Design and Application of Liquid Silicone Rubber Light Guide in Compact Automotive Headlamps

Abstract

Traditional car headlights use halogen or high-intensity discharge (HID) lamps paired with a reflector cup, fisheye lens, and shading plate to comply with ECE112B (Headlamps with an Asymmetrical Passing Beam) regulations. This design has issues such as large and bulky volumes, and it sacrifices optical efficiency due to the shading plate. This paper proposes a solution using an LED light source coupled with a Liquid Silicone Rubber-Light Guide (LSR-LG) for compact automotive low-beam headlights. By employing the principle of total internal reflection, the light beam is confined within the LSR-LG to reduce the volume of the car lamp. Simultaneously, optimized the structure and adjusted the light distribution. In the prototype measurements, the LED combined with the LSR-LG in the compact automotive headlamps module exhibited light intensities of 11,942 cd, 12,898 cd, and 298.2 cd at measurement points 75 R, 50 R, and BR, respectively, by ECE R112B standards. Simultaneously, the design effectively reduced the car lamp module’s volume to 90.63 mm × 164.53 mm × 73.37 mm. This design offers advantages such as energy efficiency, lightweight construction, and compact size. It also complies with ECE R112B regulations, showing promising potential as an excellent choice for the next generation of automotive low-beam headlights.

Keywords:

• Light emitting diodes
• ECE R112B
• liquid silicone rubber
• light guide
• projection lens
• compact
• low beam
• headlamp

1. Introduction

In recent years, there has been rapid technological development, but it comes at the cost of an increasing amount of greenhouse gas emissions, such as carbon dioxide and methane. Consequently, this exacerbates the severity of climate change. Therefore, in order to effectively address climate change, there is an urgent need for more targeted measures to reduce energy consumption and greenhouse gas emissions.^[1–4] The existing headlight housing is bulky, and the production process involves materials and processes that are not environmentally friendly. Therefore, there is a need for a novel headlight design. LEDs light sources applied in automotive lighting offer numerous advantages, including energy efficiency, excellent electrical-to-optical conversion efficiency, compact size, fast response time, wide color gamut, and long lifespan.^[5,6] Compared to incandescent and HID lamps, LEDs demonstrate higher energy efficiency in automotive lighting, making them a significant trend in the automotive lighting industry. They have been widely utilized in high-power automotive lighting applications such as headlights, fog lights, and exterior projection lighting.^[7–10] Simultaneously, LEDs exhibit high luminous flux, luminous efficiency, and adjustable color temperatures. More importantly, they boast excellent reliability. When comparing different light sources for automotive headlights, the illuminance of LEDs shows almost no decay after 10,000 h of operation. Due to this characteristic, new types of automotive headlights prefer LEDs over traditional halogen lamps and HIDs. Compared to halogen bulbs, LEDs have twice the electrical-to-optical conversion efficiency, longer beam distances, and a lifespan that is 10–15 times longer. However, to maintain their luminous flux, light intensity, and lifespan, it is necessary to control the chip junction temperature to ensure that it does not accumulate excessive heat and damage the LED chips.^[11–12] The illumination of a vehicle’s low-beam headlights is a crucial factor in road driving safety. Excellent design can provide drivers with a clear field of vision, while also ensuring sufficient illuminance and a clear cutoff line to avoid glare for pedestrians or other road users.^[13–15] The front lighting of a vehicle typically includes low-beam headlights, high-beam headlights, and fog lights. Low-beam headlights require a cutoff line with high contrast to reduce glare issues. High-beam headlights provide long-distance illumination, while fog lights are used when visibility is obstructed by fog on the road.^[16]

