Method Of Manufacturing X-ray Image Intensifier Input Phosphor Screen

Abstract

A waffle-like light-reflective surface on a substrate member has slightly protruding ribs of the waffle pattern coated with a light-absorbing material. The ribs absorb low-angle rearward traveling light photons generated in a transparent phosphor layer formed on the waffle surface and thereby reduce degradation of X-ray image resolution and contrast whereas the floor of the wafffle pattern reflects the light photons traveling rearward more normal to the floor surface, back through the phosphor layer to the photocathode of the X-ray image intensifier tube. The method of fabricating the phosphor screen includes the intermediate step of forming a rubber replica of a metal master of the waffle surface from which a silicone resin replica is developed. The silicone resin replica is metal coated to provide the light reflective surface and the ribs are blackened to be light-absorbing.

Parent Case Text

This is a division of application Ser. No. 254,065, filed May 17, 1972, now U.S. Pat. No. 3,673,438.

Description

My invention relates to an X-ray image intensifier tube, and in particular, to the phosphor screen structure at the input end of the tube and method of manufacture thereof.

The X-ray image intensifier tube is especially useful in the medical field for obtaining brighter X-ray images, particularly the images of body organs which generally are of low contrast. Conventional X-ray image intensifiers employ in the input end thereof a uniform layer of a dense high atomic number phosphor for absorbing the incident X-rays which have traversed through a patient's body. The X-ray photon is absorbed in the phosphor layer and light photons in the order of 1,000 light photons for each X-ray photon are generated in the phosphor layer and emitted in all directions from the point of X-ray photon absorption. A thin photoemitting coating deposited on the surface of the phosphor layer emits photoelectrons in response to light photons incident thereon. The photoelectrons are accelerated and electron-optically focussed onto a second phosphor screen at the output end of the image intensifier resulting in a brighter image than at the input phosphor screen.

The thickness of the phosphor layer in conventional image intensifiers is typically 5 to 12 mils and is a compromise between a thick layer necessary for high X-ray absorption and a thin layer necessary for high image resolution (a 12 mil thick layer yields a resolution of 40 to 50 line pairs per inch), resolution and local image contrast being degraded due to lateral and rearward light scattering within the phosphor layer. Obviously, it would be highly desirable to reduce degradation of resolution and contrast due to lateral light scattering, and in particular that due to the low-angle rearward light scattering and thereby obtain increased resolution and contrast with a conventional thickness phosphor layer in the input end of the X-ray image intensifier tube, or alternatively, to employ a thicker phosphor layer to thereby increase the X-ray absorption (and thus the sensitivity) with less loss in resolution and local contrast than occurs in conventional image intensifiers.

Therefore, one of the principal objects of my invention is to provide a new and improved X-Ray image intensifier tube having an input phosphor screen which simultaneously can achieve both high X-ray absorption and high image resolution and contrast, and the method of manufacture thereof.

Another object of my invention is to provide a relatively thick input phosphor screen with means to substantially reduce degradation of resolution and contrast due to low-angle, rearward light scattering in the phosphor and the method of manufacture thereof.

A further object of my invention is to provide a low cost fabrication process for manufacturing the improved input phosphor screen.

Briefly stated, and in accordance with my invention, I provide an X-ray image intensifier input phosphor screen wherein a phosphor layer is deposited on a waffle-like light-reflective surface of a substrate member wherein slightly protruding ribs of the waffle-pattern are coated with a light-absorbing material. The rib height is much less than the phosphor layer thickness and the projected area of the ribs is much less than the area of the floor portion of the waffle pattern. The other side of the substrate member is bonded to the X-ray image intensifier tube face plate which may be formed of glass or a low atomic number metal such as aluminum. The outer surface of the phosphor layer, spaced from the substrate member, is smooth and substantially parallel to the major surface of the face plate and a thin film of a photoemitter material is deposited thereon. The reflective floor surface of the substrate member reflects the high angle (more nearly normal) portion of the rearward traveling light photons in the phosphor layer to thereby deliver more useful light to the photoemitter material while the slightly protruding ribs absorb the low angle (more nearly lateral) portion of the rearward traveling light photons to thereby substantially reduce degradation of the image resolution and contrast due to low-angle rearward scattering of light in the phosphor layer.

