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In this paper, we proposed a novel optical film ‘Collimation Film with Equivalent Focal Reflective Aperture’ (CFEFRA) that can collocate with the proper light guide plate (LGP) to provide a highly collimated planar light source (HCPLS) that not only has high intensity peak but also good uniformity. The CFEFRA has micro-cylindrical lenses and inverted-prism-like micro-teeth on its upper surface and lower surface, respectively. The lower micro-teeth that are aligned with the upper lenses can deflect the light emitting from the LGP at large declination into the normal, and then the deflected light with transverse fan-out is further converged by the upper lens to become the collimated light. The exact alignment between the upper and lower micro-structures is fulfilled by the method ‘auto-secure-alignment by focusing of a collimated exposure beam’. The vertical and horizontal full width at half maximum (FWHM) of angular intensity of the HCPLS for the optimal case are of 14 and 6 degrees, respectively. Moreover, it still has high optical efficiency with light output of over 90% despite the resultant emitting light is such collimated. Most importantly, CFEFRA just needs to collocate with a relatively low-cost and easily-manufactured LGP so the HCPLS adopting the CFEFRA can be extended for large-sized application. Both optical model and experimental samples are demonstrated in this paper, and the simulation results are consistent with the experimental results. The consistency proves our design concept and optical model are convincible and feasible.
© 2013 Optical Society of America
1. Introduction
All the time how to produce a highly collimated planar light source (HCPLS) with uniform distribution and slim volume has been an important research topic. For coherent light source, HCPLS can be used in metrology and diffraction elements; for incoherent light source, it can be used in illumination, projection, and display backlighting especially. As industry of liquid crystal display (LCD) is booming, the LCD dominates for all-size products. In the LCD, LC works as a light valve to control transmission of the light emitting from the light source set behind, called ‘backlight’. As energy-saving has been paid much more attention, HCPLS exhibits more merits in LCD applications. In addition to concentrate light in the normal direction toward observers to avoid energy waste, HCPLS aids achieving some advanced backlight functions such as polarized lighting, color separation (color-filter-less) and local diming, which can further increase optical efficiency greatly [1–4]. Furthermore, HCPLS can also contribute in high-contrast LCD, 3D display and blue-phase LC application [5–8]. Therefore, HCPLS has also become an important research topic for backlight.
Backlight can be divided into two types, direct-lit and edge-lit, according to arrangement of initial light source that is usually composed of light-emitting-diodes (LEDs) in backlight module. In the module, the initial light from LEDs experiences a series of processes such as diffusing, mixing, circulation and split to form a resultant uniform-distribution planar light source. In those processes, the emitting light of the resultant planar light source can be concentrated in the normal by various methods. The common-used methods can be classified as two groups. The first group utilizes the optical film to concentrate the emitting light into the normal, which is feasible for both direct-lit and edge-lit types. The second group utilizes the special-designed light guide plate (LGP) that can be used alone or collocate with the proper optical film to achieve the goal, which is feasible only for the edge-lit type. Because the angular distributions on the two mutually-orthogonal planes perpendicular to the light-emitting surface are usually used to evaluate how concentrated (or collimated) the emitting light is, for convenience, we define the angular distribution on the ‘vertical plane’ perpendicular to both the light bar and light-emitting surface as ‘vertical angular distribution’, and that on the ‘horizontal plane’ orthogonal to vertical plane as ‘horizontal angular distribution’ in this paper. After defined both vertical and horizontal angular distribution, we begin to discuss the methods of concentrating emitting light proposed in literature.
