The Calibration Method for Incomplete Charge Transfer in ToF Image Sensor

In this paper, a kind of calibration method for two-tap indirect time-of-flight (ToF) image sensor is proposed, which aims to reduce the distance error caused by the incomplete charge transfer. The phenomenon of two-phase charge confusion due to incomplete charge transfer makes a serious impact on the accuracy of ToF imaging especially under high modulation frequency. Current researches effort to improve the charge transfer ratio by changing pixel structure or fabrication process. The proposed method obtains a charge transfer ratio by reducing the frequency of the work timing. Then the true photon-generated charge of each phase can be calculated and the distance error can be calibrated. To verify the validity of the proposed calibration method, an accurate behavioral level model is established according to the actual ToF imaging system. The simulation results prove that the calibrated modulation contrast is improved by 60% under 100 MHz modulation and 0.1 charge transfer ratio. The calibrated modulation contrast is very close to the theoretical maximum value. When the signal strength increases or the times of charge transfer increases, the calibration effect on the distance error become more effective.