The trend of natural illuminance levels in 14 Chinese cities in the past 50 years

Background Maximizing natural illuminance, seeking sustainable ecological development, and saving electricity are very important. The annual total natural light illuminance values of 14 typical Chinese cities in different climate regions were obtained in this study based on solar irradiance data of the cities in the past 50 years (from 1961 to 2010). A luminous efficacy model was integrated in the analysis of data. This study analyzes the overall and inter-annual change trends of total natural light illuminance values in each city.