A “Melanopic” Spectral Efficiency Function Predicts the Sensitivity of Melanopsin Photoreceptors to Polychromatic Lights

Abstract

Photoreception in the mammalian retina is not restricted to rods and cones but extends to a small number of intrinsically photosensitive retinal ganglion cells expressing the photopigment melanopsin. These mRGCs are especially important contributors to circadian entrainment, the pupil light reflex, and other so-called nonimage-forming (NIF) responses. The spectral sensitivity of melanopsin phototransduction has been addressed in several species by comparing responses to a range of monochromatic stimuli. The resultant action spectra match the predicted profile of an opsin:vitamin A-based photopigment (nomogram) with a peak sensitivity (λ(max)) around 480 nm. It would be most useful to be able to use this spectral sensitivity function to predict melanopsin's sensitivity to broad-spectrum, including "white," lights. However, evidence that melanopsin is a bistable pigment with an intrinsic light-dependent bleach recovery mechanism raises the possibility of a more complex relationship between spectral quality and photoreceptor response. Here, we set out to empirically determine whether simply weighting optical power at each wavelength according to the 480-nm nomogram and integrating across the spectrum could predict melanopsin sensitivity to a variety of polychromatic stimuli. We show that pupillomotor and circadian responses of mice relying solely on melanopsin for their photosensitivity (rd/rd cl) can indeed be accurately predicted using this methodology. Our data therefore suggest that the 480-nm nomogram may be employed as the basis for a new photometric measure of light intensity (which we term "melanopic") relevant for melanopsin photoreception. They further show that measuring light in these terms predicts the melanopsin response to light of divergent spectral composition much more reliably than other methods for quantifying irradiance or illuminance currently in widespread use.

Modelling ‘non-visual’ effects of daylighting in a residential environment

“…The non-visual effects of light are mediated primarily by the photopigment melanopsin, which has a peak sensitivity in the blue visible range, ~480 nm, commonly named C(λ). When comparing two light sources, the source with a spectrum that more closely matches the spectral sensitivity of the melanopsin photopigment (i.e., the one which contains more blue light) will have greater 'circadian efficacy' [44] and will require less light to achieve the same physiological effects than a source with less blue light. To determine what lux values from respective illuminants would achieve a prescribed "circadian-equivalent" illuminance, known radiometric spectra for daylight and other light sources can be used to back-calculate the absolute power in watts of a given light source, as proposed in Pechacek, Andersen and Lockley in 2008 [43].…”

Unified framework to evaluate non-visual spectral effectiveness of light for human health

“…11 These cells use melanopsin as a photopigment and, as a result, the ipRGCs are characterized by a spectral sensitivity curve that peaks in the short wavelength region around 490 nm, estimated in vivo, distinguished from the spectral sensitivity of rod and cone photoreceptors. 12,13 Before the discovery of the new photoreceptor, properties of light exposure were often reported in terms of illuminance. Illuminance is not appropriate to evaluate the non-visual spectral effectiveness of ocular light exposure, because of the different spectral sensitivities of the visual and non-visual systems (555 nm and 490 nm, respectively).…”

Short Blue Light Pulses (30 Min.) in the Morning are Able to Phase Advance the Rhythm of Melatonin in a Home Setting

“…This sensitivity to blue light is due to the protein melanopsin, located in the "intrinsically photosensitive retinal ganglion cells" (ipRGC's) [24,25]. This photopigment is most reactive to ~480 nm light [25][26][27].…”