The Bidirectional Radiometric Scanner

Liliana Beltran and Konstantinos Papamichael (Building Technologies Program) prepare a sample for testing in the scanning radiometer.

Saving energy in buildings is increasingly a matter of balancing different efficiency strategies. A building uses less cooling energy during the summer when it has specially coated windows that reduce solar heat gain. But a building with increased daylighting and lighting controls uses less lighting energy throughout the year, suggesting that buildings can harvest significant energy savings with windows that transmit most of their incident visible light. During the winter, solar heat gain through windows also contributes to maintaining a comfortable indoor temperature. But the amount of energy that gets through is determined by the effect of the window plus any shading device, such as a venetian blind‹classified as a complex fenestration system.

This complicated interplay of factors requires researchers to have an accurate method of measuring the solar heat gain of any window system, which can include shades, blinds, drapes, and a variety of glazings, tints, coatings, and glass thicknesses. Scientists in the Building Technologies Program developed a solar heat gain scanner to improve research on fenestration systems and to develop a universal rating system for fenestration solar heat gain.

The device, essentially a scanning gonioradiometer, consists of a fixed source of light and a sample mounted on a plane that rotates about a fixed vertical axis relative to the source. The sample also rotates about an axis that is perpendicular to this plane (see photo). An optical collection system is mounted on a semicircular arm that rotates about a vertical axis through the center of the sample. The three elements of this detector system are a collecting mirror, an integrating sphere, and a pair of sensors. Radiation is collected by the mirror and focused onto the entrance of the integrating sphere, which contains a radiometric and a photometric sensor. The sensors collect data on both the wavelength and intensity of radiation coming from the test sample. The data are then amplified and sent to a computer to be recorded.

Joseph Klems and colleagues have been using the scanner to develop a method of calculating solar heat gain that is more reliable than older, calorimeter-based methods. Using the old technique required testing the complex window system in every possible configuration‹for example, every possible orientation of its venetian blinds‹a prohibitively expensive procedure.

The Center researchers are testing a faster, simpler way to measure solar heat gain. During the 20-minute procedure, the detector system measures the radiation distribution over the outgoing hemisphere by continuously moving over the detector's vertical arc, which in turn steps through the horizontal outgoing angles; when the outgoing scan is completed, the sample plane is rotated 15š and the measurement repeated for another incident angle. For most systems, characterization over six incident angles is sufficient; for very complicated systems it may be necessary to repeat the 20-minute measurement for multiple rotations of the sample within its plane.

This work is sponsored jointly by ASHRAE and the U.S. Department of Energy, with the goal of establishing a standardized method for measuring solar heat gain analogous to the National Fenestration Ratings Council U-value method. The results to date suggest that this radiometer-based approach provides results at least as accurate as and considerably faster than calorimetric-based methods. Although still under development, the scanner facility can be used in collaborative work with outside organizations; when development is completed, the scanner should be available for privately-funded studies.

Joseph Klems
Building Technologies Program
(510) 486-5564; (510) 486-4089 fax