Chapter 5 Experimental results and discussions
5.3 Summary and discussion
Fig. 5-8 Captured results by a CCD camera, the captured size on the backlight is 20cm by 20cm.
5.3 Summary and discussion
The optimized curvature of MPF was successfully fabricated and verified that it can suppress lamp-Mura effectively. Among fabricated MPFs of different filling ratios, the MPF filling ratio 10% of curvature 58 mm-1 was the closest to the optimized curvature 60 mm-1 of MPF and MPF-Plate case. Thus, MPF filling ratio 10% was chosen to be the optimized MPF which can suppress lamp-Mura. In MPF case, that the lamp-Mura contrast (0.010) was lower than that of the conventional backlight (0.012), agrees with the simulation result. In MPF-Plate case, the lamp-Mura contrast was 0.018, higher than 0.012. However, both MPF and MPF-Plate case resulted in an invisible lamp-Mura luminance image as the conventional backlight. Thus, we conclude the lamp-Mura contrast of 0.018 can yield invisible lamp-Mura.
Moreover, the luminance difference between simulation and experimental results of MPF case increased when MPF curvature decreased, as shown in Fig. 5-5 (b). From the optical microscope (OM) image of filling ratio 100% MPF as shown in Fig. 5-9, many particles were in the filled material. Therefore, we assume the filled UV-curable material causes the light scattering.
According to our assumption, we simulated a MPF with scattering filled material. This material was the pure UV-curable material added scattering particles ( 105 particles/mm3). In the MPF case with different curvatures, the luminance distributions with and without scattering material are shown in Fig. 5-10 (a). Additionally, the calculated result of the luminance with scattering material relative to that without is shown in Fig. 5-10 (b). Where the luminance decreased when the curvature decreased or the filling ratio increased. This result verified that the luminance is inversely proportional to the amount of scattering material.
Fig. 5-9 OM pictures of filling ratio 100% MPF.
(a) (b)
Fig. 5-10 Simulation results of (a) luminance distribution of MPF case with and without scattering filled material, (b) relative luminance of MPF case with filled material of scattering compared to that without scattering filled material.
Moreover, MPF and MPF-Plate case enhanced FWHM to 90° and 116°, respectively, in luminance viewing angle and had an 8% and a 19% improvement in output light efficiency, respectively. The higher efficiency in MPF-Plat case was also verified.
The normal luminance of MPF-Plate case is lower than that of MPF case as shown in Fig.
5-6 and Table 5-1. One possible reason is that the number of interfaces is reduced in the MPF-Plate. Because the light guiding effect exists in a pair of interfaces, the reduced number of interfaces will cause less light guiding effect and less light transferring to normal direction.
Thus, MPF-Plate case had lower normal luminance than MPF case did.
Chapter 6
Patterned-MPF
A slim LCD-TV is a trend in current display market because it is suitable to mount on a wall to free up more space. However, the thickness of LCD-TV depends on the backlight structure. To achieve a slim LCD-TV, backlight thickness has to be reduced. When the backlight thickness decreases, the lamp-Mura will become more serious. Additionally, the MPF had a limitation to suppress lamp-Mura in a slim backlight system. Therefore, compared to the MPF with uniform prism structure, a Patterned-MPF was proposed to improve the uniformity in a slim backlight system.
The Patterned-MPF has various curvatures depending on the position from lamps, as shown in Fig. 6-1. This pattern concept has been applied to diffusive films for enhancing the uniformity of side-emitting backlight system by Y.-C. Lo[23]. Because lamp-Mura is caused by a non-uniform light distribution, thus, the pattern concept can also be applied to MPF to optimize the MPF curvature at different positions. In the following designs and simulations, MPF case will be chosen to design backlight system at a thickness of 20 mm.
Fig. 6-1 Schematic of Patterned-MPF.
6.1 Design and simulation results
To design the curvature distribution of Patterned-MPF, luminance distributions of different MPF curvatures have to be determined first. The luminance simulation results of different thicknesses are shown in Fig. 6-2. Considering the uniformity at each position, we found output luminance is limited at 0.5 pitch. This luminance value was called the target luminance in this thesis.
(a) (b)
(c) (d)
Fig. 6-2 Luminance simulation results of MPF case with different curvatures at the backlight thickness of (a) 10 mm, (b) 12 mm, (c) 14 mm, and (d) 16 mm.
