1. Field of the Invention
The present invention relates to a reflection type liquid crystal display device and a display apparatus therewith, in particular, to a reflection type liquid crystal display device and a display apparatus therewith having a high contrast ratio and a high display quality.
2. Description of the Related Art
In recent years, the number of pixels of liquid crystal display devices has been increased and in particular the size thereof have been reduced. For example, a fabrication of a liquid crystal display device with a 0.7 inch panel on which as many as 300,000 pixels are formed has been studied. To do that, in such a liquid crystal display device, thin film transistors (TFTS) composed of polysilicon should be formed on a peripheral portion of a substrate with display pixel electrodes and so forth. In addition, the aperture ratio of the pixel portion should be increased. When the pixel size of the liquid crystal display apparatus is reduced, the decrease of aperture ratio should be considered as an important problem.
In other words, in a conventional liquid crystal display device, light is entered from the opposite substrate. A liquid crystal layer is used as a light shutter. Light passes through the light shutter and is emitted from the array substrate (namely, the front surface of the panel). In this case, the aperture ratio of each pixel is at most 30 to 40 %. The rest (60 to 70 %) of the pixel is a light shielding layer that shields incident light.
If a switching device that turns on and off a pixel electrode is constructed of a TFT (thin film transistor) composed of for example amorphous silicon, when light strikes the TFT, leak current takes place due to an optical excitation effect. Thus, the operation characteristics of the switching device decrease. Alternatively, the switching device may malfunction. To prevent this problem, a light shielding film that shields the switching device from the light is required. When the light shielding region includes portions such as signal lines and scanning lines composed of metal materials, the region accounts for as many as 60 to 70 % of the effective display area.
Thus, as the pixel size decreases, the luminance of the display screen decreases and the contrast ratio degrades. Particularly, in a projection type liquid crystal display device, the contrast ratio of more than 100:1 is required. Thus, when the display area of the pixel becomes small, a more critical problem takes place. Consequently, for the liquid crystal display device with small pixels, the efficiency of the use of the incident light should be further improved.
As a means for solving this problem, the advantages of so-called reflection type display device that reflects the incident light on the pixel electrode and emits the light to the incident side are becoming attractive. The reflection type display device has been studied and developed as the liquid crystal display device that should have a higher pixel aperture ratio.
In the reflection type liquid crystal display device, various structural portions including switching devices, scanning lines, and signal lines are disposed below pixel electrodes. Light reflected on the pixel electrodes is displayed on a screen. Thus, the aperture ratio of the pixels of the reflection type liquid crystal display device is theoretically 100 % except dividing region of pixels. Although the aperture ratio depends on the panel size, so far it has been improved up to 85 %.
However, in the reflection type liquid crystal display device, light entered from the image display side is reflected on the pixel electrodes and displayed on the screen. Thus, in addition to the light that is reflected on the pixel electrodes, undesired light takes place on the front surface of a base member of the liquid crystal display panel. The undesired reflected light dazzles the display screen, thereby remarkably degrading the on/off ratio of the displayed image The theoretical reflectivity on a transparent base member of the opposite substrate is given by the following equation.
xe2x80x83r2={(n1xe2x88x92n2)/(n1+n2)}2
where r2 is the reflectivity on the base member; and n1 and n2 are the refractive indexes of a first medium and a second medium, respectively.
When light enters from air (refractive index n1 =1.0) to a transparent substrate such as a glass substrate (refractive index n2 =1.5), the reflectivity is about 4%.
Thus, even if the reflection efficiency of the reflection pixel electrodes is improved to 100% and the absorption loss of light in the liquid crystal layer and the optical path is zero, the on/off contrast ratio of the displayed image that is finally observed is 96 % 4 % =24:1. Consequently, the contrast is insufficient for both the projection type liquid crystal display device that requires an on/off contrast ratio of more than 100:1 and the direct view type liquid crystal display device.
As a means for preventing light from reflecting on the interface in between different materials in the display device, a reflection protecting film may be coated on the liquid crystal display panel. However, the effect of such a reflection protecting film is limited to a region of a particular wavelength. In addition, since the fabrication of the liquid crystal display panel becomes complicated, the cost thereof increases.
In addition, even if such a reflection protecting film is used, as shown in a curve of contrast ratio v.s. aperture ratio of FIG. 18, undesired reflected light cannot be suppressed enough. Thus, the above-mentioned high contrast ratio of 100:1 cannot be actually accomplished.
For example, in a three-panel type liquid crystal projector, liquid crystal display panels are directly contacted with a dichroic prism. Alternatively, the panels are adhered to the dichroic prism with an adhesive region with almost the same refractive index as that of a glass substrate. In such a manner, the amount of reflected light can be halved.
However, a incident light in the dichroic prism is reflected on the front surface of the dichroic prism. The reflected light is emitted from the dichroic prism along with signal light reflected from the panels. The resultant light is projected to the screen through a projection lens system. Thus, the contrast ratio remarkably deteriorates due to the undesired reflected light, thereby adversely dazzling the screen.
The present invention is made to solve such problems. An object of the present invention is to provide a reflection type liquid crystal projector having a reflection type liquid crystal display device that separates light for image display from light that is simply reflected on the surface of crystal display panel and prevents a contrast ratio from deteriorating due to reflected light so as to display an image with a high contrast ratio.
According to the present invention, each pixel electrode is disposed with an inclination angle to a base member (for example, glass substrate) on the front surface side of a liquid crystal display panel (namely, on an opposite substrate side). In addition, the front surface of the opposite substrate is disposed with an inclination angle to a base member (for example, glass substrate) of a liquid crystal display panel (namely, on an array substrate side).
