DEVELOPMENT OF TRANSPARENT NONIMAGING DAYLIGHTING DEVICE USING REFRACTION AND TOTAL REFLECTION

Abstract

In recent years, various types of daylight utilization techniques are being studied for the purposes of saving the lighting energy of building space and improving workplace productivity and comfortability. To introduce daylighting from a side window of office space, various kinds of lighting louvers and ducts using curved mirrors have been proposed. These curved mirrors are designed based on non-imaging optics and their lighting performance is not significantly affected by the solar trajectory. While lighting devices using such curved mirrors are capable of controlling direct sunlight, they visually obstruct the window surface and hinder diffused sunlight in case of cloudy weather. In this paper, we propose a new fixed type daylighting louver made of transparent material with refraction and total reflection, which is characterized as having no change in light distribution in a room regardless of solar altitude. This new transparent louver gathers light rays with a profile angle of 20 to 80 degrees of sunlight and emits the light in the room at + 5° to 35° from the horizontal direction, allowing the light to be widely distributed without causing glare. The light distribution range is adjusted by using a parabola on the total reflection surface. As shown in Fig. 9, the louver is configured with three interfaces to play the roles of light taking part (Entrance surface), light guiding part (Guiding surface) and light emitting part (Emitting surface). We used the edge ray (Light rays entering through the top and bottom edges of the incident end) for the maximum and minimum sunlight incident angles (profile angle) to determine the cross sectional shape. Combining the effects of the refracting surface and the total reflection surface allows the louver to be compact as compared with other louvers using mirror surfaces. In addition, while the reflectance of an aluminum mirror used for a general mirror louver is about 80%, the reflectance close to 100% can be obtained by total reflection at the interface between the transparent material and air. Also, the decrease in reflectance due to dust does not occur in the case of total reflection. In this paper, the louver's performance was verified by the optical simulation and its 1/10 scale model experiment was conducted. From the simulation, it was confirmed that the transparent louver can control the light ray with accuracy equal to or higher than that of the conventional mirror louver. By the scale model experiment, we evaluated the perception of light in real space including the complicated effect of diffused light, light environment in the room, and the appearance of the louver, which are difficult to understand only by simulation. From the experiment, we confirmed that the louver was practically usable. In addition, even under cloudy weather conditions, the entire louver unit was not darkened since the sky light was unblocked by the louver. The present study shows the feasibility to realize a highly efficient daylighting device utilizing the characteristics of the transparent material with refraction and total reflection.