Color cathode ray tube with improved transmittance of selective wavelengths

An object of the present invention is to provide a CRT for improving brightness without changing a conventional electron gun, a light emitting layer, etc. A color cathode ray tube according to the present invention is characterized in that both or one of a panel glass 3 and a safety glass 1 used on a front face of the color cathode ray tube uses glass having characteristics in which spectroscopic transmittance to an electromagnetic wave near 580 to 700 nm in wavelength is relatively higher than spectroscopic transmittances to electromagnetic waves near 430 nm and 530 nm in wavelength.

TECHNICAL FIELD 
The present invention relates to a color cathode ray tube (hereafter, 
called a color CRT) suitably used in a color display unit. 
BACKGROUND ART 
There is conventionally a problem of an increase in brightness in a color 
CRT particularly used in a high definition display unit such as a computer 
display and a display unit of a large screen. 
A method for increasing a voltage applied to an anode of the CRT, a method 
for increasing an electronic beam current from a cathode of an electron 
gun, a method for improving light emitting efficiency of a phosphor itself 
on a phosphor screen, etc. are known in the prior art for increasing the 
brightness of the CRT. 
However, in the method for increasing the voltage applied to the anode of 
the CRT, power consumption is increased and there tends to be a critical 
defect of discharge in the CRT. Further, deflecting efficiency of a 
deflecting yoke (DY) is reduced and a problem of heat generation of the 
deflecting yoke is particularly caused in the high definition CRT. 
In the method for increasing the electronic beam current from the cathode 
of the electron gun, a focusing function is generally deteriorated and an 
emission life of the cathode is shortened, and the life of the electron 
gun is also shortened. Further, in the high definition CRT, an amplifying 
circuit of the electronic beam current is formed in a high frequency 
region and it becomes difficult to increase the electronic beam current 
itself. 
Further, the color CRT basically has three kinds of phosphor stripes (or 
dots) of red (R), green (G) and blue (B) and light is emitted from the 
phosphors by hitting the beam from the electron gun against these stripes. 
If the electronic beam is of high densities as it hits against the 
phosphor screen in a light emitting region of a visible ray actually used, 
brightness is correspondingly increased. Namely, it can be said that 
brightness of a specific phosphor is proportional to the electronic beam 
current. 
However, light emitting efficiencies (bright degrees with respect to the 
same beam electric current, i.e., brightnesses) of the red, green and blue 
phosphors are different from each other. The light emitting efficiency of 
the red phosphor is generally worse than blue and green phosphors with 
respect to phosphors developed and used at present. Therefore, it is 
necessary to increase the electronic beam current to increase the red 
brightness. However, there is also a restriction of the beam electronic 
current determined from the above-mentioned circuit or the life of the 
electron gun so that there is a limit in the allowed beam electric 
current. Accordingly, beam electric currents with respect to green and 
blue are set such that red, green and blue lights are balanced under a 
condition in which the red beam electric current is increased as much as 
possible. Namely, the beam electric currents corresponding to the green 
and blue phosphors are set to be lower than their allowed maximum electric 
currents. Thus, the entire brightness of the CRT is actually determined by 
the red light emitting efficiency. 
Further, the method for improving the light emitting efficiency of the 
phosphor itself on the phosphor screen has been continuously researched 
conventionally, but has recently attained a maximum state. 
It is also considered that brightness can be increased by coating a 
phosphor layer with a thick coating and relatively increasing a phosphor 
amount. However, the phosphor stripes on the phosphor screen have recently 
been thinned to achieve high definition of the CRT so that the phosphor 
layer is thinned and the phosphor amount is reduced, thereby reducing 
brightness. FIG. 3B shows a typical phosphor layer of a computer display 
of 20 inches in size. A size from a phosphor R to the next phosphor R is 
set to about 0.3 mm. A width wp of each of phosphor stripes of red (R), 
green (G) and blue (B) seen through a panel glass and a width wc of a 
carbon stripe are approximately set to about 0.05 mm (50 .mu.m). There is 
a constant limit in thickness of the phosphor layer when the phosphors are 
coated along this very narrow width of 50 .mu.m. 
