Lux two-minute explainer: Flicker

There are two kinds of flicker

Visible flicker is the one that we actually see. It’s the rapid change in light output from a light source. Its generally reckoned that anything below a frequency of 100Hz (cycles per second) can be seen as a physical phenomenon. And it does have health implications.

Short-term exposure to frequencies in the range of 3Hz to 70Hz. are associated with epileptic seizures, with the greatest likelihood occurring in the range 15Hz to 20 Hz. Around one in 4000 people are identified as suffering photosensitive epilepsy and there may be as many again who have not been diagnosed, so this is a serious issue for lighting in large public spaces. And it explains why TV presenters warn us that ‘the following programme contains flashing lights’.

Invisible flicker is a real problem, however. It’s the flicker that we can’t see, but which we all experience. The symptoms of invisible flicker are familiar to us because they are the symptoms associated with sick-building syndrome; headaches, migraine, dizziness, general malaise, eyestrain, impaired intellectual and/or visual performance.

Although it's suggested that frequencies above 200Hz may have negligible biological effect, the upper frequency limit is not really known. An upper limit of 5.4kHz has been mooted, but the jury is still out on that as more evidence needs to be collected.

Where flicker comes from

There is a theatrical effect called ‘strobe’ lighting. This is a deliberate flicker effect that delivers light at frequencies that cause the brain to interpret a moving object as moving in slow motion. These frequencies are typically just a few flashes per second – and dangerously close to the 3Hz limit mentioned above.

There is also an unintentional flicker effect, often seen around the red carpet leading to the latest celeb-storm. Flash photography, taken en masse, can create the conditions necessary to trigger an epileptic seizure in some, while the rest of us just get really annoyed.

Unintentional flicker in lighting equipment is a fact of life. Ever since the power companies decided that the most efficient way to distribute electricity around the country was to use alternating current (AC) rather than direct current (DC). In the UK, the power supply is generated at 50Hz, a number that is of interest to us in terms of visible flicker because it sits neatly in the visible range. And because the sine wave has a positive and negative peak for each cycle, there is also a 100Hz frequency, and that’s not only visible but audible. A question for every electrical apprentice used to be, why does a transformer hum? The answer is not; ‘because it doesn’t know the words’, it’s because human hearing registers a frequency of 100Hz. For the musically inclined, 100Hz is pitched between a low G and G#, so basses can also hum along. And most of us have heard incandescent lamps ‘singing’. This is all down to the harmonics being generated by the frequency of the power supply.

 

Solving the LED flicker problem

The extent to which an LED flickers lies in the design of the LED driver. Very basic driver design relies on a simple circuit to control output current, but with no alteration to the mains frequency, thus leaving the LED susceptible to mains-borne visible flicker. This can be alleviated by using drivers that produce a constant current, so the peaks and troughs of the sine wave are removed.

Also, flicker can be caused by the power correction components in the driver circuit creating ‘ripple currents’. We can’t get rid of power factor correction; in fact, more LED circuits need to employ it to improve their energy efficiency, but we can ensure that driver circuits include compensatory components that reduce ripple current.

In fact, the simple answer to LED flicker is to ensure that driver design meets the requirements of both a good DC current and appropriate ripple suppression. There should be no flicker problem with LED technology; all of the necessary knowledge is there to ensure a flicker-free light environment, so the problem must lie with those companies who choose not to prioritise the health of their customers.

 

​Domestic LEDs

And having said all that, domestic LEDs do present a particular problem. These retrofit lamps often contain only basic circuitry, sufficient to drive the LED but with little or none of the protection against flicker discussed above. Some of these lamps may demonstrate mains-borne flicker simply by being switched on and, clearly, these lamps are not fit for purpose.
Dimming of LED retrofit lamps presents a specific challenge because it introduces another piece of electronic circuitry, the wall dimmer.

Many domestic dimmers work by ‘phase cutting’, a method that removes part of the sine wave and reduces the overall voltage. It’s a very clever thing to do, but it can wreak havoc on an LED circuit, and on those unlucky enough to be in the same room with it. The impact of the cut sine wave can be amplified by the LED driver and that can actually increase the flicker effect to a dangerous level, as flicker frequencies in the region of 3 – 15Hz have been experienced.


There is no straightforward answer to the retrofit scenario beyond checking with the manufacturer of the dimmer and asking if the lamp that’s just been purchased from the local hardware shop is suitable for use. This will not happen.

But a more strategic position would be something like this: decide that LED lighting is the future for your home. At this point, do a bit of research on companies that have developed LED-specific dimming for the home. An internet search for ‘LED dimmer switches’ returns a long list of companies offering this type of product. But don’t stop there. Once you’ve found a company that you like the look of, give them a ring and ask for their advice on the best LED lamps to use. If they’re good at what they do, they will have tested a range of LED lamps with their dimmers and will have all the bench-test results to prove it.

Investing in LEDs is a long-term choice, so its best to spend a bit more time on it now than to rue a toss-of-the-coin decision in a few months’ time.

 

Finally: a special safety note on stroboscopic flicker

There is a specific hazard associated with light flicker and moving machinery, especially spinning motors. As mentioned above, the strobe light creates an effect of slow-motion, but that’s an intended consequence. If you put together a light that flickers at a particular frequency with a piece of machinery that spins at a sympathetic frequency it is possible that the machine appears not to be spinning at all. We see this in the cinema when the spinning wheels of the stage coach appear to slow and go backwards as the driver attempts to escape the bad guys. That happens because of the film speed synching unintentionally with the rotation of the wheels; it looks dramatic in a movie, not so in a factory.