The Brighter Choice
Flicker is the repeated and frequent variation in the output of a light source over time. Some authors speak of the “modulation” of the light output, others speak of the light output rising and falling or getting brighter and dimmer, but these are all different ways to describe the same thing. Flicker.
All artificial light sources flicker, but the extent and the seriousness of the flicker depends on several variables.
This article will discuss and explain flicker in relation to LEDs and their associated driver connected to a mains voltage AC power source.
If you are not familiar with the term “driver” in a lighting context we suggest you read the next few paragraphs.
WHAT IS A DRIVER?
In the field of LED lighting, a driver is a piece of electronics whose main functions are:
This is necessary because LEDs require a direct current (DC) supply, at a low voltage – typically 12 or 24V. However, the mains supply is an alternating current (AC) at 230V.
Drivers can be separate from a light fitting, they can be integrated inside a light fitting and they can be integrated inside an LED lamp, but where there is an LED there will always be a driver – somewhere – performing the above two functions.
LED drivers, suitable for use in or alongside many types of commercial light fitting. They are separate from the LEDs and can often be changed if required.
An LED lamp, popular in many residential buildings. The driver is hidden from view inside the base of the lamp. Though a different shape it is still performing essentially the same function as the separate drivers shown above.
WHAT ARE THE CAUSES OF FLICKER IN LEDs? T
There are three common causes of flicker in LEDs.
1.The mains supply is 50Hz AC. In the UK and Europe the mains electrical supply is 230V AC, meaning that every second the mains supply goes from +230V to -230V and back to +230V (see diagram). Twice in each of these cycles the voltage is 0V, so no current is flowing. The output of light from an LED is (broadly) proportional to power input, and instantaneous. Therefore, if no “smoothing” of the AC cycle was carried out by the driver, the LEDs would switch off and on again 100 times every second.
Mains supply.
50 times every second the mains supply varies from +230V to -230V and back to +230V. This means that 100 times every second, when the voltage is zero, there is no current flowing at all.
This characteristic of the mains supply is the most significant root cause of LED flicker.
To reduce the intense and obtrusive flickering that this would cause, drivers are designed to smooth out the DC supply to the LEDs. Largely, this is done with capacitors which act like a mini reservoir for electricity, absorbing power at the peak of the cycle and releasing it in the troughs. However, smoothing is never completely effective, in the same way that the suspension system in a car never smooths out all the bumps in a road, so there is almost always a slight 100Hz fluctuation in the output of LEDs.
Finally, capacitors do not last forever, and failed capacitors are one of the causes of more intensely flickering LEDs in older light fittings.
2.The wrong sort of driver is being used. While LEDs are all dimmable the same is not true of drivers. Mains dimming (where the mains supply to the driver is reduced to cause the light output from the LEDs to dim) only works if the driver is designed to be treated like this, and if the correct type of mains dimmer is used.
Dimmers designed for incandescent (tungsten filament) lighting are almost always NOT suitable for dimming LEDs and their drivers. Using the existing dimmers after the lighting has been converted to LEDs often causes flickering. For more detail, please check this article: What is dimming ?
3.The wrong type of dimming is being used. Pulse Width Modulation (PWM) was the first dimming method to be used with LEDs, largely because it is cheap and easy for an electronics engineer to implement. Its disadvantage is that it generates a square wave form which is more likely to be visible to the naked eye than a waveform with more curved edges. Especially when implemented at mains frequency and at low dimming levels, say below 50%, the “off” content of the waveform is so pronounced that obtrusive levels of flicker are often observed.
WHAT ARE THE FEATURES OF LIGHT FLICKER?
4 features, or variables, determine if flicker has a serious impact on the viewer:
Low frequency flicker (3Hz – 70Hz) is visible to most people and carries the risk of triggering an epileptic seizure in individuals who are susceptible, with the greatest risk being in the 15Hz – 20Hz range.
Most people will be consciously aware of flicker up to a frequency of about 100Hz, though this will depend on other factors including the amplitude of the flicker, the intensity of the light, the contrast with the background and the field of view.
Above about 100Hz most people become progressively less conscious of flicker, though their brain may still sub-consciously detect that it is present and their bodies may therefore react to it.
Above about 200Hz the human retina cannot detect flicker at all.
Examples of two different types of flicker.
One is low frequency but low amplitude, the other is high frequency and a higher amplitude, but note that no causal relationship between frequency and amplitude is implied.
Above about 100Hz humans become less and less affected by flicker as the frequency increases, so higher amplitudes of flicker are acceptable at high frequencies.
The design of an LED driver has a significant effect on the amplitude of the fluctuation of the DC supply to the LEDs, and hence of the fluctuation of the light output from the LEDs.
This is because LEDs react almost instantaneously to a change in electrical input. If the electrical input to an LED increases, the LED gets brighter immediately; if the electrical supply is cut-off the light output stops immediately. This is different from the way old incandescent (tungsten filament) lamps worked. With them, there was thermal inertia in the filament – meaning that if the electrical supply was cut-off the filament continued to glow for a short period. This meant that a crude dimmer using a triac, operating at 50Hz, even though it cut the electrical supply abruptly 100 times a second, gave a smooth dimming performance when coupled with a tungsten filament lamp.
HOW DO YOU MEASURE FLICKER?
There are two ways to measure flicker; flicker index and modulation % (also known as % flicker). Both are defined in IEEE1789 but are derived from earlier work by various authors.
Diagram from IEEE1789 explaining flicker index and % flicker.
Flicker index = (Area 1) / (Area 1 + Area 2)
Modulation % (aka % flicker) = 100 (Maximum value – Minimum value) / (Maximum value + Minimum value)
In most common lighting applications, the lower the flicker index and/or the modulation %, the better.
WHAT ARE THE HUMAN RESPONSES TO LED FLICKER?
The human response to light flicker can be divided into 3 main types:
A summary of the biological effects of flicker at different frequencies. (Wilkins, Veitch & Lehman, “LED lighting flicker and potential health concerns: IEEE standard PAR1789 update, “Energy Conversion Congress and Exposition, Atlanta Georgia, pp. 171-178, 2010).
WHAT STANDARDS REGULATE FLICKER IN LEDs?
IEEE1789 is the most widely used standard for regulating flicker in LEDs.
IEEE1789 recognises that frequency and amplitude should be considered together when regulating flicker and the recommendations of the IEEE are sumarised in the graph below:
In summary, IEEE1789 recommends that:
FREQUENTLY ASKED QUESTIONS?
What is a flicker-free driver?
Strictly speaking there is no such thing as a flicker free driver. The output from all drivers fluctuates and this will cause a fluctuation of the light output of the LEDs that are connected to it. However, it is possible to make a driver with a very stable output that falls within the recommendations of IEEE1789 and some people call this a flicker-free driver.
Does flicker matter?
Yes, flicker matters because at some frequencies it causes headaches and tiredness, and in extreme cases, with some people, flicker can induce an epileptic fit.