Nonuniformity Analysis of a 65-kpixel CMOS SPAD Imager

While CMOS single-photon avalanche diode (SPAD) technology has steadily advanced, improving noise, timing resolution, and sensitivity, spatial resolution has been increasing as well. The increase in the number of pixels has made a comprehensive analysis of nonuniformity and its effects meaningful, allowing a more accurate comparison of SPAD imagers with other high-end scientific imagers, such as electron multiplying charge-coupled device and scientific CMOS. A comprehensive nonuniformity analysis was conducted on a 512 x 128 pixel gated SPAD imager, where dark noise, afterpulsing, crosstalk, signal response, and shot noise were measured. This analysis has led to a variety of postprocessing algorithms to improve the linearity of the response as for example required by ground state depletion microscopy-based superresolution microscopy and other techniques. We derived a new correction formula for the count rate applicable to 1-b SPAD imagers, and we measured a significant improvement of photon detection efficiency using microlenses. These techniques were used to validate the suitability of the imager in fluorescence microscopy examples.