US3152900A

US3152900A - Art of making electron-sensitive mosaic screens

Info

Publication number: US3152900A
Application number: US774062A
Authority: US
Inventors: Peter E Kaus; Rosenthal Howard
Original assignee: RCA Corp
Current assignee: RCA Corp
Priority date: 1958-11-14
Filing date: 1958-11-14
Publication date: 1964-10-13
Anticipated expiration: 1981-10-13

Description

1964 P. E. KAUS ETAL 3,

ART OF MAKING ELECTRON-SENSITIVE MOSAIC SCREENS Filed Nov. 14, 1958 2 Sheets-Sheetl Z5 i F INVENTORS .PEZZ'RE. KAI/S yawm aosawau Oct. 13, 1964 P. E. KAUS ETAL 3,152,900

ART OF MAKING ELECTRON-SENSITIVE MOSAIC SCREENS Filed Nov. 14, 1958 2 Sheets-Sheet 2 :aa wra s/rr (ea/12w: u/v/n) .PE TEE .E S [19. 6 EDWARD 3055mm w %./4M

United States Patent Office 3,152,909 Patented Oct. 13, 19%4 3,152,9tlt9 ART Gil MAKING ELECTRGN=SENSETIVE MQSAIC CREENS This invention relates to improvements in photographic methods of and means for laying down a mosaic pattern upon a suitable foundation surface and, though not necessarily limited thereto, is herein described as applied to the manufacture of a tri-color phosphor mosaic screen for use in color-kinescopes of the so-called focus-mask variety.

Classified in accordance with their mode ofo'peration, there are two kinds of masked-target cathode-ray tubes: (1) the shadow-mask variety, wherein the screen and its mask are maintained at the same potential to provide a field-free space through which the electrons pass along substantially straight paths in approaching the screen and (2) the focus-mask variety, wherein the mask is operated at a potential considerably lower than the screen potential to provide a beam-focusing electron-optical lensfield in the mask-to-screen space.

Cathode-ray tubes of the focus-mask variety are more efficient than cathode-ray tubes of the shadow-mask variety. The reason for this is that the concentrating effect of the lens-field upon the electron-beams or jets in a focus-mask tube permits the use of larger maskopenings than can be used in a shadow-mask tube. Thus, other factors being equal, there are more electrons (and hence more light) available at the screen of a focus-mask tube than in a shadow-mask tube. (As to this see French PatentNo. 866,065 of 1941.) However, colorkinescopes ofthefocus-mask variety do not lend themselves readily to mass production methods. Why this is so will be apparent when it is recalled (a) that the conventional way of making an electron-sensitive mosaic screen involves the use of the tubes mask as a stencil in laying-down (or plotting the location of) the elemental phosphor-areas of which the mosaic is comprised and (12) that where, as in the case of a focus-mask tube, the mask-holes are as large or larger than the elemental phosphor-areas of which the mosaic-screen is to be formed/any attempt to use the mask as a stencil in the screen-plotting operation would result in oversize (and hence over-lapping) phosphor-areas.

The foregoing problem has long been recognized and it has previously been proposed to step-down the size of the masks apertures temporarily, i.e., during the screenplotting operation, as by coating the mask with glue (see the French patent, supra), or by electro-plating it with metal or with a cataphoretic substance and then to remove the coating or plating from the mask prior to mounting within the tube. Alternatively, the mask-holes may first be made of the (small) size required photographically to provide a mosaic screen wherein the elemental phosphor-areas are of the desired dimensions and, after the screen has been formed, to enlarge the maskholes, e.g., by re-etching.

Irrespective of the results achieved in the laboratory with the above-described step-down and re-etching methods it may be said that they are so expensive that it is unlikely that either method will be used on a commercial scale.

Accordingly, the principal object of the present invention is to provide an improved method of manufacturing screen-units of the focus-mask variety and one which shall obviate the high costs and other disadvantages of the step-down and re-etching methods dictated by the prior art.

