Illumination – Revolving
Reexamination Certificate
1999-07-08
2002-02-19
Cariaso, Alan (Department: 2875)
Illumination
Revolving
C385S146000, C385S901000, C362S561000, C362S558000
Reexamination Certificate
active
06347873
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is directed generally to backlight assemblies for liquid crystal displays and the like, and more particularly to a backlight assembly having a light pipe with an integral diffuser surface structure formed on a top or exit surface of the light pipe.
2. Description of the Related Art
Many backlight assemblies for illuminating displays, such as a liquid crystal display panel for a laptop computer, include a light pipe that internally reflects light via what is known as total internal reflection or TIR. When the light within the light pipe is incident on a front or exit surface at the proper angle, the light exits the front or exit surface. The light then typically passes through one or more layers of additional light diffusing, directing, and/or turning films before entering and illuminating the display. The light pipe typically includes a back or reflective surface that reflects light upward until the light eventually exits the light pipe through the front surface. A reflective layer is sometimes disposed adjacent the back surface of the light pipe to reflect light back into the light pipe that has exited through the back surface. The back surface of the light pipe often includes a number of structures or elements thereon that aid in reflecting light upward toward the front surface and in changing the angle of light reflected thereby.
FIG. 1
illustrates one type of prior art backlight assembly and display
20
. The assembly
20
has a pair of light sources
21
located along opposite edges of a light pipe
22
with a plurality of silk screen dots
24
disposed on a back surface
26
. These dots
24
are typically white or light colored so as to scatter and reflect light upward from the back surface
26
toward a front surface
28
of the light pipe
22
. A reflective surface
30
is disposed adjacent the back surface
26
to reflect light back into the light pipe
22
that escapes through the back surface. The density and size of the dots
24
are manipulated in order to control the amount of light reflected from a particular area or region of the back surface
26
of the light pipe
22
. Control of the dot density and size therefore is utilized to produce desired illumination or brightness characteristics for light exiting the front surface
28
of the light pipe
22
.
The backlight assembly
20
also includes a separate diffuser film layer
32
disposed adjacent the front surface
28
of the light pipe
22
. The diffuser film can vary but in one known embodiment, the diffuser is a 60° circular diffuser. A pair of brightness enhancing film or BEF™ layers
34
are disposed over the separate diffuser film layer
32
and arranged orthogonal to one another. These film layers are available from 3M Corp. of St. Paul, Minn. and the term BEF™ is a TRADEMARK of 3M. Each BEF™ layer typically has a plurality of optical elements such as prisms
35
on a front or top surface
36
facing away from the light pipe. Each BEP™ layer
34
collimates light in one direction or axis so that all light exiting the light pipe
22
and the separate diffuser film layer
32
is collimated and redirected to near the normal direction of the backlight assembly by one of the BEF™ layers.
An LCD panel
38
, shown in phantom view in
FIG. 1
, is typically added to the backlight assembly over the second brightness enhancing film layer
34
. The LCD panel
38
reduces the brightness of the overall backlight assembly by a factor of
10
after the light emitted by the backlight passes through the LCD panel.
The separate diffuser film
32
has a smooth back surface
40
facing toward the light pipe
22
and a diffuser surface structure on a front surface
42
facing away from the light pipe. When placed against the smooth front surface
28
of the light pipe
22
, very little or no air gap is present between the light pipe front surface
28
and the diffuser film back surface
40
.
FIG. 2
illustrates a graphic representation of the light output or brightness of the various components, as they are stacked together, for the silk screen dot light pipe and backlight assembly of FIG.
1
. The graph represents brightness at virtually any point on the surface area of the components. The brightness value at any given point will vary when compared to other points on the surface area of the components, with the brightest area or region being close to the center of the backlight assembly. The brightness curve of this graph is not limited to only a vertical or horizontal axis measurement.
The LP curve shows the comparative brightness for light exiting only the light pipe for any given point over a range of angles from the normal to +/−90° relative to the normal. The brightness is somewhat evenly distributed over the entire range of angles because of the reflecting and scattering characteristics of the dots. The DF
1
curve shows the brightness over the range of angles, for the given point, of light passing through both the light pipe and the separate diffuser film. The diffuser film evens out the light dispersion but at the same time directs more light toward the normal. The BEF™1 curve shows the brightness over the same range of angles, for a given point, of light passing through the light pipe, the first diffuser layer, and the first BEF™ layer. The first brightness enhancing film layer collimates light in only one direction or plane such as along the vertical axis of the display. The second BEF™ layer further enhances the brightness of the backlight assembly by further collimating light in a direction orthogonal to the direction of the first BEF™ layer. This final brightness output is identified in
FIG. 2
by the BEF™2 curve and shows that a substantial portion of light emitted by the backlight assembly is normal to the front surface.
One problem with the silk screen dot light pipe and backlight assembly is that the brightness or luminance of light exiting the backlight assembly is not particularly high. The efficiency of the light pipe is merely adequate in directing light out of the front surface. This is because much light is scattered by the dots so that the light has Lambertian qualities and thus exits the light pipe as scattered. Also, the diffuser film is placed against the front, smooth surface of the light pipe leaving little or no air between the two surfaces. Much of the light incident on the front surface is at a relatively high angle, nearly parallel to the front surface. Because the light pipe and the diffuser film have very similar indexes of refraction, the light exits the light pipe at the same high angles. This light is either not directed to the display at all, or is not collimated sufficiently toward the normal. This light is therefore directed away from a desired viewing area normal to the surface and thus does not enhance the display brightness where needed.
FIG. 3
illustrates the other above-described prior art backlight assembly
50
. The backlight assembly
50
has a light tapered wedge TIR light pipe
52
and at least one light source
54
disposed along one edge. A plurality of grooves
56
are formed parallel to one another and parallel to the light source along a back surface
58
of the light pipe. The backlight assembly
50
also includes a reflective surface
60
adjacent the back surface
58
of the light pipe that is similar to the surface
30
described above. The light pipe also has a front surface
62
opposite the back surface. A directional turning film or DTF™
64
is disposed adjacent the light pipe front surface
62
. The term DTF™ is a TRADEMARK of the assignee of the present invention and DTF™ products are sold by the assignee of the present invention, Physical Optics Corp. of Torrance, Calif. The DTF™
64
has a plurality of optical elements
66
such as prism structures on a back surface
68
that faces toward the light pipe. The DTF™ has a surface diffuser structure formed on a front surface or exit surface
70
of the DTF™.
FIG. 3
shows exemplary light rays L traveling through and out o
Chung Hyun-Sun
Hosseini Abbas
Savant Gajendra D.
Vasiliev Anatoly
Cariaso Alan
Nilles & Nilles SC
Physical Optics Corporation
Sawhney Hargobind S
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