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In this paper, we propose an integrated micro-optical light guide plate (MOLGP), of which the top surface is designed as aspheric semi-cylindrical micro-concentrator structure (ASCMCS) arrays and the bottom surface is fused with micro-prism arrays coated with a high-reflective film. And we also present the optimized structural parameters and distribution pattern of the MOLGP. By the simulation of the professional optical software Lighttools, it’s verified that the integrated MOLGP we proposed can achieve the functions of five complex-structure films in current typical backlight module (BLM), and the Five Parameters (light energy utilization efficiency, average illuminance and luminance, uniformity of illuminance and uniformity of luminance) in the BLM with integrated MOLGP are respectively 1.49, 1.40, 1.07, 0.91 and 0.97 times than those in the typical BLM. Obviously, the performance parameters of the MOLGP exceed the traditional design. Moreover, we design two sets of four-step masks of the ASCMCS by the graphical user interface (GUI). At last, we fabricate a 1.8 inch integrated MOLGP sample. Comparative experiments show that the Five Parameters of the fabricated MOLGP sample are respectively 1.43, 1.43, 0.97, 0.89 and 0.70 times than those of the typical BLM. The experimental results verify the feasibility of the concept of the integrated MOLGP proposed in this paper.
©2013 Optical Society of America
1. Introduction
The small and medium size liquid crystal displays (LCDs) have advantages of light-weight, thin-thickness, high light energy utilization efficiency and non-radiation, so they are widely applied in mobile phones, digital cameras, vehicle-bone and other applications [1]. A typical backlight module (BLM) consists of light sources, a flat reflective film (RF), a light guide plate (LGP), a diffusion film (DF) and double crossed bright enhancement films (BEFs) [2, 3]. As the light source of LCD, the typical BLM has some disadvantages, such as complexes of devices structures and manufacturing processes, difficulty to be integrated, etc [4]. Therefore, integration of the BLM becomes research focus among domestic and foreign research institutes and manufacturers. In October 2004, we first proposed the initial idea of the integrated LGP with a separate RF [5]. After that, from December 2004 to 2006, the nanotechnology research center of Tsinghua-Foxconn proposed an integrated LGP with regular-distribution micro-prism arrays fused on the top surface and orthogonal gradient micro-prism arrays distributed on the bottom surface [6–8]. But it hadn’t mentioned the performance parameters, such as light energy utilization efficiency, illuminance and luminance. In 2005, Hong Kong Polytechnic University proposed a new high-uniformity mobile phone LGP [9] with contour-gradient groove structures fused on the top surface and with ink dots distributed on the bottom surface, but it still needed an additional prism film. In 2006, Chao Heng Chien et al. proposed an integrated LGP [10–13] with regular-distribution micro-pyramids fused on the top surface and with gradient micro-prism arrays distributed on the bottom surface, but the illuminance was not high enough, and it didn’t report the values of luminance and luminance uniformity. In 2007, Korea Advanced Institute of Science and Technology proposed an integrated LGP with inverted trapezoidal micro-structures on its top surface [14], but the light energy utilization efficiency and luminance uniformity were relatively low. Therefore, research on the integrated LGP has important theoretical significance and application values.
Based on our research foundations on binary optics [15–17] and the initial idea of the integrated LGP by micro-optics and binary optics which we first brought forward in 2004 [5], an integrated micro-optical LGP (MOLGP) is proposed in this paper. Only one piece of the integrated MOLGP can achieve the functions of five complex-structure films in current typical BLM, including double crossed BEFs, a DF, a LGP and a flat RF. The top surface of the MOLGP is fused with aspheric semi-cylindrical micro-concentrator structure (ASCMCS) arrays, which are designed and optimized by iterative algorithm. And the bottom surface of the MOLGP is fused with micro-prism arrays which must be matched with the ASCMCS arrays on the top surface. By analyzing the effect of the two micro-prism base-angles on luminance and illuminance of the output light from the MOLGP, the optimized ranges of the micro-prism base-angles are achieved. The BLM model with the integrated MOLGP we proposed, called integrated BLM as follows, is simulated and verified by Lighttools optical software. And the results indicate that the Five Parameters (light energy utilization efficiency, average illuminance and luminance, uniformity of illuminance and uniformity of luminance) are respectively 1.49, 1.40, 1.07, 0.91 and 0.97 times than those in the typical BLM. Moreover, based on the graphical user interface (GUI) in Matlab software, the masks are automatically outputted and verified. And we fabricate two overlay masks of the ASCMCS arrays by use of lithography technology. At last, we fabricate a 1.8 inch integrated MOLGP sample. And by comparative experiments between the typical BLM and the fabricated integrated BLM, it’s verified that the integrated MOLGP concept we proposed is feasible.
