US3198634A - Method of depositing particulate solid material on selected portions of a substrate - Google Patents

Description

Aug. 3, 1965 P. D. PAYNE, JR

METHOD OF DEPOSITING PARTICULATE SOLID MATERIAL ON SELECTED PORTIONS OF A SUBSTRATE Flled June 25, 1956 IN VEN TOR. PHI/L a PflX/VE, Jk.

United States Patent 3,198,634 METHGD (BF DEPGEiKTlNG PARTICULATE 891.1% MATEREAL UN SELEtITEB PQRTEGNS OF A SUBSTRATE Paul D. Payne, in, Chalfont, Pa, assignor to Philco Corporation, Philadelphia, Pa, a corporation of Pennsylvania Filed Jinn-e 25, 1956, Ser. No. 593,682 14. Claims. or. 9635) This invention relates to methods for making screens for cathode ray tubes and in particular to methods for producing improved fluorescent screens for television image-reproducing tubes.

Cathode ray tubes presently employed for the reproduction of images of scenes which have been televised in color are desirably constructed so that their screens produce a light output of maximum brightness in response to the impingement of an electron beam thereupon. It is desired to obtain a maximum brightness for several reasons among which may be enumerated the following. First, when a color television image is bright, the desaturating influence of ambient light and halation upon the component colors of the image is less evident. Second, when the image is bright there is no need for darkening of the viewing area to permit convenient viewing of the tube face. In fact it is believed that, if the viewing areas is made so dark as to present a sharp contrast to the brightness of the image, the combined elfect is harmful to human vision. constructing color television receivers of the projection type since much of the light of the image on the cathode ray tube may be lost in transmission through the optical components of the projection system.

It is very diflicult to obtain images of the desired brightness when certain types of color television display tubes, such as tubes of the well-known so-called aperture mask type, are used. These tubes are very inefiicient since most of the electrons in the three electron beams produced therein never reach the fluorescent screen because a perforated mask of metal or other appropriate material is interposed between the three electron guns and the screen. The screen contains phosphor dots emissive of the three primary colors and the perforations of the mask are so aligned with the dots that they assist in insuring that each of the electron beams strikes only phosphor dots emissive of one of the three primaries. In so doing, however, the aperture mask prevents approximately 85% of the electrons emitted from the electron guns from ever reaching the fluorescent screen so that the brightness of the image produced is limited by the number of electrons which actually impinge on the phosphor dots. The brightness of the image is also a function of the density of the phosphor material in the phosphor dots which are actually struck by electrons in the beam.

In color television display tubes other than tubes of the aperture mask type it is also desirable to increase the brightness of the images produced, as, for example, in a tube of the type in which a plurality of sets of strips of phosphor materials respectively emissive of the three additive primary color are disposed on the internal face plate of the tube. In this type of tube a single writing beam may be produced which may be deflected in scanning paths either parallel to, or transverse to the direction in which the phosphor strips individually extend. The single beam is modulated in intensity by signals which represent colors emitted by the strip on which the beam is scanning at any given instant. tubes of this general construction in which substantially all of the electrons emittedfrom the cathode strike the screen, to achieve a maximum image brightness for man of the reasons previously given. a

Third, a bright image is especially helpful in It is desirable, even in Maximum image brightness is sought for in still other types of cathode ray tubes, such as ones containing a screen which comprises a number of sets of mutually interleaved phosphor strips and a plurality of spaced electrodes intermediate the screen and an electron gun, said electrodes being arranged so that they are parallel to the phosphor strips. A single writing beamis deflected over the screen thereof in a plurality of scanning paths which are usually substantially parallel to the spaced electrodes. The beam is additionally deflected transversely to the scanning paths by potentials applied to the latter electrodes which cause the beam to impinge on selected ones of the phosphor strips at selected intervals.