My X-ray image intensifier input phosphor screen is fabricated by the following method. A sheet of metal mesh is formed by photoetching a thin metal sheet to produce an array of small holes therethrough wherein the hole width is much greater than the width of the walls separating adjacent holes. The photoetched sheet is then diffusion bonded to a heavy planar metal substrate to thereby obtain a waffle-like surface wherein the slightly protruding wall projections define the ribs of the waffle pattern. This metal substrate having a waffle-like surface is used as a master from which silicone rubber replicas are made. Each silicone rubber replica has the rib indentation surface thereof coated with a silicone resin and such coated surface is drawn toward the concave side of the X-ray image intensifier face plate. Upon hardening of the silicone resin, the silicone rubber replica is removed, and the concave-shaped resin structure has the rib-like projections of the metal master and is adhered to the face plate. The waffle surface of the resin replica is then coated with a light-reflective material such as aluminum, and a black light-absorbing material is deposited only on the ribs of the waffle surface. A layer of transparent phosphor material is then deposited on the waffle-surface of thickness much greater than the rib height and forms a smooth outer surface upon which a thin uniform coating of a photoemitter material is deposited to form the photocathode of the X-ray image intensifier tube.

The features of my invention which I desire to protect herein are pointed out with particularity in the appended claims. The invention itself, however, both as to its organization and method of operation together with further objects and advantages thereof may best be understood by reference to the following description taken in connection with the accompanying drawings wherein like parts in each of the several figures are identified by the same reference character, and wherein:

FIG. 1a is an elevation sectional view of a conventional X-ray image intensifier tube;

FIGS. 1b and 1c are elevation sectional views of two conventional phosphor screens utilized in FIG. 1a, but to a larger scale;

FIGS. 2a and 2b are top views of two geometries of an array of small holes photoetched in a metal sheet to form a waffle-like surface on a metallic substrate utilized in fabricating a master in accordance with my invention;

FIG. 3 is an elevation sectional view of the photoetched metal sheet shown in FIGS. 2a, 2b bonded to the metallic substrate, but to a larger scale;

FIG. 4 is an elevation sectional view, to the same scale as FIG. 1a, of a silicone rubber replica of the master illustrated partially in FIG. 3, retained on the X-ray image intensifier tube face plate, and a silicone resin coating on the rubber replica;

FIG. 5 is an elevation sectional view of the silicone resin replica subsequently formed on the face plate in FIG. 4, and also shows a light-reflective material coated on a first part of the resin replica, and the silicone rubber replica being peeled from the resin replica;

FIG. 6 is an elevation sectional view, to the same scale as FIGS. 1a, 4, 5 of a first means for coating the ribs of the resin replica with a light-absorbing material;

FIG. 7 is an elevation sectional view, to the same scale as FIGS. 1b, 1c after phosphor and photoemitter coatings are deposited on the coated resin replica;

FIG. 8 is an elevation sectional view of a second means for obtaining the light-absorbing ribs; and

FIG. 9 is an elevation sectional view of a third means for obtaining the light-absorbing ribs.

Referring now in particular to FIG. 1a, there is shown a conventional X-ray image intensifier tube comprised of a glass envelope 10 having an input end (face plate) 10a which has a uniform phosphor layer 11 of thickness in the range of 0.005 to 0.012 inch deposited on the inner surface thereof. The phosphor may be zinc cadmium sulfide or cesium iodide as typical materials onto which a thin film 12 of photoemitter material is deposited of thickness of approximately 100 Angstroms. The photoelectrons emitted by the photoemitter coating 12 are focussed by electrode 13a maintained at a potential of several hundred volts and are accelerated to approximately 25 kilovolts by means of electrode 13b (connected to a suitable D.C. voltage source) at the output end of the image intensifier tube, the electrodes being suitably shaped to provide electron-optical focussing of the accelerated photoelectrons onto a second uniform phosphor screen (layer) 14 deposited on the inner surface of the glass envelope at the output end 10b thereof. The image appearing on the second phosphor screen 14 is a brighter version of the image on the first phosphor screen 11 and can be viewed directly by the physician or be subjected to further processing. The paths of two photoelectrons between the photoemitter coating 12 and the second phosphor screen 14 are indicated by dashed line and arrowheads. As stated hereinabove, the thickness of the input phosphor screen 11 in conventional X-ray image intensifier tubes is a compromise between a thick screen for high X-ray absorption and thin screen for high resolution which is determined primarily by lateral and low-angle rearward light scatter in the phosphor.

