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Study of Bright “Fringes” near the Incident Surface of a Wedge-Type Light Guide Plate in an Edge-Type Backlight Unit Chih-Chieh Kang, Jeng-Feng Lin, Yu-Chang Wu, Fang-Yi Chou, Jun-Zhi Huang, Cho-Wei Chen Department of Electro-Optical Engineering, Southern Taiwan University, Yung-Kang, Tainan Taiwan, 710, R.O.C. TEL:06-253-3131 ， FAX:06-2549400 ， E-mail:[email protected] Abstract In this paper, the optical phenomenon of bright “fringes”, appearing near the incident end surface of a wedge-type light-guide plate (LGP) in an edge- type backlight unit (BLU), is studied. It is observed in the case of longitudinal V-groove microstructure fabricated in a wedge-type LGP as well as a reverse prism employed. Optical simulations using ASAP modeling software are performed to study this problem. The simulation results are presented, explained, and discussed. Introduction Though LCD displays are gaining their edge in display market, whereas the demand on product quality as well as performance is more string than ever, especially for the high-end products. As the key component in LCDs, backlight units (BLUs) have great influence on the performance of LCDs products. To better the quality of a LCD product, it is important to identify the causes of defects (muras) of a BLU and resolve the problems. A particular type of mura, bright “fringes”, sometimes called bright lines1-3, as shown in Fig. 1, occurs in an edge-type BLU, which is composed of a frame integral with a CCFL, a lamp reflector, a wedge-type light-guide Simulations Of Bright “Fringes” An ASAP ray-tracing model of a typical 7” BLU with a wedge-type LGP, in which all its end surfaces are smooth planes initially, is developed. With the concern of simulation computation time, a truncated version of this model is implemented in simulation. The LGP dimension is of 5 96 2.3 - 0.9 mm with inclination angle of = 0.8365°. The existence of a gap region between the light transmitting (reflecting) surface of a LGP and the distal end portion of lamp reflector (reflection film) in the constructed ASAP model is crucial for a successful demonstration of bright “fringes”. The width of the gap is assumed to be of 0.01 mm in simulation. The geometric shape of lamp reflector is assumed to be heptagonal, of which the side end surface against the LGP is opened, as shown in Fig 2. Simulation Results The gap width is a variable in simulations. One of the simulation results of such a structure is shown in Fig. 3. The observation plane is disposed at a distance of 0.1 mm above the emitting end surface of the LGP. The observation plane is further divided into two regions: A and B, to distinguish whether or not there is any contribution to bright “fringes” by emitting light coming from either region. Region A encompasses the area of which supposedly bright “fringes” occur; the rest is region B. There is no visible bright “fringes” in Fig. 3a, only a wider bright line is observed on the left side of the observation plane that is in accordance with the observation from a physical BLU. In Fig. 3c, the angular distribution of emitting light from region A is quite different from region B, as shown in Fig. 3d, which is consistent with the observation where the angular distribution of emitting light is quite close to grazing angle. The last step in simulation model development is the implementation of a reverse prism film. To generate an ox-axis emitting light from a BLU, a 68° reverse prism film is employed. A specific simulation result is illustrated in Fig. 4, which is corresponding to the image shown in Fig. 1. Bright “fringes” appear on the left side of the observation plane, as shown in Fig. 4a. Similar to the above case (without a reverse prism), the angular distribution of emitting light from region A and region B are shown in Fig. 4c and Fig. 4d respectively for the purpose of comparison. By Fig. 4d, it is clear that bright “fringes” can be seen by eyes at a viewing angle of close to 0°. Conclusion A simple illustrated explanation about how bright “fringes” occur by using Path Explorer in ASAP, as shown in Fig. 7. The bright “fringes”, appearing near the incident end surface of a wedge-type light-guide plate (LGP) in an edge-type backlight unit (BLU) is illustrated through ASAP modeling. It can only be observed with the disposition of a reverse prism. Through the analysis, the cause of bright “fringes” can be concluded as light leakage through a gap region between the light transmitting (reflecting) surface of a LGP and the distal end portion of lamp reflector (reflection film). References 1.K. S. Ha, US Patent 6443583, (2003). 2.Y. Hara, et al., US Patent 6974241 (2005). 3.T. Ishikawa, et al., US Patent no. 6024463, (2000). 4.X. P. Zheng, et al.,SID06 Digest, (2006). 5.F. Y. Chou, Master Thesis, STUT (2007). Fig. 1. The image of successive bright “fringes” near the incident end surface of a wedge-type LGP with longitudinal V-groove structures in an edge-type BLU Fig. 3. (a) Simulation result of emitting light from the LGP on an observation plane of 16-mm in length without the present of a reverse prism film. Angular distribution of emitting light from (b) region A + B, (c) region A, and (d) region B. Fig. 4. (a) Simulation result of emitting light from the LGP on an observation plane of 16-mm in length with the present of a reverse prism film of 68°. Angular distribution of emitting light from (b) region A + B, (c) region A, and (d) region B. Fig. 2. (a) Schematics of a wedge-type LGP with longitudinal V-groove structures in an edge-type BLU. (b) Cross-sectional view of (a) illustrating a small inclination angle of . Fig. 7. Ray tracing of the formation of bright “fringes” in a BLU with Path Explorer, an ASAP tool.
