Abstract
Subjective experiment was conducted to identify the relationship between brightness sensation and the percentage of the subjects who turned on the ceiling luminaire in an experimental room which introduced diffused direct sunlight by using reflecting panel beside the window. Brightness sensation judged by the subjects just after they entered the room with diffused direct sunlight became higher than that without diffused direct sunlight. However diffused direct sunlight introduced into the room had no significant effects on the subjects' brightness sensation judged after they stayed in the room for 5 minutes. It was also identified that in the case when the illuminance on the desk was higher than about 200-400 lx, half of the subjects turned off the ceiling luminaire.