In the above-mentioned types of image-reproducing tubes, as well as in other single or plural beam varieties of display devices, given a certain value of beam current an increase in the density of the phosphors on the screens thereof would materially assist in producing an optimum light output therefrom. One known way of achieving.

greater phosphor density is to form the screen by depositing each set of phosphor elements in several layers; Whenever greater phosphor density is achieved by resort' of phosphors for increasing phosphor density. One of these methods involves photographic processes in which photosensitive coatings or emulsions are exposed to the desired light pattern, the desired phosphor material is next deposited on the photographic emulsion, and then the unexposed portions of the phosphor material, together with the phosphor particles adhering thereto, are washed away, whereupon the process is repeated for the second coating. A more complete description of such a process is contained in my copending US. application, Serial No. 376,345, filed August 25, 1953, now US. Patent No. 2,950,193. In order to wash the unexposed portions away, it has hitherto been customary to remove the screen from the position in which it was exposed to the particular light pattern involved. After the first washing, the screen is recoated with the photosensitive material, dried, and then returned to its original position for re-exposure to the same light pattern. Accuracy and precision of a very high order are required in order to reposition the screen of the tube,

on which the first of two layers of phosphor materials has been deposited, exactly in the same position for exposure prior to the application of the second layer of phosphors.

If the light pattern projected upon the photosensitive emulsion preliminary to the application of the second layer of phosphors is not in exact register with the phosphor pattern produced by the first application, the phosphor strips produced may have irregular widths.

Since the multiple application of phosphors as a means of producing screens possessing greater phosphor density in costly and demands a high degree of precision, new

approaches to the problem of increasing phosphor density have been attempted. Before embarking upon these new approaches a study of the features of the previous method and materials used therein was made. In particular, a study was made of the process of depositing phosphors and other screen elements in which a light pattern is projected, from the rear, onto a photosensitive layer containing ammonium dichromate as the photosensitizing 7 agent.

Patented Aug. 3, 1965 In order to obtain the best results possible with a photosensitive layer containing ammonium dichromate, it is important, first, that the layer should adhere very well to the internal surface of the particular substrate used, as for example, to the internal surface of the glass faceplate of a cathode ray tube. Unless there is good adhesion of the photosensitive material to the substrate, there is a possibility that, when the unexposed portions thereof are washed away together with the phosphor material deposited thereupon in subsequent washing operations, some of the exposed portions of the :hotosensitive material and the phosphor material in contact therewith may also be washed away. Good adhesion of the photo sensitive material to the substrate is a function of the hardness of the photosensitive material when fixed by exposure thereof to the form of illumination to which the material is sensitive.

Another desideratum in using such a photosensitive material is that, when the photosensitive material is deposited upon the substrate, the hardness of selected portions of the rear surface thereof (i.e., the surface closest to the light source) be such, after exposure, that the maximum number of phosphor particles adhere thereto. This condition is attained when the rear surface of th photosensitive material is somewhat softer than the front surface after exposure.

- It is extremely difiicult, however, when using photosensitizing materials which contain ammonium dichromate as the photosensitizing agent, to produce on the substrate a layer of the photosensitive material whose front surface, after exposure, is rendered relatively hard and whose rear surface is rendered somewhat softer than the front surface thereof, but which is rendered harder and less soluble than the portions of the layer which are not exposed.

One reason for this difiiculty is that the ammonium dichromate is somewhat orange-yellow in color and the layer of photosensitive material becomes colored thereby. Therefore, when the photosensitive layer is exposed from the rear the particles of ammonium dichromate near the rear surface of the photosensitive layer act like a number of minute orange-yellow filters which absorb a large proportion of light in the ultra-violet region, i.e., light having a Wave length of about 3,650 angstroms, this being light of the region to which the layer is mainly responsive. Thus only a small amount of ultraviolet light is transmitted to the front surface of the photosensitive layer. As a result the front surface of the layer is rendered semi-hard because it is relatively underexposed whereas the rear surface is rendered very hard because it is relatively overexposed. This is a condition which is just the opposite of the one desired. Consequently the front part of the photosensitive layer does not adhere to the substrate so Well and there is a possibility that it and the phosphor material later deposited in contact therewith may be dislodged from the substrate in subsequent washing operations. On the other hand, since the rear surface of the exposed portions of the photosensitive ,layer has been made hard by exposure, fewer phosphor particles will adhere thereto than if it were soft. If increased phosphor density is desired, the customary technique of multiple phosphor applications with its attendant disadvantages may then be resorted to.

