There are two types of photoreceptors in the human retina. Cones, which are responsible for color vision, are most sensitive to green, red, and blue. Rods are "color blind", but are sensitive to light changes. Rods detect light by isomerization of the rhodopsin molecules they contain. More specifically, rods absorb light (photons), thus energizing and changing the shape of these molecules. While rods do not mediate color vision, isomerization is neither constant nor a monotonic function of the light emission wavelength . Experimental studies have shown that rods are most sensitive to wavelengths of light around 498 nm (green-blue) and insensitive to wavelengths longer than about 640 nm (red).
Top-left figure: interpolated rod spectral sensitivity
Bottom-left figure: A simulation of isomerization and detection of rods as a function of light wavelength ranging from 401 to 571 nm, with ranging from 1 to 30 nm in unit steps. The expected photon catch (middle plot) is expressed as , where is the sensitivity for a rod at wavelength . Intensity was fixed at 300 for the purpose of our experiment.
Bottom-right figure: Within a signal detection theory framework, we analyzed discrimination as a function of in the right-most figure. The sensitivity shown was computed using , where is the variability in photon absorption set at 20.0.