US3685994A - Photographic method for printing a screen structure for a cathode-ray tube - Google Patents

Abstract

A PHOTOGRAPHIC METHOD FOR PRINTING A SCREEN STRUCTURE FOR A CATHODE-RAY TUBE HAVING A SUPPORTING SURFACE, A SCREEN STRUCTURE ON THE SURFACE AND A APERTURED MASK SPACED FROM THE SCREEN, SAID METHOD COMPRISING (A) DEPOSITING A COATING COMPRISED OF A PHOTOSENSTIVE BINDER ON THE SURFACE, (B) EXPOSING PRESCRIBED AREAS OF THE COATING THROUGH THE APERTURE OF THE MASK TO LIGHT FROM A FIRST LIGHT SOURCE HAVING AN EQUIVALENT DIAMETER OF ABOUT 0.160 TO 0.200 INCH, (C) EXPOSING THE PRESCRIBED AREAS OF THE COATING THROUGH THE APERTURES OF THE MASK TO LIGHT FROM A SECOND LIGHT SOURCE HAVING AN EQUIVALENT DIAMETER OF ABOUT 0.080 TO 0.120 INCH, (D) AND THEN DEVELOPING THE EXPOSED COATING.

Description

Aug. 22, 1972 H. R. FREY PHOTOGRAPHIC METHOD FOR PRINTING A SCREEN STRUCTURE FOR A CATHODE-RAY TUBE 5 Sheets-Sheet 1 Filed May 5, 1971 [\VEATOR Hmmv 2. F zsv Bl r AT TORNEY Aug. 22, 1912 Filed May 5, 1971 PENUMBRA SIZE IN MILS H. R. FREY PHOTOGRAPHIC METHOD FOR PRINTING A SCREEN STRUCTURE FOR A CATHODE-RAY TUBE 3 Sheets-Sheet 3 SIZE OF LIGHT SOURCE IN MILS WITH I90 MIL SOURCE 4 5 e 7 8IOIII2 DISTANCE FROM CENTER OF PANEL IN INCHES INVENTOR. Han/2v A. Frzsv AT TORNEY Aug. 22, 1972 H. R. F'REY PHOTOGRAPHIC METHOD FOR PRINTING A SCREEN STRUCTURE FOR A CATHODE-RAY TUBE 3 Sheets-Sheet 3 Filed May 5, 1971 FOR IOO MIL SOURCE FOR I90 MIL SOURCE 'ITO 5 4 5 DISTANCE FROM CENTER OF PANEL IN INCHES 3OOOIIZOIEIJEI3IYIBSIIIOIIIZ DISTANCE FROM CENTER OF PANEL IN INCHES "2 INVENTOR.

Han/2v 2. F25 v BY I 6 '7 5 5 DISTANCE FROMCENTER OF PANELIN INCHES 2; z MSW 80 $2 58.: 8x322 ATTORNEY 3,685,994 PHOTO GRAPHIC METHOD FOR PRINTING A SCREEN STRUCTURE FOR A CATHODE-RAY TUBE Harry Robert Frey, Lancaster, Pa., assignor to RCA Corporation, New York, N.Y. Filed May 5, 1971, Ser. No. 140,345 Int. Cl. G03c 5/00 US. Cl. 96--36.1 5 Claims ABSTRACT OF THE DISCLOSURE A photographic method for printing a screen structure for a cathode-ray tube having a supporting surface, a screen structure on the surface and an apertured mask spaced from the screen, said method comprising (a) depositing a coating comprised of a photosensitive binder on the surface,

(b) exposing prescribed areas of the coating through the apertures of the mask to light from a first light source having an equivalent diameter of about 0.160 to 0.200 inch,

(c) exposing the prescribed areas of the coating through the apertures of the mask to light from a second light source having an equivalent diameter of about 0.080 to 0.120 inch,

((1) and then developing the exposed coating.