It is considered, in order to to improve brightness that a ratio of the 
carbon stripe width w.sub.c and the phosphor stripe width w.sub.p on the 
phosphor screen can be changed and the carbon stripe width w.sub.c can be 
thinned and the phosphor stripe width w.sub.p can be correspondingly 
increased. However, in this case, a color shift results from caused by a 
slight mislanding (a shift in position between an electronic beam and each 
of the phosphor stripes) so that an image quality is reduced. In 
particular, the phosphor stripe width w.sub.p and the carbon stripe width 
w.sub.c are thin in the high definition CRT so that a marginal degree of 
error with respect to the mislanding is slight and it is difficult to 
increase this phosphor stripe width as a trial. 
Therefore, it is considered as a trial that the same phosphors are used and 
brightness of the CRT is improved by increasing transmittance of each of a 
panel glass 3 and a safety glass (SP safety glass ) 1 of the CRT arranged 
between the phosphors and man's eyes shown in FIG. 3A. However, in this 
case, a problem of a reduction in contrast by the second power of 
transmittance is caused so that it is difficult to increase this 
brightness. Accordingly, general transmittance of the safety glass 1 and 
the panel glass 3 is generally reduced to about 40 to 80% (these contents 
will be explained later in detail by using FIG. 3C). 
An object of the present invention is to provide a CRT for improving 
brightness without changing the conventional electron gun, the light 
emitting layer, etc. in the above-mentioned situation. 
DISCLOSURE OF THE INVENTION 
In a CRT according to the present invention, both or one of a panel glass 
and a safety panel glass are used on the front face of a color cathode ray 
tube uses glass having characteristics in which spectroscopic 
transmittance to an electromagnetic wave near 580 to 700 nm in wavelength 
is relatively higher than spectroscopic transmittances to electromagnetic 
waves near 430 nm and 530 nm in wavelength. 
Light emitting efficiencies of the red, green and blue phosphors are 
different from each other. The light emitting efficiency of the red 
phosphor among the phosphors used at present is generally worse than that 
of green and blue phosphors. As mentioned above, both or one of the panel 
glass and the safety panel glass uses glass having characteristics in 
which spectroscopic transmittance to an electromagnetic wave near 580 to 
700 nm in wavelength is relatively high. Accordingly, light emitted in a 
red light emitting region efficiently passes through this glass so that 
brightness of this light can be increased. Red, green and blue lights are 
balanced in green and blue light emitting regions by increasing beam 
electric currents since there are margins with respect to the beam 
electric currents.

BEST MODE FOR CARRYING OUT THE INVENTION 
One embodiment of a CRT in accordance with the present invention will next 
be described with reference to the accompanying drawings. FIG. 1 provides 
a visual explanation of the CRT in accordance with this embodiment. The 
CRT in this embodiment shown in FIG. 1A uses a "bronze glass" as a safety 
glass 1 arranged on the front face of a phosphor screen. The bronze glass 
used in this embodiment is a single plate glass and a glass classification 
is set to a heat absorbing plate glass, and a glass kind is set to a 
bronze vane. Further, a kind abbreviation is set to BZFL3, and visible ray 
general transmittance can be specifically set to 72.9% (in the case of a 
plate thickness 3 mm). The other elements such as an unillustrated 
electron gun, a phosphor layer 5, etc. constituting the CRT are the same 
as the conventional CRT. 
In contrast to this, the conventional CRT uses a "gray glass" as the safety 
glass. The gray glass is a single plate glass and a glass classification 
is set to a heat absorbing plate glass and a glass kind is set to a gray 
vane. Further, a kind abbreviation is set to GRFL3 and visible ray general 
transmittance is specifically set to 72.4% (in the case of a plate 
thickness 3 mm). 
For example, the bronze glass (this embodiment) and the gray glass (prior 
art) can be commercially obtained from NIPPON ITA-GLASS Co., Ltd. 
FIG. 1B is a graph showing spectroscopic transmittance characteristics 
(broken line) of the bronze glass used as the safety glass in the CRT in 
this embodiment. In this figure, the abscissa axis shows a wavelength 
.lambda. [nm] and the ordinate axis shows transmittance [%]. FIG. 1B 
simultaneously shows spectroscopic transmittance characteristics (solid 
line) of the gray glass used in the conventional CRT as a comparison 
example. 
As shown in FIG. 1, transmittance [%] in each of light emitting regions of 
red, green and blue (RGB) relative to the bronze glass and the gray glass 
is set as follows. 
