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Abstract
An advanced laser headlight module (LHM) employing highly reliable glass phosphor is demonstrated. The novel glass-based YAG phosphor-converter layers fabricated by low-temperature of 750°C exhibited better thermal stability. The LHM consisted of a 5 × 1 blue laser diode array, an aspherical lens, a glass phosphor-converter layer with an aluminum thermal dissipation substrate, and a dichroic filter to allow pass blue light and reflect yellow phosphor light. The 5 × 1 blue laser array was packaged with five blue lasers having optical power of 1.2 W per laser. The LHM exhibited total output optical power of 6 W, luminous flux of 1860 lm, relative color temperature of 4100 K, and efficiency of more than 310 lm/W. The high-beam patterns of the LHMs were measured to be 45,000 luminous intensity (cd) at 0°, 31,000 cd at ± 2.5°, and 12,500 cd at ± 5°, which were well satisfied the ECE R112 class B regulation. The proposed high-performance LHM with highly reliable glass-based phosphor-converter layer fabricated by low temperature is favorable as one of the promising LHM candidates for use in the next-generation automobile headlight applications.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Solid-state lighting (SSL) has revolutionized the illumination industrial and is steadily progressing in terms of power, efficiency, and diversity for future applications. Currently, white light-emitting diodes (LEDs) are widely used for the SSL market [1]. Most of these white LEDs are basically made of a blue emitting LED chip with a coating of yellow phosphor. A part of the blue light is absorbed by yellow phosphor, which generates a yellow light. The mixing of the residual blue light together with the yellow light produces the desired white light LEDs. However, high luminous flux, compact size, and low power consumption are important features for designing light engines of automotive headlamps and smart headlights, and therefore the white laser diodes (LDs) may have advantages over LEDs in such applications [2,3]. Since the etendue of a focused laser diode array is much small than LED array, the beam performance of laser diode headlight with glass phosphor is much longer and better than the LED headlight. Therefore, the primary benefit for drivers using laser diode headlights is that the beam range can be up to 600 m [4]. This offers the driver improved visibility, contributing significantly to road traffic safety.
Most of white LD engines are integrated using blue LD and phosphor-converter layer. The laser headlights based phosphor-converted layers had been fabricated using ceramic [5–8], single crystal [3], and glass materials [9]. Table 1 lists the performance comparison of the fabrication temperature, thermal stability, advantages, and disadvantages of the silicone-, ceramic-, single crystal-, and glass-based phosphor-converter layers. The fabrication temperatures of the ceramic- and single crystal-based phosphor were over 1200°C and 1500°C, respectively. These high-temperature fabrications were difficult to be applied in the commercial production. There was a bubble issue in glass-based phosphor as indicated in Table 1. A significant reduction of bubbles in glass-based phosphor has been demonstrated by using densification process with glass powders under ball mill [10]. The densification process for bubble reduction in glass-based phosphor will be adopted in this study. In previous report, the glass-based phosphor-converter layers had shown better thermal stability than the silicone-based color conversion layers [11–13]. Therefore, the glass-based phosphor with better thermal stability fabricated by low-temperature of 750°C is one of promising materials for use in the LD light engines.
Table 1. Comparison of silicone-, ceramic-, single crystal-, and glass-based phosphor-converter layers
In this study, an advanced laser headlight module (LHM) employing highly reliable glass phosphor is reported and demonstrated. The novel glass-based YAG phosphor-converter layers fabricated by low-temperature of 750°C exhibited better thermal stability. The LHM consisted of a 5 × 1 blue laser array, an aspherical lens, a glass phosphor-converter layer, and a dichroic filter to pass the blue light and reflect the yellow phosphor light. The measured high-beam patterns of the LHMs were well satisfied the ECE R112 (Economic Commission Europe R112) class B regulation. The proposed high-performance LHM with highly reliable glass-based phosphor-converter layer fabricated by low temperature is favorable as one of the promising LHM candidates for use in the new-generation automobile headlight applications.
2. Fabrication and thermal aging test of glass-based phosphor-converter layer
2.1 Fabrication of glass phosphors
The fabrication procedures of Ce3+: YAG yellow glass phosphor included the preparation of sodium mother glass by melting the mixture of raw materials at 1300°C and dispersing Ce3+: YAG powders into the mixture by gas-pressure and sintering under different temperatures [11–13]. The composition of the sodium mother glass was 60 mol% SiO2, 25 mol% Na2CO3, 9 mol% Al2O3, and 6 mol% CaO. The resultant cullet glass of the SiO2-Na2CO3-Al2O3-CaO were dried and milled into powders. The Ce3+:YAG crystals were uniformly mixed with the mother glass and sintered at 750°C for 1 hour and then annealed at 350°C for 3 hours, followed by cooling to room temperature. The concentration of Ce3+:YAG with 40 wt% exhibited the higher luminous efficiency and provided better purity for yellow color phosphor-converter layers [13]. Then, the glass phosphor bulk was cut into the disks of phosphor-converter layer with a diameter of 25 mm and thickness of 0.35 mm. In this study, the densification process of fabricated glass-based phosphor was used [10].