The luminance at each position is designed as the same as the target, which becomes the initial curvature distribution for Patterned-MPFs, as shown in Fig. 6-3. As a result, when the thickness decreased, the variation of curvature distribution increased because of more serious lamp-Mura. In contrast, in a thick backlight system, a uniform MPF can result in a uniform light output.
Fig. 6-3 Initial curvature distribution design of Patterned-MPF at the backlight thickness of 10 mm, 12 mm, 14 mm, and 16 mm.
The Patterned-MPF with the initial curvature distribution was then simulated to evaluate the uniformity of light output. The simulation results of output luminance distribution are shown in Fig. 6-4. The initial designed Patterned-MPFs yielded a not uniform enough luminance distribution. Therefore, the curvature distribution needs to be future modified for a more uniform light output. For example, in Fig. 6-4 (a), the luminance at 0 and 1 pitch were higher than the target luminance (5500 nits). Thus, the corresponding curvatures have to be increased for decreasing luminance. This is because the luminance of 0 or 1 pitch (above the lamp) decreased with increasing curvature, as shown in Fig. 6-2. Accordingly, the uniform light outputs (blue curves) were then yielded, as shown in Fig. 6-4, and the corresponding optimized curvature distributions are shown in Fig. 6-5.
(a) (b)
(c) (d)
Fig. 6-4 Luminance simulation results of MPF case with designed Patterned-MPF at the backlight thickness of (a) 10 mm, (b) 12 mm, (c) 14 mm, and (d) 16 mm.
Fig. 6-5 Optimized curvature distribution design of Patterned-MPF at the backlight thickness
Moreover, lamp-Mura contrasts calculated from the luminance distributions in Fig. 6-4 are summarized in Table 6-1. All thicknesses using optimized Patterned-MPFs resulted in a lamp-Mura contrast smaller than 0.018 (MPF-Plate case with uniform MPF). Because the MPF-Plate case yielded invisible lamp-Mura, thus, lamp-Mura in a slim backlight system can be suppressed by optimized Patterned-MPFs. For different backlight thicknesses, the corresponding average luminance was normalized to the conventional backlight system (6875 nits), as shown in Fig. 6-6. When backlight thickness decreased, the average luminance also decreased. However, the optimized backlight thickness whose output luminance is close to that of the conventional backlight system was found about 13.3 mm.
Table 6-1 Summary of lamp-Mura contrasts in MPF case with initial and optimized Patterned-MPFs.
Fig. 6-6 Relationship between relative output luminance of MPF case with optimized Patterned-MPF and the backlight thickness, where the relative average luminance is normalized to the conventional backlight system (6875 nits).
6.2 Summary
In a slim backlight system, lamp-Mura becomes more serious and MPFs have a limited capability to suppress lamp-Mura. Thus, the Patterned-MPF with a variational curvature distribution was proposed to suppress lamp-Mura. According to output luminance distributions of different curvatures in MPF case, the curvature distribution was designed by assigning a target luminance value and optimized for a uniform output luminance. Then, according to the optimized curvature distribution, the optimized Patterned-MPF was designed.
In the simulation results of using optimized Patterned-MPFs, the relationship between the output luminance and the backlight thickness was found. The result showed that the optimized backlight thickness of 13.3 mm resulted in an output luminance close to the conventional backlight. Thus, we conclude Patterned-MPF can be applied to MPF-Plate case for a slim, invisible lamp-Mura backlight system. Moreover, screen printing process can also be used to fabricate Patterned-MPF by a patterned-mask with corresponding aperture ratio distribution.
Chapter 7
Conclusions and future work
To display a high quality image from LCD-TVs, direct-emitting backlight systems are used to provide a sufficiently bright and uniform light source. However, direct-emitting backlight systems result in a lamp-Mura issue which adversely yields period luminance patterns to degrade the display image quality of LCDs.
In this thesis work, we developed two optical films for suppressing lamp-Mura in a direct-emitting backlight system. The ray-tracing method was used to design proposed backlight systems. Moreover, the screen printing process was adopted to fabricate proposed optical films. Compared with a general imprint process using a mold, this process is simple, lower cost, and flexible in design for different applications. In addition, the fabricated optical films are possible to transfer into a mold for roll-to-roll mass production.