The front surface of the opposite electrode can be a curved shape having a inclination angle being changed continuously. The curved shape includes a convex curved shape and a concave curved shape. Those area on the front surface of the opposite electrode where the curved shape is formed, are faced with a pixel electrode, a plurality of the pixel electrodes adjacent, or a part of the pixel electrode.
The relation between the reflecting surface of the pixel electrode and the main surface of the base member of the liquid crystal display panel is defined so that the difference between the reflecting angle of light that is entered as light of a light source and reflected on the front surface of the base member and the leaving angle of the light emitted as light that is displayed through a liquid crystal display panel is equal to or greater than the collection angle of the display device optics. Thus, the undesired reflected light on the main surface of the base member of the display is separated from the light that is displayed. Thus, since the dazzling of the display screen due to the undesired reflected light and the deterioration of the contrast ratio are prevented, an image with a high contrast ratio can be accomplished.
Inclination of the front surface of the opposite substrate is not restricted being formed of a linear surface but also being formed of a curved surface having a distribution of inclination angle. The curved first surface (the front surface) of the opposite substrate separates light so that a leaving direction of the light that is reflected by the first surface of the opposite substrate is different from a leaving direction that is emitted from the first surface of the opposite substrate after having been reflected by the pixel electrodes through the liquid crystal layer. Those aspect of the invention can be applicable to a projection type display apparatus having the liquid crystal display apparatus employes a light separating means such as the curved surface.
In the liquid crystal projector according to the present invention, the incident plane to a dichroic prism is inclined to the optical axis (not perpendicular thereto). Thus, signal light that is reflected on each reflection type liquid crystal display panel can be separated from undesired light that is reflected on the surface of dichroic prism. Consequently, an image with a high contrast ratio can be displayed.
In particular, since a reflection type liquid crystal display device applying a polymer dispersed liquid crystal layer has twice as long an optical path as that of a transparent type liquid crystal display device, a cell thickness for obtaining the desired contrast ratio can be reduced. In addition, such advantages as to reduce both the driving voltage and the response time are accompanied.
Accordingly, a projection type display apparatus applying a reflection type liquid crystal display device can make a collecting angle of the schlieren optical system larger than a projection type display apparatus with a transparent type liquid crystal display device, realizing a bright display while keeping a high contrast ratio.
Furthermore, the present invention eliminates the reflecting light on the surface of a liquid crystal display panel or the surface of a dichroic prism, thereby realizing a display further excellent in display quality with a higher contrast ratio.
An inclination angle of the incident plane of the dichroic prism to the optical axis and a relative inclination angle of each pixel electrode to the base member on the front surface side (namely, on an opposite substrate side) are preferably in the range from 1 to 10 degrees. The inclination angle is more preferably in the range from 2 to 5 degrees. When the inclination angle is larger than the conventional angle (ranging from 5 to 15 degrees), the difference of the thicknesses of the dichroic prism or the opposite substrate becomes too large. Alternatively, it becomes difficult to control the variation of the cell gap in the liquid crystal display panel. So, due to the collection angle, the contrast ratio is remarkably varied in polymer dispersed liquid crystal display device. In addition, uneven display takes place due to the difference of cell gap. Experimental results conducted by the inventors of the present invention show that when the inclination angle is not in the range of the preferable angles, the resultant liquid crystal display device may not be practically used.
In a reflection type liquid crystal display device that is a related art reference disclosed as Japanese Patent Laid-Open Publication No. 4-147215, light from a light source is linearly polarized by a polarizing beam splitter. In addition, light that is reflected on a polarizing converting plane is vertically entered into the liquid crystal display device. And light that is reflected by the liquid crystal display device and rotates a direction of polarization is guided to the polarizing beam splitter. And then, the reflected light is transmitted through the polarizing beam splitter and projected as an enlarged image. In this related art reference, to prevent the contrast ratio from decreasing due to the mixture of the reflected light on the main surface of the opposite substrate of the liquid crystal display device with the light that is displayed, the opposite substrate is formed in a wedge shape with an inclination angle ranging from 5 to 15 degrees. Thus, the light reflected at the interface of the opposite substrate is prevented from being entered into the polarizing beam splitter. To do that, the distance between the reflection type liquid crystal display device and the polarizing beam splitter should be increased. Alternatively, the inclination angle should be increased. The former has a disadvantage in which the size of the optical system becomes larger. On the other hand, the latter has disadvantages in which the thickness of one edge of the substrate is much larger than that of the other edge. In addition, when liquid crystal cells are fabricated, they are pressured in upper and lower directions. Thus, it is very difficult to control (or prevent) the variation of the thickness of cells in a liquid crystal display panel. In an optical system of the related art reference, since the collection angle of the light of the light source is not controlled, the collection angle of the incident light of the optical system that uses a conventional light source and a conventional reflector is around 14 degrees or greater. Actually, in the art reference, the polarization direction of light reflected on the front surface of the panel does not change. Thus the main light reflected on the front surface of the panel is vended to the light source by the polarizing beam splitter and not entered to the projection lens.
However, according to the present invention, both the collection angle on the light incident side and the collection angle on the light leaving side are predefined. Thus, even if the inclination angle that causes the reflection on the pixel electrodes to be separated from the reflection on the opposite substrate is small, the contrast ratio can be easily improved. Consequently, the variation of the thickness of cells in a liquid crystal display panel can be prevented. In addition, the size of the optical system can be reduced.
These and other objects, features and advantages of the present invention will become more apparent in light of the following detailed description of best mode embodiments thereof, as illustrated in the accompanying drawings.
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