Note: Descriptions are shown in the official language in which they were submitted.
~ ~ ~ ~ RCA 68,805
1 Background of the Invention
~.
This invention relates to cathode-ray tubes
having apertured shadow masks therein, and particularly to
a shadow mask construction that reduces misregister between
electron beams and phosphor elements of the tube screen
caused by doming of ~he shadow mask during tube warmup.
In a shadow mask type cathode-ray tube for
producing a color image, a plurality of convergent electron
beams are projected through a multi-apertured color
selection shadow mask to a mosaic screen. The beam paths
through the mask are such that each beam impinges upon
and excites only one kind o-f color-emitting phosphor on
the screen. Generally, the shadow mask is attached to a
rigid frame, which in turn is suspended within the
picture tube envelope.
When a color cathode-ray tube is operated, the
electrons that strike the shadow mask cause it to heat up.
Since the edges o-f the shadow mask are attached to a
somewhat heavy frame which serves as a heat sink, a
temperature differential developes between the center ancl
peripheral portions of the mask. Because of the
temperature differentials, the mask center, the mask edge
and the frame expand at different rates. This difference
in expansion rates ~auses a doming of certain portions of
the mask toward the screen. In the center of the screen,
doming causes little effect on the register between the
electron beams and phosphor elements because the straight
line projection of the beams to the elements remains
unchanged with changes in mask to screen spacing. Since
the edges of the mask are fixed to a peripheral :Erame,
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RCA 68,805
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l there is no doming at the mask edges. Therefore, maximum
misregister caused by doming occurs approximately halfway -
between the mask center and mask edge. Misregister is
defined as being the amount an electron beam is off-center
5 from its respective phosphor element. Because of this ;
doming, the electron beams passing through the mask
misregister with the phosphor elements of the screen. The
misregister effect of doming peaks after 3 to 5 minutes of
tube operation but continues to have a diminishing affect
on tube performance for an additional 10 to 15 minutes.
Once the tube has reached steady state temperatures, general
electron beam misregister caused by expansion of the mask
is compensated by temperature sensitive frame supports
which move the mask-frame assembly toward the screen. Such
temperature compensating support is disclosed in U. S.
Patent 3,803,436 issued to Morrell on April 9, 1974.
Another problem somewhat related to doming is
blister warpage. Blistering occurs during operation of the
tube and is caused by a video pattern, such as a sustained
white spot in the TV image, that developes localized heating
of a part of the mask.
Summary of the Invention
A cathode-ray tube of the apertured mask type
includes a mask wherein the horizontal spacing between the
centers of adjacent apertures in the mask and the spacing
between the mask and screen both vary proportionally from
the center to the edge of the mask. The invention reduces
doming and blistering and thereby also reduces electron
beam misregister caused by these problems.
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~o~0747 RCA 68,805 ~ `
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1 Description of the Drawings
FIGURE l (sheet l) is a plan view, partly in
axial section of a prior art shadow mask cathode-ray tube.
FIGURES 2, 3 and 4 (sheet l) are enlarged schematic
views of portions o-E a line screen showin~ an electron beam
impinging thereon;
FIGURE 5 ~sheet l) is a graph of electron beam
misre~ister at a point halfway between the center and edge
of a shadow mask versus time.
FIGURE 6 (sheet l) is an enlar~ed view of a portion
of the mask and screen in the area indicated by the numeral
6 in FIGURE l.
FIGURE 7 (sheet 3) is a schematic side view illus-
trating geometric relationships between an electron beam, a
mask and a screen.
FIGURE 8 (sheet 2) is a rear view, partly cutaway,
of a tube faceplate having a prior art shadow mask mounted
therein.
FIGURES 8A, 8B and 8C (sheet 2) are enlar~ed views
of indicated portions of the mask of FIGURE 8.
FIGURE 9 is a rear view, partly cutaway, of a tube
faceplate havin~ a shadow mask mounted therein that incorpo-
rates one embodiment of the present invention.
FIGUR~S 9A, 9B and 9C are enlarged views of indicated
portions of the mask of FIGURE 9.
FI~URE lO (sheet 3) is a plan view, partly in
axial section, of a shadow mask cathode-ray tube having a flat
faceplate.
FIGURE ll (sheet 3) is a plan view, partly in axial
section, of another shadow mask cathode-ray tube having a flat
faceplate.
