Liquid crystal cells – elements and systems – Liquid crystal system – Projector including liquid crystal cell
Reexamination Certificate
1998-10-16
2001-12-18
Sikes, William L. (Department: 2871)
Liquid crystal cells, elements and systems
Liquid crystal system
Projector including liquid crystal cell
C349S057000, C349S095000, C349S061000, C353S122000
Reexamination Certificate
active
06331878
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a light reflection preventing configuration such as a reflection light absorbing plate for preventing incident light from being diffused in reflection or for reducing the reflection light, in particular to a display panel having a reflection preventing configuration and a light modulation layer at which incident light is diffused to generate a diffusion light, and more particular to a projection type display apparatus employing the display panel as a light valve, and to a display apparatus employing the display panel as an image pick-up monitor for a video camera (hereinafter referred to as, “a view-finder”).
2. Description of the Prior Art
In recent years, research and development of a liquid crystal display panel is actively pursued because the liquid crystal display panel has many advantageous features such as light in weight and thin-in thickness as compared to a cathode ray tube (CRT). Also, the liquid crystal display panel is employed as a display portion of a pocket TV or a view-finder of a video camera. However, it has many problems such that, for instance, it is difficult for use in a large scale screen.
Therefore, many interests and attention have been recently focused on a projection type display apparatus which is adapted to project and enlarge an image picture displayed on a small display panel, through a projection lens or the like for displaying an enlarged image picture on a large scaled screen. It is to be noted here that, for a projection type display apparatus and a view-finder now placed on the market, employed is a twist nematic (hereinafter, referred to as, “TN”) display panel utilizing a light twisting characteristic of liquid crystal.
The display panel using TN liquid crystal is required to linearly polarize incident light by using a polarizing plate (polarizer) disposed on the light incident side of the panel and also required to dispose a polarizing plate (analyzer) on the outlet side of the panel for detecting the modulated light. Namely, it is necessary to arrange, in total, two polarizing plates on the front and rear sides of the TN display panel, that is, one is a polarizer for linearly polarize incident light and the other is an analyzer for detecting the modulated light.
Assuming that the pixel opening ratio of the liquid crystal display panel is 100% and the light amount incident on the polarizer is 100%, the light amount outgoing from the polarizer is 40%, the transmittance of the display panel is 80% and the transmittance of the analyzer is 80%, and therefore, the transmittance as a whole becomes 0.4×0.8×0.8=about 25%, resulting in the effective utilization of only 25% of the incident light. As a result, the TN liquid crystal display panel can realize merely a low luminance image display.
The most of light lost in the polarizing plates and the like is primarily absorbed by the polarizing plates and converted into heat while heating the polarizing plates. The heat converted from the light also heats the display panel through radiation heat. In the case of a projection type display apparatus, the incident light amount applied to the polarizer is of more than several tens of thousands luxes. Therefore, in the case of using a TN display panel as a light valve for a projection type display apparatus, the polarizing plates and the panel are heated to a high temperature, resulting in a remarkable deterioration of performance in a short time period.
In addition, the TN display panel is required to be applied to coating with an orientation film and then subjected to rubbing process. The rubbing processing increases the number of processes, resulting in an increase of a manufacturing cost. Furthermore, the number of pixels of the display panel for use in the projection type display apparatus has recently been increased to a large capacity of more than three hundred thousand pixels and the size of each pixel is in the trend of miniaturization. The miniaturization of the pixel size results in formation of a large number of surface irregularities caused by signal lines, thin film transistors (TFTs)
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, and the like per a unit area. It is obvious that rubbing process can not be made advantageously due to the surface irregularities.
In addition, the miniaturization of each pixel in size makes the area for forming the TFTs and signal lines in one pixel relatively large, resulting in a decrease of the pixel aperture ratio. As an example, in the case where three hundred fifty thousands pixels are formed in a display panel of three-inch diagonal, the pixel aperture ratio is about 30%. In the case of a display panel formed with one million and five hundred thousand pixels, there is also estimated a value of less than 10% as the pixel aperture ratio. These decreases of the pixel aperture ratio not only decreases the luminance of the displayed image but also results in heating of the display panel by the light irradiated on a part other than an incident light aperture, which accelerates deterioration in performance of the TN display panel.
In the TN display panel, light modulation is effected by varying the orientation state of liquid crystals by applying voltage to pixel electrodes. Polarizing plates are disposed respectively on the incident and outgoing sides of the TN display panel, with the polarizing axes of the polarizer and analyzer being intersected at right angles with each other. Generally, the TN display panel is used in the mode (referred to as, “NW mode” hereinafter) wherein a black display can be effected in the voltage application state.
The displayed image on the display panel under the NW mode is excellent in color reproductibility, but it has a problem of leakage of light from the periphery of the pixel. This phenomenon takes place due to the fact that liquid crystal molecules are oriented not in the regular direction but in the opposite direction, which the orientation state is called an opposite chilled domain. This takes place due to the fact that the rising direction of the liquid crystal molecule becomes partly opposite due to the electric field generated between the pixel electrode and the source signal line, and the like. In the portion where the rising direction of the liquid crystal molecules are opposite, light passes through the analyzer located on the light outgoing side of the panel despite the application of voltage. Namely, leakage of light takes place. If the rising direction of liquid crystal molecule is regular, no leakage of light will take place.
As a method for preventing light leakage, there is available a method of increasing width of a black matrix (referred to as, “BM” hereinafter) to be formed on the confronting electrodes, but it also reduces the pixel aperture area of the matrix and thus reduces the luminance of a display image, and therefore, it cannot be regarded as a useful method.
In the display panel using TN liquid crystals as described above, it is required to use polarizing plates. In addition, because light leakage tends to take place at the peripheral portion of the pixel, the BM must be made thick. Therefore, the light utilizing rate is poor and the display image is low in luminance. The light applied to the BM heats the display panel and consequently raises the panel temperature, deteriorating the life of the display panel.
Similarly, the projection type display apparatus employing a TN display panel as a light valve is also poor in the light utilizing rate, and the luminance of the projection image picture is low in the screen.
In view of this fact, there has been proposed a projection type display apparatus employing a polymer dispersed (referred to as, “PD” hereinafter) display panel instead of using polarizing plates. As an example, an apparatus disclosed in Japanese Patent Laid-Open Unexamined Publication No. 94225/1991 can be enumerated. In the PD display panel used as a light valve in the projection type display apparatus, light modulation is effected by scattering or transmitting the incident
Duong Tai V.
Matsushita Electric - Industrial Co., Ltd.
Sikes William L.
Wenderoth , Lind & Ponack, L.L.P.
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