This study aims at: (1) examining visual responses to the environment illuminated by fluctuating and constant light; (2) investigating if the change of light significantly impairs task performance; (3) determining the fluctuation range of illuminance controlled by a daylight dimming system.; Field measurements and surveys were performed in full-scale mock-up spaces. Three blind conditions and three photosensor shielding conditions were used. Visual annoyance and mood responses were examined under constant and fluctuating illuminance. Six fluctuation ranges were used under three task illuminance conditions. 36 subjects participated in the survey and two types of reading tasks were used.; The daylight dimming control system worked well under clear sky days. The fluctuation range was within 6.6% of the maximum light output of fixtures when unshielded and shielded conditions were used. When an unshielded photosensor was used under partly cloudy sky days, the fluctuation range was greater than 40% of the maximum light output.; The fluctuation range caused by partially-shielded and fully-shielded conditions did not exceed 6.6% of the maximum light output from fixtures under clear and partly cloudy sky days, but energy savings was not excellent. In a small office, only the fixture on the first row from a window should be connected to the control circuit of daylight dimming systems to keep the illuminance within recommended ranges in rear area.; The more task illuminance the subjects had, the less annoyance they reported when equal amount of illuminance was changed. When a lighting design is considered to utilize a daylight dimming control system in an office space, the task illuminance should not be lower than the medium range of recommended illuminance to reduce visual discomfort.; The maximum fluctuation of illuminance should not exceed 40% of total light output to prevent significant visual annoyance, when task illuminance was on the lower and medium limit of the recommended values for office spaces. The feelings of eye fatigue, distraction and annoyance were significant contributors to visual discomfort under fluctuating light. The mean responses of mood under constant medium task illuminance showed higher ratings toward better mood than under lower constant illuminance.
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