Contrast-based evaluation of the reflected daylight glare on the blackboard in a classroom with complex fenestration systems

Abstract

Visual comfort within a classroom determines students’ concentration, productivity and well-being. The existing glare-based metric (e.g. Daylight Glare Probability, DGP) cannot reveal the visual comfort level when students look at notes written on the blackboard in a daylit classroom. Contrast-based metric has been proven as a more appreciate method for evaluating the vision within the area of a blackboard. The aim of this article is to explore the applicability of a contrast-based metric in evaluating classroom daylight design. The contrast-based metric was proposed based on the statistical results of a previous study on disturbing reflections on a blackboard. The evaluation method using contrast ratios within the blackboard area was established based on RADIANCE simulation and validated against experimental measurements. Bidirectional scattering distribution function (BSDF) data was used to model the complex fenestration systems (CFS) in the studied classroom. The validation results show that the error of simulated contrast ratio is within 17%. When using the contrast-based metric for assessing the studied classroom with different fenestration designs, the simulation results indicate that applying vertically placed daylight redirecting louver can significantly improve the contrast performance while maintain the daylight availability, providing design guidance for fenestration systems of classrooms. The simulation also illustrates that using DGP in a classroom leads to an underestimation of the glare level. This study contributes to expanding the knowledge on visual environment evaluations of classrooms, demonstrating the capacity of using contrast-based metric on determining disturbing glares in a classroom daylight design.