2. Integrated MOLGP
Figure 1 shows the integrated BLM we proposed, which is fused with size-equal and close-distributed ASCMCS arrays on the top surface and with size-equal micro-prism arrays on the bottom surface. The micro-prism arrays are orthogonal to the ASCMCS ones and matched with the ASCMCS ones. And a high RF is coated on the surface of the micro-prism arrays. The following analysis shows that the integrated MOLGP can achieve the functions of five complex-structure films including double crossed BEFs, a DF, a LGP and a flat RF in the typical BLM.
2.1 Design of the ASCMCS
The main function of the double crossed BEFs in the typical BLM is to converge the output light in the vertical direction of the output face for increasing the luminance. Micro-concentrator structures are directly fused on the top surface of the integrated MOLGP to converge the lights, which are parallel to the x-z plane, to z-axis direction and reduce the use of one BEF. The lights, which are parallel to the y-z plane, are converged by the micro structures on the bottom surface of the integrated MOLGP, and another BEF can be reduced. The half cylinder (“Toroidal lens” in ZEMAX) is used as the initial single model. Then by the method of non-sequential ray tracing, a single micro-optical model can be iteratively optimized by using optical software ZEMAX. The profile curve expression of the ‘Toroidal lens’ is following
Where, z represents the height coordinate of the micro structure (ordinate), x represents the width coordinate of the micro structure (abscissa), H is the height in the middle of the micro structure, c is the curvature, k is the conical coefficient, and α1, α2, α3, α4, α5 and α6 are the coefficients of the function, respectively.In the model, any arbitrary polarized light is used as the light source, and the typical PMMA is used as the material of semi-cylindrical micro-structure. After the initial model has been built, the optimization goal is set to make the output beam converging within ± 25° in the vertical direction by ZEMAX, and the micro-optical structure has better converging function. By the method of iterative algorithm, a micro-structure unit is optimized as shown in Fig. 2, and the optimized parameters in Eq. (1), c, k, H and α1 to α6, are shown in Table 1. Where, the width W is 180 μm, and L is the length of micro-structure unit along y direction of the LGP, as shown in Fig. 2.
Fig. 2 Stereogram of the optimized micro-structure unit fused on the top surface of the integrated MOLGP.
Table 1. The coefficients of optimized ASCMCS
A single optimized unit called ASCMCS mentioned above is fused on the top surface of LGP, on whose bottom surface there is not any structure. Compared with a LGP without any structure, it’s found that ASCMCS can converge the output beam within ± 25° in the vertical direction, and its condensing-light capacity is 1.16 times than that of the LGP without any structures.
Compared with prism, the top of the ASCMCS is relatively smooth, so the light with small incident angle can be directly refracted out to the viewing area within ± 25°, which will improve the luminance. While, in the prism case, the light with small incident angle may be reflected back into the LGP after several times of total internal reflection or refracted outside the viewing area. The light paths are shown in Fig. 3. The profile curve of ASCMCS is shown in Eq. (1), controlled by the coefficients of c, k, α1, α2, α3, α4, α5 and α6. By adjusting these coefficients, arbitrary spherical or aspherical structure can be obtained. According to different structural parameters of integrated MOLGP, the coefficients of ASCMCS in Eq. (1) could be adjusted to make the design results of integrated MOLGP suitable for the performance requirements. So the ASCMCS is much better than other basic optical structures, such as prism, sphere, and so on.
