Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
2001-01-12
2002-11-05
Sikes, William L. (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S113000, C349S118000, C349S121000
Reexamination Certificate
active
06476891
ABSTRACT:
FIELD OF THE INVENTION
This invention pertains to a reflective liquid crystal display device that has a phase difference film and a liquid crystal layer and a reflecting sheet, wherein the above mentioned incident light is reflected by the above mentioned reflecting sheet, and the reflected light passes through the above mentioned liquid crystal layer and the above mentioned phase difference film.
BACKGROUND OF THE INVENTION
Existing Technology
Because reflective liquid crystal display devices keep a high luminance, even outdoors, and backlight is not necessary, they are desirable from the viewpoint of low (electric) power consumption, and in recent years development has been advancing.
An example of such reflective liquid crystal display devices is a device wherein a reflective function has been given to the pixel electrodes themselves. This device has 1 polarizing sheet and at least 2 sheets of phase difference film and a liquid crystal layer and pixel electrodes that have the function of a reflecting sheet. These at least 2 sheets of phase difference film have the functions mentioned below.
FIG. 9
shows the correlation of the voltage that is applied to the liquid crystal layer and the reflectance of the liquid crystal layer. Herein the reflectance is the proportion of the quantity of light that is obtained when linearly polarized incident light has been incident on the phase difference film and liquid crystal layer, has been reflected by the reflecting sheet, has left the said liquid crystal layer and said phase difference film again and thereafter has again passed through the polarizing sheet, when the quantity of the above mentioned incident light that has been linearly polarized after having passed through the above mentioned polarizing sheet, is set at 100. Graph 1 is the case wherein these at least 2 sheets of phase difference film are not present. When a high voltage is applied to the liquid crystal layer, almost 100% of the incident light is reflected, and sufficient white can be displayed. Since at the low voltage side, however, the reflectance does not drop to 0%, the black luminance is floating. By the fact that herein the ideal ¼&lgr; sheet is formed with the use of at least 2 sheets of phase difference film, the white characteristics turn black, and on the other hand, the black characteristics turn white, viz, the low voltage side was made white. Graph 2 is the case wherein at least 2 sheets of phase difference film were used and turned into the ideal ¼&lgr; sheet, and the reflectance at the high voltage side is almost 0%, viz., a sufficient black level is obtained.
In order to form the ideal ¼&lgr; sheet, however, at least 2 sheets of phase difference film were necessary in such existing reflective liquid crystal display devices. Moreover, because a sufficient white level was not obtained, even if a sufficient black level had been obtained, the contrast was low. Moreover, by the angle under which the reflective liquid crystal display device is observed, it had the unfavourable situation that the black luminance was floating, and that the white luminance became dark. Moreover, it had The unfavourable situation that even in the case that colour image is displayed, the colour changes, depending on the characteristics of the angle of vision.
SUMMARY OF THE INVENTION
Problems that Should be Solved by the Invention
This invention has the aim to offer a reflective liquid crystal display device with a high contrast, wherein floating of the black luminance is prevented. Moreover, this invention has the aim to offer a reflective liquid crystal display device That has sufficient wide angle of vision characteristics.
Means to Solve the Problems
The reflective liquid crystal display device of this invention has the characteristic feature that it is the reflective liquid crystal device that has been described in claim
1
.
The inventors of the present application discovered that the cause of floating of black luminance is the fact that, de-pending on the wavelength, the light after reflection of incident light and passage through the liquid crystal layer and the phase difference film, has not become linearly polarized light. The reason is that, because only the quantity that has slipped away from the linearly polarized light passes through the polarizing sheet, even if a voltage is applied to the liquid crystals and the polarization plane of the light is changed, it is no longer converted to a black display. The inventors of the present application discovered the conditions of the phase difference film and the conditions of the liquid crystal layer as conditions to turn light, after passage through the phase difference film, into linearly polarized light. Below, this is explained.
Because in the case of a reflective liquid crystal display device, the angle of the incident light in the reflecting sheet and the angle of the reflected light are equal, the effect of auto-compensation is obtained with respect to the angle of the incident light. Consequently, it is sufficient to observe either the process from the incidence of light in the reflective liquid crystal display device until it has arrived in the reflecting sheet, or the process from reflection by the reflecting sheet until the light has arrived at the said reflective liquid crystal display device. In this application, the latter is observed, as shown below.
In reflective liquid crystal display devices, light that has been incident is linearly polarized by the fact that it passes through a polarizing sheet. In order to obtain a black display in the reflective liquid crystal display device, based on this incident light, it is necessary that the above mentioned linearly polarized light that has been incident, becomes circularly polarized light on the reflecting sheet, and again becomes linearly polarized light after having left the phase difference film. The reason is that the direction of polarization of the reflected light has to perpendicularly cross the optical axis of the above mentioned polarizing sheet. Here, the state of polarization in the case that the circularly polarized light from the reflecting sheet has been incident on the liquid crystal layer, and has left the said liquid crystal layer, is considered.
In the case that the state of polarization is observed, in general a Poincare sphere and Stokes parameters are used. The Poincare sphere is a spherical surface that displays the state of polarization, corresponding to a point on a spherical surface whereof the radius is proportional to the strength. The Stokes parameters are the 4 parameters S
0
, S
1
, S
2
and S
3
, that present the state of polarization, also including partial polarization. S
0
presents the strength of the partial polarization, and S
1
, S
2
and S
3
are 3-dimensional components in the case that the polarization has been displayed with a Poincare sphere, and they coincide with The x, y and z components. With the state of polarization as a point on this Poincare sphere, the state of polarization can be presented with the use of Stokes parameters S
1
, S
2
and S
3
. Below, this point is called the Stokes parameter point.
It is an object to provide a reflective LCD apparatus to obtain a wider view angle and a desired black- view. A reflective LCD apparatus according to the invention comprises a phase difference film (retardation film), a liquid crystal layer and a reflector for reflecting incident light. The reflective LCD apparatus suffices in a plurality of Stokes parameter points of said reflected light are plotted on a approximately straight line on a Poincare sphere, indicative of light polarization state by said liquid crystal layer, sighted in a plane cross section, and the relation between sad liquid crystal layer and said phase difference film with respect to said reflected light substantially satisfies:
cRet(&lgr;)=&lgr;&thgr;(&lgr;).
The state of polarization of the light that includes in the liquid crystal layer is observed with the Poincare sphere.
FIG. 1
shows the S
1
-S
2
plane that appears
Shibazaki Minoru
Uchida Tatsuo
Chowdhury Tarifur R.
Koninklijke Philips Electronics N. V.
Sikes William L.
Waxler Aaron
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