______________________________________ 
Glass kind 
B (430 nm) G (530 nm) 
R (580-700 nm) 
______________________________________ 
bronze (this 
71.0 71.0 74.0-80.7 
embodiment) 
gray (comparison 
73.3 72.0 71.2-80.7 
example) 
______________________________________ 
It should be understood from FIG. 1B that transmittance of the bronze glass 
is relatively lower than that of the gray glass in each of the green and 
blue light emitting regions. It should be also understood that 
transmittance of the bronze glass is relatively higher than that of the 
gray glass in the red light emitting region. 
It was conventionally normal to select a glass material in which 
spectroscopic transmittance characteristics of the safety glass are 
approximately equal in the red, green and blue light emitting regions and 
no glass material is specifically colored. However, as shown in FIG. 1B, 
this embodiment is characterized in that a safety glass having 
spectroscopic transmittance characteristics in a desirable RGB light 
emitting region corresponding to light emission of each of red, green and 
blue phosphors is selected. 
An operation and effects of the CRT using such a bronze glass as the safety 
glass of the CRT will next be explained by using FIG. 2. FIG. 2A is a 
graph showing spectroscopic characteristics (spectrum form) of each of the 
red, green and blue color phosphors. In this figure, the abscissa axis 
shows a wavelength .lambda. [nm] of light emitted from each of the 
phosphors and the ordinate axis shows relative energy e of this emitted 
light. The red, green and blue phosphors themselves are the same as the 
conventional ones so that spectroscopic characteristics of these phosphors 
are the same as the conventional CRT. 
FIG. 2B is similar to the graph of FIG. 1B and shows spectroscopic 
transmittance characteristics of the safety panel 1. In this figure, the 
abscissa axis shows a wavelength .lambda. [nm] and the ordinate axis shows 
transmittance [%]. FIGS. 2A and 2B are arranged such that values of the 
wavelength .lambda. on the abscissa axes are in conformity with each other 
in position to easily read the relative energy e of each of the phosphors 
and the transmittance t of the safety glass with respect to the wavelength 
.lambda. of light emitted every phosphor. 
With reference to FIG. 2A, man's eyes sense a visible ray (an 
electromagnetic wave from 380 nm to 780 nm in wavelength) as light 
approximately in an illustrated range of the wavelength .lambda. [nm] on 
the abscissa axis. A color is sensed by a value of the relative energy (in 
a spectrum form) on the ordinate axis emitted from each of the phosphors 
in this range of the wavelength .lambda.. The man's eyes sense an 
electromagnetic wave near 430 nm in wavelength as a blue color, an 
electromagnetic wave near 530 nm in wavelength as a green color and sense 
an electromagnetic wave near 580 to 700 nm in wavelength as a red color. 
Accordingly, a characteristic curve having a peak of the relative energy 
near 430 nm in wavelength shows spectroscopic characteristics of the blue 
phosphor. A characteristic curve having a peak of the relative energy near 
530 nm in wavelength shows spectroscopic characteristics of the green 
phosphor. A characteristic curve having plural peaks of the relative 
energy near 580 to 700 nm in wavelength shows spectroscopic 
characteristics of the red phosphor. 
If an area (an integral value of the spectroscopic characteristic curve) at 
each of the peaks is large in FIG. 2A, light is basically sensed brightly, 
namely, brightness of this light is high. Further, if each of the peaks is 
thin, the light colors are clearly seen, namely, color purity becomes 
high. In contrast to this, if each of the peaks is thick, the light colors 
become impure and the red phosphor looks impure whitish red, the green 
phosphor looks impure whitish green and the blue phosphor looks impure 
whitish blue. Namely, the color purity is reduced. 
Next, please see the spectroscopic transmittance characteristics of the 
safety glass shown in FIG. 2B. In this embodiment, the bronze glass is 
used as the safety glass so that spectroscopic characteristics of this 
bronze glass are different from the conventional spectroscopic 
characteristics. As explained by using FIG. 1, it should be understood 
with respect to transmittance of the bronze glass (in this embodiment) 
relative to each of the red, green and blue light emitting regions that 
the transmittance of the bronze glass is relatively lower than that of the 
gray glass (in the conventional example) in each of the green and blue 
light emitting regions. It should be also understood that the 
transmittance of the bronze glass is relatively higher than that of the 
gray glass in the red light emitting region. 
