Study On Mesopic Vision And Measurements Of Road Illuminance Under LED Light Sources

Recently, for road lighting applications the attention of illuminatingengineering has turned towards the research of suitable light sources fornight-time, which may lead to safer and more comfortable driving conditions,and also meet the need of sustainable energy. According to several countries’standard of city road lighting design, the average luminance recommendationfor road lighting is between0.3and2cd/m2, which is under the mesopic lightlevels. However, to this date, there are still no international standards for thesystem of mesopic photometry and the conventional photometry relying onthe photopic luminous efficiency function is not a completely satisfactory atthose low light levels for road lighting in night-time. Thus, in this paper westudied a method to measure the road illuminance under the LED streetlighting lamps, and proposed a modified coefficient of mesopic equivalentillumination. Moreover, the results provide a basis for further studies on theilluminometer, which might be suitable for mesopic vision.First, based upon a number of recent mesopic photometry studies, an equation isdeduced for the tramsition from the photopic illumination to mesopic equivalentillumination, and one kind of modified coefficient for different light sources undermesopic light levels is studied. The modeified coefficient can be used easily tocalculate the mesopic equivalent illumination by using the measuring results ofphotopic illumination for different correlated color temperature (CCT) light sourcesunder mesopic light levels. At the same time, by means of subjective evaluationthrough two part of experimental, we combined the visual subjective feeling withphysical parameters measurement. The results of these experiments also comply withthe theoretical calculation results, and show the accuracy and practicality of themodified coefficient for mesopic equivalent illumination.Second, according to the results of experiments and calculations, we analyzedthe mesopic equivalent illumination of the different correlated color temperature LED light sources under mesopic vision levels. At the same luminous conditions, theillumination modified coefficient grows when the correlated color temperature of thewhite LED light sources increased, then the values of the equivalent mesopicillumination is also increasing. Thus, we concluded that in road lighting or residentialdistrict lighting, the higher correlated color temperature light sources have bettervisual effects at the same illumination conditions (photopic light levels). In addition,the high luminous correlated color temperature light sources can increase the level ofexcitement of the brain, so be more responsive to transient events.At last, a measurement of illumination of different correlated color temperatureLED light sources under mesopic vision levels is designed. Based on twoembodiments to describe and analyze, the method provides the basis for furtherstudies on the mesopic illuminometer, which might be suitable for mesopic vision.