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A prism-based brightness enhancement film (BEF), has been investigated through computer-aided optical analysis and experiments in this study. The prism film was fabricated on a 100μm thick PET film substrate using roll-to-roll (R2R) process with UV curable resins of different refractive indices. The results from both optical analysis and experiments showed that resins with higher refractive indices have better on-axis luminous gain. The prism structures fabricated on the substrate achieved a replication rate of 97.54% in this study. Compared with the 3M BEF, the prism films developed in this study are improved in on-axis luminous gains when the refractive index of the resin is higher than 1.53.
©2009 Optical Society of America
1. Introduction
Since liquid crystal is a non-emission material, a Liquid Crystal Display (LCD) depends on the back light module (BLU) as a light source. A BLU is generally composed of a light source, a light guide plate, and optimal films such as diffuse film and prism film. The function of the prism film is to reshape the backlight’s luminance distribution by collimating the light through the refraction of rays on the surfaces of one dimensional micro prism and thus enhances the on-axis luminance. Though different patterns were proposed and studied [1–4], 3M’s prism-based brightness enhancement film (BEF) is the standard for small single-viewer LCDs. The typical 3M BEF is a micro replicated prismatic structured film, with a prism peak angle of 90° and a prism peak spacing of 50μm. The optical performances of the BEF depend on the refractive index of the material and the replication rate of the profile designed in the manufacturing process. Lee’s investigation [2] shows that the angular distribution of the luminous intensity became wider and the gain of the on-axis luminance decreased with increasing diameter of the curved regions and decreasing prism pitch. That also indicates the importance of replication rate of the prism profile. As one major element affecting costs within BLU, the prism sheet must be manufactured in a mass replication process for the sake of fabricating low-price and highly-competitive LCDs.
For the cost-effective fabrication of large area products, roll-to-roll (R2R) manufacturing is always an ideal method [5]. R2R manufacturing process is the process of creating micro-structures or electronic devices on a roll of flexible plastic or metal foil. Early efforts of different researchers have laid a good foundation for R2R to the current stage. Han [6] employed a roller with micro structures fabricated on its surface to imprint the substrate coated by PMMA (Polymethylmethacrylate), PMMA was heated up over its glass transition temperature (Tg) first, then the patterns on the surface of the roller was transferred to the PMMA on the substrate through roller printing process. Terho Kololuoma [7] mentioned R2R can fabricate products with a width of 20cm at an imprinting speed of 100m/min and cured via the UV or IR. With the nickel electroplating technology adopted for manufacture of the ultra thin nickel mold, Gale [8] completed the non-planar micro-structure fabrication by taking the nickel mold wrapping the roller and materialized a mass replication of producing plastic diffractive optical components by integrating the concept of R2R production with the roller wrapped by the nickel mold as one die for hot rolling.
With the fast development in the flexible display industry, R2R process exhibits more potential in LCD applications. Liang et al. [9] pointed out that the well-known Sipix Technology Inc., the professional manufacturer of Electro Phoretic Display (EPD), successfully fabricated the MicroCup structure carrying the electrophoretic liquids on flexible PCs by employing the rolling process technology for photo curing materials in the R2R EPD Manufacture System. Combining the roller imprinting and the CRIP technique, Kaoa et al. [10] imprinted patterns of large-area Organic Light Emitting Diode (OLED) on flexible films and eventfully fabricated one matrix structure of 500μm×300μm with a line width of 20μm. Micro optical films for LCD applications such as microlens array film, lenticular lens sheet, and antireflective optical films have been successfully fabricated using R2R.[5,11,12]. In 1983, 3M [13] produced micro structures on thin films by employing the planar molds plus the roller imprinting and realized curing of micro structures on a transparent carrier with UV irradiation. In 1992, 3M [14] adopted these techniques in making of prism films. With planar molds, the process has to stop for the molds to imprint the substrate, thus this is not a continuous imprinting process. Using a roller mold instead of a planar mold, it is possible to fabrication BEF with continuous R2R fabrication techniques.
This study is devoted to developing and demonstrating continuous R2R fabrication process in the making of prism films. 3M BEF II 90/50 based prism films will be studied through optical analysis and experiments. Resins with different refractive indices will be applied in this study to investigate the effect of refractive index on the on-axis performance of the prism film. A roller mold with 90/50 prism profile processed by the diamond cutting technique will be installed onto the self-developed R2R fabrication equipment for forming of prism sheets for the purpose of investigating effects of forming parameters on the quality of imprinted prism films. Both profile replication quality and optical quality will be measured and discussed in this study. The on-axis luminance performance will be measured by installing the formed prism films on one 2.5” BLU, and results will be compared with data regarding the 3M BEF 90/50.
