Since the discovery that intrinsically photosensitive retinal ganglion cells influence the body’s circadian rhythms, there has been a desire to quantify this effect in terms helpful to lighting designers. The effect of lighting on occupants’ circadian rhythms has implications for their health and well-being, and, for the workplace, this can affect productivity and absenteeism. There is not yet a universally agreed way to account for how different lighting choices might determine these effects. Various models have been proposed that attempt to quantify, at least relatively, these effects; in each case adopting some form of action spectrum associated with melatonin suppression, as this hormone is known to be critical in the circadian process. It is has been established that the effects vary according to the wavelength of the light, and the total effect is normally modelled as a weighted sum, despite evidence that effects from different wavelengths combine non-linearly or even in opposition. Different metrics are compared for various real light sources but these are shown not to agree, thus different design choices would be made according to the model adopted. The WELL Building Standard has adopted the use of Equivalent Melanopic Lux to formulate lighting criteria. Consequences for the selection of light sources and the specification of internal illuminance are examined
