为了计算自然光照度，需要从CIE定义的15种天空类型中正确选择一种对应当前实际天空。首先根据朝向太阳与背向太阳的垂直天空照度的比值将15种天空类型划分为6组，然后根据天空光透过窗口在空间模型两个不同测量点形成的照度的比值对6组天空类型做进一步划分，最终实现对CIE定义的15种天空类型的选择。将天空类型的选择结果与实际天空类型相比较，结果表明，这种天空类型选择方法所得结果可以准确地对应当前实际天空类型。

In order to compute the natural illuminance,it is necessary to select one correctly among the 15 sky types defined by CIE to correspond to real sky. Firstly,according to the ratio of the sky illuminance on a vertical surface facing the sun to the sky illuminace on a vertical surface back the sun,the 15 sky types can be divided into 6 groups. Secondly,the 6 groups sky types can be further divided according to the ratio of the window sky light illuminance formed by the result on the two different measuring points in the space model. And ultimately,the correct type can be chosen from the 15 sky types of CIE standard general sky model. The result from the comparison between the selected sky type and the real sky type shows that this method works accurately on choosing the correct sky type for the real sky.