The foregoing and related objects are achieved in accordance with the present invention by an improved photographic screen-plotting method which makes use of: (l) a finished mask, i.e., one wherein the mask-apertures are of the same dimensions that they are in the finished tube; (2) a unique light-source, located in the tubes plane-of-deflection, and from which the distribution of the light is such that the pattern which it projects onto the screen-plate, through the mask apertures, is substantially a negative replica (which may be slightly enlarged) of at least a portion of the pattern of apertures in the mask; and (3) a negative photosensitive material, i.e., one which when exposed to light is rendered soluble, instead of insoluble. The individual color phosphors may be mixed with the negative photosensitive coating material or they may be applied (e.g., by dusting") to the elemental areas which make up the pattern of the developed photograph. When the photosensitive coating on the screen-plate is exposed, through the mask, to the above-described unique pattern of light, the unexposed areas of the coating make up the desired mosaic screen-pattern. Increasing the exposure time decreases (instead of increases) the size of the elemental areas of the mosaic; hence they may be made of any desired small size.

The invention is described in greater detail in connection with the accompanying two sheets of drawings, wherein:

FIG. 1 is a partly diagrammatic fragmentary view in perspective of the screen-unit of a 3-bearn tri-color tube of the focus-mask dot-screen variety; the drawing being marked with lines indicative of the axes of the three beams in their transit from the tubes plane-of-deflection and through the mask holes to the relatively smaller phosphor dots on the screen-plate;

FIG. 2 is a sectional view of a screen-plotting apparatus or lighthouse having an optical system including an extended light source, as dictated by the present invention, and showing a focus-mask and photosensitized screen-plate set up on said apparatus in a position to be illuminated by one of the peculiar patterns of light shown in FIGS. 3, 4 and 5;

FIGS. 3, 4 and 5 are plan views of various lattice-like patterns of light which may be employed in the practice of the invention;

FIG. 6 is an enlarged fragmentary view of the screenplotting apparatus of FIG. 2 and including thering-like light-source of FIG. 5; the drawing being marked with lines indicative of the paths of some of the light rays in transit to the screen-plate through the mask-holes;

FIG. 7 is a diagram of a number of dots (of the same color) produced on the screen-plate by the action of the light rays of FIG. 6; and

FIG. 8 is a chart showing the relative intensity distribution of the lightrays reaching the screen-plate of FIG. 7 from the ring-like primary source of FIGS, as measured along the two lines A() and OB marked on FIG. 7.

As mentioned in the sixth paragraph of this disclosure, the subject method of photographically applying a mosaic of color phosphors to the screen-plate of a kinescope of the focus-mask variety differs in three (3) important respects from prior art methods of making such mosaicscreens. These three distinguishing features of the invention are herein described under the following headings: (1) The Aperture-Mask; (2) The Photosensitive Screen-Coating; and (3) The Light Source.

The Aperture Mask The present invention dispenses with the necessity of stepping down the mask-apertures during the screenplotting operation. This is indicated in FIG. 1 wherein it will be observed that the apertures a in the thin-metal mask 1 are larger than, or at least as large as, the elemental dot-like areas R (red), B (blue) and G (green) of the metallized color-phosphor mosaic on the glass screen-plate 3. As is known, the principal factor to be considered in selecting the proper relative size of the mask-holes and phosphor-dots in a focus-mask tube is the screen-to-mask potential ratio to be employed in the finished tube. This potential ratio may be as high as 9 to 1 or as low as about 2 to l. The theoretical maximum potential ratio (i.e., 9 to 1), if used, may give rise to voltage supply and insulation problems. Accordingly, a lower (partial focus) ratio is ordinarily to be preferred. In one case where the mask was spaced about 0.5 from the electrically conductive screen and wherein the recommended mask-to-screen potential was about 4 to l (the screen or ultor potential being of the order of 18 to 20 kilovolts) and the desired diameter of the phosphor dots was about 0.016", mask-holes approximately 0.018 in diameter proved to be satisfactory.

T he Photosensitive Screen-Coating The photographic screen-plotting method of the present invention dictates the use of a negative photosensitive screen-coating material, i.e., one which when exposed to light is rendered soluble, and which remains insoluble in unexposed areas. Satisfactory results have been achieved with a mixture comprising:

30 cc. 10% by weight of polyvinyl alcohol, e.g., Elvanol 5222 (DuPont).

30 cc. H 0.

