Craig DiLouie, LC, CLCP recently had the opportunity to interview Yan Rodriguez, VP, Product & Technology, Acuity Brands Lighting for an article about LED flicker for Electrical Contractor Magazine.
DiLouie: In your opinion, how big a problem is LED product flicker in the lighting industry as of 2016? What types of products (lamps versus luminaires, high- versus low-end products) are most likely to exhibit flicker? What types of applications are most likely to experience visual and/or stroboscopic flicker?
Rodriguez: The problem of flicker has been part of the lighting industry for decades. The older T12 fluorescent tubes, when used with magnetic ballasts, resulted in a flicker percentage of about 35% (no flicker = 0%) and a flicker index of .09 (best = 0). For 100W metal halide, still used today, has a 49% flicker and an index of .14. LED products can range anywhere between nearly 0% and 0 index to 99% flicker and .5 index. That range is what makes the subject of flicker so difficult to manage. Generally speaking, low-end residential products due to their cost constraints will use a power supply technology that is more prone to flicker, as compared to more expensive multi-stage switching power supplies found in commercial products. Lamps, due to their size constraints, will also employ topologies which are more prone to flicker. There are of course exceptions in a few high-end architectural lamps.
The problem with flicker in luminaires\lamps really does not fall into applications but rather cost, size, and dimming requirements (in that order). When cost is the most important criteria, designers quickly move away from switching supplies to rectified reverse parallel LED arrangements or AC direct (step drivers). When these are used flicker is more prevalent. Line voltage dimming, the most widely used method for dimming in residential applications, has the potential to introduce or amplify flicker in products. We see cases where products demonstrate excellent flicker performance (constant current power supplies) when not dimmed, flickering as much or worse as a direct drive solution when used with a line voltage dimmer. This particular problem is typically not present in commercial applications given that 0-10V dimming is the preferred method for dimming in these environments.
DiLouie: How often is flicker an issue of either 1) LED lighting systems operating at full output or 2) LED lighting systems either dimming or reaching a certain dim level? Which is the greater concern?
Rodriguez: For constant-current power supplies flicker is not much of an issue when operated at full output. For step drivers or reverse-parallel designs flicker is present regardless, but some techniques such as valley fill help in reducing flicker percentage. Dimming generally makes things worse, but in very poor systems, the fact that there is less light output present out of the source tends to lessen the visible effect. From a technical perspective, however, the numbers look much worse. Again, generally speaking, poor performing products at full output are of greater concern especially when used in environments where there is motion present (motors turning, water flowing). Another aspect which is creating concern is the effect these poor performers have with video cameras (e.g., surveillance, video conferencing, video recording) and the mismatch between the flicker rate of the luminaire and the frame rate of the camera. When these are encountered we see video artifacts such as flashing, scrolling lines, and uneven illumination. When the luminaire is dimmed the flicker changes making a marginal system inoperable. As the Internet of Things gains popularity the use of sensors in luminaires is increasing. Some of these sensors may not operate as intended with luminaires with high flicker.
DiLouie: How does visual and/or stroboscopic flicker manifest in traditional light sources and in LED light sources? What is it about the LED source that makes flicker a significant issue?
Rodriguez: Even though flicker in luminaires has always been present, conventional sources such as incandescent generally did not have any significant flicker on the output due to thermal inertia. LEDs do not have any persistence or inertia, so when the forward current or voltage falls below its conduction threshold, photon emission stops instantaneously. The net result is that we see higher levels of flicker percentage and flicker index as compared to conventional sources. Triac-based dimmers, which worked well with conventional, are still used with LED. So naturally the flicker effect will be greater. This is not to say that LEDs are inferior to conventional sources. A well-designed power supply in combination with the right dimming system will produce outstanding results and, in many cases, outperform conventional sources. What we have today is a result of economics, and legacy technology (dimmers). Placing rules and regulations are extremely difficult as the science needed to determine exactly what is acceptable or not is still being debated today. Furthermore, in the case of line voltage dimming products, it is impossible for a luminaire manufacturer to predict the flicker performance of their product given the multitude of options and variability of the dimming systems already installed.
DiLouie: What are common causes of flicker that are internal to the LED lighting system? What components are involved?
Rodriguez: The LEDs themselves are not the cause of flicker. All LEDs basically behave in the same manner. It is the power supplies that cause the flicker in most products. Power supply topology as explained above is the main cause. A simple rectifier power supply is the most cost-effective LED power source. It is also the worse for flicker. In fact, simple rectifier sources are generally not used in general illumination because of their poor performance. LED Christmas lights are an example of products using this topology. Reverse-parallel topologies are the next most cost-effective power system and they exhibit more flicker than other topologies, but it is better than simple rectification.
