US3767396A

US3767396A - Method of screening a color image reproducer

Info

Publication number: US3767396A
Application number: US00179921A
Authority: US
Inventors: S Kaplan
Original assignee: Zenith Radio Corp
Current assignee: Zenith Electronics LLC
Priority date: 1971-09-13
Filing date: 1971-09-13
Publication date: 1973-10-23
Anticipated expiration: 1990-10-23
Also published as: CA972606A

Abstract

The faceplate section of a tri-color cathode-ray tube of the shadow mask type is coated with a photosensitive layer and is simultaneously exposed through apertures of the shadow mask with actinic energy from six separate energy sources symmetrically located relative to a reference position which simulates the source of one of the three electron beams of the tube. For example, in screening with green phosphor, the light sources are symmetrically positioned relative to a reference position which simulates the source of the electron beam assigned to excite green phosphor in the operation of the tube.

Description

1451 Oct. 23, 1973 METHOD OF SCREENING A COLOR IMAGE REPRODUCER [75] Inventor: Sam l-l. Kaplan, Chicago, Ill.

[73] Assignee: Zenith Radio Corporation, Chicago,

Ill.

22 Filed: Sept. 13, 1971 21 Appl. No.: 179,921

Primary Examiner-Norman G. Torchin Assistant Examiner-Edward C. Kimlin Attorney-Pederson John J. and Corneluis J. OConnor [57] ABSTRACT The faceplate section of a tri-color cathode-ray tube of the shadow mask type is coated with a photosensitive layer and is simultaneously exposed through apertures of the shadow mask with actinic energy from six separate energy sources symmetrically located relative to a reference position which simulates the source of one of the three electron beams of the tube. For example, in screening with green phosphor, the light sources are symmetrically positioned relative to a reference position which simulates the source of the electron beam assigned to excite green phosphor in the operation of the tube.

8 Claims, 8 DrawingFigures PATENIEnum 2 3 ma Wang Q s m H FwQMK/Q 1 METHOD OF SCREENING A COLOR IMAGE REPRODUCER BACKGROUND OF THE INVENTION The present invention is directed to screening a color cathode-ray tube with a plurality, usually three in number, of different phosphors. While of general application, it concerns most particularly screening of tubes in which the phosphor deposits are desired to be smaller than the transparentportions of the color-selection electrode through which electron beams are permitted to impinge upon assigned ones of the various phosphor materials in the synthesizing of an image in simulated natural color.

A particular need for the subject invention is in the production of tri-color cathode-ray tubes in which the screen is a mosaic of phosphor-dot deposits defining a multiplicity of dot triads each of which includes a dot of green, a dot of red and a dot of blue phosphor. The shadow mask of such a tube has essentially circular apertures with one aperture in juxtaposition relative to each phosphor dot triad so that three electron beams generated from a gun cluster pass through the holes of the mask, as the beams are scanned by the usual deflection fields, and arrive at the screen in such directions that each beam excites only the color phosphor to which it has been assigned. In one type of tube currently' produced commercially, it is highly desirable that the phosphor dots be smaller in size than the electron beams. This is true, for example, in the so-called black surround type of tube wherein each phosphor dot is surrounded by graphite or some other light-absorbing material for enhancement of both brightness and contrast as described in U.S. Pat. No. 3,146,368, issued Aug. 25, 1964 to Joseph P. Fiore et al. and assigned to the assignee of the present invention. A similar size relation of phosphordot to electron beam is desirable for post-'deflection-focus or post-deflection-acceleration color tubes wherein the electron beams are subject to a focusing field after passing through the plane of deflection.

An attractive method for screening such tubes to achieve phosphor dots smaller in size than the apertures of the mask is described and claimed in applicants aforeidentified copending application. In accordance with the disclosure of that application, when the screen is being processed with one particular phosphor, such as green, by means of photographic printing, a photosensitive slurry coating is exposed twice, once with the exposing light source positioned to simulate e e r n g n f thelu and On -th he? posing light source positioned to simulate the blue electron gun of the tube. These exposures occur sequentially and, in effect, they expose elemental areas of the screen that totally surround each elemental area that is to receive a deposit of green phosphor. The exposure ;time is long enough that the change in solubility of the 5 coating resulting from the two exposures leaves a series of unexposed elemental areas that individually are smaller than the apertures of the mask. These unexposed areas are latent images of the green phosphor deposits and are developed in the usual way. Repeating this process for each of the three colors of the screen provides the desired screen structure with phosphor dots smaller in area than the apertures of the shadow mask. This is an acceptable screening process but is improved through the present invention by permitting the W two exposures to take place simultaneously rather than sequentially.

