Design and assessment of an anidolic light-duct

The system presented here, called Anidolic Ceiling, was developed to show the viability of intensive use of daylight by overcast outdoor conditions in nonresidential buildings. This device consists of a light-duct that is integrated in a suspended ceiling and leads midway into the office. Anidolic (nonimaging optics) elements are placed on either end of the duct, on the outside to collect light rays from the sky and on the inside to control the direction of the emitted light. The present paper describes the system design, as well as an experimental assessment of its daylighting performance in a comparison of a prototype and a full-size conventional facade, for a 6.6-m deep room. Measurements by overcast sky have established that the daylight factor on the work plane 5 m from the window is more than doubled. In addition, a monitoring campaign has shown that 30% of the energy for lighting can be saved. The system was also assessed with regard to the user. Visual comfort measurements (Laboratoire d'Ergonomie de la Vision's (LEV) method) were carried out showing that for both clear and overcast outdoor conditions, the visual environment quality is objectively improved at the rear working place. Furthermore, 33 people tested both rooms one after the other. They were submitted to a series of visual acuity tests on printed paper and on a computer screen and had to fill in a questionnaire. A comparative study showed that the personal appreciation of the luminous environment is better in the room with an Anidolic Ceiling, with a significant reduction of reading errors both on paper and on the screen. (C) 1998 Elsevier Science S.A. All rights reserved.