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What causes LED flicker – and how can I stop it happening?

Why makes LED lights flicker?

Well… put simply, flickering LEDs are caused by the regular fluctuation of light output, as your light-emitting diodes switch on and off at very high speed.

We’ll dig deeper below.

Let’s begin by debunking a common misconception: because LED flicker is caused by your electricity supply, it can be avoided completely as long as the power input (current and voltage) remains constant.

This simply isn’t true.

Even though you don’t always see it, all mains-powered light sources – whether incandescent, halogen, fluorescent or LED – are affected by flickering.

In the UK, mains electricity is an alternating current (AC) supply, delivered at a frequency of 50 hertz. Which means that the current powering your lights – any lights – switches backwards and forwards 50 times each second.

We didn’t used to notice it with our old incandescent bulbs, because residual heat kept the filament glowing between flickers… this was a side-effect of the inefficiency of traditional bulbs, which typically wasted around 90 percent of input energy through heat.

Keeping an eye on your LEDs

Because LED lighting requires a direct current (DC) rather than an AC power supply, the key to eliminating LED flicker is the transformer you use to power your lights.

A high-quality transformer will supply a constant current to your LEDs, which will produce light without any visible flicker.

A lower-quality, no-frills transformer doesn’t provide a constant current though; instead, it simply converts current from AC to DC. This most basic kind of power-supply conversion produces an oscillating current, albeit one that typically doubles the input-voltage frequency. In the UK, that will result in a frequency of 100 potential flickers a second.

100 flickers per sec is obviously much better than 50 flickers per sec. But it still sounds like a problem, doesn’t it? Luckily, in most situations it’s nothing to worry about – because the human eye isn’t perceptive enough to see it. Most of us only register light fluctuating at well below 100 flickers per second – typically 50 or slower. (Computer screens usually flicker in the range of 60 to 70 hertz, which we barely notice.)
So even though there’s certainly a tiny minority of people who can see faster strobing, for most of us it’s not an issue. In many project installations, a basic no-frills transformer is all you need. There are even some applications where LED flicker can actually be a desirable effect: think of nightclubs for example, or oscillating bicycle lights.

Constant current = constant brightness

But if a simple transformer isn’t enough for your project, then an excellent alternative for you would be a constant-current power supply.

These higher-spec transformers can practically eliminate flicker, by varying voltage across the circuit in order to generate a constant electric current. This ensures the current delivered to your LEDs doesn’t fluctuate at all, which mitigates the effects of the transformer’s AC/DC conversion.

Even a constant-current transformer can’t eliminate flicker in all situations, though. Interference caused by incompatibility issues with your control circuitry are one common cause; before installing, you should confirm your LED products are suitable for the control circuits and power supply you’re using.

Loose wiring and other faulty connections can also cause problems.

But the single most frequent cause of visible flicker in LED lighting installations is the implementation of a dimming function.

Dealing with dimming

Dimming can cause problems even in an otherwise perfect LED lighting installation. That’s because conventional dimmers work by lengthening the ‘off’ part of each on-off flicker cycle, in order to reduce the total amount of light that’s being output.
This technique is called pulse width modulation (PWM). PWM works very effectively – just as long as the switching frequency doesn’t drop to a level that the human eye can perceive.

Some manufacturers are working to solve this problem by developing LED dimmers with a much faster flicker cycle. A cycle of thousands of hertz is the goal, simulating the solution used by the electronic ballasts that have powered fluorescent lighting for many years. There’s a downside, though: the higher the flicker frequency, the nearer to your transformer your LEDs will need to be. Which is not always practical.

Rather than installing these less flexible (and more expensive) dimmers, you can very easily avoid the visible flicker created by pulse width modulation. By simply not dimming your LEDs so low, you’ll prevent their flicker cycle becoming visible to the eye.

Just a few years ago, that often meant not dimming below around 50% of full brightness. But now. the dimmers in our current range all allow you much greater flexibility. You may discover that you can dim all the way without any visible flicker!

Generally you’ll find that zero-to-10V dimmers will be less prone to flicker than, for example, a mains TRIAC control.

 

Pulse width modulation dimming

The checklist

Over the past decade, LEDs have been adopted by the lighting industry as the energy-efficient lighting solution of the future.

With all the advantages they offer, that’s not surprising.

But to avoid the effects of LED flicker, you and your electrician will need a basic understanding of the issues behind it. So always bear in mind the points we’ve discussed above.

Let’s finish with a summary:

  • Make sure your LED products are compatible with the control circuits and power supply you’re using.
  • Check for loose wiring and other faulty connections.
  • Consider using a constant-current power supply.
  • When installing a dimming system, experiment to see if there’s a minimum dimming level that you shouldn’t go below.
  • For dimming systems, consider using a zero-to-10V or digital volt-dimming system rather than a TRIAC alternative.

 

Lighting Ergonomics - Light Flicker

What is light flicker?

