Quantifying Circadian Light and Its Impact

The lighting community is currently considering the development and use of metrics that characterize circadian light—or light that acts as a stimulus for the human circadian system—to establish a design-performance standard (for a particular time of day) and to further research the suppression of melatonin production. Both light and dark set the timing of the master clock in our brains’ suprachiasmatic nuclei (SCN), and disruption of this clock has been shown to negatively affect many health outcomes, from breast cancer to diabetes.

Among the proposed metrics is “melanopic lux,” a term that can be loosely described as flux density weighted not by the photopic luminous efficiency function—or V(λ), which peaks at 555 nanometers and is based on the response of foveal, long- and middle-wavelength sensitive cones—but by a luminous efficiency function, which peaks at 480 nanometers and is based on the action spectrum of melanopsin. (Melanopsin is the retinal photopigment within our eyes’ intrinsically photosensitive retinal ganglion cells, or ipRGCs, which form the main neural conduit from the retina to the brain’s master clock.)

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