Abstract
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).
Highlights
A considerable number of studies published in recent years have proven the benefits of daylight and view out in indoor spaces for the building occupants [1,2]
We considered using the more conservative effect size thresholds proposed by Ferguson [49], yet, the cross-validation study on glare metrics [15] mentioned earlier showed that, when comparing the correlation analysis with Receiver Operating Characteristic (ROC) analysis, Cohen’s effect size thresholds are in better agreement with ROC interpretations [50] than the one from Ferguson
After a strict verification of the data and a thorough examination of High Dynamic Range (HDR) images and vertical illuminance measurements of all the scene following the rules detailed in section 2.6, we removed 7 data points to ensure the reliability of the data collected
Summary
A considerable number of studies published in recent years have proven the benefits of daylight and view out in indoor spaces for the building occupants [1,2]. Windows in workplace environment providing access to sunlight and outside view have been associated with alleviated stress at work [3] and improved productivity [4]. Research in this area have greatly impacted standards and guidelines in defining the recommendations for daylight availability, visual comfort and view out [5,6]. An increasing number of commercial buildings are utilizing glass as the main facade element to have a larger window to wall ratio for facilitating a view to the outdoors and access to daylight.
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