FMCp - 40
Monolithically Fabricated Micropatterned Light Guide Plates
for Sheet-less Backlight Unit
Hong-Seok Lee, Joo-Hyung Lee*, Byung-Kee Lee*,
Won-Seok Choi*, Hoon Song, Hwan-Young Choi, and Jun-Bo Yoon*
Display Optics & System PT, Display Device & Processing Lab.,
Samsung Advanced Institute of Technology,
14-1 Nongseo-dong, Giheung-gu, Yongin-si, Gyeonggi-do 446-712, Korea
*School of Electrical Engineering and Computer Science,
Korea Advanced Institute of Science and Technology,
373-1 Guseong-dong, Yuseong-gu, Daejeon 305-701, Korea
ABSTRACT
A light guide plate (LGP) with monolithically fabricated micropatterns on its surface was suggested
and fabricated. The fabricated 2 inch LGP showed
an average luminance 2878 nit and 73.3% uniformity with four side view 0.85cd LEDs. The suggested monolithic LGP can reduce cost and thickness of back light units (BLU) because a
sheet-less BLU can be obtained with it, and also
due to flexible property of material, its application
fields can be extended to a flexible light source for
flexible displays.
1. INTRODUCTION
In the business field of liquid crystal display
(LCD), cost reduction and premium design are hot
issues. A back-light unit (BLU) without any additional sheet is good solution for those two issues.
Sheet-less BLU is cost effective, thinner, and
lighter due to elimination of additional sheets that
should be included in the conventional BLU.
Several approaches have been tried to make a
BLU sheet-less, such as a highly scattering optical
transmission (HOST) polymer backlight system [1],
a hologram patterned light guide plate (LGP) [2],
and a BLU using optically patterned film [3]. The
light extracting structures in Ref 3 are preferable
due to high efficiency of total internal reflection.
But, an optically patterned film should be carefully
attached to an LGP for the proper operation.
To make a sheet-less BLU, light extracting
structures should be monolithically fabricated on
the surface of LGP. By introducing 3D backside
lithography and plastic replication [4], we can successfully make such micro structures on the surface of LGP monolithically [5,6].
In this paper, the optimization of LGP design
and the method of monolithic fabrication will be
described, and then the measurement results of a
fabricated 2 inch LGP sample will be shown.
Monolithically fabricated micropatterned LGP
can be applied to sheet-less BLUs, it is
cost-effective, thin, and light solution for LCD displays. And also, the flexibility of the LGP due to
material characteristics enlarges the application
fields to the flexible displays.
2. MODELING AND SIMULATIONS
The cross section of proposed monolithically
fabricated LGP is shown in Fig. 1. The cross section of micropattern on the LGP is inverse
-trapezoidal shape. To obtain optimal performance,
many parameters should be considered such as
refractive index (n), the thickness of LGP (t), shape
parameters (A, B, h,T), and the distance between
each micro structures (d). Among them, refractive
index and the thickness of LGP are fixed when
material and light source are chosen. The distance
d should be adjusted to obtain uniform output. The
minimum distance between micropatterns is determined within possible process of fabrication.
The longitudinal distances between structures are
proportional to the luminance distribution of the
uniformly distributed LGP, where the lateral distance is fixed to 40 Pm.
The shape parameters are critical to the angular
distribution of extracted light. For the sheet-less
BLU, the normal direction extraction of light should
be obtained without any additional films. Because
the light inside LGP has angular distribution, we
should determine dominant incident angle for
choosing an inclined angle of micro structure. The
dominant incident angle is calculated 30 degrees
considering a window function as shown in Fig. 2,
and the inclined angle 54.5 degrees is followed
from it. The window function is defined by the projected length of opening (2B) on the transverse
plane of incidence light direction, 2BxSin(incident
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angle). Therefore, the inclined angle of micro
structure depends on the angular distribution of
light source and opening of micro structures.
Designed micropatterns that have a bottom diameter of 12.9 Pm, a top diameter of 30 Pm, are
distributed unevenly for the sake of light uniformity
in an area of 2 inch diagonal. A mirror is located at
the far-end side of the LGP to enhance the light
efficiency. The luminance distribution and the angular distribution of the designed LGP are calculated using LightTools and shown in Fig. 3(a) and
3(b), respectively.
3. FABRICATIONS
We fabricated a proposed micropatterned
Polydimethylsiloxane (PDMS) LGP by below sequences. The photoresist micropatterns with the
inverse-trapezoidal cross section were fabricated
by backside 3-D diffuser lithography and transferred to the PDMS LGP by photoresist-to-PDMS
and PDMS-to-PDMS replications. SEM images of
each step are shown in Fig. 4(a)-(d), and the detail
fabrication method is shown in Ref. 5.
