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A new scheme of a highly-reliable glass-based color wheel applied to a laser light engine (LLE) for a high-power laser operation is demonstrated for the first time. The glass-based color wheel showed better thermal stability than the silicone-based color wheel, about 13.6 times less lumen loss and 3 times less chromaticity shift after being operated under a 30 Wopt laser for 2000 hours, respectively. The excellent thermal stability can be attributed to the high glass transition temperature up to 570 °C exhibited by the glass-based color wheel. The easy fabrication and the good reliability on optical performance under thermal stress benefit the novel glass-based color wheels as promising candidates to replace the silicone-based color wheels in the LLE modules for the next-generation laser projector.
© 2017 Optical Society of America
1. Introduction
Strong luminance with uniformly spatial distribution and widely angular extension in the light engine of projection displays is usually required to provide high projection quality [1]. The requirement of the high optical power in the light engine can be efficiently satisfied by inherently bright laser diodes cooperates with color-complementary color wheels [2, 3]. The most cost-efficient type of color wheels is realized by phosphor-doped silicone due to the ease of fabrication. However, the silicone-based color wheels suffer poor thermal stability because the low glass transition temperature of silicone (150°C). Under the illumination in the laser light engine (LLE) of projection displays, high heat flux radiates from the high-power laser yellows the silicone-based color wheel, followed by affecting the optical performance of the color wheel modules, including decayed luminous efficiency and chromaticity stability [4]. Therefore, novel carrier materials of color wheels for high power operation are essentially required.
Recently, glass-based carrier material has been proposed as a new solution for both LED- and laser-driven solid state lightings [5–8]. In our previous study on glass-based color conversion layer for LED lightings, we have also successfully demonstrated that glass-based color conversion layers show much better thermal stability than the silicone-based color conversion layers [9–13]. However, in the application of color wheel for projection system, only the optical performance has been presented and compared between glass-based color wheels and silicone-based color wheels [14]. In this study, a new scheme of high-reliable glass-based color wheel applied in LLE for high-power laser illuminating for a long-time operation is experimentally presented and demonstrated for the first time. The LLE module in this work consists a blue-light laser array and a color wheel with yellow and green phosphor-converted layers. The long-term thermal stability of both glass-based and silicone-based color wheels under the illumination of laser with 5 Wopt and 30 Wopt optical powers at room temperature for 2000 hours are measured. The results show that the glass-based color wheel showed much better thermal stability than the silicone-based color wheel characterized by lumen loss and chromaticity shifts. The high-reliable glass-based color wheel under high power operation is essentially critical for next-generation LLE module, especially for the laser-based projection applications.
2. Design of laser light engine (LLE)
LLE module includes optical mirrors, diffuser glasses, a blue laser diode, and a color wheel [15]. The color wheel is a key component for generating white light in the LLE. In this study, the color wheel can be divided into two segments based on the color of the doped phosphors, Y-segment (yellow) and G-segment (green). The Y-segment and G-segment are bonded on an aluminum substrate and fixed on a micro-motor. Figures 1(a) and 1(b) show the images of silicone and glass-based color wheel. A laser array is used as the light source to excite the color wheel to generate white light.
Fig. 1 (a) The silicone-based and (b) glass-based color wheel, (c) the silicone-based color and (b) glass-based color wheel after 30 Wopt laser power for 2000 hours.
3. Fabrication of a glass-based phosphor-converted layer
A composition of borosilicate mother glass, SiO2-B2O3-K2O-Na2O-ZnO-Al2O3-TiO2, was uniformly mixed and fabricated at 1300°C. The resulting cullet glass was ground into glass powders and then screened to under 125 μm in size. Figure 2 illustrates the fabricated process of yellow and green phosphor-converted layers [9]. The resulting borosilicate mother glass was ground and then mixed yellow or green phosphor to make each of the glass phosphor powders. Phosphor crystals, such as Ce3+:Y3Al5O12 and Ce3+:Lu3Al5O12, with the particle size of 13 μm were chosen as the yellow and green color phosphor-converted layers, respectively. The doping concentrations of yellow and green color phosphor-converted layers were 40 and 37.5 wt%, respectively. The results showed that the 40 wt% (Y-segment) and 37.5 wt% (G-segment) doping concentrations at the 0.35 mm-thick exhibited the highest luminous efficiency and better purity for yellow and green color phosphor-converted layers, respectively [9]. The precursor was sintered at 850°C for 1 hour and then annealed at 350°C for 3 hours, followed by cooling to room temperature. Then, the glass phosphor bulk was cut into the disks of phosphor-converted layer with a diameter of 25 mm and thickness of 0.35 mm, respectively. The yellow (Y-segment) and green (G-segment) glass-based phosphor-converted layers in this work named as CeYDG and CeLuDG, respectively. The yellow and green silicone-based phosphor-converted layers in this work named as CeYDS and CeLuDS, respectively.
