U.S. patent number 3,653,939 [Application Number 05/000,580] was granted by the patent office on 1972-04-04 for screening of black-surround color picture tubes.
This patent grant is currently assigned to Zenith Radio Corporation. Invention is credited to Charles J. Prazak, III.
United States Patent |
3,653,939 |
Prazak, III |
April 4, 1972 |
SCREENING OF BLACK-SURROUND COLOR PICTURE TUBES
Abstract
The screen of a black-surround color picture tube is provided
with a discontinuous coating of light-absorbing material,
discontinuous in that the light-absorbing material is applied to
surround those elemental areas of the screen which are to receive
deposits of phosphor. The screen is then heated so that the exposed
elemental screen areas and the light-absorbing coating attain a
uniform temperature. A slurry, including a chosen one of the three
phosphor materials, is then dispensed onto the screen and
distributed to establish a layer of slurry of a desired thickness
over the entire screen surface. The excess slurry is removed by
tilting the screen to effect dumping. The slurry layer is then set,
without the application of heat, and after the layer has been set,
heat is applied to dry the layer. Thereafter, screening continues
in conventional fashion.
Inventors: |
Prazak, III; Charles J.
(Elmhurst, IL) |
Assignee: |
Zenith Radio Corporation
(Chicago, IL)
|
Family
ID: |
21692119 |
Appl.
No.: |
05/000,580 |
Filed: |
January 5, 1970 |
Current U.S.
Class: |
427/68;
427/72 |
Current CPC
Class: |
H01J
29/327 (20130101); H01J 9/22 (20130101); H01J
9/2278 (20130101) |
Current International
Class: |
H01J
9/22 (20060101); H01J 9/227 (20060101); C03c
003/28 (); C03c 017/00 (); F21v 009/16 () |
Field of
Search: |
;117/33.5CP,33.5CM,101 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Leavitt; Alfred L.
Assistant Examiner: Glynn; Kenneth P.
Claims
I claim:
1. In the screening of a color cathode-ray tube in which a
light-absorbing material is first applied to the screen in
circumscribing relation to elemental areas of said screen which are
to receive deposits of phosphor material, the improvement in
applying phosphors to said elemental areas which comprises the
following steps:
heating the screen to establish said elemental screen areas and
said previously coated screen portions at a uniform temperature
which exceeds ambient temperature;
dispensing onto said screen a coating composition in an amount
exceeding that required to cover said screen with a coating layer
of a preselected thickness, said coating composition comprising at
least a volatilizable liquid;
distributing said coating composition over said elemental screen
areas and over said previously coated circumscribing screen
portions to form a layer of coating composition having a
substantially uniform temperature and a substantially uniform
thickness of a desired preselected value and removing the excess of
said coating composition;
evaporating a sufficient amount of said liquid, without the
application of heat to said screen and while maintaining said
elemental screen areas and said previously coated screen portions
at a substantially uniform temperature, to increase the viscosity
and fix said layer of coating composition to all surface portions
of said screen;
and thereafter applying heat to dry said layer of coating
composition.
2. The screening improvement in accordance with claim 1 in which
said coating composition is applied to said screen at substantially
ambient temperature during said dispensing step and in which said
evaporating step takes place without the application of heat to
said screen other than that preceding said dispensing step.
3. In the screening of a color cathode-ray tube in which a
light-absorbing material is first applied to the screen in
circumscribing relation to elemental areas of said screen which are
to receive deposits of phosphor material, the improvement in
applying phosphors to said elemental areas which comprises the
following steps:
heating said screen and establishing said elemental screen areas
and said previously coated screen portions at a uniform temperature
which exceeds ambient temperature;
dispensing onto said screen a quantity of a coating composition,
which is at a substantially lower temperature than the heated
screen, and in an amount exceeding that required to cover said
screen with a coating layer of a preselected thickness, said
coating composition comprising a sensitized volatilizable liquid in
which particles of phosphor material are suspended;
tilting said screen to an angle less than 90.degree. while rotating
said screen at a first rate to distribute said coating composition
and to form over said elemental screen areas and over said
previously coated circumscribing screen portions a substantially
uniform layer of coating composition of said preselected thickness;
thereafter, tilting said screen to an angle exceeding 90.degree.