External Quantum efficiency (EQE) is one of essential parameters used as a criterion of phosphor conversion materials in solid state lighting. The EQE is defined as the ratio of the number of photons emitted from phosphor conversion materials to the number of incident photons from the light sources. The number of photons in each wavelength was derived by dividing spectrum distribution of photon energy. The EQE was calculated by EQE = Σ(E/hυi)/(Eincident/hυi), where the E and Eincident are the emissive energy and the incident energy, respectively. The results of the emissive energy, the number of photons emitted by the phosphor, and the incident energy were 0.28 Joule/sec, 1.61x108, and 0.47 Joule/sec, respectively. Therefore, the EQE was measured about 70%.
2.2 Thermal aging tests of glass and silicone phosphors
The yellow glass-based phosphor converter layer in this work was named as CeYDG, whereas the yellow silicone-based phosphor-converted layers were named as CeYDS. For fabricating Ce:YAG-doped silicone (CeYDS), uniform mixture of Ce:YAG and silicone was baked and cured at 150°C. The ratio between Ce:YAG dopants and silicon host was controlled to keep the chromaticity of the finished CeYDS as same as that of CeYDG. All the CeYDG and CeYDS samples were polished to the thickness of 0.35 mm. The luminous loss, chromaticity shift, and quantum efficiency tests were carried out for both glass and silicone samples under thermal aging tests.
For thermal aging tests, six samples of CeYDG and CeYDS were aged at 150, 250, 350 and 450°C for 1008 hours [11,12]. The optical properties were measured every 250 hours during thermal aging to characterize the degradation of the samples. Figure 1(a) shows lumen loss of CeYDG samples after thermal aging for 1008 hours at 150, 250, 350, and 450°C. Figure 1(a) indicated that the lumen loss after thermal aging of CeYDG compared favorably with the CeYDS. The lumen losses of CeYDS were about 5 and 10 times higher than that of CeYDG at 150 and 250°C, respectively.
Fig. 1 The CeYDG and CeYDS as a function of aging temperature for 1008 hours of (a) the lumen loss, (b)the chromaticity shift, and (c) the quantum efficiency.
Figure 1(b) shows thermal aging results of chromaticity shift for the CeYDG samples under thermal aging tests at 150, 250, 350, and 450°C for 1008 hours. The chromaticity shifts after thermal aging of CeYDG compared favorably with the CeYDS. The chromaticity shifts of CeYDS were 3 and 40 times larger than that of CeYDG at 150 and 250°C, respectively. Figure 1(c) shows thermal aging results of quantum efficiency loss for the CeYDG samples under thermal aging at 150, 250, 350, and 450°Cfor 1008 hours. The quantum efficiency loss of CeYDS was 3 and 11 times larger than that of CeYDG at 150 and 250°C, respectively. The results of Figs. 1(a)-1(c) clearly demonstrated that the glass-based phosphor-converter layers (CeYDG) exhibited better thermal stability in lumen degradation, lower chromaticity shift, and higher quantum efficiency than those of the silicone-based phosphor-converter layers (CeYDS) [11,12]. These were due to that the glass-based phosphor-converter layers layer exhibited higher transition temperature, smaller thermal expansion coefficient, higher thermal conductivity, and higher Young’s modulus than silicone-based phosphor-converter layers, as shown in Table 2.
Table 2. Thermal comparison of silicone- and glass-based phosphor-converter layers
The fluorescence spectrophotometer was used to measure the characterization of emission and excitation spectra of CeYDS and CeYDG [14]. The intensity comparison of emission and excitation spectra of the CeYDS and CeYDG before and after thermal aging at 250°C was obtained. The temperature-dependent variation of the CeYDG was smaller than the CeYDS [14]. This was due to the transmittance of the CeYDG was lower than the CeYDS.
3. Design, simulation, and fabrication of laser headlight module (LHM)
3.1 Design of laser headlight module (LHM)
Figure 2 shows a schematic diagram of laser headlight module (LHM). This LHM consisted of a blue laser, an aspherical lens, a glass phosphor-converter layer, an Al substrate, and a dichroic filter. OSRAM blue lasers with wavelength of 453 nm were used in this work. A blue laser with electrical power of 5 W produced an optical power of 1.2 W and then converted to a white light of 370 lm. The glass-based phosphor was fabricated by low-temperature of 750°C and then mounted on an Al thermal dissipation substrate. The infrared thermal imaging camera showed that the temperature profile of the LHM was average temperature of 33°C after long operation time more than 1 hour. The brightness of the LHM did not change after long operation time. Therefore, the Al thermal dissipation substrate might solve the thermal effect of the LHM.
The dichroic filter was used to allow pass blue light and reflect yellow phosphor light. The aspherical lens could improve the light pattern of the LHM to satisfy the ECE R112. The material of the lens was BK7, which had the refractive index of 1.51 and absorption coefficient of 0.002086 cm−1.
