Lookup-table-based time-bin correction for SPAD arrays in non-line-of-sight imaging

Abstract

Single-photon avalanche diode (SPAD) arrays operated in time-correlated single-photon counting (TCSPC) mode are key enablers for non-line-of-sight (NLOS) imaging [1], yet manufacturing tolerances in their on-chip time-to-digital converters (TDCs) [2] introduce pixel-dependent timing errors that degrade reconstruction quality. We identify three distinct error sources – gain error (pixel-to-pixel mean bin-width variation of 6 %), relative offset error, and differential non-linearity (bin-to-bin width fluctuations of 0.6 %) – and present a fast, deterministic correction based on a pre-computed lookup table (LUT). The absolute width of every time bin is characterised from two simple calibration measurements: a continuous-illumination acquisition that yields relative bin widths and a multi-peak acquisition that anchors the absolute time scale. A backward-resampling LUT then maps measured histograms onto a uniform temporal grid, correcting all three TDC error types simultaneously. Compared with the Monte-Carlo resampling strategy of Nedbal et al. [3], the LUT approach is deterministic and executes in 4 µs per frame, making it directly compatible with real-time NLOS imaging. Applied to experimental NLOS data from a 32 32 SPAD camera and reconstructed with the phasor-field algorithm [4], the correction produces visibly sharper reconstructions and raises the structural similarity index (SSIM) of the recovered scene [6]. Simulations on a confocal dataset [5] further confirm a significant improvement in contrast transfer across all spatial-frequency bands. The method is equally applicable to other SPAD-TCSPC applications such as fluorescence-lifetime microscopy and LIDAR.