FIG. 1b illustrates a first approach conventionally utilized for obtaining improved performance from the input phosphor screen. In this case, the inner surface of face plate 10a is coated with a suitable material to provide a light-reflective surface 16 to all of the rearward traveling light photons in the phosphor layer and thereby deliver more light to the photoemitter layer 12. However, as shown in FIG. 1b, the low angle portion of the rearward directed light photons deliver (scattered) light to the photoemitter layer 12 which degrades image resolution and local image contrast. This degradation of image resolution and contrast due to low-angle rearward light scatter is avoided by a second approach illustrated in FIG. 1c wherein the face plate inner surface is coated with a suitable material to provide a light-absorbing surface 17 to all of the rearward traveling light photons. However, this second approach results in the loss of one half the light generated in the phosphor and thereby decreases the sensitivity by an equal amount.

My invention provides a new and improved high resolution X-ray image intensifier input phosphor screen which is adapted to absorb only the low-angle portion of the rearward traveling light photons while reflecting the high-angle (more nearly normal to the surface) portion thereof to thereby deliver more useful light to the photoemitter layer (i.e., improve sensitivity) without any substantial loss in resolution. These desirable characteristics are obtained by the use of a waffle-like surface formed on the inner surface of the face plate wherein the floor portions of the waffle pattern are light-reflective to the high-angle portion of the rearward traveling light photons in the phosphor layer, and slightly protruding ribs of the waffle pattern are light-absorbing to the low-angle portion thereof. My invention thus permits the use of a thicker phosphor screen for achieving higher X-ray absorption with substantially less attendant degradation of resolution and local contrast than is obtained in conventional image intensifiers, or a phosphor screen of conventional thickness but with a significantly higher resolution and local contrast. The waffle-like surface is readily achieved by fabrication processes to be described hereinafter.

The fabrication process is initiated by selecting a sheet of metal suitable for photoetching such as nickel or stainless steel in the order of 1 to 2 mils thick. An identical pattern (array) of holes is then photoetched through the metal sheet, the etched holes in the metal mesh sheet preferably having a square or hexagonal shape as illustrated in FIGS. 2a and 2b, respectively, with a typical center-to-center hole spacing of at least 4 mils and separation (wall thickness) of approximately 1 to 2 mils as indicated on the drawing. The holes are of equal size and equally spaced from each other and form an array of identical rows and columns of holes to maximize the hole area in the mesh. Other shape holes, such as triangular or circular could be used, however, such shaped holes produce less open area in the mesh.

