Daylight glare evaluation with the sun in the field of view through window shades

This paper provides new insights on daylight glare evaluation for cases with the sun in the field of view through window shades. 41 human subjects (n = 41) were tested while performing specific office activities, with 14 shade products of different openness factors and visible transmittance values (direct and total light transmission characteristics) installed on the windows. The measured variables and survey results were used to: (i) associate discomfort glare with measured and modeled parameters (ii) evaluate the robustness of existing glare indices for these cases (iii) examine alternate illuminance-based criteria for glare assessment through fabrics, extract discomfort thresholds and suggest a new related index and (iv) propose corrections in the DGP equation coefficients when the sun is visible through the shades. The modified DGP equation resulted in the best fit; the findings show that the general form of the DGP equation is reasonable and can be adjusted to account for different cases, by clustering different sets of coefficients for different environmental conditions or fenestration systems. The new alternate glare discomfort index developed in this study, based on direct and total-to-direct vertical illuminance on the eye, captures the impact of sunlight as well as the interdependence between the fabric color, overall brightness, and the apparent intensity of the visible sun. It can simplify annual simulations, eliminating the need for detailed luminance mapping of the interior, and can be directly associated with fabric optical properties for development of design guidelines and glare-based shading controls. (C) 2016 Elsevier Ltd. All rights reserved.