Note: Descriptions are shown in the official language in which they were submitted.
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BACKGROUND OF THE INVENTION
- Field of the Invention
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The present invention relates generally to a
color cathode ray tube and more particularly to a current-
sensitive color cathode ray tube.
Description of the Prior Art
In a color cathode ray tube used in an ordinary
color television receiver and so on, in opposed relation to
its phosphor screen, located is a means such as a shadow `;
mask, aperture grill or the like which functions to determine
the landing position of an electron beam on the phosphor
screen, whereby the electron beams corresponding to the
respective colors are landed on the respective color phosphor
dots or stripes of the phosphor screen to thereby provide
respective color emissions and h'ence reproduce a color
- picture on the phosphor screen.
On the contrary, in a current-sensitive color
cathode ray tube, there is provide no means to determine the
landing position of the electron beam but itc phosphor
screen is formed of more than two color phosphors, which
are mixed and then coated, and emits necessary color light
or lights within a predetermined limited color gamut in
response to the beam current density.
Since such the current-sensitive color cathode
ray tube has no means to determine the electron beam landing
position, it performs such an advantage that its weight
becomes light, its assembling or manufacturing process
becomes simpli~ied and its resoLution is improved and also
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such an advantage to avoid various problems, ~or example,
mislanding caused by the relative positional displacement
between the ph~sphor screens and the means to de~ermine the
electron beam landing position.
BRIEF DESCRIPTION OF THE DR~WINGS
Fig. 1 is a graph showing the current density versus
luminance characteristic used to explain a prior art current-
sensitive color cathode ray tube;
Fig~ 2 is a schematic diagram showing an example
of the current-sensit ~e color ca~hode ray tube according to
the present inv~ntion;
Figs. 3, 5, 9, 10 and 11 are respectively graphs
showing the beam current density versus luminance characæ~istics;
Fig. 4, ap~¢ing with~Figs. 1 and 2, ~s a ch~ticity diagram;
Fig. 6 is a graph showing a measured ourve repre-
senting the selation between Eu amount in red-emitting
phosphor and luminance thereof;
Fig. 7 is a graph showing the simil~r Eu amount and
chromaticity value; and
Fig. 8 is a cross-sectional view of the phosphor
screen used in the cathode ray tube shown in Fig. 2.
: A prior art curren~-sensi~ive colox cath~de xay
'! tube, mentioned just above, includes a phosphor screen made
of mixed color phosphors of-a green phosphor showing a
so-called sub-linear characteristic represented by a curve 1
in the graph of Fig~ 1 which shows the luminanee character-
.~ istic of the phosph~r to the electron beam ~urrent density,
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with a red phosphor showing a 50- called super-linear
characteristic xepre~entea by a ~urve 2 in the graph of Fig.
1. In this case, within a range where the beam current
density is small, the light emission of green is dominant,
while within a range where the beam current density is large,
the light emission of red is dominant, whereby a color image
having a color gamut of synthesized two colors in response
to tha beam current density is produced.
As the green-emitting phosphor whose current density
versus luminance characteristic is the above sub-linear
characteristic, phosphor of Zn2SiO~: Mn is generally used,
while as the xed-emitting phosphor showing the super-linear
characteris~ic, ~CdZn)S: Ag, Ni is usually employed. In
this case, for making the green-emitting phosphor Zn2SiO4 :
Mn represent th~ sub~linear characteristic, it is necessary
to select t~e adding amount of Mn small. When the adding
amount of Mn is selected small, a problem of burn by the
beam irradiatio~ occurs. On the other hand, if the super
-linear char~cteri~tic i8 pr~sented by the red-emitting
phosphor (ZnCd)5: Ag, N$, the amount of Cd must be
increased. When, howe~er, the amount of Cd i~ increased,
the luminance thereof becomes low and its color purity
becomes also low. This low color purity of the above prior
art color cathode ray tube xesults in tha~ its color gamut,
which can be re~roducedt becomes relatively narrow.
