The daylight performance of anidolic ceiling for diffuse daylight is the subject of this article. The performance is assessed over a wide range of sky conditions following the new CIE Standard General Sky and compared between locations in Singapore, Japan and the United Kingdom. The criteria illuminance ratio (IR) and daylight glare index (DGI) are used to quantify the daylight performance of a default ribbon window facade with and without an anidolic ceiling. The difference is expressed in new terms IR improvement factor (IR IF) and DGI reduction. These factors are charted over all 15 sky conditions and various sun altitudes and may serve as general references indicating conditions under which anidolic ceilings perform best. The application of these new terms are demonstrated for Singapore, Fukuoka and Sheffield, representing three main latitude bands and different sets of prevailing sky types. It can be concluded that the daylight performance improvement through anidolic ceilings is most significant in Singapore. (c) 2006 Elsevier Ltd. All rights reserved.