4. Results and discussion
4.1 Characteristics of color wheel modules
To characterize the color wheel modules, the lumen loss and chromaticity shift were measured by reflected way inside an integrating sphere. An integrating sphere equipped with a detector was employed to measure the reflected fluorescence from the phosphor-converted layers. A blue laser with peak emission at 445 nm was used as a pumping source. The optical power of the exciting blue laser was fixed at 5 Wopt. The spot size of the blue laser was 3 mm. The blue laser was placed at the outside of the integrating sphere. The glass phosphor layer was placed at the center of a reflected layer. The reflectance of reflector layer was over 99%. As shown in Fig. 3, the sharp emission band around 445 nm was the transmitted blue light from the exciting blue laser, and the emission band between 480 nm and 700 nm was due to the fluorescent yellow and green light from the CeYDG and CeLuDG, respectively. The luminous efficiency of CeYDG and CeLuDG were 291.1 lm/Wopt and 303.5 lm/Wopt, respectively. The CIEx,y of CeYDG and CeLuDG were (0.431,0.554) and (0.329,0.594), respectively.
The optical properties of glass-based color wheel was investigated and compared with the commercial silicone-based color wheel. The angular of Y-segment and G-segment conversion layers on high-reflected aluminum substrate were 219° and 60°, respectively, as shown in Fig. 1(a) and 1(b). The silicone-based color wheel with conversion layer thickness of 0.2 mm and concentration of 72 wt% (Y-segment) and 82 wt% (G-segment) was usually used in commercial products [9]. The Y-segment and G-segment of silicone-based phosphor-converted layers in this work named as CeYDS and CeLuDS, respectively. The 5 Wopt and 30 Wopt blue lasers with peak emission at 445 nm were used as pumping sources. The measured optical properties of the lumen loss and chromaticity shift for the glass-based color wheel were compared with the commercial silicone-based color wheel.
4.2 Lumen loss of the color wheel modules
In thermal stability study, the glass- and silicone-based color wheel modules of LLE were tested at different operating laser power of 5 Wopt and 30 Wopt for 2000 hours. In this study, the batches of glass-based color wheel of the CeYDG and CeLuDG with high-power operation tests were investigated and compared with silicone-based color wheel of the CeYDS and CeLuDS. The lumen loss and chromaticity shift were measured every 250 hours to exam the degradation of two type color wheels.
Figure 4 shows the lumen loss as a function of test time of CeYDG, CeLuDG, CeYDS, and CeLuDS with high-power operation of 5 Wopt and 30 Wopt for 2000 hours. The lumen loss of glass- and silicones-based color wheel of the CeYDG, CeLuDG, CeYDS, and CeLuDS were less than 0.7, 1.0, 9.8, and 9.3% after high-power operation of 5 Wopt for 2000 hours, respectively. The lumen loss of glass- and silicones-based color wheel of the CeYDG, CeLuDG, CeYDS, and CeLuDS were less than 3.3, 3.3, 45, and 44% after high-power operation of 30 Wopt for 2000 hours, respectively. Compared with the silicone-based color wheels of the CeYDS and CeLuDS, the results showed that the glass-based color wheels of the CeYDG and CeLuDG exhibited lower lumen loss over test time. This indicated that the lumen loss of the silicone-based color wheels of the CeYDS and CeLuDS were about 13.6 and 13.3 times higher than the glass-based color wheels of the CeYDG and CeLuDG after high-power operation of 30 Wopt for 2000 hours.
Fig. 4 Lumen loss as a function of test time of CeYDG, CeLuDG, CeYDS, and CeLuDS with high-power operation of (a) 5 Wopt and (b) 30 Wopt for 2000 hours.