6 cc. 4% ferric chloride solution.

Other colloids or gels, e.g., photographers glue or albumin, may be used instead of polyvinyl alcohol.

As in the conventional photographic screen-plotting method the preferred practice is to mix phosphor particles of the desired color-response characteristic with the photosensitive material prior to applying it to the screen-plate 3 in the form of a coating 5, FIG. 2. The phosphorhowever may be added to the photosensitive coating on the screen-plate either prior or subsequent to its exposure, or its development. The development involves simply washing the exposed coating with water. Subsequent to development, the exposed photosensitive material may be volatilized and removed from the phosphor coated unexposed areas (which constitute the desired mosaic pattern) by baking the screen plate, e.g., at a temperature of, say 400 C., for about one hour. The quantity of phosphor in the photosensitive material is preferably about milligrams of phosphor per square centimeter of the screens target surface area. In FIG. 2 the screenplate 3 and its mask l are shown set up in a lighthouse '7, in the same relative position with respect to the tubes plane-of-deflection PP that they are to occupy in the finished tube.

The Source of Light The method of the present invention instead of employing a point source-of-light (as required in the manufacture of focus-mask screens by the step-down and re-etching methods) employs an extended pattern of light corresponding to a part of the deflection plane PP (FIG. 2) which is not seen by those areas of the screenplate that correspond to the phosphor dots of one color. The entire pattern actually seen under the conditions just mentioned may embrace several dark areas, in the form of circles 9, or segments 9' of a circle, as shown in FIG. 3. However, the broad objects of the invention can be achieved by using a symmetrical part of the pattern shown in FIG. 3, such as the hexagonal rosette pattern 10 of FIG. 4, or the simple ringlike pattern 11 of FIG. 5.

The desired shape (e.g., circular or hexagonal) of the elementary areas (R, B and G) of the color-phosphor mosaic will dictate the particular pattern of light which is to be employed in the lighthouse. The selected pattern is perfectly marked, e.g., with ink, metal or other opaque substance, on one end of a light conduit comprising a block 12 (FIG. 2) of quartz, or the like. The stenciled end of the light-conduit 12 is made to coincide with the tubes plane-of-deflection PP (FIG. 2) and derives its light from a primary source, such as a one kilowatt mercury arc lamp 313 (e.g., a GE. Co. bulb type EH6), which may be provided with a suitable reflector 14- for directing the light toward the screen-plate, through the quartz block or conduit 12. The center yy (FIG. 2) of whichever pattern is selected is made to coincide with the color-center of an electron-beam in the finished tube. This optical system may be supported on a turntable (not shown) at the base of the lighthouse to permit the center of the pattern to be shifted, e.g., in steps of from one color-center to another when the mosaic-screen is to be made up of elemental areas of three difierent (e.g., red, blue and green) color-phosphors. Alternatively, the light-source may be fixed in position, and the mask and screen assembly may be shifted between exposures; or separate lighthouses may be used, each With the center of its light source disposed at a position individual to one of the (red, blue or green) color-centers. When the moisaic is to be comprised of two or more color-phosphors, the coating, exposure and development steps, above described, are repeated for each phosphor.

As previously mentioned, the function of the peculiar patterns of light (FIGS. 3, 4, and 5) employed in the practice of the invention is to cause those elemental areas of the screen-plate 3 which should be covered with phosphors, in the finished tube, to be in total darkness, or at a reduced light-level, during the screen-plotting operation.

Referring now to FIG. 6 and having in mind that the photosensitive coating 5 on the screen-plate is of the negative variety and assuming that it contains, say a red phosphor, it will be seen that the elemental areas on the screen-plate which are allotted to the red phosphor (R, FIG. 7) will be relatively dark when the exposure is made and hence will be retained on the plate when the remainder of the coating is washed away during the photographic developing operation. It will also be seen that the dimensions of the dark areas R (on which. the phosphor is retained) can be made as small as desired either by increasing the size of the light source or the length of exposure.