2. Optical Simulation
2.1 Setup of the optical model
Since 3M prism films are employed most frequently by the industry, the prism film studied in this study is based on 3M BEF II 90/50. For the 3M BEF II 90/50, the PET substrate material is with a thickness of 127μm and a refractive index n of 1.64, and the prism structure on the substrate fabricated in PMMA material with the refractive index n of 1.49 has a V-cut shape with a width of 50μm, a height of 25μm, and an apex angle of 90°. Figure 1 shows the prism film structure for this study, the thickness of the PET substrate is 100μm, and the prism on the substrate fabricated in UV-curable resin. Trace Pro software was adopted for the optical simulation in this study. Prior to analyses, the prism sheet optics analysis model will be setup; the dimensions of the prism sheet are 60mm×44mm to match one 2.5” BLU; the light source as an input for analyses is a planar light source from four 0.6 lm LEDs uniformly outputted through one light guide plate and the diffuse film; an observer plane is disposed on the irradiation plane of the prism film for observing analytic results; the refractive index between 1.45 and 2.1 of the UV-curable resin is adopted for inputs of micro structures of the prism film for the sake of investigating effects of different refractive indexes on the luminous intensity.
2.2 Analysis of simulated results
In general, a BLU can be without any prism film. To improve its on-axis luminous intensity, a BLU can be equipped with single prism film or with vertically aligned dual prism films. The simulation results listed in Table 1 are the luminous intensity at on-axis with optical simulations for the luminous intensity from single prism film and dual prism films. From the analysis data in Table 1, it can be discovered that the 90/50 type prism film with a larger refractive index has a higher on-axis luminous intensity as a result of a big refractive angle from a high refractive index followed by the increased luminous intensity of normal irradiation. However with the refractive index of the single prism film increased from 2.0 to 2.1, the on-axis luminous intensity decreases from 1.339cd to 1.3258cd instead. In the case of the dual prism sheets, the on-axis luminous intensity from sheets with a refractive index of 1.68 or higher will decay since higher refractive index will cause the light to refract beyond the on-axis, thus will split light toward both sides without light gathering but diffusion. Figure 2 including correlations between the refractive angles of BLU and the luminous intensity for single prism film significantly indicates the single prism film’s better luminous intensity compared with the case of without a prism film.
Table 1. On-axis luminous intensity of single and dual prism films according to optical simulations.
Fig. 2. The angular distribution of the luminous intensity along the perpendicular direction for BLU with single prism film of different refractive indices and for without prism film.
3. Experimental works
3.1 Materials and setup
The self-developed R2R process equipment shown in Fig. 3 is used for prism film imprinting in this study. The prism structures were formed on an aluminum roller with a precision diamond lathe. The dimensions measured for the processed micro-structure roller are showed in Fig. 4. The plastic substrate is NANYA optical-level PET CH285 with a thickness of 100μm and the transmittance rate of 91%; UV photo resists are three types of UV curable resins with different refractive indices of 1.45, 1.53, and 1.55 respectively.
3.2 Effects of process parameters on micro structures
3.2.1 Effects of imprinting pressure
As one of the important parameters that affects the transforming the prism’s structure from roller to the resin, the pressure acting on the resin by the roller is tested with one Fuji pressure sensing film. The resulted micro structures on the PET substrate are measured with Form Talysurf 635 surface profiler. Different pressure levels were applied to the resins in the experiments. The prism micro structures results from pressure less than 2kgf/cm2 are with a height of 22.9722μm and an apex angle of 44°, which expresses a difference of 2.03μm in height compared with the designed height of 25μm, and the replication rate is 91% or so. When there is a pressure over 6kgf/cm2, the roller mold presses and damages the PET substrate, causing deformation of the substrate’s structures as well as the micro structures of the formed prism. For a pressure with a magnitude of 2~6kgf/cm2, very good forming effects on the prism’s micro structures with a height of 24.3850μm, an apex angle of 44.9°, and the transforming rate of 97.54% are observed, as shown in Fig. 5. In addition, the imprinting pressure also affects the residual layer without one micro structure replicated on it after rolling: the residual layer has a thickness of over 60μm, which influences flexibility of one prism film and reduces the transmittance rate of one prism film, if the imprinting pressure is less than 2kgf/cm2. For instance, with pressures from 2 to 6kgf/cm2 exerted, the residual layer is reduced to under 15μm, as shown in Fig. 6. Thus, it can be seen from above results that the imprinting pressure is important for forming of prism micro structure and is set between 2 to 6kgf/cm2 in this study accordingly.
