Terry K. McGowan, FIES, LC

ALA Director of 

Engineering & Technology

This is my second column on lighting performance factors as applied to LED light sources. My October column covered the subject of color, and particularly, the color rendering index, or CRI. In this column, I will discuss flicker.  In future columns, I plan to discuss such things as LED dimming, audible noise from LEDs, color shift, lamp life and maintenance of light output over time.

There are two reasons to consider LED flicker in the context of lighting performance factors. First, there are several new measurements and test procedures in development that apply particularly to LEDs, and these are different from what we have been using for traditional incandescent and fluorescent light sources. Second, when incandescent sources were thought of as the only source for residential lighting, flicker, like most of the other performance factors I have listed, took care of themselves. For example, we did not concern ourselves much with color rendering or flicker when we chose a table or floor lamp designed to use a couple of 60 watt bulbs. Now, those things matter, as we've learned - often the hard way - through direct experience. And, we understand that the market success of an LED product and its sale to a particular customer depends upon our knowledge of a broader range of factors, including whether such factors as flicker will change when a light source is used with dimmers or other additional system components.

All of us routinely experience flicker from lighting situations. Motion pictures flicker at 24 frames/second and, of course, standard alternating current (AC) is supplied at 60 Hertz, which means that all lighting operating directly from the AC line flickers - turns on and off - 120 times per second. Most people don't notice flicker with incandescent lighting because the light output of the bulb depends upon the average temperature of the filament and the "thermal inertia," or the time that the filament requires to heat up and cool down is longer than 1/120th of a second. 

Flicker problems with light sources didn't start with the use of LEDs, but rather, with fluorescent lighting, which used magnetic ballasts because fluorescent phosphors have what is called a "short persistence time." Therefore, the light output of the lamp closely tracks the variation of the 60 Hz sine wave. There is zero light output when the power to the lamp is zero and it takes a fraction of a second to come back on as power returns for the next half of the AC cycle. LEDs respond even more rapidly than fluorescent lamps; but flicker from both fluorescent and LED lighting today is really determined by the design of the electronic ballast and LED driver circuitry being used because such circuitry can smooth out peaks and valleys of power flow to the light sources.

To ensure that flicker is not a problem for today's LED lighting systems, flicker should be specified and lighting systems tested accordingly using a standard test procedure. Unfortunately, testing is not always done and there are no general, agreed-upon test procedures. Worse yet, the two defined metrics for flicker - Percent Flicker and the Flicker Index - were designed for incandescent and fluorescent light sources and do not report flicker reliably for LED light sources. If you would like to learn more about those metrics and how they apply to LEDs, take a look at the DOE Fact Sheet on "Flicker." It is available here, on the DOE website.

Until better metrics and testing practices are available, what should you do to ensure flicker-free lighting?

I suggest the following:

• Evaluate flicker for yourself. Look at LED light sources out of the corner of your eye. There is flicker of those sources jump or "strobe." Try this test looking at a string of multicolored holiday lights. Such lights typically use AC LEDs (no driver), and since they do not use phosphors either, they have a strong flicker component, which most people will easily see.
• Check the DOE publication referenced above to see how to use a pen or pencil as a simple flicker tester.
• Look on LED fixture and lamp specification sheets to see if either the Flicker Index or Percent Flicker values are reported. For standard incandescent bulbs, the Flicker Index will typically be in the 0.02 - 0.04 range, and the Percent Flicker will be about 6-7%. Higher values for each metric indicate more flicker. 
• If you are a lighting retailer, ask your manufacturer suppliers about the flicker ratings of their LED products. Not everyone sees or is sensitive to flicker; but my experience is that those who are sensitive to it are strongly affected and will complain about lighting products that flicker.
• If you are a manufacturer, talk to your component suppliers about the flicker characteristics of their driver products and check them visually yourself.  

Special thanks to Underwriters Laboratories, Inc., (UL) for their sponsorship of this column. You should know that the UL sponsorship comes without strings regarding editorial content or review. You and I determine the content. 

And, of course, your comments and questions are always welcome.

Sincerely, 

Terry McGowan, FIES, LC

Director of Engineering & Technology

[email protected]