Reconstruction of Daylight Spectral Power Distribution Based on Correlated Color Temperature: A Comparative Study between the CIE Approach and Localized Procedures in Assessing Non-image Forming Effects

Spectrally and spatially resolved information on daylight is critically important when planning for non-image forming (NIF) responses. Nevertheless, the availability of such data is scarce given the high initial costs and complex on-site maintenance of high-end spectral measurement devices. The CIE (Commission Internationale de l'eclairage) reconstruction procedure allows for the derivation of the daylight spectral power distribution (SPD) from the chromaticity coordinates or the correlated color temperature (T-cp). However, several studies have suggested that both the daylight locus and the reconstruction procedure are erroneous, and specifically SPDs with a higher T-cp cannot be reproduced accurately. This paper studies the reconstruction accuracy of the SPD of daylight, and contextualizes the findings in relation to NIF effects. The analysis comprises a comparative study to determine the accuracy of the CIE procedure compared to two localized reconstruction procedures, and a sensitivity study to examine the impact of accuracy on the assessment of NIF responses, as represented by all five retinal photoreceptors and expressed in the alpha-opic efficacy of luminous radiation. The results indicated that a localized procedure, adjusting both the daylight locus and the PCA components of daylight, outperformed the CIE reconstruction method. However, improvement in the reconstruction accuracy had no effect on NIF assessment. The RSMPE for alpha-opic quantities did not exceed 4% for any procedure. In practical terms, this implies that cost-effective sensors and the representation of spectral properties in sky models with a single value - the correlated color temperature - can be used for NIF purposes. These findings bridge theory and practice by opening up new insights into the understanding of simplified methods used to determine NIF effects of daylight. HIGHLIGHTS Using a localized procedure to define spectral power distribution (SPD) based on correlated color temperature (T-cp) outperforms the CIE method. Accuracy depends on the computation procedure rather than the daylight locus location.. Higher accuracy does not affect the alpha-opic responses used in defining non-image forming (NIF) effects. Findings confirm the applicability of simplified measuring and representation methods for daylight SPDs. T-cp can be used to represent daylight SPD in planning software to assess NIF effects.

Automated summary

This study examines the reconstruction accuracy of the spectral power distribution (SPD) of daylight and compares different reconstruction procedures. The results show that a localized procedure outperforms the CIE method in terms of accuracy. However, the improvement in reconstruction accuracy does not affect the assessment of non-image forming (NIF) effects. This implies that cost-effective sensors and simplified representation methods can be used for NIF purposes.