P-65 / Y.-J. Wang
A Novel Highly Collimating Backlight Module
Using a Double Wedge-Shaped Light Guide Plate
1
Yi-Jun Wang1,2, Jian-Gang Lu1, and Han-Ping D. Shieh1,2
Dept. of Electronic Engineering, Shanghai Jiang Tong University, Shanghai, China
Dept. of Photonics & Display Institute, National Chiao Tung University, Hsinchu, Taiwan
2
Abstract
A novel highly collimated backlight module transforming a point
light source into a planar light source by means of a double
wedge-shaped light-guide plate (LGP) was designed. The
uniformity of spatial luminous can reach 80.5% at view cone of
±1.8 deg.. The Gray-half-tone technology can be used to fabricate
the microstructures for mass productions.
1.
Introduction
Liquid Crystal Display (LCD) is dominating the display market, yet
the demands of green technologies have urged power saving and
image quality enhancement. To meet the above requirements in LCD,
backlight module plays a key role. For a conventional edge-type
backlight module, several optical films such as BEF-DBEF-diffuser
and reflector are used to achieve the high optical performance, with its
obvious drawback, such films increase costs and reduce optical
efficiencies. Therefore, collimated backlight module with fewer
optical films clearly shows its necessity and urgency.
Figure 1. Illustration of collimated backlight module (a)
Front, (b) Left Side, and (c) Isometric.
Recent years, IBM used multi-index material to control the light
angular distribution of entering the microstructures [1-4]. Kodak
used a dual-turning film to collimate the light [5]. K.K designed a
monolithic block-wise functional light guide and combined with
an inverted prism to get the collimated backlight [6-7]. However,
all these methods still cannot achieve a highly collimating within
±2deg. at FWHM. To resolve this issue, we proposed a backlight
module which had a double wedge-shaped LGP and a collimating
light source to suit this backlight module.
2.
Collimated Backlight Module
Design
2.1.
Components of Backlight Module
Figures 1 (a-c) illustrate the new collimated backlight module in
different view angles. It composes of two parts: the first part is a
collimated light source; the second part is a double wedge-shaped
LGP which can be divided into a side-LGP with an incline of 45
deg. on the top surface and a main-LGP.
2.2.
Collimated Light Source
It is clearly indicated that the conventional LED with Lambertian
angular distribution is not suitable for collimated backlight design
[5-7]. Therefore, we designed a collimated light source including
an LED chip and a parabolic reflector immerged in a high index
material, as shown in Figure 2. According to the Snell’s Law, the
light comes out from chip can be compressed to a small
divergence angle when it enters the high index material from the
air. The parallel light can be obtained by the reflection of
parabolic reflector when the chip is positioned at the focus of a
parabolic reflector based on parabolic reflection law. Thus, a
highly collimated light source which had an optimized proportion
between LED chip and focal length of parabolic can be achieved.
Figure 2. Concept of a collimated light source.
2.3.
Wedge-Shaped Light Guide Plate Design
2.3.1. Side-Light Guide Plate Design
The proposed wedge-shaped LGP is based on the reflection–
refraction characteristics of the prismatic pattern on LGP. In order
to convert the output light from a collimated point light source to
a line light source, prism microstructures were designed on the
bottom surface of the side-LGP. Suitable pitch and tilt angle of
microstructures were set to change the direction of light
propagating in side-LGP and keep it spatial uniformity. As shown
in Figure 3(a), for propagating the light to the end of the sideLGP, the tilt angelαof wedge-shape side-LGP is determined by
using triangular relations as follows:
 H1 -H 2 

 W 
  tan -1 
ISSN 0097-966X/12/4303-1305-$1.00 © 2012 SID
(1)
SID 2012 DIGEST • 1305
P-65 / Y.-J. Wang
where H1 is the height of light source which is half height of the
side- LGP, H2 and W is the height and width of thin side of sideLGP, respectively.
where H3 is the width of LED, H4 is the height of the thinner side
of main-LGP, and L is the length of main-LGP, as illustrated in
Figure 4.
Due to the fact that light energy emitted from the LED was not
uniform, causing energy distribution spreading from center to the
peripheral and reduced gradually which would affect the
uniformity of spatial luminous obviously. To optimize the spatial
uniformity, the discontinuous prismatic microstructures were used
to modify the distribution of light energy in the main-LGP, which
was different from the continuous prismatic in side-LGP. The tilt
angle Φ of prismatic microstructure in main-LGP was
determined by the method mentioned above, which was used to
fix the prism angle βin side-LGP.