The following effects are provided with respect to a viewer seeing light 
emitted from each of the red, green and blue phosphors through the safety 
glass having the spectroscopic transmittance characteristics of FIG. 2B by 
using this safety glass. 
(1) In the red light emitting region having the worst light emitting 
efficiency, the red phosphor itself is the same as the conventional one so 
that light emitting energy e.sub.R of this red phosphor is the same as the 
conventional one. However, when this emitted light is seen through the 
bronze glass relatively high in transmittance t.sub.R in the red light 
emitting region, this emitted light can look relatively bright in 
comparison with a case in which this emitted light is seen through the 
conventional gray glass. 
(2) The green and blue phosphors are the same as the conventional ones in 
the green and blue light emitting regions having relatively good light 
emitting efficiency so that light emitting energies e.sub.G and e.sub.B of 
these green and blue phosphors are respectively the same as the 
conventional ones. However, when these emitted lights are seen through the 
bronze glass relatively low in transmittances t.sub.G and t.sub.B in the 
green and blue light emitting regions, these emitted lights look 
relatively dark in comparison with a case in which these emitted lights 
are seen through the conventional gray glass. 
Conventionally, a beam electric current is maximized as much as possible in 
the red light emitting region and beam electric currents are respectively 
set to be low in the green and blue light emitting regions by balancing 
these beam electric currents with respect to red brightness at this time. 
In this embodiment, viewer's eyes see the red brightness as if the red 
brightness is increased, and see green and blue brightnesses as if these 
green and blue brightnesses are decreased in comparison with the 
conventional case. Accordingly, each of the green and blue beam electric 
currents is increased to brighten each of green and blue lights in 
comparison with the conventional case so as to balance the red, green and 
blue brightnesses. Each of the green and blue beam electric currents is 
not a maximum electric current as mentioned above, but has a margin so 
that each of these beam electric currents can be increased. Thus, each of 
the red, green and blue brightnesses is increased by an increase in the 
red brightness (seen by the viewer's eyes) and increases in the green and 
blue brightnesses provided by increasing the green and blue beam electric 
currents (to compensate an increase in brightness provided by a rise in 
transmittance of the bronze glass relative to the red light emitting 
region and a reduction in brightness caused by a reduction in 
transmittance of the bronze glass relative to the green and blue light 
emitting regions). Thus, brightness of the CRT is increased as a whole. 
This increase in brightness will be next confirmed by making a calculation. 
There are D65 (average white of sun light, about 6500 K in color 
temperature) and D93 (slight bluish white in consideration of general 
taste, about 9300 K in color temperature) with respect to reference white 
in an NTSC system. With respect to calculated results using D93, 
transmittance is 0.739 when transmittance is set to 1.0 at a nonexisting 
time of the safety panel and the bronze glass (this embodiment) is used. 
Further, transmittance is 0.715 when transmittance is set to 1.0 at the 
nonexisting time of the safety panel and the gray glass (prior art) is 
used. Accordingly, the red brightness, namely, white brightness is 
increased by 0.739/0.715=1.0336. 
In this embodiment, color purity is relatively improved in comparison with 
the prior art. This explanation will next be described. This color purity 
is improved in the red light emitting region. As shown by a spectroscopic 
characteristic curve of each of the phosphors in FIG. 2A, there are plural 
peaks of relative energy e.sub.R near 580 to 700 [nm] in wavelength in the 
red light emitting region. As mentioned above, with respect to the color 
purity, light looks bright if a peak of the relative energy is thin. 
Namely, the color purity becomes high. Accordingly, the red color purity 
is improved if transmittance t.sub.R relative to a peak at 580 to 640 [nm] 
in wavelength is increased, or if transmittance t.sub.R relative to a peak 
at 680 to 710 [nm] in wavelength can be decreased. 
As shown in FIG. 2B, in the CRT using the bronze glass as the safety glass 
in this embodiment, transmittance (about 80 to 81.5%) with respect to a 
peak in a region from 680 to 720 [nm] in wavelength is equal to 
transmittance of the conventional gray glass. However, transmittance 
(about 74.0 to 74.5%) with respect to a peak in a region from 590 to 640 
[nm] in wavelength is relatively high in comparison with transmittance 
(about 71.0 to 71.6%) of the conventional gray glass. Namely, transmitted 
light of the red phosphor in the region from 590 to 640 [nm] is relatively 
large in the bronze glass according to this embodiment in comparison with 
the gray glass. Therefore, it should be understood that an influence of 
the transmitted light of the red phosphor in the region from 580 to 640 
[nm] in wavelength is relatively reduced. As a result, the color purity is 
improved in the red light emitting region. 
