Illumination – Revolving
Reexamination Certificate
2000-11-13
2003-11-18
O'Shea, Sandra (Department: 2875)
Illumination
Revolving
C362S331000, C362S561000
Reexamination Certificate
active
06648485
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to light guides for illumination, and more particularly to a tapered collimating light guide for flat panel displays.
2. Description of the Related Art
In a typical portable display backlight unit, a triband cold cathode fluorescent lamp (CCFL) is coupled to one edge of a tapered Acrylic light guide where a white reflector and a variable density pattern of printed dots or molded microstructure such as ridges, etc. are formed on the bottom surface of the light guide to scatter light and generate a uniform distribution of light which exits the light guide.
Referring to
FIG. 1
, a side view of one display arrangement is shown. A light source
10
is coupled to a tapered light guide
12
for introducing light into light guide
12
. Light source
10
includes a cold cathode flourescent lamp (CCFL) which is elongated into and out of the plane of the page. Source
10
also typically includes a reflector
14
for directing light into light guide
12
. A dot pattern or microstructures
16
are formed on a surface
18
to diffuse light incident on surface
18
. Light
11
exits guide
12
in twin peaks at angles of about 50 degrees and 80 degrees off normal with a full width half maximum distribution (FWHM) of ±30 degrees in the case of a printed white “dot” pattern on surface
18
and in twin peaks at angles of about 60 degrees and about 80 degrees off normal with a FWHM distribution of ±20 degrees in the case of microstructured ridges on surface
18
. A white reflector sheet
20
is spaced apart from and runs parallel to surface
18
to redirect light back towards guide
12
. Typically, a pair of crossed ridge sheets
22
and
24
with a diffuser sheet
23
are used to redirect the light normal to light guide
12
and to somewhat collimate the light (about ±25 degrees FWHM horizontal & vertical). Ridge sheets
22
and
24
include parallel running spaced ridges. Ridges on sheet
22
run perpendicular to ridges on sheet
24
.
Recently, an alternate approach using a prism bending sheet has been disclosed by Mitsubishi Rayon in U.S. Pat. Nos. Re 35,704, 5,711,589, and 5,863,113. In these patents, the pair of crossed “ridge sheets” is replaced by a single prism bending sheet
30
, as shown in FIG.
2
.
Referring to
FIG. 2
, prism bending sheet
30
redirects the light which exits the light guide into a direction normal to the light guide where surface texture or microstructure variation along the light guide on surface
34
is used to produce a uniform light distribution. The use of a bending sheet
30
has significant advantages in that only one sheet is required instead of two, and it can result in more collimated light in the direction along the length of the guide, but the degree of collimation which can be achieved is limited by the surface texture or microstructure along the guide which controls the extraction of light to achieve uniform illumination. The prism bending sheet preserves the degree of collimation of the light output by the light guide in the total internal reflection (TIR) direction unlike the ridge sheets as shown in FIG.
1
.
A very high degree of collimation in the direction along the guide can be achieved by a tapered light guide with both major surfaces flat and with a mirror underneath instead of a white reflector, but the light distribution along the length of the guide is very nonuniform.
Conventional backlight designs provide a flat entrance surface
13
angled at about 90 degrees with the top surface (exit surface
15
) of the light guides
2
of
FIGS. 1 and 2
. With reference to the coordinate system of
FIG. 2
,
FIGS. 3A and 3B
shown the interface between air and light guide
12
at entrance edge
13
. Light enters at entrance edge
13
for −90 degrees ≦&agr;≦90 degrees and −90 degrees ≦&bgr;≦90 degrees, where &agr; is the input angle of light from the x-axis in the x-y plane and &bgr; in the input angle of light from the x-axis in the x-z plane. &agr;′ and &bgr;′ are corresponding angles in the light guide after refraction. The angular distribution of light is reduced according to Snell's law when light propagates into light guide
12
, which has a higher index of refraction than air (e.g., n=1.49 for acrylic). If entrance edge
13
is 3.5 mm thick, taper angle is 0.5 degrees surface
16
and n=1.49 for light guide
12
, then the initial light distribution in the acrylic light guide is −42 degrees ≦&agr;′≦42 degrees and −42 degrees ≦&bgr;′≦42. Considering only the x-y plane, &agr;′ is increased each time propagating light is reflected from surface
16
until the angle for TIR (e.g. 48 degrees) is exceeded. If structures of the prior art did not include a dot pattern or microstructure on surface
16
(See FIGS.
1
and
2
), light must propagate about 40 mm in the x direction before TIR is exceeded. This creates an undesirable dark region adjacent to entrance edge
13
.
There are a number of applications where it is desirable to have very highly collimated light exit the light guide. The primary application is to make a “collimate and post diffuse” (CPD) type of display as has been described previously in Zimmerman et al. in SID '95 Digest, pp. 793-796 and McFarland et al. in Asia Display '95 Digest, pp. 739-742. The system as described in these publications and their respective patent (e.g., U.S. Pat. No. 5,598,281) is not practical for many applications due to the thickness and cost required to manufacture the complicated structures described therein.
A number of recent publications including Kalantar et al. SID '99 Digest, pp. 764, Kalantar et al. SID '00 Digest pp.1029-1031, have described that by forming ridges along one major surface of the light guide perpendicular to the axis of the CCFL (axis running into the page for example in FIGS.
1
and
2
), the output light can be collimated in the direction across the guide (where “across the guide” is perpendicular to “along the guide” which is perpendicular to the axis of the CCFL). See also U.S. Pat. No. 5,926,601.
Therefore, a need exists for a light guide system which efficiently provides a more uniform light distribution at the output of a light guide while maintaining a highly collimated light output.
SUMMARY OF THE INVENTION
A light guide system has a light guide having a first end portion opposite a second end portion. The light guide provides a first surface and a second surface between the first and second end portions, and the second surface is inclined relative to the first surface. A light source is disposed along the first end portion on a first axis. A light redistribution device is disposed at an entrance of the light guide for receiving light from the light source and redistributing a portion of the light perpendicular to the first axis to provide a uniform light distribution from the first surface.
Another light guide system includes a light guide having a first end portion opposite a second end portion, and the light guide includes a first surface and a second surface between the first and second end portions. The second surface is inclined relative to the first surface. A light source is disposed along the first end portion wherein the first end portion defines a first axis and the light source extends along the first axis. A structured entrance is formed in the first end portion of the light guide, and the structured entrance includes a plurality of angled surfaces extending along the first axis for redirecting light received from the light source.
A light guide system for liquid crystal displays includes a light guide having a first end portion opposite a second end portion, the light guide including a first surface and a second surface between the first and second end portions, the second surface being inclined relative to the first surface. A light source is disposed along the first end portion wherein the first end portion defines a first axis and the light
Colgan Evan G.
Doany Fuad E.
Nishikai Akiko
Singh Rama N.
Suzuki Masaru
DeRosa Frank V.
International Business Machines - Corporation
O'Shea Sandra
Trepp Robert M.
Ward John Anthony
LandOfFree
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