Design of Counter Beam Tunnel Lights for CIE 88 : 2004 Regulation in Threshold Zone

Department of Electrical Engineering, National Kaohsiung University of Science and Technology, Kaohsiung 80778, Taiwan Department of Technology, Dong Nai Technology University, Bien Hoa 830000, Dong Nai, Vietnam Department of Aviation & Communication Electronics, Air Force Institute of Technology, Kaohsiung City 820, Taiwan Department of Materials Science and Engineering, National Tsing Hua University, No. 101, Section 2, Kuang-Fu Road, Hsinchu 30013, Taiwan Department of Graduate Institute of Clinical Medicine, Kaohsiung Medical University, Kaohsiung 807, Taiwan


Introduction
e main objective of tunnel lighting is to allow traffic to enter, pass through, and exit the enclosed section of the tunnel safely [1][2][3][4]. ese aims are achieved through the adequate illumination of the tunnel interior, which allows drivers to quickly adjust to the light and identify possible obstacles on the road in tunnels. e threshold zone is the first zone within the actual tunnel that extends for the same length as the stopping sight distance for the design [5][6][7][8]. When using the L 20 method, the target luminance level L th (threshold zone luminance) for this zone is derived from the portal luminance (L 20 ) factored by the k value associated with the class of tunnel [9,10,11]. is level is maintained at 100% for the first half of the threshold zone and reduces to 40% by the end of the zone. e k factor is inversely proportional to the contrast revealing coefficient q c , so if q c can be elevated, the required L th can be decreased, which can provide the necessary lighting conditions for the tunnel road while decreasing the consumed electrical power and the maintenance cost of lighting. However, since the new CIE 88 : 2004 regulation was published, updating tunnel lights to achieve a high contrast revealing coefficient is rarely performed, so few commercial tunnel lights are qualified as counter beam lights [12,13,7]. Many newly built tunnels, such as the Conway Eastbound tunnel on the A55 in the U.K., adopt symmetrical lights in threshold zone, guaranteeing a q c of only more than 0.2 [14,15,16]. To decrease the L th level and follow the CIE 88 : 2004 regulation and as counter beam lights can guarantee a q c higher than 0.6, they are always considered for tunnel lighting.
In this study, a freeform surfaced luminaire is proposed to handle 400 W high-pressured sodium lamps to act as the counterbeam lights for CIE 88 : 2004 tunnel road lighting. e efficiency and lifespan of white light emitting diodes (LEDs) have improved, but the golden light of high-pressured sodium lamps has the specific advantage of high fog permeability and low insect attraction, which increase driver safety in the threshold and exit zones, where the dangerous black-hole effect or white hole effect need to be addressed. As a result, lights with high-pressured sodium lamps are still commonly used in the entrances and exits of tunnels. e proposed counterbeam light was prototyped and its intensity distribution was measured using a goniophotometer to obtain its far field data. By importing the data into tunnel lighting analysis software, we found that the counter beam light combined with conventional fluorescent lights can meet tunnel road lighting CIE 88 : 2004 regulations in the threshold zone, yielding an average road surface luminance L av of 130 cd/m 2 , which is greater than the minimum regulation level of 105 cd/m 2 , a luminance uniformity U o of 0.89 > 0.4 (minimum regulation level), a longitudinal luminance uniformity U L of 0.99 > 0.6 (minimum regulation level), a glare factor T I of 5.5% < 15% (maximum level), and a contrast display factor q c of 1.24 > 0.6 (minimum level) in the threshold zone.

