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Abstract
A novel laser-assisted LED for adaptive-driving-beam (ADB) headlights employing an ultra-reliable Ce3+: YAG-based single crystal phosphor (SCP)-converter layer for use in autonomous vehicles is demonstrated. The SCP fabricated at a high-temperature of 1,940°C exhibited better thermal stability than other phosphor-converter materials, evidenced by a thermal aging test. The high-beam pattern of the ADB is measured at a luminous intensity of 88,436 cd at 0°, 69,393 cd at ± 2.5°, and 42,942 cd at ± 5°, which well satisfies the ECE R112 class B regulation. The advantage of introducing the laser-assisted LED system employing the highly reliable SCP is to produce the high intensity for the ADB, which enables the increase of the field of view by 20% and the brightness by 28% for the ADB headlight and results in improving the visibility from ± 7° to ± 8.5° and the illumination distance up to 200 m. This proposed advance ADB headlight with the ultra-reliable SCP and the novel laser-assisted LED is favorable as one of the most promising ADB light source candidates for use in the next-generation autonomous vehicle applications.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Vehicle headlights are becoming smarter and more intelligent [1,2]. The safety feature of the headlight is very critical, especially when vehicle is driven at night or in bad weather. The design of vehicle headlights should meet the performance requirement and the strict automotive and the highway safety standards, such as the United Nations Economic Commission for Europe (ECE). The adaptive driving beam (ADB) headlights have been approved as one of the advance headlamp technologies. Recently, the ADB headlights have been developed using a blue light-emitting diode (LED) arrays with silicone-based phosphor, a digital mirror device (DMD), and a projection optics system [3]. Although LED dominates the automotive market due to its high efficiency, high reliability, long lifetime, and smaller dimension which is ideal to save space in headlamps, however, the power conversion efficiency of LED drops with the increase of the input power density. This feature is a drawback of LED to be used in high power applications, such as the headlamps. Furthermore, the nearly Lambertian emission of the LEDs limits the optical system efficiency [4–6]. Therefore, it is necessary to develop the ADB headlight with a Laser-Assisted LED system, which can increase the field of view (FOV) and the brightness of the headlight. However, due to the thermal stability problem caused by the silicone-based phosphor, the degradation of the silicone resins due to heating from the blue light source adversely affects the optical properties and chromaticity characteristics of the white light source [7–9]. Therefore, alternative matrix materials with the high thermal stability are essential for the ADB headlights with a Laser-Assisted LED system.
Recently, the single crystal-based phosphor (SCP) has been reported for high thermal stability in lower chromaticity coordinates shift, and less lumen degradation for high-power laser lighting applications [10,11]. However, the required high-temperature fabrication process has been difficult for commercial production. Therefore, it is necessary to use a design of single crystal growth to produce the SCP with higher yield and better uniformity, thereby overcoming the problem of higher manufacturing temperature [12]. In this work, the ultra-reliable Ce3+: YAG-based SCP-converter layer is introduced. The SCP fabricated at a high-temperature of 1,940°C exhibited better thermal stability than other phosphor-converter materials.
In this study, a novel Laser-Assisted LED for adaptive-driving-beam (ADB) headlight employing an ultra-reliable Ce3+: YAG-based single crystal phosphor (SCP)-converter layer for use in autonomous vehicles is proposed. The ADB headlight consists of a white LED with an additional SCP-converter layer, a TI digit mirror device, a projection optics system, and a Laser-Assisted LED system. The high-beam pattern of the ADB is measured at a luminous intensity of 88,436 cd at 0°, 69,393 cd at ± 2.5°, and 42,942 cd at ± 5°, which well satisfies the ECE R112 class B regulation. The advantage of introducing the Laser-Assisted LED system employing the highly reliable SCP is to produce the high intensity for the ADB, which enables to increase of the field of view by 20% and the brightness by 28% for the ADB headlight, and resulting in improving the visibility from ± 7° to ± 8.5° and the illumination distance up to 200 m [13]. This proposed advance ADB headlight with the ultra-reliable SCP and the novel Laser-Assisted LED is favorable as one of the most promising ADB light source candidates for use in the next-generation autonomous vehicle applications.