Light flicker refers to quick, repeated changes in light intensity - light that appears to flutter and be unsteady. It is caused when the voltage supplied to a light source changes or when the power line voltage itself fluctuates. The severity of the flicker depends on several factors such as:

  • How often and regularly the voltage fluctuates.
  • How much of a voltage change occurs.
  • The kind of light (incandescent, fluorescent, or HID - high intensity discharge lighting systems).
  • The gain factor of the lamp [gain factor is a measure of how much the light intensity changes when the voltage fluctuates - (% relative change in light levels) divided by (% relative fluctuation in voltage)].
  • The amount of light in the lighted area (ambient light levels).

Lamps operating on AC electric systems (alternating current) produce light flickering at a frequency of 120 Hertz (Hz, cycles per second), twice the power line frequency of 60 Hz (50 Hz in many countries outside North America). Essentially, the power is turning on and off 120 times a second (actually the voltage varies from +120 volts to -120 volts, 60 times or cycles a second and is at zero volts twice in one cycle).

Can you actually see lights flicker?

It depends on the frequency of the flicker. People can see lights flashing on and off up to about 50 flashes per second (50 Hz) - they are most sensitive to time-varying illumination in the 10-25 Hz range. The actual critical flicker frequency increases as the light intensity increases up to a maximum value, after which it starts to decrease. When a light is flickering at a frequency greater than 50 or so Hertz, most people can no longer distinguish between the individual flickers. At this frequency - the critical flicker frequency or flicker fusion threshold - the flashes appear to fuse into a steady, continuous source of light. This happens because the response to the light stimulus lasts longer than the flash itself.

Most people cannot notice the flicker in fluorescent lights that have a flicker rate of 120 cycles per second (or 120 Hz).

The light flicker may be detected by its stroboscopic effect. When objects move or rotate rapidly, they may be lit at or about the same position during each cycle or rotation. This makes objects look as if they are moving more slowly than their actual speeds - they may even appear stationary if the object is moving at the same rate as the flicker frequency (or a multiple of it). This fact is the principle behind a strobe light but it is not the desired effect in general lighting. In fact, it could be a safety hazard if someone mistakenly thought that some equipment was stationary or was moving slowly.

Are there any health effects associated with light flicker?

Although humans cannot see fluorescent lights flicker, the sensory system in some individuals can somehow detect the flicker. Ever since fluorescent lighting was introduced in workplaces, there have been complaints about headaches, eye strain and general eye discomfort. These complaints have been associated with the light flicker from fluorescent lights. When compared to regular fluorescent lights with magnetic ballasts, the use of high frequency electronic ballasts (20,000 Hz or higher) in fluorescent lights resulted in more than a 50% drop in complaints of eye strain and headaches. There tended to be fewer complaints of headaches among workers on higher floors compared to those closer to ground level; that is, workers exposed to more natural light experienced fewer health effects. [ Fluorescent lighting, headaches and eye-strain. A. J. Wilkins, I. Nimmo-Smith, I., A. Slater & L. Bedocs. Lighting Research and Technology, 1989. Vol. 21, 11-18]

What kind of things can cause light flicker or dimming?

Voltage changes can be caused by dimmer switches or when electrical equipment drawing heavy currents are turned on or when being used (e.g., resistance welding machines; motors in refrigerators, air conditioners; arc furnaces; medical imaging machines (x-ray, CAT scan, MRI); motors subject to variable loads; large capacity photocopiers). Resistance welding machines that repeats welding at a rate of one or more per second can cause repetitive voltage fluctuations and may result in a noticeable light flicker.

Usually voltage fluctuations are small and do not have adverse effects on electrical equipment. However, in offices, for example, voltage fluctuations of just a few tenths of one percent can produce very annoying flickers in the lighting, especially if they are regular and repetitive in the 5-15 Hz range.

What kind of lighting is likely to cause a flicker problem?

Flicker is usually a potential problem only with lighting that requires the use of ballasts, like fluorescent lights. Incandescent lights usually do not cause a flicker problem since the light filaments generally do not cool quickly enough (and make the light dimmer) during the "off" time as the voltage changes in the AC power line.

The type of ballast, which controls the electrical supply to fluorescent lights, affects the amount of flicker. Magnetic ballasts change the voltage supplied to the fluorescent lamps but do not alter the frequency - the power line frequency of 60 Hz. The ultraviolet (UV) light produced inside the fluorescent light tube also fluctuates 120 times per second. The phosphorescence (the fluorescent light) resulting from the UV shining on the phosphor coatings inside the light tube is sufficiently stable (i.e., lasts long enough) to even out the variations in the fluorescent light output.

What can be done to reduce or eliminate light flicker?

Some types of ballasts can reduce flicker considerably. New, energy-efficient electronic ballasts take the 60 Hz supplied power and convert it to a much higher frequency (20,000 - 60,000 Hz). The resulting flicker frequency (twice the supplied power frequency, 40 -120 kHz) is so high that the human eye cannot detect any fluctuation in the light intensity - essentially flicker-free. An added benefit is that electronic ballasts produce less hum than that emitted by other kinds of ballasts.

To correct flicker:

  • Replace bulbs on a scheduled basis. Old bulbs tend to flicker more and they are not as bright.
  • Ensure that all parts of the light fixture, especially the ballast, are functioning properly.
  • When replacements are needed, upgrade to fluorescent lighting that uses electronic ballasts.

Document confirmed current on August 1, 2013

 
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