CA1117176A - Cathode ray tube with a concave surface - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE:

A cathode ray tube has a faceplate of a concave surface.
The radius of curvature with the concave surface on the viewer's side is selected that it renders the reflected image of brightly lighted objects inconspicuous, whereby the viewer's attention is concentrated to the screen image.

Description

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BACKGROUND OF T~ INVENTION
The present in~ention relates to a cathode ray tu~e of which the viewing screen has a concave surface to prevent the viewer's attention from being di~st~acted by reflected images of ~rightly lighted ambient objects.
Since the faceplate of a cathode ray tube con~en-tionally has a convex surface on the viewer's side, brightly lighted objects located in the neigh~orhood of the screen are reflected from the convex surface to form a luster on the screen, wh'`ch consti`tutes an obstacle to normal:television viewing.
Many attempts have hitherto been made to mitigate thïs problem. One approach is to provide a rough or fro$ted surface on the outer face of the vle~ing screen on which,ls coated a layer of a light absor~ing material.
Although, this method is effective in making the reflected mage indistinct in detail, the ~righ,tness of the luster is not substantially reduced and the wanted screen image is also slightly,diffused with the resul'tant loss of its detail. A second approach is to employ an anti-- re~lection film which is used for coating the sur~ace , of an opti`cal lens. Alth,ough th,is met~od is effective in reducing the brightness of the luster ~ithout causing ~' a loss of detail in screen Lmage, problems still exist which concern the difficulties in manufacture with a .
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consecluent:Lal hlgh prod~lction cost.
Accordinq to the invention, there ls pxo~ided a cathode ray tube cornprislng a faceplate and means.,sealed thereto for forming an evacuated envelope therewith. The faceplate has a concave surface toward the outside. The radius of curvature o~ the surface is selected such that images reflected from the surface are made to appear incon-spicuous to a viewer seated a dis-tance ~rom the concave surface. The radius of curvature is selected to satisfy the following relations:
C/~< Q.~ C ~ n Q.H< C:X/2 + n where, C is the radius of curvature;
X is -the distance of an ob~ect from the concave surface, X being equal to or greater than C/2;
H is the height of the viewing screen of the cathode ray tube; . .
Q is a constant ranging from 1 to 10 depending on the number of scanning lines; and n is the minimum eye focusing distance of said viewer.
The invention will be further described in detail by way of example with reference to the accompanying drawings, in which:
Fig. 1 is a cross-sectional view of a cathode ray tube of the inventlon;
Fig~ 2 is a cross-sectional view of a transparent plate embodying the lnvention shown attached to the faceplate of a conventional cathode ray tube;
Figs. 3A and 3B are cross-sectional ~iews of a modified transparent plate of the invention respectively illustrating its configuration before and after being attached --2~

11~L7~ 6 to the faceplate of ~ con~entional cathode ra~ tub~;
Fig. 4 to 6 are further modifications of the invention;
Fig. 7 is a schematic Illustration useful for describing the location oE re1ected images Fiy. 8 is a graphic illustration of the ~iewer'~
position with respect to the positions of objects and th,elr reflected image to determine the range of viewer's posltions with respect to the vlewing surface in which the viewer's own reflected image is obscured;
Fig. 9 appearing on the same sheet of drawings as Fig. 7 is a graphic illustration useful to determine the range of objects other than the viewer's own image in which the object's reflected image is obscured;
Fig. 10 is a graphic illustration of a range of obscurity which satIsfles the ranges determined by Figs. 8 and 9; and Fig. 11 is a graphic illustration of a total range of obscurity.
Referring now to the drawings, particularly to Fig. 1 in which a first embodiment of the invention is il-lustrated. Numeral 11 is a funnel portion of a cathode r ~; ~117~6 . . .

ray tuhe which is sealed at the front edge by a aceplate 12 to pro~ide an evacuated envelope 10. Th.e ~aceplate 12 is formed of the same transparent material as the funnel portion 11 and shaped to provide a front surface 13 which is concave Ln accordance with.th.e inYention.
The inner face o~ the ~aceplate 12 is aoated with a phosphorescent materi~al in conventional manner, so that it has a frostl~ke sur~ace.
Consider now the formation of images reflected from the concave surface 13 with.reference to Fig. 7.
Assuming that an object 30 is located in front of the surface 13 in a position to the left of the focal point " of the concave surface 13, an inverted real image 30a will be seen. Since the inner side of the faceplate has a frostlike surface, it is not necessary to consider ~ reflections from the inner surface. The size of the ;~ reflected image 30a wl11 increase as the o~ject 30 moyes toward the surface 13 until it reaches th.e focal point "F", whereupon the reflected ~mage is a virtual ~ ~ 20 image, so that if an o~ject 31 is located ~etween the focal point and the surface 13,a ~irtual image 31a will be seen as illustrated. Experiments showed that when the reflected image, whether it be real or ~irtual, is at least tw~ce the size of the object itself, vi`ewers :fail to recognize it as an intelligible image, so that : ~ .

