High Dynamic Range Imaging with TDC-Based CMOS SPAD Arrays

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.

Quanta Burst Photography

“…1 shows the sensor parameters we used for the simulation. The parameters for the conventional sensor are for a high-end machine-vision camera 4 . The parameters for the SPAD camera are based on the SwissSPAD2 sensor we use for our experiments.…”

“…8 .φ min is defined as the lowest photon flux for which the SNR is above certain threshold. Here we choose the threshold to be 1 (0dB) which is consistent with previous works on SPAD [4,5] and common definition for conventional sensors. Fig.…”

Silicon-Based Avalanche Photodiodes: Advancements and Applications in Medical Imaging

“…One of the key advancements in APDs for medical imaging is the improvement in quantum efficiency and responsivity. Efforts have been made to enhance the APD's ability to detect and convert incoming photons into measurable electrical signals [90]. This includes optimizing the material properties and structure of the APD, as well as incorporating advanced photon management techniques [91].…”

Passive Inter-Photon Imaging

“…A saturated CMOS pixel's output is simply a constant and meaningless, but if the time taken to reach saturation is also available [13], it provides information about scene brightness, because a brighter scene point will reach saturation more quickly (on average) than a dimmer scene point. The idea of using photon timing information for HDR has also been discussed before but the dynamic range improvements were limited by the low timing resolution of the pixels [53,31] at which point, the photon timing provides no additional information over photon counts.…”