Principles
e major difference between tunnel lighting and conventional road lighting is in the need for lighting by day [17,18,19]. A driver needs to be able to see a certain distance ahead so that if an unexpected hazard appears, the driver can react and stop within that distance. When this distance extends into a tunnel, the lighting level inside should be sufficiently high to maintain visibility. If the lighting level is not high, the driver will be unable to see into the tunnel, which is called the black-hole effect. During approach and entry into a tunnel, drivers' eyes adapt to the darker surroundings. is adaptation is a continuous process, so the further into the tunnel the driver travels, providing the tunnel is of sufficient length, the lighting level may be steadily reduced until it reaches a constant level in the tunnel interior zone. On emerging from a tunnel into daylight, the eye adapts far more quickly to the higher luminance level.
e lighting of a tunnel should be sufficient to prevent the black-hole effect when a driver is unable to see into the tunnel. As such, tunnel road lighting must satisfy some stringent requirements, such as the CIE 88 : 2004 regulation.
In order to avoid encountering the black-hole effect when approaching the portal of a tunnel and to help drivers adapt to the lighting environment in a tunnel, there are five lighting zones distributed throughout a tunnel such as access zone, threshold zone, transition zone, interior zone, and exit zone. e transition zone is given by where L tr is the road surface luminance in the transition zone, L th is the road surface luminance in the threshold zone, and t is the driving time in seconds. e necessary driving time in the transition zone t tr is then given by e threshold zone is the first inner region encountered when entering a tunnel and where the black-hole effect critically dominates. Based on the CIE 88 : 2004 regulation, the threshold zone luminance is usually set to be higher than L th. : where L m � (τ ws × L atm + L ws + L seq )/(τ ws × τ atm ), C m is the minimum required perceived contrast (-0.28), ρ is the reflectance factor of the target (0.2), τ ws is the windscreen transmission factor (0.8), τ atm is the atmosphere transmission factor (1.0), L seq = 5.1 × 10 −4 Σ L ije with L ije � (τ ws × L ij ) + L ws is the equivalent veiling luminance, L ije is the luminance of each section in front of the eye (cd/m 2 ), L ij is the average luminance of each section measured outside the car in front of the windscreen (cd/m 2 ), L atm is the atmospheric veiling luminance (200 cd/m 2 , medium veiling level), L ws is the windscreen veiling luminance (100 cd/m 2 , medium veiling level), and q c is the contrast revealing coefficient (≥0.2 for symmetrical light systems or ≥0.6 for counterbeam light systems). Based on the Holladay-Stiles formula, the equivalent veiling luminance L seq can be determined busing a graphical method to identify L th. e equivalent veiling luminance L seq is found from L seq � 5.1 × 10 −4 Σ L ije with L ije � (τ ws × L ij ) + L ws . e contrast revealing coefficient (q c � L/E v ) is the ratio between the luminance of the road surface and the vertical illuminance Ev at a specific location in the tunnel. C m is the minimum required perceived contrast, where 28% is recommended. is contrast is mostly negative (for any q c greater than 0.06 with a reflectance factor of the target equal to 0.2). To determine the threshold road luminance, the designer should start from the standardized figures for the contrast revealing coefficient (either 0.2 for symmetrical light systems or 0.6 for counterbeam light systems). To identify a more precise threshold luminance value, an iterative process is necessary. After selecting an initial estimated figure for the average q c of the installation and calculating the correlated L th , the real average q c of the installation may need to be calculated to verify initial assumptions. In general, the atmospheric transmissivity (τ atm ) for design purposes is assumed to be 1.0 and the transmission factor for the windscreen (τ ws ) is assumed to be 0.8. Disability glare reduces visibility and shall be minimized. If disability glare is controlled under tunnel lighting conditions, then discomfort glare will also be controlled. Disability glare effects shall be quantified by means of threshold increment (T I ). Good uniformity of luminance must be provided on the road surface. e recommended minimum to the average value of the luminance on the road surface in clean conditions of the tunnel is 0.4. A longitudinal uniformity of 0.6 along the center of each lane is recommended for the road. It is recommended that these values are independently reached on the length of the step. e values of 0.4 and 0.6 are those corresponding to the values for normal road lighting provided in CIE 115-1995 [7,13].
Asymmetrical directional lighting, such as counterbeam and probeam lighting, distributes patterns in only one direction, either with or against traffic. Counterbeam lighting directs the maximum optical intensity against traffic along the driver's line of vision, creating a high negative contrast. By minimizing glare, drivers can clearly see the contours of the vehicle ahead. Probeam lighting directs the maximum candlelight with the traffic away from the driver, providing high object luminance and low road luminance, creating a positive contrast. is system operates by minimizing luminaire glare and increasing distance visibility.

