specially designed handheld meters.
Due to the importance of this issue, the lighting industry required metrics and guidelines to help electrical professionals evaluate and specify appropriate products. In 2015, the Institute of Electrical and Electronics Engineers (IEEE) published IEEE PAR1789-2015, providing recommendations for minimizing flicker based on existing flicker metrics. These recommendations can be summarized as three major application needs: prevent seizures among light-sensitive people, limit other biological effects, and prevent these other effects. For each, IEEE recommends maximum percent flicker based on frequency.
After IEEE published its recommendations, the National Electrical Manufacturers Association (NEMA) released a position paper stating the IEEE recommendation is overly stringent for many applications, which could result in unnecessary additional cost to products due to more robust electronics required. In April 2017, the National Electrical Manufacturers Association (NEMA) published NEMA 77-2017, a new standard recommending a method for quantifying visibility of temporal light artifacts such as flicker and recommending application-based limits. The measurement methods and recommendations are applicable to all types of lighting (lamps, luminaires, etc.) and controls, though control methods and recommendations are limited to phase-cut dimming. It addresses visibility among human observers with limited speeds of motion. It does not address interference with equipment such as cameras, nor stroboscopic flicker.
Standards provide manufacturers a basis for testing and reporting and electrical professionals a basis for product evaluation, comparison and application. Recommendations give electrical professionals guidance to properly select products. This is important to the industry because if a lighting installation suffers from objectionable flicker, and that flicker is part of the LED product’s normal operation, typically the only recourse is product replacement. For this reason, distributors should vet LED products as posing a low risk of producing flicker before commitment. New methods and recommendations provide valuable tools to facilitate this vetting.
