# HCL Control Strategy for an Adaptive Roadway Lighting Distribution
###### Abstract
Much of the literature has further discussed the improvement of uniformity and efficiency of lighting [18; 19; 20; 21; 22]; the design method of freeform surfaces is also suitable for road lighting [23; 24; 25], and provides various calculation methods when designing rectangular beam patterns. Besides using freeform lens analysis for secondary optics design, it can also be applied on LED packaging [26], chromaticity space uniformity [27] and offer concrete improvement guidelines. For the secondary optical design of road lighting, the main requirement is often the uniformity of illuminance, hence the designs are all aimed at forming a uniform rectangular beam pattern. However, from a driver's point of view, the level of luminance of the road surface is the most important factor that determines their visual experience. In addition, different pavement materials, such as asphalt, cement, etc., will have different brightness coefficients on the road surface. In ideal conditions, different road surfaces have their ideal photometric curves, and the optical parameters can be obtained through actual driving experience. The purpose of roadway lighting is to provide road users with a comfortable and quality light source while driving. We have referred to luminance-related road lighting publications including the Taiwan Expressway Lighting Code CNS-16069, International Road Lighting Code CIE-115-2010 [28] and the North America Lighting Association Road Lighting Code RP-8 -19 [29]. In addition, when using luminance as an evaluation standard for lighting quality, drivers' visual perceptions can serve as a reference when designing lighting. As shown in Figure 1, the relationship between users' viewing angles and the angle of the road surface scattering characteristics is clearly defined in the CIE-140-2000 [30] measurement regulations. When calculating the level of luminance for the road surface, its brightness coefficient (r-table) should be taken into consideration. The reflection coefficients of incident light from different angles are non-identical. Even if the luminance is measured from the same observation point, different luminance levels will be generated due to the difference in the angle of the observation position. Therefore, when it comes to road surface, the luminance measurement is relatively more complicated than the illuminance measurement. Recent years, the measurement methods for road lighting have made significant progress. Studies [31] have established a complete measurement system that can assess the luminance and illuminance of road images on continuing roads, which can be considered as a reliable method and of reference value when it comes to on-site road lighting testing and acceptance. Some researchers [32] have also combined the illuminance measurement system with vehicles so that the road surface illuminance level can automatically be recorded while in motion. Although the method is quite convenient, the interference factors, such as vehicle lights, would be hard to avoid during the whole measurement process. The designs of most of the measurement equipment are based on the theory of photonic vision. Since the roadway lighting at night is often in a mesopic-vision environment, the S/P ratio of the street light spectrum will greatly affect the measured value. This theory is typically suitable for road lighting that has a dark surrounding environment [33]. Pedestrian demands are one type of issue that has received attention in recent years. Besides considering the visual experience (clarity) of the driver, roadway lighting should also take into account the safety of the pedestrians. Researchers [34] used ILM to analyze the contrast of pedestrian, road surface and background luminance from the drivers' fields of view, which are different from the traditional measurement design, which only evaluates the road surface illuminance of the crosswalk. Another study [35] introduced some practical measurement experience.