The Effect of Luminance Reduction on Foveal Task Performance while Implementing Mesopic Photometry in Street Lighting Standards

<p>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.</p>