A wide dynamic range CMOS image sensor with dual charge storage in a pixel and a multiple sampling technique

Abstract

This paper presents a dynamic range expansion technique of CMOS image sensors with 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 photodiode can be manipulated to expand the dynamic range of the sensor. It allows flexible control of the dynamic range since the accumulation time in PD2 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-nose ratio dip at the switching point of the PD1 signal to the PD2 signals in the synthesized wide dynamic range image. A wide dynamic range camera with 320x240 pixels image sensor has been tested. It is found that the sampling of 4 times 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 dip at the switching point of the PD1 signal to the PD2 signal is improved by 6 dB using 4 short-time exposures.