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The increasing replacement of traditional lamps by LED technology opens the door to research into visual performance under their multiple possible spectral power distributions. The correlated color temperature and S/P-ratio are considered parameters characterizing the spectrum of a light source. The lack of firm conclusions motivates the analysis in this work of the influence of LED spectrum, which is defined by the correlated color temperature and S/P-ratio, on a representative night-driving task, visual reaction time. A two-channel and four primaries photostimulator was used as set-up for measuring reaction time in off-axis vision, at mesopic illumination conditions and for a range of stimulus contrasts. The experimental conditions included a wide range of correlated color temperatures, ranging from 1870 to 6350 K, and different S/P-ratios for the same temperature. A total of 16 young subjects participated in the experiments. The results show significant shorter reaction times for those conditions in which the spectrum has greater short-wavelength content. This is corroborated by the greater stimulation of rods and S-cones obtained in the calculation of their excitation level. However, the definition of the spectrum using the correlated color temperature and S/P-ratio does not equally define the results obtained. For the same temperature, a higher S/P-ratio provides lower reaction time values, but for the same S/P ratio reaction time is independent of the temperature. LED light sources with high short-wavelength content provide faster response on the basis of a greater excitation of rods and S-cones. The S/P-ratio is the best parameter to justify the effect of spectrum on reaction time, as considers the spectral sensitivity of the visual system in its calculation.

Effect of Correlated Color Temperature and S/P-ratio of LED Light Sources on Reaction Time in Off-axis Vision and Mesopic Lighting Levels