2.2 Effect of the micro-prism base-angles of integrated MOLGP on luminance and illuminance
Not only the ASCMCS arrays on the top surface of the integrated MOLGP, but also the micro-prism arrays with specific base-angles on the bottom surface of the integrated MOLGP can improve the luminance of the integrated BLM, and they mach with each other and co-improve the luminance. The lights, which are parallel to the x-z plane, are mainly converged by the ASCMCS arrays, which can reduce the use of one BEF. And the lights, which are parallel to the y-z plane, are converged by the micro-prism arrays on the bottom surface of the integrated MOLGP, which can reduce the use of another BEF.
Based on the research of ASCMCS fused on the top surface of the integrated LGP, furthermore, micro-prism arrays are fused on the bottom surface of the same LGP. We investigate the relationships between the micro-prism base-angles and the luminance or illuminance of the output light. As examples, 1.8 and 2.8 inch integrated MOLGP are built in optical software Lighttools respectively, where the width of the micro-prism unit is 50 μm. The effect of the micro-prism base-angles α and β on luminance and illuminance of the integrated MOLGP is respectively investigated by optical software Lighttools. As shown in Figs. 4(a) and 4(b), convex and concave micro-prism arrays are respectively fused on the bottom surface of the LGP, α is the micro-prism base-angle near to LED and β is the micro-prism base-angle far to LED.
Fig. 4 Schematic of integrated MOLGPs when the bottom surfaces are fused with (a) convex micro-prism arrays or (b) concave micro-prism arrays.
After numerous simulations by Lighttools, it’s proved that the above two LGPs have similar characteristics in the effect of the micro-prism base-angles α and β on luminance or illuminance of the BLMs. Considering the luminance and illuminance of the integrated BLMs simultaneously, for the convex micro-prism, the better range of β is 30°~40°, and the better range of α is 20°~40°. When β is near to 85°, the better range of α is 35°~45°. However, for the concave micro-prism, the better range of α is 30°~40°, and the better range of β is 20°~40°. When α is near to 85°, the better range of β is 35°~45°.
2.3 Comparison of simulation results in typical BLM and integrated BLM
Based on the above studies, the models of 1.8 inch integrated BLM and typical BLM are both built up in Lighttools, and the structural parameters of these two BLMs are shown in Table 2. In order to improve the uniformity of luminance and the uniformity of illuminance in the BLMs, the dot optimization function in Lighttools is used in the typical BLM. While, in the integrated BLM, the structure and the distribution of the micro-structures fused on the bottom surface are used to replace the dots, the DF and one BEF in the typical BLM. In this paper, the micro-prism arrays, for example, are arranged from sparse to dense along the direction far away from the light source in the integrated BLM (The algorithm study of the distribution law of the micro structures will be deeply discussed in another special paper). In case to optimize the structures and distributions of both micro-prisms, it’s found that the simulation result of average luminance in the integrated BLM with the convex micro-prism arrays is relatively higher than that in the integrated BLM with the concave ones. So, for the ideal integrated MOLGP, the micro-structures on the bottom surface must be the convex micro-prism arrays, as shown in Table 2. The simulation results of illuminance, luminance and angular luminance of the output light in the integrated BLM and the typical one are shown in Fig. 5. Where, the vertical and horizontal coordinates indicate the structure direction x and y of the LGP that are corresponding to Fig. 1, and different colors represent the different values of luminance or illuminance.
Table 2. Structural parameters of integrated BLM and typical BLM
Fig. 5 Diagram of (a) illuminance, (c) spatial luminance and (e) angular luminance of the output light in typical BLM, of (b) illuminance, (d) spatial luminance and (f) angular luminance of the output light in integrated BLM.
The above simulation results are listed in Table 3, where the uniformity of luminance or the uniformity of illuminance equals to the ratio of the minimum and maximum measured by 9 point measurement method.
