Liquid crystal display device

• US 5,920,367 A
• Filed: 10/10/1997
• Issued: 07/06/1999
• Est. Priority Date: 11/12/1996
• Status: Expired due to Term

• Current Assignee: Sharp Kabushiki Kaisha (Hon Hai Precision Industry Co., Ltd.)
• Original Assignee: Sharp Kabushiki Kaisha (Hon Hai Precision Industry Co., Ltd.)
• Inventors: Seike, Takeshi, Kajimoto, Koichi
• Primary Examiner(s): Sikes, William L.
• Assistant Examiner(s): NGUYEN, DUNG T
• Application Number:
  US08/949,128
• Time in Patent Office: 634 Days
• Field of Search: 349/51, 349/62, 349/96, 349/113, 349/162, 349/114, 349/68
• US Class Current: 349/162
• CPC Class Codes: G02F 1/133555 Transflectors
• CPC Class Codes: G02F 1/133614 using photoluminescence, e.g. phosphors illuminated by UV or blue light
• CPC Class Codes: G02F 1/133626 providing two modes of illumination, e.g. day-night

Abstract

In a liquid crystal display device of the present invention, a white plate light source such as a cool cathode fluorescent tube is provided on a back surface of a liquid crystal cell, i.e., on a surface opposite to a displaying surface of the liquid crystal cell. On a surface opposite to the displaying surface of the liquid crystal panel, a polarizing plate-equipped semitransmitting reflecting plate is provided. The liquid crystal panel is composed of active elements such as MIM (Metal-Insulator-Metal) elements or TFTs (Thin Film Transistors). With this arrangement, discharge of a charge is suppressed by switching of the active element when a voltage is applied to two types of electrodes sandwitching a liquid crystal layer. By setting reflectance and transmittance of the polarizing plate-equipped semitransmitting reflecting plate such that contrast of the liquid crystal panel is higher when the cool cathode fluorescent tube is not lighted than when the cool cathode fluorescent tube is lighted, it is ensured that visibility both in a reflecting mode and a transmitting mode are improved.

Claims

1. A liquid crystal display device, comprising:

a liquid crystal panel including (1) a first substrate having first transparent electrodes provided in a form of matrix, (2) a second substrate, having second transparent electrodes, provided so as to face the first substrate, and (3) a liquid crystal layer sandwiched between the first substrate and the second substrate;

a polarizing plate-equipped semitransmitting reflecting plate provided on a side opposite to a displaying surface of said liquid crystal panel; and

illuminating means for illuminating said liquid crystal panel through said polarizing plate-equipped semitransmitting reflecting plate,

wherein said liquid crystal panel includes active elements which control supplying of a charge to the first transparent electrodes by their switching, and reflectance and transmittance of said polarizing plate-equipped semitransmitting reflecting plate are set so as to make contrast of said liquid crystal panel higher when said illuminating means is not lighted than when said illuminating means is lighted.

2. The liquid crystal display device as set forth in claim 1, wherein the contrast of said liquid crystal panel is not less than 10 when said illuminating means is not lighted, whereas the contrast of said liquid crystal panel is not less than 3 when said illuminating means is lighted.

3. The liquid crystal display device as set forth in claim 1, wherein the contrast of said liquid crystal panel is not less than 20 when said illuminating means is not lighted, whereas the contrast of said liquid crystal panel is not less than 10 when said illuminating means is lighted.

4. The liquid crystal display device as set forth in claim 1, wherein luminance of the displaying surface of said liquid crystal panel is in a range of not less than 5 cd/m² and not more than 200 cd/m² when said illuminating means is lighted.

5. The liquid crystal display device as set forth in claim 1, wherein said illuminating means is a light-emitting diode.

6. The liquid crystal display device as set forth in claim 5, further comprising wavelength converting means for converting into white light a wavelength of light emitted from said light-emitting diode.

7. The liquid crystal display device as set forth in claim 6, wherein said wavelength converting means is made of a fluorescent substance which emits fluorescence upon activation by the light emitted by said light-emitting diode, and a white substance which scatters the fluorescence.

8. The liquid crystal display device as set forth in claim 6, wherein said light-emitting diode emits blue light.

9. The liquid crystal display device as set forth in claim 1, wherein said illuminating means is an electroluminescence light-emitting element.

10. The liquid crystal display device as set forth in claim 1, wherein the active elements have a metal-insulator-metal structure.

11. The liquid crystal display device as set forth in claim 1, wherein each of the active elements is a thin film transistor.

12. The liquid crystal display device as set forth in claim 1, wherein said illuminating means is a cool cathode fluorescent tube.