Optical waveguides – Noncyclindrical or nonplanar shaped waveguide
Reexamination Certificate
1998-12-31
2001-07-03
Healy, Brian (Department: 2874)
Optical waveguides
Noncyclindrical or nonplanar shaped waveguide
C385S036000, C385S147000, C385S901000, C362S559000, C362S560000, C362S561000, C349S056000, C349S057000, C349S058000, C349S064000, C349S065000, C349S067000
Reexamination Certificate
active
06256447
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to a backlight such as is used in backlighting a flat panel liquid crystal display (LCD), and more particularly to a backlight having an optical input arranged to provide a uniform light distribution to the LCD.
2. Description of the Related Art
Flat panel displays, such as liquid crystal displays or LCD's used in laptop computers, generally incorporate a backlighting system to illuminate a liquid crystal based display panel. Important requirements of the backlighting system are to provide a substantially uniform light distribution and to provide a sufficiently intense light distribution over the entire plane of the display panel. To accomplish these requirements, the backlighting system typically incorporates a light pipe to couple light energy from a light source to the LCD panel. In scattering backlighting systems, an array of diffusing elements are disposed along one surface of the light pipe to scatter light rays incident thereto toward an output plane. The output plane is coupled to the LCD panel, coupling the light rays into and through the LCD panel. While a scattering backlighting system offers the ability, by controlling the distribution of the scattering media on the scattering surface, to control the light distribution, it does not offer an ability to control the angle of light distribution. Much of the light energy produced by the backlighting system is wasted because it is scattered in directions that are not useful to a viewer or user of the LCD display. Because much of the light energy is not directed to the user and thus wasted, scattering backlighting systems lack the desired light energy intensity or brightness.
Non-scattering backlighting systems offer the advantage that both the light distribution and the angle of distribution may be controlled. Thus, the light energy may be directed in a way to make more efficient use of the available light energy. For example, the light energy may be directed so that substantially all of the light energy is emitted toward the user. A term often used to describe non-scattering backlighting systems is “deterministic” because the output point of a light ray is known based upon its input position. Thus, it may be said that a non-scattering backlighting system correlates the rays of input light energy and the rays of output light energy.
This correlation is advantageously used in the design of a backlighting system to ensure that a majority of the light energy is directed to the user. The correlation of input light rays to output light rays in a non-scattering backlighting system may also lead to a potential disadvantage arising from imaging at the light input appearing at the light output. If there is any distortion of the light energy at the input, this distortion will also appear at the output. The distortion may result from roughness or discontinuity in the light source or the input optics. Generally, such distortions will result in an area of non-uniform light intensity or a shadow in the output. Another source of distortion is the construction of the walls of the light pipe perpendicular to the light source. These walls must be made extremely smooth and flat or else they result in a distortion or shadow in the output.
A particular distortion that is observed in non-scattering backlighting systems is the formation of a diagonal line across the output plane of the backlight. It has been observed that distortions at the corner or interface between each side wall of a light pipe and the input surface of the light pipe are imaged into the output as a dark, diagonal line. The distortions in each corner are due to manufacturing limitations in the construction of the light pipe. While it is possible to polish and smooth the surfaces to reduce the appearance of such a distortion, these processes are time consuming and labor intensive and, therefore, are impractical in mass production of light pipes.
Additionally, it has been observed that non-uniformity in the direct output of the light source may cause distortions and shadows in the output. More particularly, a cold cathode fluorescent light (CCFL), for example, has inherent dim regions adjacent the electrodes at each end of the CCFL tube. These dim regions are areas where the light output from the CCFL is not uniform, and the light output is substantially diminished as compared to a center portion of the tube. Not only do these dim areas image into the output plane, but they also contribute to and exacerbate the appearance of the diagonal line.
Other light sources may be utilized in an LCD light pipe such as light emitting diodes (LED's), incandescent bulbs, laser diodes, and virtually any other point light source. These light sources each typically exhibit some non-uniformity in the light output energy as well, creating a distortion problem in the LCD output.
Co-pending and commonly assigned U.S. patent application Ser. No. 09/137,549, pending, entitled “Light Pipe for a Backlighting System” discloses a light pipe construction which addresses the diagonal line problem. The disclosed light pipe includes extensions beyond the perimeter of the output surface dimensions for use with an oversized light source. The extensions and elongate bulb correct the diagonal line problem but disadvantageously increase the size of the light pipe.
SUMMARY OF THE INVENTION
What is needed is a light pipe or backlight apparatus for a backlighting system which eliminates distortions in the output of a liquid crystal display but does not require costly, time consuming and labor intensive efforts in producing the backlight. What is also needed is a light pipe wherein the light pipe and light source are conventional in size and do not require larger dimensions than are needed for the LCD display itself. One object of the present invention is to provide a backlight apparatus for an LCD that is of the same size and configuration of a conventional backlight and yet essentially eliminates the diagonal distortion problem. Another object of the present invention is to provide a backlight apparatus which requires virtually no additional manufacturing processes or efforts than a conventional backlight construction and yet eliminates the diagonal line distortion problem.
These and other objects and advantages are achieved by the particular backlight construction of the invention. In one embodiment, a backlight apparatus includes a collimating waveguide with a light input end, a top surface, a bottom surface, opposing sides, a far end opposite the light input end, and a total internal reflection critical angle for the material of the waveguide. The apparatus also has a plurality of first facets formed in the bottom surface distributed along the collimating waveguide between the light input end and the far end. The plurality of first facets extend at least partway between the opposing sides. Each of the first facets has a first facet bottom surface converging toward the top surface in a direction away from the far end at an angle relative to the top surface of less than about 10°. A light scattering reflective surface is disposed adjacent the far end of the collimating waveguide. The first facet bottom surfaces are arranged so that they cause light rays entering the light input end to be totally internally reflected to the far end of the collimating waveguide without being leaked from the top surface. The reflective surface at the far end reflects and scatters the light rays incident thereon back toward the light input end. The first facet bottom surfaces are arranged so that they cause light rays reflected from the far end and at an angle near the total internal reflection critical angle to exit the top surface of the waveguide. Such a construction is different than previous constructions in that all of the light entering the input end travels the entire length of the collimating waveguide prior to being leaked through the top surface that is adjacent a liquid crystal display.
In one embodiment of the invent
Healy Brian
Nilles & Nilles S.C.
Physical Optics Corporation
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