Design of a backlight module with a freeform surface by
applying the Taguchi method
Yingcong Chen (陈颖聪)
1
, Shangsheng Wen (文尚胜)
1,2,
*, and Pengcheng Song (宋鹏程)
1
1
Institute of Polymer Optoelectronic Materials and Devices, South China University of Technology,
Guangzhou 510640, China
2
State Key Laboratory of Luminescent Materials and Devices, South China University of Technology,
Guangzhou 510640, China
*Corresponding author: shshwen@scut.edu.cn
Received October 14, 2014; accepted December 22, 2014; posted online February 24, 2015
A novel structure, called a “freeform surface,” is integrated into a direct type light-emitting diode backlight. By
applying the Taguchi method, the performance of this backlight is optimized. The Taguchi experiments are
configured in L
9
ð3
4
Þ orthogonal arrays and are simulated via LightTool analysis software. After that, the in-
fluence of the design parameters on the luminance and uniformity are separately evaluated by analysis of vari-
ance (ANOVA). Next, the parameters are optimized, and a new backlight structure with desirable performance
is designed at last. LightTool simulation shows that this new type of backlight is just 15 mm thick and has 310.3
nits luminance and 83.5% uniformity.
OCIS codes: 230.3670, 220.2945.
doi: 10.3788/COL201513.032302.
Recently, we have witnessed the rapid development of the
panel display, and the demand for slim and large-scale dis-
plays is growing quickly
[1]
. The large-size panel display
usually adopts the direct type backlight modules with a
20–50 mm thickness
[2]
. The direct type backlight module
has light-emitting diode (LED) sources positioned directly
onto the bottom of the backlight
[3]
. In order to achieve a
uniform lighting effect, a sufficient light mixing distance
must be preserved, which conflicts with the trend of a slim
panel display. Therefore, a new backlight module struc-
ture with a freeform surface is proposed.
In this Letter, a backlight module with a freeform sur-
face is designed and optimized by using the Taguchi
method
[1,4]
with its experiments conducted through Light-
Tool analysis software
[5]
. The simulation shows that the
new type of optimized backlight module yields a uniform-
ity of 83.5%, a luminance of 310.3 nits, and a thickness of
just 15 mm.
The simplified module of a traditional direct type LED
backlight is shown in Fig.
1
[6]
. It consists of a metal (alu-
minum) housing containing some LED bars on the bot-
tom. Directly above the LED bars are a diffuser plate
and brightness enhancement films (BEFs) that increase
color mixing and enhance brightness. When the light mix-
ing distance is short, the direct type backlight cannot sat-
isfy uniformity and cannot avoid leading the spot issue
[6–8]
.
Therefore, the freeform surface is integrated into the hous-
ing to redistribute the light of the LEDs, as shown in Fig.
2.
By increasing the radian of the freeform surface, the opti-
cal path is longer, which translates into a longer light mix-
ing distance. In addition, a microstructure is used to
change the direction of the horizontal light in order to en-
hance the light-extraction efficiency of the backlight.
As shown in Fig.
2, the freeform surface can be
described as a piecewise function. The normal plane,
where the LEDs are mounted, is described as
~
n ¼
ðx
0
; y
0
; tan θ
0
x
2
0
þ y
2
0
p
Þ. Every point ðx; y; zÞ on the plane
is defined as Function 1. The ordered parts of the freeform
surface have no definite shape. Transformed to spherical
polar coordinates, the freeform surface is described as
Function 2:
x
0
ðx − x
0
Þþy
0
ðy − y
0
Þþtan θ
0
x
2
0
þ y
2
0
q
ðz − z
0
Þ¼0;
(1)
8
<
:
R; θϵð0; θ
1
Þ∪ðθ
2
; πÞ; φϵð0; φ
1
Þ∪ðφ
2
; πÞ
x
0
R
cos θ cos φ
− x
0
þ y
0
R
cos θ sin φ
− y
0
þ tan θ
0
x
2
0
þ y
2
0
p
R
sin θ
− z
0
¼ 0; θ ∈ ðθ
1
; θ
2
Þ; φϵðφ
1
; φ
2
Þ
: ð2Þ
Before applying the Taguchi method to design the ex-
periment, the characters of backlight performance and the
rules to measure them need to be defined.
In this Letter, luminance and uniformity are the char-
acters of the backlight’s performance
[9–12]
and are supplied
by the client or defined by ourselves. According to the test
standard of a panel display
[13]
, a TV with a uniformity
>80% can be considered an acceptable product. In
COL 13(3), 032302(2015) CHINESE OPTICS LETTERS March 10, 2015
1671-7694/2015/032302(5) 032302-1 © 2015 Chinese Optics Letters