  Detecting light is fundamental to all optical experiments and applications.
At the single photon level, the quantised nature of light requires specialised
detectors, which typically saturate for more than one photon, rendering the
measurement of bright light impossible. Saturation can be partially overcome by
multiplexing single-photon-sensitive detectors, enabling measurement up to tens
of photons. However, current approaches are still far from bridging the gap to
bright light levels. Here, we report on a massively-multiplexed single-photon
detector, which exhibits a dynamic range of 123 dB, from optical energies as
low as $\mathbf{10^{-7}}$ photons per pulse to $\mathbf{\sim2.5\times10^{5}}$
photons per pulse. This allows us to calibrate a single photon detector
directly to a power meter. The use of a single-photon sensitive detector
further allows us to characterise the nonclassical features of a variety of
quantum states. This device will find application where high dynamic range and
single-photon sensitivity are required.