Another screening disclosure relevant to the subject invention is found in U.S. Pat. No. 3,152,900, issued Oct. 13, 1964 to P. E. Kaus et al. It also concerns preparing phosphor dot deposits on the screen of a color tube with an area that is less than the area of the apertures in the shadow mask through which photographic screening takes place. In achieving its objective, the process of the patent utilizes a ring or annular type light source in conjunction with a positive resist by which is meant a photosensitive material which is rendered solu- :ble in a given solvent in response to exposure to actinic energy, which usually is ultraviolet light. While the disiclosure permits photographically printing phosphor dots of the desired dimension relative to the cross section of the electron beams of the tube, it is confined to forming dots of circular configuration. In contradistinction, both the process of applicants aforementioned copending application and that described herein gphosphor depos it is approximately 10 percent larger in area than the circular dot of Kaus et al. with the TPQlfiHfiidiFiQLlbfiBWPQfi? the eme -H i Accordingly, it is an object of the invention to pro- .vide a novel method of coating the screen of a color image reproducer, such as a cathode-ray tube, with a ipattern of at least three different phosphor materials.

It is a further object of the invention to improve photoprinting of color'cathode-ray tube screens by multi- 4 ple simultaneous exposures of elemental screen areas.

It is a particular object of the invention to provide a Q'screening method to attain elemental phosphor deposits smaller in size than the transparent portions of the fcolor-selection electrode in much the same fashion as the above-mentioned copending application but featuring simultaneous as distinguished from sequential exposures from a plurality of energy sources.

More specifically, it is an object of the invention to' provide a novel method of screening a shadow mask type of color tube with phosphor dots of essentially hexagonal configuration and smaller in area than the apertures of the shadow mask.

The method of the invention in its broadest aspect is for coating the screen of a color image reproducer with at least three different phosphor materials arranged in an interlaced pattern with a deposit of any one phosphor surrounded by like deposits of the remaining phosphors for selective energization by at least three electron beams having access to the phosphor deposits through transparent portions of a color-selection electrode, such as a shadow mask. The method comprises the step of forming over the screen a layer of a material having a surface characteristic that is subject to change in response to impingement by actinic energy. Thereafter elemental areas of the layer are simultaneously exposed through transparent portions of the colorselection electrode with actinic energy from an even number of separate energy sources symmetrically located relative to a reference position which simulates the source of the one of the electron beams of the tube to create in the layer a latent image. Finally, that latent image is developed.

In one aspect of the invention for screening a tricolor tube having a shadow mask with circular apertures, a series of six light sources are symmetrically positioned about a reference point simulating the position of the electron gun assigned to excite the phosphor in process. Simultaneous energization of the six light sources exposes six elemental areas of the screen which, in processing green phosphor by way of illustration, constitute the areas that are to receive red and blue phosphors. These surrounding elemental areas completely encircle an area assigned to receive green phosphor and conjointly reduce the unexposed elemental area in size so that it is smaller than the apertures of the shadow mask.

DESCRIPTION OF THE DRAWINGS The features of the present invention which are believed to be novel are set forth with particularity in the appended claims. The invention, together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawings, in the several figures of which like reference numerals identify like elements, and in which:

FIG. 1 is an enlarged showing of a fragmentary portion of a color tube screen;

FIG. 2 is a schematic representation ofa screen in an exposure position;

FIGS. 3-5 are illustrative exposure diagrams;

FIG. 6 represents an exposure light source;

