Illumination – Revolving
Reexamination Certificate
2002-09-13
2004-09-07
Cariaso, Alan (Department: 2875)
Illumination
Revolving
C362S560000, C385S901000
Reexamination Certificate
active
06786613
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a low-profile spread illuminating apparatus used as an illuminating means for signs, various display devices and so on, and used as a back illuminating means and a front illuminating means for a liquid crystal display device.
2. Description of the Related Art
A liquid crystal display (hereinafter referred to as “LCD”) operating in low power consumption and characterized by its small thickness and lightweight has been extensively used in electric products including personal computers and cellular phones, and demand for the LCD has been increasing. However, since a liquid crystal which is a structural element of the LCD does not emit light by itself unlike a light emitting element such as a cathode-ray tube (CRT), the LCD requires a separate illuminating means to observe an image when the LCD is used in a dark place. In particular, in recent years, to satisfy a demand for downsizing and energy saving, a spread illuminating apparatus of side light type (light conductive plate type) is often used as an illuminating means for the LCD.
FIG. 9
is a schematic sectional view of a spread illuminating apparatus
1
used as a back illuminating means. The spread illuminating apparatus
1
is mainly composed of a light conductive plate
2
and a lamp
11
. A top surface
6
of the light conductive plate
2
is arranged so as to face a bottom surface of a transmission-type liquid crystal display element (not shown). A cold-cathode fluorescent lamp (CCFL) or a hot-cathode fluorescent lamp (HCFL) is used as the lamp
11
. Also, recently, a lamp is often used which is configured such that a spot-like light source such as a light emitting diode is arranged at at least one end of a light conductive bar made of a transparent material.
A light reflection pattern
7
is formed on a bottom surface
4
of the light conductive plate
2
. The light reflection pattern
7
is composed of grooves
8
each substantially rectangular in section and flat portions
9
each parent between adjacent grooves
8
, and is adapted to reflect and emit light rays from the lamp
11
substantially vertically from the top surface
6
of the light conductive plate
2
. In the light reflection pattern
7
, depths of the grooves
8
differ depending on their position so that luminance is almost uniform at any positions of the light conductive plate
2
independent of a distance from the lamp
11
. Specifically, the depth of the grooves
8
is set to increase gradually with an increase in distance from an end surface
3
facing the lamp
11
such that a groove
8
thereof closest to an end surface
5
opposite to the end surface
3
has a maximum depth. In this connections since the grooves
8
of the light reflection pattern
7
formed on the light conductive plate
2
are very fine, they cannot be recognized with the naked eye when a screen is observed. Alternatively, the distance between two adjacent grooves
8
or the width of the flat portions
9
may be set to gradually decrease as the distance from the lamp
11
increases, while the depth of the grooves
8
is set to be constant regardless of the position.
How the light rays travel at the light reflection pattern
7
will now be described with reference to FIG.
10
. In order to define the cross-section of each of the grooves
8
, it is assumed that a portion corresponding to the bottom surface
4
of the light conductive plate
2
is a virtual plane S, an inclination angle of an inclined surface
30
closer to the lamp
11
(the right side of
FIG. 10
) is &agr;, and that an inclination angle of an inclined surface
31
closer to the end surface
5
(the left side of
FIG. 10
) is &bgr;.
The inclined surface
30
is set such that a light ray
32
traveling from the right side of
FIG. 10
is totally reflected at the inclined surface
30
and travels substantially vertically toward the top surface
6
of the light conductive plate
2
so as to be oriented substantially vertical to the screen. It is proved experimentally that in order to make the light ray
32
travel as above described, the inclination angle &agr; of the inclined surface
30
may be set to range approximately 40° to 55°. In this case, since the optimum inclination angle a varies depending on the size of the light conductive plate
2
, it should be selected appropriately in accordance with the size of the light conductive plate
2
.
The inclination angel &bgr; of the inclined surface
31
is set in consideration of a light ray
33
incident on the inclined surface
30
at an angle smaller than a critical angle. Most of the light ray
33
incident on the inclined surface
30
at an angle smaller than a critical angle are not reflected thereat but pass therethrough to exit out from the light conductive plate
2
. Here, the inclination angle &bgr; is set such that the light ray
33
having exited out from the light conductive plate
2
reenters the light conductive plate
2
to be incident on an inclined surface
30
of a next groove
8
. It is proved experimentally that in order to make the light ray
33
as described above, the inclination angle &bgr; is set to range approximately 60° to 90°.
Here, since the inclination angle &agr; of the inclined surface
30
is set such that a light ray traveling like the light ray
32
is totally reflected at the inclined surface
30
to be directed toward the screen substantially vertical to the screen as described above, the light ray
33
, which reenters the light conductive plate
2
through the inclined surface
31
and reaches the inclined surface
30
is totally reflected thereat to travel in a direction perpendicular to the screen.
When the inclination angles &agr; and &bgr; are set as described above, light rays going toward the light reflection pattern
7
travels similar to the light ray
32
or
33
, and finally exit out from the top surface
6
of the light conductive plate
2
substantially perpendicular to the screen. Also, since there are very few light rays exiting out from the bottom surface
4
of the light conductive plate
2
, there is no need to provide a reflection plate, which is an essential component in the conventional spread illuminating apparatus and disposed on the bottom surface
4
of the light conductive plate
2
.
The light rays are totally reflected mainly at the inclined surfaces
30
of the grooves
8
to exit out from the top surface
6
of the light conductive plate
2
. Since areas of the inclined surfaces
30
and
31
are set to increase gradually as the distance from the lamp
11
increases, an amount of the light rays incident on the inclined surfaces
30
also increases gradually. Accordingly, the light rays can be spread in a uniform manner on the screen in spite of the intensity of the light rays attenuating with an increasing distance from the lamp
11
.
A front illuminating means is structured with the light conductive plate
2
configured upside down and with a reflection type liquid crystal display element, instead of a transmission type, being placed under the light conductive plate
2
. The mechanism for achieving a uniform spread light emission on the screen is same as that of the back illuminating means described with reference to FIG.
10
.
In the spread illuminating apparatus of side light type mentioned above, the luminance of the screen varies largely depending on the viewing angle to the screen at an increased distance from the lamp
11
, thereby narrowing its viewing field angle.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a spread illuminating apparatus of side light type, in which the direction of light rays exiting out from the light conductive plate
2
is controlled so as to obtain a minimum viewing field angle for n increased viewing field angle irrespective of the distance from the lamp
11
thereby further improving the image quality of an LCD illuminated by the spread illuminating apparatus.
In order to achieve the above object, a spread illuminating apparatus of side light type a
Cariaso Alan
Minebea Co. Ltd.
Oliff & Berridg,e PLC
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