2. Experimental methodology
2.1 Manufacturing the single crystal phosphor
Czochralski (CZ) technique was used to fabricate the ultra-reliable SCP-converter layer. The commercially available oxides as raw materials were mixed and melted in an iridium crucible at a temperature of 1,940°C. Figure 1 shows the various steps in the fabrication of Ce3+: YAG-based SCP-conversion layer. The schematic diagrams of the furnace with RF heater coil and crystal phosphor seed were shown in Fig. 1(a) and (b) using the CZ growth technique. The CeO2, Al2O3, and Y2O3 raw materials were placed in an iridium crucible located in the center of furnace heated by a RF heater. All materials were melted at the liquidus temperature and turned into a homogeneous melt. Figure 1(b) shows a seed crystal attached to the rotating puller rod being dipped into the furnace through the aperture. The seed crystal exhibited the desired crystal orientation of the preform to be grown. When the descending seed crystal touched the phosphor melt and stopped, the CZ growth process started. In order to promote the crystallization of the phosphor on the seed crystal, the temperature of the melt was slowly lowered to promote crystallization of the melt onto the seed crystal and becoming the preform. The crystallization process continued as the preform was slowly pulled out of the melt. Since the growth process of the Ce3+: YAG-based SCP-converter layer was long and delicate, the pulling speed and temperature drop were very precisely controlled. The seed, now the preform, was continually rotated such that a round finished crystal phosphor preform, which was in the form of a rod was produced as shown in Fig. 1(c). The perform as a length of 200 mm with a diameter of 55 mm, and the doping concentration of Ce3+ in the single crystal was about 0.5 at%. The next step was to cut the preform into thin wafers using a circular saw. Depending on the final product, the doping concentration of Ce3+ in the single crystal ranged from 0.02 at% to 1.4 at%, the thickness of each wafer ranged from 0.2 mm for the SCP-converter layer and to more than 2 mm for waveguide application. One or both sides of the wafers in Fig. 1(d) were polished and finally diced into small chips, as shown in Fig. 1(e).
2.2 Characterization of the single crystal phosphor
An integrating sphere measurement system [8] was used to evaluate the light emission spectra, International Lighting Commission chromaticity coordinate (CIE), and correlated-color temperature (CCT) of a 55 mm diameter SCP-converter layer with a thickness of 0.7 mm and excited by 449 nm blue laser were measured. Figure 2(a) shows the normalized light emission spectra of a SCP-converter layer. The light emission spectra peak, (x, y) coordinate, and CCT of the SCP-converter layer were 537 nm, (0.417, 0.547), and 4090 K, respectively. We further used a Hitachi U4150 spectrometer to measure the transmittance of a SCP-converter layer in 400-700 nm wavelength range. The refractive index of a SCP-converter layer was 1.82 at 550 nm and the Fresnel reflection loss of the double-sided surface was 16%. As a result, the transmittance value was only 84% in 525-700 nm wavelength range as shown in Fig. 2(b). In the 400-525 nm wavelength range, the lower transmittance was due to the absorption by the SCP-converter layer, which covered the blue excitation laser wavelengths. Based on the results, the ultra-reliable SCP-converter layer was a highly transparent material with virtually no scattering in 525-700 nm wavelength range.
Fig. 2. (a) Normalized light emission spectra and (b) measured transmittance of a 0.7 mm-thickness SCP-converter layer.
2.3 Thermal stability study of different phosphor-converter layers
In order to compare the thermal stability of different phosphor-converter layers, the silicone-based phosphor (SP), glass-based phosphor (GP), and single crystal-based phosphor (SCP)-converter layers are studied by thermal aging test at different temperatures under 1008 hours. The fabrication of the SCP was described in previous Section 2.1, whereas the fabrications of the SP and GP were described in Ref. [8]. The six samples of each SP, GP, and SCP-converter layers are evaluated by thermal aging tests at different temperatures of 150, 250, 350 and 450°C for 1008 hours. The optical properties were measured every 250 hours during thermal aging to characterize the degradation of the test samples. An integrating sphere measurement system [8] was used to evaluate the chromaticity coordinate, luminous flux, and quantum efficiency of the test samples excited by a 449 nm blue laser to assess the thermal stability of the SP, GP, and SCP- converter layers before and after the thermal aging tests.