4. MEASUREMENTS
The optical properties of the fabricated LGP
were measured by a luminance colorimeter (EZContrast 160 from ELDIM S.A.) with four 0.85 cd
LEDs at 3 points (near, center, far from the light
source) throughout the LGP. Fig. 5(a) and 5(b)
show the measured luminance value at 0 degrees
and 90 degrees in each position respectively, with
a mirror at the far-end side from the LEDs. The
far-end mirror improves the luminance and uniformity of the fabricated LGP by recycling light.
The average luminance was observed as 2878 nit
with a uniformity of 73.3%, which are comparable
to those of the conventional BLU with a reflective
film, one diffuser, and two prism sheets on a polycarbonate LGP (approximately 4000 nit with a uniformity of 88% with four 0.85 cd LEDs). Fig. 5(c)
shows the angular luminance distribution observed
at the center point. It has still some lateral wide
distribution of extracted light compared to the calculated one, and it causes from the difference of
the inclined angle of micro structures.
Further improvements can be possible by optimization of the pattern shape and distribution.
5. CONCLUSIONS
The suggested monolithic LGP can reduce cost
and thickness of BLU because its optical performances are comparable to those of a conventional
BLU with three additional optical sheets. And also,
its application fields can be extended to a flexible
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light source for flexible displays due to flexibility of
LGP as shown in Fig 6. It can be applied to the
front-light of flexible reflective displays and the
backlight of plastic LCDs.
6. REFERENCES
[1] A. Tagaya, M. Nagai, Y. Koike, and K. Yokoyama, “Thin liquid-crystal display backlight
system with highly scattering optical transmission polymers,” Appl. Opt. 40, 6274-6280
(2001).
[2] H.Y. Choi, M.G. Lee, J.H. Min, and J.S. Choi,
“Hologram based light-guide plate for
LCD-backlights,” in Proceedings of International Display Workshops, 20-23 (2001).
[3] K. Fujisawa, I. Onishi, and Y. Fujiwara,
“Edge-light backlight unit using optically patterened film,” Jpn. J. Appl. Phy. 46, 194-199
(2007).
[4] Sung-Il Chang and Jun-Bo Yoon,
“Shape-controlled, high fill-factor microlens
arrays fabricated by a 3D diffuser lithography
and plastic replication method,” Opt. Express
12, 6366-6371 (2004).
[5] J.-H. Lee, H.-S. Lee, B.-K. Lee, W.-S. Choi,
H.-Y. Choi, and J.-B. Yoon, “Simple LCD
Backlight Unit Comprising Only a Single-sheet
Micropatterned Polydimethylsiloxane (PDMS)
Light-guide Plate,” Opt. Lett. 32, 2665-2667
(2007).
[6] J.-B. Yoon, “3-D Diffuser Lithography and Its
Application to LCD/LED Backlight Unit and
Flexible Front-light Unit,” IDW’07, to be presented (MEMS2-2).
d
A
T h
Incident angle
B
t
Window = 2 x B x Sin(Incident angle)
Fig. 1 Cross-sectional view of monolithically
fabricated micropatterned LGP.
1
window
0.9
Intensity [Arb.]
PR patterns
LED
0.8
LED*window
0.7
0.6
27.3u
0.5
0.4
0.3
12.3u
0.2
o
55
0.1
0
0
10
20
30
40
50
60
70
80
11.7u
90
Incident angle [Degree] (inside)
Fig. 2 Dominant incident angle of internally
confined light of LGP.
(a)
PDMS mold
13.7u
o
56
13um
27.2u
(a)
(b)
PDMS LGP
25.9um
12.9um
60o
14.3um
(b)
Fig. 3 Simulation results of the designed 2 inch
micropatterned LGP with far-end mirror, (a) the
calculated luminance distribution and (b) the
angular luminance distribution at 3 points
(near(2), center(5), far(8) from the light source).
(c)
Fig. 4 SEM images of (a) photo resist master
pattern, (b) elastomer mold, (c) monolithically
fabricated micropatterned LGP.
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P2
P5
P8
Luminance [nit]
3000
2500
2000
1500
1000
500
0
-80
-60
-40
-20
0
20
40
60
80
Altitude angle [degree]
(a)
3500
P2
P5
P8
Luminance [nit]
3000
2500
2000
Fig. 6
1500
1000
500
0
-80
-60
-40
-20
0
20
40
60
80
Altitude angle [degree]
(b)
(c)
Fig. 5 Optical properties of 2 inch LGP sample with far-end mirror. The measured luminance value at (a) 0 degrees and (b) 90 degrees
at 3 points (near(2), center(5), far(8) from the
light source) (c) the angular luminance distribution at the point 5 (center).
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Picture of fabricated flexible LGP.