SKY LUMINANCE DISTRIBUTUION MODEL FOR SIMULATION OF DAYLIT ENVIRONMENT

It is important to apply the natural light effectively for the low energy consuming daylighting design. A reliable standard sky is absolutely required that copies after the real sky luminance distribution for the simulation and evaluation of the daylit environment. Any applicable standard sky, however, has never found that shows accurately the actual state of daylight. A research work was carried out by the authors in order to establish a suitable sky model that can represent all the sky luminance distribution from the clear sky to the overcast sky continuously. The model proposed here was named “All Sky Model”. The detailed descriptions on the research are explained in this paper as well as All Sky Model. INTRODUCTION An excellent standard of the sky luminance distribution is necessary for daylighting design. CIE Standard Clear Sky and Overcast Sky (1994A, 1996) show the two extreme sky conditions. Most of the real skies lie between the two CIE Standard Skies. They show various aspects of the sky conditions and the diverse distribution of sky luminance. The goal of this research work is to standardize all sky conditions by formulizing their luminance distribution. A numerical equation is introduced in this paper by which the absolute values of the luminance and luminance distribution of all kinds of skies from the clear sky to the overcast sky can be estimated. The sky proposed in this paper as an advanced standard sky is named “All Sky Model”. SKY LUMINANCE MODELS RECOMMENDED AND PROPOSED Moon and Spencer (1942) surveyed and arranged the previous research works, and proposed luminance distribution of the overcast sky as the standard. This overcast sky model was simplified a little, and has been recommended as the CIE Standard Overcast Sky (1955). The CIE Standard Overcast Sky is similar to the considerably dark sky covered with thick clouds. Kittler (1967) proposed luminance distribution of the clear sky model as a standard. This clear sky model has been adapted as the CIE Standard Clear Sky (1973). The CIE Standard Clear Sky shows very close luminance distribution of the completely clear up sky. There is no recommended zenith luminance of the overcast and clear skies by CIE, though both of the CIE Standard Skies are shown in the relative values to the zenith luminance. There are some proposals (1994B) about the zenith luminance of the overcast and clear skies by several research workers. Both CIE Standard Skies show the extreme sky conditions of the cloudy condition and the clear up condition. The frequency of occurrence of the luminance distribution close to both CIE Standard Skies is considerably small. Most of the real skies lie between the two CIE Standard Skies. Nakamura (1985) proposed the Intermediate Sky. An equation concerning the zenith luminance of the Intermediate Sky was proposed at the same time. The Intermediate Sky is a kind of the average sky models of each solar altitude of all the skies except similar sky to the two CIE Standard Skies. Littlefair (1981) proposed a sky luminance distribution model of each solar altitude called BRE AVERAGE SKY based on the measurement data by Wegner (1975). Kittler (1985) assumed that the sky condition fluctuates homogeneously from the clear sky to the overcast sky, and proposed the HOMOGENEOUS SKY to show the absolute value of the sky luminance distribution. Perraudeau (1988) classified all the skies into five categories, that is, the overcast sky, the intermediate overcast sky, the intermediate sky, the intermediate clear sky, and the clear sky. And he proposed equations by which the sky luminance distributions were expressed. Perez (1993) proposed the ALL WEATHER MODEL as the function of Sky Clearness and Sky Brightness by which all the skies were classified into eight categories. The relative diffusion indicatrix of this model is a little different from the CIE Standard Clear Sky. Recently, Kittler, Darula and Perez (1997) proposed a