## 1 Introduction  
Driving at night in adverse weather conditions can be a threat to safe driving by reducing the driver's forward visibility [1]. The mesopic and glaring conditions of the night driving environment, especially due to age-related deterioration of vision, make night driving more difficult for the elderly [2]. Since August 2017, Korea has become an aging society in which the proportion of the population aged 65 and over accounts for more than 14% of the total population [3], and the proportion of elderly drivers is increasing every year [4]. As the number of older drivers increases, the incidence of traffic accidents among older drivers is also increasing [4, 5]. According to statistics from the Road Traffic Authority, as of 2016, the total number of traffic accidents in Korea decreased by 4.8% compared to the previous year, but the number of traffic accidents involving elderly drivers increased by 5.9% [6]. The number of traffic accidents involving elderly drivers is increasing every year due to deterioration of cognitive ability while driving and deterioration of driving ability due to geriatric diseases [4]. In this regard, there is a case in which a headlamp was developed and evaluated to improve the visibility of the elderly in consideration of the glare and sensitivity characteristics of the elderly [7]. Considering the traffic environment in which the proportion of elderly drivers is increasing, the development of technology that enables safe driving by increasing driver visibility and securing reaction time will be helpful not only to elderly drivers but also to general drivers [6]. In addition, it can be expected that the overall accident rate will be reduced, and thus, direct and indirect reductions in social costs caused by traffic accidents are expected [6]. If the luminance of the target object can be made relatively higher than the background luminance in bad weather conditions such as fog or rain, the likelihood of recognizing the target object at a distance greater than the vehicle stopping distance will increase [8]. In addition, there is a case in which the visibility can be improved in bad weather comparedto the color temperature of about 3000 K and the color temperature of 6000 K [9]. The visibility performance of headlamps is generally evaluated based on the luminous intensity or illuminance of the headlamps [8]. Illuminance is evaluated by directly measuring the light intensity of the light source, and luminance measures the intensity of light emitted from the light source that is reflected on a specific incident surface and reaches the human eye [8]. Therefore, rather than the illuminance, which is the amount of light from the headlamp itself, the road surface luminance, which is the amount of light that is reflected from the road surface and enters the driver's eyes, and the luminance that enters the driver's eyes after being reflected by obstacles on the road are more suitable indicators for the evaluation of visibility related to the headlamps [8]. From the perspective of automobiles, in the case of studies related to night visibility in bad weather conditions, there are studies on subjective and objective evaluation methods and research cases for improving visibility [9; 10; 11; 12; 13; 14; 15; 16; 17]. In the case of studies on visibility evaluation methods, there is a study on the performance of LED streetlights with different color temperatures [9], and there is a case where the visual performance with different CCTs (correlated color temperatures) in mesopic conditions is evaluated [10]. In some cases, subjective evaluations were conducted through participants for each different CCT [11], and in other cases, LED (light-emitting diode) headlamps were evaluated through objective and subjective methods [12]. Among the interesting studies related to the objective visibility evaluation method, there are cases where the contrast between the road surface and lane markings was measured using a mobile retroreflectometer mounted on a vehicle [13; 14].