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LETTHEREBEHEALTHYLIGHTINGPerspectives onhow healthy lighting can beachieved through designClearly, “healthy lighting” should address the needs ofboth the circadian system—which has been linked tosleep disorder, seasonal affective disorder (SAD) anddepression—and the visual system, in terms of migraines andeyestrain.However, the lighting needs of the visual and circadian systemare so different and contradictory that it seems almostimpossible to design healthy lighting to serve both. Mark Rea(2002) provides an excellent summary of lighting characteristicsof the circadian and the visual systems (Table 1). What followsis an analysis of these characteristics from the perspectiveof a lighting designer.• The quantity of light needed for the visual system is moderatecompared to the needs of the circadian system. Thevisual system operates as a spotlight requiring optimallight level at the task. By the law of diminishing returns,increasing the illuminance further on the task will notimprove the visual performance. On the other hand, thecircadian system requires 10 times more light “at the eye”and a vision field, focal and peripheral, completely filledwith light. These differences raise legitimate concerns forvisual discomfort caused by over brightness and spaceslighted as integrated sphere, without light and shadowpatterns.• The spectrum for the visual system has a peak of 555 nm,which results in a generally warm light. The peak sensitivityfor the circadian system is at 465 nm, perceived asbluish light. Lighting efficiently for the circadian systemwill make spaces appear cold and bluish and generallyunacceptable to the visual system.• Spatial distribution is important to the visual system.The geometry of eye, task, light source, contrast and sizeof the task all define the visual system’s performance. Thecircadian system only cares about retinal illuminance. Infact, it can be compared to a giant photocell, monitoringfor large amplitude light/dark signals. However, the lowerarea of the retina, where the image of the sky is formed, ismore sensitive to light and thought to contribute more tothe circadian system.• The timing and the duration are interconnected, especiallyfor the circadian system. Timing for the visual systemis not important, because the visual systemresponds quickly and the response is similar from oneLighting characteristics Application Vision Circadian day shift work Circadian night shift workQuantity Low (300-500 lux on task High (~1000 lux at eye) High (~1000 lux at eye)and ~100 lux at eye)Spectrum Photopic Short-wavelength Short-wavelength(peak sensitivity 555nm) (peak sensitivity 420-480 nm) (peak sensitivity 420-480 nm)Spatial Distribution Distribution important Independent of distribution Independent of distribution(task luminance, contrast and (illuminance at eye) (illuminance at eye)size determine visibility)Timing Any time Subjective morning Periodically throughout the shiftDuration Very short (less than 1s) Long (1 - 2 h) Short (15 min) pulsesTable 1: Summary of research findings on visual and circadian systems. (Source: Lighting Research Center, RPI.)76 LD+A/May 2003 www.iesna.org