OBJECTS AND SI~ RY OF THE INVENTIO~
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Accordingly, it is an object of the present
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invention ~o provide a curren~-sensitive color cathode ray
tube in which color purity is improved to thereby widen
light emission color gamut.
Acc~rding to an aspect of this invention there
is provided a current-sensitive color cathode ray tube
having a phosphor screen scanned by an electron beam from
an electron-source, with the current density in said beam
varied at substantially constant accelerating voltage,
said screen comprising a red-~mitting phosphor having a
sublinear characteristic in the intensity-current density
relationship and a different color-emitting phosphor other
than red having a linear or superlinear characteristic in
the intensity-current density relationship, characterized
in that said red emitting phosphor has a following formula:
(Lnl_xEux)2O2s
wherein Ln is a material selected from the group consisting
of Y, La, Gd and Lu, and x ranges from 0.05 to 0.10 and
further, concentration of a rare earth impurity such as
terbium Tb and praseodymium Pr is less than 10 ppm.
The other objects, eatures and advantages of the
present invention will become apparent from the following
description taken in conjunction with the accompanying
drawings through which the like references designate the
: same elements and part~.
DESCRIPTION OF THE PR~FE~RED ~ODIMEnTS
: The present invention will be hereinbelow described
with reference to the attached drawings.
In general, according to the present invention, by
using the combination of one color-~mitting phosphor, which
has its current density versus luminance characteristic
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representing the sub-linear characteristic, with another
color-emitting phosphor whose current density versus
luminance characteristic represents the super-linear or
linear characteristic, there is generated light emission
with hue or color gamut represented by the synthesized color
of the lights emitted from both color phosphors by controlling
the current density, for example, modulating the cathode :
current.
~ . - In this invention, especially, as the phosphor
which shows the above sub-linear characteristic a red-emitting
p~osphor, which is high in color purity and has the compo-
sition formula of (Lnl xEux)2O2S is used. In this formula Ln
is formed of at least one kind in the group consisting of
yttrium Y, lanthanum La, gadolinium Gd and luletium Lu, and
x is selected to satisfy the condition of 0.05 - ~ - 0.10.
Further, in this red-emitting phosphor rare-earth impurities
such as terbium Tb, praseodymium Pr and so on are selected
less than 10 ppm.
The phosphor screen used in the current-sensitive
color cathode ray tube of this invention is made by such a
manner that the above red-emitting phosphor with sub-linear
characteristic is mixed with another color, for example,
green-emitting phosphor or blue-green-emitting phosphor with
the super-linear or linear characteristic and the mixed
phosphor is laminated as a layer, or each phosphor is re-
spectively and sequentially laminated as a plurality of layers.
The reason why the value of x in the red-emitting
phospnor (Lnl_xEux)2O2s is selected to satisfy 0.05 ~ x _ 0.10
in the invention is that such a fact was noted that if x was
less than 0.05 the color pu:ity was lowered and ycllowish
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light emission is caused, while if x exceeded 0.10 the
luminance became low. Further, the reason why the rare-
earth impurity concentration of Tb, Pr and so on is selected
less than 10 ppm is that it was ascertained that when the
rare-earth impurity concentration of Tb, Pr and so on
exceeded 10 ppm, the luminance was difficult to be saturated
upon high beam current density and hence the sub-linear
characteristic was not presented.
Now, the examples of this invention will be
explained as follows:
Example 1
The red-emitting phosphor is made as follows:
Rare-earth oxide Ln203 is added with europium oxide
Eu203 at a predetermined amount and then they are mixed to
prepare a mixed rare-earth oxide; the mixed rare-earth oxide
is further added with sulphur S at 30 to 80 weight %~ sodium
carbonate Na2C03 at 30 to 80 weight %~ which serves as flux,
and so on and they are mixed; and the mixture is burned in
air at 900 to 1300C in 0.5 to 5 hours to provide phosphor
represented by the composition fQrmula of (Lnl xEux)202S.
