Re: Who turned the lights out ?
See the incredibly beautiful study of Hecht, Shlaer, and Pirenne (1942) Energy, quanta and vision. J. Gen. Physiol. 20 819-840. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2142545/ Dark adapted human rod cells require between 5 and 8 quanta on average to produce a neural signal. After the effects of absorption by parts of the eye between the outside world and the retina this corresponds to an ability of the humans tested to detect between 54 and 148 quanta (i.e. that's all you need to see a light under optimal conditions - the integration time is about 100ms, so all the quanta have to arrive within 100ms for optimal performance).
Here is the relevant section of the paper's abstract:
"With these three corrections, the range of 54 to 148 quanta at the cornea becomes as an upper limit 5 to 14 quanta actually absorbed by the retinal rods. 3. This small number of quanta, in comparison with the large number of rods (500) involved, precludes any significant two quantum absorptions per rod, and means that in order to produce a visual effect, one quantum must be absorbed by each of 5 to 14 rods in the retina. 4. Because this number of individual events is so small, it may be derived from an independent statistical study of the relation between the intensity of a light flash and the frequency with which it is seen. Such experiments give values of 5 to 8 for the number of critical events involved at the threshold of vision."
A Vision Scientist.