3.2.2 Effects of speeds of delivering flexible PETs
Because curing occurs with UV curing resins absorbing energy from UV light, adequate exposure under UV is indispensable for a prism’s structures with a good quality. The exposure can be expressed with light source power multiplying exposure time. As a result of one UV source with the fixed power of 45.14mw/cm2 in this study, the delivery speed for the forming process is the major factor controlling exposure energy from UV to resins. For this curing experiment, there are three types of UV curable resins with the individual refractive index n of 1.45, 1.53, and 1.55. In this regard, the resin with a refractive index n of 1.45 causes one unformed micro structure because of sluggishness at an imprinting speed of 1.51m/min. Comparatively, a prism’s structure with a good quality will appear at an imprinting speed of below 0.75m/min, i.e., energy of 302.62 mj/cm2 at least absorbed by resins is imperative for curing. For either of the other two resins with refractive indexes of 1.53 and 1.55, good quality prism structures can be formed under the best pressures between 2 to 6 kgf/cm2 at imprinting speeds of 0.75~4.04m/min where the best speed is 1.01m/min, i.e., equivalent exposure of 225.7mj/cm2. The structures of formed prism films indicated in SEM photos under different magnifications are shown in Fig. 7.
4. Optical performance measurement and discussion
To evaluate the optical effects of prism films formed in the experiment, we install both the single prism film and the vertically disposed dual prism films developed in this study and those by 3M on one 2.5” BLU and measure optical characteristics with BM7 BLU Analyzer according the industrial criteria, nine-point measurement, as shown in Table 2. Furthermore, an extra BLU with the backlight plate and the diffuser rather than the prism film installed is taken as one standard module to compare the optical gain with other BLUs including prism films. From 2 Table 2, the formed prism films having both UV curable resins as materials with refractive indices of 1.53 and 1.55, either the single or the dual prism films vertically disposed, lead to better effects of the optical gain compared with the same combinations from 3M products. In this regard, the single prism film with the refractive index of 1.53 has 8.34 nit (0.02 in the gain) greater than the single 3M prism film and the dual prism films have 54.98nit (0.1 in the gain) greater than the dual 3M prism films; on the other hand, the single prism film with the refractive index of 1.55 has 32.97nit (0.1 in the gain) greater than the single 3M prism film and the dual prism films have 58.49nit (0.18 in the gain) greater than the dual 3M prism films. For one structure constructed by 3M prism film with a refractive index of 1.49, data in Table 2 are consistent with the simulated analyses that a higher refractive index leads to a better gain. For the sake of exhibiting results of the optical gain via combinations of various prism films, significant effects can be observed through those combinations installed on one BLU, as shown in Fig. 8.
Table 2. Comparisons regarding the gain from the nine-point luminance means for simulated and actually measured results.
Fig. 8. Optical effect for different combination of prism films with resin refractive index of 1.55.
5. Conclusions
In this paper, we conduct optical analyses and relevant processing steps by selecting 3M BEF II 90/50 as a reference prism film. From optical analyses of the 90/50 prism film, the fact indicates that the optical gain increased by the ascending refractive index inherently will decrease instead as the refractive index is greater than 2. In our experiment, the event that the formed V-cut micro-structure prism with a width of 50μm and a height of 25μm fabricated from the self-developed R2R fabrication equipment reaches a replication rate of 97.54% for various resins with different refractive indices has demonstrated that this self-developed equipment is available to forming of prism films. With applications of this technique, the forming of large prism films can be materialized in the future. In the case of the measurement of optical characteristics, the feature of a high refractive index causing a high optical gain has been proved in the 90/50 prism film. For the prism film’s performance in the optical gain, both prism films with refractive indices of 1.53 and 1.55 are superior to the 3M prism film with the refractive index of 1.49. As a result of differences in materials, the quality of formed prisms fabricated with dissimilar materials is diversified. In this study, we exhibit superior-quality prism films fabricated from the R2R continuous rolling process with potential applied to manufacture of low-price, high-quality, and commercialized wide prism films.
Acknowledgement
This work was supported by the National Science Council of Taiwan (Under the grant of NSC97-2221-E-151-015). The financial support is acknowledged.
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