FIG. 7 shows a fragment of an aperture mask; and

FIG. 8 is a light source arrangement for exposing a screen through a mask of the type shown in FIG. 7.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now more particularly to FIG. 1, the arrangement there represented is an enlarged fragmentary portion of the screen of a tri-color cathode-ray tube. It is formed on the faceplate or cap portion of the envelope which is normally separate from the remaining part of the envelope structure to facilitate screening. The faceplate is processed by first forming over the screen a layer or coating of a material having a surface characteristic that is subject to change in response to impingement by actinic energy. A choice is available, depending on the nature of the screening process to be employed. In one process the screen is coated with a photosensitive material or resist having a solubility in a solvent that is changed by exposure to actinic energy. In another, the screen is coated with a conductive layer and then with a superposed photoconductive layer which is uniformly charged and thereafter selectively discharged in response to the impingement of light. For convenience, let it be assumed initially that the layer applied to the cleansed faceplate as the first step of the screening process is a photosensitive material. These materials are characterized as either positive or negative. The positive type has the characteristic that is rendered soluble in a solvent upon exposure to ultraviolet light whereas otherwise the material is insoluble. The negative type, on the other hand, is normally soluble in a particular solvent but is rendered insoluble in response to such exposure. The process under consideration may be advantageously utilized with either type material but, again for convenience, it will be assumed that the coating layer is a positive photosensitive resist, such as one of AZ resists available from the Shipley Company of Newton, Massachusetts. It is also convenient to consider that the photosensitive coating material includes a phosphor in particulate form as an ingredient. Since the screen is to be coated with green, blue and red phosphors, let it be further assumed that the green phosphor is to be applied first and is an ingredient of the photoresist coating.

After the entire screen area has been covered with a layer of such material and after that layer has been permitted to dry, the screen is exposed through transparent portions of the color-selection electrode. In other words, the screen is exposed by directing actinic energy to the coating layer through the apertures of the shadow mask. For the assumed conditions it is necessary to expose the entirety of the screen except the elemental areas thereof assigned to the particular phosphor material in process, green for the case at hand. In other words, the exposure step requires that the elemental areas of the screen assigned to receive blue and the elemental screen areas assigned to red be exposed, leaving unexposed only the set of elemental screen areas intended to receive green phosphor.

In practicing the prior art, specifically that of the aforementioned copending application, this exposure is accomplished in two steps in conventional exposure chambers or lighthouses having a small area or point source of ultraviolet light and a suitable collimator by means of which the light is directed to the screen through the apertures of the shadow mask. In the first exposure step, the light source is positioned to simulate the electron gun of the tube assigned to excite the blue phosphor which results in exposure of all elemental areas of the screen that are to receive a deposit of blue phosphor. In a similar but second exposure step, the light source is positioned to simulate the electron gun assigned to excite the red phosphor so that all elemental areas of the screen to receive red phosphor are exposed.

The conditions of the screen at this juncture will be readily understood by reference to FIG. 1. Each of the circles designated B represents an image projected on the coated screen with the light source simulating the beam assigned to excite the deposits of blue phosphor. Similarly, the circles designated R indicate the images projected on the screen through the shadow mask with the light source simulating the gun assigned to energize the red phosphor. It will be observed that each circle or elemental screen area designated G is completely surrounded by a series of six similar elemental areas alternately designated R and B. The dash-dot circles around each of these latter areas represent that the exposed elemental areas R and B, giving due regard to the penumbra effects of the light source, which are suffciently large if the exposure time is adequately long to partially overlap the elemental area G that the exposed areas R & B surround. Collectively, they cause the nonoverlapped portion of the surrounded elemental area G to have a hexagonal configuration with the hexagon enscribed within the circle G. In short, the described sequential exposure steps leave unexposed cusp-shaped areas G which are smaller in size than the apertures of the shadow mask. The mask apertures are essentially the same size as the cross-hatched circles R and B. All elemental areas of the screen designated G concurrently receive a similar treatment, that is to say, a set of cusp-shaped hexagonal areas G distributed over the screen area constitute the unexposed elemental areas and represent a latent image of the set of elemental areas to be developed in green phosphor. This is true even though the drawing has been simplified to particularize a showing of the unexposed cusp hexagon G for only a single elemental screen area located centrally of the fragmentary portion illustrated in FIG. 1. If the composition of the coating area be Shipley resist AZlll in the amount of 75 per cent, having green phosphor suspended therein in the amount of 25 per cent, an expos'ure interval in the order of 8 minutes will produce the desired small sized cusp-shaped hexagonal green phosphor dots.

