Alternating currentThe ‘European’ electricity is an alternating current with a frequency of 50 Hz. Various home appliances, televisions and lamps are connected to this. Incandescent lamps need to be hot in order to radiate light, and because thermal processes are relatively slow, changes in light intensity as a consequence of the alternating current are not noticeable. LEDs, however, react very quickly, with the result that light variations are sometimes visible.
DriverMost LED lamps are (therefore) not directly connected to the mains, but contain a handful of electronics – the driver – which regulates the current through the LED. This driver can reduce the visible consequences of the alternating current. However, with this there is always a trade-off between cost, size, reliability and efficiency. As a result, different driver configurations can lead to various degrees of undesirable effects in the light that is emitted from the LED lamps.
What you can't see won't affect youFlickering is immediately visible, but the so-called stroboscopic effect is more challenging to observe if you don't know what you are looking for. When the flickering light strikes a moving object, the effect is visible as a series of stationary images. The stroboscopic effect affects our perception of the quality of the light. That is because the human eye perceives this effect, which then can lead to annoyance, fatigue, but also headaches. It is important to determine when people are still able to sense that the light is stroboscopic. The research in this area indicates that only the visible flickering and the stroboscopic effects have biological reactions that potentially could affect your health. In short: what you can't see also won't affect you.
Visibility thresholdThis research searched for the so-called ‘visibility threshold’ of specific light characteristics at which the stroboscopic effect was still just perceptible to the human eye. For this purpose, 200 people were invited to look at a black disc with a white dot under an LED light source in a simulated office environment. The disc rotated at a constant speed, comparable to fast hand gestures in an office environment. During the experiments, various light parameters were changed: frequency, amplitude and wave shape. The test subjects indicated for each change whether they perceived a stroboscopic effect or not. Researchers from the Southeast University in China repeated the experiments to exclude cultural differences.
ModelBased on the thresholds that were found for every combination of the tested parameters, Perz developed an open-source model that quantifies the visibility of the stroboscopic effect of any light waveform. LED manufacturers can use this model to test their products. The method that was developed, is called SVM (Stroboscopic Visibility Measure) and is now recommended to quantify the visibility of stroboscopic effects in general lighting applications.
Testing your own LED lampsIf you know what you a looking for it is very easy to determine whether an LED lamp exhibits a stroboscopic effect. Wave a pen quickly up and down in front of the lamp. Subsequently, wave the pen – at the same speed – in front of a window. If you do not see a difference, then the driver in the LED lamp has been designed such that no stroboscopic effect is visible. If you see a continues blur (blurry image) in front of the window, while in front of the lamp you can see a series of stationary images, then the driver exhibits the stroboscopic effect.
(Source: TU Eindhoven)