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~7~6 such images do not constitute an obstacle to normal television vie~ng under lighted or equivalent cond~tions.
More specifi`cally, when the o~ject of interest is located in a range of one hal of the focal distance "f" from the focal point "F", the si~ze of the re:Elected image of the object ~s enlarged to at least twi'ce i`ts own size.
Since the radius "C" of curvature of the surface 13 equals twice the focal dlstance (i.e. C = 2f~, this occurs when the o~ject i`s located within a range of 0.25C
to 0.75C. W~th the object of interest ~eing located in an typical range of distances to the viewing screen or surface 13, and the viewer being seated in a position which falls wi~thin a range o~ vi`ewi`ng distances which are conYentionally determined as optimum ~ased on the number of scanning lines and the height of the viewing screen, the experi~oents sho~ that preferable values of the radius "C" are 1 to 10 meters for cathode ray tubes with screen sizes o~ from 5 to 32 inches measured diagonally across the screen surface, as seen in the

2 0 f ollowing Tàble I.

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, TABLE I

Screen S~e ~iewin~ Distance Radius "C"
~¦~inches) ~meters~ (meters) 32 3.4 - 4.g 10 26 2.8 - 4.~ 8 22 2.~ - 3.4 7 18 2.0 - 2.8 6 16 1.7 - 2.5 5 14 1.5 - 2.2 1.1 - 1.5 3 7 0.8 - 1~1 2 0.6 - a.a .
Unobstructed Yiewing can also be achie~ed if the reflected images are blurred to an unintelligeable de-gree.~ Fig. 8 is a graphic representation of the relation between the distance X of the object to the ie~ing screen and the distance Y of the location at which the reflected ;mage i`s formed wit~ respect to the Yie~ing screen, with a plot of a ~athemat~cal relation Y = Xf/(X - f) which is derived ;n a manner ~ell-kno~n in the art of optics. Curve I is a plot o~ distances on which inYerted real i~ages are formed and curve II
is a plot of distances on which noninverted virtual ~mages are fo De~.

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~7~76 An object positioned at X = 2f forms its ~n~erted real image at a posit~on Y = 2f and an object ~ositioned at X < f forms noninverted virtual'images. .. Li~ne IIX
represents the distance between the viewer and surface 13.
As regards the viewer`s own image when he observes h.is own reflected image at various positions with respect to the surface13, the observation can ~e considered to be made at positions along the Z axis whic~. is at 45 with respect to th.e X and Y coordi~nate axes. The range of viewer~s own images which.are sh.arply focused is defined between the point zero ,and a point ~ which corresponds to the f,ocal distance "f" on the X and Y
axes~ If he is seated i`n positions ~etween a poLnt A
and a poInt B wh~c~ corresponds to 2f on the X and Y
axes, his own reflected ;lmage is out of ~ocus to such. a degree that it consti`tutes no obstacle. to normal vie~ing.
Th.is out-of-focus area e~tends along th.e Z axis to a : point C which is located ~eyond point B ~y the minimum eye focus~ng d~stance "n", so that the relation : 20 f < z ~ 2f + n sh.ould h~old to ensure unobstru~ted viewing.
When objects other than.the viewer are located at 'a position at infinite di`stance from the v~ew~ng screen, the reflected image i~s formed at the focal distance as seen from Fig. 8. When this reflected image is vïewed in a position between th.e vie~ing screen and a point -1~17~7~