The next step of the process is to develop in the green phosphor the unexposed elemental areas G. This is done simply by washing the screen in a solvent for the photosensitive coating which removes the exposed areas B and R because they are soluble in the solvent whereas the unexposed portions G are insoluble. The process as thus far described is the same as that of the applicants copending application and in like fashion deposits of blue and red phosphors, smaller in size than the apertures of the shadow mask may be applied to the screen. It should be noted in passing that, after the application of the first two sets of phosphor dots, each such set should be treated so that its resist component is rendered insensitive to actinic energy lest one set of phosphor dots be destroyed in processing the next succeeding set. For example, after developing the green phosphor dots, the screen is coated with a photosensitive resist including blue phosphor and is then exposed from the green and red positions to create latent images of the blue dots. If the previously applied green phosphor dots retain their photosensitivity, they will wash off in developing the blue dots. This may be avoided by treating the green phosphor dots with a desensitizing chemical or protecting them with a barrier layer before laying down the blue phosphor slurry. A resin that is not soluble in the solvent of the slurry resist is suitable; nitrocellulose is an acceptable material to use for the barrier layer.

The significant processing change introduced by the subject invention is that the multiple exposures employed in developing a single phosphor are made simultaneously from at least two, but preferably more, separate sources of actinic energy. In order to permit such simultaneous exposure, it is necessary to position the energy sources symmetrically relative to a reference position which simulates the source of the gun assigned to excite the phosphor in process. This is a distinct departure from prior practices as will be readily understood by reference to the schematic diagram of FIG. 2 which illustrates the more conventional setup of the exposure chamber. The subassembly of a screen 10 with its shadow mask 11 installed in position is supported on the lighthouse for exposure from a light source L which is usually a mercury arc lamp or other ultraviolet light generator. In the conventional lighthouse source L is positioned off the axis of the lighthouse by a distance S and at a distance p from the mask which, in turn, is spaced from the screen by an amount q. The spacing S is chosen so that the light source L simulates the electron gun assigned to excite the particular phosphor that is being applied. This is the appropriate position for source L if the phosphor is included as an ingredient of a photosensitive layer of the negative type. When the photosensitive material is positive and phosphor depos-' its smaller in area than the mask apertures are desired, one exposes the red and blue elemental areas in processing green phosphor, as described above, rather than exposing the set of green elemental areas. One approach that suggests itself is indicated in FIG. 3 where the triangle interconnects centers of one phosphortriad and where light sources L and L are positioned in the lighthouse to project ultraviolet light on the elemental areasB and R intended to receive blue and red phosphors. This lighthouse arrangement indeed exposes the six elemental areas clustered about the single elemental area G to receive green phosphor and would appear on the surface to accomplish the objective of simultaneous exposure. A practical difficulty however is encountered, one attributable to the geometry of the tube in question. Because of such things as the variation in q distance with deflection angle there is a tendency to what is known as a degrouping effect of the photographically printed phosphor triads particularly at the edges of the screen. In conventional exposure techniques, processing phosphor in a negative resist, such degrouping effects are minimized by the use of a correction lens interposed in the optical path between the light source and the shadow mask but each such lens is tailored to one particular position of the light source in the lighthouse. For thiS reason, a correction lens for exposing blue phosphor areas is inappropriate for use with respect to a light source positioned to expose red phosphor areas. Applicant has discovered that through a different physical arrangement of a plurality of separate light sources, simultaneous exposures from all such sources can indeed take place. One arrangement, employing only two light sources, is represented schematically in FIG. 4 wherein one light source is positioned to exposed elemental screen areas B outlined in dash-dot construction lines, while the other is positioned to expose elemental screen areasR outlined in broken-construction lines. A projection of the point light sources is designated L and L which, for this arrangement are symmetrically positioned relative to the reference position which simulates the green electron gun. With respect to FIG. 4, this reference position may be considered to be the center of cusp-shaped hexagon G. With the light sources thus positioned, a single correcting lens appropriate for utilization with a light source positioned to simulate the green electron gun is equally efficacious for two light sources positioned in the manner indicated in FIG. 4. Accordingly, the light sources may be energized together to effect simultaneous exposure of the setof elemental areas R assigned to red phosphor and the set of elemental areas 8' assigned to blue phosphor. Aside from this modification having to do with the position of the two light sources L and L and the fact of simultaneous multiple exposures, the process is the same as described and claimed in the above-identified copending application.

