CIE 110-1994

The terms of reference of TC 3-09 were “to study standard reference skies between the two skies already standardized, clear sky and overcast sky, and to propose a method of composing average and mean skies”. Accordingly, this technical report reviews studies on various sky luminances and specification of reference skies, with a method of composing average sky and mean sky as an appendix. Several reference skies including the average sky and mean sky are given, definitions and various studies on these sky luminances are introduced, with a bibliography, in three appendices.

Although several kinds of average and mean skies have been proposed in Europe, USA and Japan since 1981, their definitions and the methods of composing them were not standardized. Also the two CIE Standard skies (Clear sky and overcast sky) are reviewed in the present publication, thus CIE 22-1973 is now outdated and withdrawn. The standardization of reference skies between the two standard skies was left to a new technical committee, TC 3-15 (Standardization of intermediate sky luminance), which was formed at the CIE Venice Session in June 1987. In this report a references sky means a clearly defined sky to be used for daylighting design and calculation. For example, one reference sky is the uniform sky which has been used for a number of theoretical applications and for certain design purposes. The skies treated in this report do not include direct sunlight, though the BRE average sky developed by Littlefair consists of two components, direct sunlight and the sky.

In order to estimate annual energy consumption of electrical lighting, the effect of direct sunlight should be taken into consideration in calculating illuminance on the interior working plane, whether this is done for every weather condition or for a certain average condition. Hence, each standard sky luminance has to be combined with a standard direct sunlight illuminance. These items are to be decided in another technical committee in future, but a proposal is tentatively shown.

The report is written in English, with a short summary in English, French and German. It contains contains 33 pages, 10 figures and 2 tables.