Flat color cathode-ray tube

A flat color CRT wherein a single beam emitted from an electron gun scans the inner surface of a tube wall and which includes a phosphor screen formed on the walls inner surface. The phosphor screen comprises a nonluminescent substance and phosphor index stripes with the substance interposed between the stripes. Red, green, blue primary phosphor color stripes have a sufficient thickness and are arranged repeatedly on the nonluminescent substance at a spacing in a definite relation to the phosphor index stripes.

TECHNICAL FIELD 
The present invention relates to a flat, color cathode-ray tube 
(hereinafter referred to as "CRT") having a beam-indexing system 
incorporated therein. 
PRIOR ART 
As is well known, beam-indexing color CRTs have a phosphor screen 
comprising a multiplicity of phospher index stripes arranged on the inner 
surface of a panel. In addition, three primary phosphor color stripes are 
repeatedly arranged on the inner surface panel of the in a definite 
relation to the phosphor index stripes. When the phosphor screen is 
scanned by a single electron beam, an index light signal is obtained. The 
screen is utilized by the electron beam to excite the desired phosphor 
color with a specified amount of the electron beam for the reproduction of 
color images. 
There are two types of flat, color CRTs incorporating such a beam-indexing 
system. FIG. 9 shows the structure of one of these types. With reference 
to FIG. 9, a flat glass tube 1 comprises a neck 3 accommodating an 
electron gun 2, a funnel 4 and a panel 6 provided with a phosphor screen 5 
on an inner surface. The phosphor screen 5 is inclined with respect to the 
central axis of the electron gun 2 (to the direction of propagation of an 
electron beam 7 when the beam is not deflected). 
The phosphor screen 5 comprises red, green, blue, i.e., three primary 
phosphor color stripes 8 repeatedly arranged on the inner surface of the 
panel 6. A metallic layer 9 of aluminum is formed over the resulting inner 
surface of the panel. Phosphor index stripes 10 are provided on the 
metallic layer 9 in a definite relation to the primary phosphor color 
stripes 8 (FIG. 10). 
In the aforementioned flat, color CRT, a single electron beam 7 is emitted 
by the electron gun 2. The beam is deflected by a deflection yoke 14 and 
scans the phosphor screen 5 to produce index light. The index light 
strikes a light collector plate 11 disposed on the rear side of the funnel 
4. The index light collected by the plate 11 is converted to a wavelength 
matching the sensitivity of a photodetector, from which the light is led 
to a photodiode 12 provided at one end of the light collector plate 11. 
The photodiode 12 produces an electric signal upon conversion. Published 
Unexamined Japanese Patent Application No. SHO 57-65651 discloses a light 
collector plate which is usable as the plate 11. 
On the other hand, the electron beam 7 passes through the metallic layer 9 
and excites a color phosphor stripe 8, whereupon the stripe luminesces. 
The luminescence can be observed through the panel 6. 
However, a drawback of the flat, color CRT of the above construction is 
that an insufficient luminance color image is reproduced because the 
electron beam 7 excites the color phosphor stripe 8 through the metallic 
layer 9. Insufficient luminance is due to the phosphor screen 5 being 
arranged inclined to the axis of the electron gun 2. Therefore, the beam 7 
is incident on the screen 5 obliquely, resulting in the electron beam 
traveling a longer distance as it passes through the metallic layer 9. An 
increased proportion of the electron beam energy therefore attenuates 
within the metallic layer 9 to further reduce the luminance of 
luminescence of the phosphor color stripe 8. 
Published Unexamined Japanese Patent Application No SHO. 57-27541 discloses 
a flat, color CRT of another type which is adapted to overcome the above 
drawback. FIGS. 11 and 12 schematically show the construction of the CRT. 
With reference to FIG. 11, a phosphor screen 5' comprises phosphor index 
stripes 10 provided on the inner surface of a panel 6. A metallic layer 9 
of uniform thickness is formed over the panel's inner surface to cover the 
stripes 10. Primary phosphor color triplet stripes 8 are provided on the 
metallic layer 9. In this case also, the phosphor index stripes 10 are of 
course arranged in a definite relation to the arrangement of phosphor 
color stripes 8. 
