Under dark-adapted conditions, the pigment molecules have an extremely high sensitivity to light. Thermal energy is thought to assist pigment activation at the extremely low light levels under dark conditions, but is not quiet and produces noise. Barlow  proposed that the higher energies required for blue and green pigment activation are less affected by thermal noise than the lower energy red and IR pigments. But a mechanism by which the pigments are activated and thermal noise produced was not identified.
In this regard, a thermal mechanism was recently proposed  to assist pigment photoisomerization. See "Vision by QED Radiation,"at http://www.nanoqed.org Since the activation energy of rhodopsin is above that of dark rhodopsin, the amount of thermal energy necessary to assist pigment isomerization in the dark was estimated at about 1.2 eV. However, the Boltzmann distribution showed it is extremely unlikely that thermal energy of this magnitude can be accessed at ambient temperature. To circumvent the Boltzmann limitation, Hinshelwood's statistical mechanics relation  was adopted allowing each vibration mode in the pigment to contribute 1 unit of kT energy to the available thermal energy for pigment isomerization. Here, k is Boltzmann's constant and T is absolute temperature. At ambient temperature, kT ~ 0.0258 eV, and therefore it was concluded that about 45 vibration modes provided the necessary ~ 1.2 eV to assist pigment isomerization.
Statistical mechanics is only applicable to the collective properties of a large number of molecules. Individual pigment molecules in non-bonded contact with rhodopsin receptors do not follow the rules of statistical mechanics. What this means is Hinshelwood's derivation allowing each vibration mode in the pigment molecule to contribute a unit of kT energy to the pigment is of questionable validity. Unfortunately, the unlikely Boltzmann probability of the pigment accessing the necessary level of thermal kT energy from the ambient surroundings cannot be overcome. Another mechanism is required for the pigment to access the thermal energy to assist isomerization.
QED Induced Radiation
Unlike statistical mechanics, QM allows the individual pigment molecule to access thermal energy upon intermittent contact with the rhodopsin receptors in the retinal membrane. QM stands for quantum mechanics. Contact occurs repeatedly as the pigment molecule vibrates against the rhodopsin receptor. Since the rhodopsin is integral with the macroscopic membrane, the rhodopsin provides a source of kT energy at body temperature for thermally activating pigment isomerization. Each time the pigment contacts the receptor it momentarily becomes a part of a macroscopic body and spontaneously acquires thermal kT energy.
Upon loss of contact, however, the pigment molecule is free while still having the thermal kT energy acquired during contact. But in the free state, the acquired kT energy cannot be conserved by an increase in temperature because the pigment like any nanostructure is precluded by QM from having heat capacity. Conservation may only proceed by QED inducing the pigment molecule to emit the acquired kT energy as non-thermal radiation given by its EM spectrum, the VIS content of which randomly activating the pigment . QED stands for quantum electrodynamics, EM for electromagnetic, and VIS for visible.
Comparison with Olfaction
In vision, QED induced activation of vibrating pigment molecules in continual random contact against rhodopsin receptors is required to reach the VIS levels for pigment activation. EM emission occurs at the spectrum of the pigment, but depending on the vibration contact frequency may randomly accumulate to VIS levels. Olfaction by QED induced contact is similar to vision, but differs in that only FIR emission is required. FIR stands for far infrared. Since excitation of the VIS is not required, only a single contact of the odorant with the olfactory receptor is required to produce the odorant's unique FIR spectrum that signals the olfactory receptor that it is present in the nose. See http://www.prlog.org/
Available Thermal Energy
At body temperature, the Einstein-Hopf relation for the QM harmonic oscillator shows thermal kT energy is only available at FIR wavelengths greater than about 40 microns. Measurements  showing temperatures from 25 to 37.5 C do not affect the spectral sensitivity of pigments below 800 nm are consistent with the Einstein-Hopf relation for the available FIR energy at body temperature. The thermal noise in [Fig. 3, G and H of (2)] is not NIR radiation from the thermal surroundings but rather the QED induced radiation of the pigment molecule from the frequency up-converted thermal FIR energy acquired upon contact with the rhodopsin receptor NIR stands for near infrared..
Unlike Hinshelwood's derivation based on vibration modes, QED assume atoms in pigment molecules have 3 degrees of freedom, each atom with 3 kT / 2 ~ 0.04 eV units of thermal energy at body temperture. For a pigment retinal having about 100 atoms, the total thermal energy acquired in a single contact with the rhodopsin receptor is about 4 eV, but may randomly accumulate as the pigment molecule vibrates against the receptor. Hence, QED induced assistance of pigment isomerization can only be random as found in measurements shown in [Ibid of (2)].
However, only a fraction of the thermal energy absorbed in a single contact may be partitioned to the VIS to activate the pigments. Assuming the Planck energy is uniform in the filling of both VIS and NIR states, the VIS states at shorter wavelengths would be less filled than the NIR states. Beyond the NIR at 800 nm, the Planck energy is bounded at about 1.5 eV, and therefore it is reasonable to speculate that rapid intermittent contact of the pigment against the receptor randomly accumulates to the ~ 1.2 eV levels necessary to assist pigment isomerization.
1. Activation of rhodopsin pigments under dark-adapted conditions occurs because thermal energy is acquired in intermittent non-bonded contact of the pigments with their rhodopsin receptors as the pigment molecule vibrates against the receptor. During momentary loss of contact, QED induces the acquired thermal energy to be emitted by the EM spectrum of the pigment, the VIS content of which assists photoisomerization but the content beyond the NIR at 800 nm produces thermal noise.
2. The EM spectrum is emitted from the pigment upon loss of contact because the thermal energy acquired during contact cannot be conserved by an increase in temperature as the heat capacity of the pigment vanishes by QM.
3. At this time, the partition of the absorbed thermal kT energy acquired by the pigment into VIS modes that assist pigment isomerization in combination with the random accumulation of kT energy under repeated contact of the vibrating pigment against the receptor can only be conjecture and requires further study to be conclusive.
1. H. B. Barlow, "Purkinje Shift and Retinal Noise," Nature, 179, 255 (1957).
2. D-G Luo, et al., "Activation of Visual Pigments by Light and Heat," Science, 332,307,
3. H. N. Hinshelwood, The Kinetics of Chemical Change, Clarendon Press, Oxford, 1940.
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About QED Induced EM Radiation: Classically, absorbed EM energy is conserved by an increase in temperature. But at the nanoscale, temperature increases are forbidden by quantum mechanics. QED radiation explains how absorbed EM energy is conserved at the nanoscale by the emission of nonthermal EM radiation.