Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
1998-12-21
2002-02-19
Sikes, William L. (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S111000, C349S153000
Reexamination Certificate
active
06348959
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to silicon based reflective liquid crystal displays (LCDs) and projection devices comprising a light source, such LCDS having a layer of liquid crystalline material between a first and second substrate which may be provided with orientation means, preferably the molecules of the liquid crystalline material having a twist angle &phgr;, means for guiding light from the light source to the first substrate, the second substrate being provided with means for reflecting light passing the layer of liquid crystalline material, said projection device further comprising polarizing means in the light path between the light source and the first substrate and analyzing means in the light path, after reflection, between the liquid crystal display device and a display plane.
BACKGROUND OF THE INVENTION
Silicon based reflective LCDs are potentially the most cost effective solution for high performance, high resolution digital projection in both the business and consumer markets. To maintain this cost effectiveness, a single panel approach is desirable. Reflection offers an advantage to this approach because LCDs can be made thinner and faster, thus offering the brightness and color quality expected in the high performance market via the single panel approach. LC effects which can switch to dark as fast as possible are desirable for single panel operation because they can block the unwanted colors during scanning most effectively with minimal use of dark regions between the scanning colors (guard bands). This results in high color purity and higher brightness when compared with slower responding devices, such as transmissive LCDS. The physical properties of liquid crystals imply that the electric field applied transition should be to a dark state in order to meet this requirement. Thus, optimally, all candidate effects should be “normally white”, i.e., fully transmissive with no field applied. This means that with the highest voltage on the electrodes, no light is directed to the viewer from these areas/pixels and therefore they appear dark.
Normally white LC effects can be separated into two different categories: those requiring external foil compensation and those which do not require external foil compensation. A disadvantage of both categories, however, is that areas of inactivity (passive regions where no voltage can be applied) in the border of the display device appear bright.
Furthermore, it may be desirable to integrate driving electronics into the silicon immediately outside the active area. These areas must be shielded from light in order to function properly. A simple layer of aluminum above these circuits would most effectively shield them from light but this layer would act as a mirror reflecting most of the incident flux directly to the viewer with a “normally white” LC effect. Certain precautions have been recommended to render these areas dark to the viewer, for example, by replacing the aluminum with a black chromium layer. This requires an extra masking step which makes the display more expensive, while it also adds difficulty to the process of coupling the two substrates. Furthermore, chromium is not a commonly used material in silicon processing as it can render circuitry inoperable due to contamination.
Such a light absorbing mask could be used on the passive plate, but then it must be critically aligned to the active region of the silicon chip. Additionally, in order to absorb sufficiently, a minimum thickness of material is required and as such will have to be compensated for in order to maintain uniformity in a thin cell gap. Furthermore, the light energy absorbed will be converted to a significant amount of heat under the intensities expected in any projection system.
There is a need in the art for rendering these undriven display regions dark with a normally white LC effect in which these disadvantages are obviated as much as possible while still effectively shielding any underlying circuitry from the incident light.
SUMMARY OF THE INVENTION
An object of this invention is to provide a display device which uses a normally white LC effect that is free of or in which the above-described disadvantages are obviated as much as possible.
Another object of the invention is to make the regions that integrate driving electronics into the silicon immediately outside the active area appear dark to a viewer while still offering effective light shielding for the underlying circuitry.
Another object of the invention is to provide a display device, for use in reflection, that is provided with a border structure around an active display area, said border being effective to shield areas of said display device from incident light and to render said shielded areas dark to a viewer in the non-driven state.
These and other objects of the invention are accomplished by the provision of a projection display device, according to one embodiment of the invention, which comprises a layer of liquid crystalline material between a first and a second substrate, which substrates are optionally and preferably provided with orientation means and the molecules of the liquid crystalline material optionally and preferably having a twist angle &phgr;, means for guiding light from the light source to the first substrate, the second substrate being provided with means for reflecting light passing the layer of liquid crystalline material, preferably a pixel array active area of said substrate, said projection device further comprising polarizing means in the light path between the light source and the first substrate and, optionally and preferably, analyzing means in the light path, after reflection, between the liquid crystal display device and a display plane,
wherein a portion of the liquid crystalline material is displaced by a dam comprising a substantially transparent, non-birefringent material such as SiN
x
, said dam forming a border, preferably, a substantially continuous border which substantially surrounds the pixel array active area of the display device, and wherein, most preferably, the liquid crystal display device is provided with a retardation foil or other compensation device, substantially all areas of said display device being dark in a non-driven state without absorbing substantial energy of the incident light flux.
As used herein, the phrase “non-driven state” means any voltage below the threshold for reorientation of the liquid crystalline material that is applied across the cell gap.
In another embodiment of the invention, there is provided a projection display device which comprises at least one light source whose light is incident on a display device as described hereinabove and is reflected dependent on the optical state of pixels defined by picture electrodes surrounded by a substantially continuous border comprising a dam of non-birefringent material, the light thus modulated being imaged by projection means, the areas situated between the electrodes and also other non-switching areas, for example, electronics integrated along the edge of the picture sections of said display device, being effectively shielded from the incident light yet appearing substantially dark in the non-driven state.
Thus, the invention involves the concept of a dark border employing a non-birefringent material on the active plate, around the active matrix of the reflective LC display device, and the use of LCDs comprising such borders. The border may be formed from a “dam” of any non-birefringent material, preferably a photodefinable spacer material already patterned or otherwise provided on the active plate. Preferably, the thickness of the dam is such as to allow a thin layer of LC material on top of the dam, and most preferably, the border structure shields the areas of underlying circuitry between the glue seal and the active region.
By design, this border has proven effective for both foil compensated and non-foil compensated effects.
As stated hereinabove, the invention is particularly suitable for devices using a “normally white LC mode” in which, in the
Melnick George A.
Pinker Ronald D.
Bartlett Ernestine C.
Chowdhury Tarifur R.
Philips Electronics North America Corporation
Sikes William L.
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