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What is flicker and stroboscopic effects with LEDs?

Online meeting with camera

Especially during filming or online meetings, flicker-induced streaks can be seen.

Flicker is the unwanted periodic or non-periodic change of light and its effect on visual perception. These changes can relate to the luminance or the spectral distribution. These phenomena are summarised under the term "Temporal Light Artefacts" (abbreviation: TLA)

Overview of the topic flicker (LED)

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What causes flicker?

Flickering can lead to irritation and mood disorders. Flicker is often perceived particularly clearly and unpleasantly at the edge of the field of vision. If someone suffers from photosensitive epilepsy, flickering at a certain frequency can lead to epileptic seizures.
Below 60Hz, flicker is often consciously perceived, an example here is the mains frequency, which was clearly noticeable in earlier fluorescent lamps for example. Flicker with a frequency > 60Hz is perceived more unconsciously.

What is stroboscopic effect?

The stroboscopic effect can also be detrimental to filming by causing streaks to run across the image, or by falsely displaying the movement of objects. This effect results from interference between the frequency of the light and the frame rate.

Another possible effect of the stroboscopic effect is that moving parts are perceived as being stationary, or with slowed movement, or even moving backwards. These effects always occur when a moving part, e.g. the saw blade of a circular saw, is illuminated with a light source that itself has a certain frequency. The rotation of the saw blade interacts with the frequency of the light, and depending on the ratio, the saw blade is perceived to rotate the wrong way, it rotates more slowly or it is seen to be stationary – which can be fatal in terms of safety. Finally, with the stroboscopic effect, falling raindrops for example may be seen as a dotted line or a string of pearls.

What can trigger flickering or stroboscopic effect in an LED luminaire?

The LED light source itself is always without flicker. Whether the luminaire is flicker-free depends on the quality of the control gear used. The above effects do not usually occur with high-quality control gear.
There are various causes that can trigger flickering in an LED luminaire:

  1. One possible cause of flickering is load fluctuations in the mains grid that are not picked up by the electronics of the LED control gear.

  2. Another cause is that low-quality control gear converts the (mains) alternating current into direct current but does not sufficiently smooth it. This can cause periodic fluctuations in brightness that are perceived as flickering.

  3. Interference within LED control gear can cause the LED luminaire to flicker or flash briefly when switched.

  4. Another cause of flickering LED light can be the dimming mode of the control gear. Constant current regulation (CCR) is not critical with high-quality control gear. In contrast to pulse width modulation (PWM), the operating current is regulated when dimming the LED with the CCR method. PWM on the other hand switches the LED on and off in high frequency cycles, thus regulating the perceived brightness. The switching frequency is usually not perceptible to the human eye. Film cameras are more sensitive here, which can lead to artefacts (imperfections) such as horizontal streaks in the image, especially with strongly dimmed luminaires.

What are the measured variables for flicker in LED luminaires?

Regulation EU 2019/2020 on Directive 2009/125/EC (Ecodesign Directive) defines two evaluation variables for Temporal Light Artefacts (TLA):

  • The PstLM for the evaluation of visible flicker in the frequency range 0.3Hz to 80Hz. "st" stands for "short term", and "LM" stands for "light flicker meter method".

  • The "stroboscopic visibility measure" SVM for evaluating the stroboscopic effect on moving objects in the frequency range 80Hz to 2000Hz.

For both parameters, a value of 1 means that an average observer recognises the flicker with a probability of 50%. The permissible limits are 1 for PstLM and 0.4 for SVM.

An LED luminaire is considered flicker-free if the SVM value is <= 0.4 and if the PstLM value is <=1.0.

When is a luminaire suitable for video recording?

If a luminaire is dimmed by pulse width modulation (PWM) and filming is being done, streaks may appear in the image. Whether and how strong the streaks are depends on the frame rate and the frequency with which the control gear switches the LED on and off.

Good to know:
An LED luminaire is considered suitable for film if the SVM value is <= 0.1 and if the PstLM value is <=1.0.

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