The introduction of pulse width modulated LED lighting in automotive applications has created the phenomenon of LED flicker. In essence, LED flicker is an imaging artifact, whereby a light source will appear to flicker when image by a camera system, even though the light will appear constant to a human observer. The implications of LED flicker vary, depending on the imaging application. In some cases, it can simply degrade image quality by causing annoying flicker to a human observer. However, LED flicker has the potential to significantly impact the performance of critical autonomous driving functions. In this paper, the root cause of LED flicker is reviewed, and its impact on automotive use cases is explored. Guidelines on measurement and assessment of LED flicker are also provided.
High-frequency flickering light sources such as pulse-width modulated LEDs can cause image sensors to record incorrect levels. We describe a model with a loose set of assumptions (encompassing multi-exposure HDR schemes) which can be used to define the Flicker Signal, a continuous function of time based on the phase relationship between the light source and exposure window. Analysis of the shape of this signal yields a characterization of the camera’s response to a flickering light source–typically seen as an undesirable susceptibility–under a given set of parameters. Flicker Signal calculations are made on discrete samplings measured from image data. Sampling the signal is difficult, however, because it is a function of many parameters, including properties of the light source (frequency, duty cycle, intensity) and properties of the imaging system (exposure scheme, frame rate, row readout time). Moreover, there are degenerate scenarios where sufficient sampling is difficult to obtain. We present a computational approach for determining the evidence (region of interest, duration of test video) necessary to get coverage of this signal sufficient for characterization from a practical test lab setup.