7.1 Multi-performance film for suppressing lamp-Mura
Compared with BEFs, multi-performance films (MPFs) had concave-parabolic prism structures on the top surface. The concave-parabolic prism structures offered the brightness enhancement and the light scattering functions simultaneously. Thus, MPF with the scattering function could improve lamp-Mura and increase luminance viewing angle. Moreover, the output light efficiency also could be enhanced.
In this thesis, two experiment backlight structures using MPFs, MPF and the MPF-Plate case, were designed and compared with the conventional backlight of invisible lamp-Mura.
Both cases can do without a diffusive film yet, results in similar functionality. In the simulation results, for MPF case, the optimized MPF curvature range from 57 to 81 mm-1 has smaller lamp-Mura contrast than the conventional backlight; for MPF-Plate case, the optimized value about 60 mm-1 is the closest to the conventional backlight. Thus, the curvature 60 mm-1 is the optimized value in both MPF and MPF-Plate case.
According to the fabrication results, the relationship between filling ratio and fitting curvature of MPFs were obtained. Additionally, the fabricated MPF with filling ratio 10% had the fitting curvature of 58 mm-1 closed to the optimized curvature. Thus, the MPF of filling ratio 10% is the optimized MPF which can suppress lamp-Mura effectively.
To verify whether lamp-Mura can be suppressed by the MPF with optimized curvature, the MPF of filling ratio 10% was then applied to MPF and MPF-Plate. In the lamp-Mura contrast result, the value of MPF case was 0.010 smaller than 0.012 (conventional backlight), but that of MPF-Plate case was 0.018 higher. However, according to real luminance images and contours, the MPF and MPF-Plate case with filling ratio 10% MPF can result in invisible lamp-Mura. Thus, the lamp-Mura contrast of 0.018 also can yield invisible lamp-Mura.
From the measurement results, MPF and MPF-Plate case enhanced luminance viewing angle to 90° and 116° FWHM and had an 8% and a 19% improvement in the output light efficiency, respectively. Additionally, the results revealed that the MPF-Plate case can enhance more output light efficiency than the MPF case. The experimental results of the three experiment backlight structures are summarized and compared in Table 7-1. Both MPF and MPF-Plate case with optimized filling ratio 10% MPF successfully achieved higher quality display image with suppressed lamp-Mura, and they also enhanced the luminance viewing angle and output light efficiency.
Table 7-1 Comparison of experimental results between the conventional backlight, MPF case, and MPF-Plate case.
7.2 Patterned multi-performance film for slim backlight systems
For achieving a slim backlight system, a slim backlight is necessary. When the backlight thickness was reduced, the lamp-Mura is more serious. Moreover, MPF has a limited capability to suppress lamp-Mura in a slim backlight system.
Accordingly, the Patterned-MPF which has variational curvature distribution is proposed to suppress lamp-Mura. Considering the uniformity at each position, the curvature distribution was optimized for a uniform output luminance. Then, the Patterned MPFs with the optimized curvature distribution was designed. In the MPF case using Patterned-MPFs, the relationship between the output luminance and the backlight thickness was found. This result showed that the optimized backlight thickness was about 13.3-mm which yielded output luminance closed to that of the conventional backlight.
In conclusion, Patterned-MPF was designed to reduce the backlight thickness and to suppress lamp-Mura simultaneously. Additionally, we conclude Patterned-MPF also can be applied to MPF-Plate case for a slim, compact, suppressed lamp-Mura backlight system.
7.3 Future work
Patterned-MPF also can be designed to further do without the bottom diffuser for lower backlight cost. To realize Patterned-MPFs in slim backlight systems, a patterned mask with corresponding aperture ratio distribution has to be designed and fabricated. In addition, compared with CCFLs, light-emitting diodes (LEDs) have some advantages such as low power consumption and small volume. Thus, LED backlight systems will replace CCFL ones and become the mainstream backlight product in the future. Because the LED backlight system has a hotspot issue, the light output distribution is non-uniform. We think MPF can be further extended to a 2-D concave paraboloid structure to apply to LED backlight systems for improving the backlight uniformity. One possible kind of 2-D MPF is concave-quadrangular pyramid, as shown in Fig. 7-1. The screen-printing process also can be utilized to fabricate the concave paraboloid structures of 2-D MPF. Thus, MPF is also a candidate to apply for next generation LED backlight systems of low cost and high optical performance.
Fig. 7-1 Concave-quadrangular pyramid structure of 2-D MPF.
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