Accordingly, it is a primary object of my invention to provide an improved method of manufacturing screen structures for cathode ray tubes.

It is another object of my invention to provide an improved method of manufacturing the screen structures of cathode ray tubes used for reproducing images televised in color.

Still another object of my invention is to provide a method for manufacturing improved screen structures for obtaining brighter images in color televsion display tubes.

Another aim of the invention is to reduce the cost of manufacturing screen structures-for cathode ray tubes used for reproducing images of scenes televised in color.

A further object of my-invention is to provide a method for manufacturing screen structures for cathode ray tubes for color television receivers in which elements previously formed are not washed away in subsequent steps of the manufacturing process.

My invention is based on the realization that, if the rear surface region of the photosensitive layer is made somewhat less photosensitive prior to exposure than the front surface region, the rear surface region will be rendered less hard than the front surface region when exposed. In order to produce a gradient of photosensitivity throughout the photosensitive layer, I have taken advantage of the fact that the photosensitivity of any portion of the layer depends on the concentration of photosensitive particles therein.

Accordingly the layer is so prepared prior to exposure that there is a greater concentration of the photosensitizing substance toward the front surface than toward the rear surface thereof. When the photosensitive substance is so distributed, the front surface area receives and absorbs more of the actinic rays from the source of illumination than has hitherto been possible so that it becomes fixed to a fairly high degree of hardness, and thus its adhesion to the substrate is enhanced. Since there are fewer particles of the photosensitizing substance toward the rear surface of the layer, fewer of the actinic rays will be absorbed therein and the rear surface area will not be fixed to the same degree of hardness as is the front surface area. As a result, the rear surface area is rendered relatively soft so that, after exposure, more phosphor particles can adhere thereto.

I11 order to produce a photosensitive layer having the -esired gradient of photo-sensitivity according to my invention the photosensitive layer is deposited, prior to exposure, on the substrate as has previously been customary. Then. a solvent for the photosensitizing substance of the photosensitive layer is applied to the rear surface of the layer. This solvent causes the particles of the photosensitizing substance near the rear surface to enter into solution. The applied solvent, together with the particles of the photosensitizing substance dissolved therein, is then drawn off. When the solvent has been drawn off, the remaining photosensitive layer has a greater concentration of photosensitizing substance near the front surface than near the rear surface. Next the prepared layer is exposed to the desired pattern of light, then coated with selected phosphor or other materials, and finally washed in the customary manner. It has been found that, by employing the process according to my invention, the density of the phosphor material in the phosphor elements deposited in one application approximates that achieved by convention- 211 double application techniques. It is thus possible to produce at a smaller cost a cathode ray tube whose screen will produce images of the desired brightness.

FIGURE 1 is a schematic and perspective view of a rear-projection photographic deposition system in which my invention may be used; and

FIGURES 2A to 2F inclusive are a six-pant schematic and sectional representation of the steps of a photographic deposition process which embodies my invention.

Solely for convenience in exposition, the invention will be explained in connection with a color television cathode ray tube of a type previously mentioned, i.e., one having a number of strips of phosphor materials emissive of the additive primary colors, i.e., red, green and blue, arrayed in a spaced-apart and substantially parallel relation on the faceplate of the tube. This type of screen may be scanned by a single Writing electron beam which is modulated in intensity by signals corresponding to the color of the element impinged upon. In some forms another electron beam having low current may be provided which is deflected in unison with the writing beam and which is caused to impinge upon portions of the screen having secondary-emission ratios, for example, which differ from the secondary-emission ratios of other portions thereof. As a result, so-called indexing signals are generated which are used to coordinate the modulation of the writing beam with the position thereof. The construction of such a tube is described in more detail in the US. Patent No. 2,725,421 issued November 29, 1955 to S. F. Valdes.

