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Description  |
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BACKGROUND AND SUMMARY OF THE INVENTION
This invention relates to an improved direct photographic process for
preparing a screen structure for a shadow-mask-type cathode-ray tube.
Direct photographic processes for preparing a screen structure for a
shadow-mask-type cathode-ray tube have been described previously, for
example, in U.S. Pat. Nos. 3,406,068 to H. B. Law and 3,685,994 to H. R.
Frey. The tube is usually comprised of a faceplate panel which includes a
viewing window and peripheral sidewalls extending from the margins of the
window. A mask assembly comprising an apertured mask and
temperature-compensating mounting means is mounted in the panel, usually
on three or four studs implanted in the panel sidewalls, with the mask
spaced a desired distance from the inner surface of the window.
Temperature-compensating mounting means for the mask have been described
previously, for example, in U.S. Pat. Nos. 3.803,436 to A. M. Morrell and
3,330,980 to T. M. Shrader. The temperature-compensating feature of the
mounting means operates to move the mask towards the screen as the tube
heats up (to a maximum of about 80.degree.C) during the operation of the
tube, so that a projection of electrons through each aperture remains on
its associated screen element. Heating causes the mask assembly to expand,
moving the off-center apertures outwards from the longitudinal axis of the
tube. By moving the mask forward towards the screen, the projection from
the off-center apertures upon the screen is moved inward towards the tube
axis, thereby compensating for the outward movement caused by heating.
In one method for making a screen structure for a cathode-ray tube having a
shadow mask mounted on temperature-compensating mask-mounting means in a
faceplate panel, the panel is coated with a layer comprising a
light-hardenable material, (with or without phosphor particles), the panel
and layer are heated to dry the layer, and the mask is mounted in the
panel. Then, actinic light is projected through the mask from a small-area
source to expose selected areas of the dry layer to the light so as to
harden (insolubilize) the exposed portions of the layer. The exposing step
is assumed to take place with all parts of the system at about
22.degree.C. In many situations, the panel is still hot (above
40.degree.C) just prior to and during the exposing step. Heat from the
panel warms the temperature-compensating mask-mounting means and causes
the mask to move forward towards the layer. This produces off-center
exposed areas that are located inwards, resulting in misplaced
light-hardened areas, which may later be misregistered with respect to the
electron beams impinging on the screen. In some cases, because of the
geometry of the mounting means, as with some three-spring structures, the
mask assembly is rotated or twisted, as well as screen elements being
shifted. Also, a hot panel may cause the mask to dome or become distorted
when the mask becomes warm faster than the frame to which it is attached.
The novel method is similar to the prior method except that, after the mask
assembly is mounted in the panel, and while the panel is still above
40.degree.C, the temperature-compensating mounting means is cooled to
provide a desired spacing between the mask and the panel. The cooling is
conducted during the period of the light exposure, and may be commenced
before or after the start of the exposure. One convenient method for
cooling the mounting means is to pass a stream of air over the mounting
means during the exposure. Preferably, the mounting means is cooled with
room-temperature air to temperatures below 25.degree.C as desired.
Practice of the invention can be used to avoid shifting of screen elements
and/or twisting of the mask assembly resulting from the use of a hot panel
during the light-exposing step. Additional benefits in dimensional
stability of the mask may be achieved in some cases by also cooling the
mask itself, as with one or more streams of cooling air. Cooling of the
mask can be used to reduce or eliminate doming or anti-doming movements of
the mask which may later cause misregister of the electron beams on the
elements of the viewing screen.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a partially-broken-away elevational view of a lighthouse on which
the exposure steps of the novel method may be practiced. The lighthouse
has a faceplate-panel assembly thereon in position for exposure.
FIG. 2 is a schematic diagram illustrating a lighthouse geometry and some
thermally-induced movements in the panel assembly.
DESCRIPTION OF THE PREFERRED EMBODIMENT
As an example of the novel method, the invention is applied to printing the
phosphor elements for a screen for a 17-inch 90.degree.-deflection
shadow-mask-type cathode-ray tube for a television receiver. Since
shadow-mask-type cathode-ray tubes are described in the prior art, they
need not be described in detail here. Generally, however, the tube is
comprised of an evacuated glass envelope including an electrongun-mount
assembly, a funnel assembly, and a faceplate-panel assembly.