By hydraulic elutriation method, small size particles are
removed from the above phosphor and, by use of a sieve large
size particles are removed from this phosphor. While, green
-emitting phosphor (ZnO 64Cdo 36)S: Ag,Ni which contains
50 ppm of Ag and 10 ppm of Ni is prepared. Then, the red-
emitting phosphor and green-emitting phosphor are respectively
weighted to be 35 weight % and 65 weight % and then mixed.
Then, the phosphor thus mixed is coated on the
inner surface of a panel 3a of a cathode ray tube envelope 3
by the sedimentation method to form a phosphor screen 4 as
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shown in Fig. 2. In Fig. 2, reference numeral 5 designates
an electron gun which will emit an electron beam to the
phosphor screen 4 and reference numeral 6 denotes its
horizontal and vertical deflection means. This current-
S sensitive color cathode ray tube is provided with no means
to determine the landing position of the electron beam on
the phosphor screen 4j for-example, no shadow mask or ~~
aperture grill and so on and hence this color cathode ra~
~ tube can take the similar construction to that of an
ordinary monochromatic cathode Yay tube.
Fig. 3 is a graph showing measured current
density versus luminance characteristics of the phosphor
screen 4 of the invention for such a case where in the
Example 1 the red-emission phosphor is Y202S: Eu. In the
graph o Fig. 1, a broken line curve 11 represents the
current density to luminance characteristic of the red
light emission by the red-emitting phosphor Y202S: Eu, a
-- solid line curve 12 shows that of the green light emission
by the green-emitting phosphor ~ZnO.64CdO.36) g
a one-dot chain line curve 13 represents the current density
to luminance characteristic of this phosphor screen,
namely the total luminance versus current density characteristic
of respective color lights of the color cathode ray tube.
As will be apparent from the graph of Fig. 3, the red-
emitting phosphor Y202S: Eu shows the sub-linear characteristic
while the green-emitting phosphor (ZnO 64Cdo 36)S Ag, Ni
shows the super-linear characteristic, respectively.
Now, the color gamut of the light emission by the
red and green-emitting phosphors will he considered with
reference to the graph of Fig. 4 which is a MacAdam's u,v
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chromaticity diagram. In the graph of Fig. 4, reference
letters R, G and B respectively shows the chromaticities of
red, green and blue. In this case, the line RlGl from a
point Rl to a point Gl shows a color gamut within which the
light emission of the color cathode ray tube with the
phosphor screen made by the mixture of the red-emitting
phosphor Y202S: Eu with the g~een-emitting phosphor (ZnO 6~ ;
Cdo 36)S: Ag, Ni according to Example 1 is possible. In
~ - this case, in accordance with the beam current density,
namely as the beam current density becomes large, such light
emission with hue is generated in which the hue goes to the
point Gl in the line RlGl, while as the beam current density
becomes low the hue of light emission goes to the point Rl.
In the graph of Fig: 4, letters Rl' and Gl' respectively
designate chromaticity points of red and green colors of a
prior art current-sensitive color cathode ray tube mentioned
previously. In this case, the color gamut which can be
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reproduced rests on the line connecting the points Rl' and
G '. As will be apparent from the comparison of both lines
RlGl and Rl'Gl', it is understood that the color gamut of
this invention which can be reproduced is wider than that of
the prior art since the line RlGl is longer than that Rl'Gl'
and hence the color purity of, especially red color is
improved.
In the graph of Fig. 5, a solid line curve 21
shows the current density to lumir,ance characteristic of
Y202S: Eu which corresponds to the broken line curve 11 in
the graph of Fig. 3, and a solid line curve 22 in the graph
of Fig. 5 shows the similar characteristic of Y202S: Eu, Tb
from which it will be clear that when Tb is added to the
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phosphor, it beco~es di~ficult to present the sub-linear
characteristic.
The graph of Fig. 6 shows reIative values of the
luminance for the amount of Eu ~n the Y2O2S: Eu phosphor
Cnamely the value x in the Composit~on formula (Lnl xEux)202S)
which were measured, and curVes 31 and 32 in the~ graph of
F~gO 7 respectively show values of x and y in the x-y
chromaticit~ diagram of the amoun~ of Eu in similar phosphors.