With the above described flat, color CRT, an electron beam 7 directly 
excites the color phosphor stripe 8 for luminescence. The luminescence is 
reflected from the metallic layer 9 toward the interior vacuum space of 
the flat glass tube 1, so that a bright color image can be observed 
through a window formed in a funnel 4. 
Nevertheless, the phosphor screen 5' has a drawback. FIG. 12 shows the 
screen 5' in greater detail. The electron beam 7 passes through the 
metallic layer 9 and excites the phosphor index stripe 10, which therefore 
produces weak luminescence. Consequently, the index light incident of a 
light collector plate 11 through a panel 6 is low in intensity. 
Beam-indexing color TV receivers require an index signal be obtained at all 
times in order to detect the position of the electron beam, so that even 
when a black image is reproduced, a beam current not lower than a 
specified level is passed. Accordingly, a smaller amount of electron beam 
is needed for producing the index signal for the black level in order to 
improve the contrast to the image. However, because the metallic layer 
attenuates the energy of the electron beam as mentioned above, a larger 
amount of electron beam is required to reproduce the black level even with 
a CRT of the second type than when the electron beam directly excites the 
phosphor index stripes. This invariably results in a lower contrast. 
To assure improved contrast, a nonluminescent carbon layer is formed in the 
spaces 5a' the phosphor color stripes other than those spaces where the 
phosphor index stripes are provided. However, since the aluminum forming 
the metallic layer is generally porous, it is extremely difficult to form 
the carbon layer over the metallic layer. The nonluminescent substance 
commercially available generally comprises a mixture of carbon and an 
aqueous solution of ammonia or alkali material to render the carbon 
effectively separable. However, the solvent is not compatible with the 
metallic layer of aluminum. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide a beam-indexing flat CRT 
for reproducing bright images with an improved contrast ratio. 
Another object of the invention is to provide a color CRT which has no 
metallic layer and in which the electron beam directly excites phosphor 
index stripes. 
Still another object of the present invention is to provide a phosphor 
screen for CRTs wherein the position of a mask pattern and 
position-matching marks relative to each other is easily adjustable. 
More specifically, the present invention provides a CRT having a phosphor 
screen comprising phosphor index stripes and a black nonluminescent 
substance both of which are provided on the inner surface of a panel. The 
stripes are interposed between portions of the substance. Primary phosphor 
color triplet stripes are arranged at a predetermined spacing on the 
nonluminescent substance layer and have a sufficient thickness. The 
phosphor index stripes are provided in some of the spaces between the 
phosphor color stripes in a definite relation thereto.

DETAILED DESCRIPTION OF THE INVENTION 
FIG. 1 schematically shows a flat, color CRT of the present invention. The 
CRT is similar to the construction shown in FIG. 11 of a flat glass tube 
1, an electron gun 2, deflection yoke 14 and light collector plate 11. The 
CRT has a phosphor screen 15 which is characteristic of the invention and 
which therefore will be described below with reference to FIGS. 2 and 3 
which are sectional view taken along the line II--II in FIG. 1. 
FIG. 2 shows a first embodiment. A layer 16 of carbon or like black 
nonluminescent substance is formed in the shape of stripes on the inner 
surface of a panel 6. Also formed on the panel's inner surface are index 
stripes 17 of a phosphor such as P47 phosphor (brand name of Y.sub.2 
SiO.sub.5.Ce, product of KASEI OPTONICS K.K.). The layer 16 is interposed 
between the index strips 17. Primary phosphor color triplet stripes R 
(red), G (green) and B (blue) are arranged at a specified spacing on the 
nonluminescent substance layer 16 in a definite relation to the phosphor 
index stripes 17. The phosphor index stripes 17 are disposed in some (18a) 
of the spaces 18 between the phosphor color stripes. The phosphor color 
stripes have a thickness sufficient for these stripes to reach saturation 
luminance when luminescing to the highest luminance. 
