Discharging and light emitting device

Details Discharging and light emitting device Discharging and light emitting device

2000-07-17

2002-08-27

Patel, Vip (Department: 2879)

Electric lamp and discharge devices

With luminescent solid or liquid material

With gaseous discharge medium

C313S631000

Reexamination Certificate

active

06441548

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a discharge and light emitting device in which a discharge gas such as xenon enclosed between electrodes is discharged for emitting light.
2. Description of the Background Art
Various light emitting devices have been proposed and reduced to practice as a light source. One of such conventional devices is a light source used for a contact image sensor (hereinafter as CIS) which reads a content such as diagram.
FIGS. 11 and 12
show an example of such a CIS, CIS
100
including a conventional light source.
FIG. 11
is a plan view of CIS
100
disclosed by Japanese Patent Laying-Open No. 4-360458 (Japanese Patent No. 2953595), and
FIG. 12
is a cross sectional view of CIS
100
shown in FIG.
11
.
As shown in
FIGS. 11 and 12
, CIS
100
includes an LED (Light Emitting Diode) array
101
as a light source, a casing
102
, a sensor IC (integrated Circuit)
103
, a rod lens array
104
, and a glass plate
105
.
A document
106
between a platen
107
and glass plate
105
is irradiated with light by LED array
101
, and reflected light is passed through rod lens array
104
to reach sensor IC
103
. The reflected light is then converted into an electrical signal by sensor IC
103
and the content of document
106
is read.
The use of LED array
101
as a light source for a contact image sensor as described above is encountered with the following various disadvantages.
When an LED is used as a light source, the necessary light amount of the light source changes depending upon the time required by the image sensor to read one line of information in the case of a line sensor. This means that the signal output I of the sensor has the relation represented as I∝T×B relative to the reading speed (reading time T per line) and the brightness B of the light source. Therefore, if reading time T is large (a document is read by a facsimile machine for example at the speed of ~10 ms/line), an output from the sensor is tolerable for use.
Note however that reading time T would be very small for high speed reading at a speed of 0.5 ms/line or less, and therefore sufficient sensor output does not result.
When LED chips are arranged, the optical output of the LED chips has strong directivity, the light amount is much different between forward and diagonally forward directions, and therefore the following problem is encountered. When a light source is manufactured using an arrangement of LED chips, a gap is present between LED chips by the restriction of the mounting pitch, which causes difference in the light amount between the region over the LED chips and the region over the gaps. As a result, a corrugation is generated in the amount of light at the LED mounting pitch in the direction of the arrangement of the LED chips.
Furthermore, because of variation in the LED mounting precision (precision at which the light emitting center of the LEDs is aligned on one line) and the directivity of the light amount as described above, the above corrugation could be larger.
In addition, there is variation in the brightness itself of LED chips, and therefore, using an arrangement of LED chips, the brightness variation is reflected upon the brightness distribution on a line. Therefore, the light amount could not be uniform for the entire illumination length.
When a high brightness is to be achieved, LED chips must be mounted in a high density to increase current contributing to light emission, which however causes the light source to generate heat and hence reduces the useful life of the LED chips.
As a light source for a CIS, a conventional, cylindrical lump such as a hot cathode tube (fluorescent lamp) and a cold cathode tube could be used. In this case, a sufficient amount of light as a light source could be obtained.
However, the inner shape of the CIS must have such a form to receive the cylindrical light source, and therefore the cross sectional shape would be large. Since such a lamp has electrodes at both ends, a lower brightness portion as long as several centimeters called “cathode dark space” is necessarily formed. As a result, the percentage of the region having a stable light amount relative to the entire length of the light source is reduced.
The inventor has eagerly studied and come to conceive the use of a light source which emits light by discharge for the light source of the contact image sensor, and succeeded in the development of a light source of this type.
FIG. 1
shows an example of a discharging and light emitting device
1
which can be used as the light source.
As shown in
FIG. 1
, discharging and light emitting device
1
includes a substrate
2
, a transparent substrate
3
, an inner electrode
4
, an outer electrode
5
, a metal bus
6
, an insulating layer (dielectric layer)
7
, a first fluorescent substance
8
, a second fluorescent substance
9
, a sealing layer
10
, and a discharging space
11
.
Substrate
2
and transparent substrate
3
are made for example of glass. Transparent substrate
3
is placed upon substrate
2
, and has a wall
3
a
extending toward substrate
2
. Wall
3
a
is connected to substrate
2
through sealing layer
10
and insulating layer
7
. Thus, a discharging space
11
is formed between substrate
2
and transparent substrate
3
. A discharge gas such as xenon is enclosed in discharging space
11
. Note that sealing layer
10
is made of a glass layer formed for example by melting frit.
Inner electrode
4
is formed on substrate
2
, and covered with insulating layer
7
. Insulating layer
7
is for example made of a glass layer. First fluorescent substance
8
is formed on insulating layer
7
, and second fluorescent substance
9
is formed on the surface of transparent substrate
3
.
Outer electrode
5
is for example made of ITO (Indium Tin Oxide) or SnO
2
, and has transmittancy. Outer electrode
5
formed on the outer surface of transparent substrate
3
forms metal bus
6
on the periphery of outer electrode
5
.
In order to allow discharging and light emitting device
1
having the above construction to emit, a prescribed voltage (for example, about 1000V) is applied between inner electrode
4
and outer electrode
5
. Thus, a discharge gas is electrolytically dissociated to discharge ultraviolet rays, which are then directed upon first and second fluorescent substances
8
and
9
and these substances emit light.
The inventor has confirmed that the brightness of light thus obtained is higher than the conventional case using the LEDs. The brightness distribution is homogeneous, the useful life of discharging and light emitting device
1
is significantly longer than that of the LEDs. The percentage of the effective illumination length is much increased, which makes it easier to reduce the size in the longitudinal direction. Furthermore, since no toxic substance such as mercury is used, the risk of environmental destruction can be avoided.
While discharging and light emitting device
1
shown in
FIG. 1
may provide various, more excellent effects than those of the conventional device as described above, the inventor has further advanced his study to come across the following, new problem to be solved for such discharging and light emitting device
1
. The problem will be now described.
FIG. 2
shows a discharging path
12
a
in discharging space
11
when discharging and light emitting device
1
emits light. Note that the arrow in
FIG. 2
represents the direction in which light is emitted.
As shown in
FIG. 2
, since inner electrode
4
and outer electrode
5
are placed opposing each other, discharging path
12
a
is positioned vertically to the main surface of each of substrates
2
and
3
. The length of discharging path
12
a
is therefore as short as the shortest distance between substrates
2
and
3
.
In a light source using gas discharging, the brightness and light emission efficiency typically increase as a function of the length of the discharging path length. As a result, the short discharging path length as described above could

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