The newly discovered retinal ganglion cell photoreceptor melanopsin absent in the central fovea of the eye but distributed throughout the remaining human retinal body provides both non-visual biological/physiological input inducing circadian entrainment, and visual input affecting perceived brightness； this perceived brightness is not the object brightness commonly associated with luminance and perceived color of an object in central view, but the perception of brightness of a whole space or task background. Discussed are improvements to circadian lighting systems based on melanopsin stimulation whereby ambient and/or device background lighting may be temporally tuned over a range of prescribed color temperatures from a first subset of lighting having a higher melanopic content to a second subset of lighting having a lower melanopic content or vice versa in accordance with a desired circadian cycle, and in a manner where net light output is of a constant perceived brightness and color throughout temporal tuning.
