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Seeing in the dark; a well engineered device

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After being in the dark for 30 minutes or more the mammalian eye 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 put this into perspective, it can be calculated that, if it could be efficiently harvested and converted into light, the energy provided by dropping a pea just a few millimetres could provide a visual sensation to everyone in the world. A more useful description of this remarkable sensitivity 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 has been known for nearly 100 years, the retinal mechanisms that make it possible for this sensitivity to be achieved are only just being elucidated. The key to this remarkable performance is the realisation that the photosensitive cells of the mammalian retina (rods) are connected to the post-receptoral cells by synapses that can discriminate between the signal generated by the activation of a single molecule of photo-pigment (rhodopsin) by a single photon and the inevitable random noise generated by the biochemical processes that serve to amplify the effect. This discrimination makes it possible for the retina to operate in the same photon-counting mode that is now used in physical devices to measure extremely weak lights. In this respect the operation of the retina resembles that of recently developed multipixel avalanche diode (MAD) detectors though preceding it by two or three hundred million years. As with all photo-detectors the ultimate 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 active is rather less than 10-10 per second, corresponding to a half-life of about 400 years.

This talk is part of the Caius MCR/SCR research talks series.

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