BACKGROUND OF THE INVENTION This invention relates to an improved photographic method for printing a screen structure for a cathode-ray tube, such as a color television picture tube.

A commercial picture tube for color television, or a color kinescope as it is sometimes called, is a cathode-ray tube which includes a viewing screen comprised generally of a multiplicity of red-emitting, green-emitting and blue-emitting phosphor elements. These elements are usually arranged on the inner surface of the faceplate panel of the picture tube in a regular cyclic array. In a shadowmask-type picture tube, the phosphor elements are usually dots arranged in groups of threes or triads, each triad having a red-emitting dot, a green-emitting dot and a blueemitting dot. Each triad is associated with a particular aperture in the shadow mask (also called apertured mask).

In order to produce a television picture with suitable resolution, brightness and color purity, the process for forming the phosphor elements must be capable of producing a large number of phosphor elements with relatively small and prescribed sizes which are accurately positioned with respect to one another and with respect to their associated mask apertures. In one preferred process for printing phosphor elements for a shadow-mask type viewing screen, the inner surface of the faceplate panel is coated with a mixture of phosphor particles and a photosensitive binder. A light field is projected from a small area light source upon the coating through the shadow mask of the tube, which mask functions as a photographic master or negative in the process. The exposed coating is subsequently developed to produce phosphor elements of the first phosphor; for example, the blueemitting phosphor dots. The process is repeated for the green-emitting phosphor elements and again for the redemitting phosphor elements using the same shadow mask in the same position as a photographic master. The light source is appropriately olfset from the center line of the tube during the exposure steps so that the phosphor elements are displaced from one another to form the prescribed triads.

United States Patent 0 ice The shape of any particular phosphor element is determined by the shape of the projected light spot which, in turn, is determined primarily by the shape of the mask aperture thorugh which the light spot is projected. The size of any particular phosphor element is determined in part by the size of the projected light spot, which is defined by the relationship where R is the. size of the light spot including the penumbra projected through the particular aperture in the mask, q is the spacing between the mask and the coating, p is the spacing between the light source and the mask, M is the size of the light source and B is the size of the mask aperture. It has been found empirically that, with a normal exposure, the phosphor element size R equals about 0.88R. Overexposure produces larger elements, and underexposure produces smaller elements. As used herein, the degree of exposure is the ratio of the linear distance across the phosphor element to the linear distance across the light spot that produced the element, or R/R', and is called the adherence ratio.

In order to design certain operating tolerances into the tube, the mask apertures grade in size from largest at the center of the mask to smallest at the corners of the mask. The projection of these apertures covers areas which grade from about of the respective phosphor elements at the center of the screen to about 50% of the respective phosphor elements at the corners of the screen. Thus, the elements increase in size relative to their associated apertures from the center to the corner of the screen. Such a design may be printed using only one light exposure if the center of th light field is underexposed yielding smaller elements relatively and the corners are overexposed yielding larger elements relatively. With such an exposure the corner phosphor elements are printed using larger adherence ratios relative to those of the center element.

Because of the optical geometry of the lighthouse used for exposure, the brightness of the light field grades in the opposite way, being brightest at the center of the screen and dimmest at the corners. In order to provide the opposite distribution, it is necessary to have the light pass through a light attenuation filter which exhibits a graded transmission from least transmission at the center (typically 15 to 25%) to greatest transmission at the corners (typically Exposures through the filter require relatively long exposure times. The exposure time for the entire field is determined by the time required for overexposure in the corners of the field. The greater the overexposure at the corners, the longer will be the exposure times. In some designs, the differential between underexposure at the center and overexposure at the corners on the same viewing screen is so great that the phosphor elements at the center laok sufficient adherence when the desired element size is achieved at the corners.

