Liquid crystal cells – elements and systems – Particular structure – Particular illumination
Reexamination Certificate
1998-03-24
2001-07-24
Sikes, William L. (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Particular illumination
C349S061000, C349S062000, C349S067000, C349S113000
Reexamination Certificate
active
06266108
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a reflective display device to perform a display by using external light such as natural light, and more specifically, it relates to an illuminating structure of a reflective display device, which is used as an auxiliary when external light is scarce.
2. Description of the Related Art
Among current various modes of display devices, mainly adopted are a TN mode or an STN mode in which a nematic liquid crystal having a twisted or super twisted alignment is used. However, these modes require a pair of polarizers for operation, and because of the light absorption thereof, they have a low transmittance, incapable of achieving a bright display screen. In addition to the above modes, a guest-host mode which uses a dichroic dye has been developed. A liquid crystal display device having a guest-host mode takes advantage of the anisotropy of the absorption coefficient of the dichroic dye added to the liquid crystal, in order to perform the display. By using a rod-shaped dichroic dye, the alignment direction of the dye changes as the molecular alignment of the liquid crystal is changed by applying a voltage to the electric field since the molecules of the dye are aligned in parallel to the molecules of the liquid crystal. The dye does or does not develop a color depending on the direction, and therefore by applying a voltage, the coloring mode of the liquid crystal display device can be switched.
FIG.
5
A and
FIG. 5B
show a HEILMEIER type guest-host liquid crystal display device.
FIG. 5A
shows the state in the absence of an applied voltage, while
FIG. 5B
shows the state in the presence of an applied voltage. This liquid crystal display device includes a p-type dye and a nematic liquid crystal having a positive dielectric anisotropy (N
p
liquid crystal). The p-type dichroic dye having an absorption axis which is substantially parallel to the molecular axis, strongly absorbs the polarization component Lx which is parallel to the molecular axis, and hardly absorbs the polarization component Ly which is perpendicular to it. In the state shown in
FIG. 5A
when no voltage is applied, the polarization component Lx included in the incident light is strongly absorbed by the p-type dye, resulting in the coloring of the liquid crystal display device. On the other hand, in the state shown in
FIG. 5B
when a voltage is applied, the N
p
liquid crystal having a positive dielectric anisotropy rises in response to the electric field and accordingly the p-type dye is perpendicularly aligned. Therefore, the polarization component Lx is only slightly absorbed, resulting in the liquid crystal display device being substantially colorless. The other polarization component Ly included in the incident light is hardly absorbed by the dichroic dye whether the state of the voltage is being applied or not being applied. Accordingly, in the HEILMEIER type guest-host liquid crystal display device, a polarizer is provided beforehand to remove the other polarization component Ly for improving the contrast.
Although the guest-host liquid crystal display device shown in
FIG. 5
is a transmissive type, a reflective liquid crystal display device is also known. For example, a reflective guest-host liquid crystal display device, as shown in
FIG. 6
, has been proposed, in which a polarizer is removed on the side of the incident light, while a quarter-wavelength plate and a reflector are provided on the emission side. In this device, the polarization directions of the two polarizing components Lx and Ly which are orthogonal to each other are rotated by 90 degrees at both incident light and reflected light paths by the quarter-wavelength plate in order to exchange the polarizing components with each other. Therefore, in the off-state (absorption state) shown in
FIG. 6A
, individual polarizing components Lx and Ly are absorbed either at the incident light path or at the reflected light path. In the on-state (transmission state) shown in
FIG. 6B
, both polarizing components Lx and Ly are hardly absorbed. Thus, the utilization efficiency of the incident light can be improved.
In the transmissive display device shown in
FIG. 5
, a panel holding a liquid crystal as an electro-optical material is provided between a pair of transparent electrodes, and a light source (backlight) for supplying illumination light is arranged on the rear of the panel. The image is viewed from the front of the panel. A backlight is essential to the transmissive type, and, for example, a cold cathode fluorescent tube or the like is used. Accordingly, from the viewpoint of the display device as a whole, the backlight consumes most of the electric power, which is unsuitable for displays of portable apparatuses. On the other hand, in the reflective type shown in
FIG. 6
, a reflector is arranged on the rear of the panel. External light such as natural light enters from the front and the image is viewed also from the front of the panel by making use of the reflected light. Differing from the transmissive type, the reflective type does not use a light source for supplying illumination light in the back, resulting in a relatively low rate of electric power consumption, which is suitable for displays of portable apparatuses. However, in the reflective display device, the image cannot be viewed in an environment where external light is scarce, for example, at night, which remains to be a problem to be solved.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide a reflective display device provided with an illumination structure which enables the viewing of an image in a dark environment while not spoiling the image quality in a bright environment.
A reflective display device, in accordance with the present invention, includes a panel, a light guide plate and a light source as fundamental components. The panel includes a transparent first substrate lying on the side of the external incident light, a second substrate joined to the first substrate with a predetermined gap therebetween and lying on the reflection side, an electro-optical material held in the gap, and an electrode provided on at least one of the first substrate and the second substrate for applying a voltage to the electro-optical material. The light guide plate is composed of a transparent material and arranged on the outside of the first substrate. The light source is arranged on the end of the light guide plate and generates illumination light as required. Notably, the light guide plate normally transmits external light onto the first substrate and emits the light reflected from the second substrate, and also as required guides illumination light onto the first substrate and emits the illumination light reflected from the second substrate.
Preferably, the light guide plate includes a planar section divided into bands and an inclined step lying between each band of the planar section. The thickness of the light guide plate decreases stepwise from the end where the light source lies toward the front. The light guide plate reflects the illumination light guided forward at each step so as to guide it onto the first substrate, and emits the illumination light reflected from the second substrate through the planar section. In such a case, the step of the light guide plate inclines from 40 to 50 degrees toward the planar section. Further, the panel may use a guest-host liquid crystal layer, including a liquid crystal as a host to which a dichroic dye is added as a guest, as the electro-optical material. In such a case, the panel includes a reflecting layer lying on the side of the second substrate for scattering and reflecting external light, and a quarter-wavelength layer provided between the guest-host liquid crystal layer and the reflecting layer. Or, the panel may include a polarizing plate provided on the side of the first substrate and may use a liquid crystal layer, which functions as a quarter-wavelength plate in response to the state of an applied voltage, as t
Bao Yang Ying
Fujioka Takayuki
Kataoka Hideo
Kinoshita Yukio
Urabe Tetsuo
Chowdhury Tarifur R.
Sikes William L.
Sonnenschein Nath & Rosenthal
Sony Corporation
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