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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.
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
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).
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. 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.
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. 7. Ray tracing of the formation of bright
“fringes” in a BLU with Path Explorer, an ASAP
tool.