In the x y chromaticity value, when the Eu amount is less
10 than 5 mole %, the p~osphor represents yellow. Thereforè,
it will ~e easily understood that the Eu amount is selected
more than 5 mole % and less than 10 mole ~ so as to select
the relative value of the luminance more than 50 ~ in view
of the luminance characteristic in the graph of Fig. 6,
15 namel~ the x ~alue in t~e above composition is selected to
satisfy 0.05 _ x _ 0.10.
- In the above Example 1, although the red and green
phosphors are mixed to form the phosphor screen 4, it is
possible that the res~ective color phosphors are r~spectively
20 laminated as layers to form the phosphor screen 4. ~ow, an
example of such case will be described.
Example 2
90 weight % of the gr~en-emittin~ phosphor used in
Example 1 is first coated on the inner surface of the panel
25 3a of the cathode ray tube envelope 3 by the sedimentation
method and the phosphor layer thus formed is dried sufficiently.
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Thereafter, 10 weight % of the red-emitting phosphor used
in ~xample 1 is coated on the above green-emit~ing phosphor
layer by the similar sedimentation method. Thus, the phosphor
- --- screen 4 consisting of green and red phosphor layers 4G and
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4b laminated in this order on the inner surface of the
panel 3a is provided as shown in Fig. 8 and hence a current
-sensitive color cathode ray tube similar to that explained
in connection with Fig. 2 is provided.
The current density to luminance characteristics
of the red and green colors in Example 2 become as indicated
by broken line and solid line curves'!41 and'42 in t'he'graph ' '''-~
of Fig. 9 and the total characteristic thereof is shown by
a one-dot chain line curve 43 in the same graph. In this
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case, as will be apparent from the comparison of the curve
12 in the graph of Fig. 3 with that 42 in the graph of Fig.
9, in case of the laminated phosphor screen o~ Example 2,
the current density versus luminance characteristic of the
lower layer, namely the green-emitting phosphor layer 4G
located on the side opposite to the electron beam impinging
side represents lower luminance at the low current range so
that the purity of t~e red can be enhanced much.
~' It is needless to say that the present invention
is not limited to the combination of the~red-emitting
phosphor having sub-linear characteristic with the green-
emitting phosphor having the super-linear or linear
characteristic, but the invention can be applied to the
combination of the red-emitting phosphor having the sub-
linear characteristic with another color phosphor having the
super-linear or linear characteristic such as blue-green
-emitting phosphor. An example o~ this case will be now
described.
Example 3
Similar to Example 1 but in place of the green
-emitting phosphor, a blue-green-emitting phosphor
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(ZnO 82Cdo l8)S: Ag, Ni is used. The current density
versus luminance characteristics of xed light emission,
blue-green light emission and total light emission of this
case are respectively measured as shown by broken, solid ::
and one-dot chain line curves 51, 52 and 53 in the graph
of Fig. lO.
Example ~
Similar to the laminated structure as in Example
2, but in place of the green-emitting phosphor, a blue-
emitting phosphor (ZnO 82Cdo l8)S: Ag, Ni is used. The
current density versus luminance characteristics of red
light emission, bluish. green light emission and total
light emission of this case are respectively measured as
indicated by broken, solid and one-dot chain line curves 61,
62 and 63 in the graph of Fig. 11.
As will be apparent from the oregoing explanation,
according to the current-sensitive color cathode ray tube of
~` the present invention, its color purity is Pnhanced as
compared with the conventional color cathode ray tube of
this kind and accordingly, light emission colox gamut can be
widened.
The above description is given on the preferred
embodiments of the invention, but it will be apparent that
many modifications and variations could be effected by one
skilled in the art without departing from the spirits or
scope of the novel concepts of the invention, so that the
scope of the invèntion should be determined by the appended
claims only.
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