The optical films used in the first group include the light-refracted micro-structure film (simply called ‘LRMSF’ hereafter) that utilizes light circulation of refraction and total internal reflection (TIR) by its micro-structures to concentrate emitting light; collimation lens film (simply called ‘CLF’ hereafter) that has microlenses on the upper layer and the corresponding focal apertures on the lower reflective layer; multilayer-integrated optical film (simply called ‘MIOF’ hereafter) [9–11]. Types of micro-structures of the LRMSF include the prism (e.g. BEF, 3M Ltd. Corp.), microlens, pyramid, and hemi-cylinder (lenticular lens). The LRMSF can concentrate emitting light in various degrees depending on its design parameters, and stacking multiple LRMSFs together can further concentrate emitting light. However, because light will be partially absorbed during light circulation, the emitting light is more concentrated but might reduce as the LRMSF increase more. For some case of higher concentrated emitting light, the resultant head-on luminance (or intensity peak) decreases instead. Therefore, the full-width-at-half-maximum (FWHM) of the angular intensity distribution of emitting light for the practical case of using the LRMSF is about 40 degrees. As for the CLF, it can provide FWHM of 20 degrees or less by reducing the focal aperture size on the reflective layer. Similarly, reducing aperture size increases probability of light reflected on the reflective layer, which inevitably results in energy loss due to absorption. Therefore, FWHM of the angular intensity distribution of emitting light for the practical case of using the CLF is about 15 to 20 degrees. However, the reflective layer is usually made of silver that tends to absorb more light of short wavelength to make emitting light yellowish so the CLF is of low feasibility. The MIOF is to integrate the stack of multiple LRMSFs into one film by gluing them, which converges emitting light by the mechanism same as the LRMSF. In the MIOF, the gap between each LRMSF is fulfilled by gaps or low-refractive-index material. The performance of the MIOF on light-concentrating is lower than the stacked multiple LRMSFs due to the joints between layers.
The designs in the second group can be further classified as two types: LGP alone and LGP collocating with the proper optical film. The designs of the first type further include five cases. The first case is a LGP with inverted-cone micro-structures that are directly formed by a flexible mold made of PDMS [12]. The inverted-cone micro-structure can deflect the light propagating in the LGP to emit out about the normal, and FWHM of the angular intensity of emitting light is about 25 degrees both in the vertical and horizontal. The second case is a LGP with prism-like-tooth micro-structures thereunder that have a sunken major facet with a slope of 47 degrees and a protruding minor facet with a slope of 20 degrees prior to the major facet [13]. In the case of a single LED light source, FWHM of the angular intensity of emitting light is about 23 and 38 degrees in the vertical and horizontal, respectively. The third case is a LGP with diffractive micro-structures (grating) thereon that diffract the light propagating in the LGP to emit out about the normal. For example, a LGP has many grating-dots thereon, and each grating-dot has one dimensional gratings distributed in its spot area [14]. Both modulated depth and direction of the gratings in each grating-dot is fine-tuned in order to obtain the optimal collimated and uniform emitting light. In the case of a single LED light source, FWHM of the angular intensity of emitting light is about 8 and 20 degrees in the vertical and horizontal, respectively, but uniformity is only 62%. The similar method was also claimed in the patent proposed by Nanogate [15]. It uses UV imprint to directly form grating dots on LGP, and FWHM of the angular intensity of emitting light is about 18 degrees in the vertical. The above three cases all have difficulty in obtaining a highly precise mold. In addition, angular dispersion caused by diffraction is a fatal drawback. The fourth case is a LGP with an extra optical component to pre-collimate the light source before the light entering the LGP, which can obtain the better collimated emitting light [16]. However, it needs larger space to accommodate the optical component if we want to get much more collimated emitting light, which degrades its practicability. The fifth case is a LGP with multi-layer, and each layer is made of different material. This concept was claimed in IBM patent [17]. In the embodiment, the upper is a wedge layer; the lower is a reflective layer with V-grooved micro-structures; the middle is a transparent layer with lower refractive index. FWHM of the angular intensity of light emitting from the LGP is below 10 degrees in the vertical. For better light convergence in the horizontal and uniformity, a modified design was also proposed, and FWHM of the angular intensity of emitting light is about 34 degrees in the horizontal [18]. The major drawback of such design is hard to find the matched material of low refractive index.