and rotating said screen at a rate greater than said first rate to
dump the excess of said coating composition while retaining said
layer in situ;
continuing the rotation of said screen at the angle of tilt and
rate of rotation established in the last-described step, and
without further heating said screen, to evaporate a sufficient
amount of said liquid, while maintaining said elemental screen
areas and said previously coated screen portions at a substantially
uniform temperature, to increase the viscosity and fix said layer
of coating composition to all surface portions of said screen;
and thereafter further heating said screen to dry said layer of
coating composition.
Description
BACKGROUND OF THE INVENTION
The present invention is addressed to an improved screening process
for use in the manufacture of black-surround color picture tubes as
described and claimed in U.S. Pat. No. 3,146,368, issued on Aug.
25, 1964 to Joseph P. Fiore et al. Such a tube has light-absorbing
material covering those portions of the screen which are interposed
between its various phosphor deposits. Advantages of such a
structure are very real and include a substantial enhancement of
both brightness and contrast.
In preparing the screen of such a tube, there is a choice of
applying the green, blue and red phosphors and filling the spaces
between the phosphor deposits with light-absorbing material or,
alternatively, a grille of the light-absorbing material, having
holes into which the phosphors may be deposited, may be formed on
the screen initially. There is a preference to forming the grille
of light-absorbing material first and then filling its holes with
the various phosphors arranged, of course, in the necessary
interspersed relation to result in a repeating pattern of the three
different color phosphor deposits distributed over the entire
screen surface. This, in fact, is the procedure currently employed
in commercial practice but it has manifested an undesired loss of
brightness which is corrected by the present invention. In
particular, it has been found that the phosphor deposits are not as
well as packed as desired and may even be characterized as porous
in comparison with the phosphor deposits of a standard tri-color
picture tube that does not have a grille of light-absorbing
material. Applicant has discovered the failure of current practices
to achieve the theoretical brightness capability of a
black-surround tube and teaches herein an improved method for
overcoming the deficiency of past screening processes used in
making black-surround color picture tubes.
Accordingly, it is an object of the invention to provide an
improved process for screening a black-surround color cathode-ray
tube.
It is a specific object of the invention to improve the screening
of such a color picture tube to attain the brightness output of
which the screen of such a tube is theoretically capable.
In particular, it is an object of the invention to improve the
screening of a black-surround color tube to achieve phosphor
deposits that provide at least the same brightness as attained from
similar screening processes used in the manufacture of a
conventional color picture tube that does not have the
black-surround feature.
SUMMARY OF THE INVENTION
In screening a color cathode-ray tube of the black-surround
variety, a light-absorbing material is first applied to the screen
in circumscribing relation to those elemental areas of the screen
that are to receive deposits of phosphor material. Thereafter, in
accordance with the invention a liquid coating composition is
dispensed onto the screen in an amount exceeding that required to
coat the screen with a coating layer of a preselected thickness.
That liquid coating composition comprises at least a volatilizable
constituent which is usually water. The coating composition is
distributed, as by rotating and tilting the screen, to form over
the elemental screen areas and also over the previously coated
circumscribing screen portions a layer of coating composition
having a substantially uniform temperature and of the desired
thickness and then the excess of the coating composition is
removed. Thereafter, a sufficient amount of the volatilizable
constituent of the coating composition is evaporated, while
maintaining the elemental screen areas and the previously coated
screen portions at a substantially uniform temperature. This
evaporation increases the viscosity of the remainder of the coating
composition so as to fix the layer of coating composition to all
surface portions of the screen. After having fixed that layer in
position, the screen is heated to dry the layer thoroughly and from
this point forward the screening is conducted in conventional
manner.