In manufacturing the tube, the faceplate-panel assembly is completed as a
unit. The panel assembly A shown in FIG. 1 comprises a glass faceplate
panel 72 and an apertured-mask assembly B mounted in the panel 72. The
panel 72 includes a viewing window 73 and sidewalls 74. Mounting studs 76
are implanted in spaced positions around the inside sidewalls 74. The mask
assembly B includes a mask 77 having an array of apertures 79 therein. The
mask 77 is attached along the margins thereof to a mask frame 78 which has
mounting means 80 attached thereto at three spaced positions. Each
mounting means 80 includes a bimetal portion and a spring portion. The
extended ends of the mounting means 80 are adapted to fit on the studs 76
in a predetermined relationship. The bimetal portion is adapted to adjust
the spacing of the mask 77 with respect to the inner surface of the
viewing window 73 with changes in temperature of the mask assembly B.
The panel assembly A includes a light-hardenable layer 75 on the inner
surface of the window 73. In this example, the light-hardenable layer 75
includes particles of a green-emitting phosphor, polyvinyl alcohol, and a
dichromate photosensitizer for the alcohol.
The lighthouse C illustrated in FIG. 1 is comprised of a light box 21 and a
panel support 23 held in position by bolts (not shown) with respect to one
another on a base 25 which in turn is supported at the desired angle by
lugs 27. The light box is a cylindrical cup-shaped casting closed at one
end by an integral end wall 29. The other end of the light box is closed
by a plate 31 which fits in a circular recess 33 in the light box 21. The
plate 31 has a central hole therein through which the light pipe 35
(referred to as a collimator in the tube-making art) in the form of a
tapered glass rod extends. The narrow end 37 of the light pipe 35 extends
slightly above the plate 31 and constitutes the small area light source of
the lighthouse. The wider end 39 of the light pipe 35 is held in position
by a bracket 41 opposite an ultraviolet lamp 43 within the light box 21. A
light reflector 45 is positioned behind the lamp 43.
The lens assembly 51 is mounted on a lens-assembly support ring 53 and
standoff spacers 55 with bolts 57. The support ring 53 is clamped in
position between the light box 21 and the panel support 23. The lens
assembly 51 is comprised of a correction lens 61 and a wedge lens 63 held
and spaced from each other by separator ring 65, an upper clamp 67 and a
lower clamp 69. The upper surface of the wedge lens has thereon a light
intensity correction filter 71 which has a neutral gray transmittance that
varies from point to point, so that point-to-point variations in
brightness in the light field are reduced according to a prescribed plot.
An eclipser 22 normally blocks the upward path of the light emitted from
the end 37 of the light pipe 35, but can be swung out of the light path
when it is desired to expose the coating 75 on the window 73.
A length of 1/4-inch plastic tubing 81 extends around the inside wall of
the panel support 23. Both ends of the tubing terminate in two legs of a
metal "T" fitting (not shown) near the higher side of the panel support
23, which is tilted. The third leg of the T fitting is connected to a
source of compressed air. Air may be supplied continuously, or may be
supplied only when the eclipser 22 is swung away from the light pipe 35,
or may be supplied according to some other program. The tubing 81 has a
hole 83 opposite the position of each bimetal portion of the mask-mounting
means 80, so that a stream of cooling air (shown by the arrow) can be
passed thereover as desired.
To practice the invention, on the lighthouse C shown in FIG. 1, the inner
surface of the window 73 is coated with a layer 75 of light-hardenable
material; for example, a layer of slurry comprising water, green-emitting
phosphor particles, polyvinyl alcohol and ammonium dichromate. The panel
72 and a layer 75 are heated above 50.degree.C to dry the layer 75. Such
temperatures can be as high as 80.degree.C. Then, after drying, but while
the panel 72 is above 40.degree.C, usually at about 45.degree. to
50.degree.C, the mask assembly B, which is at room temperature, is
inserted in the panel 72 with the mounting means 80 on the studs 74. Upon
insertion, the temperatures of the mounting means 80 rise to above
40.degree.C due to the radiation of heat from the relatively larger mass
of the panel 72 and particularly the sidewalls 74. The mask 77 and the
mask frame 78 also rise in temperature due to radiation of heat from the
panel 72. A rise in temperature of the mounting means 80 causes the mask
to move towards the window 73.
The panel 72 with the mask assembly B mounted therein is placed on the
panel support 23 of the lighthouse C as shown in FIG. 1. The eclipser 22
is swung out of the light path permitting light from the source 37 to pass
upward through the lens assembly 51, then through the apertures 79 of the
mask 77 incident upon the layer 75. When the eclipser 22 is swung out of
the light path, compressed air at room temperature is fed into the tubing
81 and out through the holes 83, producing streams of cooling air which
pass over the mounting means 80. The cooling air has the effect of
reducing the temperature of the mounting means 80 below 25.degree.C,
although the mask 77 and a mask frame 78 may be at higher temperatures.