In order to produce the phosphor strips of such a tube on the faceplate, a rear-projection photographic deposition method, such as described in the co-pending U.S. application of M. Sadowsky and S. Parsons, Serial No. 488,219, filed Februray 4, 1954, now U.S. Patent No. 2,870,010 is often used. In this method, which is illustrated in FIGURE 1, the inner surface of the faceplate 11 of a cathode ray tube is coated with a layer 18 of a photosensitive material. A photosensitive layer which has proved very useful in this process is one produced by depositing on the faceplate 11 a solution containing ninety parts by volume of a stock solution comprising 600 cc. of water, 200 cc. of a denatured alcohol such as that sold under the trademark Solox, grams of a polyvinyl alcohol such as that sold under the trademark Elvanol 52-22, and 1% grams of ammonium dichromate to which ten additional parts by volume of Solox are added. The ammonium dichromate is the photosensitizing substance in this solution and impart-s an orange-yellow color thereto. This solution is flowed onto the inner surface of the faceplate 11 and is dried at room temperature unit it be comes a layer 18 as shown in FIG. 2A resembling dried sheet gelatine in its consistency. When the solution is dried the sensitizing substance, ammonium dichromate, is distributed uniformly throughout the layer 18 in the form of a number of minute particles 19. When the ammonium dichromate particles are struck by actinic ray, i.e., rays of light in the ultra-violet region of the spectrum (around 3960 angstroms), the ammonium dichromate is broken down into compounds which include chromium oxides which cause the portions of the layer in proximity thereto to become hard and relatively insoluble in water.

In accordance with the present invention the concentration of ammonium dichromate particles 19 toward the rear surface of the photosensitive layer 18 is made less dense than elsewhere in the layer by flowing onto the rear surface of the dried photosensitive layer 18 additional amounts of the aforementioned Solox. The minute particles 19 of ammonium dichromate within the layer are dissolved by the additional Solox, whereas the polyvinyl alcohol component of the layer adsorbs some of the additional Solox and is swollen somewhat thereby. The excess Solox and the ammonium dichromate dissolved therein is then drawn oif. The photosensitive layer is then dried so that the Solox, which has swollen the polyvinyl alcohol component of the layer, is evaporated thereform. The photosensitive layer 18 now has a greater concentration of the photosensitizing ammonium dichromate particles 19 near the surface thereof closer to the substrate than it does near the rear surface of the layer 18 as shown in FIG. 2B.

The layer 18 has now been prepared for the conventional steps of exposure and phosphor deposition such as have hitherto been known in the art. A source of light, indicated schematically (in FIG. 1) at the numeral 12, provides light rays which pass through the grill 13 which has a plurality of alternately opaque andtransparent sections 14 and 16 respectively. Light rays transmitted through the transparent sections 16 are focused upon the prepared layer 18 by an optical system indicated at the numeral 15. When the image of the grill 13 is projected on the prepared photosensitive layer 18, those portions of the latter on which light transmitted through the transparent portions 16 falls, such as the portion 20 shown in FIG. 2C, become fixed or harder than the other portions thereof which are unexposed such as the portions 21. In the exposed portion 24) there is a greater concentration of the photosensitizing particles 19 closer to the faceplace 11 than there is toward the rear surface so that the front region of the portion 211 will be rendered harder than the rear region thereof.

After exposure, a slurry of a desired phosphor material 22 is flowed onto the rear surface of both the eX- posed and unexposed portions 20 and 21 of the photosensitive layer 18 as shown in FIG. 2D. This slurry is then dried and the screen is then washed with water. It is characteristic of the photosensitive layer 18 that the portions 21 thereof which have not been exposed to light are very soluble in water whereas the portions 20 which have been are relatively insoluble therein. Therefore, when subjected to the washing action of water, the unexposed portions 21 of the layer 18, together with the portions of the phosphor 22 in contact therewith, will be washed away leaving only the exposed portions 20 and the portions of the phosphor in contactftherewith as shown in'FIG. 2E. 7