It has been suggested that each photosensitive coating be twice exposed at the same locations; once with a standard-sized light source (about 0.160-inch diameter) with no filter and once with an over-sized light source (about 0.220-inch diameter) with a graded filter as described above. The total effect is to produce smaller dots at the center of the screen, whose size is governed primarily by the exposure from the smaller source, and larger dots at the corners of the screen, whose size is governed primarily by the exposure from the larger source. This double-exposure technique may provide improvements over the prior single-exposure technique with previous tube designs. However, new tube designs have made new demands on the printing process requiring still further improved exposure techniques.

3 SUMMARY OF THE INVENTION In the novel method, prescribed areas of the coating on the supporting surface are exposed through the aperatured mask of the tube to light from a first small area light source having an equivalent circular diameter of about 0.160 to 0.200 inch, and also are exposed through the mask to light from a second small area light source having an equivalent circular diameter of about 0.080 to 0.120 inch. The sizes of the phosphor elements produced at the peripheral and corner portions of the screen are determined primarily by the exposure from the first light source. Using a first light source in the defined size range avoids raggedness of the elements produced and other adverse effects of using too small a light source. The sizes of the phosphor elements produced at the central portions of the screen are determined primarily by the exposure from the second light source. Using a second light source in the other defined size range avoids minimal or insufiicient adherence of the elements produced and other adverse eifects of using too large a light source while, at the same time, producing smaller elements relatively than previously produced. Both exposures may be adjusted to provide a smooth grading of element size from the center to the edge of the screen.

The novel method can be used to print a screen structure which requires even greater ditferences in exposure to produce the elements of the screen. The method is versatile enough to be applied to a variety of designs and can be conducted with a practical amount of process control in the factory.

In preferred embodiments of the novel method, both exposures are made through light attenuation filters. The combined filtered exposures may be designed to provide a tailored distribution of element sizes, which may be nonsymmetrical with respect to the screen center. The com bined filtered exposures may be tailored to print screens wherein the gamut of screen exposures required for all elements of the screen is too great for a single-exposuretype process. In some situations, the total time required for multiple exposures using the novel method is less than the time required for a single exposure.

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 bepracticed. The lighthouse has a faceplate panel thereon in position for exposure.

FIG. 2 is a schematic diagram illustrating a tube geometry and some of the nomenclature used in this specification.

FIG. 3 is a graph illustrating the calculated exposure level factor for a particular example of the novel method.

FIG. 4 is a diagram showing the calculated and desired values of penumbra size for a particular example of the novel method.

FIG. 5 is a diagram showing the relative brightness across the light field for a particular example of the novel method.

FIG. 6 is a diagram showing the calculated total exposure for several triads in a particular example of the novel method.

FIG. 7 is a diagram showing the measured phosphor element size produced by a particular example of the nevel method.

DESCRIPTION OF THE PREFERRED EMBODIMENT Example As an example of the novel method, the invention is applied to printing the phosphor elements for a screen for a 25-inch 1l0-deflection shadow-mask-type picture tube. Since shadow-mask-type picture tubes are described in the prior art, they need not be described in detail here. Generally, however, the tube is comprised of an evacuated 4 glass envelope including an electron gun mount assembly, a funnel assembly, and a faceplate panel assembly.

In manufacturing the tube, the facepate panel assembly is completed as a unit. The panel assembly includes the faceplate panel, an apertured shadow mask mounted therein, and a viewing screen. The panel includes a viewing window, sidewalls and mask mounting studs extending from the sidewalls. The mask mounts on the studs in a predetermined spaced relation with the inner surface of the viewing window. The viewing screen is made up of various structures, some of which are deposited on the inner surface of the viewing window by the photographic printing method of the invention. The green-emitting dots for the viewing screen are made by coating the inner surface of the viewing window with a photosensitive material comprising polyvinyl alcohol, a dichromate photosensitizer for the alcohol, and particles of green-emitting phosphor. The mask is inserted into position on the studs and then exposed according to the novel method on a lighthouse.

