The early strategy of R&D at the Department of Energy Solid-State Lighting Program was to get rid of problematic flicker to make sure that LED products were not going to cause undue harm to people under those lighting systems, and also to make sure that people were going to accept LEDs as a light source in their offices, homes, streets, etc.

So PNNL set up a hand-built flicker meter—you know, this big box of electronics—that was able to take the light signal from the LED fixtures or LED chips themselves paired with a driver, and turn that into a waveform. So if we know the maximum amplitude and the minimum amplitude and how that changes over time, we get a waveform of how that LED performs. That’s very useful; from that information we can get all kinds of values to input into a metric to try to figure out which LEDs are problematic and which ones are not.

PNNL did all of this testing. A lot of this was CALiPER testing looking at commercial products, publicizing the waveforms, and speaking to manufacturers, to standards-making bodies, and even to the medical community, to talk about what kinds of products were producing flicker that might be a problem and how to quantify it.

So now we’ve come quite a long way. Manufacturers of instrumentation started developing their own flicker meters, and now, these flicker meters are proliferating. We now have much smaller and more accurate meters, so the measurement of flicker can be done quite easily these days.

There are different approaches to standardizing the measurement of flicker and setting appropriate levels for acceptable flicker depending on the standards body that you’re talking about. The first standards body that set a standard was the IEEE. The IEEE developed a standard that was published in 2015. It’s called the P1789 Standard. This was a document that had as background information all of the research information we could get our hands on. And that was looking at medical research on epilepsy, on headache frequency, on electronics and the kinds of waveforms that those can produce.

The effort was to look at the kinds of waveforms that are produced by particularly LED products and characterize them in simple ways so that we can set a minimum standard for products that are not going to cause neurological problems in users. They looked at setting a ‘no effect level.’ It’s called N-O-E-L, which means No Observable Effect Level. So there is a safety-level recommendation from this organization, which basically says take the flicker frequency of the product, let’s say it’s 120 Hertz, multiply that by .03, and if your percent flicker, which is also known as the percent modulation, doesn’t exceed that multiplied value, then you’re going to be safe with all populations.

There is also a low level of risk set by that organization that basically is similar. It says take the flicker frequency of the product, multiply that by .08, and that gives you a maximum percent flicker or percent modulation that is very, very unlikely to cause problems in the general population.

So that was the intent of that first standard. It’s a sliding scale based on frequency, because we know that lower levels of frequency are much more likely to cause problems in the population than higher frequencies.

Then there were other groups that started looking at additional factors to consider. They said this is a very good first step, but we really need to consider duty cycle, which contributes to the visibility of that flicker. Another thing that they said is, sometimes the waveform isn’t straightforward. It isn’t a simple sinusoidal wave or a square wave. It is more complex than that, so there has to be a different way to evaluate it.

There’s one from the Lighting Research Center that’s called M_p for example. What they have said is, you need to look at the raw waveform itself and do some kind of fast Fourier transformation on it in order to figure out what the fundamental frequencies are, and then analyze those frequencies.

So those are changes that have been suggested by these different groups. We have several groups that are developing standards. All of these are taking slightly different approaches at characterizing flicker in different ranges. We don’t have one that everybody agrees on yet, but at least we agree for the most part on the elements of the waveform that contribute to the perception of flicker.