Brightness of the CRT is normally measured with white prescribed by D93 or 
D65. This white is conventionally obtained when light is approximately 
emitted in a ratio of R:G:B=2:7:1. When the red color purity can be 
increased as in this embodiment, a net red light portion is increased so 
that a red ratio can be relatively reduced. For example, it is assumed 
that the same white can be obtained in a ratio of 1.8:7.1:1.1. Red light 
can be conventionally brightened until a brightness corresponding to value 
2. Therefore, the ratio of (1.8:7.1:1.1) can be entirely multiplied by 
2/1.8=1.1 times. Accordingly, in this case, general brightness, namely, 
white brightness is multiplied by 1.1 times by the improvement of the 
color purity. 
Thus, the use of the bronze glass having relatively high transmittance in 
the red light emitting region is similar to an application of a band pass 
filter to a spectrum of the red light emitter. Accordingly, the red color 
purity is improved so that brightness is increased. 
The increase in brightness caused by this color purity is confirmed by 
making a calculation. The red brightness is calculated by paying attention 
to chromaticity at a time of white 100 [cd/m.sup.2 ] of D93. This red 
brightness is 21.3 in the case of the bronze glass and is 21.8 in the case 
of the gray glass. Accordingly, when the red brightness is set to 100 
[cd/m.sup.2 ] in use of the conventional gray glass, the red brightness is 
increased by 2.35% from 100 .times. (21.8/21.3)=102.35 [cd/m.sup.2 ] in 
the case of the bronze glass. 
These effects of the increases in transmittance and color purity in total 
provide 102.35 [cd/m.sup.2 ] .times.1.0336=105.7 [cd/m.sup.2 ]. Finally, 
the brightness is increased by 5.7%. Namely, the increase in brightness 
has also been confirmed by making a calculation. It is also confirmed in 
the case of D65 that brightness is increased by 5.0% by making a similar 
calculation. 
Contrast will next be considered. A CRT image quality greatly depends on a 
ratio of a highlight brightness and a black level brightness on the 
screen, i.e., a contrast ratio. The image quality is reduced if contrast C 
is reduced even when brightness is increased. The contrast C is 
represented as follows. 
C=brightness of CRT/brightness of light reflected on the CRT screen 
=brightness of CRT/(external light x reflectivity) 
As shown in FIG. 3C, when the CRT with a safety glass is seen and external 
surrounding light (external light) such as interior illumination, etc. is 
incident to the screen, reflected light (1) on a safety glass surface, 
reflected light (2) on the surface of a panel glass 1 on a phosphor layer 
side, and reflected light (3) on the phosphor are caused. The reflected 
light (2) and the reflected light (3) are mixed with light emitted from a 
phosphor surface. Accordingly, the black level brightness is increased so 
that the contrast ratio is apparently reduced, thereby causing a problem. 
In this case, the reflected lights (2) and (3) passing twice through the 
panel glass and the safety glass in going and returning are damped 
(namely, reflectively is reduced) in the second power of transmittance by 
reducing transmittances of the panel glass and the safety glass 1 in the 
CRT so that the contrast C is improved. (Accordingly, the safety glass and 
the panel glass are generally used in a state in which general 
transmittance of the safety glass and the panel glass is reduced to about 
40 to 80%. This is because a CRT having a reduced transmittance called a 
black face is used.) In this embodiment, ;the general transmittance in the 
case of the bronze glass is approximately equal to that in the case of the 
gray glass. Accordingly, the contrast C is increased by the 
above-mentioned increase in brightness of the CRT. Strictly speaking, 
transmittance of the bronze glass is higher by 0.33% than that of the gray 
glass. However, it is considered that there is no influence of this 
difference since it can be expected that brightness is improved by about 
5%. 
