Improving the Luminous Environment of Underground Spaces Using an Active Multistage Light Shelf Integrated with a Light Well

Underground spaces often experience poor luminous conditions because daylight access is limited. Although a light well (LiW) can introduce daylight, its passive geometry may not provide stable illuminance under seasonal changes in solar altitude and azimuth. This study proposes an active multistage light shelf (LiS) integrated with an LiW to improve daylight penetration and indoor luminous conditions. A Radiance-based model was validated using a 1/5 scale model, yielding a normalized mean bias error of +7.0% and a coefficient of variation of the root mean square error of 22.4%. The system was then evaluated under representative summer solstice, equinox, and winter solstice conditions using 300 and 500 lx illuminance thresholds, uniformity ratio (UR), useful daylight illuminance (UDI), and daylight-linked lighting reduction potential. A lower light shelf improved performance during the summer solstice but was less effective at lower solar altitudes. Combining upper and lower active shelves provided two-stage daylight redirection and improved daylight penetration, target illuminance satisfaction, UR, and UDI compared with the baseline and single-shelf cases. The results indicate that an active dual-shelf configuration can improve daylighting performance under the three representative seasonal conditions evaluated in this study.