1. Introduction
LED light sources are currently widely used in the lighting market. They have many advantages, such as high luminous efficiency, low power consumption, multi-color and longevity [
1], but on the contrary there are heat dissipation and cost issues that must be resolved at the same time [
2]. In addition, the optical characteristics of LEDs and traditional light sources are very different [
3]. LEDs have become a new-generation light source and its directional light-emitting features hold a number of optical advantages [
4]. Therefore, when this new generation of LED lights are used in commerce, or to illuminate places such as homes and roads, they must have a good optical design to achieve the required lighting quality and efficiency. Professional roadway lighting requires precise light distribution in order to provide a safer driving environment for road users. In order to achieve the goals mentioned above, LEDs must change the output of the beam angle with the help of secondary optical components to provide appropriate lighting for all kinds of places, so as to meet the expectations and needs of road users.
When it comes to LED secondary optical design, there are some important factors that should be taken into consideration in the current LED optical industry, such as manufacturing materials, production techniques, beam pattern application, freeform surface calculation and efficiency evaluation, etc. These factors will directly affect the quality of lighting and only by fully considering the correlation between various indicators, and making a comprehensive evaluation of the whole system, can a lighting design be thoroughly planned out.
There are many types of LEDs on the market at the present time, and different package forms of light sources display their beam pattern differently [
5]. Although the light-emitting area of most LED lights are smaller than traditional light, it cannot be regarded as a real dot lighting source. The recent literature has proposed a few lighting uniformity designs for the LED light source [
6,
7], however, they often assume that LEDs are an ideal light source, which causes further constraints when designing. Currently, there are some studies on LED secondary optical design that proposes that the design process be conformed to its application requirements and needs. The most extensive research, which is still in the early stage of its development, is the design of the total internal reflection lens [
8,
9,
10,
11]. Based on the physical characteristics of the total internal reflection principle, components of the curved surface are designed to change the LED’s light-emitting angle.
There are diverse research directions on the application of freeform surfaces regarding LED secondary optics, and there are many discussions on the establishment of formulas and methods of evaluation [
12,
13,
14,
15]. The research includes collimating lens design [
16,
17], which defines the basic general formula for freeform surface design. Under ideal light sources, perfect collimated light can be output, as well as a narrow-angle beam pattern in practice. 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. Since the measured value of the luminance depends on the on-site road surface condition, the study points out that the road surface reflectivity and road curves are both influencing factors. Therefore, in addition to the lighting simulation software data, the theoretical basis should also take into consideration of the on-site road surface conditions for verification.
4. Discussion
The engineering design requirements of roadway lighting usually take both the road surface illuminance and the luminance conditions into consideration. Among them, road surface luminance is highly related to the visual experience of the road users. Hence, road surface luminance performance has been regulated around the world as an important evaluation index. Since the luminance value is inseparable from the reflection characteristics of the pavement material, if the scattering characteristics of the road surface change it would also cause a significant visual difference. Hence, the luminance value of the roadway lighting should be adjusted according to the road conditions. For example, the reflectivity of wet and slippery roads in rainy days is higher than that of dry roads where uneven reflection spots on the road are likely to cause indirect glare or poor uniformity, which contribute to the degrees of risk while driving.
Traditional roadway lighting optics are all designed with fixed light distribution. Usually, the road surface material is preset to R3 pavement characteristics as the standard parameter when evaluating the luminance distribution. Therefore, it is relatively difficult for traditional lighting to adapt to various environmental conditions. In situations where the weather or road surface conditions change abruptly, drivers will be exposed to poor lighting quality since the uniformity of the light distribution is usually designed for standard road surface conditions. Hence, when it comes to bad road conditions, the visual uniformity will drop drastically also. This paper proposes a LED human-centric roadway lighting system that can adapt to different road conditions, hoping to provide drivers with a safer road environment at all times.
In this paper, an imaging luminance meter (ILM) is used for detecting and monitoring the luminance uniformity of road lighting. During 2007, due to the increasing awareness for energy saving and safety, one study [
38] used an ILM to measure and analyze the luminance level of different kinds of road surface and of intelligent street lighting. In that particular article, measurement results of different road surface conditions including dry, wet and snow-covered roads are proposed. The results show that the luminance of the snow-covered roads is significantly higher than that of dry and wet roads. In addition, dry roads and snow-covered surfaces are less susceptible to height and distance when measuring and the standard deviation of luminance is around 0.03 and 0.02 respectively. While the measurement result of the wet road is much more susceptible to the observation point, which leads to a different measurement value. Therefore, when comparing the luminance level measured from the driver’s position with the installation point of the ILM, the luminance values of dry roads are relatively close while the luminance level of wet roads can reach a difference of more than 10%, which is close to the results of this paper. However, they did not mention the road pavement materials in their study. In our paper, we have stated clearly that we have used the R3 dry road surface and the W1 wet road surface for optical design and luminance measurement, so as to solve the zebra effect by improving the longitudinal uniformity of the road surface. Moreover, the adaptive roadway lighting system we established is able to monitor the road surface condition when it is affected by the weather in real time, through the ILM, and automatically switch to the most suitable type of lens to maintain high uniformity and ensure the safety of road users.
This research integrated many kinds of technology, from optical design to smart lighting control, to contribute and establish a lighting uniformity on the road surface. This is also the first project that imports an ILM system to roadway lighting in Taiwan. The project prior to this research focused more on energy saving, auto failure report, fast maintenance, etc., but this study focused on drivers’/road users’ safety, especially when it comes to driving on wet or slippery roads. Therefore, we designed LED fixtures with two types of photometric distribution, which are able to adapt to various road surface types, and used an ILM for sensing the lighting uniformity status on the road surface, enabling it to switch to the most suitable types of optic lenses accordingly.
The next step of this HCL LED system will be discussing the dimming strategy; due to maintenance issues, when undergoing the lighting design step, a higher level of illuminance and luminance at the initial step is applied in practice. Therefore, we need to further study the suitable dimming level based on the roadway lighting standard. This paper aims to propose a systematic method that makes roadway lighting intelligent by integrating an ILM device, which will hopefully garner and ensure the safety of the drivers.
5. Conclusions
This article proposes a design and control strategy for LED human-centric intelligent roadway lighting. First, it is based on the theoretical analysis of zonal flux. A simple and fast secondary optical design method for LED roadway lighting was established, aiming to achieve both road illuminance uniformity and luminance uniformity of the photometric light curve as the design goal. Thus, two types of optical lenses were produced and assembled to a street lamp to meet various lighting needs. In addition, they were equipped with an imaging luminance meter to detect the luminance level of the road in real time. Then, the return signal was used to adjust the output of the street lamp in a timely manner through the ZigBee wireless transmission control interface to meet the lighting demands, whether the road was wet or dry road. Then, a roadway lighting evaluation in practice was carried out where we found out that the lens designed for luminance uniformity did indeed provide quality road lighting, giving drivers a safer and more comfortable road environment.
Another future application of this research is for an autonomous car in the future. Taoyuan City in Taiwan has always aimed to be an advanced smart city and a demonstration field for an autonomous car. However, non-uniform luminance will cause not only the zebra effect but also unclear road markings. An autonomous car will configure the path of travel using camera images to check road markings. Based on the result of this research, we can now lower the chances of the zebra effect on the dry and wet road surface by using an ILM to automatically sense and switch optic lenses. With the novelty of this article’s findings, we can collaborate and integrate it with the future autonomous car system, which combines luminance sensing with imaging recognition by using an advanced ILM device.