The impact of melanopic illuminance and CCT on spatial brightness perception of illuminated interiors and energy-saving implications

Abstract

Modern interior lighting design pays increased attention to spatial brightness appearance of illuminated spaces. However, spatial brightness is affected by spectral power distributions (SPDs) of lighting in a manner that cannot be predicted based on the traditional photometric quantities such as illuminance and luminance. Understanding the other contributing factors of lighting spectra to spatial brightness is important for improving energy efficiency of lighting in built environment, especially in indoor spaces where natural ambient illumination by daylight is lacking and electric lighting is heavily relied on.In this work, the experiments of spatial brightness comparisons were performed in a windowless room equipped with spectrum-tunable LED luminaires that delivered indirect light to observers with individually controllable parameters at the eye-level, such as photopic illuminance, cyanopic illuminance, melanopic illuminance, and correlated color temperature (CCT). A method was developed to quantitatively compare spatial brightness between lighting scenes with different SPDs. The results showed that great differences in the effectiveness of spatial brightness exist among SPDs. Both melanopic illuminance and CCT make contributions to spatial brightness, but with different intensity-dependent behaviors. Furthermore, the M/P ratio (melanopic to photopic ratio) is likely to have a clear positive correlation with spatial brightness at a fixed photopic illuminance. The results of this study could help improve energy efficiency in indoor lighting.