P-76 / Y.-J. Wang
Distinguished Student Poster
Viewing Angle Switchable Display with a
Compact and Directional Backlight Module
Yi-Jun Wang1, Jian-Gang Lu1, Wei-Chung Chao2, and Han-Ping D. Shieh1, 2
1
National Engineering Lab for TFT-LCD Materials and Technologies, Department of Electronic
Engineering, Shanghai Jiao Tong University, Shanghai, 200240, China
2
Dept. of Photonics & Display Institute, National Chiao Tung University, Hsinchu, 300, Taiwan
Abstract
A compact light guide plate (LGP) with special designed
microstructures and dual collimated light sources was proposed
to achieve an adjustable viewing angle backlight for eco-display.
The micro-prisms were utilized to guide the emitting rays toward
normal viewing cones while a set of strip patterned diffuser
toward wide viewing cones. The uniformity of spatial luminous
can reach 91% / 87% at view cone of ±3/ ±60 deg. in each mode.
Only 5% of power consumption in narrow viewing angle mode
was needed to keep the same luminance in wide viewing angle
mode.
Introduction
Liquid Crystal Displays (LCDs) are currently dominating the
display market due to their thin form factor and light weight. As
the demand of multimedia information applications, LCDs are
supposed to be used for various purposes and situations. Two
different situations are often encountered: group viewing and
privacy viewing. The former demands a wide viewing angle for
sharing the contents of the display, but the latter prefers a narrow
viewing angle in order to protect personal or confidential
information and sometimes for low power consumption.
Therefore, backlight module with viewing angle switchable is a
good feature to meet various needs.
However, the weight and the thickness of the system became
undesirable for mobile displays [5-6]. To simplify the module
structure, a compact backlight module was proposed in this work
to achieve switchable viewing angle with only one light guide
plate (LGP) with special designed microstructures and dual
collimated light sources.
Design Principle
In spite that LEDs are widely used as the light source for
backlight in LCDs, the conventional LED of Lambertian angular
distribution is not suitable for narrow viewing angle backlight
design with eco-display[7-9]. Thus, the collimated point light
source
becomes
a
good
candidate
for
the
direction-dependent-displays. Two collimated point light sources
are used in this proposed structure to expand a point collimated
light source to a planar light source without changing the high
directional characters. The proposed structure of viewing angle
switchable backlight module with isometric view, top view, and
front view are demonstrated in Figure 1.
Typical viewing angle switchable displays can be classified into
two categories, namely, by liquid crystal (LC) components and by
backlight module design. The LC technologies for viewing angle
switchable such as an ultra-super-twisted cell with a
polymer-network LC cell[1], and a hybrid aligned nematic (HAN)
LC cell with a negative C-plate[2], have shown some notable
success for low power consumption, high-quality image and low
distortion, but with the major drawback of additional viewing
angle control panel and high cost. The other LC technologies such
as a patterned vertical alignment (PVA) LC display using only
one panel[3], an optically isotropic LC[4] with the feature of
dividing a pixel into a main pixel and a sub-pixel for display
images and controlling the viewing angle respectively, exhibit the
merits of high contrast ratio (CR) and possibility of using similar
fabrication process of conventional LC displays. Nonetheless, due
to spatial-multiplex of one pixel, the transmittance and resolution
are decreased. In order to maintain the luminance of the panel, the
power consumption of the backlight module has to be increased.
On the other hand, backlight with stacked dual light guide plate,
that one offered narrow viewing angle, and the other offered wide
viewing angle was also proposed with the benefit of low cost.
1270 • SID 2014 DIGEST
ISSN 0097-966X/14/4503-1270-$1.00 © 2014 SID
Distinguished Student Poster
P-76 / Y.-J. Wang
Fig. 1. Sketch of backlight in different viewing angle
(a) Isometric, (b) top, and (c) front view.
The backlight module includes two parts: a light guide plate and
dual collimated light sources located at the diagonal of LGP, as
shown in Figure 1(a). The micro-prisms which can be divided into
three groups and coated with silver reflective film, were located
on all the surfaces of LGP except the top one, as shown in Figure
1(b) and (c) respectively. The strip patterned diffusers on top
surface were located corresponding to the left half of
micro-prisms of group C as illustrated in Figure 1(c).
In x-z plane, the LGP is not regular rectangle but incline to z-axis
with the angle of α1. According to the total internal reflection
(TIR) theory, Figure 2(a) illustrates the process of light
propagation in the x-z plane: the incident light propagating alone
z-axis is reflected twice by the micro-prisms which function as
transfering a point light source to a line light source. In y-z plane,
the left/right surfaces of LGP also incline to y-axis with an angle
of α2 except the incident region of light source. The light
propagation in the y-z plane can be described as: the light from
group B propagating alone negative z-axis with a tilt angle, ψ, is
reflected toward the normal direction by the micro-prisms of
group C on bottom surface of LGP as illustrated in Figure 2(b).
Therefore, a line light source is expanded to a planar light source.