As for the designs of the second type in the second group: a LGP collocating with the proper optical film, the most successful and well-known case is proposed by Mitsubishi Rayon [19]. It is mainly composed of a well-designed LGP that can emit highly-concentrated light at a large angle departing from the normal and an optical film with micro inverted-prisms thereunder (simply called ‘IPF’ hereafter) that can directly deflect the concentrated light emitting from the LGP into the normal by TIR. Because of much less light circulation, the light absorption can reduce greatly. Therefore, the case of using the IPF can provide more 30~40% head-on luminance as compared with crossed-stacked BEFs, and FWHM of the angular intensity of emitting light is below 18 degrees in the vertical [20,21]. The LGP collocating with the IPF usually has micro-structures on its two surfaces: the micro-structure with little slope and tiny slope variation on one surface; the micro-structures longitudinally extending on the opposite surface. The former can emit highly-concentrated light at a large angle departing from the normal in the vertical; the emitting light concentrates more as the slope variation decreases. The later can make emitting light converge in the horizontal. Generally, the convergence of the emitting light in the vertical depends on the IPF and the micro-structure with tiny slope variation, and the convergence in the horizontal depends on the longitudinally extending micro-structure. The common-used longitudinally extending micro-structures include V-groove and hemi-cylinder, but the related revised designs were also proposed [22,23]. Although IPF has good performance on both FWHM of the angular intensity and head-on luminance, it still needs a well-designed LGP to collocate with. However, these LGPs are currently fabricated through complex processes such that it is high-cost and cannot be applied to large-sized product.
After review the above literatures, we know there were many proposed designs of the HCPLS, and some of them have good performance. However, the above-mentioned designs all have their own limit, and some of them lack practicability, especially for large-sized application. Generally, the design of a LGP alone encounters difficulty in high manufacture cost due to complex micro-structures and surface defect. Thus, the design is limited only for small-sized application or even impracticable. Based on the above reasons, a well-designed LGP collocating with the proper optical film is a reasonable and practical solution for the HCPLS. Therefore, how to design an optical film that can collocate with a relatively easily-manufactured LGP to generate a HCPLS is our study target. The HCPLS in this study should have good uniformity, lower cost, ability to partially hide surface defects, and further potential for large-sized application. In this paper, we proposed a novel optical film ‘collimation film with equivalent focal reflective aperture’ (simply called ‘CFEFRA’ hereafter), which has micro-structures on its both surfaces and can generate a uniform HCPLS when it is put on the well-designed but relatively easily-manufactured LGP. Such HCPLS can provide collimated light emitting with FWHM of the angular intensity below 18 degrees both in the vertical and horizontal. Moreover, it also reveals potential for large-sized application because of easy manufacture of the LGP used for CFEFRA. Finally, both principle model and experimental measurement are also demonstrated.
2. Principle model and simulation analysis
2.1 Design concept for the micro-structure on the CFEFRA
The CFEFRA proposed in this study has micro-structures on its both surfaces, which can highly collimate the light emitting from the LGP and partially hide surface defects. The micro-structures on the lower surface are inverted-prism-like teeth that can deflect the concentrated light emitting from the LGP at large declination into the normal similar to the IPF. The micro-structures on the upper surface are lenticular lenses that can converge light in the horizontal and partially hide surface defects. The lower micro-structure is aligned to the upper so that the lower micro-structure works as an equivalent focal reflective aperture (simply called ‘EFRA’ hereafter); i.e. a line of teeth is aligned to a longitudinally-extending lenticular lens. Figure 1 and Fig. 2 show the scheme of the CFEFRA and its single unit cell of the micro-structures, respectively. The most important is to secure the alignment between the upper and lower micro-structures, which determines if the CFEFRA actually works. Therefore, we adopt a method ‘auto-secure-alignment by focusing of the collimated exposure beam’ described as later. We first finish the upper micro-structure of the CFEFRA and then exposure the UV resin filled in the mold of prism arrays to a collimated UV beam vertically through the finished upper micro-structures. In the meantime, the extending direction of the prism arrays in the mold is orthogonal to the extending direction of the lenticular lenses. The exposed portion of the resin is cross-linked to form into arrays of inverted-prism-like teeth ‘EFRAs’ after the unexposed resin is removed, and thus each line of teeth is definitely aligned to a longitudinally-extending lenticular lens. Because the width of the EFRA is determined by the UV beam focused by the upper lenticular lens, the EFRA has taper side facets. Thus, the width of its local region becomes narrower as the local region approaches closer to the focal plane of the lenticular lens. If the concentrated emitting light from the LGP at a large angle departing from the normal is incident on the portion of the EFRA close to the focal plane, the light not only is deflected into the normal by TIR on the second back facet of the EFRA, but also converged in the horizontal by the upper lenticular lenses, as shown in Fig. 3. Because the reflected light from the portion of the EFRA close to the focal plane can be approximated as the light emitting from the focal point of the upper lenticular lens, the reflected light can be highly collimated in the horizontal by the lenticular lens. According to the above mechanism, there are three main impacts on convergence of the emitting light in the horizontal described as follows. The first is local width of the EFRA region in which the light is deflected by TIR; the narrower the width of the EFRA region is and the more collimated the resultant emitting light is. The second is the position of light-deflecting region in the EFRA; the resultant emitting light diverges as the light-deflecting region of the EFRA departs further from the focal plane. The third is the slope of the left and right facets of the EFRA. The left and right facets of the EFRA should have a proper slope to keep the deflected light from incidence on the both facets before the deflected light reaches the upper lenticular lens; incidence on the both facets degrades collimation of the resultant emitting light. As for the convergence of the emitting light in the vertical, it mostly depends on the micro-structures with tiny slope variation on the LGP surface because the EFRA just deflects the light emitting form the LGP into the normal. In addition, the emitting light can be converged more if the slopes of the front and back facets of the EFRA are further optimized. Therefore, if we want to generate a highly collimated light source by the CFEFRA, the LGP collocating with the CFEFRA must have the optical characteristic similar to the LGP used for the IPF so that it can emit highly-concentrated light at a large angle departing from the normal in the vertical. Differently, the LGP used for the CFEFRA does not need longitudinal microstructures to converge the emitting light in the horizontal so the LGP can be manufactured at lower cost and applied for the large-sized products.
2.2 Simulation and analysis
In this study, we select the proper geometric parameters for the CFEFRA to be analyzed in order to obtain the optimal. First, we select a lenticular lens with 44 um radius and 22 um height as the upper micro-structure of the CFEFRA. Then, the proper ranges of the parameters about the substrate thickness and EFRA are determined. For simplicity, the refractive index of the CFEFRA material is assumed as 1.5. The LGP used in this study to collocate with the CFEFRA has the optical characteristics similar to the LGP used for the IPF to emit highly-concentrated light from the surface at a large angle departing from the normal in the vertical. In a different way, we utilize laser to directly carve the very shallow micro-concavities with tiny slope variation on the LGP surface. We call this kind of the LGP as ‘digital laser-blastering LGP’ (DLB LGP) [24]. In addition, because the CFEFRA can effectively converge the emitting in the horizontal, the DLB LGP used for CFEFRA does not need the longitudinally extending micro-structure. Therefore, the DLB LGP is relatively easy to be manufactured and extended for large-sized applications. In order to obtain highly precise simulation, we use the practically-measured ray data as the emitting light from the DLB LGP in simulation. For simplicity, the total flux of the light source is assumed as 1 lumen, and dimension of the LGP is assumed as 70 mm X40 mm. All the simulation in this study is implemented by the commercial ray-tracing software ‘LightTools’.