In one aspect of the invention, the screen with its previously
developed light-absorbing grille is preheated to the extent
required to bring the grille and the exposed elemental screen areas
that are to receive phosphor material to a uniform high temperature
and then a phosphor-containing light-sensitive slurry at room
temperature or at an elevated temperature is dispensed onto the
screen to form a desired slurry layer. Thereafter, the screen is
tilted to such an angle that the excess slurry is dumped out and
this occurs with the screen rotating at sufficient speed to retain
the slurry layer in situ. The rotation is continued without further
heating of the screen until, through evaporation, the slurry layer
attains a set after which heat is applied to dry the layer
preparatory to its exposure and further processing.
BRIEF DESCRIPTION OF THE DRAWING
The features of the present invention which are believed to be
novel are set forth with particularity in the appended claims. The
invention, together with further objects and advantages thereof,
may best be understood by reference to the following description
taken in connection with the accompanying drawing, in the several
figures of which like reference numerals identify like elements,
and in which:
FIGS. 1 to 5 are fragmentary cross sectional views, on an enlarged
scale, of a color cathode-ray tube showing sequential process steps
in screening a black-surround tube; and
FIG. 6 is a curve relating brightness to the size of the apertures
of the light-absorbing grille of such a tube.
DESCRIPTION OF THE PREFERRED EMBODIMENT
The envelope of a shadow mask color tube has a face section that is
initially separated from a conically shaped envelope portion which
is a convenience in screening. A small or fragmentary portion 10 of
such a faceplate is represented in FIG. 1 and its specific
dimension and configuration are of no special consequence. The
invention may be practiced in screening large or small sized tubes
of round or rectangular form. It is equally applicable to the
manufacture of tubes in which the phosphor deposits are stripes or
the more conventional dot triads. For the purpose of a specific
disclosure of the invention, it will be assumed that faceplate 10
is part of a 25 inch rectangular color tube having a mosaic-type of
screen, that is to say, having a multiplicity of dot triads
distributed over the internal screen surface with each triad
comprising a dot of green, a dot of blue and a dot of red phosphor.
It should also be noted that the invention is applicable to any of
the various screening processes which utilize photographic printing
in which the inner surface of the screen is provided with a layer
of a light-sensitive material for exposure and developing in order
to lay down any of the three color phosphors. The phosphor may be
included as a component of the light-sensitive coating material or
it may be added subsequently, as in the case of dusting. Again, for
purposes of the disclosure it will be assumed that screening is
accomplished through the use of a water soluble photosensitive
slurry comprised of water, polyvinyl alcohol, sensitized with
ammonium dichromate and bearing phosphor particles in suspension.
Before this slurry is utilized, however, screen 10 is provided with
a grille of light-absorbing material and its application will be
discussed initially.
After the screen has been made chemically clean, it is coated with
a removable layer 11 of clear polyvinyl alcohol sensitized with
ammonium dichromate. After layer 11 has been dried, selected
portions thereof are exposed to ultraviolet light to establish in
the layer interleaved sets of images of those elemental areas of
the screen that are to receive assigned ones of the phosphor
materials. This is accomplished in a step analogous to that
conventionally taken in photoresist screening to define the
elemental areas of the screen which are to receive a particular
phosphor and to distinguish them from the other portions of the
screen. This discrimination is easily attained by exposure to
ultraviolet light through round holes or apertures of the shadow
mask 12 of the tube in process. For that purpose, such a mask is
installed within the faceplate portion of the tube envelope after
which this subassembly is placed in an exposure chamber so that
ultraviolet light is directed to selected elemental areas of the
screen through the apertures of mask 12. If the light source is
positioned, for example, to simulate the electron gun of the tube
in process which is to excite the green phosphor material, the
ultraviolet light will be confined to expose only those portions
11g of layer 11 which overlie elemental areas of screen 10 assigned
to receive deposits of green phosphor. After this exposure step,
portions 11b of layer 11 are similarly exposed, these constituting
the portions of layer 11 that overlie elemental areas of screen 10
to receive deposits of blue phosphor. To achieve their exposure, it
is only necessary to change the position of the light source in the
exposure chamber so that it now simulates the electron gun of the
tube intended to energize the blue phosphor dots. In a third
position of that light source, where it simulates the red electron
gun of the tube, a third set of small portions 11r of layer 11 are
exposed and these portions overlie elemental areas of the screen
intended to receive red phosphor. As a consequence of the multiple
exposures, there are established in layer 11 interleaved sets of
images of circular, elemental areas of the screen that are
separated from one another and are to receive assigned ones of the
phosphor materials.