The inventive process is not confined to the use of two light sources, such as L, and L an advantage is realized by using a larger number although there should be an even number of light sources symmetrically positioned relative to the simulated reference position of the green gun, again, assuming for convenience that the green phosphor is being processed. A preferred arrangement employs six light sources and establishes the exposure pattern of FIG. 5 where the designations L -L represent projections of the exposing light sources. All are symmetrical about the simulated position of the green gun (the center of hexagon G) with the odd numbered sources positioned to expose elemental areas B assigned to blue and the even numbered sources positioned to expose elemental areas R assigned to red.

In FIG. 5 each of the exposed elemental areas B and R is enclosed in three concentric circles represented respectively by a full construction line, a dash construction line, and a dash-dot construction line. This signifies that each of the six elemental areas for the illustrated cluster represented in FIG. 5 does, in fact, receive three superimposed or simultaneous exposures which may have the added benefit of a much faster exposure rate. The reason that each such elemental area receives three such exposures will be understood by further reference to FIG. 1.

The elemental area designated B as explained above and as is evident from the crosshatching, is included in a group of six similarly crosshatched elemental areas completely surrounding elemental area 6,. Inspection of the figure shows that this same elemental area B is included in a like cluster of six surrounding elemental area G and is further included in a third such cluster surrounding elemental area G All three of these clusters are exposed at the same time from which it is clear that elemental area B, is subjected simultaneously to three superimposed or registered exposures. The same is true of all elemental areas B and all elemental areas R during the processing of the green phosphor.

As a practical matter it is not necessary to have physically separate sources of light or actinic energy; they may be derived from a common energy source in the manner of FIG. 6 which shows the usual ultraviolet lamp backed by a reflector 21 and distributing light through a diffusion plate 22 to a series of holes or transparent areas 23 provided in a plate 24 located between lamp 20 and the aperture mask. The number of transparent elements 23 corresponds to the number of simulated separate light sources desired. As stated, there should be an even number which means that it is convenient to employ two or four, although six is preferred. Where six are used, the transparent portions 23 must be provided to satisfy the geometry requirements described in relation to FIG. 5 in the discussion of light sources Is -L If desired, at each location 23 of plate 24 there may be a diffusion tip to achieve a better distribution of light especially at the edge portions of the screen. Of course, the pattern of light sources must properly relate to the aperture pattern of the mask especially if the latter is arranged to minimize moire.

It is known that moire effects may be experienced when scanning a shadow mask color tube and in order to minimize moire it is conventional practice to arrange the apertures of the mask to define a series of regular hexagonal patterns individually having a major dimension extending in the vertical direction in the manner represented in FIG. 7. This figure shows a small portion of the shadow mask including seven apertures in which the center to center spacing of adjacent holes is designated W. If the center aperture is ignored, the remaining six define a regular hexagon and its greatest dimension is disposed vertically. The appropriate arrangement of the six-point exposure system for an aperture mask having this aperture array is indicated in FIG. 8. In this figure, the broken-line circle G at the center denotes the place where the point light source is located for conventional screening. It is spaced a distance S from the axis, as explained in discussing FIG. 2, and simulates the green electron gun. For the case at hand, however, no light source is used at that location; instead a series of six-light source are arrayed in a regular hexagonal pattern with the light sources positioned at each apex. The separation between light sources is given on the drawing in terms of the spacing S which, in turn, is related to the spacing W of the mask apertures as follows:

The major dimension of the hexagonal array of light sources is disposed horizontally, that is to say, in the direction of horizontal scan and has a value of 2 3 times S.

Use of the preferred array of six-light sources results in three simultaneous exposures of each exposed elemental area of the screen as described in relation to FIG. 5. There is an advantage here in that each such area is simultaneously exposed through three apertures of the mask so that if one of the three has an out-ofround imperfection the effect of the defect tends to be suppressed by exposure through other apertures admitting light to the same elemental area of the screen. An even greater benefit of this nature is described and claimed in a concurrently filed application Ser. No. 179,920 of William Rowe et al assigned to the assignee of the present invention. In Rowe et al there may be as many as seven, and even more, simultaneous exposures of selected elemental areas of the screen in process.

It has been stated above that the screen may be printed photographically utilizing photosensitive coating materials or, alternatively, it may be printed electrostatically. The processes are similar in that exposure of the coated substrate through the shadow mask causes a change in a surface characteristic to create a latent image of a pattern of elemental areas that is subsequently developed. Utilizing photosensitive resists causes the surface phenomenon to be a change in solubility of the layer coating the substrate, whereas in electrostatic printing the change in surface characteristic is a change in a charge pattern. It is not necessary to burden this disclosure with the details of electrostatic screening because it is described in U.S. Pat. No. 3,475,169, issued Oct. 28, 1969 in the name of Howard G. Lange and assigned to the assignee of the present invention.

