Development of a Light Duct Using Relay Lenses and Diffusers

Due to the recent increase in building energy consumption, daylighting technologies such as light ducts are becoming increasingly important. However, conventional light ducts have limitations, such as light loss, uneven illumination, and spectral distortion as the transmission distance increases, restricting the development of a comfortable lighting environment. This study developed technical alternatives for transmission and diffusion parts to overcome these limitations and improve the daylighting performance of light ducts. The performance of these alternatives was verified through testbed experiments. The proposed light duct design minimized light loss through the arrangement of multiple relay lenses in series in the transmission part and improved indoor illuminance uniformity in the diffusion part using a double-reflection structure with upper and lower reflectors. Consequently, for a transmission distance of 20 m, the average illuminance increased by ~27.3% and the uniformity improved by an average of 47.8% compared to a conventional plastic optical fiber (POF)-based light duct. Even under intense summer sunlight conditions, a transmission distance of 30 m showed a high useful daylighting illuminance (UDI) ratio and considerbly reduced glare risk, indicating characteristics favorable for maintaining a comfortable visual environment. Furthermore, the proposed light duct exhibited a spectral distribution similar to that of outdoor sunlight, demonstrating the potential to ensure the continuous spectral characteristics of natural light transmitted indoors. Finally, it also exhibited the potential to maintain its higher daylighting performance even at a transmission distance of 30 m compared to conventional technology.