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Detailed nescription
FIGURE l illustrates a prior art rectangular color
picture tube, having an evacuated glass envelope 20 comprising
a rectangular panel or cap 22 and a tubular neck 24 connected
by a funnel 26. The panel 22 comprises a viewing faceplate
28 and a peripheral flan~e or sidewall 30 which is sealed to
the funnel 26. A mosaic three-color phosphor screen 32 is
located on the inner surface of the faceplate 28. The screen
32 is a line screen, i.e., comprised of an array of parallel
phosphor li.nes or strips, with the phosphor lines extending
substantially parallel to the vertical axis of the tube. The
area between phosphor lines is :Eilled with a light absorbin~
material. A multiapertured color selecti.on electrode or shadow
mask 34 is removably mounted in predetermined spaced relation-
ship to the screen 32. An inline electron gun 36, shownschematically by dotted lines in FIGURE l, is mounted within
the neck 24 to generate and direct three electron beams 38B,
38R and 38G along co-planar convergent paths through the mask
34 to the screen 32. When appropriate voltages are applied
to a yoke 40, the three beams 38B~ 38R and 38G are subjected
to vertical and horizontal magnetic fields that cause the
beams to scan horizontally and ve~tically in a rectangular
raster over the screen 32. ~ ~:
For simplicity, the actual curvature o~ the paths
25 of the deflected beams in the deflection zone--is not shown in ~ ~ .
FIGURE l. Instead the beams are schematically shown as having
an instantaneous bend at the plane of d.eflection P-P.
Although the present invention is described herein
Iwith respect to an inline gun, line screen type cathode-ray
tube, it should be appreciated that the broader concept o~ the
invention is also app~icable to the delta gun, dot
~CA 68,8Q5
~Cr7~ 7 ~
1 screen cathode-ray tube as well as to other cathode-ray
tube types.
For a full understanding of the present
invention, it is desirable to know what electron beam
misregister is. FIGURES 2, 3 and 4 show the electron beam
38G impinging on a port~on of the screen 32. Each phosphor
line (42R, 42G and 42B) is separated from its adjacent line
by a gap that is filled in with a light absorbing
substance 44. The width of the beam 38G is slightly wider
than its associated phosphor line 42G. This arrangement
is commonly referred to as a negative tolerance matrix and
is a preferr0d screen construction -for practicing the
present invention. The present invention also is equally
applicable to positive tolerance matrix tubes (phosphor
lines which are separated by a light absorbing substanc~
and which are wider than their associated beams) and to non-
matrix tubes. In FIGIJRE 2, the electron beam 38G is exactly
centered on its associated phosphor line 42G. This is the
desired beam position for accurate color output. As the
tube ~egins to warm up, doming of the shadow mask will occur
moving the center of the mask toward the screen and the beam
38G will begin to misregister with its associated phosphor
line 42G as in FIGURE 3. In this case, the green phosphor
line does not receive full excitation and the green color
output falls off in intensity. FIGURE 4 shows a more
extreme case where the electron beam 38G has become
misregistered to the extent that it is impinging on an
adjacent phosphor line 42B, thus causing a color purity
problem.
As previously noted, the doming effect is caused
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1 by uneven heating of the shadow mask assembly. FIGURE 5
presents a graph of misregister as a -function of time o-f an
electron beam with a corresponding phosphor line located
halfway between the center and edge of the screen. The
solid curve 50 represents misregister for a prior art tube
and the dashed curve 52 represents the misregister in a tube
using one embodiment of the present invention. The peaks
of the curves 50 and 52 occur from 3 to 5 minutes after
tube activation. The misregister then decreases as the mask
continues to warm up
It should be noted that doming is a movement of
a portion of the mask toward the screen while the periphery
of the mask is held stationary. The effect o:E this
movement is llustrated in FIGURE 6. The shadow mask is
indicated in two positions, its unheated, undomed position,
designated 3~ and its heated, domed position, designa~ed
34'. The boundaries of a portion of a beam 38G that passes
through an aperture of the unheated mask 34 are shown by
dashed lines 39 and the boundaries of the beam portion that
~0 passes through the same aperture of the domed mask 3~' are
indicated by the dot and dash lines 39'. The distance "x"
indicated in FIGURE 6, represents misregister occuring
because of doming. The result of doming misregister is a
shi-ft of beam landing position on the screen toward the
center of the screen 32.
