Re: dark adapted

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Hal Murray 16 years ago

> 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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Paul Keinanen 16 years ago

That claim is quite strange, since many animals active at night have much better sensitivity than humans.

If the human eye would be as sensitive as claimed, then why do passive night vision systems create a usable picture even in a cloudy night.

A quick search showed quite a few references of 1-5 % quantum efficiency, one even claimed up to 10 % quantum efficiency.

CCD claims quantum efficiencies in the 50-80 % range, so there would at least one order of magnitude.

Paul

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John Devereux 16 years ago

They can have mush larger sensitive area than that of the eyes, so are proportionally more sensitive in that sense.

John Devereux

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Phil Hobbs 16 years ago

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

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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

Dr Philip C D Hobbs Principal ElectroOptical Innovations 55 Orchard Rd Briarcliff Manor NY 10510 845-480-2058 hobbs at electrooptical dot net http://electrooptical.net

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Jim Thompson 16 years ago

My family owns Seneca Caverns (Pendleton County, WV, **). There are locations in there where I've waited for 20-30 minutes, and still couldn't see my hand in front of my face.

** My father (James R.) and an uncle (Wilmer) wired that cavern for lighting when they were teenagers (~1936). They used to play games with cutting the lights and see who could get out quickest ;-) ...Jim Thompson

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Charlie E. 16 years ago

Also, it take time, and lots of it, for the human eye to get that light adapted, and it only takes a few moments of bright light to ruin it!

Charlie

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Paul Keinanen 16 years ago

Looking at the newsreels for some night vision systems in Irak/Afganistan, the man portable systems did not have any large apertures.

In some countries, conscripts are trained to avoid loss of light adaptiotation starting at their first week of service.

Paul

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Re: dark adapted

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