Behind electrochromic glazing: Assessing user's perception of glare from the sun in a controlled environment

The adaptable transmittance of electrochromic glazing allows to control the solar radiation entering buildings, yet the level of transmittance needed to protect from glare is still an unanswered question. To bridge this gap, this study evaluates the level of visible light transmittance (τv) required for blue-tinted low transmittance glazing to prevent discomfort glare when the sun is visible through the glazing. Twenty participants were exposed to four visual scenarios with varying viewing directions and window transmittance. Results indicate that when the sun is close to the central field of view, a normal-hemispherical transmittance, τv, n-h of 0.6% prevents disturbing glare for most users but does not provide a comfortable situation (this condition corresponds to a “seen” sun disc’s luminance of 4.8 M cd/m2). To achieve comfortable situations, a τv, n-h of 0.14% was found suitable. For non-critical viewing directions, τv n-h of 0.6% is sufficient to achieve visually comfortable space for most participants. This study also examined the reliability of five discomfort glare metrics by comparing their objective output to subjective responses for the tested conditions. The contrast-based metrics (Daylight Glare Probability, CIE Glare Index, Unified Glare Probability, Daylight Glare Index) possess a valid positional sensitivity and show higher Spearman’s rank correlations (ρ – 0.56–0.59) compared to solely saturation-based metrics as the vertical illuminance (Ev) (ρ – 0.44).