Table 3 shows the safety accreditation of the high beam (ECE R112). The luminous intensity (cd) of the high-beam patterns will be measured at 0° (center), ± 2.5°, and ± 5°. As an example for ECE R112 Class B, the intensity of the high beam needs to be higher than 40,500 cd at 0°, 20,300 cd at ± 2.5°, and 5,100 cd at ± 5°. In this study, the design of the high-beam patterns for the LHMs should be satisfied the ECE R112 class B regulation.
Table 3. Safety accreditation of high beam (ECE R112)
3.2 Design of an aspherical lens
The equation of an aspheric lens can be described as [15]:
where the k, r, and c = 1/ro are the constant of conic, radius, and constant of curvature, respectively. Figure 3 shows a schematic diagram of an aspherical lens with variable surfaces. In Fig. 3, the k = 0, −1< k <0, and k = −1 are the spherical, ellipsoidal, and parabolic surfaces, respectively. We changed the k and substituted R = ro = 13.5 mm into the Eg. (1) of aspherical lens, which the optimized result could be obtained. The optimized projection result could be obtained for k = −0.5. The designed result is shown in Fig. 4 (a). Figures 4(b)-4(d) show the simulation of ray tracing, 2-D intensity of distribution, and 1-D intensity of distribution with 8600 cd at the focus of 11 mm, respectively.Fig. 4 (a) 3D schematic of aspherical lens. (b) Ray tracing diagram. The blue rectangle the white light monitor. (c) 2D intensity distribution pattern. (d) 1D crosscut of intensity distribution as indicated by the horizontal line in (c).
In this work, the eye safety should be an issue since high power lasers are used, A white light source (as indicated by a blue rectangle below the lens in Fig. 4(b) will be install to monitor if the lasers and glass phosphor layer function properly. If there is overheating damage or function failure caused by a car accident, the monitor will sense these problems and send a signal to terminate the blue lasers, preventing the risk of laser leakage.
3.3 Simulation of LHM by SPEOS software
Based on the optimizing parameters of aspherical lens, as shown in Fig. 4, the simulation with SPEOS software and measurement of the LHM for the high beam were showed on Table 4. Table 4 indicated that the measurement and simulation were in good agreement at 0°, ± 2.5°, and ± 5°. The difference between the measurement and simulation might be caused by fabrication and assembly error.
Table 4. The simulation and measurement of LHM high beam
3.4 Fabrication and measurement of LHM array
Because the measured luminous intensity with a single blue laser of the LHM was 8400 cd, we should design a 5 × 1 LHM array to satisfy the ECER 112 Class B regulation, as shown in Table 2. The estimated intensities of a 5 × 1 LHM array are 8400 × 5 = 42000 cd at 0°, 6400 × 5 = 32000 cd at ± 2.5°, and 3500 × 5 = 17500 cd at ± 5°. The designed 5 × 1 LHM array is expected to satisfy the safety accreditation of the high beam of the ECER 112 Class B.
The LHM array consisted of a 5 × 1 blue laser array, aspherical lens, glass phosphor-converter layers with an Al thermal dissipation substrate, and dichroic filters to allow pass blue light and reflect yellow phosphor light. The 5 × 1 blue laser array was packaged with five blue lasers, each providing 1.2 W of optical power. Figure 5 shows a schematic design of 5 × 1 LHM array. Figure 6 shows a photo of fabricated LHM with 5 × 1 blue laser array.
The LHM array exhibited total output optical power of 6 W, luminous flux of 1860 lm, relative color temperature of 4100 K, and efficiency of more than 310 lm/W. The high-beam patterns of the LHMs were measured to be 45,000 luminous intensity (cd) at 0°, 31,000 cd at ± 2.5°, and 12,500 cd at ± 5°, which were well satisfied the ECE R112 class B regulation.
4. Discussion and conclusion
Due to the important features of high luminous flux, compact size, and low power consumption, white laser diodes (LDs) may have advantages over LEDs for designing light engines for automotive headlights and smart headlights. Currently, most of white LD engines have been integrated using blue LDs and ceramic-based phosphor-converter layers [4–7]. However, the fabrication temperature of the ceramic-based phosphor-converter layer was over 1200°C. It is difficult for the high-temperature fabrication to be applied in the commercial production. In contrast to the glass-based phosphor-converter layer, the glass-based phosphors were fabricated by low-temperature of 750°C. Furthermore, the material cost of glass is compatible with low-cost silicone polymer. The glass-based phosphor-converter layer fabricated by low-temperature of 750°C with low cost and better thermal stability may be one of favorable materials for use in the LD light engines.
In this study, the proposed LHMs exhibited better luminous flux, excellent relative color temperature, and higher efficiency. The measured high-beam patterns of the LHMs were well satisfied the ECE R112 class B regulation. Therefore, the proposed LHMs with high performance and low-cost fabrication are favorable for use in the next-generation automobile headlight applications.
Funding
Ministry of Science and Technology (MOST) (106-2622-E-005-018-CC2).
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