Automotive lighting technology is continually advancing, with recent developments including advanced light sources like Mini/Micro LEDs. Due to their smaller size, Mimi LED and Micro LED allow users to conduct optical designs with greater flexibility.^[17–19] These designs can be more compact; however, the luminous efficiency and reliability of Mini/Micro LEDs for automotive use need improvement. Additionally, they come with a relatively higher cost. Therefore, LEDs remain the most popular choice for automotive lighting sources at present.^[20] LED low-beam headlights primarily have three design approaches: projection type, refraction type, and reflective type. Among them, the projection type is the most widely used and is mainly composed of a reflector, shield, and lens.^[21–23] In recent years, many researchers have proposed various designs for projected automotive low-beam headlights to achieve high efficiency and compliance with relevant lighting regulations. These designs include the application of elliptical reflectors, parabolic reflectors, micro lenses, complex curved lenses, prism arrays, and double freeform lenses, among others. Some of these designs also involve stacking illumination and overlaying reflectors to achieve both low-beam and high-beam lighting.^[24–29] Numerous studies also explore different design approaches for reflective car lights, aiming to achieve high efficiency and compliance with various automotive lighting standards. Examples include aluminum-coated double parabolic reflectors, total internal reflection (TIR) freeform lenses, Oliker composite elliptical reflectors, and freeform reflectors that combine high-beam and low-beam functions.^[30–33]

Some literature also investigates refractive car light design methods to achieve high efficiency and compliance with diverse automotive lighting standards. These include cylindrical lens arrays (CLA), Adaptive Front-lighting System (AFS) low-beam headlights, micro lens arrays, and structures incorporating freeform lenses for collimation.^[34–37] Laser diodes applied in low-beam headlight design can offer a longer illumination distance. Relevant research includes designs incorporating laser light sources with glass phosphors, gradient refractive index lenses, rod lenses, freeform lenses, etc.^[38–39] However, their drawback lies in the overly concentrated energy density, leading to potential issues with the aging of phosphors. Additionally, numerous studies explore the possibility of changing the light path through lighting design to achieve high uniformity, reduced glare, and high light efficiency. Examples include adaptive micro-lens road lamps, freeform designs, and the use of reflectors to alter the light trajectory.^[40–43] Liquid Silicone Rubber (LSR) is characterized by low viscosity, flame resistance, rapid curing, cold resistance, heat resistance, and stress-relieving properties, making it considered a material with great potential.^[44] Previous studies have demonstrated the use of LSR with Fresnel structures for the primary encapsulation of LEDs, effectively reducing the volume of automotive lamp modules.^[45]

In summary, previous research has proposed various optical design methods for automotive lamp modules, including reflectors, parabolic surfaces, micro lenses, and freeform lenses, to meet the requirements of the ECE R112B automotive lighting regulation. However, the existing studies lack the effectiveness of reducing the volume and weight of automotive lamp modules while improving optical efficiency to achieve energy savings. Compared to glass, it is lighter and easier to process. Additionally, compared to PC (polycarbonate), it has the advantage of heat resistance and UV resistance. Therefore, we chose to use LSR as the light guide material. Therefore, this paper introduces the Design and Application of Liquid Silicone Rubber Light Guide (LSR-LG) in Compact Automotive Headlamps. This approach utilizes LED light sources combined with LSR-LG for compact low-beam headlights in automobiles. By employing the principle of total internal reflection to confine the light within the LSR-LG, the design aims to reduce the volume of the automotive lamp module. This is further complemented by a projection lens for light field adjustment, resulting in an efficient, lightweight, and compact design for automotive low-beam headlights.

2. Method

Traditional designs of automotive low-beam headlights can be broadly categorized into three types: projective headlights with double-sided reflectors, reflective headlights with only a single-sided reflector, and refractive headlights that use lenses to deflect light rays. The drawback of such designs is that the light source first disperses and then passes through a reflector or reflectors, often combined with a fisheye lens, requiring a larger lamp volume. This is illustrated in Figure 1(a). Low-beam headlights must provide drivers with a clear and safe field of vision, necessitating the design of a sharp and asymmetrical cutoff line to prevent interference with oncoming drivers from the entry of a significant amount of light into their eyes. The detection area and measurement points for compliance with the ECE R112B (Headlamps with an Asymmetrical Passing Beam) regulation are shown in Figure 1(b).