In FIG. 6, the pattern of light which has been selected by illustration is the simple ring-like pattern 11 shown in the plan view, FIG. 5. In FIG. 6, as in FIG. 2, the screen-plate is designated 3; the negative photosensitive coating on the target surface is 5; and the apertured mask, 1. Other reference characters marked on this drawing are PP=the plane of deflection; q=the maskto-screen spacing; p spacing between the plane-of-defiection and the mask; and I =the internal diameter and L, the outside diameter of the ring-like pattern l1 shown in FIG. 5.

The outer diameter I and the inner diameter 1 of a ring-shaped source projecting completely nonluminous tangent dots of a diameter d on the screen plate through mask apertures of diameter D can be shown to be given by:

and

Here s is the mutual separation of the three light source centers (for the projection of the three sets of phosphor dots) in the deflection plane, 1 is the distance from the deflection plane to the mask and q is the distance between =3 the mask and the screen plate. It s is the distance of the source centers from the axis of symmetry,

Further examination shows that patterns of completely nonluminous dots which, in superposition, form a tangent array, can be produced only with relatively small mask apertures, corresponding to mask transmissions less than about 17 percent. However, the processes described will result in phosphor deposition not only at completely unilluminated portions of the screen plate, but also in portions Where the illumination is finite and does not exceed a certain maximum level. For example, FIG. 8 shows the intensity distribution on the screen plate along a line A (FIG. 7) passing through the centers of nearest neighbor dots, and OB, passing through the center of one dot and halfway between nearest-neighbor dots, computed under the following assumptions +q/p) =(P/q) Bi-i6) i=(p/q) (v This corresponds to a mask transmission of 46 percent. If the exposure is made such that the maximum light level for which phosphor is deposited on the screen plate corresponds to the horizontal dotted line, a system of tangent phosphor dots is produced by three successive exposures. If the exposure is increased, the dotted line is lowered and the dot size shrinks, Whereas for smaller exposures the dot size increases. The dot shape, in this case, has the approximate shape of a regular hexagon, closely approximating that of a circle; the corners of the hexagon, corresponding to the directions of lowest illumination, occur on the line OA and its five analogs.

From the foregoing it should now be apparent that the present invention provides an improved photographic method of manufacturing phosphor screens and one which, when applied to the manufacture of a screen-unit for use in color-tubes of the focus-mask variety, obviates the high costs and other disadvantages of the step-down and re-etching methods heretofore employed in the manufacture of such tubes.

What is claimed is: I

1. Method of laying down a mosaic pattern upon a foundation surface, said method comprising: coating said foundation surface with a photosensitive material of the type that is rendered soluble instead of insoluble when exposed to light, placing adjacent to said coated surface a mask containing a pattern of apertures corresponding to the desired pattern of said mosaic, creating a pattern of light corresponding substantially to a negative replica of at least one elemental area of said first mentioned pattern and exposing said photosensitive coating to said pattern of light through the apertures in said mask.

2. Method of laying down a mosaic pattern upon the target surface of the screen-plate of a cathode-ray tube of the masked-target variety, said method comprising: coating said target surface with a photosensitive material of the type that is rendered soluble instead of insoluble when exposed to light, creating a pattern of light corresponding substantially to a negative replica of at least a portion of the pattern of apertures in the mask of said tube, exposing said photosensitive coating to said pattern of light through the pattern of apertures in said mask, and developing the resulting photograph.

3. Method of laying down a phosphor mosaic upon the target surface of the screen-plate of a cathode-ray tube of the masked-target variety, said method comprising: coating said target surface with phosphor particles and with a photosensitive material of the type that is rendered soluble instead of insoluble when exposed to light, creating a pattern of light corresponding substantially to a negative replica of at least a portion of the pattern of apertures in the mask of said tube, exposing said photosensitive coating to said pattern of light through the pattern of apertures in said mask, washing said light-exposed photosensitive coating to remove the phosphor particles and photosensitive material from the light-exposed areas of said screen-plate, and thereafter baking said screenplate to volatilize and to remove the remaining photosensitive material from the phosphor coated unexposed areas of said screen-plate.