Upon completion of the photoetching step, the sheet 30 of metal mesh is positioned on a heavy planar substrate 31 of the same metal as the mesh and is subsequently diffusion bonded thereto. The approximately 1 to 2 mil thick metal mesh 30 is diffusion bonded by bolting the photoetched sheet between two massive planes of metal, the upper one of which is thinly coated with an oxide such as MgO to prevent sticking, and this assembly is heated to a suitable temperature (e.g., approximately 1,000.degree.C when bonding nickel or stainless steel) in a hydrogen atmosphere or vacuum to accomplish the diffusion bonding. The diffusion bonding results in a "waffle-like" structure shown in FIG. 3 wherein the slightly projecting walls 32, 33, 34 from the surface of substrate 31 are rectangular in the section taken vertically through the projecting walls although it should be obvious that the ribs may be rounded off at the top in some applications. The wall projections will hereinafter be referred to as the ribs of the waffle surface to indicate the relatively small protrusion thereof, the floor portion of the waffle surface having an area relative to the projected area of the rib portion in the ratio of 2 to 1 or greater and being in accordance with the above-described hole spacing and hole separation in the etched metal sheet. Even more important is the rib height-to-spacing ratio which determines the "cut-off" point for the low-angle rearward light radiation absorption, and is approximately 1 to 4 for a cut-off angle of 14.degree.. Obviously, increasing the rib height-to-spacing ratio will decrease the cut-off angle, resulting in greater absorption of the rearward traveling light photons and therefore less reflection thereof to the photoemitter layer. It is obvious that the diffusion bonding step results in a master substrate structure provided with a plurality of identical ribs wherein FIG. 2a or FIG. 2b represent the top view of the waffle surface shown in elevation sectional view in FIG. 3. The ribs 32, 33, 34 of the waffle surface can be thinned to less than 1 mil thickness by a chemical etching process if a larger ratio of floor portion to rib area is required by the particular application. Alternatively, the master substrate structure can be formed by directly photoetching the heavy planar substrate 31 to obtain the desired waffle surface, i.e., without using any additional metal sheet 30.

A phosphor material could be deposited on the waffle-like surface of the metal master structure in FIG. 3 to form a phosphor screen, however, the process hereinabove described is relatively expensive and in accordance with my invention, I fabricate many inexpensive silicone resin replicas of such original master whereby the cost per X-ray intensifier tube will be small. Also, at some stage in the process it is necessary to sag the planar surface of substrate 31, that is, to obtain it in a concave-shape conforming to the shape of the face plate 10a of the image intensifier tube.

In order to replicate the master illustrated in FIG. 3, an intermediate step of making one or more silicone rubber replicas is utilized. The silicone rubber replica is fabricated by vacuum impregnation wherein the master is covered with a layer of liquid silicone rubber (e.g., General Electric RTV-11) to which a small amount of a suitable curing catalyst has been added. The coated master is then placed in a vacuum chamber for a few minutes in order to pump away all air bubbles and insure that the silicone rubber contacts all the crevices of the master. The rubber is then allowed to cure for an appropriate period, e.g., 24 hours, in order to form an elastic, rubbery solid. The silicone rubber replica is approximately 50 mils thick in order to remain somewhat flexible so that it can be subsequently easily removed by peeling from the silicone resin replica to be described hereinafter.

Referring now to FIG. 4, the rib-indented side 45a of the silicone rubber replica 45 is substantially uniformly coated with a silicone resin 46. The face plate 10a of the image intensifier tube is then placed over the rubber replica 45, positioned in its proper orientation, and the two margins 45b, c along the rib-indented side 45a of the rubber replica are suitably retained against corresponding planar margins of the concave face plate 10a. The entire assembly is then placed within a chamber wherein a vacuum is drawn between the rubber replica toward the concave face plate (as shown in part) to produce a silicone resin replica 46 of the FIG. 3 master waffle-like surface except that the resin replica 46 is curved into the concave shape of the image intensifier tube face plate 10a rather than being planar. The resin-coated rubber replica and face plate assembly is then baked at approximately 250.degree.C to harden the silicone resin.

Upon hardening of the silicone resin, the silicone rubber replica 45 is removed therefrom by peeling it from the resin replica 46 as shown in FIG. 5, and the resin replica and face plate assembly is additionally air baked and then vacuum baked to outgas and cure the silicone resin. The rubber replica may be reused to form additional resin replicas. A possible problem may occur in peeling the rubber replica off the silicone resin replica if the two replicas tend to stick together. This sticking effect can be minimized, if it occurs, by treating the surface of the rubber replica with a "parting agent" such as a thin film of silicone oil. The face plate 10a is fabricated of glass or a low atomic number metal such as aluminum.

