While LED is fast-becoming the pervasive form of lighting these days, there are still challenges to be resolved as the technology evolves.
by David LaVigna
Thanks to the proliferation of LED retrofit lamps and LED luminaires on the market – in homes, cars, hotels, warehouses, stores, and streetlights – the general public is getting comfortable with LED. As this technology evolves, there are still some challenges that need to be overcome. One of these is flicker.
Having the unique history of successfully designing many global architectural projects and then transitioning into high-quality private label manufacturing, I have seen LED flicker destroy otherwise great projects and product designs.
Not all flicker is bad. There are instances in products and applications where one would want flicker; such as warning devices like a bicycle light or personally worn headlights. When flicker is present in areas where machinery is present and where people need to be aware of rotating blades or other dangerous elements, however, flicker can impede what we think we see — and that makes the situation incredibly risky.
In a circumstance of public assembly (such as restaurants or hospitality locations) flicker is mostly an annoyance unless a specific flicker frequency is present in which medical conditions (i.e. seizures, migraines, epilepsy) can become triggered or exasperated. Technological advances are progressing at such a rapid rate that I dare state that most try to keep up as best they can with the real issues of LEDs.
What Causes Flicker
Understanding the basics of how flicker is defined and measured should help designers and specifiers become more aware and confident to ask questions and engage in conversations to prevent this – sometimes just annoying and other times, medically harmful – LED pain point.
Most all AC power-based fixtures flicker. I’ve heard terms such as “flicker,” “flutter,” “strobe,” and “shimmer” to describe this light modulation. IESNA defines flicker as “the rapid variation in light source intensity.” For purposes of this article, we will only be talking about light modulation (photometric) flicker caused by luminaire or lamp design and only with LED as an illumination source.
LEDs are primarily direct-current sources, therefore when constant current is supplied, the diodes function without flicker. Without constant-current regulation, the LEDs’ intensity will change with the cycle of the AC input current. Herein lies the basic topography of the most common drive system; constant voltage driver systems – in which the driver is used to regulate voltage, and in series diodes and resistors – are used to regulate current. Inverting the AC to DC causes a ripple in the voltage and the current output, and that ripple is typically twice the frequency of the input voltage (60Hz to 120Hz;
50 Hz to 100Hz).
When you add dimming to the LED system, the complexity of this [problem] increases. Both PWM and triac-style dimming will stretch the time on/time off cycle and increase the percentage and possibility of seeing flicker.
With two metrics for measurement – percent flicker and flicker index – we can start to organize and compare LEDs relative to other sources and each other. Percent flicker is a measure of the variation in light output. This relative measure is based on the minimum (A) and maximum (B) output levels.
Flicker index is a “reliable relative measure of the cyclic variation in output of various sources at a given power frequency” and this measure accounts for amplitude and waveform.
Having read through the many articles written on the impacts of flicker on the human factor, we can summarize through the collection of data and theories that light frequencies between 3Hz and 70Hz have the most probable potential impact related to severe medical conditions such as photosensitive epileptic seizures. Between 70Hz and 160Hz there are reports of eye strain, fatigue, headache, low performance and dizziness – both are impacted by longer exposure, area of retina stimulation, brightness of the flash, contrast, and color.
Can Flicker be Measured?
There are ways of measuring flicker, although no standard is yet set (that I could find). A technical system can be made; but must include a discrete and controlled environment. A photosensor, trans-
impedance amplifier, and digital oscilloscope are required to measure and digitize flicker.
Or one can use the “chopstick” method, as I call it. If you have ever taken wooden chopsticks and rubbed the tips together to get the excess splinters off, you are qualified to perform this test. By waving a rigid object in the beam of the questioned source, you will either see the illusion of a strobe or a solid line. When gaps are seen, you have confirmed you have flicker — although at what level and Hz would require the earlier test setup.
As a manufacturer, I offer one key tip to the design community: Make sure to test sources at a level other than full on. I have seen systems that promote “no flicker,” but that is only true when at full on. When at a dimmed level, the flicker is prevalent and annoying at best.
So where does this leave us? Well, the short version is that LEDs are prone to flicker and as professionals in each of our disciplines, we must take action to further educate and talk with one another in order to continue shedding some light (pardon the pun) on the issue of LED flicker.
David LaVigna is President of 0energyLIGHTING, a company offering fully designed and integrated lighting solutions from product development & OEM fabrication to energy retrofits and design/build applications.