Illumination – Revolving
Reexamination Certificate
2001-01-19
2002-11-05
Sember, Thomas M. (Department: 2875)
Illumination
Revolving
C362S026000, C349S062000, C349S065000, C349S095000
Reexamination Certificate
active
06474827
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to image display apparatus, including automatic teller machines and game tables, using a new surface light source device, in particular, of an edge light type.
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 3B
. Alternatively, the diffusion layer
15
may take the form, as shown in
FIGS. 4A and 4B
, of indentations (or protrusions) provided on the lower surface of the light conducting plate
9
. In this case, the diffusion layer
15
becomes wider as the distance from the linear light source
7
increases. The light which is passing through the light conducting plate
9
is diffused. by the diffusion layer
15
either after it is totally reflected at the upper surface of the light conducting plate
9
, simply reflected by the light-reflecting plate
10
or at the upper surface of the light conducting plate
9
or directly by entering the diffusion layer
15
. Only that small portion of the light which did not undergo total reflection at the top surface escapes. Since the area of the diffusion layer
15
increases as the distance from the linear light source
7
increases, diffused light is emitted out of the light conducting plate
9
at a uniform brightness over the whole of the light conducting plate
9
.
The diffusion plate
11
and the pair of converging lens plates
12
and
13
are stacked on the upper surface of the light conducting plate
9
. The diffusion plate
11
comprises a synthetic resin sheet or film with its surface processed to provide fine roughness. The portion of light which escaped through the upper surface of the light conducting plate
9
is diffused by the diffusion plate
11
. The converging plates
12
and
13
each have a parallel array of sectionally triangular prisms such that the, arrays on the upper lens plate
13
and the lower lens plate
12
are perpendicular to each other. Thus, the light which has been diffused by the diffusion plate
11
is focused by the lens plates
12
and
13
in two directions and emitted out of the light conducting plate
9
nearly perpendicularly to its upper surface.
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 &thgr;
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 und
Aoyama Shigeru
Shinohara Masayuki
Omron Corporation
Sember Thomas M.
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