As the mask warms up, the effect of doming
decreases because the temperature gradients in the mask
decrease. Furthermore, heating of the mask causes the
mask to expand thereby moving the apertures in the mask
laterally outwardly (i.e., parallel to the screen) from
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their original locations. Such outward movement produces
a misregister away from the center of the screen. It is
then this combination of doming reduction and heating of
the mask that causes the mask apertures to return toward
alignment with the associated phosphor lines. However, the
expansion of the mask causes more severe misregister problems
at the edge of the screen. In order to~correct for the mis-
register problem a~ the edge of the screen, it is common to
support the mask-frame assembly on heat sensitive supports
that move the mask-frame assembly toward the screen to reduce
or eliminate the misregister caused by mask expansion. Since
the compensation provided is correct only when there is no
heat gradient between the portions of the mask in the support
frame some residual misregister at the halfway point as shown
by the curves of FIGURE 5 will exist. It also should be
noted that because the mask has a greater heat sink at its
edg~, ~,e~,mask frame, some temperature transient will
a~ways exist in the mask during tube operation, and therefore
some degree of doming will alwa~s be present.
FIGURE 7 illustrates the geometry of a shadow mask
tube. Line P-P again represents the plane of de-flection
(at zero deflection) as in FIGURE l. The distance from the
plane P-P to the screen 32 is designated "L" and the spacing
between the shadow mask 34 and the screen 32 (measured
parallel to the axis A-A) is designated "q". The distance
"S" represents the distance from the tube central axis A-A
; to the center 54 of an off-axis electron beam as it
passes through the deflectior plane P-P, and "a"
represents the center-to-center spacing between apertures
in the mask 34. The foregoing dimensions
~7~74 ~ RCA 68,805
1 are approximately related as shown in the following
equation:
q = La
In the present in~ention, in order to reduce the
effects of doming, the shadow mask 56 is given greater
contour or curvature than found in prior art tubes of
similar construction thereby providing greater variation
in "~". At the same time, the value of "a" is also varied
proportionally to "q". This is a deviation from prior art
line screen cathode-ray tubes wherein "a" was made uniform
over the entire mask and "q" was permitted to vary on:Ly with
"L" and "S".
FIGURF.S 8, 8A, 8B and 8C present a prior art
shadow mask having a radius of curvature of lOOOmm. Values
for "a" and slit width for this mask are given in
millimeters. In the center 60, edge 62 and halfway 64
between the centerand edge, the value of "a" is shown to be -:
a constant 0.77 mm. The slit width is graded in
decreasing size from the center 60 to the edge 62 of the :~
mask 34.
In an embodiment of the present invention wherein
the radius of curvature of a shadow mask 50 is 850 mm. shown
in FIGURES 9, 9A, 9B and 9C, the aperture spacing in the -
mask 56 having greater curvature increases from 0.77 at the
center 66 of the mask, to 0.885 at the halfway point 68, to
l.000 mm. at the edge 70 of the mask. If the same slit
widths as used in the prior art mask 3~ of FIGURE 8 were
used in the mask 56 of FIGURE 9, the transmission of the
mask would be reduced beyond a desired level. Th~efore,
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to maintain the desired mask transmission, the slit width
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l is increased relative to the slit width of the prior art
mask. In fact, if the values for "a" were varied from
0.77 mm. at the mask center to 1.14 mm. at the edge o~ the
mask, the slit width could be held at a constant ~.15 mm.
over the entire mask for a given grading factor. An
increase in slit width is highly desirable since it eases
manufacturing of the mask.
Table A presents the ratios of mask to screen
spacings ~q - measured parallel to central axis of tube)
for two prior art tubes and for two tubes constructed in
accordance with the present invention. The first column
shows the ratio of q spacing at an edge of a mask alcng
its major axis to the q spacing at the center o-E the mask.
The second column shows the same ratio taken along h~diagonal~
TABLE A
MaJor axis q Diagonal
enter q Center
19"-90 Prior Art Tube 1.13 1.12
25"-110 Prior Art Tube 1.10 1.09
25" Tu~e 1 Incorporating 1.47 1.45
20Present Invention
25" Tube 2 Incorporating 1.58 1.~8
Present Invention
It can be seen that the edge-to-center q spacing
ratios are substantially larger than the same ratios in
the prior art tubes. For the two examples of tubes
incorporating the present invention, it can be seen that
all edge-to-center q spacing ratios are greater than 1.15.
