This paper presents the quantitative measurement to thesky luminance distribution with the help of the figure and numericalsystem of disposal

and suggests that the actual sky

with exception ofa few particular weather conditions

such as rain in which artificiallighting is needed

may be divided into CIE clear sky

pseudo-clearsky

pseudo-overcast sky and CIE overcast sky according to the skyluminance distribution

and verify the sky classification is reasonablewith fuzzy mathematics.