Thus, in this embodiment, the red light emitting efficiency is relatively 
low in comparison with the green and blue light emitting efficiencies in 
the red, green and blue phosphors developed at present. Therefore, the 
red, green and blue phosphors use a safety glass in which spectroscopic 
transmittance of the safety glass in the red light emitting region is 
relatively higher than each of spectroscopic transmittances of the safety 
glass in the green and blue light emitting regions. A beam electric 
current with respect to the red phosphor is set to be high as much as 
possible. Each of beam electric currents with respect to the green and 
blue phosphors is set by balancing the red brightness and the green and 
blue brightnesses. 
In this embodiment, the safety glass is explained, but the panel glass is 
similar to the safety glass in that the panel glass is arranged between 
the phosphor layer 5 and a viewer. Accordingly, the matters relative to 
the glass transmittance explained with respect to this embodiment can be 
also applied to both or one of the safety glass and the panel glass. 
It is conventionally considered that two kinds of beam electric currents 
(i.sub.R, i.sub.G, i.sub.B) and light emitting energies (brightnesses) 
(e.sub.R, e.sub.G, e.sub.B) of the phosphors are parameters with respect 
to the CRT brightness. However, this embodiment is characterized in that 
it is found that the improvement of brightness can be achieved by 
adjusting transmittances t.sub.R, t.sub.G and t.sub.B of the safety glass. 
It can be said that this embodiment is a technique for compensating 
dispersions between the light emitting energies (brightnesses) e.sub.R, 
e.sub.G and e.sub.B of the present phosphors by transmittances t.sub.R, 
t.sub.G and t.sub.B of the safety glass. Accordingly, the essence of the 
present invention is considered as follows. 
There is a possibility that a development relative to the improvement of 
the brightnesses of the red, green and blue phosphors themselves is 
advanced by engineers skilled in the art in the future, and a balance of 
the light emitting efficiencies of the red, green and blue phosphors in 
the future is different from the present balance. In the future, for 
example, the light emitting energy e.sub.R of the red phosphor might 
become relatively higher than the light emitting energies e.sub.G and 
e.sub.B of the green and blue phosphors. In this case, the CRT brightness 
can be improved by selecting a safety glass having a transmittance ratio 
t.sub.R :t.sub.G :t.sub.B respectively approximately proportional to a 
ratio 1/e.sub.R :1/e.sub.G :1/e.sub.B of inverse numbers of the light 
emitting energies (brightnesses) of the respective phosphors. 
Further, if optical characteristics of glass can be freely controlled in 
the future, the following operation is considered. The contrast C is 
represented as follows. 
C=brightness of CRT/(external light x reflectivity) In general, an image 
quality is preferable when the contrast C is high. In the embodiment 
described so far, reflectivities are approximately the same and the 
contrast C is improved by improving the CRT brightness. However, in the 
future, if glass having transmittance higher by 5% can be obtained without 
changing the contrast C (since the CRT brightness, the external light and 
the reflectivity are respectively independent variables), 1.052=1.10, 
i.e., a CRT having a brightness increased by 10% is obtained from 
brightness of CRT=contrast C.times.(external light x reflectivity) since 
the reflectivity is proportional to the second power of transmittance as 
explained with reference to FIG. 3C. 
Further, the CRT brightness can be improved by adjusting three kinds of 
parameters of the above beam electric currents, the light emitting 
energies (brightnesses) and transmittances of the safety glass. The 
respective beam electric currents i.sub.R, i.sub.G and i.sub.B are 
desirably equal to each other or close to each other as much as possible 
in view of life of an electron gun. The light emitting energies 
(brightnesses) e.sub.R, e.sub.G and e.sub.B of the phosphors are desirably 
close to each other as much as possible or are desirably set to values in 
a desirable ratio in view of generation of other colors such as white, 
etc. Further, the transmittances t.sub.R, t.sub.G and t.sub.B of the 
safety glass are desirably equal to each other or close to each other as 
much as possible in view of prevention of accidental coloring caused by 
glass transmission of light. 
Finally, it is considered that values of these three kinds of parameters 
between red, green and blue are adjusted to balance desirable red, green 
and blue brightnesses at a developing time point of the CRT by providing a 
priority order of the parameters according to needs at this time point, a 
technical situation of glass, phosphors, etc. and fixedly providing a 
parameter having a relatively high priority order, and varying a parameter 
having a relatively low priority order. 
In the above-mentioned explanation, the present invention is not limited to 
the above embodiment. A technical range of the present invention is 
determined on the basis of the description of claims. 
In accordance with the present invention, brightness of the CRT can be 
further improved.