Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
1999-11-29
2002-01-22
Parker, Kenneth (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S099000, C349S180000, C349S181000
Reexamination Certificate
active
06341001
ABSTRACT:
The invention relates to a reflective active matrix liquid crystal display mixed with mixed twisted nematic and birefringent modes.
FIELD OF THE INVENTION
The present patent deals with reflective mode liquid crystal displays that combine the waveguiding effect and the birefringent effects of twisted nematic liquid crystals. By optimizing the various optical arrangements of the liquid crystal display such as the twist angle, the retardation and the polarizer angle, a series of mixed twisted nematic—birefringent (MTB) display modes has been invented.
There has previously been considerable activity in the study of reflective liquid crystal displays (RLCD). Broadly speaking RLCD can be divided into 2 categories: those that do not rely or polarizers and those that do. Examples of the former are reflective cholesteric displays and absorptive guest-host displays. The latter categories are necessarily nematic liquid crystal displays. These are displays that are based on polarization manipulation, as in ordinary twisted nematic LCDs. However, unlike ordinary LCD, there is only one front polarizer and the rear polarizer is eliminated.
The main applications for such RLCDs are in direct view displays with no backlighting, and in projection displays using crystalline silicon backplane with integrated CMOS drivers, or in reflective liquid crystal light valves (LCLV) in general.
Reflective nematic LCD has been investigated. One of the successful inventions is the so-called TN-ECB mode. A variation of this has been reported recently by Wu et al. It has a 90° twist angle. There are many names given to display modes that operate on a combination of the waveguiding TN effect and the pure birefringent effect, e.g. the 45°, hybrid-field-effect (HFE) mode, the 63°, TN-ECB mode the 90° mixed TN (MTN) mode, the self-compensated TN (SCTN) mode and the 52° RTN mode.
A generalized picture of reflective twisted nematic LCD is disclosed herein that encompasses all of these mixed mode displays, and provide a method of optimizing them all at the same time. Many new operating conditions can be found that have not been reported and are the subject of the present invention.
These reflective liquid crystal displays can be fabricated on passive matrix or active matrix backplanes. The active matrix backplane can be fabricated on glass or on silicon wafers.
SUMMARY OF THE INVENTION
As shown in
FIG. 1
, the reflective nematic LCD consists of a polarizer, a twisted nematic liquid crystal cell, and a reflector, which can be part of a circuit in an active matrix device. The polarizer can either be a sheet type polarizer or a polarizing beam splitter as shown. In this invention, the PBS case is generally described as it is the most popular geometry for silicon microdisplays.
As discussed in the paper by H. S. Kwok, [“Parameter space representation of liquid crystal display operating modes, J. Appl. Phys., 80(7), 3687-3693 (1996)], all nematic RLCD modes can be represented in the parameter space diagram The parameter space in the case of twisted nematic RLCD is particularly useful, as it shows the relationship between the TN-ECB, MTN, SCTN and ECB modes. The reflectance R of the RLCD is a function of 3 major parameters: twist angle &phgr;, polarizer angle &agr; between the polarizer and the input director of the LC cell, and the LC cell retardation d&Dgr;n where d is the cell thickness. The wavelength &lgr; always appears together with the retardation as d&Dgr;n/&lgr; in the Jones matrix. Therefore it can be treated as just a scaling of d&Dgr;n. Hence, if one of the 3 parameters (&agr;, &phgr;, d&Dgr;n) is fixed, R can be plotted as a function of the other two parameters in a 2D parameter space using contour lines.
FIG. 2
shows a series of parameter spaces for the RLCD, with &agr; varying from 0 to 45°. A wavelength of 550 nm is assumed in the calculations. The contours indicate constant reflectance in steps of 0.1. The wells in
FIG. 2
are the so-called TN-ECB minima. The center of the well corresponds to either maximum reflectance for crossed polarizers or minimum reflectance for parallel polarizers. For example, with a polarizing beam splitter (PBS) in the display, (&agr;, &phgr;, d&Dgr;n)=(0, 63.5°, 0.181 &mgr;m) will give R=1. This corresponds to the first TN-ECB minimum. It is marked in
FIG. 2
The SCTN mode and the MTN mode are also indicated in
FIG. 2
for the appropriate &agr;.
