The variation of daylight under clear and partly cloudy sky conditions was analyzed to examine the fluctuation of electric light controlled by a daylight dimming control system. Field measurements were performed in a full-scale mock-up office space that faced south. Horizontal and 35 degrees blind angle conditions were tested. Three different shielding conditions for a photosensor were examined to predict the fluctuation of electric lights. The fluctuation of vertical illuminance under partly cloudy sky conditions was 23.8 times as greater than that under the clear sky conditions. A multiple linear regression model was employed to determine the fluctuation range of outdoor daylight illuminance according to sun positions under partly cloudy sky conditions. The fluctuation of electric light output under clear sky conditions was less than 6.6% of target illuminance. In contrast, it varied greater than 50% of target illuminance under the partly cloudy sky conditions. Shielding condition effectively blocked the direct influence of daylight fluctuation on photosensor. The linear correlation between the fluctuation of outdoor vertical illuminance and photosensor illuminance was the weakest when the photosensor was partially-shielded. From this, it was found that partial shielding of photosensors is a desirable strategy for reducing the fluctuation of light output from electric lights integrated with daylight dimming systems. (c) 2006 Elsevier B.V. All rights reserved.