After the final silicone resin baking step, the resulting structure consists of the silicone resin replica 46 adhered to the concave side of the image intensifier face plate 10a as illustrated in FIG. 5 wherein the slight rib projections of the silicone resin replica 46 extend normal to the surface of face plate 10a. The thickness of the floor portion of the resin replica is not critical and is generally in a range of 0 to 3 mils. The whole waffle surface is then coated with a light-reflective material such as evaporated aluminum to obtain a relatively highly light reflective surface 50. The light-reflective coating is of thickness in the range of 100 to 2,000 Angstroms (A) and typically may be in the order of 1,000 A. Alternatively, only the floor portions of the waffle surface are coated with the light-reflective material, and the ribs are incidentally and only partially coated in the process of coating the floor portions.

After the light-reflective material is deposited on the waffle surface, a relatively highly light-absorbing material such as carbon is evaporated obliquely on the waffle surface substrate member so as to blacken only the protruding ribs thereof. FIG. 6 illustrates a first means for accomplishing the rib-blackening process wherein the face plate 10a is supported and rotated relatively rapidly about a central vertical axis by means of rotatable vertical shaft 60 while an evaporation source 61 of suitable black matter retained in a suitable container 62 slowly pivots about the center of curvature of face plate 10a by means of pivot arm 63. Container 62 is an open-ended hollow chamber connected to the free end of pivot arm 63. The shape of container 62 is not critical but requires a suitable opening for the black matter to be emitted at the desired low angle. Container 62 is oriented relative to the surface generated by the ends of the ribs such that the angle at which the evaporated black matter exits from the open end of container 62 remains constant while pivot arm 63 swings back and forth diametrically across such surface in slightly spaced apart relationship therefrom. By this process of source 61 pivoting slowly back and forth across the rotating waffle surface, all sides of the rib projections are thinly coated with the light-absorbing material to a thickness in the range of 100 to 2,000 A. The feature of obtaining blackening only on the rib projections and not on the floor portions of the waffle surface (which are light-reflecting) is accomplished by selection of a small enough angle of container 62 relative to the surface defined by the rib ends, as well as by the proper spacing of adjacent ribs for a particular height thereof. A change in any of such parameters can generally be compensated for by a change in the angle of container 62. Thus, a closer spacing of the ribs would generally require an increase in the angle of container 62 in order to obtain blackening of the entire surfaces of the ribs.

After the ribs of the waffle surface have been made light-absorbing, a suitable phosphor material is deposited on the waffle surface using conventional techniques to form a uniform phosphor layer 62 which extends beyond the ends of the rib projections of the silicone resin replica 46 to a thickness greater than twice the rib height. The phosphor 65 can be any transparent phosphor such as evaporated cesium iodide (CsI). Evaporation of the CsI from vertically above the resin replica 46 may result in the outer surface of the phosphor layer 65 having a slight undulating form due to the projecting ribs of the resin replica, however, such undulations are generally not so severe as to upset either the electron-optics or the formation, and, or surface resistivity of the photocathode (to be described hereinafter). If desired, or necessary, such undulations may be readily smoothed out. The phosphor layer 61 is approximately 12 mils thick as one typical example, and obviously can be made thicker if higher X-ray absorption is desired.

Referring now to FIG. 7, a thin uniform coating of a suitable photoemitter material 70 is deposited on the smooth surface of the phosphor layer 65 during the evacuation of the image intensifier tube to form the photocathode of such image intensifier tube. The photoemitter material may be of the common types known as S-20 (a compound of antimony, cesium, sodium and potassium) or S-11 (a compound of cesium, antimony and oxygen) as two typical examples and is a very thin coating in the order of 100 Angstroms. If desired, an isolating layer of transparent alumina, as one example, may be deposited between the phosphor 65 and photoemitter 70 layers in order to isolate the alkali metal of the photoemitter material from the phosphor, however, such isolating layer is not essential to the successful operation of my input phosphor screen.

