The QIS performs no time-stamping. Each pixel delivers a simple binary output based on the detection (or not) of a single photon. The simplified binary frame readout scheme of such a device not only allows for high pixel fill factor (61%), but it also enables the use of a large format array (256 × 256) giving acquisition of depth images with high resolution in all three dimensions. Timing information on the arrival of photons is based on the readout window of each frame and its delay relative to the outgoing optical pulse. By iterating the delay with very short increments (10 ps), the team at Heriot-Watt have managed to achieve millimeter depth precision, despite the fact that the laser pulse is 600 ps wide and the range gate is 10 ns. Such a scheme offers huge advantages in terms of building cheaper (thanks to not requiring expensive ps pulse lasers or scanning optics), large-scale depth imaging systems.
To improve the quality of images taken with fewer photon returns per pixel (for example at shorter acquisition times or for low reflectivity targets), the authors also present a bespoke algorithm that takes advantage of spatial correlations within the scene to reconstruct depth maps even from noisy data. In all, this paper presents a dramatic step forward in depth imaging.
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