Retinal ganglion cells (RGCs) are involved in the processing of visual information, and the varying ratio of photoreceptors to each cell is related to visual sensitivity across the retina. Recent research has also shown that some of the RGCs (~3%) are intrinsically photosensitive (ipRGCs), and provide the most significant input to our circadian cycle. Disruption of that circadian cycle appears to be related to such diverse health issues as mood, obesity, diabetes and even some forms of cancer.
Since before the turn of the century, information has been developed about the role of retinal ganglion cells in the control of melatonin and the associated health issues. An excellent summary of the science as of 2007, written by JP Hanifin and GC Brainard, is available here. Additional information is available here. Based on published data, in 2004 J. Hollan developed a corresponding action spectrum for this non-visual system which responds to radiation but does not contribute to the visual image. This action spectrum is consistent with the information in the IES publication TM-18 and numerous other publications (see the bibliography here.) The following diagram shows the "meltopic" action spectrum as the blue line on the left side, along with the conventional photopic (white and gray) and scotopic (black) sensitivity functions.
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