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SUMMARY:Seeing in the dark\; a well engineered device - John Robson
DTSTART:20170530T200000Z
DTEND:20170530T203000Z
UID:TALK70075@talks.cam.ac.uk
CONTACT:Michael Kosicki
DESCRIPTION:After being in the dark for 30 minutes or more the mammalian e
 ye becomes exquisitely sensitive\, making it possible for humans to see  a
  small  flash of light whose energy  is  no more than  10-16 Joules.  To p
 ut this into perspective\, it can be calculated that\, if it could be effi
 ciently harvested and converted into light\, the energy provided by droppi
 ng a pea just a few millimetres could provide a visual sensation to everyo
 ne in the world.   A more useful description of this remarkable sensitivit
 y is in terms of the number of photons that must enter the eye to evoke a 
 visual sensation\; the number is rather less than 100.  While this fact ha
 s been known for nearly 100 years\, the retinal mechanisms that make it po
 ssible for this sensitivity to be achieved are only just being elucidated.
 \nThe key to this remarkable performance is the realisation that the photo
 sensitive cells of the mammalian retina  (rods) are connected to the post-
 receptoral cells by synapses that can discriminate between the signal gene
 rated by the activation  of a single molecule of photo-pigment (rhodopsin)
  by a single photon and the inevitable random noise generated by the bioch
 emical processes that serve to amplify the effect.   This discrimination m
 akes it possible for the retina to operate in the same photon-counting mod
 e that is now used in physical devices to measure extremely weak  lights. 
  In this respect the operation of the retina  resembles that of recently d
 eveloped multipixel avalanche diode (MAD) detectors though preceding it by
  two or three hundred million years.  As with all photo-detectors the ulti
 mate sensitivity is set by the level of thermally induced activity that is
  indistinguishable from light- evoked activity.    In this context we find
  that the probability of a rhodopsin molecule becoming spontaneously activ
 e  is  rather less than 10-10 per second\, corresponding to a half-life of
  about 400 years.\n
LOCATION:Senior Parlour\, Gonville and Caius College
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