Empirical Assessment of a Prismatic Daylight-Redirecting Window Film in a Full-Scale Office Testbed

ABSTRACT

Daylight-redirecting systems with vertical windows have the potential to offset lighting energy use in deep perimeter zones. Microstructured prismatic window films can be manufactured using low-cost, roll-to-roll fabrication methods and adhered to the inside surface of existing windows as a retrofit measure or installed as a replacement insulating glass unit in the clerestory portion of the window wall. A clear film patterned with linear, 50- to 250-μm-high, four-sided asymmetrical prisms was fabricated and installed in the south-facing, clerestory low-e, clear glazed windows of a full-scale testbed facility. Views through the film were distorted. The film was evaluated in a sunny climate over a 2-year period to gauge daylighting and visual comfort performance. The daylighting aperture was small (window-to-wall ratio of 0.18) and the lower windows were blocked off to isolate the evaluation to the window film. Workplane illuminance measurements were made in the 4.6-m (15-ft)-deep room furnished as a private office. Analysis of discomfort glare was conducted using high dynamic range imaging coupled with the EvalGlare software tool, which computes the daylight glare probability and other metrics used to evaluate visual discomfort.

The window film was found to result in perceptible levels of discomfort glare on clear sunny days from the most conservative view point in the rear of the room looking toward the window. Daylight illuminance levels at the rear of the room were significantly increased above the reference window condition, which was defined as the same glazed clerestory window but with an interior Venetian blind (slat angle set to the cut-off angle), for the equinox to winter solstice period on clear sunny days. For partly cloudy and overcast sky conditions, daylight levels were improved slightly. To reduce glare, the daylighting film was coupled with a diffusing film in an insulating glazing unit. The diffusing film retained the directionality of the redirected light spreading it within a small range of outgoing angles. This solution was found to reduce glare to imperceptible levels while retaining for the most part the illuminance levels achieved solely by the daylighting film.

• building energy efficiency
• daylighting
• microstructure film
• prismatic film
• windows

ACKNOWLEDGMENTS

We thank Raghunath Padiyath and Doug Huntley at the 3M Company for their in-kind contributions of time and material and our LBNL colleagues Dennis DiBartolomeo, Howdy Goudey, Daniel Fuller, and David Parker for their technical support throughout the duration of this project. This work was supported by the Assistant Secretary for Energy Efficiency and Renewable Energy, Building Technologies Program, of the U.S. Department of Energy, under Contract No. DE-AC02-05CH11231 and by the California Energy Commission through its Public Interest Energy Research (PIER) Program on behalf of the citizens of California.

Notes

1. 1Wienold and Christoffersen [2006] indicated that the correlation of vertical eye illuminance to percentage of disturbed persons was expected to be valid within 1000 lx to almost 10,000 lx. For the DGP correlation, DGP was valid for DGP values greater than 0.2 and E_v greater than 380 lx [Wienold 2009b]. DGP values are given for 20-year-old persons. Discomfort will increase with age.

2. 2Wienold and Chistoffersen [2006] found no significant change in the DGP correlation for a search radius within a range of 0-0.8 sr.