Measured daylighting potential of a static optical louver system under real sun and sky conditions

TitleMeasured daylighting potential of a static optical louver system under real sun and sky conditions
Publication TypeReport
Year of Publication2011
AuthorsKyle S Konis, Eleanor S Lee
Date Published09/2011
TypeDOE/ LBNL FY10 Technical Report Deliverable
Keywordsdaylighting, field measurements, high dynamic range luminance images, optical louver system, sunlight redirecting system
Abstract

By utilizing highly specular surfaces and engineered geometry, optical sunlight redirecting systems integrated into the overhead "clerestory" zone of the building facade present the potential to enlarge the daylighting zone by redirecting the luminous flux incident on the window deeper into the space than conventional shading systems. In addition, by developing system geometry to redirect daylight to specific regions within the space, optical light redirecting systems have the potential to avoid the glare conditions commonly produced by conventional facade shading systems that diffuse significant amounts of daylight below head height into the occupant's field of view.In this case study, side-by-side comparisons were made between an optical louver system (OLS) and a conventional Venetian blind located inboard of a south-facing, small-area, clerestory window in a full-scale office testbed. Daylight autonomy (DA), window luminance, and ceiling luminance were used to assess performance. The performance of both systems was found to have significant seasonal variation, where performance linearly improved as maximum solar altitudes transitioned from summer to winter solstice under clear sky conditions. Although the OLS produced fewer hours (between 0.25 to 1.5 h) per day of DA than the horizontal Venetian blind, the OLS never exceeded the designated 2000 cd/m2 threshold for window glare. In contrast, the Venetian blind, which was set a horizontal slat angle, significantly exceeded this threshold between 61% and 80% of the day for clear sky conditions over the test interval. Ceiling luminance was analyzed using calibrated high dynamic range luminance images. Under clear sky conditions, the OLS was found to produce a more uniform luminance distribution over the depth of the room as well as significantly increase the luminance of the ceiling during the middle of the day (10:00-14:00). Performance was comparable to or slightly worse when the surface solar azimuth was greater than approximately ±45° from the facade normal. The OLS always occluded direct sun but blocked direct views to the outdoors.