Experimental Setup and Results
e tunnel for the study is located in the eastern part of Taiwan, and its entrance is shown in Figure 1. e geometry of the tunnel is shown in Figure 2, and its details are as follows: motorway tunnel, 2 unidirectional bores; two traffic lanes, 3.75 m each; total width, 10 m; ceiling height, 8.05 m; and length of tunnel, >500 m. e tunnel orientation is south to north. e amount of lighting required within a tunnel is dependent on the level of ambient lighting at which visual adaptation for the driver is possible on the tunnel approach and inside the tunnel. To achieve this, the lighting of a tunnel is divided into specific zones; the threshold zone and the transition zone are shown in Figure 3. Using the Holladay-Stiles formula, the equivalent veiling luminance Lseq can be determined using a graphical method embedded in LiteStar4D software (OxyTech, Milan, Italey), as shown in Figure 4. e overlay lines and segments are related to the Holladay-Stiles formula [1], which results in a luminance reduction curve and L th � 178 cd/m 2 , as shown in the green curve of Figure 5. In the threshold zone, considering the daylight shining into the tunnel, the minimum average road surface brightness L av due to tunnel light greater than 105 cd/m 2 is sufficient. Based on CIE 115-1995, the brightness uniformity U o and the longitudinal brightness uniformity U L should be higher than 0.4 and 0.6, respectively; the glare factor T I needs to be larger than 15%; and the contrast revealing coefficient must be larger than 0.6 for economic reasons. e criteria of the tunnel lighting design are as follows: ( To meet the abovementioned targets, counterbeam lights (CBLs) with one OSRAMVIALOX Nav-T Super 400 W/ 56,500 lumen sodium lamp and linear symmetrical lights with two T5 MASTER TL5 HE 35 W/840 SLV/20 fluorescence tubes with efficacy of 94 lumen/W were arranged regularly in the threshold zone. e layout of the lighting design is shown in Figure 6. e spacing of the lights was set to 1.6 m with one linear light followed by three counterbeam lights in, and then repeating the same arrangement until the end of the zone. For the experiments, the design process of the proposed CBL with sodium lamps is presented. e CBL was prototyped to demonstrate its feasibility for tunnel road lighting for CIE 88 : 2004.
e new CBL is composed of a freeform surfaced reflector and one 400 W ORSAM high-pressured sodium lamp. With the aim of achieving a contrast revealing coefficient qc higher than 0.6, the new CBL model was built using SolidWorks (DASSAULT SYSTEMES, USA) mechanical software, analyzed using TracePro (Lambda Research, USA) optical software, and optimized with LightTools (Synopsys, USA) software. e design workflow is shown in Figure 7.
During the design process, the optical models of the lamp and reflector were built first, as shown in Figures 8(a)  and 8(b), respectively. To determiend the accuracy of the lamp model, the model file was imported into TracePro software for evaluation. en, we connected the lamp model to the reflector model in TracePro and LightTools as the complete counterbeam light, as shown in Figure 9, to conduct ray tracing analysis and optimization, respectively. e initial reflector model was built, as shown in Figure 10(a), in which surface radii R 1 and R 2 were set to 137 and 115 mm, respectively, and the lamp position was set to (49, -36,100) xyz .
Using TracePro, the intensity distribution of the CBL was analyzed, as shown in Figure 10(b), and its far field source file was obtained. However, based on the geometry of the trial tunnel and the criteria of the tunnel lighting design, after importing the source files of CBL and the T5 fluorescence light into LiteStar 4D for tunnel road lighting  International Journal of Optics simulation, the output report showed that its contrast revealing coefficient q c was 0.24, which is less than 0.6. at means that the initially designed CBL was insufficient.
erefore, the CBL model was optimized using LightTools with the target of a q c larger than 1.0 in the threshold zone. rough the global searching method with reflector surface radius and lamp position acting as the variables, the CBL was optimized, as shown in Figure 11     distribution is shown in Figure 11(b). After optimization, the CBL had an R 1 of 1000 mm and R 2 of 85 mm, and the lamp position shifted to (74, -65,90) xyz from (49, -36,100) xyz . We input the far field source of the optimized CBL into LiteStar 4D; the output report showed that its contrast revealing coefficient q c was 1.11, which is above 0.6, indicating that the new design would qualify as a counterbeam light. e tunnel road lighting performance evaluation items, including average road surface luminance L av , brightness uniformity U o , longitudinal brightness uniformity U L , glare factor T I , and q c of the initial and final designs of the new CBL are listed in Table 1 for comparison. To show that the optimized CBL allows the tunnel lighting to meet CIE 88 : 2004 specifications, a reflector composed of stainless steel was prototyped for optical analysis, as shown in Figure 12. e new CBL fixture installation with the 400 W sodium lamp with 56,500 lumens is shown in Figure 13, which was measured by imaging goniophotometers (Radiant Imaging Co. Ltd.) to obtain its intensity distribution ( Figure 14) and its far field source file, which shows the fixture output power was 37,054 lumens. erefore, the efficacy was calculated as 37,054/400 � 93 lumen/W. erefore, after the source file of the optimized CBL sample was loaded in LiteStar 4D, the output report showed that its q c was 1.19 (>0.6), which means that the optimized CBL could act as a counterbeam light for tunnel lighting in practice. e tunnel lighting evaluation items, including average road surface luminance L av , brightness   Figure 6: e tunnel lighting configuration of one 75 W T5 fluorescence light followed by three 400 W discharging lights with sodium lamp for each traffic lane is repeated from the entry to the end of threshold zone.
International Journal of Optics uniformity U o , longitudinal brightness uniformity U L , glare factor T I , and q c of the optimized CBL design and the measurements from the sample are both listed in Table 2 to compare the performance differences.
To demonstrate the advantage of the optimized new CBL, the 400 W sodium lamp tunnel light GE17734 (General Electric Co., Ltd.), broadly used in tunnels, was used to compare their performance in the threshold zone. e  measured intensity distribution of the GE17734 (General Electric Co., Ltd.) is shown in Figure 15; the output power was 36,865 lumens, and the efficacy was calculated as 36,865/ 400 � 92 lumen/W. e lighting performance between the optimized new CBL and the GE17734 (General Electric Co. Ltd.) in threshold zone of tunnel is compared in Table 3. e q c of GE17734 was 0.55, which is below the standard and much smaller lower than that (1.08) of the optimized CBL.

Discussion and Conclusions
To We compared the performance of the GE17734 tunnel light and the new CBL before and after optimization. e experimental results (Tables 1-3) showed that the road luminance produced by the optimized new CBL is the lowest but has the highest q c , which means cars, people, or other objects observed on road are the most contrasted, revealed by the optimized CBL. e results shown in Figures 15,10(b), and 11(b) show that the optimized new CBL has a weaker light intensity at 40°-45°, meaning light dominates the road luminance of drivers, so the optimized new CBL cannot provide as much luminance as the other fixtures. [20]

Data Availability
No data were used to support this study.

Conflicts of Interest
e authors declare that they have no conflicts of interest.