Methodology to modify and adapt the standardised spectral power distributions for daylight to account for geographical, seasonal and diurnal variations for practical applications.

In recent years, the spectral properties of solar radiation and daylight have become increasingly important in lighting design and research, and various approaches to implement these have been applied. This paper proposes to modify and adapt the CIE reconstruction method, a procedure developed in the early 1960s to define standardised spectral power distributions (SPDs) of daylight, for this purpose. The CIE D Illuminants resulting from the reconstruction procedure are widely used for standardisation purposes but are based on a smaller number of measurements and do not consider geographical, seasonal and diurnal variations. In order to be able to use the CIE reconstruction method specifically in daylight planning, research and application, a technical committee of the CIE has launched a worldwide measurement campaign to collect spectral daylight measurements. The aim of the committee is to formulate a customised reconstruction method that more accurately reflects the local SPDs of daylight. This paper contributes to the discourse on the improvement of daylight estimation methods and emphasises the importance of accurate daylight data in various scientific and practical contexts.