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Vertically collimating backlight and 1-D diffuser for improved vertical contrast ratio on portable AMLCDs

IP.com Disclosure Number: IPCOM000015847D
Original Publication Date: 2002-Jul-30
Included in the Prior Art Database: 2003-Jun-21
Document File: 3 page(s) / 72K

Publishing Venue

IBM

Abstract

Disclosed is a structure for a portable active matrix liquid crystal display (AMLCD) with an improved vertical contrast ratio. A backlight is used which somewhat collimates the light in the vertical direction in combination with a low reflectivity exit diffuser which only expands the output distribution slightly in the direction in which the light was collimated. The light can be collimated in one direction with a conventional tapered light guide in a portable AMLCD by adding ridges to the top (or bottom) surface. The ridged light guide, which incorporates a mirrored surface below the light guide, collimates and directs the output light to the AMLCD with a vertical angular distribution of about 10 degrees full width half maximum (FWHM) or less and has the desired final distribution (about +/-20 to +/-25 degrees FWHM). In the horizontal direction (where the cold cathode fluorescent lamp (CCFL) is along the top or bottom of the display), the resulting angular distribution remains the deired +/-20 to +/-25 degrees FWHM, Figure 1. A low reflectivity exit diffuser which only expands the output distribution (to about 15 to 25 degrees FWHM) in the vertical direction can be formed by using a polymer sheet containing parallel curved or faceted ridges overcoated or planarized with a different optical index polymer where the ridges are arranged parallel to the CCFL axis.

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  Vertically collimating backlight and 1-D diffuser for improved vertical contrast ratio on portable AMLCDs

   Disclosed is a structure for a portable active matrix liquid crystal display (AMLCD) with an improved vertical contrast ratio. A backlight is used which somewhat collimates the light in the vertical direction in combination with a low reflectivity exit diffuser which only expands the output distribution slightly in the direction in which the light was collimated. The light can be collimated in one direction with a conventional tapered light guide in a portable AMLCD by adding ridges to the top (or bottom) surface. The ridged light guide, which incorporates a mirrored surface below the light guide, collimates and directs the output light to the AMLCD with a vertical angular distribution of about +/- 10 degrees full width half maximum (FWHM) or less and has the desired final distribution (about +/-20 to +/-25 degrees FWHM). In the horizontal direction (where the cold cathode fluorescent lamp (CCFL) is along the top or bottom of the display), the resulting angular distribution remains the deired +/-20 to +/-25 degrees FWHM, Figure 1. A low reflectivity exit diffuser which only expands the output distribution (to about +/- 15 to 25 degrees FWHM) in the vertical direction can be formed by using a polymer sheet containing parallel curved or faceted ridges overcoated or planarized with a different optical index polymer where the ridges are arranged parallel to the CCFL axis.

   With current portable AMLCDs, which typically use a 90 degree twisted nematic (TN) liquid crystal mode, the vertical viewing angle is much less than the horizontal viewing angle due to the electro-optical response of the liquid crystal. The contrast ratio is 100:1 to more than +/- 20 degrees in the horizontal direction from the display normal, but the region of 100:1 contrast ratio is less than half of that size in the vertical direction[1]. Gray scale inversion also degrades more rapidly when viewing off normal in the vertical direction than in the horizontal direction. Collimate and post diffuse (CPD) type displays have been proposed [1] to improve the off axis viewing performance of TN LCDs but these methods are not suitable for portable displays due to the reduced optical efficiency from the light collimation and post diffusion systems. A key consideration with portable flat panel displays is low power consumption (i.e. high optical efficiency) to maximize the system battery life. A low reflectivity diffuser is also required for a CPD type display. If the diffuser reflects ambient light back to the viewer, the display contrast ratio will be greatl...