If it is desired to lay down phosphor strips emissive of other colors in the spaces formerly occupied by the unexposed portions 21 of the layer 18, the foregoing process is repeated, i.e., a photosensitive layer is first applied to the substrate and to' rear surface of the exposed portions 20, then the layer is dried, then additional Solox is applied and drawn off to remove part of the ammonium dichromate toward the rear surface of the new layer of photosensitive material, the prepared photosensitive layer is again dried and then exposed to the desired pattern of light rays, the additional phosphor material is deposited on the rear surface of the prepared photosensitive layer, the phosphor slurry dried and the screen subjected once again to a wash in water. As many different sets of phosphor materials as is desired may be deposited in the preceding manner to form the screen. 'When'all the elements of the screen have been deposited in this manner, the residual layer 18 is removed, usually by subjecting the screen to a high heat in a baking out operation leaving the phosphor strips in contact with the faceplate as shown in FIG. 2F.

It has been found that, when photosensitive layers prepared in accordance with my invention are used in making screens, much greater phosphor density per given unit of area of the phosphor element is obtained. In fact, phosphor densities comparable to those obtained by the customary double application procedure have been attained. Previously, as many as twenty-four different steps were required (prior to baking-out the screen) to lay down three different sets of phosphor elements on the faceplate, each set of phosphor elements being deposited twice. By using the process according to my invention as described herein, three sets of phosphor elements may be deposited on the faceplate in approximately fifteen steps rather than the approximately twenty-five steps required to arrive at the same point in the manufacture of this screen according to the prior method. The elimination of nine steps helps to cut down the cost of production of color television tubes employing such screens. Also, since only half as many layers of photosensitive materials are used there is less waste of this material. Furthermore, less phosphor material is wasted since the excess phosphor material is washed away only three times rather than six times as in the previous process.

-The invention is also applicable to the deposition of materials other than phosphors. It has been stated above that, in tubes such as those having phosphor strips arranged as shown in FIG. 1, the screen thereof may also contain portions having different secondary-emission responses to the impingement of the indexing beam thereupon. The portions having a greater secondary-emission ratio may consist of vertical strips of a material such as MgO, for example, which are arranged in a predetermined spatial relation to selected ones of the phosphor strips. The density of the indexing strips is one factor in determining the aplitude of the indexing signals produced.

amass-s It is often desirable to make the amplitude of the indexing signals large so as to facilitate detection of them. By using the process according to my invention, a single application of the MgO will usually suffice to produce the desired density of MgO and hence the desired amplitude of indexing signals.

The invention may also be used to lay down the socalled guard lines which. are sometimes incorporated in the screens of such tubes. The guard lines are strips of an opaque and substantially non-reflective material disposed between adjacent ones of the phosphor elements in tubes having screens such as shown in FIG. 1. These guard lines help to prevent desaturation of the colors of the reproduced image by cutting down halation and adverse effects of ambient light falling on the faceplate of such tubes. They also facilitate the mass fabrication of tubes. A fuller explanation of guard lines and methods of obtaining the same are contained in the co-pending US. patent application of Frank I. Bingley entitled Electrical Systems, Serial No. 551,648 filed December 7, 1955, now US. Patent No. 2,842,697. The guard lines are most effective when they are completely opaque. Their opacity is somewhat dependent upon the density of the material used. Therefore, if the guard lines are deposited according to the process hereinbefore described, an optimum density of the opaque material can be achieved.

I It will be understood that still other applications of the processes according to the diverse forms of my invention described herein will occur to those skilled in the art. Consequently, I desire the scope of this invention to be limited only by the following claims.

