Illumination flicker correction and frequency classification methods

In many modern CMOS imagers employing pixel arrays the optical integration time is controlled by the method known as rolling shutter. This integration technique, combined with fluorescent illuminators exhibiting an alternating light intensity, causes spatial flicker in images varying through the sequence. This flicker can be avoided when the integration time of the imager is adjusted to a multiple of the flicker period. Since the flicker frequency can vary upon the local AC power frequency, a classification must be performed beforehand. This is either performed utilizing an additional illumination intensity detector or, in the case we focus on, restricting to image information only. In this paper we review the state of the art techniques of flicker detection and frequency classification, and propose two robust classification methods based on a clear mathematical model of the illumination flicker problem. Finally we present another approach for compensating for flicker in single images suffering from these artifacts by inversing the flicker model function. Therefore, the flicker phase, amplitude and frequency are to be adjusted adaptively. This however compensates for the fact, that the shutter width is no longer limited to a multiple of the flicker period. We present our simulation results with synthesized image series as well as with real captured sequences under different illumination frequencies, whereas our approaches classify robustly in most imaging situations.