While not essential, it is preferred that the exposed elemental
areas of layer 11 be smaller in size than the apertures of the
shadow mask as that mask is finally and permanently installed
within the completed tube. The correct relative size may be
realized in a variety of ways. The mask may be provided initially
with a coating to temporarily close down the size of its apertures
so that the dimensions of the phosphor dots determined by exposure
through the shadow mask are properly related to the final size of
the mask apertures. If closing down of the mask apertures by a
coating technique is resorted to, the mask coating is removed after
screening has taken place so that the final hole size of the mask
is properly related to the phosphor dots.
A more attractive process which is currently used commercially is
one in which the apertures originally formed in mash 12 have the
size required for screening. After screening has taken place, the
mask is re-etched to open up or enlarge its apertures to a desired
final size. This has an advantage in precisely controlling the
dimensions of the phosphor dots and also in attaining uniformity of
size and configuration of the dots.
By whichever approach the selected portions 11g, 11b and 11r of
prpva layer 11 are exposed, the interleaved sets of images are next
developed by removing all unexposed portions of photosensitive
layer 11 producing the screen condition of FIG. 2. Inasmuch as the
photosensitive material as applied to screen 10 is soluble in water
and all exposed portions thereof have been rendered insoluble,
washing the screen with water after the three exposure steps
removes all of the unexposed portions of layer 11. The screen of
FIG. 2 may be described as having clear deposits or dots of pva
separated from one another by screen portions which are bare and
are to receive a pigment or a material having light-absorbing
capabilities.
The next step of the screening process constitutes depositing in
the spaces between the elemental screen areas covered by the dots
of clear pva a coating 13 of an inorganic pigment having
light-absorbing capabilities and having the property that its
adherence to screen 10 is substantially immune to attack by an
active agent which may be employed to destroy the adherence of pva
dots 11g, 11b and 11r to the screen. While this light-absorbing
material may be applied only to the areas surrounding the clear pva
dots, it is far more convenient to apply the coating 13 over the
entirety of the screen as indicated in FIG. 3 in which case the
coating of light-absorbing material is also applied over the clear
pva dots in the nature of an overcoat. Preferably, layer 13 is
applied as a slurry and also preferably it is a colloid having in
suspension a fine pulverulant material such as black iron oxide,
powdered mica, molybdenum disulphide, manganous carbonate, ceramic
black or graphite. Colloidal graphite is especially useful as the
material of coating 13. The commercial form of colloidal graphite
available under the tradename "Aquadag" diluted to 3 per cent solid
with deionized water is an acceptable slurry material. After the
slurry coating 13 is applied, it is fixed by drying through the
application of heat. Drying for a period of 5 minutes under a 750
watt heater, while rotating the panel at a rate of 5 to 8 r.p.m.
and directing an air blower on the screen, has given satisfactory
results.
It is next necessary to remove all of the coating materials from
screen 10 except for the graphite coating 13 over those portions of
the screen which intervene the dots 11g, 11b, and 11r of clear pva.
This may be achieved by the application of a chemical stripper
which reacts with pva to free or lift the clear pva dots off the
panel. A suitable stripper is 30 per cent hydrogen peroxide and 70
per cent water which may be poured onto the center of the screen
and slurried to uniformly coat the screen. After a processing
interval of about 30 seconds, the excess stripping solution is
poured off and the screen is washed with a spray of deionized water
for an interval of 30 seconds to a minute at a pressure of 35
p.s.i. In this washing, the clear pva dots 11g, 11b and 11r wash
off the screen and take with them their individual overcoat of
graphite. In this manner, a light-absorbing grille is produced on
screen 10 having holes that are to receive deposits of phosphor
material. For convenience, the elemental screen areas that are to
receive phosphor deposits are designated 11g', 11b' and 11r' in
FIG. 4 which further illustrates that these elemental areas of the
screen are totally surrounded by the light-absorbing grille, that
is to say, by the residue of the colloidal graphite layer 13a. So
far as the present invention is concerned, the processing up to
this point is in the prior art and the improvement of the invention
concerns the screening process by which the phosphor deposits
designated 11g", 11b" and 11r" in FIG. 5 are made.