While particular embodiments of the invention have been shown and described, it will be obvious to those skilled in the art that changes and modifications may be made without departing from the invention in its broader aspects and, therefore, the aim in the appended claims is to cover all such changes and modifications as fall within the true spirit and scope of the invention.

I claim: 1. In the manufacture of shadow mask-type color cathode ray tubes having a phosphor screen on an inside surface of a faceplate for the tube including interlaced patterns of red, blue and green phosphor elements, in the formation of a first of said patterns of phosphor elements, a method comprising:

depositing on the faceplate a coating including phosphor material and a positive photoresist material;

supporting adjacent the faceplate a color selection electrode having a pattern of apertures corresponding in geometry to the desired patterns of phosphor elements;

forming on the coating a negative image of said first pattern of phosphor elements by exposing the coating through said color selection electrode to one or more pairs of sources of radiation actinic to said coating disposed in a diagonal relationship about a reference position corresponding to the first order color centerlocation for said first pattern of phosphor elements, one source of said pair or pairs being disposed at a first order color center location associated with a second of said patterns of phosphor elements and the other source of said pair or pairs being disposed at a second order color center location associated with the third of said patterns; and

developing said coating to produce a pattern of unexposed photoresist areas corresponding to the desired first pattern of said phosphor elements.

2. The method defined by claim 1 wherein the exposure of said coating through said color selection electrode is accomplished using three simultaneously activated, diagonally related pairs of sources arranged in a hexagonal array centered about said reference posi- 10 tion.

3. The method defined by claim 2 wherein the time of exposure of said coating to said three pairs of sources is such that upon development of said coating said areas of said pattern of unexposed coating areas are smaller than said apertures in said color selection electrode.

4. The method in accordance with claim 2 in which the apertures of said color-selection electrode define a series of regular hexagonal patterns having a major dimension in the vertical direction,

and in which said sources are positioned at selected apices of a similar regular hexagonal pattern centered on said reference position but with its major dimension extending in the horizontal direction.

5. The method in accordance with claim 4 in which one of said sources is positioned at each apex of said hexagonal pattern centered on said reference position.

6. The method in accordance with claim 5 in which the center-to-center spacing of adjacent holes in said color-selection electrode is W,

and in which the spacing of adjacent apices of said hexagonal array centered on said reference position is approximately 3 times S, where S is related to the distance p of the light source from said electrode, to the spacing q of said electrode from said screen and to W as follows:

than said transparent portions of said electrode.

Claims

2. The method defined by claim 1 wherein the exposure of said coating through said color selection electrode is accomplished using three simultaneously activated, diagonally related pairs of sources arranged in a hexagonal array centered about said reference position.
3. The method defined by claim 2 wherein the time of exposure of said coating to said three pairs of sources is such that upon development of said coating said areas of said pattern of unexposed coating areas are smaller than said apertures in said color selection electrode.
4. The method in accordance with claim 2 in which the apertures of said color-selection electrode define a series of regular hexagonal patterns having a major dimension in the vertical direction, and in which said sources are positioned at selected apices of a similar regular hexagonal pattern centered on said reference position but with its major dimension extending in the horizontal direction.
5. The method in accordance with claim 4 in which one of said sources is positioned at each apex of said hexagonal pattern centered on said reference position.
6. The method in accordance with claim 5 in which the center-to-center spacing of adjacent holes in said color-selection electrode is W, and in which the spacing of adjacent apices of said hexagonal array centered on said reference position is approximately Square Root 3 times S, where S is related to the distance p of the light source from said electrode, to the spacing q of said electrode from said screen and to W as follows: 3 Sq (p + q) W.
7. The method in accordance with claim 6 in which said major dimension of said hexagonal pattern of energy sources is equal to 2 Square Root 3 times S.
8. The method in accordance with claim 2 in which the exposure interval is of such duration that the portions of said layer that experience a change in surface characteristic overstep one another and reduce said elemental areas of said third set to a size smaller in area than said transparent portions of said electrode.