One lighthouse suitable for practicing the novel method is disclosed in patent application Ser. No. 844,852, filed July 25, 1969, by Harry R. Frey. That lighthouse, which is 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 which in turn is supported at the desired angle by lugs 27. The light box is a cylindrical cup-shape casting closed at one end by an integral end wall 29. The other end of the light box 21 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 a 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 beyond 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 54 is positioned behind the lamp 43.

A lens assembly 51 is mounted on a lens assembly support ring 53 and stand 011 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 the correction lens 61 and a wedge lens 63 held and spaced from each other by a separate 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. The filter is in the form of a relief image comprised of preformed carbon particles in gelatin or other clear colorless binder. The filter has essentially a neutral gray transmittance varying only in the intensity of grayness. The intensity of grayness varies from point-to-point so that the point-to-point variations in brightness in the light field are reduced according to a prescribed plot.

In one mode for operating the lighthouse shown in FIG. 1, a faceplate panel 73 having a layer 75 comprised of a light-sensitive binder and phosphor particles on the inner surface thereof and a mask 77 mounted therein is placed in position on the panel support 23 as shown in FIG. 1. A light field from the narrow end 37 of light pipe 35 passes upwardly through the wedge lens 63, the filter 71 and the correction lens 61. The light field then passes upwardly through the apertures 79 on the mask 77. The light passing through the apertures falls incident upon the phosphor layer 75 exposing the light-sensitive binder, thereby changing its solubility characteristics. The light source 37 has a diameter of about mils (0.100 inch). The exposure continues for a desired time interval and then the light from the light source 37 is eclipsed.

Next, the panel assembly is placed on a second lighthouse similar to the first lighthouse except that it has a diiferent filter 71 and the diameter of narrow end 37 of the light pipe 35 is about mils (0.190 inch). The

second exposure continues for a desired time interval, and

then the light from the light source 37 is eclipsed. Then, the panel assembly is removed from the second lighthouse, the mask 77 is removed from the assembly, and the coating 75 is developed by flushing with an aqueous solvent. Unexposed areas of the coating 75 are flushed away by the solvent, and the exposed areas are retained in place.

The method is then repeated as described above for making the blue-emitting phosphor elements by substituting blue-emitting phosphor particles for the greenemitting phosphor particles in the coating 75. This latter coating is applied over the green-emitting phosphor elements. The mask 77 is again inserted in the faceplate panel 73 and the coating is exposed on a third and then a fourth lighthouse. The third and fourth lighthouses are similar to the first and second lighthouses except that they each have a different lens assembly 51 and filter tailored for that field of phosphor elements. After exposures on the third and fourth lighthouses, the coating 75 with the blue-emitting phosphor therein is developed as described above to remove the unexposed portions of the coating 75. The exposed portions are retained in place.

The method is then repeated again as described above for making the red-emitting phosphor elements by substituting red-emitting phosphor for the green-emitting phosphor in the coating 75. This latter coating is applied over the green-emitting and blue-emitting phosphor elements. The mask 77 is again inserted in the faceplate panel 73, and the coating is exposed on a fifth and then a sixth lighthouse. The fifth and sixth lighthouses are similar to the first and second lighthouses except that they each have a different lens assembly 51 and filter tailored for that field of phosphor elements. After exposing the coating with the red-emitting phosphor therein on the fifth and sixth lighthouses, the coating is developed to remove the unexposed portions of the coating 75, and the exposed portions are retained in place.

After the phosphor elements have been printed, the structure is filmed, aluminized and baked out at about 420 C. by methods known in the art. The completed screen structure is then assembled, with other parts, into the faceplate panel assembly, and the panel assembly incorporated into a completed tube.

Feasibility study A feasibility study for making the phosphor elements for the screens of a 25-inch shadow mask picture tube with 110 deflection used the relationships shown in FIG. 2 for each particular phosphor element during any particular exposure. The light pipe 35 terminates in a light source 37 that is circular and has a diameter M. The mask aperture 79 is circular, has a diameter B and is located a distance p from the light source 37 and a distance q from the inner surface of the viewing window 73. Light from the source 37 projects light through an aperture 79 producing a substantially circular light spot. The penumbra of the projected light spot has a diameter R at the inner surface of the window 73. After development, the retained phosphor element has a diameter R. The ratio R/R' is referred to as the adherence ratio.

