Daylight Redirecting Window Film

3M offers a daylight redirecting film and SerraLux offers their SerraGlaze redirecting film. Both involve a thin passive acrylic film that contains micro-structured louvres to re-direct incident daylight arriving on the upper portion of the window toward the ceiling of a room. Daylight is then reflected down to light the room.  The redirecting films can be incorporated into windows for new construction or installed as retrofits.  Electricity savings occur as lighting can be turned off or dimmed.  This technology provides little savings during cloudy, overcast days. 

Daylight redirecting films (DRF) can be installed on south, east, and west facing building windows. Generally, the film is installed above a sill height of 6 feet to keep redirected sunlight out of the eyes of occupants. Performance is a function of angle of incidence of sunlight (depends on latitude, orientation of the building façade, season, time of day) and sky conditions. Outdoor views are not possible through the DRF--the appearance is similar to frosted glass. The building façade must not have deep recessed windows, trees, and nearby buildings that block the sunlight. Windows should not have tinted glass or sun control window films.

The films will work as intended but several modifications must be made can reduce cost-effectiveness and/or technical potential. First, the indoor space must have minimal vertical obstructions (for 20 feet to 25 feet). Windows area must be available above six feet from the floor. Ceilings and walls must be light colored to promote reflectance and a dimmable lighting control system is required for affected lights. Sites with vertical shades may have to install horizontal shades just below the DRF level. HVAC cooling and heating loads may increase or decrease based upon the magnitude of the solar gain versus the reduction in heat rejection from lighting sources. Users have found that a diffusing film must also be used to eliminate introduction of glare problems.

The Department of Defense ESTCP program conducted a cost and performance study for DRFs at six sites nationwide including the Bremerton Naval Hospital.  The simple payback for the northwest application (windows for examining rooms) ranged from 11 to 35 years for south, east, and west-facing windows assuming an electrical energy rate of $0.086/kWh.  Savings were found to be heavily dependent upon successful use of lighting controls, plus building location and orientation.  Savings are also dependent upon the light type (32 W fluorescent versus LED) and the existing light level that is maintained in the affected space.