Cooling the mounting means 80 causes the mask 77 to move away from the
window 73.
FIG. 2 shows schematically light rays from the narrow end of the light pipe
35 passing through an off-center mask aperture 79 of the mask 77 incident
upon the layer 75, whereby the region R is insolubilized by the light. The
mask 77 is spaced a distance q from the inner surface of the window 73. If
no cooling air were employed, the mounting means, being
temperature-compensating and being at a much higher temperature than
25.degree.C, would maintain the mask 77 closer to the window 73, as shown
by the distance q'. The effect of the different mask position is to
insolubilize a region R', which is displaced laterally inwardly along the
layer 75. When the tube is later completed, the screen element R' would
not then be properly located with respect to the mask aperture 79, whereas
the screen element R would be properly located.
After the layer 75 has been exposed for a sufficient period of time, light
from the light source 37 is eclipsed and the supply of air to the tubing
80 is stopped. Then, the panel assembly A is removed from the lighthouse
C, the mask assembly B is removed from the panel 72, and the coating 75 is
developed by flushing the layer 75 with an aqueous solvent. Unexposed
areas of the layer 75 are flushed away by the solvent, and the exposed
areas with the greenemitting-phosphor particles therein are retained in
place.
The novel method may then be repeated as described above for making the
blue-emitting-phosphor elements by substituting blue-emitting-phosphor
particles for the greenemitting-phosphor particles in the layer 75. This
latter layer is applied over the green-emitting-phosphor elements. The
mask assembly B is again inserted in the faceplate panel 72, and the
second layer is exposed on a second lighthouse. The second lighthouse is
similar to the first lighthouse C except that it has a different lens
assembly and there is a different relative location for the light pipe 35.
After exposure on the second lighthouse, the layer with the blue-emitting
phosphor therein is developed as described above to remove the unexposed
portions of the coating and to retain in place the second exposed
portions, which are the blue-emitting-phosphor elements.
The novel method may then be repeated again as described above for making
the red-emitting-phosphor elements by substituting red-emitting-phosphor
particles for the green-emitting-phosphor particles in the layer 75. This
latter layer is applied over the green-emitting- and
blue-emitting-phosphor elements that are retained from the prior steps.
The mask assembly B is again reinserted in the faceplate panel 72, and the
third layer is exposed on a third lighthouse which is similar to the first
and second lighthouses except that it has a different lens assembly and
the light pipe 35 is located in a different relative position. After
exposing the third layer with the red-emitting phosphor therein on the
third lighthouse, the third layer is developed as described above to
remove the unexposed portions of the layer and to retain in place the
exposed portions, which are the red-emitting-phosphor elements.
After the phosphor elements have been printed, the screen structure is
filmed, aluminized, and baked out at about 420.degree.C by methods known
in the art. The completed screen structure is then assembled, with the
mask assembly and other parts, into the faceplate-panel assembly and the
panel assembly incorporated into a completed tube.
The novel method may be used with a system employing circular-mask
apertures and light sources to yield circular-screen elements as in the
example. The novel method may also be used to make line, elliptical, or
rectangular screen elements, in which case the geometry of the mask
apertures, the light source and the lens assembly may be modified in the
manner known in the art.
As was mentioned above, the mask 77 may be heated by the heat radiated from
the window 73 above about 40.degree. to 50.degree.C. In the normal case,
the mask 77, which has a relatively low mass, heats at a faster rate than
does the mask frame 78, which has a relatively greater mass. As a result
of this difference in heating rates, the mask frequently will dome due to
the different changes in size due to the different heating rates. Doming
has the effect of moving portions of the mask towards the window 13. In
order to reduce doming of the mask, a jet or a plurality of jets of
cooling air; for example, at room temperature, may be played upon the
inner surface of the mask 77 to cool the mask below about 25.degree.C.
This can be achieved with the tube 81 or with another air-supplied tube
adjacent to the tube 81 and having openings adapted to pass air against
the mask 77.
It has also been noticed that, because the lighthouse panel support 23 is
tilted at an angle from the horizontal plane, heat from the panel 72 tends
to concentrate towards the higher side of the panel 72, with the result
that the mask 77, the mask frame 78 and the mask-mounting means 80 tend to
be heated faster at the higher side than at the lower side. This
differential in heating rates between the higher side and the lower side
of the panel assembly A may cause distortions, twisting, or localized
doming in the mask assembly B. The differential effect can be overcome by
introducing cooling air or other cooling means at a greater rate at the
higher side of the panel assembly than at the lower side of the assembly.
In fact, the cooling pattern may be tailored across the surface of the
mask assembly to overcome, locally, the differentials in heating rates
across the assembly.
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