The preferred design procedure for the optical exposure for each field of phosphor elements is as follows. First, calculate the relationships of adherence ratios and light source sizes for the center and the edge elements of the viewing screen. Second, determine the optimum light source sizes so that the calculated adherence ratio is about 0.88 at the corners for the exposure with the larger light source, and is about 0.88 at the center of viewing screen for the smaller of the light sources. Third, design filters for each exposure to provide a tailored size plot for the elements on the viewing screen after both exposures are complete. And finally, determine empirically the optimum combination of exposures (timexbrightmess) to provide the desired plot of sizes of screen elements.

Generally, the center of a viewing screen requires a smaller area light source, and the corners of the viewing screen require a larger area light source for optimum exposure geometry. In the novel method, two sequential exposures are made on one photosensitive coating prior to developing the coating. One exposure is made on a lighthouse equipped with a smaller area light source in combination with a filter that provides the greatest light intensity at the center of the viewing screen. The other exposure is made on another lighthouse with a larger area light source in combination with a filter that provides the greatest light intensity at the corners of the viewing screen. The filter intensities are designed to smoothly blend together the exposures.

The adherence ratio can be calculated from the desired tube geometry, and the relative difliculty of printing a given screen structure can be estimated. This factor is the ratio of the desired screen element size to the penumbra (light spot) size on the coating producing that element. The adherence ratio is plotted for the center and the corners of the viewing screen as a function of light source size as shown in FIG. 3 by the curve 81 for the center and 83 for the corners. Based on experience, the normal and desired exposure level exhibits an adherence ratio of about 0.88. Experience has also indicated that the minimum adherence ratio should not be less than 0.80 nor more than 1.06 as indicated by the dotted lines in FIG. 3. For a 25-inch, deflection viewing screen, FIG. 3 indicates that the optimum light source sizes for the screen are about 100 mils for the cented of the screen and about mils for the corners of the screen. Using a larger light source for the screen center would tend toward poor screen element adherence because lower exposure would be required to maintain the desired phosphor element size. A smaller light source for the screen corners would tend toward small, ragged and oval phosphor element size.

In FIG. 4, the calculated penumbra sizes are given for exposures from a 100-mil and a l90-mil light source as a function of distance from the center of the panel by the curves 85 and 87 respectively. The desired penumbra sizes calculated from the bogie green-emitting, blue-emitting and red-emitting phosphor element sizes are plotted as broken lines by the curves 89G, 89B and 89R respectively. It is apparent that the exposures from the two light sources should cross over at approximately 7 to 10 inches from the viewing screen center. The curves 85 and 87 do not cross over. However, in practice a smooth transistion is achieved and is thought to be aided, at least in part, by the proximity of the curves, by light scattering in the phosphor coating, and by the brightness profiles of the light spots.

Intensity compensating filters 71 have been fabricated on the wedge lens 63 to produce the desired intensity variations in the light field. The filter for the smaller light source produces a relative brightness in the light field from center to edge according to the curve 91 of FIG. 5. The filter for the larger light source produces a relative brightness in the light field from center to edge according to the curve 93 of FIG. 5.

Design data indicates that the bogie screen element sizes should be between about 14 and 15 mils in all parts of the viewing screen with the blue-emitting phosphor dots being somewhat smaller at the screen corners. The curves of FIGS. 6 and 7 show the relationship between total exposure (the sumof the two exposures on increments of the viewing screen) and screen element size for the green phosphor dots. The total exposure shown by the curves in FIG. 6 is the sum of the exposures (time in minutes times brightness in arbitrary units) from the two light sources and plotted as a function of distance from the center of the screen. Four total exposures shown by the curves A, B, C and D are plotted in FIG. 6. As exposure between curve C and curve D provides the desired screen element size distribution.

