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高帧频CIS是高速成像系统的核心器件,器件的性能参数决定了成像系统的性能。文中选定的高帧频图像传感器采用5T像素结构,空间分辨率为1280×1024,像素量化精度为12 bit,在全分辨率条件下,最大帧频达到1000 fps [7]。其主要性能指标如表1所示。
Parameter Performance Parameter Performance Resolution 1280×1024 Response 10 V/Lux-s @525 nm Pixel size 5.5 μm×5.5 μm Conversion gain 75 μV/e− Pixel type 5T Shutter speed ≥500 ns Max frame rate 1000 fps Shutter type Global shutter Throughput 1440 Mpixel/s MAX SNR 42.5 dB ADC resolution 12 bits Dynamic range 56.8 dB Full well 18000 e− PRNU 0.9% rms Table 1. Performance of the high-speed CIS
从表1可以看出,与高灵敏度的科学级CIS通常采用卷帘快门(Rolling Shutter)不同,高帧频CIS通常采用全局快门(Global Shutter)结构。卷帘快门读出噪声低,配合高精度的ADC进行量化,有利于实现高灵敏度的成像性能,而高帧频则主要关心图像推出速度,其列处理电路的噪声较大,数字化精度较低,对读出噪声不太敏感,因此高帧频CIS大多采用全局快门工作模式。高帧频CIS的电荷转移时间短,通常在百纳秒量级,浮置栅级电容较小,为了实现高帧频图像推出,像素列处理电路的工作频率较高,因此读出噪声较大。为实现较高的帧频率和较短的曝光时间,通常采用5T或更多晶体管的像素结构,以实现良好的全局曝光控制。
基于该款高帧频CIS,设计实现了一种瞬态成像系统,系统主要由前端高速相机及远端高速图像采集系统构成[8]。前端高速相机包括光学镜头、快响应像增强器、高压快脉冲发生器、光纤锥耦合CIS、以及传感器驱动电路等。该瞬态成像系统的构成和实物如图1所示。
瞬态成像系统通常需要实现很高的时间分辨能力,通过快响应像增强器配合高压快脉冲发生器,该系统实现了纳秒量级的时间分辨性能,同时像增强器可以对目标场景进行光增强,获得较好的图像信噪比。文中所设计的瞬态成像系统通过将快响应像增强器的开门时间与高帧频CIS的曝光时间相配合,实现了超短曝光的连续两帧瞬态图像获取功能。前端相机获取的瞬态图像通过光纤传输至后端图像采集系统进行处理和显示,有利于提高成像系统对各类不同科学实验的适用性。
Study on the performance of high-speed CMOS image sensors in transient imaging mode
doi: 10.3788/IRLA20210694
- Received Date: 2021-09-19
- Rev Recd Date: 2021-12-10
- Publish Date: 2022-08-31
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Key words:
- high-speed CMOS image sensor /
- transient imaging /
- performance analysis /
- PRNU
Abstract: High-speed CMOS image sensors (CISs) have the advantages of high integration, high frame rate, low power consumption, and radiation difficulty. It is widely used in scientific experiments. When used in image measurement and diagnosis, CIS usually works at a synchronized mode triggered by an external signal. This mode, called the transient imaging mode, is quite different from the continuous imaging mode in which the sensor outputs images frame-by-frame at a specific frame rate. In this paper, the performance of a high-speed CIS that has 5T pixels and a global shutter is analysed in transient imaging mode, and the key performance of the sensor is tested using an EMVA1288 compatable device and compared with continuous imaging mode. The results show that in transient imaging mode, CIS has a larger dark current and a lower signal-to-noise ratio and dynamic range. However, the temporal readout noise and photo response nonuniformity are better. The test results could be used in scientific imaging system design and performance optimization.