Polarizer angles larger than 45° are not depicted in FIG.
2
. It is because that beyond 45°, the parameter space repeats itself, except for a reflection of the x-axis, i.e. the parameter space for &agr;=90°−&agr; is the same as the one for &agr;, with &phgr; changed to −&phgr;. From
FIG. 2
, it can be seen that there are 2 sets of operating modes for reflective LCD. One set of modes are the “in-well” kind which correspond to the islands in the parameter space, such as the TN-ECB, MTN, SCTN modes. The other set of modes are the “out-well” modes which are located outside the TN-ECB wells, such as the RTN, RSTN and HFE modes.
The “in-well” modes are disclosed herein. It can be seen in
FIG. 2
that the various TN-ECB minima move systematically in the parameter space as &agr; is changed. In particular, the first TN-ECB mode with +&phgr; is examined. It can be seen that this mode becomes the MTN mode at &phgr;=900°, &agr;=22°, then it becomes the SCTN mode at &phgr;=60°, &agr;=30°. Finally, this first TN-ECB minimum becomes the true ECB mode at &phgr;=0° and &agr;=45°.
The situation is clearly shown by a plot of the trajectory of the center of the first TN-ECB minimum for the +&phgr; case as shown in FIG.
3
. In this plot, &agr; goes from 0 to 45° in steps of 5°. It can be seen the first TN-ECB minimum first moves out and then towards the y-axis. The retardation increases monotonically as &agr; increases. The maximum twist angle reaches 70.2° at a polarizer angle of 15°.
FIG. 4
is a similar plot of the 0.9 reflectance contours for &agr; ranging from 0 to 90°, again in steps of 5°. This plot is different from
FIG. 3
because we also include &agr; from 45° to 90°. As can be seen from
FIG. 4
, as &agr; goes from 45° to 90°, the originally −&phgr; TN-ECB minimum moves into the positive &phgr; side, thus forming a complete loop in the parameter space. This is more easily seen in a parameter space showing both positive and negative twists (FIG.
5
).) Notice that the parameter space for &agr; and &agr;+90° are identical so that a complete trajectory is formed in
FIG. 5
as a goes from 0 to 90°.
FIG. 3
indicates that for twist angles from −70° to +70°, there always exists 2 first order TN-ECB minima at different polarizer angles, one with a smaller d&Dgr;n value and one with a higher d&Dgr;n value.
The operating points of the MTN mode, the TN-ECB mode and the SCTN mode are also indicated in FIG.
4
. Thus
FIG. 4
unifies the entire picture for the TN-ECB, the MTN and the SCTN modes. They all operate with a combination of polarization rotation (TN) and birefringence (ECB) effects. They differ by a rotation of the polarizer relative to the input director, or, in other words, by the proportion of TN to ECB effects. Therefore, it should be possible to perform an optimization of these modes in a general sense, allowing for variations of all 3 parameters simultaneously.
The nomenclature of these nematic reflective LCDs will now be defined. Since all of these modes operate with a combination of TN effect and ECB effect, they can be called a hybrid mode or a mixed mode. They have been called TN-ECB, MTN, SCTN or HFE in the literature. Instead of calling them the TN-ECB/MTN/HFE mode, such LCD operating modes are hereinafter referred to as the generalized mixed TN-birefringence mode, or MTB mode in short.
In the optimization of the MTB mode, it can be assumed that high reflectance is desirable. If the desired reflectance or light efficiency is set to be 0.9, then the solution will be bound by the 0.9 reflectance contours depict
Parker Kenneth
Qi Mike
Varintelligent (BVI) Limited
Wood Phillips VanSanten Clark & Mortimer
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