The flickering of lighting in a scene can produce undesirable behaviors because of the interaction of a temporally modulated light source (e.g. an LED light with pulse-width modulation) and the image sensor.
For row-wise exposure architecture (rolling shutters) of many imaging sensors. Fundamentally, the issue is caused by the fact that the different rows of the imaging sensor integrate light over different periods of time and those different periods of time are also lit differently because of a temporally modulating light source.
Global shutter sensors do not see as extreme problems but are not immune to temporally modulated lighting effects, which can exhibit itself as a sort of “breathing” of exposure, where the light pulse may be synchronized with the sensor integration period (brighter), or synchronized with the sensor readout period (darker).
Flickering lighting can also confound HDR multi-exposure image-fusion.
This is a known and major concern, especially for the automotive
industry. This is why there is an entire subgroup devoted to this one effect in the IEEE P2020 Automotive Image Quality Working Group
(+ extra link to the more active P2020 site
). This group is currently putting together a white paper for publication which describes the scope of the standard and its subgroups- we can send a copy to you whenever it is published. Imatest has a large role in LED flicker subgroup.
Testing the effect
Currently, Imatest doesn’t have a solution for measuring this effect. No one really does- in fact, part of the P2020 subgroup’s challenge is even figuring out appropriate metrics to measure and describe it robustly. Imatest is working towards enhanced temporal testing.
Lab testing of this effect will almost surely use an LED light source with an adjustable frequency PWM control, in the range of 50Hz to 10kHz+. We do not have any hardware solutions for this yet, please contact us
if you are interested in testing flicker.
By using high-frequency pulsed LED’s (>100Khz), or a high-frequency ballast for fluorescent, this allows several cycles of pulsing to appear during the row or sensor integration time, the effects of flicker can be minimized.
LED’s that use phosphors can dampen the flickering effect of the underlying diode.
Using constant current controlled power supplies can completely eliminate the appearance of flicker.