As the backlight works in different viewing angle, rays are
coupled out from LGP directly or scattered by strip patterned
diffuser according to the position where rays emit to the top
surface of LGP. Compared with conventional backlight with
LEDs, the light propagating in novel structure LGP has less
scattering and trapping after the three reflections, which is
benefitted for high optical efficiency.
Fig. 2. Light propagation in LGP with different planes
(a) x-z plane and (b) y-z plane.
According to the geometrical optics theory, the parameters of
LGP can be deduced by the following equations:
In x-z plane:
1  tan 1  H cls L 
  tan 1  W L 
  tan 1  L W 
1   2  180    2
3   4   90    2
1  90  1  1
 2  90     3
In y-z plane:
  tan 1  L H 
  tan 1  H L 
1  2   90    2
3  4   90   2
 2  90    2
 3   4  90   3
where, Hcls is the height of collimated light source, H，L and W is
the height, length and width of the LGP respectively, as shown in
Figure 2.
Results & Discussion
Optical efficiency, uniformity of spatial luminance, and angular
luminance distribution are important parameters to evaluate the
performance of viewing angle switchable display. In order to
verify the effect of this novel structure, a 3D geometric model of
the backlight module was established by ray tracing software
Light tools 7.2, as illustrated in Figure 1(a). According to the
fundamental principle of geometrical optics and equations in last
section, the parameters of the backlight module were calculated
and listed in Table I.
TABLE I: PARAMETERS OF BACKLIGHT MODULE
Material / Refractive Index
PMMA / 1.49
Backlight Dimension
（L*W*H）(mm)
α1 /α1 (degree)
59*84*3
5 / 22.5
SID 2014 DIGEST • 1271
P-76 / Y.-J. Wang
Distinguished Student Poster
β1 / β2 (degree)
22.5 / 27.5
β3 / β4 (degree)
43.5 / 43.5
In order to verify the high optical efficiency of proposed structure,
the backlight with different viewing angle modes were simulated
with the conventional backlight (BL) as the reference. The
different conditions and the simulation results were summarized
in Table II.
TABLE II: DIFFERENT CONDITIONS OF BACKLIGHT MODULE
Type
Optical
Efficiency
(%)
A
Normal BL
B
Novel BL
with Wide
Viewing Angle
C
Novel BL
with Narrow
Viewing Angle
65
86.8
95.4
Compared with Type A, Type B and Type C could improve the
optical efficiency by more than 33.5% and 46.7% respectively
owing to the effect of novel micro-structure on LGP. As the
scatter points of conventional backlight simply diffuse the rays
from light source without controlling the directions of light
transmission, a part of rays cannot be coupled out of the LGP if
the incident angle is larger than the total reflection angle.
However, the micro-prism of the proposed structure controls the
most of rays in a precise manner and guides them to the normal
direction instead of trapping in the LGP. Besides, in order to keep
the high uniformity of spatial luminance distributions, the scatter
points are only covered a part of bottom surface resulting in the
energy loss, thus the optical efficiency of normal backlight is
always lower than 70%. On the other hand, the novel backlight
shows the high optical efficiency due to the collimated light
source and micro-structures with compact arrangement which
reduces the energy loss effectively. The simulation results in
Table II demonstrated that the configuration could improve the
optical efficiency obviously, consistent with our analyses
mentioned in last section. The spatial luminance distributions
were calculated to evaluate the uniformity of the novel backlight
in different viewing angle mode (Narrow viewing angle mode /
Wide viewing angel mode), as illustrated in Figure 3. The
uniformity, defined as the minimum luminance divided by the
maximum luminance, were of 91% in narrow viewing angle mode
and 87% in wide viewing angle mode.
(a)
(b)
Fig. 3. The spatial luminance distributions of the backlight
(a) narrow and (b) wide viewing angle mode.
The light distribution is the most important property for viewing
angle switchable display. The characteristics of viewing angle in
different modes were depicted in Figure 4. These results
demonstrated that the novel backlight module offered the same
functionality as a conventional backlight in wide viewing angel
mode with ±60° of FWHM. Besides, the backlight offered ±3° of
FWHM in narrow viewing angle mode with the ultra-low power
consumption (only 5% power consumes at the same luminance)
compared with conventional backlights.
Fig. 4. Viewing angle of novel backlight.
1272 • SID 2014 DIGEST
Distinguished Student Poster
1.
Conclusion
The viewing angle switchable display with special designed
microstructures and dual collimated light sources was
demonstrated for the application of eco-display. Compared with
the conventional backlight module, the novel configuration can
achieve high uniformity (91% and 87% in N/W mode
respectively) with the optical efficiency increased to 46.7%. It can
offer the same FWHM at ±60° as the conventional backlight in
wide viewing angle modes and offer FWHM at±3° in narrow
viewing angle mode with ultra-low power consumption (5%
power consuming of conventional backlight) at the same
luminance.
2.
Acknowledgements
This work was supported in part by 973 project (2013CB328804)
and by NSFC (61275026).
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