Fig. 3.

Photon specific energy contribution to opsin activation throughout the visible and IR spectra. The wavelength-dependent energy of light is depicted in blue; red bars correspond to the thermal energy contribution of 310.15 K (37 °C), $70.3\frac{kJ}{\mathit{\text{mol}}}\left(16.8\frac{k\mathit{\text{cal}}}{\mathit{\text{mol}}}\right)$. The black horizontal line corresponds to the thermal activation energy of bovine rhodopsin, $E_A^T=184.1\frac{kJ}{\mathit{\text{mol}}}\left(44.0\frac{k\mathit{\text{cal}}}{\mathit{\text{mol}}}\right)$ (Lythgoe and Quilliam, 1938). At 1050.9 nm the combined thermal and radiation energy crosses the threshold which implies the theoretical limit of mammalian light perception.