Abstract
This paper investigates the use of image sensors based on complementary metal–oxide–semiconductor (CMOS) single-photon avalanche diodes (SPADs) in high dynamic range (HDR) imaging by combining photon counts and timestamps. The proposed method is validated experimentally with an SPAD detector based on a per-pixel time-to-digital converter (TDC) architecture. The detector, featuring 32 × 32 pixels with 44.64-µm pitch, 19.48% fill factor, and time-resolving capability of ~295-ps, was fabricated in a 150-nm CMOS standard technology. At high photon flux densities, the pixel output is saturated when operating in photon-counting mode, thus limiting the DR of this imager. This limitation can be overcome by exploiting the distribution of photon arrival times in each pixel, which shows an exponential behavior with a decay rate dependent on the photon flux level. By fitting the histogram curve with the exponential decay function, the extracted time constant is used to estimate the photon count. This approach achieves 138.7-dB dynamic range within 30-ms of integration time, and can be further extended by using a timestamping mechanism with a higher resolution.
Highlights
Applications such as security surveillance, astronomy, automotive, and scientific imaging demand detectors with single-photon sensitivity to achieve a high signal-to-noise ratio (SNR) in all light conditions and with a high dynamic range (HDR) to cope with large variations of the background [1,2,3,4,5].For example, in automotive applications, a highly demanding condition occurs when entering a tunnel on a sunny day
A range of pixel technologies are currently available with single-photon sensitivity, for instance electron-multiplying charge-coupled device (EMCCD), photo-cathode-based intensified CCD (ICCD), and intensified complementary metal–oxide–semiconductor (ICMOS)
We propose a new method for HDR imaging that exploits the great time-resolving capability of single-photon avalanche diodes (SPADs) to estimate the photon arrival time information and, overcome the maximum count limit
Summary
Applications such as security surveillance, astronomy, automotive, and scientific imaging demand detectors with single-photon sensitivity to achieve a high signal-to-noise ratio (SNR) in all light conditions and with a high dynamic range (HDR) to cope with large variations of the background [1,2,3,4,5]. These technologies, present some limitations; EMCCDs require a cooling system, ICCDs are costly, and all these technologies can tune the gain to offer either high single-photon sensitivity or HDR but not both [10] Another pixel paradigm achieving single-photon detection capability is the quanta image sensor (QIS), which avoids the need to use avalanche multiplication. We propose a new method for HDR imaging that exploits the great time-resolving capability of SPADs to estimate the photon arrival time information and, overcome the maximum count limit. With such a method, which can be classified as a compression technique, the average photon arrival rate is reconstructed from the histogram of the generated TDC codes.
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