(a)
(b)
Figure 3. (a) Relations between the inclined angle and the
height of side-LGP and (b) Prism microstructure for light
shaping on the bottom surface of side-LGP.
Based on Total Internal Reflection (TIR), light propagating along
z-axis was changed to y-axis under the condition of α+β=450
(βis prism angle). The prism angleβcould be fixed with the
given tilt angle α. Additionally, the prism angle γand the pitch
of microstructure (P), as shown in Figure 3(b), were set at 90
degree and a given value. From this design, only a small portion
of collimated lights (such as S1 and S2) were reflected by the
microstructures. Thus, a collimated light point source was
transformed to a line source, which would propagate along y-axis
and be directed to x direction by the reflection of an incline of 45
degree on the top surface，as illustrated in Figure 4.
2.3.2. Main-Light Guide Plate Design
3.
Simulation Results
3.1.
Collimated Light Source Simulation
All these modules were built and simulated by commercial
software Lighttools 7.1. The emitting area of collimated light
source is 1.6 x 1.6 mm2. The shape of an LED chip is a circle with
a diameter of 150 m. The focal length of paraboloid is 4 mm.
The parabolic reflector is immersed in a high refractive index
(index=2.1478, K-PSFn173 from SUMITA) material. The angular
luminance distribution of the LED chip is limited to a divergence
angle of θin which is determined by the following equation:
in  sin 1  (nair *sin  air ) nm 
(3)
where nair and nm are the refractive index of air and material,
respectively. θair is the divergence angle of an LED chip in the air.
Thus, the range of θin is ±27.7 deg.. The simulated results are
shown in Figures 5(a-c) where Figure 5(a) shows the light
distribution of spatial luminous, Figure 5(b) illustrates the angular
luminance distribution of the collimated LED, and Figure 5(c)
depicts the luminance of the LED has a cone shape both on
horizontal and vertical with ±1.5 deg. at FWHM.
The collimated line light source was coupled into the main-LGP
by the method mentioned above. The tilt angel θ of main-LGP
is determined by using triangular relations as shown in equation
(2):
 H 3 -H 4 

 L 
  tan -1 
(2)
Figure 4. Discontinuous prismatic microstructures for light
collimation on the bottom surface of the main-LGP.
1306 • SID 2012 DIGEST
Figure 5. (a) The spatial luminance of an LED, (b) Angular
luminance distribution of the collimated LED, and (c) the
profiles of angular luminance in horizontal and vertical.
P-65 / Y.-J. Wang
3.2.
Backlight Module Simulation
The dimension of backlight module is 40 mm in length, 30 mm in
width and 1.6 mm in thickness on the thick side of main-LGP.
The material of LGP is PMMA. The efficiency of the LGP is 70%.
The panel’s uniformity is defined as U= Lmin / Lmax where Lmin
and Lmax are the minimum and maximum luminance, respectively.
The uniformity of 80.5% was achieved as depicted in Figure 6(a).
The average luminance Lavg was 1.38e+5 nits. The angular
luminance distribution of the backlight unit was plotted in the
polar coordinate as shown in Figure 6(b). Figure 6(c) illustrates
the profiles of the cone. The luminance of the backlight module
had a cone shape with ±1.8 deg. at FWHM, implying that the
lights emitted from backlight were confined in a narrow angle
both on horizontal and vertical directions. Thus, most of the lights
could be coupled out from the top surface and the brightness on
normal direction was greatly improved.
Specially, it was observed that except the top surface, almost no
lights can emit from other surfaces of main-LGP. In this case, it is
not necessary to use the reflectors to improve the light efficiency.
In addition, large-scale could be fabricated by split joint because
of no light leakage issue.
(a)
(b)
Figure 7. (a) Schematic of a 4 inch backlight module and
(b) The structure of a larger-sized backlight module.
4.
Discussion
The comparisons of the typical backlight unit with the novel one
were listed in Table.1. It is found that the new LGP allowed a
fewer number of LEDs, a higher light extraction efficiency, and a
sharply higher brightness. In general, such high brightness on
screen is not necessarily required. Therefore, using an LED with
smaller luminous flux could still meet the average needs yet
reducing the power consumption. Unlike the traditional backlight
module which uses four films (Reflector + BEFI + BEFII + DP),
the new backlight module needs no additional optical film.
