Spectral Drivers of Circadian Daylight Performance in Architectural Glazing under Different Gaze Directions

Windows affect indoor thermal conditions, visual comfort, and non-visual circadian light exposure. Standard window metrics, including U-factor, solar heat gain coefficient (SHGC), and visible transmittance (Tvis), are commonly used; however, they offer limited insight into the impact of glazing spectral transmission on circadian daylight performance. This study investigates which wavelength ranges within the visible spectrum most strongly influence circadian daylight outcomes and examines how these effects vary with window orientation and occupant gaze direction. Using a representative library of glazing spectra, transmitted light was divided into nine wavelength bands covering the blue, green, and red regions. Circadian performance was quantified using Spatial Circadian Daylight Autonomy (sCDA) for north- and south-facing windows across eight gaze directions. Correlation analysis and best-subset regression identified the spectral bands and viewing conditions most strongly associated with sCDA. Results indicate that circadian daylight performance is most sensitive to transmission in the blue-cyan (460-498 nm) and green (550-586 nm) ranges, while Tvis alone does not reliably predict circadian outcomes. Furthermore, indirect viewing directions are more sensitive to glazing spectral properties than direct window-facing views, underscoring the combined importance of glazing spectrum and occupant-window geometry. These findings support performance-based glazing selection and interior planning strategies that incorporate spectral transmission to enhance circadian health.