Illumination – Revolving
Reexamination Certificate
1996-10-11
2001-05-15
Sember, Thomas M. (Department: 2875)
Illumination
Revolving
C362S026000, C362S035000, C362S027000, C362S326000, C362S327000, C362S328000, C362S330000, C362S331000, C362S339000, C385S901000, C385S147000
Reexamination Certificate
active
06231200
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates firstly to surface light source devices and in particular to such devices of the edge light type. This invention relates additionally to optical elements such as prism sheets which are used in such surface light source devices, as well as to apparatus using such surface light source devices such as image display apparatus, automatic teller machines and game tables.
Liquid crystal (LC) display apparatus, because they have the favorable characteristics of being light and thin, have been used as display devices not only for lap-top and book-type personal computers and word processors but also for electronic notebooks, portable telephones, LC television sets, various portable terminals and video cameras. More recently, they are also being used as display apparatus for measurement instruments such as time counters, overhead display of virtual reality and LC projectors.
Among these LC display apparatus, there are those having a vertically downward-facing surface light source device disposed on the back surface of a LC display panel (hereinafter referred to as the LCD panel), as well as those having an edge-light type surface light source device.
FIGS. 1A and 1B
show a surface light source device
1
of the former kind, having a linear light source
4
such as a cold cathode ray tube (a fluorescent tube) disposed on the back surface of diffusion plates
2
and
3
and a reflector
5
further behind the linear light source
4
such that the emitted light from the linear light source
4
can be diffused by the diffusion plates
2
and
3
and uniformly projected out from the projecting surface. Because a plurality of linear light sources can be disposed behind the diffusion plates, an LC display apparatus using such a vertically downward-facing surface light source device can provide a high degree of brightness. For obtaining a uniform brightness over the entire light-emitting surface, however, a certain distance must be maintained between the light source and the diffusion plates, causing the overall thickness of the surface light source device to increase. This makes it difficult to produce thin LC display apparatus.
Edge-light type surface light source devices have the advantage that the light source can be made thin because the linear light source is positioned at a side of a light conducting plate. Because of this advantage, more and more apparatus are coming to use edge-light type surface light source devices, as the demand to reduce the thickness of LC display apparatus is becoming greater.
FIG. 2
shows an edge-light type surface light source device
6
, with a portion removed, including optical elements such as a linear light source
7
, a reflector
8
, a light conducting plate
9
, a light-reflecting plate
10
, a diffusion plate
11
and a pair of converging lens plates
12
and
13
. The linear light source
7
and the reflector
8
are disposed by a (light-incident) side surface of the optically transparent light conducting plate
9
such that the light emitted from the linear light source
7
enters the light conducting plate
9
through this side surface either directly or after being reflected by the reflector
8
. Side-surface reflecting plates (shown at
14
in
FIG. 5
) of a metallic dielectric material with a rough surface are provided on side surfaces of the light conducting plate
9
other than the light-incident surface. A cold cathode ray tube (fluorescent tube) is shown as the linear light source
7
. A straight single tube or an L-shaped tube may be used, depending on the brightness of display required of the LC display apparatus
6
.
A diffusion layer
15
is formed on the lower surface of the light conducting plate
9
, and the light-reflecting plate
10
is disposed therebelow. The diffusion layer
15
may be produced by depositing dots of light-diffusing paint or the like by a screen-printing method such that the area of the diffusion layer
15
increases gradually as the distance from the linear light source
7
increases, as shown by examples in
FIGS. 3A and 4B
.
The efficiency, by which light from the linear light source
7
can be led to the upper surface, will be discussed next. Assume now that the diffusion layer
15
did not exist on the lower surface of the light conducting plate
9
. Light beam F
1
shown in
FIG. 5
indicates a beam which made incidence onto the light incident side surface
16
of the light conducting plate
9
with an angle of incidence 90 degrees from its normal line, that is, its angle of refraction &thgr;
1
equals the critical angle for the total reflection inside the light conducting plate
9
. If the index of refraction for air is n
1
and that of the light conducting plate
9
is n
2
, it is known that &thgr;
1
=sin
−1
(n
1
2
), and the angle of incidence &thgr;
2
of the beam F
1
at the lower surface of the light conducting plate
9
is given by &thgr;
2
=90 degrees−&thgr;
1
. If the light conducting plate is of polycarbonate, n
2
=1.59 and hence &thgr;
1
=38.97 degrees and &thgr;
2
=51.03 degrees. Since this angle of incidence &thgr;
2
is greater than the critical angle &thgr;
1
for total reflection, light beam F
1
will undergo total reflection at the lower surface of the light conducting plate
9
if the diffusion layer
15
is not present on the lower surface of the light conducting plate
9
. Similarly, total reflection will take place also at the upper surface of the light conducting plate
9
.
Consider another light beam F
2
entering from the linear light source
7
into the light conducting plate
9
. Since its angle of refraction &thgr;
3
is smaller than &thgr;
1
, its angle of incidence E
4
at the upper and lower surfaces of the light conducting plate
9
is larger than &thgr;
2
. Accordingly, light beam F
2
from the linear light source
7
undergoes total reflections at both upper and lower surfaces of the light conducting plate
9
if there is no diffusion layer
15
.
Since the reflecting plates
14
are provided on the other side surfaces of the light conducting plate
9
(that is, other than the light incident side surface
16
), light which is reflected on them is nearly entirely reflected back into the interior of the light conducting plate
9
. Since the angle of incidence at the upper and lower surfaces does not change by such reflections, light beam F
2
continues to undergo total reflection. Loss of light may be considered negligible by reflection by the reflecting plates
14
made of a metallic dielectric material.
Next, consider the light source. If a cold cathode ray tube is used as the linear light source
7
, the surface of the glass tube of such a cold cathode ray tube is covered with a fluorescent layer having a property of total diffusion against light from outside. In other words, light which is made incident onto the linear light source
7
is reflected therefrom without any loss.
Thus, the light conducting plate
9
without the diffusion layer
15
on its lower surface can seal in with a very high efficiency any light which enters from the linear light source
7
, but a plate which seals in incident light cannot serve as a light source. The sealed light must be allowed to come out through a light emitting surface
17
(the upper surface of the light conducting plate
9
). This is why the diffusion layer
15
is provided on the lower surface of the light conducting plate
9
such that light which is incident on the diffusion layer
15
is diffused and that portion of the light which does not satisfy the condition for total reflection is allowed to escape. This escaped portion of light is further diffused by the diffusion plate
11
on the upper surface of the light conducting plate
9
.
In summary, light from the linear light source
7
is emitted with a very high efficiency towards the display surface of the LC display apparatus. Even light coming from the display surface is similarly re-emitted towards the display surface without any loss.
Diffusion of light from such an edge-
Aoyama Shigeru
Shinohara Masayuki
Coudert Brothers
Omron Corporation
Sember Thomas M.
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