Photon-counting achieves several orders of magnitude higher dynamic range than conventional CCD or CMOS pixels
Image sensors capable of counting individual photons are conventionally used in specialized low photon flux applications such as LiDAR and fluorescence lifetime imaging microscopy. Here we present a new imaging modality that enables single-photon avalanche diode (SPAD) sensors to operate at extremely high photon flux levels and hence provide unprecedented dynamic range. We call this technique passive free-running SPAD or PF-SPAD. Our scene brightness estimator uses the number of photons detected by a dead time-limited SPAD sensor over a fixed exposure time. Our key insight is that due to the random nature of photon arrivals, a PF-SPAD does not suffer from a hard saturation limit, even for extremely bright scene points. Coupled with high sensitivity at low flux, this enables a PF-SPAD to achieve extreme dynamic range. We present theoretical analysis, simulation results and experimental demonstration of an improvement of over 2 orders of magnitude over conventional sensors by imaging scenes spanning a dynamic range of 1,000,000:1. We envision that this will expand the applicability of SPADs as general-purpose passive image sensors for a broad range of applications including consumer photography, computer vision and microscopy.
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