how many peak sensitivity of the human eye?
I'm not sure I understand te question, but, as far as I know, the human eye is sensitive enough to detect 15 quanta (photons) of light.
Is that the info you're looking for?
That info is taken from 'Sensory Systems: Anatomy and Physiology' by Aage R. Moller. I presume it means that 15 quanta is the minimum required to elicit an action potential in a ganglion cell (these are the cells whose axons form the optic nerve).
Anyway, if this isn't the info you're looking for, http://webvision.med.utah.edu/facts.html page - which contains various facts and figures about the human retina - might help you. In fact, that entire site is a very good resource on vision; it might be worth bookmarking!
There was an experiment decades ago that showed the dark adapted human eye could respond to just one quantum of radiation. I don't know how the textbook squares with that - might be two different definitions of "respond".
Yeah, probably is! That's pretty amazing when you think about it, IMO - i.e. responding to a single photon of light!
IIRC, the human eye has a 'quantum efficiency' - which is pretty much the same as what SamLuc posted - of ~<10%, also in line with SamLuc's post.
Better than photographic film, but very limited ability to integrate (but apparently the human visual system does have some ability; am not sure if it's in the eye or the brain).
Then along came CCDs.
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