PATH - A Public Private Partnership for Advancing Housing Technology


PATH: Video-based Assessment of Advanced Light Redirecting Components in Windows and Luminaires to Optimize Lighting in Buildings

Massachusetts Institute of Technology

* Marilyne Andersen, Principal Investigator

Start: October 1, 2005
Expires: September 30, 2008


Technical description:

To efficiently control visual comfort in housing and reduce heating and cooling loads, an optimized use and distribution of lighting in buildings remains a major objective for advanced fenestration systems such as novel solar blinds, new glazing or coating materials and daylight-redirecting devices, as well as for energy-efficient luminaires.

Their directional optical properties are described by Bidirectional Transmission (or Reflection) Distribution Functions, abbreviated BT(R)DF, that express the emerging light flux distribution for a given incident direction. Their accurate assessment requires an appropriate experimental equipment.

To answer this need, the team developed a new measurement device, able to achieve truly time-efficient bidirectional measurements of coatings or materials. This innovative bidirectional goniophotometer, also convertible into a fully automated heliodon for qualitative studies on scale models, collects the light flux emitted by the analyzed component on a half-mirrored hemi-ellipsoid and redirects it towards a calibrated digital camera equipped with a fish-eye lens. Only one set of images taken at different integration intervals is therefore needed to achieve a complete and continuous visualization of the emerging light distribution, which is a unique feature compared to conventional, point-per-point goniophotometers.

The other major innovations of this research are to offer a wavelength-dependent investigation and to do this over the full solar spectrum. Extending the applicability of bidirectional functions to the near-IR spectral range opens new perspectives in managing and controlling solar gains, and leads to refinements in the location of the building's thermal mass.

Non-technical description:

Daylighting, or more generally lighting, not only has to adequately respond to our needs for visual comfort and for a healthy environment, it can also greatly contribute to reduce the environmental impact of buildings, considering that they represent about a third of the total energy use and that 40 percent of this energy today is generally dedicated to lighting. A carefully planned lighting strategy can also significantly reduce heating and cooling needs for housing by increasing solar gains in the winter and decreasing them in the summer. At the same time, numerous scientific studies have demonstrated the strongly positive impact of daylight availability on human productivity and well-being.

To answer the consequential increasing incentive to design buildings more adapted to daylighting, this team developed an original, leading-edge and time-efficient measurement device for investigating how light is distributed inside a space after being redirected or altered by advanced daylight collection and redirecting systems or luminaires components. The instrument relies on digital imaging techniques to reduce the measurement time to a minimum, while offering a wavelength-dependent analysis over the whole solar spectrum to extend the materials' investigation to the thermal aspects of solar radiation.

Having access to such a detailed information is critical for manufacturers that want to quickly develop and optimize their products, for architects to get guidelines in the judicious selection of the proper window and lighting components already at the project's design level, and for daylighting simulation tool developers to extend the capabilities of such software by including advanced light redirecting systems in connection with thermal control. Despite the complexity of its outcomes, the assessment process remains here rapid and cheap to efficiently promote more healthy and sustainable design.

For additional details on this NSF award.

Content updated on 9/26/2006

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