at a dis-tance "~ -t n", the reflected ima~e is seen as a blurred or o~scured y.irtual image, and ~h.en vi~ewea in a position ~eyond that ~oint, it is se~n as a sharply ~ocused inverted real image, as plotted ~n F~g. Y. If such objects are located ~n a range between infinity ~nd the focal poi~nt, -the imaye will bP seen as an blurred yixtual image when viewed at a d~stance which.lLes within the shaded area defined by the curve Z = ~XXf f~ ~ n.
When the object's posit~on i`s wîthi~n a range ~etween zero and f, reflected image is blurre.d regardless o~
the ~iewing position as indicated by the sha~ed area of Fig. 9.
From the fore~oing d~,scussion, the area in wh.ich the reflected Lmage of both.vie~er and surrounding objects are both blurred ~s defined as $ollo~s and visualized by a shaded area ~n F~g. 10.
f < Z < 2f + n ..... ~............... ~1 Z < X Xff ~ n .~.... ~............... (21 where, X is greater than f. Since C = 2f, E~uations 1 and 2 can be rewritten as ~ollows:
- C/2 < Z < C ~ n ................. ... (3 Z < x-CX=_~7~ ~ n ............... ... C4 where, X is greater th.an C/2.
It is generally known that the optimum viewing 2 distance $rom the screen is given ~y Q.H', where Q,i`s ~117~ 6 a coefficient inversely proportional to the num~ex of scanning li`nes as listed i`n Table II and H, the heigh.t of the screen.
TA~LE II
Scanning Llnes . Q

1,00Q . 3 10,000 Equations 3 and 4 can ~e rewritten as follows:
C~2 < Q.H < C ~ n .................. (51 Q-H < xXC/c/2 ~ n .-------~ .. (6~
Optimum values of radius C in relation to various screen sizes are given in Table III.
TABLE III
: Optimum Viewing Screen Size Screen HeightDistance _ Radius "C"
:~ (inches) lmeters~ (meters~ (meters) 32 0.49 - 3.43 6.86 : 26 0.39 2.73 5.46 22 0.30 2.1 4.2 : . , 18 0.27 1.89 3.78 - 16 0.24 1.68 3.36 ~:~ 14 0.21 1.47 2.94 0.15 1.05 2.1 7 0.1~ 0.77 1.54 0.08 0.56 1.12 :~ 2.5 0.04 ~.25 0.5 .
_ g _ With Q = 7 and the viewer bei.ng positioned at a distance C/2 (.=f), the reflected image is blurred to a maximum degree, and the preferred values of radius o curvature C range from 0.5 to about 7 meters for screen sizes of from 2.5 to 32 inches, as seen from Table III, where the m~nimum value of xadius is determined by the minimum eye focusing distance (=0.25 meters~.
As previously descri`bed, the reflected image can be made to appear as..an. unrecogni~zable image when the object, including the viewer, is located in a range between 0.25C to 0.75C if the refle~cted image size is greater than twice th.e size of the object. Therefore, the shaded area of Fig. 10 extends to the minimum distance 0.5f as illustrated in Fig. 11, and it will be seen that Equation 5 is rewritten as C/4 < Q.H < C + n ............... ~7~
Experiments showed th.at in so far as the radius of curvature C is so selected as to satisfy Equations 6 and 7, the reflected images are rendexed unnoticeable to the eyes of the viewer so that th.e viewer's attention : --- is not distracted by reflected images.
Returning to Fig. 1, the faceplate 12 has its edges covered with a layer 14 of an opaque material to prevent introduction of light rays to the interior of the face-plate since such rays might otherwise be reflected in .

11~7~76 a zigzag pattern and interfere wi~th the screen ~ma~ebeing vie~ed.
Various mod~ficati`ons of the Eaceplate 12 are possi~le. Fi`g. 2 is an ill~stration o~ another en~odi-ment of the ~nventi~on which ~s useful for apply~ng thepresent i~nvention to ex~sti~ng telev~si`on ~ecel~vers having a conventional convexed front surface. In Fig.
2, the conventi~onal cathode ray tu~e 15 has a conve~
f~ceplate 16 to the front face of whLch is secured a transparent panel 17 ~y a su~ta~le means suchas trans-parent adhesivefor example polyester resin, having the same index of refraction as that of the front panel 17.
The panel 17 has one surface so concaved as to conform to the outer surface of the ~aceplate 16 and the other surface of the panel 17 ~s formed in a manner as descr~bed abo~e.
A modification of the Fig. 2 em~odiment is il-lustrated ~n F~gs. 3A and 3B. In Fi`g, 3A~ the panel 18 IS formed of an elasti~c transparent mater~al hav~ng 20- prefera~ly the same index of refraction as that of the faceplate 16 such as acxylic resin. The panel 18 fS
~o formed that Lt has a smaIler radrus of curvature under normal conditions than that of the front surface of the faceplate 16 so that when the panel 18 îs secured to the faceplate as shown in F~g. 3B ~ecause 1~

