Can a Human See a Single Photon?
math.ucr.edu
math.ucr.edu
"There was a way to make individual alpha particles visible using zinc sulfide... A small glass plate coated with zinc sulfide and bombarded with alpha particles briefly fluoresced at the point where each particle struck, a phenomenon known as "scintillation" from the Greek word for spark. Under a microscope the faint scintillations in the zinc sulfide could be individually distinguished and counted. The method was tedious in the extreme. It required sitting for at least thirty minutes in a dark room to adapt the eyes, then taking counting turns of only a minute at a time -- the change signaled by a timer that rang a bell -- because focusing the eyes consistently on a small, dim screen was impossible for much longer than that. Even through the microscope the scintillations hovered at the edge of visibility; a counter who expected the experiment to produce a certain number of scintillations sometimes unintentionally saw imaginary flashes."
Thank god they invented the photomultiplier before I arrived on the scene.
This was actually done for toads. The experimenter would train a toad to lash its tongue at faint flashes of light and the performance of the toad at discriminating light from no light was measured as the toad's body temperature was cooled. As the toad was chilled the quality of its guesses improved. Interestingly it's error rate as a function of temperature matched the change in signal to noise ratio for the currents in the retinal cells. Bizzare! The toad's brain controlled it's tongue at the threshold of being able to statistically distinguish signal from noise.
Unfortunately this idea is impractical for warm-blooded creatures, whose body temperature is strictly regulated... at least until it isn't, at which point they're no longer much good for seeing photons because they're busy being treated for hypothermia.
Although I'm sure that Rutherford and Geiger, or their grad students, were quite happy to avoid the necessity of performing their human photomultiplier experiments while sitting in a chilled room.
Aho AC et. al. "Low retinal noise in animals with low body temperature allows high visual sensitivity." Nature. 1988 Jul 28;334(6180):348-50.
Aha! A new spam filter!
According to the book, after using the counter to confirm that properly trained and rested humans were as accurate as the electronics, the next step was to put the electronic counter away because the humans could also give you spatial information: They could not only tell you that a flash had happened, but could tell you where it was in the visual field. Multichannel PMTs or (god bless 'em) chilled semiconductor-based multi-element low-noise photodetectors were still dozens of years in the future...
A close friend of mine has HPPD, and although I can't speak directly from experience on that, it seems to me that much of HPPD is an anxiety reaction to normal visual phenomena that most people grow used to and filter, but for which people can develop hypersensitivity and then become convinced that something is wrong. (This would be much like how, during the onset of a severe panic attack, especially in a person who has never had one before, normal semi-liminal physical sensations morph into "evidence" of impending doom/insanity.)