Liquid crystal cells – elements and systems – Particular structure – Particular illumination
Reexamination Certificate
1999-07-08
2001-04-24
Parker, Kenneth (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Particular illumination
C349S064000, C349S096000, C349S057000
Reexamination Certificate
active
06222598
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to liquid crystal displays, and particularly to a backlight device for liquid crystal displays.
The technology of liquid crystal display, particularly color liquid crystal display, has been remarkably progressed in recent years. There are now many different liquid crystal displays of which the display quality is as excellent as CRT. In addition, notebook-type personal computers have been widely used which have always backlight devices incorporated as illuminators. The backlight device is indispensable to the direct viewing type color liquid crystal display.
The color liquid crystal displays can be roughly classified into two types: the TN (twisted nematic) liquid crystal display of active matrix drive using TFT (thin film transistor) and the STN (super-twisted nematic) liquid crystal display of multiplex drive. Either one of these types has a liquid crystal layer held by glass substrates, and polarizing plates that are disposed on both sides of crystal layer to modulate the polarized state of linearly polarized incident light. The backlight devices for these liquid crystal displays have various luminance levels depending on the use. Particularly in the color notebook type personal computers, small thickness, light weight and low power consumption are absolutely necessary as well as the required brightness.
In the conventional liquid crystal displays, since the light emitted from the backlight device that is disposed on the back of the liquid crystal display is non-polarized light, more than half of the incident light to the display is absorbed by the polarizing plate provided on the incident side of either one of TN and STN type displays, that is, the light utilization efficiency is low and thus displaying is dark. Therefore, for bright displaying, the power consumption must be increased.
To solve these problems, there is proposed a backlight device which emits polarized light as disclosed in, for example, Japanese Patent Application JP-A-6-265892. This backlight device has polarizing means provided on the light emitting side of a plane type waveguide for the emitted light to be substantially perpendicular to the surface of the plane type waveguide, and a polarizing beam splitter provided on the polarizing means. The polarizing beam splitter has polarizing beam splitting layers laminated on a column-shaped prism array with a triangular cross-section.
In order to achieve a high-performance, polarized light source capable of highly polarizing, it is necessary to make highly parallel light incident to the polarizing beam splitting layers. This requirement is satisfied by a proposed backlight device which has a thin waveguide pipe provided nearby and has a microprism structure as disclosed in Japanese Patent Application JPA-6-202107.
However, it is very difficult to simultaneously achieve a highly-parallel light source and a high degree of light uniformity in a plane. Any light uniformity in a plane is not described in this gazette. In addition, the polarizing beam splitter (or retroreflective polarizing sheet) using a dielectric multilayer needs lamination of multiple layers for high degree of polarization, and the multilayer-thickness control is severe, thus leading to high cost.
SUMMARY OF THE INVENTION
It is an object of the invention to provide a backlight device having high brightness and high uniformity.
According to the invention, there is provided a backlight device including a light source, a waveguide provided close to the light source and of which the thickness is decreased with the increase of the distance from the light source, polarizing beam splitting means provided on the light-exiting side of the waveguide, and light converting means disposed on the polarizing beam splitting means so as to convert the path of light from the polarizing beam splitting means and thereby to make it exit in the direction substantially normal to the light-exiting surface of the waveguide.
Also, there is provided a liquid crystal display having a liquid crystal display element and the same backlight device as given above which is disposed on the back of the liquid crystal display element so that the average polarization axis of the light exiting from the backlight device is made substantially coincident with that of the polarizing plate of the liquid crystal display element on the incidence side.
According to another aspect of the invention,. there is provided a liquid crystal display having a liquid crystal display element, and a backlight device including a light source, a waveguide provided close to the light source, and light converting means provided at the light source and the waveguide so that the light from the light source can be made substantially parallel, the waveguide for guiding the light from the light converting means including polarizing means for making substantially linearly polarized light exit in the direction substantially perpendicular to the guiding direction, the backlight device being disposed on the back of the liquid crystal display element so that the average polarization axis of the light exiting from the backlight device is made substantially coincident with that of the polarizing plate of the liquid crystal display element on the light-incident side.
The light converting means is the means that is disposed between the lamp as the light source and the waveguide so as to make the diffused light from the light source highly parallel. This light converting means is preferably constructed so that a mirror (particularly a paraboloidal mirror) is provided around the light source and that the thickness is increased with the increase of the distance from the lamp, whereby the light is reduced in its diffusion and made parallel by utilizing total reflection.
The polarizing means is preferably constructed so as to be formed by laminating transparent media of which the index is different from that of the waveguide itself in a tilted manner relative to the light-incident direction. Particularly, polarization eliminators should be laminated after the transparent media, respectively.
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Funahata Katsuyuki
Hiyama Ikuo
Kondo Katsumi
Antonelli Terry Stout & Kraus LLP
Duong Tai V.
Hitachi , Ltd.
Parker Kenneth
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