Figure 1. (a) The types of traditional automotive low-beam headlight designs. (b) The detection area and measurement points specified by the ECE R112B regulation for Headlamps with an Asymmetrical Passing Beam.

This paper proposes the design and application of liquid silicone rubber light guide (LSR-LG) in Compact Automotive Headlamps. LSR is employed as the light guide in the automotive lamp module, utilizing light guiding to confine the light within the light guide, preventing light dispersion and effectively reducing the volume of the lamp module. By designing the LSR-LG structure to control the light path’s emission angle and generate an illuminance distribution with distinct brightness and darkness cutoff lines, the light is then focused and projected onto a flat surface at 25 meters using a projection lens. This achieves compliance with the ECE R112B Headlamps with an asymmetrical passing beam regulation. Figure 2 illustrates the principle of the LSR-LG automotive lamp design. Figure 2(a) shows a schematic diagram of the structure, including LED light source, LSR-LG, and projection lens. Figure 2(b) depicts the Light guide mechanism of LSR-LG. n_s is the refractive index of LSR-LG, n_a is the refractive index of the outgoing medium. When the width (L_S) of the LSR-LG is fixed, its thickness (W_S) will affect the area of total internal reflection, as shown by the yellow region in Figure 2(b). If the angle of incidence θ_i exceeds the critical angle θ_c, the light beam can effectively be confined within the LSR-LG. This yellow region is defined as the width of the Total Internal Reflection (TIR) region (2 T_P), and it can be calculated using Equation (1)(1) $$T_P=\mathrm{\text{tan}}\left({\mathrm{\text{sin}}}^{-1}\frac{n_a}{n_s}\right)\times W_S$$(1) . (1) $$T_P=\mathrm{\text{tan}}\left({\mathrm{\text{sin}}}^{-1}\frac{n_a}{n_s}\right)\times W_S$$(1)

Figure 2. illustrates the design principle of the Liquid Silicone Rubber Light Guide (LSR-LG) automotive lamp. (a) Schematic diagram of the structure of the LSR-LG in compact automotive headlamps. (b) Light guide mechanism of LSR-LG.

We employed the OSRAM OSTAR Headlamp Pro’s white LED chips, with the model LE UW U1A3 01, as the light source for the automotive lamp. In Figure 3(a), a 3D schematic diagram of the light source is depicted, consisting of three LEDs with dimensions of 0.85 mm × 0.85 mm × 0.5 mm each, mounted on an aluminum substrate measuring 20 mm × 20 mm × 1 mm. Figures 3(b)–(d) display actual measurements, including the luminous intensity distribution (LID) curve, electroluminescence (EL), and Luminance-Current-Voltage (LIV) curve. At a driving current of 1.15 A, the forward voltage is 9.27 V, the output luminous flux is 1100 lm, the peak wavelength is 446.5 nm, and the Full Width at Half Maximum (FWHM) of the emission half-angle is 67.5°.

Figure 3. The actual measurement data of the OSRAM OSTAR Headlamp Pro, (a) 3D schematic diagram, (b) Luminous intensity distribution, (c) electroluminescence (EL) spectrum, (d) Luminance-Current-Voltage curve.

Figure 4 illustrates the schematic diagram of the Liquid Silicone Rubber Light Guide (LSR-LG) module in Compact Automotive Headlamps. It includes OSRAM Headlamp LEDs, LSR-LG, projection lens, and fixture. The distance from the vertex of the projection lens to the light-emitting surface of the LSR-LG is 30.5 mm, as shown in Figure 4(a). The dimensions fixed for the LSR-LG include the input surface width (W_i), input surface height (H_in), total length (L), total width (W_o), and cutoff line angle (θ_CL). The output light distribution of the LSR-LG is adjusted through the optimization of the base tilt angle (Rise angle, θ_R) and side wall angles (Side angle, θ_S), as depicted in Figure 4(b). The dimensions of the projection lens, including length (L_p), height (H_p), and thickness (D_p), are 73 mm, 40.7 mm, and 27.93 mm, respectively, as shown in Figure 4(c).