With the arrangement described above, an electron beam directly excites the 
phosphor color stripes R, G, B and the phosphor index stripes 17, enabling 
the viewer to observe bright images through an observation window 13 and 
giving index light of high intensity through the panel 6. Moreover, images 
of improved contrast ratio can be obtained because the black 
nonluminescent substance layer 16 is present in the spaces 18 between the 
phosphor color stripes other than the spaces 18a where the phosphor index 
stripes 17 are positioned. 
Furthermore, the nonluminescent substance layer 16, on which the phosphor 
color stripes R, G, B are arranged blocks the luminescence of the color 
phosphors that otherwise would strike the light collector plate 11 through 
the panel 6. Therefore, only the luminescence of the phosphor index 
stripes 17 is emitted toward the collector plate. Thus, the index light 
alone can be separated effectively. Another advantage of the phosphor 
screen 15 is that it is easy to fabricate because there is no need to form 
a metallic layer and further because the nonluminescent layer 16 has a 
large stripe width. 
The phosphor color stripes R, G, B have a sufficient thickness, so that the 
deficit of luminance due to the absence of a metallic layer can be fully 
compensated. Our experiments have shown that satisfactory luminance is 
available for a beam current of 30 .mu.A and anode voltage of 6 kV when 
the color stripes, R, G, B are at least 20 .mu.m in thickness. 
FIG. 3 shows a second embodiment. Throughout FIGS. 2 and 3, like parts are 
referred to by like numerals. 
According to the second embodiment, the phosphor screen 15 of the first 
embodiment is entirely covered with a protective transparent thin film 19 
of silicon dioxide (SiO.sub.2), which is further entirely covered with a 
very thin electrically conductive transparent film 20, such as a thin film 
of ITO (indium oxide doped with tin oxide), formed by vacuum evaporation. 
The CRT, although a vacuum in its interior, still contains remaining 
undesirable substances in the form of a gas. Thus, anions will be produced 
upon ionization when electrons collide with such substances. Accordingly, 
in the first embodiment such anions can strike the phosphor to break and 
scorch the phosphor. With the second embodiment, however, the phosphors 
are covered with the protective transparent thin film 19 of SiO.sub.2. 
Thus, the film prevents the anions from stinking and scorching the 
phosphor. The film also prevents the phosphor stripes from peeling off. 
When the SiO.sub.2 film 19 is provided, negative charges can accumulate in 
the film 19. The film is an insulator. Thus, the luminescence efficiency 
of the phosphors is reduced. According to the present embodiment, however, 
a high voltage, approximately at the same level as the positive voltage 
applied to the conductive nonluminescent layer 16, is applied to the 
conductive transparent film 20. This eliminates the reduction of 
luminescence efficiency due to the accomplished negative charge the film. 
The protective film 19 is several hundred angstroms in thickness, while the 
conductive film 20 has a thickness of 200 to 300 angstroms. The very small 
thickness of these films will not substantially attenuate the electron 
beam. 
Referring to FIGS. 4 to 8, processes for preparing the phosphor screen of 
the above embodiments will be described. FIG. 4 shows the phosphor screen 
15 as viewed through the observation window 13 of the glass tube 1. FIG. 5 
is a sectional view taken along the line A--A' in FIG. 4. FIG. 5 shows an 
exposure procedure for forming the phosphor index stripes. FIG. 6 is a 
sectional view taken along the line B--B' in FIG. 4 and shows an exposure 
procedure for forming the phosphor color stripes. 
The mask patterns to be used is formed with apertures in conformity with 
the phosphor index stripes 17, position matching marks MR, MG, MB, 
phosphor color stripes R, G, B or run-in phosphor index stripe 17a. The 
light from an exposure light source passes through the apertures as seen 
in FIGS. 5 and 6 and impinges on a photosensitive coating. 
First, a glass panel is prepared. The panel is coated with a carbon coating 
composition everywhere except where the index stripes, position matching 
marks and run-in index stripe are to be formed. 
A photoresist is uniformly applied to the glass panel 6 and then dried. 
Next, the coating is exposed to light through a mask pattern only where 
the index stripes, position matching marks (the function of which will be 
described later) and run-in index stripe are to be formed. The photoresist 
is then cured. When the panel is then washed with water, only the exposed 
cured resist portions remain on the panel's inner surface. 
