Transflective liquid crystal display with enhanced contrast ratio

• US 4,541,692 A
• Filed: 05/31/1983
• Issued: 09/17/1985
• Est. Priority Date: 05/31/1983
• Status: Expired due to Fees

• Current Assignee: Amespace Incorporated
• Original Assignee: General Electric Company
• Inventors: Haim, Elias S., Collins, Robert P., Rowen, John A.
• Primary Examiner(s): Corbin, John K.
• Assistant Examiner(s): Gallivan, Richard F.
• Application Number:
  US06/499,572
• Time in Patent Office: 840 Days
• Field of Search: 350/339 R, 350/339 F, 350/345, 350/337, 350/339 D, 350/349
• US Class Current: 349/68
• CPC Class Codes: G02F 1/1335 Structural association of cells with optical devices, e.g. polarisers or reflectors
• CPC Class Codes: G02F 1/13725 based on guest-host interaction G02F1/13762, G02F1/13737, take precedence

Abstract

A back lighted transflective liquid crystal display with improved contrast ratio has a light mask positioned between the transflective element and the source of back lighting. The mask has light transmitting sections which are aligned with and have the same configuration as the indicia-forming electrodes in the cell. The mask has opaque or light blocking sections that are aligned with the background portion of the cell. In the transmissive mode light impinges on the cell only in the area of the indicia-forming electrodes, thereby enhancing the contrast ratio between the indicia and background portions of the cell.

Claims

1. A transflective liquid crystal display operable in transmissive and reflective modes with enhanced contrast ratio in the transmissive mode between the display indicia and the background portions comprising:

(a) A single liquid crystal cell with display indicia,

(b) A light source positioned to the rear of the single cell, said source being energized when the cell is operated in the transmissive mode

(c) A transflective element between the rear of cell and said source, the rear of said transflective element being illuminated by said source when said source is energized,

(d) Passive masking means positioned between said source and the transflective means for blocking rear illumination of the background portion of said cell, said masking means having light transmitting and light blocking portions, the light transmitting portions being aligned with the cell display indicia whereby said indicia are illuminated from the rear during the transmissive mode but light is blocked from the rear of the background portion of the cell to enhance contrast ratio between these portions.

2. The liquid crystal display according to claim 1 wherein said cell contains a host-guest solution in which the host liquid crystal contains an optically active element which imports a helical order to the crystal molecules and the guest is a pleochroic dye whose molecules spontaneously align with the crystal molecules.

3. A liquid crystal display device comprising:

(a) A cell selectively operative in a transmissive or reflective mode comprising a pair of substantially parallel, spaced apart, substantially transparent substrates retaining a liquid crystal solution,

(b) A plurality of optically transparent conductive electrode segments positioned on the surface of the substrate defining the front face of the cell, said electrode segments having configurations of the cell indicia,

(c) A light source positioned behind the rear substrate element, adapted to be energized when the display is operated in the transmissive mode,

(d) a transflective element positioned between light source and the rear of said cell, for reflecting ambient light entering the cell through the front face in the reflective mode and for transmitting a portion of the light striking rear face thereof when said light source is energized in the transmissive mode,

(e) Passive contrast enhancing means positioned between said light source and said transflective means having light transmitting portions of the same configuration as, and aligned with, the optically transparent electrode segments of said liquid crystal cell, the remaining portion of said passive contrast enhancing means being essentially non-transmitting whereby light from the source behind the cell is transmitted through the display segments but is blocked from illuminating the background portion thereby enhancing the contrast ratio between the background and display portions of the liquid crystal device in the transmissive mode.

4. The liquid crystal display according to claim 3 wherein said cell contains a host-guest solution in which the host liquid crystal contains an optically active element which imports a helical order to the crystal molecules and the guest is a pleochroic dye whose molecules spontaneously align with the crystal molecules.

5. A liquid crystal display device comprising:

(a) A liquid crystal cell having a display portion for visually defining the shape of one or more indicia and a background portion, the display portion being defined by conductive light transmitting electrodes deposited on substrate of said cell,

Conductive energy light transmitting electrodes deposited on the liquid crystal surface of the other substrate element for selectively establishing electric fields across liquid crystal and controlling the light transmitting character of the liquid crystal solution in the area of the indicia defining electrode,

(b) means for selectively enhancing the contrast ratio between the indicia of the display portion and the background portion in the transmissive mode by controlled back lighting of the cell including;

(1) a transflective means positioned behind said other substrate for reflecting ambient light passing through said cell in the reflective mode, said transflective means being capable of transmitting a portion of the light illuminating the rear of said transflective means in the transmissive mode,

(2) A selectively energized light source,

(3) and a masking means between said transflective means and said selectively energized light source,

said masking means having light transmitting portions aligned with and of the same configuration as the light transmitting electrodes and non-transmissive portions for blocking light from the background portion of said cell.

6. The liquid crystal display according to claim 5 wherein said cell contains a host-guest solution in which the host liquid crystal contains an optically active element which imports a helical order to the crystal molecules and the guest is a pleochroic dye whose molecules spontaneously align with the crystal molecules.