Another component that causes flicker in luminaires is the line voltage dimmer (phase cut). These dimmers chop the AC waveform which can result in the interruption of power flow at the LED, thus causing the LED to turn on and off rapidly causing flicker. Better power supplies will have enough buffering to prevent or lessen this effect while others will perform poorly.
DiLouie: What detailed recommendations should electrical contractors follow to minimize flicker when selecting an LED driver?
Rodriguez: Flicker percent or index is generally not advertised. It is hard to do so when the product is intended for phase voltage dimming. If flicker is of extreme importance the selection of other dimming schemes such as 0-10V will generally work better, but that is not guaranteed. Best strategy is to consult with the luminaire manufacturer and have a clear understanding of the operation requirements: dimming/not dimming, PWM dimming/constant current dimming, AC direct driver/constant current driver, etc.
DiLouie: What detailed recommendations should electrical contractors follow to minimize flicker when pairing lighting controls with LED lighting systems?
Rodriguez: It all begins with the power supply (driver). If the driver is not designed well to deal with flicker there are no controls that will make it better. Digital controls, whether wireless or wired, will not generally induce flicker in the system. These digital controls will interface with the driver in an analog fashion (0-10V), or in a digital form (Dali, nlight, I2C, LedCODE, Zigbee). To the driver network, controls or local controls will look the same.
DiLouie: What detailed recommendations should electrical contractors follow to minimize flicker that may be produced by other external sources such as voltage fluctuations on the power line? What applications present the greatest risk?
Rodriguez: It can be argued that voltage fluctuations cause visual effects in the light output. Strobing or light pulsing is the net result. These fluctuations are typically slow moving and extremely noticeable by the observer. They are generally annoying and will often result in callback to the electrical contractor. On the other hand, flicker may not be readily noticeable by the observer and, while it may not result in a callback, the observer may feel something is “not right” with the installation. Part of the problem here is the lack of knowledge. Consumers generally are unaware of flicker.
The greatest risk for the introduction of flicker in an installation is the use of phase-cut dimmers that are not approved by the luminaire manufacturer. Look into the list of approved dimmers before making the selection. Avoid phase-cut dimmers as much as possible and opt for digital controls or, at a minimum, 0-10V. Beware that “budget” or “value” products will likely be more prone to flicker than high-end quality products. Consult with the manufacturer when making the selection.
DiLouie: How can electrical contractors test for flicker in the field? What specific testing can they undertake to evaluate potential installations?
Rodriguez: Before undertaking tests at the site an electrical contractor should begin with the right products in the first place. Consult with the manufacturer and ask questions before taking product to the site.
There are several “quick” test tools for flicker that every electrical contractor should have in their toolbox. The simplest one is a flicker checker wheel. It is basically a flat wheel with a point at the bottom that resembles a top. When the wheel is spinning a pattern printed on the wheel will become visible or “choppy” as the wheel spins. If there is no pattern present, then flicker is minimal. Electrical contractors should do the test at full output and at several dim points to see if flicker is being introduced by dimming.
There are other more expensive tools available. Some come in the form of apps for a smartphone (but some in the industry question their effectiveness).
The simplest test however is waving a pencil near the luminaire. If there is no stop- motion on the pencil as you wave it back and forth flicker is minimal.
Also, don’t forget cameras. If the room is used for video conferencing, ask for it to be turned on before leaving. There may be little noticeable flicker in the space but it may be completely incompatible with the video system.
DiLouie: What can electrical contractors do to mitigate flicker after installation, if anything? What is the basic troubleshooting process and typical remedies?
Rodriguez: If flicker is there when the lighting products are not on a dimmer, there is not much to be done except look for a different product. When a dimmer is present, then determine if the flicker is there when the luminaire is not dimmed. If it is there then take the dimmer out and see if it goes away. If it does, then there is the possibility of getting a different dimmer that may lessen the effect.
DiLouie: What metrics are available to electrical contractors seeking to evaluate flicker potential of LED products?
Rodriguez: Standards are still being developed. Flicker percent and flicker index are the best two criteria. CEC, DOE, IEEE are publishing recommendations on minimum flicker performance. As time goes on we expect to see the standards moving from voluntary to mandatory.
DiLouie: What are the shortcomings of these metrics? What is the industry doing to develop suitable new metrics? How do you anticipate they will be used?
Rodriguez: The matrices are fine but they are hard to understand by the general public. The whole concept of flicker is often hard to explain given that the effects are perceived differently by different people.
DiLouie: If you could tell the entire electrical industry just one thing about the flicker and LED lighting, what would it be?
Rodriguez: All products flicker. The level or threshold of what is acceptable is widely debated. In many cases products with some flicker are perfectly fine and in other cases the same product is completely unacceptable. Be aware of the application. Ask your luminaire provider to educate you and show you what is available. Many times what looks like equivalent products will vary greatly in the area of flicker. The industry is currently generating standards which ultimately will remove a lot of these questions and, more importantly, help the electrical contractor identify good and bad performers.