Screening apparatus with which the invention may be practiced is
described in U.S. Pat. No. 3,319,556, issued to Joseph P. Fiore and
in U.S. Pat. No. 3,319,759 issued to Hajduk et al., both of which
are assigned to the assignee of the present application. Briefly,
to apply any given phosphor material, a charge of slurry including
such material is dispensed onto the central portion of the screen
in an amount exceeding that required to cover the screen with a
slurry layer of a preselected thickness. The screen is then tilted
slightly to distribute the slurry coating and form over the screen,
including both the elemental screen areas desired to receive
phosphor as well as the light-absorbing grille, a substantially
uniform slurry layer of the desired thickness after which the
excess slurry is removed. At this juncture, the slurry layer is
fixed but without the application of heat so that all elemental
areas of the screen are retained at the same uniform temperature.
Fixing of the slurry layer is accomplished by evaporating a
sufficient amount of the water constituent to increase the
viscosity of the slurry material to a value that the layer,
especially that which covers the elemental screen areas intended to
receive phosphor, is not subject to migration but rather, is firmly
secured over the entire screen area. After the slurry layer has
been fixed, by evaporating water, it is dried by the application of
heat as with an infrared heater.
In practicing the invention with apparatus of the type illustrated
in the above-identified patents, the screen with its previously
applied grille having apertures to receive phosphor material is
supported and rotated at a relatively slow rate of the order of 5
to 15 r.p.m. Heat from infrared heaters is applied to the screen to
bring it to a uniform temperature which exceeds the ambient or room
temperature. It is necessary that the grille as well as the
elemental areas of the screen circumscribed by the grille be
brought to the same temperature and it has been found that they may
be brought to a uniform temperature of 120.degree. F. by subjecting
the screen to an infrared heater of about 12 kw. for an interval of
a minute to a minute and 30 seconds and then allowing the
temperatures to equalize for 30 seconds to 1 minute.
After all portions of the screen have been brought to this
relatively high temperature and with the screen rotating at a slow
rate in a horizontal plane, a charge of pva slurry having, by way
of illustration, green phosphor in suspension and being
approximately at room temperature, is dispensed onto the center of
the screen. The amount of the charge exceeds that which is required
to cover the entire inner surface of the screen with a slurry layer
of a desired dry thickness of approximately 0.0005 inch. After the
pva slurry has been dispensed, the screen is tilted slightly to an
angle of approximately 15.degree., while its slow rotation is
continued, in order to distribute the slurry and form over the
screen a substantially uniform slurry layer of the desired
thickness. Thereafter, the screen is tilted to an angle exceeding
90.degree. and its rotation is increased to attain a balance
between gravitational and centrifugal forces to the end that the
excess slurry is dumped out of the screen into a catcher or
container while the desired uniform layer remains in situ. The
panel may be tiled, for example, to approximately 130.degree. and
its rotation increased into the range from 80 to 100 r.p.m. to dump
the excess slurry.
After dumping, rotation of the screen is continued under the
conditions of tilt and speed of rotation established during the
dumping step and, importantly, these conditions are continued
without further heating of the screen. In this continued rotation,
some of the water component of the slurry evaporates, increasing
the slurry viscosity and thereby fixing the slurry layer to all
surface portions of the screen. Fixing of the slurry layer in this
manner may be accomplished in from 11/22 to 2 minutes because of
the assist attributable to preheating the panel before dispensing
the slurry. This part of the process may, if desired, be speeded up
in any fashion that increases the rate of evaporation provided it
does not involve differential heating of the graphite coating and
glass surface before the slurry layer has been fixed or set. For
example, a stream of gas such as air at room temperature may be
caused to flow over the screen.