Figures 3(a), 3(b), and 3(c) shows the thermal aging test results of the SP, GP, and SCP-converter layers at different temperatures under 1008 hours for the lumen loss, the CIE shift, and the quantum efficiency, respectively. Obviously, the thermal aging results of the Figs. 3(a), 3(b), and 3(c) clearly demonstrated that the SCP-converter layers exhibited better thermal stability in lumen degradation, lower CIE shift, and lower quantum efficiency loss than those of the SP and GP-converter layers. These were due to that the SCP-converter layer exhibited higher melting temperature, smaller thermal expansion coefficient, higher thermal conductivity, and higher Young’s modulus than those of the SP and GP-converter layers, as shown in Table 1.
Fig. 3. The thermal aging tests of the SP, GP, and SCP-converter layers for the (a) lumen loss, (b) CIE shift, and (c) quantum efficiency.
Table 1. Thermal properties comparison of SP, GP, and SCP-converter layers.
3. Design and measurement of the ADB headlight
Recently, the continuous development and improvement of the luminous efficiency for LEDs have reached various limitations. In order to further increase the visibility and the illumination distance of the ADB headlight, a Laser-Assisted LED system is introduced. An integrating sphere measurement system [8] was used to evaluate the light emission spectra, International Lighting Commission chromaticity coordinate (CIE), and correlated-color temperature (CCT) of the white LED. Figure 4(a) shows the normalized light emission spectra of the white LED. The light emission spectra peak, (x, y) coordinate, and CCT of the white LED were 547 nm, (0.321, 0.340), and 6025 K, respectively. Figure 4(b) shows the schematic structures of the white LED with an additional SCP-converter layer. A SCP-converter layer was mounted on the heat sink and located on top of the white LED. The blue laser with collimating lens illuminated on the surface of the SCP-converter layer and the beam size was about 3.5 × 1.4 mm and the luminous flux of the blue laser beam on the SCP surface was approximately Gaussian distribution. With the blue laser beam collimated onto the SCP-converter layer on top of the white LED, as shown in Fig. 2(a), most of the blue laser beam was absorbed by the SCP-converter layer and may not be absorbed by the silicone-based phosphor of the white LED, thus eliminating the overheating, or burning issues. This was due to the SCP-converter layer was a highly transparent material with virtually no scattering in 525-700 nm wavelength range, as shown in Fig. 2(b). Therefore, the yellow portion of the white LED passed through the SCP-converter layer and the blue light was also absorbed and converted to yellow light adding to the photonic output of the system. Figure 4(c) shows the schematic structure of the ADB headlight. The ADB headlight consisted of a Nichia white LED, a TI DMD with 1 million pixels, a projection optics system, and a Laser-Assisted LED system, which included a SCP-converter layer (10 × 10 × 0.7 mm) on top of the white LED emission area and two blue lasers for the addition excitation of the SCP-converter layer. As shown in Fig. 4(c), a white LED with output of 3,500 lm passed through the SCP-converter layer, which was transparent to yellow light. Part of the blue light from the white LED was also converted to yellow light by a SCP-converter layer, increasing the yellow output of the system. A novel approach was developed directing the two blue laser beams as shown in the bottom of Fig. 4(c), passing through small apertures in the aspherical reflector to illuminate the SCP-converter layer located on the top of the white LED, passing through a SCP-converter layer, and then reflected by the top surface of blue LED. The two blue laser beams were converted into 1,000 lm white light and added to the white LED output with a total of 4,500 lm, which was then collected by the light tunnel, and the aspherical reflector reflected. The white light was reflected and modulated by the DMD, and eventually projected onto the roadway.
