Passive Inter-Photon Imaging

Digital camera pixels measure image intensities by converting incident light energy into an analog electrical current, and then digitizing it into a fixed-width binary representation. This direct measurement method, while conceptually simple, suffers from limited dynamic range and poor performance under extreme illumination -electronic noise dominates under low illumination, and pixel full-well capacity results in saturation under bright illumination. We propose a novel intensity cue based on measuring interphoton timing, defined as the time delay between detection of successive photons. Based on the statistics of interphoton times measured by a time-resolved single-photon sensor, we develop theory and algorithms for a scene brightness estimator which works over extreme dynamic range; we experimentally demonstrate imaging scenes with a dynamic range of over ten million to one. The proposed techniques, aided by the emergence of single-photon sensors such as single-photon avalanche diodes (SPADs) with picosecond timing resolution, will have implications for a wide range of imaging applications: robotics, consumer photography, astronomy, microscopy and biomedical imaging. Measuring Light from Darkness Digital camera technology has witnessed a remarkable revolution in terms of size, cost and image quality over the past few years. Throughout this progress, however, one fundamental characteristic of a camera sensor has not changed: the way a camera pixel measures brightness. Conventional image sensor pixels manufactured with complementary metal oxide semiconductor (CMOS) and chargecoupled device (CCD) technology can be thought of as light buckets (Fig. 1(a) ), which measure scene brightness in two steps: first, they collect hundreds or thousands of photons and convert the energy into an analog electrical signal (e.g. current or voltage), and then they digitize this † Equal contribution.