As a component of our cost-effective core daylighting research program, we have developed a hollow light guide that can be used to distribute sunlight collected by a heliostat device, and can also operate as an efficient fluorescent light fixture when the sunlight is insufficient to illuminate the workspace. This dual-function prism light guide structure has a cross-section that is similar to standard commercial fluorescent light fixtures, but with an increased depth in order to pipe the sunlight. The bottom surface of the guide is lined with a thin polycarbonate film having prismatic surface features, while the top and sides are lined with a highly-reflective multilayer optical film having a luminous reflectance greater than 98 percent. The angular constraints of the concentrated sunlight, combined with the high reflectance of the surfaces, enable the sunlight to be efficiently transported within the guide along a distance of greater than 10 m. The guide can also illuminate the room using electric sources, since the light from the lamps strikes the prismatic bottom surface at angles for which it is an efficient transmitter. Photometric characterization of the new dual-function guide has been performed, resulting in illuminance levels, uniformity and efficiency values well within recommended standards. The dual-function design approach has yielded a daylighting system that can be cost-effectively incorporated into standard office building construction.