With the screen surface at a uniform temperature and the slurry
layer thus fixed thereto, infrared heaters are energized to heat
the screen and thoroughly dry the slurry layer preparatory to
exposure and development of so much of the layer as has been
deposited in grille openings 11g'. In this way, the green phosphor
deposits 11g" are made in the assigned ones of the openings of
grille 13a.
In similar processing steps, differing only in the location of the
exposure source and the phosphor component of the slurry coating
material, both the blue phosphor deposits 11b" and red phosphor
deposits 11" are made to complete the formation of phosphor dot
triads on the screen. From this point forward, including filming
and aluminizing, the processing of the screen is conventional and
need not be described.
The significant difference in the described screening process,
compared with that heretofore practiced in the art, is in fixing or
setting the phosphor slurry layer which, for the described process,
occurs while all incremental areas of the screen are at the same
temperature and with no application of heat, whereas previously the
slurry layer was formed and then dried in conventional manner by
the application of heat. This difference in setting the slurry
layer is responsible for the formation of well packed phosphor
deposits or the formation of porous deposits.
If the phosphor slurry is applied in the usual way over the grille
and then set and dried by the application of heat as has been
practiced heretofore in the art, an undesirable temperature
gradient is established with the grille at a high temperature and
the elemental screen areas circumscribed by the grille at a lower
temperature. This results from the fact that the graphite grille is
essentially non-transmissive to infrared and, therefore, achieves a
high temperature when exposed to infrared heaters. On the other
hand, the incremental areas of the screen that are to receive
phosphor and the phosphor slurry itself are relatively transmissive
to infrared and therefore the incremental areas achieve a
significantly lower temperature in the heating process. This
temperature differential manifests itself in a migration of the
slurry out of the holes of the grille and onto the surrounding
grille surface. As a consequence, the grille accumulates a thicker
slurry layer than the elemental screen areas intended to be coated
with the phosphor in process. If the screen is exposed and
developed in this condition, the resulting phosphor deposit is
porous or at least lower in coating density than that on the
graphite.
The described invention avoids this undesirable consequence of the
prior art method of screening black-surround tubes. It is to be
emphasized that in practicing the invention the grille and the
exposed elemental areas of the screen to be coated with phosphor
are preferably heated and brought to the same uniform temperature
before the layer of slurry including phosphor is deposited over
them. While all portions of the screen are thus at a uniform
temperature, the water constituent of the phosphor slurry is
permitted to evaporate in part so that the viscosity is increased
to fix the slurry layer. This avoids the difficulty of past
practices by precluding the possibility of migration of slurry from
the holes of the grille onto the grille surface. Of course, once
the slurry layer has thus been fixed, the application of heat to
accomplish complete drying of the layer, as required for the
exposure step, does not destroy the uniformity of the slurry layer
as between the elemental areas of the screen to receive the
phosphor coating and the surface of the grille. It is for this
reason that subsequent exposure and developing of the slurry layer
in the conventional way results in a compact deposit of phosphor
and avoids the porous dots that have been experienced in the
past.
The curves of FIG. 6 show the variation of brightness with changes
in the size holes formed in the grille. Curve A indicates how the
brightness of the screen varies, theoretically, with the aperture
size of the grille. Broken curve B represents measured brightness
output of black-surround tubes with varying aperture sizes of the
grille when those tubes have been screened in accordance with prior
practices. It will be noted that for apertures in excess of 11 mils
the measured brightness is distinctly less than the theoretical
value. At an aperture size of 131/2 mils which is used in
commercial black-surround tubes the brightness is down 10 to 15 per
cent or more. This results from the prior practice of heating the
screen to dry the phosphor slurry layer without previously fixing
the layer to avoid migration. Where the drying is delayed until
such time that the slurry layer shall have fixed itself over the
entire screen surface, as taught by this invention, the measured
brightness is essentially the same as that indicated by curve A and
the brightness loss, as indicated by curve B, is recaptured.
While particular embodiments of the invention have been shown and
described, it will be obvious to those skilled in the art that
changes and modifications may be made without departing from the
invention in its broader aspects, and, therefore, the aim in the
appended claims is to cover all such changes and modifications as
fall within the true spirit and scope of the invention.
* * * * *