What I claim is:

1. In the method of depositing particulate solid material on selected portions of a substrate, comprising the steps of 2 forming on said substrate a layer made of a composition containing a photosensitizing substance, said photosensitizing substance impeding the transmission of radiant energy of a given form through said composition and said composition having a hardness which increases and a solubility in a given solvent which decreases, in response to exposure of said composition to radiant energy of said given form, by respective amounts directly dependent on both the concentration of said photosensitizing substance and the quantity of radiant energy of said given form to which said composition is exposed;

exposing those regions of said layer overlying said selected portions of said substrate to radiant energy of said form throughout the thickness of said layer, said exposure being sufficient to render said regions substantially less soluble in said given solvent than the unexposed regions of said layer;

7 depositing said particulate material on at least the exposed portions of the surface of said layer opposing and remote from said substrate,

and washing said layer with said given solvent sufficiently to remove selectively both said unexposed regions of said layer and particles of said material deposited on said unexposed regions,

the improvement which comprises forming said layer so that the the concentration of said photosensitizing substance therein decreases along at least part of its Thickness from a given value adjacent said substrate to a lesser value adjacent said opposing surface, whereby all parts of said layer can be hardened approximately uniformly even though the part thereof adjacent said opposing surface is exposed to more radiant energy of said given from than other parts thereof.

2. A method according to claim 1, wherein said step of forming said layer comprises the steps of coating said substrate with said composition, applying to the surface of said coating which opposes and is remote from said S substrate a solvent for said photosensitizing substance, thereby to dissolve a part of said photosensitizing substance in said coating, and removing from said coating a solution comprising solvent applied to said coating and photosensitive material dissolved therein.

3. A method according to claim 1, wherein said exposure step is carried out so that said radiant energy enters into said layer at said opposing surface.

4. A method according to claim 1, wherein said photosensitizing substance is a dichromate salt.

5. A method according to claim 1 of forming a screen structure for a cathode ray tube, wherein said particulate solid material is a substance emissive of radiation in response to electron impingement thereon and said substrate is composed of a substance transmissive of said radiation.

6. A method according to claim 1 of forming a screen structure of a cathode ray tube, wherein said particulate solid material is a substance emissive of secondary electrons in response to electron impingement thereon.

7. A method according to claim 1, wherein said composition principally comprises a polyvinyl alcohol and said photosensitizing substance is ammonium dichromate.

3. A method according to claim 7, wherein said given solvent is water.

9. in the method of depositing particulate solid material on selected portions of a substrate, comprising the steps of:

forming a photosensitive layer on said substrate by flowing onto said substrate a mixture containing a photosensitizing substance and a first solvent therefor and drying said mixture, said photosensitizing substance impeding the transmission of radiant energy of a given form through said dried mixture and said dried mixture having a hardness which increases and a solubility in a second solvent which decreases, in response to exposure of said dried mixture to radiant energy of said given form, by respective amounts directly dependent on both the concentration of said photosensitizing substance and the quantity of radiant energy of said given form to which said dried mixture is exposed;

exposing those regions of said layer overlying said selected portions of said substrate to radiant energy of said form throughout the thickness of said layer, said exposure being sufficient to render said regions substantially less soluble in said second solvent than the unexposed regions of said layer;

depositing said particulate material on at least the exposed portions of the surface of said layer opposing and remote from said substrate,

and washing said layer with said second solvent sufiiciently to remove selectively both said unexposed regions of said layer and particles of said material deposited on said unexposed regions,

the improvement which comprises the additional steps,

performed before said exposing step, of flowing onto said opposing surface of said layer a quantity of said first solvent sutficient to dissolve a given amount of said photosensitizing substance from the part of said layer adjacent said opposing surface, removing from said layer most of said quantity of first solvent together with said photosensitizing substance dissolved therein, and redrying said layer, whereby all parts of said layer can then be hardened approximately uniformly even though the part thereof adjacent said opposing surface is exposed to more radiant energy of said given form than other parts thereof.

I A method according to claim 9, wherein said particulate matter is deposited by flowing a slurry thereof over said mixture and drying said slurry.

ILThe method according to claim 9 wherein said photosensitizing substance is ammonium dichromate,

9 10 wherein said first solvent is a denatured alcohol, and 2,683,769 7/54 Banning. wherein said second solvent is water. 2,747,997 5/56 Smith et a1.