Screen element size distributions for the blue-emitting phosphor dots and red-emitting phosphor dots followed exposure characteristics generally similar to the greenemitting phosphor dots but at different total exposure levels. The exposures from the two light sources to produce a single field of screen elements blend together smoothly without sudden changes in size as shown by the measured values plotted by graphs A, B, C' and D of FIG. 7. There was normal edge definition and cross contamination for these screen structures.

Typical exposure data with a mercury arc ultraviolet lamp operated at an average 1,000 volts are given in the table. Exposure time is given in minutes. Exposure is the product of exposure time (T) in minutes times brightness (1) in arbitrary units measured at the coating.

TABLE 100 collimator 190 collimator Expo- Exposure sure Total Expotime Expotime time sure (minsure (min- (min- Color (IT) utes) (IT) utes) utes) Green- 435 3. 7 950 10. 7 14. 4 Blue 450 3. 8 800 9. 12. 8 Red 330 2. 8 773 8.7 11. 5

The novel method may be used in any process for printing a viewing-screen structure which involves the shadowing principle; that is, where the photographic master, mask or stencil is spaced from the photosensitive coating a distance q and spaced from the light source a distance p during the exposure step. In the example, the ratio of q/p is about 0.0570 at the center of the screen structure and about 0.0362 at the corners of the structure. In practice, circular apertures in a shadow mask may range in size from 6 to 14 mils, but usually do not vary more than 3 mils in any particular mask. In the example, the mask apertures are about 10.6 mils at the center of the mask and about 8. 1 mils at the corners of the mask. With a q/ p ratio about 0.0570 at the center of the mask, a p of about 14 inches, a 100-mil light source yields an R" of about 16.9 mils and a 190 light source yields an R of about 22.0 mils.

The novel method may be used with a system with circular mask apertures and light sources to yield circular screen elements as in the example. The novel method may also be used to make elliptical or rectangular screen elements, in which cases the shortest sectional dimensions are used for M, B, R and R in the relationships described above. The diameter for circular geometry or this shortest dimension for non-circular geometry is referred to as the equivalent circular diameter herein. In the case of rectangular screen elements, the light source may be rectangular with the value of M defining the short side of the rectangular screen elements. With a rectangular light source and a mask having slits that are about 55 mils by 4 mils in size and slits aligned and spaced so that the short sides are spaced about 19 mils apart and the long sides are spaced about 6 mils apart, the novel method produces screen elements that are lines with a width R.

The novel method may be used on photosensitive coatings which contain particulate material or which are free of particulate material. US. Pat. No. 3,269,838 to T. A. Saulnier discloses suitable type formulations of phosphor particles and photosensitive binders for producing the coatings usable in the novel method. The novel method may be applied to the method of making a light-absorbing matrix for a viewing screen described in U.S. Pat. No. 3,558,310 to E. E. Mayaud, which method may use a coating that is free of particles.

The novel method requires at least two exposures, one with a smaller light source having an M value of about to mils (0.080 to 0.120 inch) and the other with a larger light source having an M value of about to 200 mils (0.160 to 0.200 inch). Either exposure may occur first or last. The smaller light source should not be smaller than about 80 mils because diffraction and other interference effects degrade the quality of the elements produced. The larger light source should not be larger than about 200 mils because the penumbra produced is so large as to render the method difficult to produce well-defined, uniform screen elements with adequate adherence.

A lighthouse filter 71 may be tailor-made for each exposure according to the method disclosed in the abovecited patent application of Harry R. Frey. Preferably, filters are used on both exposures in order to provide adequate grading of screen element size from the center to the edge of the screen and also to achieve a tailored screen element size according to a design plot. The design plot may or may not be symmetrical about an axis of the screen. The grading of screen element size is thought to be aided by other factors such as (1) scattering of light from the light spot by particles in the coating and by the supporting surface and (2) by the brightness profile of the light spots particularly in the penumbra of the light spots.

