Critical review of the daylight glare probability and proposal for modification for improved evaluation of small glare sources

EN 17037 defines essential properties of daylight in buildings: daylight provision, view out, sunlight exposure and glare protection. For assessing glare, the standard proposes the daylight glare probability (DGP) metric. A look at the formula reveals the following problem: since the luminance is included quadratically, but the solid angle only linearly, small but bright glare sources (e.g. the sun) are rated highly. This must be questioned if the glare stimulus can no longer be distinguished from larger stimuli causing equal vertical illuminance at the eye, especially in the peripheral visual field. For the unified glare rating for artificial lighting, this is avoided by limiting the minimum solid angle of the glare source. There is nothing comparable for the DGP. We present a simulation study on how the DGP changes with the bidirectional scattering distribution function resolution for complex fenestration systems or with rendering the sun at its real size. We performed subject experiments on the discriminability of small light stimuli in the periphery in a laboratory setup. Based on the findings, we propose adjusting the DGP by limiting the minimum glare source solid angle depending on its position in the field of view. We conclude with an evaluation of the simulation results with the modified metric daylight glare metric (DGM) and point out open issues that still need to be investigated.