The previous study reported by some members of the Research Group for Glare Evaluation of Outdoor LED Light Sources established in the Illuminating Engineering Institute of Japan, showed that luminance based photometric quantities have a stronger correlation to discomfort glare evaluation than those based on equivalent veiling luminance or illuminance at the observer’s eye. They proposed the m_DGI as an index to assess the degree of discomfort glare. However, the test lights that cause strong discomfort glare were not examined there.
In this study, we conducted a subjective evaluation experiment using seven types of LED streetlights including those of higher luminance in the outer environment similar to the previous study. The results revealed several findings. First, discomfort glare is strongly felt when the positional relationship between the pedestrian and lighting fixtures is in a certain range; second, light sources with spatially non-uniform luminance distribution cause a strong discomfort glare; third, GR for sports-field lighting is not suited for discomfort glare evaluation of pedestrian zones; and fourth, the relation found in the previous study was confirmed.
In addition, we confirmed the versatility of m_DGI from weak to strong discomfort glare evaluation. We proposed a new index m_GR calculated using equivalent veiling luminance. Correlation to the discomfort glare evaluation is worse than m_DGI, our data indicate that m_GR is applicable to estimate a degree of discomfort glare in practical situations.
An integrating sphere is commonly used for a total luminous flux measurement of LED lighting. According to the theory of the integrating sphere, its interior coating needs to be a perfect reflecting diffuser. Although JIS C7801-2009 and IES-LM79, in which the measurement method of the total luminous flux of a light source is standardized, recommend that the reflectance of an interior coating of an integrating sphere be 90 % or higher, its deviation from the Lambertian is not discussed. This report measures and compares the bidirectional reflectance distribution functions (BRDF) of two commercial BaSO4 coatings, made by different manufacturers, and a PTFE coating for the integrating sphere. All samples had characteristics similar to the perfect diffuse reflection if the angle of incidence was close to the normal of the surface. At an incident angle of 60 degrees or more, BRDF of some of the sample was grater out-of-perfect diffusion. It affects the spatial response distribution function (SRDF) of the integrating sphere when a 2π geometry light source mounted on the sphere wall is tested and when part of the light from the source is reflected on the sphere wall closest to the source.