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of its elasticity the front surface of the panel be-comes conca~ed 50 as to have the same radius o~ curvature as described above.
Fig. 4 is an illustra~Ion of another preferred embodiment of the invention i~n which a modifled trans-parent panel 20 is ~rovided. The panel 20 has its center axis of cur~ature 21 inclined downward by ~
degrees with respect to the longitudinal axis 23 of the cathode ray tube 22. This inclination of front surface provides downward reflection of light to the outside of the viewer's ~ision.
The periphery of the faceplate 12 of the Fig. 1 embodiment may be formed with a recess 24 as illustrated in Fig. S. The reason for this is to reduce the overall weight of the cathode ray tube 10 to facilitate instal-lation during the manufacture of television receivers as well as to reduce the amount of direct light which might otherwise enter through the periphery.
The transparent panel 17 of the embodiment of Fig. 2 is formed by injecting a liquid transparent - material into a mould and allowing the injected liquid to cure to form a hard plastic body which îs later attached to the faceplate of a conventional cathode ray tube. Instead of doing this, the front panel 17 can ~5 also be provided by attaching a mould 26, as illustrated '~ .

in Fig. 6, to the front face of a cathode ray tube 27 and injecting the liquid through an inlet opening 28.
An opening 29 is provided to escape air inside to facilitate the liquid injection. When the injected liquid is cured, the mould 26 is removed from the cathode ray tube. The mould 26 may be formed of a transparent material. In this case, the mould forms part of the front panel so that after curing of the injected liquid it is not necessary to remove the mould from the cathode ray tube.
To enhance the blurring effect,. it is preferable that the faceplate 12 have a relatively large value of spherical aberration which corresponds to a light dis-tribution of less than 2 lines per millimeter at the focal point.
The foregoing description shows only preferred embodiments of the present invention. Various other modifications are apparent to those skilled in the art without departing from the scope of the present in-vention which is only limited by the appended claims.
~- For example, the surface 13 of the front panel may be applied with a layer of semi-transparent material to serve as an optical filter for the purposes of in-creasing the image contrast~ The embodiments shown and described are only illustrative, not restrictive.

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Claims (6)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A cathode ray tube comprising a faceplate and means sealed thereto for forming an evacuated envelope there-with, said faceplate having a concave surface toward the outside, the radius of curvature of said surface being selected such that images reflected from said surface are made to appear inconspicuous to a viewer seated a distance from said concave surface, wherein said radius of curvature is selected to satisfy the following relations:
C/4? Q.H?C + n Q.H? + n where, C is the radius of curvature;
X is the distance of an object from said concave surface, X being equal to or greater than C/2;
H is the height of the viewing screen of said cathode ray tube;
Q is a constant ranging from 1 to 10 depending on the number of scanning lines; and n is the minimum eye focusing distance of said viewer.

2. A cathode ray tube as claimed in claim 1, wherein the center axis of said concaved surface is skewed relative to the center axis of said cathode ray tube.

3. A cathode ray tube as claimed in claim 1, wherein the periphery of said faceplate is covered with a layer of an opaque material.

4. A cathode ray tube as claimed in claim 1, 2 or 3, wherein said faceplate has a spherical aberration corresponding to a light distribution of less than two lines per millimeter at the focal point of said concave surface.

5. A transparent plate for use with a cathode ray tube, having a viewing screen with a convex front face, said plate comprising a body of transparent material formed with a first concave surface and a second surface opposite thereto, the second surface being so shaped as to conform to the contour of the convex front face of the viewing screen of said cathode ray tube, and the radius of curvature of said first concave surface being so selected that images reflected from said first concave surface are made to appear incon-spicuous to a viewer seated a distance from said first concave surface, wherein said radius of curvature is selected to satisfy the following relations:
C/4? Q.H?C + n Q.H? + n where, C is the radius of curvature of said first concave surface;
X is the distance of an object from said first concave surface, X being equal to or greater than C/2;
H is the height of the viewing screen of said cathode ray tube;
Q is a constant ranging from 1 to 10 depending on the number of scanning lines; and n is the minimum eye focusing distance of said viewer.

6. A transparent plate as claimed in claim 5, wherein said transparent body has a spherical aberration corresponding to a light distribution of less than two lines per millimeter at the focal point of said concave surface.