4. In the art of manufacturing a cathode-ray tube of the kind containing a screen-unit of the type comprising a mosaic screen and a mask containing a multiplicity of apertures each of a size at least as large as the elemental areas of which said mosaic is comprised; the method of laying down the mosaic upon the target surface of the screen-plate of such a tube which comprises: coating said target surface with a photosensitive material of the type that is rendered soluble instead of insoluble when exposed to light, creating a pattern of light corresponding substantially to a negative replica of at least a portion of the pattern of apertures in said mask, and exposing said photosensitive coating to said pattern of light through the pattern of apertures in said mask for a period suiticiently long to reduce the dimensions of the elementary areas of the mosaic pattern in the resulting photograph to the desired size.

5. In the manufacture of a cathode ray tube having a screen structure comprising a light transparent base and phosphors fixed thereto emissive of light of different colors in response to electron bombardment, a method of applying phosphors including the following steps: depositing a photosensitive layer having a solubility proportional to exposure thereof to light and comprising an organic gel, a photosensitizing material adapted to vary the solubility of the gel and inorganic phosphor particles emissive of light of one color; selectively exposing to light the areas of the deposited iayer where said phosphor particles are to be removed so as to render the photosensitized material in said light exposed areas relatively soluble by a solvent to which the unexposed photosensitized material is immune; subjecting said layer to said solvent to dissolve the photosensitized material in said light exposed areas; depositing a second layer having a solubility proportional to exposure thereof to light and comprising an organic gel, a photosensit-izing material adapted to vary the solubility of the gel and inorganic phosphor particles emissive of light of a different color than'the first-mentioned particles; selectively exposing to lightareas of said second layer offset in relation to the first-mentioned areas to render the exposed areas of the second layer relatively soluble, subjecting said second layer to said solvent to dissolve the light exposed areas thereof and subsequently baking said screen structure to remove said gel from the remaining phosphor covered areas.

6. Method of laying down a mosaic pattern of phosphor dots upon the target surface of the screen-plate of a cathode-ray tube of the masked-target variety, said method comprising the steps of:

(a) coating said target surface with a phosphoncon taining photosensitive material of the type that is rendered soluble instead of insoluble when exposed to light;

(b) placing adjacent to said coated surface a mask having a pattern of apertures corresponding to said mosaic pattern of phosphor dots;

(c) placing on the side of said mask remote from said material a second mask having a pattern of apertures which is so related to said mosaic pattern that light from a source disposed on the side of said second mask remote from said first mask will pass through the apertures of both'rnasks and thus create a pattern of light-and-shade upon said photosensitive coating, the shade falling on the areas of the desired phosphor dots and the light falling on the areas between the desired phosphor dots;

(d) exposing said photosensitive coating, with both said masks in place, to a light source from said remote side of said second mask to subject the material to said pattern of light-and-shade, and

(e) thereafter photographically developing said phosphor-containing photosensitive coating.

References Cited by the Examiner 2,544,905 3/51 Van Deusen 96-35 2,625,734 1/53 Law 96-116 X 2,789,905 4/57 Austin 96-116 2,817,276 12/57 Epstein et a1. 96-35 2,885,935 5/57 Epstein et a1. 96-35 2,897,089 7/59 Ahlburgh et a1. 96-35 FOREIGN PATENTS 780,218 7/57 Great Britain.

OTHER REFERENCES Levy et al.: Electrochemical Society Journal, vol. 101, pages 9 403, 1954. Copy in Div. 56.

Claims

1. METHOD OF LAYING DOWN A MOSAIC PATTERN UPON A FOUNDATION SURFACE, SAID METHOD COMPRISING: COATING SAID FOUNDATION SURFACE WITH A PHOTOSENSITIVE MATERIAL OF THE TYPE THAT IS RENDERED SOLUBLE INSTEAD OF INSOLUBLE WHEN EXPOSED TO LIGHT, PLACING ADJACENT TO SAID COATGED SURFACE A MASK CONTAINING A PATTERN OF APERTURES CORRESPONDING TO THE DESIRED PATTERN OF SIAD MOSAIC, CREATING A PATTERN OF LIGHT CORRESPONDING SUBSTANTIALLY TO A NEGATIVE REPLICA OF AT LEAST ONE ELEMENTAL AREA OF SAID FIRST MENTIONED PATTERN AND EXPOSING SAID PHOTOSENSITIVE COATING TO SAID PATTER OF LIGHT THROUGH THE APERTURES IN SAID MASK.