Illumination – Revolving
Reexamination Certificate
1999-07-08
2002-05-14
Cariaso, Alan (Department: 2875)
Illumination
Revolving
C362S330000, C362S333000, C362S558000
Reexamination Certificate
active
06386721
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to backlight assemblies for liquid crystal displays and the like, and more particularly to an improved total internal reflection light pipe for a backlight assembly.
2. Description of the Related Art
A standard backlight assembly for a liquid crystal display (LCD), such as for a lap top computer, is schematically shown in cross-section in
FIG. 1. A
backlight assembly
20
has a light propagating waveguide or total internal reflection (TIR) light pipe
22
. The light pipe has one or more light sources
24
located along its edges. A commonly used light source is known as a cold cathode fluorescent light tube or CCFL tube and is placed along an edge or along opposed edges of the light pipe. The backlight assembly
20
also has a reflector or reflective layer
26
placed adjacent a back surface
28
of the light pipe. A number of optical elements
30
are disposed on the back surface of the light pipe.
In one known construction, the optical elements
30
are a plurality of silk screen, light colored dots that reflect light upward toward a front surface
32
of the light pipe. In an alternative known construction, the optical elements
30
are a plurality of elongate grooves formed in the back surface
28
of the light pipe
22
that direct light toward the front surface
32
at controlled angles. A separate diffuser film layer
34
is disposed adjacent the front surface
32
of the light pipe. The diffuser film
34
has a smooth back surface
36
facing the light pipe and a diffuser structure on a front surface
38
facing away from the light pipe.
A pair of brightness enhancing film (BEF™) layers
40
are located adjacent the diffuser film layer
34
and are oriented orthogonal relative to one another. THE BEF™ layers are available from 3M of St. Paul, Minn. and BEF is a TRADEMARK OF 3M. One of the BEF™ layers turns light in one direction or plane and the other BEF™ turns light in another direction or plane 90° relative to the first BEF™ layer. Each BEF™ layer typically has a plurality of optical elements
42
such as prisms located on a front surface
44
. The BEF™ layers together collimate light toward the direction normal to the light pipe front surface.
The backlight assembly also has a protective layer
50
disposed over the BEF™ layers to prevent damage to the optical elements. A display panel such as a LCD panel (not shown) is typically placed adjacent the protective layer.
The silk screen dot density on the back surface of the light pipe determines the amount of light projected upward toward the front surface of the light pipe and determines uniformity of light distribution. Two alternative dot density distributions are shown in
FIGS. 3 and 4
and are discussed below. The higher the density of dots in a particular region on the light pipe back surface, the more light that is projected upward from that region toward the front surface. The front surface of the standard light pipe is left essentially smooth or flat.
Such a standard silk screen dot backlight assembly produces a light brightness output exiting the backlight.
FIG. 2
illustrates one possible light output representation. The HLP and VLP curves represent the light intensity or brightness for only light exiting the light pipe in a horizontal or a vertical direction, respectively, across the light pipe. The brightness is fairly consistent both vertically and horizontally across the light pipe. This is achieved by having a higher dot density near the middle of the light pipe back surface and a lower dot density near the light sources, as shown in FIG.
3
. More light thus travels to the middle of the light pipe before being reflected out. The HBL and VBL curves illustrated in
FIG. 2
show the brightness for the entire backlight assembly including all of the components. The diffuser film and two BEF™ layers enhance the brightness near the center of the backlight where a viewer would be situated. The brightness near the edges of the backlight assembly is lower. The diffuser film and BEF™ layers collimate the light and therefore direct more light towards the middle of the light pipe and upon exiting the light pipe, direct more of the light toward the normal and not at an angle relative to the normal. Therefore, a majority of the light and thus the greatest brightness is near the center of the screen.
FIG. 4
illustrates an alternative dot density distribution on the back surface
28
of the light pipe
22
where the dots are more dense near the edges adjacent the light sources
24
.
FIGS. 5 and 6
illustrate graphs representing the brightness intensity measured along the horizontal axis and vertical axis across the light pipe and the backlight assembly, respectively identified as curves HLP, VLP, HBL, and VBL. As can be seen, the brightness along the vertical axis for both the light pipe and the backlight follows a curve having a general “M” shape. This is caused both by the intensity being brighter nearer the light sources and the increased dot density nearer the sources. The brightness along the horizontal axis for the light pipe is fairly consistent across the surface. This is because the distance from the light sources and the dot density are consistent in the horizontal direction for any plane across the light pipe. The brightness for the backlight along the horizontal axis is somewhat higher near the center of the backlight and lower near the edges due to collimation of the light by the BEF™ layers. By changing the dot density, the brightness can be altered over portions of the backlight exit surface.
A problem with these standard backlight constructions is that changing the dot density not only affects brightness but also reduces the uniformity of light distribution. The silk screen dots also reflect light in scattered directions such that some light is not directed to a desired area or is lost in the system altogether. The overall efficiency of the standard backlight is less than ideal.
Another problem with standard backlight constructions is caused by contact between the separate diffuser film layer and the front surface of the light pipe. No air gap exists between the two since both surfaces are flat and smooth. Both components also typically have very similar refractive indexes. Any light within the light pipe incident on the front surface within the light pipe at a high angle or nearly parallel to the front surface exits the light pipe at the same high angle. The diffuser film and the BEF™ layers cannot completely collimate some of this high angle light back to the normal direction relative to the front surface. This light is either misdirected away from viewers or lost within the system, greatly reducing the efficiency of the backlight.
SUMMARY OF THE INVENTION
The present invention is directed to an improved backlight assembly having a light pipe with at least an integral diffuser formed on the back surface of the light pipe. In a separate embodiment, a second integral diffuser is located on the front surface of the light pipe. The present invention simplifies the construction of the backlight assembly and reduces its manufacturing and component cost. The simpler backlight assembly construction of the invention provides improved overall brightness, improved light uniformity, and increased lighting efficiency.
One object of the present invention is to provide a backlight assembly having improved overall brightness characteristics. Another object of the present invention is to provide a backlight assembly with improved light distribution uniformity. A further object of the present invention is to provide a backlight assembly of a simpler construction that is easier to assemble and less costly to manufacture. A further object of the present invention is to provide a backlight assembly having all of these improved characteristics that can be used in place of standard backlight assemblies without any retrofit.
In one embodiment, a backlight assembly of the invention has a pair of light sources disposed along o
Hosseini Abbas
Savant Gajendra
Nilles & Nilles S.C.
Physical Optics Corporation
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