CA1070747A - Cathode ray tube having shadow mask with variable aperture spacing and mask-screen spacing - Google Patents

Abstract

Abstract A cathode-ray tube of the vertical line screen, slit apertured mask type includes a mask wherein the horizontal center-to-center spacing between adjacent apertures in the mask, and the spacing between the mask and screen, vary proportionally from the center to the edge of the mask.

Description

~ ~ ~ ~ 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,

- 2 -RCA 68,805 ~7~7~

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.
3o ~o~0747 RCA 68,805 ~ `
., ~. .
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.

~ ~7~ ,_, R~A 68,805 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 ~07~17~ ~ RCA 68,805 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 ~7~ 7 ~7 RCA 68,~5 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, 3o to maintain the desired mask transmission, the slit width g ~ , RCA 68,805 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 - 10 ~

~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.

~rr~ 7 ~ ~ RCA 68,805 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 , ' ' ~ .

RCA 68,805 ~o~7~ 7 ~

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.

Claims (3)

Canada 9 April 1979 The embodiments of this invention in which an exclusive prop-erty or privilege is claimed are defined as follows:

1. A cathode-ray tube utilizing an apertured sha-dow mask for registering electron beams on phosphor elements of a cathodoluminescent screen, wherein said mask is such that the ratio of the edge-to-center spacing between said mask and said screen is greater than 1.15, the change in said spacing varying proportionally to the change in spacing between adja-cent apertures of said mask.

2. A cathode-ray tube according to claim 1, where-in said phosphor elements are vertical lines and said apertures are vertical slits.

3. A cathode-ray tube according to claim 2, where-in the faceplate supporting said screen is substantially flat and said mask is curved.