Mesopic photometry (CIE 191: 2010) can be applied to the road lighting practice to enhance peripheral visual performance and decrease light levels. However, mesopic photometry characterizes peripheral vision, whereas photopic photometry describes foveal vision at all light levels. Therefore, national lighting standards have been amended based on mesopic photometry, allowing lighting specifiers to decrease light levels if the S/P ratio of the light source is higher than conventional standard lamps. However, such reductions in the required light level may impair foveal visual performance. Therefore, we conducted two experiments to investigate whether visual targets detected by peripheral vision could be recognized correctly by foveal vision after moving the line of sight to fixate on the on-axis targets. Based on the results of the experiments, we examined whether reducing background luminance while replacing a conventional light source (S/P ratio = 1.16) with a light source having a higher S/P ratio (= 2.22) under a practical street lighting condition based on the mesopic photometry would not impair foveal visual performance. The results of the experiments and discussion suggested that reinforcing street lighting standards for pedestrians (e.g., JIS Z 9126) based on mesopic photometry (i.e., CIE 191: 2010) improves peripheral visual performance with little impairing the foveal vision as long as the luminance contrasts of the visual targets are high enough. However, care is needed for foveal vision when implementing mesopic photometry for visual targets with lower luminance contrasts.

It has been reported that both luminance and melanopsin stimulation affect the discomfort glare. However, in previous studies, visual stimuli were only presented to a part of the field of view. In this study, we examined the effect of melanopsin on discomfort glare in a lighting environment and attempted to formulate a discomfort glare, including its application in designing interior lighting. A white interior apparatus with a multispectral light-emitting diode light source placed on the ceiling was used, and ten participants evaluated the glare levels under several conditions. Glare perception was evaluated using nine categories corresponding to unified glare rating (UGR) values. Eight conditions were prepared by combining correlated color temperatures (2700, 5000, and 8000 K) and melanopsin stimulation. The luminance conditions of the luminous part were set to approximately 18,000 and 45,000 cd/m². We found that glare perception was affected by melanopsin stimulation when the correlated color temperature and luminance conditions were constant, but not by differences in the correlated color temperature conditions. Furthermore, we analyzed the results and derived an evaluation equation for discomfort glare as a function of melanopsin and S-cone stimulation added to the UGR parameters. The proposed equation enables the precise evaluation of discomfort glare in lighting environments.