Table 3. Comparative simulation results with integrated BLM and typical BLM
From Table 3, it can be seen that uniformity of luminance and uniformity of illuminance in the integrated BLM we designed reach the industry standards, and the efficiency of light energy utilization, average illuminance and luminance are respectively 1.49, 1.40 and 1.07 times than those in the typical BLM, which indicates that the integrated MOLGP designed has good performance.
Compared with Figs. 5(e) and 5(f), it’s found that the viewing angular of the integrated BLM in one direction is relatively bigger than that of the typical BLM. This may be the reason that the coefficients of the optimized ASCMCS in Eq. (1) are not the best matched with the structural parameters of the 1.8 inch integrated BLM, as shown in Table 2.
2.4 GUI based on Matlab with automatic output and verification of the overlay mask data of the ASCMCS
The ASCMCS is so precise, that it can be approached by multi-step micro-relief structure by using the micro-optics and binary optics technology [18].
There are three main processes to design binary optical mask [19]. Firstly, according to the relationship between optical path and phase, the continuous phase function of the micro-relief structure is calculated. Secondly, the continuous phase is compressed and quantized [20] to achieve the phase distribution of 2N steps of the binary optical element. Finally, the abscissa data versus phase of 2N steps are calculated, and they are N sets mask data. In this paper, the overlay mask data of the micro-relief structures on the top surface of the integrated LGP have been designed based on the above processes.
Firstly, the micro-relief structure (z(x) in Eq. (1)) on the top surface of the integrated LGP is transformed into continuous phase function Φ(x) by using below equation
Where, x indicates the width coordinate (abscissa) in Fig. 2, and k, c, H, α1, α2, α3, α4, α5 and α6 are shown in Table 1. Wave number k0 is equal to 2π/λ0, where λ0 of 0.466 μm is the dominant wavelength of the light source LED in BLM. The refractive index of PMMA n is 1.49.Secondly, the continuous phase function is compressed and quantized by using the below equation, and the phase distribution ΦB(x) of 2N steps of the binary optical element is achieved.
Where, int represents to round down an integer, 2N is a quantified step number, and N is the number of the masks.Finally, considering the fabrication precision of our laboratory equipments, quantified step number is set to be 4, and the number of masks is two. The value of four quantified phase steps ΦB(x) is respectively 0, π/2, π, and 3π/2. The above values and Eq. (2) are all substituted into Eq. (3), and we get the x coordinate values corresponding to the phase steps which are two sets mask data.
By the method of above mask data extraction, the GUI extracting mask data is programmed by using Matlab software, as shown in Fig. 6.
We achieve phase quantization of an integrated multiple of 2π and 2N steps by using GUI in Fig. 6 and adjusting structural parameters of the micro structure on the top surface of the integrated LGP. N sets of mask data of the 2N steps are extracted, and the mask.txt file can be generated automatically. The GUI in Fig. 6 not only can verify whether the overlay mask data are correct, but also can determine the minimum line width of the extracted mask data.
According to mask data extracted by the GUI, we fabricate the overlay masks of ASCMCS arrays of 1.8 inch integrated micro-optical LGP. In addition, we detect the mask images by the confocal laser scanning microscope (OLS4000), as shown in Fig. 7. From Fig. 7(b), it can be seen that the minimum line width of the second mask is already less than 0.7 μm. Therefore, due to the limitations of current mask manufacture, there is a little bit fabrication error at the precise locations of the second set of the overlay masks.
Fig. 7 Partial schematic of (a) the first set and (b) the second set of the overlay masks of the ASCMCS fused on the top surface of the 1.8 inch integrated LGP.
3. Fabrication of the integrated MOLGP and analysis of experimental results
The ASCMCS is so precise, that it can be approached by multi-step micro-relief structure by micro-optics and binary optics technology [18], and also can be fabricated by modern ultra-precision machining technology such as diamond ultra-precision cutting tool technology.
