Direct measurement of darkness using a standard single-photon avalanche photodiode

In experiments requiring extreme darkness, such as experiments probing the limits of human vision, assessment of the background photon flux is essential. However, direct measurement thereof with standard single photon detectors is challenged by dark counts and their fluctuations. Here we report an experiment and detailed statistical analysis of a direct measurement of darkness in a dedicated dark chamber suitable for human vision experiments, only using a standard single photon detector and a mechanical shutter. From a Bayesian analysis of 616 h of data, we find substantial to decisive evidence for absolute darkness (depending on choice of prior distribution) based on the Savage-Dickey ratio, and a light level <0.039 cnt/s (posterior 0.95-highest density interval).