After the accident of the Fukushima nuclear power plant caused by the Great East Japan Earthquake of 2011, the supply of electricity in Japan continued to be tight. In the summer 2011, the electric power consumption had to be reduced by 15 percent or more in office buildings in the eastern half of the Japanese mainland including the Tokyo metropolitan area and workers in the office spaces experienced different light environment than before the earthquake. This paper analyzes basic data on light environments in office buildings after the 3.11 earthquake in order to contribute to future lighting design.
The field measurements and workers’ evaluation surveys were conducted in 13 office spaces located in the Kanto metropolitan area from July to November 2011. Horizontal illuminance, vertical illuminance, luminance distribution in the workers’ visual field and electric power consumption were measured. The measurements were conducted for a week or two in each building to identify the effects of outside conditions on the interior lighting environment. Desktop illuminances are estimated from the horizontal illuminance data measured on the desk partitions. The workers evaluated “brightness on the desktop”, “satisfaction with lighting environment around the desk”, “spatial brightness of the workspace”, and “Satisfaction with lighting environment of the workspace”. The results are divided into three time/weather conditions, daytime of sunny days, daytime of cloudy days and nighttime.
Results are as follows:
1) In most offices the horizontal illuminance remains in the range of 300 to 500 lux in the nighttime, whereas the value is below 100 lux or above 700 lux in some offices.
2) There was some relationship between the average horizontal illuminance on desktops and the workers’ evaluation on brightness, however appropriate light environment could not be estimated only by representative values of horizontal illuminance of the space, because the nonuniformity of the light environment also had a high correlation coefficient with workers’ satisfaction.
3) Average luminance on the ceiling had a higher correlation coefficient with spatial brightness in the working space, regardless of time zone or climate.
4) The rate of dissatisfaction or brightness evaluation decreases with until a desktop illuminance reaches approximately 400 lux: above 400lx of desktop illuminance the dissatisfaction rate remains almost constant.
