Obviously, if we get this ratio to just "1", we'd get our perfect photon counting device, and clearly we aren't there yet, but with the commercially-available sensors in 2017, how close are we?
The LinCam has 1000x1000 pixel resolution with a temporal resolution of 50ps (2.5 gigasamples per second precision).
Such cameras are used for Time-Resolved Super-Resolution Fluorescence Studies of Biological Structures, a method where fluorescent chemicals are injected into plants or humans and then cell activity can be viewed.
PicoQuant's wide range of products for photon counting includes several high-end modules for time-correlated single photon counting (TCSPC) and event timing, single photon sensitive detectors and specialized analysis software for the evaluation of (time-resolved) fluorescence measurements and quantum correlations.
Some of the available single photon detectors (not an image, just a pixel) feature detection efficiency maximum of 40 to 50% at 400 to 550 nm. Indeed they do rely on receipt of a single photon to measure the time it occurred after the stimulus as multiple photons (or missing photons) affect the measurements in Fluorescent Lifetime Imaging (FLIM).
While the technology continues to improve such cameras have been available for many years, Stanford Computer Optics was founded in 1989. Their Image Intensified CCD Camera and EMCCDs (electron multiplying) operate efficiently, the photon being detected is brighter than the residual noise (with sufficient cooling).
These cameras (and single pixel detectors) are designed to count single photons accurately and without error, like all electrical devices sometimes they miss a photon and sometimes there's an error in the count - averaging the results with multiple frames can accumulate enough information to produce a histogram showing the count and occurrence frequency.
For an explanation of the math behind counting photons and the effects of noise see http://www.andor.com/learning-academy/ccd,-emccd-and-iccd-comparisons-difference-between-the-sensors or http://www.andor.com/learning-academy/electron-multiplying-ccd-cameras-the-technology-behind-emccds where Andor claims:
"In the limit of when there is less than 1 electron falling on a pixel in a single exposure the EMCCD can be used in Photon counting mode. In this mode a threshold is set above the ordinary amplifier readout and all events are counted as single photons. In this mode with a suitable high gain a high fraction of the incident photons (>90%) can be counted without being affected by the Noise factor effect.".
An EMCCD can multiply it's input by over 10K times, dividing by the same value gives you an accurate count of the photons.