Subsequently, a carbon (nonluminescent substance) coating composition is 
uniformly applied to the entire panel inner surface. The panel is dried 
and thereafter immersed in an oxidizing solution. Thus, the cured resist 
film is swollen and removed from the panel surface. Consequently, the 
panel is formed with a carbon coating over the entire surface except where 
the index stripes, matching marks and run-in index stripe are to be 
provided. 
Next, with reference to FIG. 5, a slurry 25 of phosphor index and 
photoresist is uniformly applied to the panel's inner surface and then 
dried. Subsequently, an index stripe forming mask pattern 21 is placed 
over the panel surface in register with the portions bearing no carbon. 
The slurry coating is exposed to light through apertures 22 and 22a to 
cure the photoresist as indicated at 26. The uncured coating is then 
removed by washing it with water, with the result that the cured portions 
26 provide the phosphor index stripes 17 and a run-in index phosphor 
stripe 17a corresponding to the aperture 22a. 
The primary phosphor color stripes R, G, B are formed on the carbon layer 
in a definite relation to the index stripes 17 thus formed, by the 
following method. 
Because the phosphor color stripes are to be formed on the carbon layer as 
shown in FIG. 6, the light of the exposure light source through the 
apertures 24 of a phosphor color stripe forming mask pattern 23 is not 
observable through the carbon layer and the panel 6. Thus, it is 
impossible to set the mask pattern 23 in position. 
According to the present invention, however, the position matching marks 
MR, MG, MB are provided in a corresponding relation to the positions of 
the color stripes, so that each mask pattern 23 can be set in position. 
More specifically, the three position matching marks MR, MG, MB formed 
outside the effective image area are in a corresponding relation to the 
positions where the phosphor color stripes R, G, B are to be formed. For 
example, when the red phosphor color stripes are to be formed, the mask 
pattern 23 is so positioned that the light through its matching aperture 
24a can be seen through the center of the red mark MR as shown in FIG. 6. 
Subsequently, a phosphor slurry 27 for red is formed over the carbon layer 
16 and is exposed to light through the pattern 23 and then washed with 
water. Consequently, the exposed cured portions 28 only remain on the 
carbon layer as the red phosphor color stripes. 
Similarly the green and blue phosphor stripes are formed, with each mask 
pattern 23 so positioned that the light through the aperture 24a can be 
seen at the center of the green mark MG or the blue mark MB. 
To shield the phosphors in the position matching marks MR, MG, MB from the 
exposure light and to thereby prevent the phosphors from remaining in the 
marks, the matching aperture 24a of each mask pattern 23 is usually 
provided with a film 29 for blocking ultraviolet rays. If otherwise, the 
electron beam, when overscanning, would excite the phosphor remaining in 
the mark, and the resulting luminescence would disturb the color image. 
The film 29 avoids such an objection. 
Nevertheless, the ultraviolet blocking film 29, if provided, reduces the 
amount of light through the aperture 24a, presenting some difficulty in 
positioning the mask pattern with reference to the mark. 
The desired phosphor screen can be fabricated in the following manner 
without using the blocking film 29. 
When the phosphor color triplet stripes are formed without using the 
blocking films 29, the phosphors R', G', B' are provided also in the 
matching marks MR, MG, MB as seen in FIG. 7. The phosphor portions R', G', 
B' thus formed are then completely covered with the nonluminescent 
substance 16 as shown in FIG. 8 to prevent luminescence of these phosphor 
portions. 
The position matching marks MR, MG, MB, which are provided at one portion, 
may alternatively be formed at the four corners of the panel outside the 
effective image area. 
The panel thus formed with the phosphor color stripes can be coated with 
SiO.sub.2 first and then with ITO by vacuum evaporation after baking the 
stripes to obtain the screen shown in FIG. 3. 
Although one phosphor index stripe is provided for every four phosphor 
color stripes according to the embodiments described, this arrangement is 
not limitative but is variable unless the index stripe is provided in 
every space between the color stripes. 
The foregoing disclosure and description of the invention are illustrative 
and explanatory thereof, and various changes in the size, shape and 
materials, as well as in the details of the illustrated construction may 
be made without departing from the spirit of the invention.