First, we analyze the CFEFRA with the substrate of various thicknesses. Since the upper micro-structure of the CFEFRA and its focal plane is kept unchanged, the height of the EFRA must decrease as the increase in the substrate thickness, which ensures the concentrated light emitting from the LGP incident on the region of the EFRA close to the focal plane. The slopes of the left and right facets of the EFRA are determined by exposure condition, and the angle between the facet and substrate (simply called ‘base angle’ hereafter) is about 72 degrees according to our preliminary analysis. Moreover, the base width is depends on both exposure condition and substrate thickness. The optimal angle between the front and back facets (simply called ‘vertex angle’ hereafter) depends on the condition of the light emitting from the LGP and is set as 70 degrees in this study. The related geometric parameters of CFEFRA can refer to Fig. 2. The substrate thicknesses for analysis include 25, 31, 37.5, 44, 50, 62.5, and 75 um; the other related parameters are detailed in Table 1; the related simulation results are shown in Fig. 4.
Table 1. Detailed Geometrical Parameters of CFEFRA for Analysis
Fig. 4 Simulation results of light emitting through the CFEFRA. (a) intensity of light emitting from the LGP; (b) for case A; (c) for case B; (d) for case C; (e) for case D; (f) for case E; (g) for case F; (h) for case G.
From the simulation results shown in Fig. 4, we can find some phenomena as follow: first, although the initial light emitting from the LGP is only concentrated at a large angle departing from the normal in the vertical but divergent in the horizontal, the resultant emitting light is deflected into the normal and highly concentrated both in the vertical and horizontal when the CFEFRA is put on the LGP. Second, the intensity peak reaches maximum when the substrate thickness is about 37.5 um. Third, except the case G, FWHM of angular intensity of the resultant emitting light in the horizontal decreases as the substrate thickness increases, but FWHM in the vertical almost keeps the same. Fourth, the side lobe in angular distribution of intensity becomes more obvious as FWHM in the horizontal decreases, and the peak intensity in the normal does not proportionally increases as FWHM in the horizontal further decreases. The main reason for the above phenomena can be explained as follow: FWHM in the horizontal decreases as the width of the EFRA decreases when the substrate thickness increases. However, considering the angular distribution of the light emitting from the LGP, although the narrower width of the EFRA benefits reducing FWHM in the horizontal, the peak intensity trends down because more light emitting from the LGP bypasses the EFRA instead of being deflected into the normal by the EFRA. Therefore, it is adverse to optical efficiency when the width of the EFRA is excessively narrow. In addition, part of the light bypassing the EFRA also forms the side lobe in angular distribution of intensity.
Next, we further implement analysis based on the prior simulation results. In this study, we not only obtain more collimated resultant emitting light with smaller FWHM but also maximize peak intensity of the resultant emitting light in the normal as possible. In Fig. 4, we can find case C (with substrate of 37.5 um thickness) and case E (with substrate of 50 um thickness) have higher peak intensity and collimation, respectively. Therefore, we further optimize the height of the EFRA for the case C and case E, respectively. The EFRA height for analysis includes 0.9, 0.95, 1, 1.05, 1.1, and 1.15 times the original value in Table 1 (are designated as I, II, III, IV, V, and VI, respectively); the other related parameters are detailed in Table 2; the related simulation results are shown in Fig. 5.
Table 2. Detailed Geometrical Parameters of CFEFRA for Further Analysis
From the simulation results shown in Fig. 5, we can find some phenomena as follow: for both case C and case E, FWHM of angular intensity of the resultant emitting light in the horizontal decreases as the height of the EFRA increases, and the peak intensity also increases as FWHM in the horizontal decreases until it gradually reaches saturation. For case C, the case with 1.15 times the original height of the EFRA has the maximum peak intensity of 1.91 and FWHM of 9 degrees in the horizontal. For case E, the case with 1.15 times the original height of the EFRA has much narrower FWHM of 6.13 degrees in the horizontal and still keeps comparable peak intensity of 1.83. The main reason for the above phenomena can be explained as follow: the region of the EFRA on which the maximum density of the light emitting from the LGP is incident shifts downward when the height of the EFRA increases, and thus width of the EFRA in that region decreases such that FWHM further decreases in the horizontal. Moreover, the peak intensity also rises up as FWHM decreases in the horizontal. It should be noted that the height of the EFRA is limited by its base angle. Theoretically, the cross section of the EFRA on y-z plane is generally an isosceles trapezium but becomes an isosceles triangle as the height reaches maximum.