The rib projections of the silicone resin replica 46 extend in a normal direction through less than 50% of the phosphor layer 65 thickness, and as shown in FIG. 7, typically extend through approximately 20 percent of the phosphor layer. The effect of the relatively highly light-absorbing rib projections is to absorb the low angle (more nearly lateral) portion of the rearward traveling light photons to thereby substantially reduce degradation of image resolution and especially local image contrast due to such low-angle rearward scattering of light in the phosphor layer 65 while the relatively highly light-reflecting floor portions of the waffle surface reflect the high-angle (more nearly normal) portion to thereby deliver more useful light to photocathode 70 and hence increase the tube sensitivity. The reflection and absorption of the rearward traveling light photons is depicted by the arrows in FIG. 7.

Obviously, the metal master can be made with more sheets of the metal mesh to thereby obtain a silicone resin replica having rib projections of greater height whereby a thicker phosphor layer 65 can be utilized for increased X-ray absorption, and thus increased sensitivity, while still maintaining the height of the ribs at less than the prescribed one half of the phosphor layer thickness.

FIG. 8 illustrates a second means for obtaining the light-absorbing ribs. In this second embodiment, a waffle-like surface is formed on the light-reflective surface (which can be the inner surface of an aluminum face plate 10a, or a light-reflective coating 50 on the inner surface of a nonreflective face plate) by fastening a preferably blackened metal screen 80 to the light-reflective surface. Screen 80 may be woven, as illustrated, in the manner of a window screen, from approximately 1 mil wire and having a pitch of 5 to 10 mils, that is, the dimensions and spacing of the ribs can be made the same as in the first embodiment. The screen 80 can be fabricated of aluminum or other low atomic number metal such as iron, titanium or nickel. The metal screen 80 is fastened to the light-reflective surface by a conventional process appropriate to the particular metals involved, and as one typical example, can be adhesively bonded thereto by means of a silicone resin. Alternatively, an unblackened metal screen can be first fastened to the light-reflective surface, and the screen then blackened by oblique evaporation of the black material as in the first embodiment shown in FIG. 6. The blackened metal screen, besides absorbing low-angle rearward traveling light photons in the phosphor layer 65, also aids in holding the evaporated phosphor layer on the light-reflecting substrate face plate 10a while the reflective surface reflects the high-angle rearward traveling light photons.

FIG. 9 illustrates a third means for obtaining the light-absorbing ribs. In this third embodiment, the inner surface of the face plate 10a is etched to produce the illustrated jagged, irregular (waffle) surface 90. The size of the irregularity need not be large, and can be as small as a fraction of a mil, peak-to-peak. If the face plate 10a is aluminum, the light-absorbing ribs are completed by oblique evaporation of the black material on the peaks 91 of the irregularities as in FIG. 6. If the face plate is glass or another nonreflective material, a coating of a light-reflective material such as aluminum is deposited on the irregular surface prior to the oblique evaporation of the black material on the peaks. The blackened peaks absorb the low-angle rearward traveling light photons while the unblackened portions of the irregularities reflect the high-angle rearward traveling light photons.

From the foregoing description, it is apparent that my invention attains the objectives set forth and makes available a new and improved X-ray image intensifier tube which has an input phosphor screen that simultaneously achieves both high X-ray absorption (and thus high sensitivity) and high image resolution and contrast as well as providing methods of manufacturing such input phosphor screen. The method of manufacturing the input phosphor screen is a low cost fabrication process due to the use of a silicone rubber replica which permits fabrication of many inexpensive silicone resin replicas of the original master in a first method of manufacture, the second and third described methods also being low cost. The light-absorbing ribs of the waffle surface, which are an important aspect of my invention, can be adjusted as to the height-to-spacing ratio thereof in the formation of the ribs and thereby the "cut-off" point for the low-angle rearward light radiation can be set at any desired angle. As a typical range, it would generally be desired to absorb the light photons which travel rearward at angles up to approximately 15.degree. from the plane in the phosphor layer parallel to the face plate, and containing the point at which an incident X-ray photon generates the light photons. The combination of the light-absorbing ribs and light-reflecting floor portions of the waffle surface provides an input phosphor screen which avoids the compromise in conventional X-ray image intensifier tubes between high X-ray absorption and high image resolution. Having described three specific embodiments of my waffle surface, it will be apparent to those skilled in the art that such waffle-like surface which constitutes the essence of my invention may take other forms than that specifically illustrated and described above, and the ribs may be blackened by other methods. Also, the support for the input phosphor screen, herein described as the face plate, may be slightly spaced from the input window of the tube glass envelope. Thus, it is to be understood that changes may be made in the particular embodiment of my invention as described which are within the full intended scope of the invention as defined by the appended claims.

