For Energy-saving by using daylight, automated control systems of venetian blinds have been widely used in office buildings in Japan. For controlling blind slat angle, not only sun-cut strategy but also algorithms based on various factors (energy consumption for both lighting and cooling/heating, special brightness, glare prevention, or contact to view) have been used. Previous studies conducting questionnaire surveys in offices with automated blinds controlled by such algorithms showed that the office workers were distracted by the change in brightness of the window caused by the movements of the blind slats. The purpose of this study is to reduce discomfort caused by the change in the brightness of windows by changing the speed of the slat rotation.
In order to identify the degree and the frequency of the change in luminance of the windows with blind controls, a numerical simulation was carried out by using weather data, which includes global illuminance and sky illuminance measured on the roof every one minute for 184 days (July 1 to December 31, 2015). For blind control, glare prevention strategy based on PGSV is used. The control intervals are three minutes and ten minutes. The results show that the maximum change of the window luminance ranges from 900 to 2800 cd/m2 when the slat angle decreases. That ranges from -3600 to -1600 cd/m2 when the slat angle increases.
In order to identify the effect of the change in the window brightness on the subjective response, an experiment was carried out in a test chamber. It has an artificial window (width: 2000 mm, height: 1200 mm) with venetian blinds of 0.69 of reflectance. Maximum window luminance of the artificial window without a blind is 18,000 cd/m2. Based on the simulation results, 24 conditions were tested in the experiment; they included six different slat-rotation speed (2.2 degrees/s - 37 degrees/s) and six different window luminances (1000 cd/m2 - 6400 cd/m2). Twenty-nine students (with an age range of 18-23 years) participated as subjects. In the waiting room, the subjects adapted to 450 lux at eye level for ten minutes. Subjects entered the test room and each took a seat. VDT task was carried out on the desk for four minutes. During the VDT task, the blind slats moved to prevent the discomfort glare from the window. After that, the subjects were asked to rate “Discomfort caused by the change in the window brightness” and “acceptability of the change in the window brightness”. The results of the experiment showed the effect of slat-rotation speed and the change in window luminance on “Discomfort caused by the change in the window brightness” rated by the subjects. It was expected that the higher the slat-rotation speed, the higher the “Discomfort caused by the change in the window brightness”. However, “Discomfort caused by the change in the window brightness” shows a peak. When the slat-ration speed is slow, “Discomfort caused by the change in the window brightness” is low and when the slat-ration speed is fast, “Discomfort caused by the change in the window brightness” is also low. Also the results showed the strong correlation between “Discomfort caused by the change in the window brightness” and “the acceptability”.
In this study, the method for determining slat-rotation speed of blinds was proposed based on “Discomfort caused by the change in the window brightness” obtained from the subjective experiment.