Furthermore, in order to evaluate the effect of the LGP collocating with the CFEFRA on angular distribution of resultant emitting light, we use another five LGPs to collocate with the CFEFRA in simulation, and each of them has different optical characteristic due to its micro-structure characteristic. Three of them are DLB LGPs, one is made by inject-molding, and one is processed by screen-printing. The three DLB LGPs has different slope variation and depth of the micro-structure, and the inject-molding LGP has spherical microstructures whose facet slope variation is larger than the three DLB LGPs. In general, vertical angular distribution of the light emitting from the LGP increases as variation of facet slope of its micro-structure increases. Therefore, the three DLB LGPs has more concentrated emitting light than the other two LGPs, as shown in Fig. 6. In Fig. 6, DLB 1 is the original LGP; DLB 2~4 is another three LGPs. The measurement of emitting light of the five LGPs is also input as light source in the simulation, and parameters of the CFEFRA are as case E in Table 1. The simulation results for the LGPs collocating with the CFEFRA are shown in Fig. 7. In Fig. 7, we can find vertical angular distribution of resultant emitting light strongly depends on that of light emitting from the LGP, and peak intensity decreases as divergence in vertical angular distribution. It confirms that the LGP collocating with the CFEFRA must have the optical characteristic similar to the LGP used for the IPF. In addition, we can also find convergence in horizontal angular distribution even though the light emitting form the LGP is not suitable for the CFEFRA.
Fig. 7 Simulation results of light emitting through the CFEFRA for different LGPs. (a) DLB1; (b) DLB2; (c) DLB3; (d) DLB4; (e) inject-molding; (f) screen-printing.
Finally, we further check optical efficiency of the HCPLS adopting the CFEFRA. In general, HCPLS must have not only highly collimated light emission, but also high energy efficiency. In the simulation, we have taken account of the major energy loss due to absorption of the bulk material and white reflector (reflectivity of 0.98) underneath the LGP; the simulation results show over 90% energy can output through the CFEFRA.
2.3 Manufacture of CFERA sample
In order to verify the optical model and its simulation results, we made some samples for the practical test and compared the experimental results with the simulation results. The manufacture steps are described in turn as follow:
- 1. Use a mold with micro-structure arrays of lenticular lenses and UV resin to form the micro lenticular lenses on the first surface of a substrate made of Polyethylene terephthalate (PET); then lift the mold after the UV resin is cross-linked by UV exposure.
- 2. Use a mold with micro-structure arrays of prisms and UV resin to form the micro prisms on the second surface (opposite to the first surface) of the substrate; the extending direction of the prism is orthogonal to that of the lenticular lens. Then, use a collimated UV beam to irradiate the UV resin filled in the mold through the lenticular lenses on the substrate, which makes only the part of the resin right under the lenses exposed to be cross-linked due to focusing of the UV beam, the rest unexposed. Finally, lift the mold.
- 3. Immerse the sample in isopropyl alcohol (IPA) to dissolve the uncross-linked (unexposed) resin.
- 4. Use deionized (DI) water to rinse the sample and then dry it. The sample of the CFEFRA is completed.
In this paper, with considering the simulation results and availability of mold fabrication, we select the related geometrical parameters for the experimental sample of the CFEFRA as follows: radius of the lenticular lens of 44 um, sag of the lenticular lens of 23 um, height of the EFRA of 38 um, vertex angle of 70 degrees, PET substrate of 50 um. Because the shape of the EFRA is affected by the dosage of UV exposure, we use three different dosages to fabricate the CFEFRA samples separately.
