Effective radiant flux for non-image forming effects – is the illuminance and the melanopic irradiance at the eye really the right measure?

Research indicates that intrinsically photosensitive retinal Ganglion Cells are not evenly distributed or evenly sensitive throughout the retina. Still, most research looking into non-image forming (NIF) effects uses an integral measured quantity, illuminance or melanopic weighted irradiance, to represent the amount of light at the participants’ eye level. This paper describes a theoretical approach to define the effective radiant flux for stimulating the ipRGCs, taking into account a spatially resolved sensitivity. Research on retinal sensitivity is scares and not yet substantial, but the methodology can easily be adopted when areas of specific sensitivity are set. Preliminary results indicate that, with similar vertical illuminances and spectral power distribution, typical office lighting solutions might have a lower NIF effectiveness than settings with higher luminances in the central part of the field of view. This could explain why research on NIF effects is inconclusive, even though reported lighting conditions are similar