Flicker and stroboscopic effects with LEDs

Flickering and stroboscopic effects - the special of LED lamps

How does a flicker arise in LED lamps? What should you watch out for when buying an LED workplace light? Is every LED light flicker-free?

How does a flicker arise in LEDs?

LEDs differ significantly in their functionality compared to conventional light sources such as compact fluorescent lamps. An LED is a semiconductor that only emits light when the current flows in the forward direction. With negligible exceptions, an LED cannot be operated with the pure mains voltage (alternating current) because the electrodes can no longer flow back – therefore, a rectified current is required. Special LED converters supply the LED light with stabilised direct current. They ensure that the wind no longer flows in two directions but only in one order.

In contrast to conventional light sources, we have to look at flicker with LED lights from a different perspective. LEDs flicker depends on the quality of the LED converter and the dimming process.

The quality of LED converters and the ripple current

High-quality converters (LED drivers) smooth the frequency waves with the help of input and output filters to no longer perceive the waveform of the frequency as flickering. We use the display of the ripple current to evaluate the residual ripple on the direct current side. As a ripple current, we refer to an alternating current superimposed with direct current. Fundamentally, the lower the wave of the converted AC power, the less flickers the LED lamp. However, from a technical and, above all, from an economic point of view, it is impracticable to convert the residual ripple of the reversed alternating current into a completely flattened direct current.

As a rule of thumb, LED converters with a ripple value of max. 10 % have so little residual ripple that flickering is almost invisible to the eye. Our RMD workplace lights achieve ripple values of below 5 % and are, therefore, “flicker-free”.

The dimming of the LED light

In the case of dimmable converters, the second criterion for assessing the quality is the dimming process. We have to reduce the operating current to dim a lamp. There are two ways to do this:

Dimming via pulse width modulation (PWM for short):

This method allows us to turn the LEDs on and off regularly. We make the human eye believe that the brightness is lower because it is too slow to see this flicker. We perceive the average brightness of light, i.e. the ratio between bright (LEDs switched on) and dark (LEDs switched off). The lower the dimming level, the longer the “dark phase” of the LEDs.

Fig. 1: Dimming of an LED luminaire using pulse width modulation (PWM)

Technically easy to implement, the dimming method via PWM, however, comes with some dangers. The switching frequency of the LEDs is particularly decisive. Since the LEDs’ luminous flux is repeatedly interrupted abruptly, the luminaire flickers without attenuation at the PWM frequency used. To ensure that the human eye does not see this flicker, very high frequencies are required. Especially in cheap luminaires or older models, problems occur due to PWM frequencies that are too low, often noticed late. The consequences are adverse effects on the human organism as the concentration drops rapidly and health problems such as headaches occur. It becomes particularly dangerous with moving objects. Especially with low-frequency luminaires, strong stroboscopic effects can occur, causing hazardous situations.

In addition to digital dimming via PWM, there is also the option of dimming the LED luminaire analogue. This method is called amplitude dimming.

Analogue dimming (amplitude dimming):

With analogue dimming, the luminous flux is maintained continuously. In this case, we dim the luminaire by reducing the amplitude until we have reached the desired brightness.

Fig. 2: Analogue dimming of an LED task light (amplitude dimming)

A significant advantage of this method is flickering due to the dimming method used, and we can effectively prevent a stroboscopic effect. Only at shallow dimming settings will this method reach its limits. The disadvantage is that we can’t set the brightness as precisely as with PWM. Possible consequences can be slight shifts in the colour locus or an uneven distribution of illumination. However, these restrictions are limited to shallow dimming values, which we rarely use in practice.

To enable our customers to work safely and without interference, we, therefore, rely on high-quality, state-of-the-art LED converters from the brands OSRAM and Tridonic in our RMD workplace lights. We use both high-frequency PWM dimmer circuits and analogue amplitude dimming. However, due to the advantages of amplitude dimming, we almost exclusively use amplitude dimming for our LED lights.

For example, we have been able to help our watchmaker customers see disturbances that were previously difficult to see due to low-frequency lights dimmed with PWM.