Considering current fabrication technology, the integrated MOLGP model in Table 2 is modified.
3.1 Modification of integrated MOLGP model
As a product, the integrated MOLGP with the concave micro-prism arrays on the bottom surface has many advantages, like no easiness to be scratched in assembly process and easiness to be fabricated. So, in this paper, the concave micro-prism arrays are fused on the bottom surface of the modified integrated MOLGP.
Simultaneously, according to the current processing conditions, there are other five structural parameters in the integrated MOLGP to be modified. Firstly, the base-angles of the micro-prism arrays on the bottom surface of the integrated MOLGP are changed to be α = β = 45°. Secondly, the micro-prism arrays are just simply partitioned distributed. Thirdly, the thickness of the integrated MOLGP is modified to be 4mm. Fourthly, the distribution of the ASCMCS arrays on the top surface of the integrated MOLGP is modified to be connected with 20 µm-radius rounded corners. Fifthly, there are no structures on the input-light surface of MOLGP.
So, there are six structural parameters of 1.8 inch integrated MOLGP to be modified, as shown in Table 4. The simulation results of the Five Parameters in the modified integrated BLM are shown in Table 5.
Table 4. Modified structural parameters of 1.8 inch integrated MOLGP
Table 5. Comparative simulation results with modified integrated BLM and typical BLM
Compared with the values in Table 3 and Table 5, it can be seen that luminance uniformity, illuminance uniformity and average luminance in the integrated BLM are descended after the structural parameters are modified.
Firstly, from the simulation results in Table 3 and Table 5, it’s found that the luminance uniformity in the integrated BLM (92.00%) is higher than that in the modified integrated BLM (86.13%), and the illuminance uniformity in the integrated BLM (86.84%) is higher than the one in the modified integrated BLM (57.73%). It’s mainly because that the micro-prism arrays on bottom surface of the modified integrated MOLGP are just simply partitioned distributed. So, the distribution of the micro-prism arrays is not the optimal one.
Secondly, the simulation results in Table 3 and Table 5 show that the average luminance in the integrated BLM (10014nit) is larger than that in the modified integrated BLM (6432.5nit). There are mainly two reasons. One is that the micro-prism arrays on the bottom surface of the modified integrated MOLGP are concave, but not convex. The other is that the base-angles of the micro-prism arrays are both 45°, which are not the optimal ones.
3.2 Experimental results
Then, we fabricate a 1.8 inch integrated MOLGP sample by use of precision machining technology, as shown in Fig. 8(a). By the confocal laser scanning microscope (OLS4000), 3-D diagrams of the top and bottom surface topography parameters of the sample are shown in Figs. 8(b) and 8(c). As for current fabrication technology, the micro-structures on the top and bottom surfaces of the integrated MOLGP sample basically coincide with the design as shown in Table 4.
Fig. 8 3-D diagram of 1.8 inch integrated MOLGP sample. (a) top view of 1.8 inch integrated one, 3-D diagram (b) of the ASCMCS arrays on the top surface of the integrated one, and (c) of concave micro-prism arrays on the bottom surface of the integrated one.
The comparative experimental results are the best judgments for the feasibility of the integrated MOLGP we proposed. Under the same experiment conditions, comparative experiments are done between our integrated BLM and the typical BLM by 9 point measure method. In order to ensure that the experiments are done in the same experiment conditions, an actual independent aluminum reflective film of 93.5% reflectivity is closely placed under the fabricated integrated MOLGP, which is same to that in the typical BLM. The experiment process is shown in Fig. 9. In Fig. 9(a), it’s shown that the light from our integrated BLM is brighter than that from the typical BLM. The experimental results of the Five Parameters in above two BLMs are listed in Table 6.
Fig. 9 (a) comparative experiment diagram of typical BLM (below) and integrated BLM (up), (b) luminance testing diagram.