4.3 Chromaticity shift of the color wheel modules
Figure 5 shows the chromaticity shift as a function of test time of CeYDG, CeLuDG, CeYDS, and CeLuDS with high-power operation of 5 Wopt and 30 Wopt for 2000 hours. The chromaticity shifts of glass- and silicones-based color wheel of the CeYDG, CeLuDG, CeYDS, and CeLuDS were about 2.6 × 10−3, 3.3 × 10−3, 5.5 × 10−3, and 6.6 × 10−3 after high-power operation of 5 Wopt for 2000 hours, respectively. The chromaticity shifts of glass- and silicones-based color wheel of the CeYDG, CeLuDG, CeYDS, and CeLuDS were about 3.6 × 10−3, 5.6 × 10−3, 11.1 × 10−3, and 11.4 × 10−3 after high-power operation of 30 Wopt for 2000 hours, respectively. Compared with the silicone-based color wheels of the CeYDS and CeLuDS, the results showed that the glass-based color wheels of the CeYDG and CeLuDG exhibited less chromaticity shifts over test time. This indicated that the chromaticity shift of the silicone-based color wheels of the CeYDS and CeLuDS were about 3 and 2 times higher than the glass-based color wheels of the CeYDG and CeLuDG after high-power operation of 30 Wopt for 2000 hours.
Fig. 5 Chromaticity shift as a function of test time of CeYDG, CeLuDG, CeDS, and CeLuDS with high-power operation of (a) 5 Wopt and (b) 30 Wopt for 2000 hours.
4.4 Discussion
Figure 1(c) and 1(d) show the photographs of the silicone-based and glass-based color wheels after high-power operation of 30 Wopt for 2000 hours, respectively. A significant defect was observed for silicone-based color wheel after high-power operation 30 Wopt for 2000 hours, as shown in Fig. 1(c). However, the glass-based color wheel did not observe defect, as shown in Fig. 1 (d). From Fig. 3, at high-power operation of 30 Wopt, the silicone-based color wheels of the CeYDS and CeLuDS exhibited about 45% in lumen loss which failed in operation, whereas the glass-based color wheel of the CeYDG and CeLuDG only exhibited 3.3% in lumen loss. Because the glass transition temperature of 570°C was much higher than the silicone transition temperature of 150°C [5,6]. In this study, the glass-based CeYDG and CeLuDG were fabricated by borosilicate mother glass which demonstrated high thermal stability. High thermal stability of the glass-based phosphor-converted layer had also been demonstrated for use in white light-emitting diodes [5–9]. Therefore, the adapting unique glass-based color wheel in the LLE module is beneficial to employ in the LLE module where the silicone-based color wheel fails to operate in high power and strict reliability.
5. Conclusion
In summary, an innovative glass-based phosphor-converted layer of the CeYDG and CeLuDG for use in the LLE module with high-power laser operation was proposed and demonstrated. The glass- and silicone-based color wheels of the lumen loss and chromaticity shift were measured and compared. The lumen loss of glass- and silicones-based color wheel of the CeYDG, CeLuDG, CeYDS, and CeLuDS were less than 3.3, 3.3, 45, and 44% after high-power operation of 30 Wopt for 2000 hours, respectively. The chromaticity shifts of glass- and silicones-based color wheel of the CeYDG, CeLuDG, CeYDS, and CeLuDS were about 3.6 × 10−3, 5.6 × 10−3, 11.1 × 10−3, and 11.4 × 10−3 after high-power operation of 30 Wopt for 2000 hours, respectively. The results showed that the silicone-based color wheel exhibited less thermal stability than the glass-based color wheel about 13.6 times higher lumen loss and about 3 times higher chromaticity shift after laser operation of 30 Wopt for 2000 hours, respectively. This was due to the higher glass transition temperature of 570°C than the silicone transition temperature of 150°C [5,6]. Therefore, the glass-based color wheels significantly showed an excellent thermal stability and performance, including remarkably low lumen loss and chromaticity shift after high-power laser and long-time operation tests, as theoretical expectation. This study clearly demonstrates that the novel glass-based color wheels in the LLE modules to be used in the next-generation laser projector system may replace the currently commercial usage of the silicone-based color wheels.
Funding
Ministry of Science and Technology (MOST) (MOST 104-2622-E-005-015-CC); the Advanced Optoelectronic Technology Center (AOTC), National Cheng Kung University.
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