3. Experimental results and discussion
In order to verify our model, we fabricated some CFEFRA samples and put them in an edge-lit LED backlight module for experiment. The module contains a 2 um thick LGP with very shallow micro-concavities thereon (i.e. DLB LGP). We used the Conoscope (AUTRONIC-MELCHERS GmbH, German) to measure angular luminance and then transferred the data into normalized angular intensity for easy comparison. The dimensions of the CFEFRA samples are 40 mm X 40 mm; the cross-section photography by optical microscope and related geometric parameters are shown in Fig. 8 and written in Table 3, respectively. In Fig. 8, we can find the alignment between the upper and lower micro-structures are secured well, and the method ‘auto-secure-alignment by focusing of the collimated exposure beam’ is successful. In addition, when the exposure dosage decreases, the base width of the EFRA is less, but the base angle of the EFRA becomes larger.
Fig. 8 Photographs of cross-section of the CFEFRA samples: (a) sample I; (b) sample II; (c) sample III.
Table 3. Parameters of Experimental Samples of the CFEFRA
Next, the practical optical measurement was implemented. Before the CFEFRA was put on the LGP, we first measured the angular luminance of the backlight module on which had no optical film. The angular intensity of the emitting light is the same as Fig. 4a; the emitting light is highly concentrated at angle of 78 degrees departing from the normal in the vertical and with vertical FWHM of 19 degrees. On the contrary, the emitting light is divergent with FWHM of 69 degrees in the horizontal, which can just test the converging effect of the CFEFRA in the horizontal (i.e. azimuthal). Then we put the CFEFRA samples on the backlight module and measured its angular luminance in turn; the measurement results were transferred into normalized angular intensity and compared with the corresponding simulation in the vertical and horizontal direction. The parameters set for each simulation are the same as the corresponding experimental sample. The related results are as shown in Fig. 9. In Fig. 9, we can find the measurement results are substantially consistent with the corresponding simulation results, but they have a little difference from the simulation case: the side lobe disappears in the experimental results. I think the left and right facets of the EFRA are not such smooth that some emitting light is diffused to blur the side lobe. It should be noted that both simulation and measurement results in Fig. 9 are different from the optimal cases in Fig. 5 due to the difference in geometric parameters between the optimal cases and the samples. The major difference in geometric parameters is the base angle due to exposure condition; the next difference is the base width. They both widen FWHM of the resultant emitting light in the horizontal and reduce the intensity peak.
Fig. 9 Comparison between the experimental sample and corresponding simulation for angular intensity: (a) sample I; (b) sample II; (c) sample III.
In this study, the measurement results are substantially consistent with the simulation so the proposed design concept and optical model are feasible and have been verified. Therefore, the further analysis based on the same optical model in this paper is also convincible, which is beneficial to further optimize our design in the future.
4. Conclusion
In this paper, we proposed a novel optical film ‘CFEFRA’ that can collocate with the proper LGP (e.g. DLB LGP) to provide a highly collimated planar light source. The exact alignment between the upper and lower micro-structures of the CFEFRA was fulfilled by the method ‘auto-secure-alignment by focusing of the collimated exposure beam’. The HCPLS adopting the CFEFRA has the advantages as follows: first, the emitting light of the HCPLS can be collimated in both the vertical and horizontal, so it can provide very high intensity peak (or head-on luminance). Moreover, the HCPLS still has high optical efficiency with light output of over 90% despite the resultant emitting light is such collimated. Second, the CFEFRA just need to collocate with a relatively easily-manufactured LGP such as DLB LGP so it is low-cost and can be extended for large-sized applications. Third, for the backlight of LCD, HCPLS can greatly increase the contrast of LCD except reducing energy waste in the angular range departing from the observer’s view. Moreover, the CFEFRA can partially hide surface defects on the LGP to mitigate the impact of the surface defects due to its upper cylindrical micro-structures, which makes the HCPLS feasible for backlight. Finally, FWHM of the emitting light can also be flexibly adjusted by changing the geometrical parameters of the CFEFRA to satisfy the practical requirements of various cases.
Acknowledgments
This study was sponsored by National Science Council of Taiwan under Grant No. NSC 100-2221-E-003-015& NSC 101-2221-E-003-020. We also thank Prof. Yang Chi-Rong of Department of Mechatronic Technology, National Taiwan Normal University for his support on the fabrication equipment.
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