Table 6. Experimental results of fabricated integrated BLM and typical BLM in 9 point measure method
From Table 6, it can be seen that the efficiency of light energy utilization and average illuminance in the fabricated integrated BLM are better than those in the typical BLM. However, compared with the performance parameters in Table 5 and Table 6, it’s found that the uniformity of luminance decreases in the fabricated integrated BLM.
3.3 Analysis of experimental results
For exploring the reason why the uniformity of luminance decreases in the experimental result of the fabricated integrated BLM, we establish and simulate the model of the modified integrated BLM with a separate RF. The structural parameters in the modified integrated BLM with a separate RF are same to those in the fabricated integrated BLM, and the simulation data are listed in the last column of Table 7. For comparative convenience, relative simulation data are arranged in columns 2, 3 and 4 of Table 7, which are respectively from Table 3 and Table 5.
Table 7. Comparative simulation results of different types of BLMs
From the simulation results in Table 7, it’s found that the luminance uniformity in the modified integrated BLM (86.13%) is higher than that in the modified integrated BLM with a separate RF (57.62%). Therefore, when the structure of the integrated BLM is set up with a separate RF, its luminance uniformity should be reduced in theoretically. So, the simulation result explains the reason why the experimental result of luminance uniformity decreases in the fabricated integrated BLM.
By further analysis in physics, there are mainly two reasons of the reduction in the uniformity of luminance. Firstly, an independent RF is placed under the fabricated integrated MOLGP, instead that the RF is coated on the surface of micro-prism arrays fused on the bottom surface of fabricated integrated MOLGP. But the distribution of the micro-prism arrays of the fabricated integrated MOLGP is designed in the case that the RF is coated on the surface of micro-prism arrays. Secondly, only one sample is fabricated, so the fabrication error is relatively larger.
Combined with the analysis of the reduction in the simulation results (luminance uniformity, illuminance uniformity and average luminance) from the integrated BLM to the modified integrated BLM in Section 3.1, we are further exploring the distribution and the base-angles of the micro-prism arrays which are not only suitable for current fabrication technology but also can reach high uniformity of luminance, uniformity of illuminance and luminance, what is more, the RF is coated on the bottom surface of the integrated MOLGP. Although the performances of the fabricated integrated BLM in this paper cannot completely exceed those in the typical BLM, but the experimental results show that the idea of the integrated MOLGP proposed in this paper is feasible.
4. Conclusion
In this paper, we presented and fabricated an integrated MOLGP with the ASCMCS arrays fused on the top surface, and with micro-prism arrays on the bottom surface, which can realize the functions of five complex films in the type BLM, such as double crossed bright enhancement films, a diffusion film, a LGP and a flat reflective film.
We have optimized the range of the micro-prism base-angles. Simulation results show that the Five Parameters (the efficiency of light energy utilization, average illuminance, average luminance, and uniformity of illuminance and uniformity of luminance) of the integrated BLM are respectively 1.49, 1.40, 1.07, 0.91 and 0.97 times than those in the typical BLM, which exceed the traditional design. The GUI which can extract and verify automatically the overlay mask data of the ASCMCS were set up and two sets of four-step masks have been fabricated. A 1.8 inch integrated MOLGP sample was fabricated. Comparative experimental results show that the Five Parameters of the integrated BLM are 1.43, 1.43, 0.97, 0.89 and 0.70 times than those in the typical BLM.
Since the limit of the fabrication technology, some of the performances of the fabricated integrated BLM cannot exceed the typical ones. But the experimental results show that the idea of the integrated MOLGP proposed in this paper is feasible. With the improvement of the fabrication technology and the optimization of design method, we will develop a BLM with integrated MOLGP, whose all performances are better than those in the typical BLM in the near future.
Acknowledgments
This work is supported by National Natural Science Foundation of China (No. 61275167, No. 61108053 and No. 60878036), by Foundation for Distinguished Young Talents in Higher Education of Guangdong, China (No. LYM11109), and by Science and Technology Development Funds of Shenzhen, China (No. Nankeyuan 2008008, No. JC200903120023A, No. JC201005280533A and No. JCYJ20120613174700014).
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