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Supertwisted Nematic Liquid Crystal Display Device With Improved Response Time and Optical Characteristics

IP.com Disclosure Number: IPCOM000035226D
Original Publication Date: 1989-Jun-01
Included in the Prior Art Database: 2005-Jan-28
Document File: 3 page(s) / 69K

Publishing Venue

IBM

Related People

Ong, HL: AUTHOR

Abstract

Disclosed is a supertwisted nematic liquid crystal display with improved response time and optical characteristics.

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Supertwisted Nematic Liquid Crystal Display Device With Improved Response Time and Optical Characteristics

Disclosed is a supertwisted nematic liquid crystal display with improved response time and optical characteristics.

As compared to the 90o twisted nematic (TN) liquid crystal display (LCD), considerably higher multiplex ratio and wider viewing cone can be achieved with chiral-doped TN devices having a twist angle greater than 90o . These TN LCDs, with twist angle more than 90o, are generally referred to as supertwisted nematic (STN) LCDs. In general, STNs exhibit strong interference colors in both field-on and field-off states, require very precise cell thickness tolerances, and require long response times.

Disclosed is an STN device, having a different polarizer arrangement and cell thickness as compared to the conventional STN, from which faster response time and improved optical characteristics are obtained. In the disclosed STN device, the exit polarizer is directed perpendicular to the exit polarizer orientation that is used in the conventional STN devices, and the cell thickness is then chosen to give the optimized bright field-off state. In this disclosure, an example is given using 180o STN LCDs. The invention is general, however, and can be used for other STNs having different liquid crystal (LC) twist angles.

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The device geometry (schematically shown in Fig. 1) is defined as follows: a general TN cell of cell gap d is confined between the planes z = 0 and z = d of a Cartesian coordinate system. Both surfaces, which are parallel to the xy plane, are treated to give a homogeneous alignment with a pre-tilt angle to the surface. The LC director at the entrance surface is directed along the x axis. The LC director at the exit surface is twisted at an azimuthal angle with respect to the x axis. R denotes the twist angle, DEnt the entrance polarizer angle, and DExit the exit polarizer angle, with the angles defined with respect to the x axis.

Under crossed (or parallel) polarizers, the entrance polarizer orientation DEnt = (2R - nf)/4, with n = 1,2, gives the optimal polarizer orientation with optimized transmission for field-off state. In the conventional STN LCDs, with a bright field- off state, the parallel polarizer orientation is used, and the cell thickness is chosen to give the optimized transmission. For the conventional 180o STN LCDs, the following parallel polarizer orientation is used: DEnt = DExit =+_45o . FOR A...