Fig. 4. The diagrams of (a) the normalized light emission spectra of white LED, (b) the schematic structures of the white LED with an additional SCP-converter layer, and (c) the ADB headlight.
Table 2 shows the measurement results of the ADB headlight. The safety accreditation of the high beam (ECE R112 Class B) was followed by evaluating the performance of ADB headlight. The luminous intensity (cd) of the high-beam pattern was measured to be at 0° (center), ± 2.5°, and ± 5°. The ADB pattern was measured to be 88,436 cd at 0°, 69,393 cd at ± 2.5°, and 42,942 cd at ± 5°, which well satisfied the ECE R112 class B regulation. The difference between the measurement points at + 2.5° and - 2.5° might be caused by fabrication and assembly error, and the same situation existed between the measurement points at + 5° and - 5°. The advantage of introducing the Laser-Assisted LED system employing the highly reliable SCP-converter layer was to produce high intensity of the ADB, which had increased the FOV by 20% and the brightness by 28% for the ADB headlight prototype, and resulting the improved visibility from ± 7° to ± 8.5° and the illumination distance up to 200 m. Figure 5 shows the far-field intensity distribution of the ADB headlight at 2.3 m. The blue circle was Imax about 1.3 cm to the left of 0°. It also indicated the test points for evaluating the headlight according to Class B specification of ECE R112.
Table 2. The measurement results of the ADB headlight.
Figure 6 shows the photo of the long-time laser excited measurement of the ADB headlight. Right side was an integrating sphere measurement system and left side was the ADB headlight with additional heat sinks. The heat dissipation of the ADB headlight was required additional heat sinks to obtain stable light output characteristics. The dimensions of the ADB prototype met the size requirements of autonomous vehicles. As a result, the (x, y) coordinate, CCT, and total luminous flux of the ADB headlight output were (0.345, 0.365), 5074 K, and 1375 lm, respectively. Figure 7 shows the long-time laser excited measurements of the ADB headlight for the lumen loss and CIE shift. Within 30 minutes after the ADB headlight was started, the initial variations of the lumen loss and CIE shift may be caused by the increase in the temperature of the optical components such as blue LED, blue lasers, silicone-based phosphor, and SCP-converter layer. However, the lumen loss and CIE shift with the test time became stable after 30 minutes, as shown in Figs. 7(a) and 7(b). The long-time laser excited results clearly demonstrated that the ADB headlight exhibited stable light output characteristics during long-time laser excited operation.
Fig. 7. The long-time laser excited measurements of the ADB headlight for the (a) lumen loss and (b) CIE shift.
4. Discussion and conclusion
In summary, a novel Laser-Assisted LED system for the ADB headlight employing an ultra-reliable SCP-converter layer for use in autonomous vehicles was presented and demonstrated. The Ce3+: YAG-based SCP-converter layer was fabricated by CZ technology at a high-temperature of 1,940°C, which showed better thermal stability than other phosphor-converter materials, evidenced by thermal aging test. This was due to SCP’s higher melting temperature, smaller thermal expansion coefficient, higher thermal conductivity, and higher Young’s modulus. In this study, the measured high-beam pattern of the ADB headlight well satisfied the ECE R112 class B regulation. The advantage of introducing the Laser-Assisted LED system employing the high output white LED, addition two blue lasers, and the ultra-reliable SCP-converter layer was to produce the high intensity for the ADB system, which enabled the increase the FOV by 20% and the brightness by 28% of the ADB headlight, resulting in the improvement of the visibility from ± 7° to ± 8.5° and the illumination distance up to 200 m. Therefore, the proposed high performance ADB headlight with the ultra-reliable SCP-converter layer and the novel Laser-Assisted LED is one of the most promising ADB headlight candidates for use in the next-generation autonomous vehicle applications.
Funding
Ministry of Science and Technology, Taiwan (107-2218-E-005-025, 108-2218-E-005-018, 109-2218-E-005-012, 109-2823-8-005-003).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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