Crst., 364, 479(2001). [11] G. D. Lee, G. H. Kim, T. H. Yoon, and J. C. Kim, Jpn. J. Appl. Phys., 39, 2716(2000). [12] M. Oh-e, and K. Kondo, Appl. Phys. Lett., 67 ...
4.1 / T. H. Yoon
4.1: Optical Configuration for a Horizontal-Switching Tansflective LCD Tae-Hoon Yoon, Kyoung-Ho Park, Young Jo Ko, and Jae Chang Kim Department of Electronics Engineering, Pusan National University, Busan 609-735, Korea
Gi-Dong Lee Division of Electronics Engineering, Dong-A University, Busan 604-714, Korea
Abstract We propose optical configurations of a horizontalswitching liquid crystal cell for a transflective display. Optimum configurations can be achieved by using a halfwave liquid crystal cell and three retardation films. We found that measured transmission and reflection spectra in the bright state as well as in the dark state of the proposed configurations show excellent optical performance as expected by numerical calculation.
1. Introduction Recently, liquid crystal display (LCD) manufacturers have been developing reflective and transflective LCDs because of their possibility of low power consumption and high display performance. In particular, the role of transflective LCDs is becoming more and more important in mobile display applications because of their superior performance in out-door as well as in-door environments[12]. In order to achieve high image quality, wide viewing angle and high contrast ratio are important factors even in transflective LCDs[3-7]. The transmittance of light for a non-twist liquid crystal cell with two crossed polarizers is T = T0 sin2(2ψ)sin2(π∆nd/λ),
the light is incident on the medium obliquely with respect to the optic axis. Birefringence in a vertically-switched liquid crystal (LC) cell may be affected very much by the angle of incidence. To improve the viewing angle characteristics, multi-domain or optical compensation are needed. On the other hand, horizontal switching of a LC cell will produce little change in birefringence upon switching. Therefore, a horizontal-switching cell can provide wide viewing angle characteristics. In this paper we will show that horizontal switching can be applied successfully to transflective LCDs. In this work we propose optical configurations of a halfwave LC cell for a transflective LCD with high contrast and high brightness. The configuration may be applied to horizontal-switching liquid crystal cells, such as an inplane-switching (IPS) cell[12], a fringe-field-switching (FFS) cell[13], a ferroelectric LC cell[14], an antiferroelectiric LC cell[15]. A transflective cell is composed of two polarizers, a half-wave LC layer, two quarter-wave films, a half-wave film, and a transflective film as shown in Fig. 1. In order to find the optimum device configurations, we calculated the reflectance in the dark state as well as in the bright state as a function of optical parameters, such as the angle of the LC director with or without an applied electric field, and the optic axis of the retardation films.
(1) Top polarizer Reflective part Transmissive part
where ψ is an angle between the input polarzier and the liquid crystal director, d is the cell gap, ∆n is the birefringence of the liquid crystal medium, λ is the wavelength of the incident light, and T0 is the transmittance of a half-wave (∆nd = λ /2) cell with ψ = 45 o. Horizontal switching[8-11] can be achieved by changing ψ through the in-plane rotation of the liquid crystal director by applying an electric field, while vertical switching can be achieved by changing the phase retardation ∆nd with the applied field.
LC cell (λ/2) λ/4 film Transflective film λ/4 film λ/2 film Bottom polarizer Back light
Nematic liquid crystal is an example of an optically uniaxial medium, which shows birefringence effect when
Fig 1. Structure of a transflective LCD.
ISSN/0004-0966X/04/3501-0006-$1.00+.00 © 2004 SID
4.1 / T. H. Yoon Upper λ/4 film LC cell (λ/2) for the bright state (transmissive) LC cell (λ/2) for the bright state (reflective)
60º
37.5º
Layer ①: wide-band λ/4 layer
Layer ② : mirror image of layer ①
(a) Mirror image method
LC cell (λ/2) for the dark state
75º
15º
Layer ② : compensating layer
Layer ① : wide-band top polarizer
λ /4 layer
(b) Phase compensation method Fig. 2. Configuration of the reflective part.
Fig. 4. Design methods for the transmissive part.
2. Optical configurations of the transflective LCD 2.1 The reflective part A transflective LCD is separated into two parts, a transmissive and a reflective part. These two parts should be optimized simultaneously. The reflective part is designed first because the optical configuration of reflective part is more complex to design than that of the transmissive part. We applied the configuration for the reflective part proposed in our previous works[16-19]. The configuration is shown in Fig. 2. A half wave LC cell is aligned to 15 o , which can be switchable to 37.5 o and 60o.
Polarizer
a
b
c
f
e
d λ/4 film
λ/2 cell
Transflective film
A quarter wave film is located at 75o. The polarization change of the incident light is shown in Fig. 3.
