> Please be advised that the human eye is a *lot* more sensitive than a
>> PMT.
>
>For a sufficiently crappy PMT. Good ones are several orders of
>magnitude better than the eye, and even more orders better than APDs.
I thought eyes were good enough to see a few photons. (I'm not sure about the time scale associated with that claim.)
If we are down to counting photons, how can I get "several orders of magnitude better"? Is that just spreading out the photons over a longer time chunk? So eyes can see N photons per second and PMTs can see (with some probabilty) N photons per many-many-seconds?
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In certain special circumstances, some people's eyes can see bursts of
30 photons or so in the blue-green. That's in the presence of lots of background noise in the form of phosphenes and other artifacts. If you take a photon counting PMT, jack the threshold up to 3 photons or so, and reject ion events with an upper threshold, the dark count rate of a PMT is essentially zero, and that applies to PMTs of basically any size, up to 20 inch diameter. (There are some pulse height histograms on P 9 of
formatting link
So electrically speaking, that's 20 dB just from the threshold difference, plus probably 3 orders of magnitude from the area difference--even assuming that the eye has a detection probability of 1 for those 30-photon bursts, and in spite of the inherent advantage of spatially resolved detection, where you can ignore noise in other locations.
The eye is a remarkable detector, but PMTs are pretty remarkable too.
Cheers
Phil Hobbs
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