Table 1. Typical and Novel BLU comparisons
Typical BLU
Novel BLU
Dimension
(mm)
60*40
60*40
LED Chip Size
(mil)
9*28
6*6
LED Quantity
(pcs)
6
2
LED Luminous Flux
(lm)
7.13*6=42.78
1*2=2
27.25
1.4
Output Luminous
Flux(without films)
(lm)
LGP Efficiency
(%)
63.7
70
1830
1.38e5
Figure 6. (a) The spatial luminance of BLU, (b) Angular
luminance distribution of the directional BLU, and (c) the
profiles of angular luminance in horizontal and vertical.
LGP Brightness without
films
(nits)
Diffuser Film Gain/Light
Transmittance
1.493/0.86
None
3.3.
BEFI Brightness Gain/
Light Transmittance
1.795/0.86
None
BEFII Brightness Gain /
Light Transmittance
1.655/0.86
None
LGP Brightness with films
(DF+BEFI+BEFII) (nits)
8120
1.38e5
FWHM
±30
±1.8
Backlight Module for Large-Scale
Figure 7(a) shows a 4 inch backlight formed by jointing 4 light
guide plates together. A larger-sized backlight was jointed
through the method as shown in Figure 7(b). In the X direction,
adjacent units can be seamlessly connected by cutting a section of
45 degree on the thin side of the main-LGP, while there were
spaces between adjacent units to ensure the placement of the LED
in the Y direction. With this arrangement, different number of
units can joint together into a scalable backlight to cater the
various sizes requirement.
(degree)
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P-65 / Y.-J. Wang
Furthermore, in order to achieve flexible arrangement of
microstructures on the main-LGP, Gray-Half-Tone technology [8]
can be used to overcome the fabrication where traditional methods
cannot achieve. Based on pulse width modulation (PWM)
method, Gray-Half-Tone mask can be made as shown in Figure 8.
The prismatic microstructures can be obtained by using an
excimer laser micromachining with coded Gray-Half-Tone mask
photolithography.
6.
7.
[3]
[4]
[5]
5.
Conclusion
To achieve low power consumption and high brightness, a newly
collimated light source and a highly collimating backlight module
with a double wedge-shaped light guide plate were designed. The
simulation results exhibited that the uniformity of spatial
luminous can reach 80.5% at view cone of ±1.8 deg.. Besides
small-size LCDs, this backlight module is also scalable for
medium and large size displays used in notebooks and TVs. Grayhalf-tone technology can be used to fabricate the microstructures
on the LGP. Evidently, this one-step optical exposure method
with a coded mask has great potential for rapid and cost-effective
fabrication of the backlight module.
1308 • SID 2012 DIGEST
References
[1] Y. Taira, D. Nakano, H. Numata, A. Nishikai, K. Sueoka,
[2]
Figure 8. Gray-half-tone mask with pulse width modulation
Acknowledgements
This work was supported by Coretronic Co., Ltd., Hsinchu,
Taiwan and Nano Precision Co., Ltd., Suzhou, China.
and F. Yamada, “Illumination System for Color filterless
Liquid Crystal Display”, Int’l Display Res. Conf.
/Eurodisplay, p.585-588, (2002)
Y. Taira, F. Yamada, and A. Nishikai, “Light guide
apparatus, a backlight apparatus and a liquid crystal display
apparatus”, US Patent 6867828 B2, (2003)
Y. Taira, H. Numata, K. Sueoka, and F. Yamada, “Color
Filterless LCD Illuminated with LEDs”, SID Symp. Digest,
p.1250-1253, (2003)
F. Yamada, H. Numata, and Y. Taira, “Multi-layered flatsurface micro-optical components”, J. of SID, p.525-531,
(2003)
T. Ishikawa and X. D. Mi, “New Design for a Highly
Collimating Turning Film”, SID Symp. Digest, p.514-517,
(2006)
[6] K. Käläntär, M. Okada, and H. Ishiko, “Monolithic blockwised light guide with controlled optical crosstalk for fieldsequential color/scanning LCD”, SID Symp. Digest, p.10381041, (2009)
[7] K. Käläntär, M. Okada, “A monolithic block-wise functional
light-guide for 2-D dimming backlight”, SID Symp. Digest,
p.997-1000, (2010)
[8] C. H. Tien, Y. E. Chen, Y. Chiu and H. P. D. Shieh,
“Microlens Array Fabricated by Excimer Laser
Micromachining with Gray-tone Photolithography”, Jpn. J.
Appl. Phys. 42, p.1280-1283, (2003)