Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
2000-08-11
2003-03-11
Ton, Toan (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S171000, C349S181000
Reexamination Certificate
active
06532051
ABSTRACT:
The present invention relates to a new type of liquid crystal display where the input and output light beams do not follow the usual specular relationship.
Liquid crystal displays are usually manufactured with a structure as shown in FIG.
1
. It comprises an input polarizer
1
, a liquid crystal cell
2
, an output polarizer
3
and a reflective diffuser
4
. The liquid crystal cell is commonly made of two pieces of glass
5
,
6
, alignment layers
7
,
8
conductive electrode films
9
,
10
and the liquid crystal material
11
which possesses a twisting alignment in conformance with the alignment layers
7
and
In this common reflective (or sometimes known as transflective) liquid crystal display, the light
12
enters the display from one direction at some azimuthal angle &thgr; relative to the surface normal
13
of the display. The corresponding polar angle of the incident light is &phgr; relative to some x-axis on the surface of the display. Thus the angles specifying the light propagation direction is given by (&thgr;,&phgr;). This light is scattered and reflected by the diffusive reflector and goes through the liquid crystal cell once more and is seen by the observer
14
. This light intensity is strongest at the reflection angle (&thgr;,&phgr;+&pgr;). This is called specular reflection or glare reflection. There is light observable at angles other than (&thgr;,&phgr;+&pgr;) as shown because of scattering, but its intensity drops off rapidly as the angle deviates from &phgr;. The situation is depicted in FIG.
2
. By the same scattering mechanism, at any viewing direction (&thgr;,&phgr;+&pgr;), there is contribution of light incident from (&thgr;,&phgr;), and light from incident angles near (&thgr;,&phgr;). However, a majority of the light is from the (&thgr;,&phgr;) direction.
In designing and optimizing such common liquid crystal displays, the alignment direction of the top and bottom glass plates and the placement of the input and output polarizers are crucial. If one takes the example of a 90° twisted nematic liquid crystal display, the most common configuration is shown in FIG.
3
. The input polarizer P
in
and the input director n
in
are aligned at right angles. The output polarizer P
out
is also perpendicular to the output liquid crystal director n
out
as shown. This is the so-called o-mode operation for the TN display. The light enters the liquid crystal display from the 12 O'clock direction
15
and the viewer looks at the display from the 6 O'clock direction
16
. This is in contrast to the e-mode operation where P
in
and n
in
are parallel, and P
out
and n
out
are also parallel. The viewing angle polar plot for the o-mode TN display is shown in
FIGS. 4 and 5
.
FIG. 4
is the polar plot for V=0 and
FIG. 5
is the polar plot of transmittance for 2.5V. They show clearly the optimal viewing direction which is at the 6 O'clock position The darkest part of the polar plot in
FIG. 5
indicates the light should exit the display at an azimuthal angle &thgr; of 30° and a polar angle &phgr; of 270°.
This optimization of the viewing angle of the liquid crystal display is well-known and has been discussed in the literature. For example, the books by Blinov et al (Electrooptic Effects in Liquid Crystal materials Springer-Verlag, 1994) and Bahadur (Liquid Crystals Applications and Uses, World Scientific, Singapore, 1990) have discussions on the viewing angle of liquid crystal displays. In these discussions, the light is assumed to traverse the liquid crystal cell at an oblique angle once. The viewing angle diagram plots the contrast of the display at the working voltages for light going through the liquid crystal cell at an angle of (&thgr;,&phgr;) where &thgr; is the angle between the light beam and the surface normal of the liquid crystal cell (the azimuthal angle) and &phgr; is the angle between the projection of the light bean on the liquid crystal cell surface and the reference x-axis (the polar angle). The input director of the liquid crystal is also measured referenced to this x-axis. For the case of the 90° twist TN display, as shown in
FIG. 3
, the x-axis is usually taken to be at 45° to the input director.
In the traditional optimization of the liquid crystal display, it is generally assumed that light enters at a certain angle. Many plots of the transmission-voltage curves have been shown in the literature for various combinations of the light viewing angle characterized by (&phgr;,&thgr;). Implicit in such curves, with only one value of &thgr; specified, it is assumed that light enters and exits the cell at the same azimuthal angle. The possibility of light entering and exiting the liquid crystal cell at different azimuthal angles is never considered in the numerical and experimental optimization procedures. The present invention shows that for the case of nonspecular reflection, it is important to perform the simultaneous optimization of all important LCD parameters by considering light entering and exiting the LCD at different angles.
FIG. 6
shown the transmission-voltage curves for liquid crystal displays operating in the so-called second minimum. This second minimum corresponds to a retardation value, the product of the cell thickness and the birefringence of the liquid crystal (d&Dgr;n), of 1.075 &mgr;m and a liquid crystal twist angle of 90°. Curve
17
is when the viewing angle and the light entrance angle are 0° (normal to the cell). Curve
18
corresponds to light entering at &thgr;=30°, &phgr;=90° and the display is viewed at &thgr;=30°, &phgr;=270°. This is the so-called 6 O'clock viewing condition. Curve
19
corresponds to conditions exactly opposite to curve
18
, i.e. light entering at &thgr;=30°, &phgr;=270° and the display is viewed at &thgr;=30°, &phgr;=90°. In the 6 O'clock position, the liquid crystal cell turns off at a lower voltage and the change in transmission as a function of voltage (the transmission-voltage or T-V curve
18
is sharper. This leads to a much better multiplexing capability for this display.
FIGS. 7 and 8
are similar plots for the cases of 120° and 180° twist displays.
In this present invention, we recognize the fact that it is possible to manufacture LCDs where the input light angle and the output light angle are greatly different (non-specular reflection). Such a possibility of having non-specular light reflection was pointed out in U.S. Pat. No. 5,659,408 of M. Wenyon. One way of obtaining this situation is to use the so-called holographic reflector films (see, for example, M. Wenyon et al, “White Holographic Reflector for LCDs”, SID Symp. Dig. 1997). There are additionally many types of structured scattering surfaces that can achieve such nonspecular reflections. However, such prior LCDs do not optimize the reflection.
It is accordingly an object of the invention to seek to mitigate this disadvantage.
According to the invention there is provided a liquid crystal display, characterised by the incident light direction and the direction of light exiting the display after reflection being different directions which are non-specular.
Using the invention it is possible to provide that the incident and reflected light beams to be at different angles. The transmission-voltage curves should be calculated using different values of input and output angles.
A liquid crystal display embodying the invention, thus has all of its critical parameters simultaneously optimized allowing for the input light angle and the viewing angle to be different from each other, thus yielding retardation values of the display that are significantly different from conventional liquid crystal displays. The polarizer angles, the input/output directors and/or the liquid crystal cell retardation may thus be optimized for non-specular operation.
Another significant aspect of the present invention is the recognition of the fact that most of the nonspecular reflectors are monochromatic. That is, even with white light input, the reflected lig
Kwok Hoi Sing
Liao York
Yeoh Colin T.
Blakely , Sokoloff, Taylor & Zafman LLP
Nguyen Hoan
Ton Toan
Varintelligent (BVI) Limited
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