A Dynamic Range Expansion Technique Using Dual Charge Storage in a CMOS APS and Multiple Exposures for Reduced Motion Blur

Abstract

This paper presents a dynamic range expansion technique for CMOS image sensors using dual charge storage in a pixel and multiple exposures. Each pixel contains two photodiodes, PD1 and PD2 whose sensitivity can be set independently by the accumulation time. The difference of charge accumulation time in both photodiodes can be widely controlled to expand the dynamic range of the sensor. It allows flexible control of the dynamic range since the accumulation time in the PD2 signal is adjustable. The multiple exposure technique used in the sensor reduces the motion blur in the synthesized wide dynamic range image when capturing fast-moving objects. It also reduces the signal-to-noise ratio dip at the switching point of the PD1 signal to the PD2 signals in the synthesized wide dynamic range image. To reduce the read out time, a comparator-controlled selective readout of the PD1 and PD2 signals has been tested. The synthesis of the captured images to produce wide dynamic range image is also described. A wide dynamic range image sensor with 320x240 pixels has been implemented and tested. It is found that the use of 4 exposures in one frame for the short accumulation time signals is sufficient for the reduction of motion blur in the synthesized wide dynamic range image, and the signal-to-noise ratio dipping at the switching point of the PD1 to PD2 signals is reduced by 6 dB using 4 short-time exposures.