I claim:

1. In a photographic method for printing a screen structure for a cathode-ray tube, said tube having a supporting surface, a screen structure on said surface and an apertured mark spaced from said screen, the steps comprising (a) depositing on said surface a coating comprised of a photosensitive binder,

(b) exposing perscribed areas of said coating through the apertures in said mask to light from a first small area light source having an equivalent circular diameter of about 0.160 to 0.200 inch,

(c) exposing said prescribed areas of said coating through the apertures in said mask to light from a second small area light source having an equivalent circular diameter of about 0.080 to 0.120 inch,

(d) and then developing said exposed coating.

2. The method defined in claim 1 wherein said first and second light sources are in substantially the same location with respect to said mask during each of said exposures.

3. The method defined in claim 1 wherein the sums of the exposures of steps (b) and (c) for equal increments of the coating are substantially equal over the central portions of said surface and increase to two to three times that sum of exposures at the edges of the screen.

4. The method defined in claim 1 wherein, for each mask aperture and its associated exposed area of coating, the ratio of the size of the developed area of the coating to the size of the exposed area of the coating is in the range of 0.80 and 1.06.

S. In a photographic method for printing a phosphor screen comprlsing (a) depositing upon a supporting surface a coating comprised of phosphor particles and a photosensitive binder therefor,

(b) exposing prescribed areas of said coating through an apertured mask to light from a light source having a given small area,

(c) and then developing said exposed coating, and wherein the differences between screen exposures required for all incremental areas of said screen are to great for a single exposing step, the improvement comprising conducting said exposing step in two stages:

(1) one stage comprising exposing said prescribed areas of said coating through said mask to light from a light source having an equivalent circular diameter of about 0.160 to 0.200 inch,

(2) and the other stage comprising exposing said prescribed areas of said coating through said mask to light from a light source having an 1 equivalent circular diameter of about 0.080 to 0.120 inch.

References Cited UNITED STATES PATENTS 7/1971 Frey 951 R 4/1971 Mueller 96--27 2/1969 Ratliff, Jr. 9636.1 10/1968 Law 9636.1 3/1971 Lange 9636.1 3/1971 Gallaro 9636'.1 10/1971 Kaplan 9636.1

U .S. Cl. X.R.

UNITED STATES PATENT OFFICE a 1 CERTIFICATE or connrcrrom Page 4 August 22, 1972 Inventor) Harry Robert Frey It is certified that error appears in. the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 4, line 22 after "Frey" insert now patent No.

- 3,592,112 issued July 13, 1971-- Column 4, line 26 change "lugs" to -legs Column 4, line 64 change "on" to in- Column 5 line before "viewing" insert -the- Column 6, line 32v change "cented" to --center Column 6, line .39 before "element" insert -elements requiring long exposure times to maintain desired phosphor- Column 7, line 1 change "As" to -An-- Column 7, line 15 after "lamp" insert ,such as a 1,000-watt BH-6 mercury vapor lamp,-

Column 8, line 5 after "3, 269 838" insert -issued August 30 Column 8, line 11 after "3,558,310" insert -issued January 26 l97lv FORM FO-1OSOHO-69) USCOMM-DC 60376-P69 r us. GOVERNMENT PRINTING OFFICE: 1969 o3ss-s34 V Patent No.

QERTIFIQATE or C August 22, 1972 Dated Inventor(s) Column 8, line Column 8, line (SEAL) Attest:

Attesting Officer EDWARD M.FLETCHER,JR.

Harry Robert Frey I It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Signed and sealed this 3rd day of April 1973.

ROBERT GOTTSCHALK Commissioner of Patents i FORM PO-105O (10-69) USCOMM-DC 603754 59 U.S. GOVERNMENT PRINTING OFFICE: 1959 O-355-33A,

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