Daylight Glare Evaluation When the Sun is Within the Field of View Through Window Shades

Shading fabrics have the ability to reduce daylight glare and provide privacy when needed. Recent studies have shown that glare indices such as DGP and its simplified version can be used to predict daylight glare through shades when the sun is not within the field of view. However, there are no comprehensive studies on glare sensation with the sun visible through the fabric – a situation that happens in office buildings – and therefore the applicability of glare indices for such conditions is uncertain. Shades with very low openness factors transmit only a small amount of direct sunlight due to their weave density; nevertheless, existing glare metrics may show intolerable conditions for these cases, while specific studies with human subjects are nearly non-existent. This paper presents an experimental study on daylight glare evaluation for the case of shading fabrics with the sun within the field of view. 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). The measured variables and survey results were used to: (i) associate discomfort glare with measured and modeled parameters for these cases (ii) evaluate the robustness of existing glare indices for these cases (iii) examine recently suggested alternate (direct and total vertical illuminance) criteria for glare assessment through fabrics, extract discomfort thresholds and suggest a new related index and (iv) propose corrections in the DGP equation when the sun is visible through the shades, which could be generalized for other systems following a similar approach. Combining illuminance-based metrics and existing glare indices can result in a more realistic glare evaluation covering all cases with and without the sun through shading fabrics. The new results can be inversely used as thresholds for selecting optical properties of shades to ensure glare protection, as well as for the development of glare-based shading controls.