Two-minute explainer: (In)visible random flicker

One of the yardsticks for good LED design is that the electronic circuitry should not produce any perceptible flicker. Flicker is potentially harmful to health and can trigger migraines and epileptic fits – but the difficult word here is ‘perceptible’.

Typically, we expect flicker to be something that most of us can see, but that’s the easy part and easily remedied. More difficult is the flicker that is generated but cannot be seen except via testing equipment on the lab bench. Once that flicker is identified, then, again, it can be dealt with.

But what about the flicker that is not a regular feature of a circuit and only occurs when a particular set of circumstances comes together? And what if that set of circumstances varies from circuit to circuit? Welcome to the phenomenon that is (in)visible random flicker (VRF).

It sounds like the stuff of paranoia, searching for something that may not be there, but extensive testing by one of the UK’s major driver manufacturers has uncovered this effect in some of the latest generation integrated circuits (ICs) – and these from respectable manufacturers where build quality is not in question.

It appears that the effect is created when a specific conjunction of variables in the circuit come together; say, the output setting of a dimmer meets the internal algorithm of the IC that’s been built into the circuit of the driver. And, like the flat spot in an engine, the effect disappears by altering the variables – a change of speed, a shift of gear, the temperature of the engine . . . and all is well.

What can be done about it? VRF is very difficult to measure: the levels are very low and can be buried in the noise floor of measuring equipment. The answer appears to be that vigilance is the only cure. Each driver needs to be inspected across the range of its operation, which probably makes it an impossibly expensive operation for any manufacturer to take on. But, at least, if we know that (in)visible random flicker is out there, we have something to look for when a client complains about health problems occurring around their lighting.

Thanks are due to Jerry Lister, senior design engineer at Harvard Technology, for bringing this phenomenon to our attention.

For more information on the harmful effects of light flicker:

Epilepsy Action – photosensitive epilepsy

Migraine.com – sensitivity to light

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