Illumination – Revolving
Reexamination Certificate
1996-03-27
2001-10-30
Sember, Thomas M. (Department: 2875)
Illumination
Revolving
C362S331000, C362S339000, C362S330000, C362S026000, C349S064000, C349S065000
Reexamination Certificate
active
06309080
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of Invention
The present invention relates to techniques for narrowing the visual field or increasing the brightness of a surface light source device, by making use of two prism sheets. The present invention is especially advantageously applied to backlighting of a liquid crystal display which is observed preferentially from a certain direction.
2. Related Art
A well-known optical device taking the form of a sheet and acting to modify the propagation direction characteristic of a primary light beam supplied from one surface and to cause the beam to exit from the other surface as a secondary light beam is generally known as a prism sheet.
Generally, a prism sheet consists of a member in the form of a plate made of an optical material having a surface (or prism face) provided with a number of aligned, repeating V-shaped channels (an array of convex and convex portions). Since this device has a function of modifying directions of light flux having a cross-sectional area corresponding to the plasm face, the device is employed, for example, to adjust the illumination direction light for the backlighting of a liquid crystal display.
FIG. 1
is a perspective view diagrammatically showing a surface light source device using a prism sheet for this purpose. Referring to
FIG. 1
, a light guide plate
1
of wedge-shaped cross section is made of a light scattering guide. For example, the light scattering guide is fabricated by preparing a matrix of polymethylmethacrylate (PMMA) and uniformly dispersing a substance of a different index of refraction in the matrix.
The thick end surface of the wedge-shaped light guide plate
1
forms an incident surface
2
. A light source device (fluorescent lamp) L is disposed near the incident surface
2
. A reflector
3
consisting either of silver foil of positive reflectivity or of a white sheet of diffuse reflectivity is disposed along one surface (rear surface
6
) of the light guide plate
1
. The other surface
5
of the light guide plate
1
forms an exiting surface
5
for taking out light flux supplied from the light source L.
A prism sheet
4
is disposed outside this exiting surface
5
. One surface of the prism sheet
4
has a V-shaped prism face
4
a
,
4
b
. The other surface is a flat surface
4
e
. If a well-known liquid crystal panel (liquid crystal display device) is disposed further outside the prism sheet
4
, a liquid crystal display is given.
In this surface light source device, as the thickness of the light guide plate
1
falls off with distance from the incident surface
2
, the surface light source device shows excellent efficiency of utilization of light and excellent brightness uniformity because of oblique surface multiple reflection effect occurring inside the light guide plate
1
. This effect based on the shape of the light scattering guide is described in detail in Japanese Patent Laid-Open No. 198956/1995.
Light introduced into the light guide plate
1
from the light source device L is directed toward the thin-walled end surface
7
while being scattered and reflected inside the light guide plate
1
. During this process, the light gradually exits from the exiting surface
5
. As described later, if the diameters (generally, the correlation distance regarding structures of nonuniform indices of refraction) of particles having a different index of refraction and dispersed in the light guide plate
1
are not very small, light exiting from the exiting surface
5
has a clear direction of preferential propagation. In other words, a collimated light beam is taken out from the exiting surface
5
. This nature is hereinafter referred to as the “emitting directivity” or “emission of directional light”.
As discussed in detail later, this direction of preferential propagation (the direction of the main axis of the collimated light beam) is usually upwardly spaced about 25-30° from the exiting surface as viewed from the incident surface
2
. Taking account of this, the prior art function of modifying the direction of propagation of the prism sheet
4
is described by referring to
FIGS. 2 and 3
.
FIG. 2
is a diagram associated with the arrangement of
FIG. 1
, illustrating the behavior of light the cross section taken along a direction vertical to the lamp L. “Direction vertical to the lamp L” means “direction vertical to the long axis of the lamp L”, i.e., direction vertical to the direction to which the incident surface
2
extends”. This may hereinafter be simply referred to as “lamp-vertical direction”. Similarly, “direction parallel to the direction of the long axis of the lamp L”, i.e., direction parallel to the direction to which the incident surface
2
extends, may be simply referred to as the “lamp-parallel direction”.
The prism sheet
4
shown in
FIG. 2
faces the exiting surface
5
of the light guide plate
1
with its prism face directed inward. Preferably, the prismatic vertical angle &phgr;3 made in the prism face is about 60°. Prism sheets satisfying this condition and having a prismatic vertical angle &phgr;3 of 64° are often used.
The refractive indices of materials of the matrix of the light guide plate
1
are generally about 1.4 to 1.6. Where this is taken into consideration, if light is directed to the light guide plate
1
from the direction indicated by the arrow L′, the direction of preferential propagation of light flux exiting from the incident surface
5
forms an angle &phgr;2=about 60° with respect to a normal to the exiting surface
5
. Where a PMMA matrix having an index of refraction of 1.492 is used, the incident angle to the exiting surface
5
to give 100 2=about 60° is &phgr;1=about 35° according to the Snell's law. A light beam corresponding to this direction of preferential propagation will hereinafter be referred to as a representative light beam, which is indicated by B
1
herein.
The representative light beam B
1
exiting from the exiting surface
5
travels straight through an air layer AR which can be regarded as having an index of refraction n
0
=1.0. Then, the light enters the prism face
4
a
of the prism sheet
4
at an angle (&phgr;3=about 60°) close to a right angle. This light beam enters the opposite prism face
4
b
at a smaller percentage.
Then, the representative light beam B
1
travels substantially straight through the prism sheet
4
up to the opposite prism face
4
b
and is reflected regularly. The light beam then enters the flat surface
4
e
of the prism sheet
4
at an angle close to a right angle and then goes out of the prism sheet
4
. By this process, the direction of preferential propagation of the light beam exiting from the exiting surface
5
is changed to a direction substantially vertical to the exiting surface
5
. The whole light rays are collected into a light beam traveling substantially in the perpendicular direction. As a result, the angular range in which the emitting surface is observed to be luminous is restricted. This action is herein referred to as narrowing of the visual field. The visual field will be quantitatively defined later.
The modified direction of preferential propagation is not always vertical to the exiting surface
5
. Rather, the direction can be adjusted in a considerable range of angles by selection of the vertical angle &phgr;3 of the prism sheet
4
, selection of the material (index of refraction) of the prism sheet
4
, selection of the material (index of refraction) of the light guide plate
1
, and so forth.
If the prism sheet
4
is so positioned that its prism face is directed outward, the preferential propagation direction is modified by similar prismatic action. In this case, however, the range of preferred prismatic vertical angles is wider than the range obtained where the prism face is directed inward.
FIG. 3
, which takes the same form as
FIG. 2
, illustrates this. The vertical angle &phgr;4 of the prism made at the prism face is approximately 70°.
It is assumed that the direction of incident light lies in the direction indicate
Sasako Hiromi
Watai Kayoko
Enplas Corporation
Sember Thomas M.
Staas & Halsey , LLP
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