An approach to avoid stroboscopic effects on pulse-width modulated LED headlamps

Dipl.-Ing. Dmitrij Polin, Prof. Tran Quoc Khanh, TU Darmstadt, Germany

Light emitting diode (LED) has become an increasingly important light source in most applications of automotive lighting systems. In addition to their efficiency, LED technology promises a variety of benefits in headlamp design and advanced functionality for example adaptive light distribution known as adaptive driving beam (ADB). ADB systems consist of many LEDs and adapt the light distribution of the high beam according to road conditions by dimming single LEDs.

The mostly used method for dimming is pulse-width modulation: Turning the LED on and off at frequency beyond the flicker fusion threshold prevents the perception of single light pulses and allows to control the brightness by varying the pulse duration.

Nevertheless, it is possible to perceive flicker indirectly through stroboscopic effects at frequencies above the flicker fusion frequency. A stroboscopic effect known as beads effect or phantom array occurs during a rapid eye movements. By eye movements, the pulse-width modulated light is projected on different areas on the retina. Thus, the on and off switching of the pulse-width modulation causes a spatially extended series of lights. This effect was already investigated on pulse-width modulated rear lamps based on LEDs (Fig. 1) and a frequency above 300 Hz were suggested for this use case.

Figure 1 Beads effect rear lamps

It is also possible to observe this effect on retro reflective traffic signs by using pulse-width modulated high beam (Fig. 2). Two studies were conducted addressing this case. One study deals with investigations under controlled lighting conditions in the laboratory. Based on its results an approach to reduce the stroboscopic effect was developed and thereafter another study was conducted under real traffic conditions.

Figure 2 Beads effect retro reflective traffic signs