(Issue: December 2017)
By Craig DiLouie, CLCP, LC
Light source flicker is an old lighting problem returned due to the proliferation of LED lighting. Lighting management companies concerned about flicker should get educated on the issue, learn about metrics, ask manufacturers for flicker testing data, and specify quality products that minimize it.
Photometric flicker is modulation in light source output or intensity over time. Flicker may be internal (product of how the lighting system is designed and interaction between its components) or external (product of how the system is designed and changes in supply voltage). It may be visible—perceivable in cases where neither the observer nor light source is moving—or invisible.
Another type of flicker is stroboscopic, perceptible only if the observer or light source is moving. When flicker makes rotating machinery appear to slow or stop, that is stroboscopic flicker, the product of the machinery rotating at the same or a multiple of the modulation frequency. A related problem is the phantom array effect, in which flicker is perceived by the observer’s eye moving.
In this article, we will focus on photometric flicker resulting from both product design and how products interact with each other within a system. All AC-powered light sources produce photometric flicker during operation. Some sources suggest flicker is noticeable up to about 80 Hz. However, flicker may not be noticeable but still produce a physiological response in some people up to 125 Hz or higher. Studies have linked flicker to increased eyestrain, blurred vision and poor visual task performance. A small percentage of the population is highly perceptible; for them, flicker can trigger extreme headaches and migraines.
Years ago, fluorescent magnetic ballast systems were prone to objectionable flicker. The rapid adoption of electronic ballasts, most of which operate at a frequency of 40 kHz, remedied the problem. The rapid ON/OFF cycles blend into a visually constant stream of light. In one study conducted by Wilkins et al in 1989, a small population of office workers demonstrated a significant reduction in self-reported eyestrain and headaches after high-frequency electronic ballasts were installed.
The problem returned with LEDs because unlike conventional light sources, they have no persistence. Changes in forward current produce a nearly instant light output change, which can make flicker more pronounced. Of particular concern is dimming. Phase-control dimmers chop the AC waveform, which may make the LEDs cycle rapidly and thereby produce flicker. And dimming generally can make presenting flicker more visible because it is more noticeable at lower light levels.
Generally speaking, LED products built with high-quality drivers and paired with compatible controls should not produce objectionable flicker. However, these drivers are generally larger and more costly. Cost and size, therefore, present limiting factors. Low-end residential products, for example, may feature a power supply that is more flicker prone. Of particular concern are low-cost circuitry options such as rectifier, reverse-parallel and AC direct (step driver) power supplies. And because of constraints on size, lamps aside from high-end architectural lamps may be more prone to it as well. Another factor is dimming. Digital controls, for example, generally do not induce flicker. Older phase-control (line-voltage) dimmers may be more problematic.
To minimize flicker, lighting management companies should select LED products with high-quality drivers and pair them with digital dimming controls or dimmers specifically rated as compatible with the LED product. In the field, flicker can be measured or detected using handheld meters or other methods. If flicker is present, note whether it is constant or intermittent. If intermittent, whether it occurs when another activity occurs, such as an elevator moving. Move the luminaire to another location in the building and test it. This basic troubleshooting process will help determine whether the cause of the flicker is internal or external.
If the cause is internal to the lighting system, little may be done to fix the problem aside from product replacement. If flicker occurs during dimming, evaluate the dimmer and consider replacement if it is a phase-control dimmer. Otherwise, consider LED product replacement with one that features a high-quality driver. Because a problem may entail product replacement, lighting management companies should vet LED products to mitigate risk. A test installation can be beneficial to provide confidence the proposed system will work as required.
Vetting requires metrics lighting management companies can use to evaluate, compare and select products. The electrical industry offers various metrics and guidelines to help. In 2015, the Institute of Electrical and Electronics Engineers (IEEE) published IEEE PAR1789-2015, which offers recommendations for minimizing flicker based on current metrics. To summarize, IEEE provides recommendations based on three major application needs: prevent seizures among people who are sensitive to light and limit or prevent other biological effects. For each need, IEEE recommends maximum percent flicker based on frequency.
After IEEE PAR1789’s publication, the National Electrical Manufacturers Association (NEMA) published a statement that the IEEE recommendation is overly strict for many applications, which could result in unnecessary added cost to products due to the requirement for more robust electronics. In 2017, the organization released NEMA 77-2017, a standard method for quantifying visibility of flicker and other temporal light artifacts. The document also recommends application-based flicker limits. The methods of measurement and related recommendations apply to all lighting types (lamps, luminaires, etc.) and phase-control dimmers. NEMA 77 addresses visibility among observers with limited speeds of motion. It does not address stroboscopic flicker or interference with equipment such as cameras.
Flicker is a serious issue in some applications. To mitigate risk of flicker, lighting management companies should get educated about it and properly vet products and lighting-control combinations.
Craig DiLouie, CLCP, LC, principal of
ZING Communications, Inc., is a consultant, analyst and reporter specializing in the lighting and electrical industries, and a regular contributor to
LM&M. You may contact Craig at firstname.lastname@example.org.