Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
2000-02-08
2004-02-17
Nguyen, Dung (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S096000, C349S106000
Reexamination Certificate
active
06693688
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is based upon and claims the benefit of priority from the prior Japanese Patent Applications No. 11-029493, filed Feb. 8, 1999, No. 11-066088, filed Mar. 12, 1999; and No. 2000-016482, filed Jan. 26, 2000, the entire contents of which are incorporated herein by reference.
BACKGROUND OF THE INVENTION
The present invention relates to a flat display device such as a liquid crystal display device.
For example, a reflection type liquid display device as a conventional flat display device displays an image using external light, so illumination light is short and results in a dark display screen depending on use environments. In particular, this display device cannot be used in a dark place.
There has been developed a semitransmission type liquid crystal display device using a semitransparent reflecting plate (half mirror) for reflecting external light and a backlight arranged on the back surface side of the semitransparent reflecting plate, so as to use the display device as a transmission liquid crystal display device in a dark environment. The utilization efficiency of incident light of the semitransparent reflecting plate is 50% at maximum. The brightness of the display screen is much lower than that of the transmission or reflection type liquid crystal display device.
To overcome this drawback, there has been examined a semitransmission type liquid crystal display device in which a pinhole corresponding to each pixel is formed in a reflecting plate, and a microlens corresponding to each pinhole is disposed. In this liquid crystal display device, when external light is used, the external light reflected by an area except the pinholes of the reflecting plate is used as a light source. When a backlight is used, light transmitted through the pinholes is focused by the microlenses, and the focused light is used to improve the light utilization efficiency.
Even in such a liquid crystal display device, however, light loss caused by the pinholes occurs in using the external light. The transmission type liquid crystal display device using a backlight is frequently used to increase power consumption.
The reflecting plate having pinholes has a complicated structure and must be attached as an independent member to the liquid crystal panel. This produces disparity and greatly degrades the display performance.
There has also been examined a so-called front light type display device in which a light guide plate is disposed on the observation surface side of a reflection type liquid crystal display device, and a linear light source is disposed on the side surface of the light guide plate. However, light is frequently reflected on the front light surface, and display quality such as contrast of the liquid crystal display device greatly degrades.
In the above semitransmission type liquid crystal display device can perform color display by forming a color filter layer. More specifically, the conventional semitransparent type color liquid crystal display device is constructed by stacking a polarization plate, front-surface substrate, color filter layer, drive electrodes, liquid crystal layer, back-surface substrate, semireflecting plate, and back-surface light source in this order. The color filter layer is formed on the front side of the semireflecting plate, i.e., on the observer side.
When the liquid crystal display device functions as a reflection type, external light entering from the front-surface substrate side passes through the color filter layer and liquid crystal layer, is reflected by the reflecting plate, and passes through the liquid crystal layer and color filter layer again, and emerges outside. That is, the external light passes the color filter layer twice along the forward and return optical paths. On the other hand, when the liquid crystal display device functions as a reflection type, light emitted by the back-surface light source passes through the color filter layer only once.
When the display device as the transmission type uses a color filter layer to obtain a sufficient saturation, external light is greatly absorbed by the color filter layer in the display device functioning as the reflection type because the external light passes through the color filter twice. As a result, reflection brightness lowers. Assume that the density of the color filter layer is reduced to obtain sufficient brightness as the reflection type. That is, assume that the wavelength dispersion characteristic of the transmittance of the color filter layer is designed to obtain a desired saturation upon transmission of external light through the color filter layer twice. For example, assume that the Y value of the average transmittance is set to 40% or more. In this case, light emitted by the back-surface light source in the liquid crystal display device functioning as the transmission type passes through the color filter only once, and the saturation becomes short.
As described above, in the conventional semitransmission type color liquid crystal display device, when it functions as the reflection type, the display brightness greatly degrades; when it functions as the transmission type, the display type color density greatly lowers. The conventional semitransmission type color liquid crystal display obtains either optical characteristics.
BRIEF SUMMARY OF THE INVENTION
The present invention has been made in consideration of the above situation, and has as its object to provide a semitransmission type flat display device which can solve the conventional problems and greatly improve light utilization efficiency.
It is another object of the present invention to provide a flat display device capable of displaying an image with sufficient brightness at a sufficient color density even if the display device functions as the transmission or reflection type.
In order to achieve the above object, according to the present invention, there is provided a flat display device comprising a first polarization plate having a polarization axis and adapted to transmit linearly polarized light along the polarization axis, an optical modulation layer located behind the first polarization plate to modulate incident light in accordance with an applied voltage, a selective reflecting layer located behind the optical modulation layer to selectively reflect a first circularly polarized light component of the incident light, and a backlight located behind the selective reflecting layer to output light having intensity peaks in a plurality of predetermined wavelengths,
wherein the selective reflecting layer substantially transmits, of the first circularly polarized light component, light components having wavelengths in a plurality of small regions including the respective predetermined wavelengths, and substantially reflects a light component in regions between the plurality of small regions.
According to the flat display device having the above arrangement, about 95% of the first circularly polarized light components output from a back-surface light source passes through the selective reflecting layer. The selective reflecting layer reflects about 90% of the first circularly polarized light component of the external light. Regardless of whether the flat display device operates as either a reflection type device or the transmission type device, an image can be displayed at a high color purity and high brightness.
Another liquid crystal display device according to the present invention comprises: an optical modulation layer which is sandwiched between a pair of observation-side and back-surface-side transparent substrates opposing each other and has a plurality of liquid crystal pixels arranged in a matrix, the optical modulation layer being adapted to modulate incident light in accordance with an applied voltage; a selective reflecting layer having a plurality of selective reflecting filters arrayed in a predetermined cycle, respectively, on back-surface sides of the liquid crystal pixels to selectively and partially reflect light components having different wavelength ban
Hisatake Yuzo
Nakamura Takashi
Oono Atsuko
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