Claims

What I claim as new and desire to secure by Letters Patent of the United States is:

1. A method for manufacturing an improved X-ray image intensitifer input phosphor screen comprising the steps of

forming a waffle-like surface along the concave-shaped major side of a face plate of an X-ray image intensifier tube wherein the waffle-like surface has light-reflective floor portions and rib-like projections extending outward therefrom,

coating the rib-like projections with a light-absorbing material,

depositing a phosphor layer on the waffle-like surface to a thickness wherein the phosphor layer extends substantially beyond the ends of the rib-like projections, and

depositing a thin uniform coating of a photoemitter material on the outer surface of the phosphor layer.

2. The method set forth in claim 1 wherein the step of coating the rib-like projections with a light-absorbing material consists of evaporating carbon obliquely on the waffle-like surface to thereby blacken only the rib-like projections.

3. The method set forth in claim 1 wherein

the step of forming the waffle-like surface comprises

fastening a metal screen to a light-reflective surface on the concave-shaped major side of the face plate to thereby obtain the rib-like projections.

4. The method set forth in claim 1 wherein

the steps of forming the waffle-like surface and coating the rib-like projections comprises

fastening a blackened metal screen to a light-reflective surface on the concave-shaped major side of the face plate to thereby obtain the light-absorbing coated rib-like projections.

5. The method set forth in claim 1 wherein

the step of forming the waffle-like surface comprises an intermediate step of forming a silicone rubber replica from which the waffle-like surface is formed.

6. The method set forth in claim 5 wherein the step of forming the silicone rubber replica comprises the steps of

photoetching a thin metal sheet to produce an array of small holes therein forming a mesh,

positioning the sheet of metal mesh upon a heavy planar substrate,

diffusion bonding the sheet of metal mesh to the planar substrate to obtain an array of rib-like projections and thereby forming a metal master,

coating the metal master with liquid silicone rubber,

placing the rubber coated metal master in a vacuum chamber for pumping away all air bubbles to insure the silicone rubber contacts all the crevices of the metal master,

curing the silicone rubber to form an elastic solid rubber replica having rib-like indentations along one side thereof corresponding to the rib-like projections of the waffle-like surface, and

removing the rubber replica from the metal master.

7. The method set forth in claim 6 and further comprising the steps of

coating the rib-indented side of the rubber replica with a silicone resin,

positioning the silicone coated rubber replica upon the concave-shaped major side of the face plate,

drawing a vacuum between the silicone coated rubber replica and face plate to thereby force the silicone coated rubber replica against the face plate,

baking the resin-coated rubber replica and face plate assembly to harden the silicone resin and cause adherence of the silicone resin to the face plate, and

peeling the rubber replica from the silicone resin whereby a silicone resin replica of the metal master is in adherence on the face plate to thereby form the waffle-like surface.

8. The method set forth in claim 7 and further comprising the step of

coating the waffle-like surface with a highly light-reflective material prior to coating the rib-like projections with the light-absorbing material to thereby obtain the light-reflective floor portions.

9. The method set forth in claim 1 wherein

the step of forming the waffle-like surface comprises

etching the convex-shaped major side of the face plate to thereby produce a jagged, irregular surface wherein the peaks of the irregular surface form the rib-like projections, and

the step of coating the rib-like projections comprises

coating the peaks of the irregular surface with the light-absorbing material.

10. The method set forth in claim 9 and further comprising the step of

depositing a light-reflective material on the irregular surface prior to the peaks being coated with the light-absorbing material.