2.2 The transmissive part While maintaining the configuration of each optical component in the reflective part, we optimize the configuration of the transmissive part. Two optical configurations are proposed for the transmissive part. To achieve fancy optical characteristics, the dark states of the transmissive part are put in importance. Two methods are used to achieve the perfect dark state. In one method, the mirror image of the reflective part is used for the transmissive part shown in Fig. 4(a). The light passing through the transmissive part propagates an optical path equivalent to the double pass of the reflective part. The other is a phase compensation method shown Fig. 4(b). The phase retardation can be compensated by placing a layer having the same retardation, but rotated by 90 o. The pair of layers thus can be considered optically isotropic for a normally incident light so that the perfect dark state can be achieved with crossed polarizers.
(a) Optical path of the reflective part. lower λ/4 film
b
c
d
e
bottom polarizer 75º
30°
λ/2 film
f
dark
a
bright
Polarization
Step
λ/2 film 15º
120° bottom polarizer
lower λ/4 film 165º
105º
top polarizer
75°
(b) Polarization change of the reflective part.
Fig. 3. Polarization changes of the reflective part.
(a) compensation
(b) mirror image
Fig. 5. Configuration of the transimssive part.
4.1/T. H. Yoon The optical configurations for the transmissive part which are placed under the reflective part of the transflrective LC cell are shown in Fig. 5. The dark states of these transmissive parts are achieved by using the methods of compensation and mirror imaging. To achieve the bright states of the transmissive parts, the transmittances are calculated as varying the in-plane tilt angle of LC directors, as shown in Fig. 6. As expected, the dark states are achieved at 15o. The bright states are achieved at 60o in both cases. Figure 7 shows the polarization changes of light. Because of the compensation layer, the polarization does not change in passing through b, c, d layers at the dark state and c, d layer at the bright state, as shown Fig. 7(b). The wavelength dispersion is calculated with 2x2 Jones matrix method. In both transmissive and reflective parts, the wavelength dispersion characteristics of the dark and the bright states are excellent, as shown in Fig. 8.
Transflective film
e
λ /2 cell Polarizer
STEP Dark bright
Polarizations
λ /2 film
Config.
a
b
Mirror image
c
d
e
120º
30º
Compen -sation Mirror image
120º
30º 120º 120º
Compen -sation
0.4
0.5
0.5
0.4
0.4
Dark state of reflective part Bright state of reflective part Dark state of trasmissive part Bright state of trasmissive part
0.3
0.3
0.2
0.2
0.1
0.1
0.0
Reflectance
Fig. 7. Polarization changes of the transmissive part.
mirror image compensation
0.0 400
450
500
550
600
650
700
Wavelength(nm)
0.3
0.5
0.5
0.1
0.4
0.4
0.0 0
20
40
60
80
In-plane tilt angle (degree) Fig. 6. Calculated transmittances as a function of the in-plane tilt angle.
Dark state of reflective part Bright state of reflective part Dark state of trasmissive part Bright state of trasmissive part
0.3
0.2
0.3
0.2
0.1
0.1
0.0
The parallax problem, which is inevitable since a λ/4 film should be inserted under the glass substrate in order to implement the proposed configuration, can be overcome by coating an inner retardation layer[20] on the glass substrate.
0.0 400
450
500
550
600
650
Wavelength(nm) (b) Phase compensation method
Fig. 8. Calculated transmittance and reflectance of bright and dark states.
700
Reflectance
(a) Mirror image method 0.2
Transmittance
Transmittance
Polarizer
λ /4 film
λ /4 film
(b) Polarization change of the transmissive part.
Transmittance
0.5
a
b
(a) Optical path of the transmissive part.
4. Experiments and Discussion We fabricated transflective LC cells with the two proposed configurations, whose transmission and the reflection spectra in the bright state as well as in the dark state are measured. Experimental results are compared with numerical calculation obtained by using the 2x2 Jones matrix method, as shown in Fig. 8. Figure 9 shows the measured transmittances and reflectances of the proposed transflective LCDs. As expected by numerical calculation, the light leakage of the mirror image method is little bit higher than that of the compensation method.
c
d
4.1 / T. H. Yoon 5. Conclusion
References
We proposed optical configurations for a transflective LCD with a half-wave cell. We found that measured transmission and reflection spectra in the bright state as well as in the dark state of the proposed configurations show excellent optical performance as expected by numerical calculation. The proposed transflective configurations can be applied to most of horizontalswitching LCD modes.
40
50 Dark state (reflective mode) Dark state (transmissive mode) Bright state (reflective mode) Bright state (transmissive mode)
40
30
30
20
20
10
10
0 400
500
600
Reflectance(%)
Transmittance(%)
50
0 700
Wavelength(nm) (a) Mirror image method
40
50 Dark state (reflective mode) Dark state (transmissive mode) Bright state (reflective mode) Bright state (transmissive mode)
40
30
30
20
20
10
10
0 400
500
600
Reflectance(%)
Transmittance(%)
50
0 700
Wavelength(nm) (b) Phase compensation method
Fig. 9. Measured transmittances and reflectances of the bright and the dark states.
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