| With the increasing demand of 3D imaging system, more and more researches are focused on obtaining the complete 3D information of the human eye. Relative to traditional real-time 3D imaging system of optical intensity imaging system has more and more application fields, such as: automotive safety, biomedical devices, robot, entertainment equipment, industrial control and virtual reality equipment field. The Time-of-Flight(TOF) ranging method is one of the most suitable methods for 3D imaging distance measurement. The image processing circuit and photosensitive part can be integrated in a chip of CMOS image sensor, which compare other imaging techniques have the advantages of high integration, low power consumption and low price. It can also obtain two-dimensional image and distance information and make it become the best choice for the development of 3D imaging.Firstly, this paper analyzes the feasibility of indirect TOF ranging principle, including the principle of phase demodulation and time-gated through integral method, then carries on an analysis of the accuracy of the system, summed up several important influence factors, including the source power, noise, measurement range, reflection coefficient, demodulation efficiency and intensity of background light. Thus it can be worked out an index system to aid in the design. Based on the structure of pinned photodiode to design 2-tab demodulation pixel. On the basis of traditional PPD for improvement and optimization, improving electron transfer efficiency and bandwidth of the optoelectronic mixer to design the high demodulation efficiency of demodulation pixel. The circuit part of the image sensor includes a pixel circuit, correlated double sampling circuit, drive circuit, row, and column decoding circuit. The pixel unit uses a shared type structure to improve signal-to-noise ratio in charge regional, and also can improve the pixel fill factor. And column level correlated double sampling circuit can eliminate the pixel KTC and fixed pattern noise. Based on the image sensor chip to build three-dimensional TOF ranging system, including the image sensor, LED light source, ADC, CPLD and other hardware. The power of the LED light source is 3.4 W, the frequency of the modulated light signal is 0.5 MHz, the exposure time is 20 ms, and the distance function is successfully realized. The nonlinearity error which is the average difference between measured and real depth is less than 2% within 1 m to 260 m and the best-case is below 2.5 cm at 1 m distance and the worst-case is about 3.68 m at 150 m distance. By increasing the source power and exposure time, or by inhibiting the background light, to improve the range accuracy. |