12. A method according to claim 11, wherein said 2,790,107 4/57 Bradley. mixture consists principally of a polyvinyl alcohol. 2,950,193 8/60 Payne.

13. The method according to claim 11 wherein said 5 FOREIGN PATENTS particulate SOlld material IS an electron-sensitive phosphon 6,368 5/85 Great Britaln.

14. The method according to claim 13, including the OTH REFERENCES fi gg i z g. ii Friedman, History of Colored Photography, 1944-The p or ma arms on Se ece p0 ions 0 Sal 10 American Photographic Publishing Co., Bostonpages strate.

References Cited by the Examiner NORMAN G. TORCHIN, Primary Examiner.

UNITED STATES PATENTS 15 MILTON STERMAN, PHILIP E. MANGAN, HAROLD 1,785,635 12/30 Lebedenko 96-35 N. BURSTEIN, WILLIAM B. KNIGHT, Examiners.

2,370,330 2/45 Smith et a1. 96-49

Claims (1)

1. IN THE METHOD OF DEPOSITING PARTICULATE SOLID MATERIAL ON SELECTED PORTIONS OF A SUBSTRATE, COMPRISING THE STEPS OF: FORMING ON SAID SUBSTRATE A LAYER MADE OF A COMPOSITION CONTAINING A PHOTOSENSITIZING SUBSTANCE, SAID PHOTOSENSITIZING SUBSTANCE IMPEDING THE TRANSMISSION OF RADIANT ENERGY OF A GIVEN FORM THROUGH SAID COMPOSITION AND SAID COMPOSITION HAVING A HARDNESS WHICH INCREASES AND A SOLUBILITY IN A GIVEN SOLVENT WHICH DECREASES, IN RESPONSE TO EXPOSURE OF SAID COMPOSITION TO RADIANT ENERGY OF SAID GIVEN FORM, BY RESPECTIVE AMOUNTS DIRECTLY DEPENDENT ON BOTH THE CONCENTRATION OF SAID PHOTOSENSITIZING SUBSTANCE AND THE QUANTITY OF RADIANT ENERGY OF SAID GIVEN FORM TO WHICH SAID COMPOSITION IS EXPOSED; EXPOSING THOSE REGIONS OF SAID LAYER OVERLYING SAID SELECTED PORTIONS OF SAID SUBSTRATE TO RADIANT ENERGY OF SAID FORM THROUGHOUT THE THICKNESS AND SAID LAYER, SAID EXPOSURE BEING SUFFICIENT TO RENDER SAID REGIONS SUBSTANTIALLY LESS SOLUBLE IN SAID GIVEN SOLVENT THAN THE UNEXPOSED REGIONS OF SAID LAYER; DEPOSITING SAID PARTICULATE MATERIAL ON AT LEAST THE EXPOSED PORTIONS OF THE SURFACE OF SAID LAYER OPPOSING AND REMOTE FROM SAID SUBSTRATE, AND WASHING SAID LAYER WITH SAID GIVEN SOLVENT SUFFICIENTLY TO REMOVE SELECTIVELY BOTH SAID UNEXPOSED REGIONS OF SAID LAYER AND PARTICLES OF SAID MATERIAL DEPOSITED ON SAID UNEXPOSED REGIONS, THE IMPROVEMENT WHICH COMPRISES FORMING SAID LAYER SO THAT THE THE CONCENTRATION OF SAID PHOTOSENSITIZING SUBSTANCE THEREIN DECREASES ALONG AT LEAST PART OF ITS THICKNESS FROM A GIVEN VALUE ADJACENT SAID SUBSTRATE TO A LESSER VALUE ADJACENT SAID OPPOSING SURFACE, WHEREBY ALL PARTS OF SAID LAYER CAN BE HARDENED APPROXIMATELY UNIFORMLY EVEN THOUGH THE PART THEREOF ADJACENT SAID OPPOSING SURFACE IS EXPOSED TO MORE RADIANT ENERGY OF SAID GIVEN FROM THAN OTHER PARTS THEREOF.

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