A study on daylighting metrics related to the subjective evaluation of daylight and visual comfort of students in China

Uneven daylighting distribution in classrooms is a common issue in many countries, despite meeting the standard daylighting factor (DF). The issue has been particularly acute in China. With the development of dynamic daylighting metrics, they have been incorporated into daylighting standards in various countries. The introduction of dynamic daylighting metrics in China is an opportunity to improve the classroom lighting environment, considering that the Chinese regulations have not been updated for a long time. However, there is a dearth of research investigating the validity of dynamic daylighting metrics in Chinese school buildings. Therefore, a correlation analysis of dynamic daylighting metrics and students' subjective evaluation has been carried out. This one-year study included field surveys and illuminance measurements collected in 18 classrooms in a Chinese school; the dynamic and static daylighting metrics of these classrooms were simulated. The spatial daylight autonomy (sDA) at 450 lx for 50% of annual hours (sDA450/50%; a dynamic metric) and the proportion of area with an illuminance (Ep) > 300 lx (a static metric) were highly correlated with student evaluations. Additionally, sDA450/50% ≥50% was preferred by student evaluations. Annual sunlight exposure (ASE) was not correlated with student evaluations in north-facing classrooms in the Guangzhou area. However, a correlation was found between perceived blackboard glare and the percentage of space where disability glare probability (DGP) exceeded a threshold of 0.35 for>5% of the usage time (sDGPexceed). Consequently, sDA450/50%≥50% is a recommended metric for China’s school-building daylight standards, while sDA250/50% ≥75% can be used simultaneously to improve the effective daylighting area of classrooms.