Figure 4. Liquid Silicone Rubber Light Guide in Compact Automotive Headlamps module, (a) 3D schematic diagram of the LSR-LG automotive lamp module, (b) Top, front, side, and isometric views of the LSR-LG, (c) Top, front, side, and isometric views of the projection lens.

We use SolidWorks 2019 (Dassault Systèmes, Vélizy-Villacoublay, France) and optical simulation software SPEOS 2020 (Ansys, Canonsburg, Pennsylvania, USA) to simulate and optimize the LSR-LG. The analysis involved studying its light intensity distribution (cd) and verifying compliance with the ECE R112B Headlamps with an asymmetrical passing beam regulation. The points 75 R and 50 R on the detection surface have the highest light intensity requirements exceeding 10100 cd, while the measurement point BR should be below 1750 cd.

3. Result and discussion

The parameters for the LEDs are set with an input power of 1100 lm, a luminous half-intensity angle (FWHM) of 67.5 degrees, and 50,000,000 rays. The projection lens has a refractive index of 1.59 (PC), and the LSR-LG has a refractive index of 1.41. The dimensions of the LSR-LG, including the input surface width (W_i), output surface width (W_o), length (L), and height, are 9.5 mm, 15.84 mm, 30 mm, and 3 mm, respectively. The side angle (θ_s) aims to enlarge the emitting surface area to increase the illuminated horizontal area on ECE112B. According to simulation analysis, a 9° has a better light output efficiency of 82.8%, as shown in Figure 5(a). Subsequently, an analysis of the Rise angle (θ_R) was conducted within the range of 4.7°–5.3°. This range effectively controls the light distribution projected by the projection lens within the ECE112B valid range while maintaining the phenomenon of total internal reflection in the LSR-LG, reducing the loss of light output efficiency. When θ_R is set to 4.7°, the entrance efficiency on the ECE 112B detection surface is optimal at 51.9%, as shown in Figure 5(b). Figure 5(c) illustrates the light intensity distribution on the ECE 112B detection surface for θ_R ranging from 4.7° to 5.3°. Through simulation, it was found that a better light output efficiency of 51.9%, and a uniformity of 62.5% are achieved when θ_R is 4.7°.

Figure 5. Simulation and optimization analysis of LSR-LG, (a) Side angle (θ_s), (b) Rise angle (θ_R), (c) Light intensity distribution on the ECE 112B detection surface when θ_R is in the range of 4.7°–5.3°.

Finally, we simulated the Cutoff line angle (θ_CL) of the LSR-LG light-emitting surface. The ECE112B regulation requires a cutoff line angle of 45°, designed to meet the cutoff line distribution requirements. The simulated optical path of the headlamp module is illustrated in Figure 6(a). To enhance the light intensity at ECE112B measurement points 75 R and 50 R, optimization was performed for the Cutoff line angle (θ_CL). The simulation revealed that the highest light intensity at 75 R and 50 R could be achieved with θ_CL at 35°. However, at this angle, the value at measurement point BR exceeded the regulatory requirement of being less than 1750 cd. After comprehensive evaluation considering both the light intensity at 75 R and 50 R and compliance with regulatory requirements, θ_CL of 30° was identified as the optimal result, as shown in Figure 6(b). Figure 6(c) presents the simulation results for the ECE R112B detection surface, with an entrance efficiency of 49.4% at θ_CL of 30°. The light intensity at measurement point 75 R is 14,354.6 cd, and at measurement point 50 R, it is 15,461.6 cd.

Figure 6. Simulation schematic of the LSR-LG headlamp module, (a) 3D simulated optical path of the headlamp module, (b) Simulation optimization of Cutoff line angle (θ_CL) for measurement points 75 R, 50 R, and BR, (c) Simulation results of Cutoff line angle (θ_CL) on the ECE R112B detection surface.

