The nonvisual effect of natural lighting

Abstract

The electromagnetic spectrum within the waveband (~380-780 nm) is defined as light mainly for visual sensation. In addition, the designs of natural light and artificial light are both primarily on basis of the visual demands of occupants. Thus, the minimized constant lighting level is regulated within design standards considering the health risk from radiation. However, people prefer a natural light cycle than a constant one, and the effect of lighting extends much further by recent photobiology researches. The discoveries of the third photoreceptor cell on retina and its neural pathway, which primarily relate to circadian system, indicate that lighting has a significant nonvisual effect on health, mood, and productivity. Besides, it strongly suggests that the lighting demand of nonvisual effect is very different from that of the visual one, and artificial lighting is not an appropriate means to satisfy the nonvision system. Nevertheless, daylighting is capable to stimulate both the visual and the nonvisual systems. To investigate the daylighting environment of Chinese buildings, this paper assessed the nonvisual effect of different design levels in Chinese daylighting design standard (GB/T 50033-2013). As for the evaluation method, the constant relation between vertical illuminance and horizontal illuminance was used to convert the maintained horizontal illuminance of different design levels to the illuminance which reached the eyes (vertical illuminance). Then the nonvisual effect could be calculated by a dose-response function between the nonvisual effect and the illuminance at eyes. Moreover, this function was proposed on basis of the static researches of threshold values which only considered spectrum and intensity. The nonvisual effect of the design levels I-V was respectively 100%, 100%, 71%-100%, 38%-60%, and 5%-16% with the ratio of vertical illuminance to horizontal illuminance varying from 1.5 to 2.0. Since the design standard adopted the overcast sky conditions based on the worst principle, the daylighting of levels I-III was adequate in the major rooms of public buildings where most occupational people stayed during the day considering the actual illuminance which was higher under normal sky conditions. However, if there was a consideration that the aged people who often stayed at home and needed much higher illuminance with the degradation of eye function, the level IV of the daylighting for major residential rooms should be improved. Besides, although the daylighting of level V was extremely low, its effect might be ignored as the short dwell time in transition space. What is more, a field measurement was conducted to validate the evaluation results in a typical room which adopted the design level of III, which demonstrated that the average nonvisual effect of a room in the field measurement was in accordance with the evaluation results mentioned before.