Standard observer watts: Evaluating the efficiency of circadian-effective luminaires using a standard observer methodology

The daily timings of our behavioural activities and physiological functions are regulated by the master biological clock in the brain. Disruption of these circadian rhythms can lead to poor performance and poor health. Consistent exposure to bright days and dim nights is necessary for circadian entrainment and, thus, for good sleep and good health. Since we spend most of our lives indoors, we often must rely on electric lighting for circadian entrainment. Surprisingly perhaps, current lighting practice does not purposely support circadian entrainment. A perceived barrier to promoting circadian entrainment with electric lighting is the absence of a standard test method for assessing the efficiency of luminaires for providing circadian-effective light to building occupants. Proposed is a measure for quantifying luminaire efficiency based on the electric power (watts) needed to reach the circadian stimulus criterion (CS = 0.3) at the eyes of a standard observer, termed Standard Observer Watts (SOWatt). The present paper describes how SOWatt can be applied to ceiling mounted, accent and table-top luminaires.