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Where ultimate limits can be defined, the evidence is that the evolutionary processes have pushed the sensitivity of the sense organs to their limits: The threshold of vision is a few photons, the threshold of hearing is thermal noise, the threshold of chemical detection is a single molecule as in the moth.
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The rate of the thermally assisted optical transitions was calculated under the assumption that the lower electronic state contains a broad uniform population of vibrational states that are thermally excited. Under this assumption, the total rate of transitions in the retina illuminated by spectral flux of I(v) W cm-2 µm-1 is given by A[(v)/v3]exp (hv/kT), where A is a constant that depends on the number of molecules, the mean spontaneous life time, the energy gap, and the spectral distribution of the light.
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