Next, we proceeded with the prototype fabrication and measurement analysis. Figures 7(a) and (b) respectively show the isometric and front views of the LSR-LG. Figures 7(c) and (d) present the front and side views of the LSR-LG headlamp module, with dimensions of 90.63 mm × 164.53 mm × 73.37 mm. Figure 7(e) provides a flat illumination image of the actual test field for the headlamp module, clearly displaying the cutoff line.

Figure 7. LSR-LG compact automotive headlamps prototype and testing (a) Isometric view of LSR-LG, (b) Front view of LSR-LG, (c) Top view of LSR-LG headlamp module, (d) Side view of LSR-LG headlamp module, (e) Actual test field image for the headlamp module complying with ECE R112B.

The actual measured values of the intensity distribution at various measurement points according to the ECE regulations are presented in Table 1. The measurement results demonstrate full compliance with the ECE R112B regulations.

Table 1. Intensity limits and actual measurement results for ECE R112B regulation measurement points.

Test Point	Required luminous intensity [cd]	Measurement luminous intensity [cd]
B50L	<350	282.9
BR	<1750	298.2
75 R	>10,100	11942
75 L	<10,600	6540
50 L	<13,200	12,568
50 R	>10,100	12,898
50 V	>5100	12,954
25 L	>1700	8156
25 R	>1700	9449
ZONE I	<2 × I50L or 2 × I50R	12,881
ZONE III	<625	623
ZONE IV	>2500	10,759
Points 1＋2＋3	>190	275.57
Points 4＋5＋6	>375	504.6
Point 7	>65	721.2
Point 8	>125	454

Next, the LSR-LG underwent high-temperature and high-humidity aging tests, with samples S1 to S6 subjected to conditions of 85 °C/90% RH and 150 °C for 240 h, as shown in Figure 8. The results indicate that the light attenuation of the LSR-LG is less than 2% after 240 h of aging testing.

Figure 8. High-Temperature and High-Humidity aging test of LSR-LG.

4. Conclusion

The traditional design of automotive low-beam headlights has the drawback of the light source first diverging and then being adjusted through reflector cups, mirrors, or fisheye lenses to shape the light field, requiring a larger lamp volume. This paper proposes a method for compact automotive low-beam headlights by combining LED light sources with a Liquid Silicone Rubber (LSR) light guide (LG). The advantages of combining LED light sources with LSR light guides include achieving better optical efficiency, reducing the volume of the lamp, and making it lighter. Using the principle of total internal reflection, the light is confined within the light guide, preventing the light source from diverging and reducing the size of the headlight. Simultaneously, in combination with a Projection lens, the optical distribution is adjusted to achieve the illumination effect of automotive low-beam headlights. Measurement of the prototype indicates that the optical efficiency of the LED combined with the LSR-LG in the Compact Automotive Headlamps module reaches 49.4% on the ECE 112B detection surface, with intensity measurements at points 75 R, 50 R, and BR of 11,942 cd, 12,898 cd, and 298.2 cd, respectively. This design not only effectively reduces the module size to 90.63 mm × 164.53 mm × 73.37 mm but also offers advantages such as energy efficiency, lightweight, and small size. Moreover, it complies with the ECE R112B Headlamps with an Asymmetrical Passing Beam regulation, demonstrating great potential as an excellent choice for the next generation of automotive low-beam headlights.

Authors contributions

Zhi Ting Ye and Chia Chun Hu are responsible for the structure and conception of the entire article. Zhi Ting Ye and Yang Jun Zheng is responsible for the simulation data of the article. Zhi Ting Ye, Hsu Yu Fu and Chia Chun Hu is responsible for the initial writing of the manuscript.

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All authors consent to the publication of this manuscript.

Nomenclature
LSR	=	Liquid Silicone Rubber

Disclosure statement

The authors declare no conflicts of interest.

Data availability statement

The data presented in this study are available on request from the all authors.

Additional information

Funding

This work was supported by the National Science and Technology Council of Taiwan (NTSC 112-2622-E-194-004).