By increasing the curvature of the shadow mask
rom a radius of 1000 mm. to a radius of 850 mm. both
doming and blister warpage as well as their associated
res~ultant misregisters are reduced. It is known that added
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~7~ 7~ ~ RCA 68,805
1 curvature can provide added streng~h. Therefore, mask
warpage can be reduced. ~urthermore, because of the
geometric relationships when the tube is operated and the
mask becomes heated, a point on a mask having greater
S curvature moves a smaller distance toward the screen than
does a similarly located point on a mask having lesser
curvature for a given mask expansion. For the foregoing
mask curvatures, doming or movement of a portion of the mask
toward the screen is reduced from about ~8 microns in the
1000 mm. radius of curvature mask to approximately 30 microns
in the 850 mm. mask. The increase in "a" permits increases in
the misregister tolerances of the off-center phosphor lines.
Again, as previously mentioned, the spacing between lines
on the screen cannot be too large since it would produce an
objectionable coarseness to the -viewer. Therefore 9 the
chosen spacing should be a compromise between the possible
increase in tolerance and an accepatab~e coarseness of line
trios; symaintaining a smaller value of "a" ~t the central
portions of the screen and allowing the large "a" near the
edge regions,,the subjective appearance o-f -th~ screen is
that of a fine array.
Table B presents tolerance and doming misregister
measurements for a prior art tube and for a new tube with a
shadow mask having greater curvature than the mask of the
2S prior art tube (~50 mm. vs. 1000 mm. radius) at points
halfway between the centers and edges of the tubes. All
units are in mil~imeters.
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1 TABLE B
Tolerance Doming
Available Misregister Result
Prior Art Tube .053 .079 -.026
New Tube .067 .066 .OOl
The increase in tolerance available in the new
tube is caused by the larger "a" spacing and the reduction
in doming misregister is due to the increased shadow mask
curvature of the new tube. Th~efore, by increasing mask
curvature and "a" spacing, the resultant misregister at
the point on the screen where the effects of doming are
greatest can be significantly reduced (e.g., by 0.27 mm.
in TABLE B).
Although the mask having increased curvature and
varied "a" spacing has been shown with respect to a curved
faceplate, the concept of the present invention also permits
use with a flat faceplate. ~eretofore, although shadow
masks for use with line screens have not had exactly the
same curvature as their associated faceplates, it can be
said that the mask and facep~ates were substantially parallel.
A flat faceplate is desirable since it permits greater
viewing angle without distortion of a portion of the
picturé. FIGURE lO shows a cathode-ray tube 72 having
curved shadow mask 74 but a flat faceplate 76. The "q"
spacing in this tube increases substantially from the
center to the edge of the mask and the "a" spacing o-f the
mask aperturessimilarly increases to maintain acceptable
nesting of the phosphor lines on the screen.
It should be appreciated that the concept of
increasing mask curvature over that found in prior art
~ 12 -
~ RCA 68,805
1 tubes to strengthen the mask and reduce doming is not
necessarily limited to masks of spherical or substantially
spherical shape. As shown in FIGURE ll, the curvature of
a mask 78 in a flat-face cathode-ray tube 80 may also have
a reverse curve to give greater strength to the mask. In
this case, the "q" spacing increases then decreases from
center to edge of the mask. The "a" value then is varied
prop~rtionally to the variation in "q" spacing, therefore,
it too increases then decreases from center to edge of the
mask.
The basic inventive concept on which struc~res
according to the present invention are based is the
combination of increased curvature to the mask in
combination with varied '!a" spacing as one proceeds
outwardly from the center of the tube. In some conventional
prior art tubes, the mask to screen spacing "q" is greater
at the edge of the mask than at the center. When the
present invention is applied to such a tube the mask to
screen spacing is given even greater variation. However,
it will be appreciated that the inuention is equally
applicable to a prior art tube design in which the edge
"q" may be smaller than the center "q". In this case
application of the invention to such a design would result
in varying the "q" spacing to a greater extent than what
2S it was in an otherwise identical prior art tube. Such
a variation however may not actually result in a tube
having a larger edge "q" than center "q" but instead could
result in a tube having a smaller edge "q" than center "q"
albeit not as small as it was, or perhaps in a tube having
a constant "q". Thus the invention should not be equated
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RCA 68,805
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1 with the relative size of edge versus center "q" in a tube,
but rather to the relative size and variation of the "q"
to that of an otherwise identical prior art tube. The same
relationship applies to a conceptual statement of the "a"
dimension since this dimension is varied proportionally
with variations in "q".
The relation9hip q ~ ~ permlts proper nesting
of phosphor elements on the screen. Nesting is the
relationship of phosphor element trios relative to each
other wherein the spacing between dots or lines in a trio
is the same as the spacing between adjacent dots or lines
of different trios.
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