High-efficiency color separation method based on fractional Talbot
effect for color liquid crystal display
Qiaofeng Tan
*
, Yan Zhang, Yingbai Yan, Guofan Jin
State Key Laboratory of Precision Measurement Technology and Instruments, Tsinghua University, Beijing, 100084, People’s Republic of China
article info
Article history:
Received 11 April 2008
Received in revised form 11 August 2008
Accepted 10 September 2008
Keywords:
Liquid crystal display
Fractional Talbot effect
Talbot grating
Color separation
abstract
Based on the fractional Talbot effect of Talbot grating, a new method with high light efficiency for color
separation in color liquid crystal displays (LCDs) is proposed. The light efficiency of 85% and 89% are
obtained by three- and four-step Talbot gratings, respectively, which are rather higher than that of using
color filter, the traditional color separation method in color LCDs. This method has large tolerance to the
position error of the color separation plane, the wavelength changes of red, green and blue colors, and the
step height errors of the gratings.
Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction
Liquid crystal displays (LCDs) are widely used in commercial
electronic products. As the LCD panel has no function of spontane-
ously emitting light, backlight unit is used for illumination. The
typical light throughput is around 5% for the color LCDs. This poor
light efficiency is predominately relate to the use of color filter
[1,2]. The color filter realizes red (R), green (G), blue (B) colors dis-
play with arrays of wavelength-selective filters. Each pixel of the
LCD is divided into three sub-pixels corresponding to the RGB col-
ors. The filters always work on the absorption and each color filter
can transmit only one-third of the total light energy.
To increase the light efficiency, several approaches have been
suggested. Color sequential method [3–5] is one of the preferred
methods which display RGB colors in time sequence. It has the
advantages of high resolution and less use of drive IC. However,
high-speed liquid crystal mode and quickly flash-able lamps are
necessary in this method because one image is constructed with
sequential change of the three primary color sub-images, other-
wise color break-up occurs [6]. Color separation grating based on
the far field diffraction with the light efficiency of 64% also offers
a potential solution to this problem [7]. Considering the color sep-
aration in LCDs is implemented in the near field, the Talbot grating
realizing beam-splitting in the near field based on the fractional
Talbot effect [8–10] is proposed in this paper to realize color sep-
aration with high light efficiency. The light efficiency of 85% and
89% are obtained by three- and four-step Talbot gratings, respec-
tively, which are rather higher than that of using color filters.
The light emitted from the source is regarded to be coherent,
which is a commonly accepted case in the discussion of diffractive
optics used in LCDs [7,11]. In the backlight unit of LCDs, as shown
in Fig. 2b, the light guide plate (LGP) extends point source (for
example, LED: light-emitting diode) or line source (for example,
CCFL: cold cathode fluorescent lamp) to one uniform planar source,
but unfortunately the coherence cannot be maintained with the
use of microprisms [11,12], micorlens [13,14] or other traditional
structures. With the use of new technique, the coherence of the
light output from the LGP might be assured, for example, by using
holographic LGP [15]. Accordingly in this paper the light output
from the LGP is supposed to be an ideal plane wave. For the pur-
pose of description, the RGB wavelengths are chosen as
k
b
= 0.45
l
m, k
g
= 0.54
l
m and k
r
= 0.63
l
m, respectively, the
refractive index of the grating substrate is chosen as n = 1.52 and
the dispersion is ignored.
2. Three-step Talbot grating for color separation
Firstly three-step Talbot grating is designed to realize RGB color
separation. If the phases of the three-step Talbot grating are u
1
, u
2
,
u
3
respectively, the period of the grating is d, then the transmit-
tance function of the Talbot grating is
tðx
0
Þ¼ rect
x
0
þ d=3
d=3
expði
u
1
Þþrect
x
0
d=3
expði
u
2
Þ
þrect
x
0
d=3
d=3
expði
u
3
Þ
1
d
comb
x
0
d
; ð1Þ
0030-4018/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.optcom.2008.09.057
* Corresponding author. Tel.: +86 10 62781187.
E-mail address: tanqf@mail.tsinghua.edu.cn (Q. Tan).
Optics Communications 281 (2008) 5949–5953
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Optics Communications
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