Abstract: A process of manufacturing a shadow mask includes the steps of preparing a plurality of metal plates having a surface deviation characterized in that its Rsk value is not more than 0.3 .mu.m, preferably less than 0, its Sm value is not less than 60 .mu.m, and its Pc value relative to the band of 1 .mu.m width centered about the center line of a roughness profile is not more than 60/cm, forming a plurality of apertures in each of the plates, registrating and piling up the plates upon each other, annealing the plates piled upon each other, and pressing and molding the annealed metal plates into a predetermined curvature. Each of the metal plates preferably has Ra in the range from 0.1 to 0.7 .mu.m.

Abstract: A method for manufacturing a color CRT includes the step of coating a shadow mask of the color CRT in black. It further includes the step of forming an electron beam reflecting film by depositing tungsten oxide with a large electron beam reflection coefficient on the side of an electron beam irradiated surface of the shadow mask coated in black.

Abstract: A method for manufacturing a shadow mask is used for color television picture tubes and the like. The surface of a shadow mask body, made of Fe-Ni type alloy, is ion-nitriding processed so as to make the most of the low thermal expansion property inherent in the alloy. The result is that the color purity degradation caused by the thermal expansion at temperature increases of the mask, and by the resonance of the mask itself, can be prevented, thereby realizing a shadow mask serving to render a highly refined CRT.

Abstract: A grid apparatus which has a multiplicity of ribbon-shaped grid elements stretched in tension and attached to a frame which comprises a pair of support bars and a pair of resilient support members. Metallic members which have a larger thermal expansion coefficient than the resilient support members are connected to the reverse surfaces of the resilient support members opposite the side to which the grid-elements are attached. In the grid apparatus during heat treatment, thermal creep of the grid elements is prevented so that the tension of the grid elements will remain high when the normal temperature state is resumed. A method of making such grid apparatus includes an additional heat treatment such as a blackening process after the reduction of the tension applied to the grid elements by the resilient support members and the support bars, hence preventing thermal creep of the grid elements to produce a high-reliability with high tension of the grid elements.

Abstract: A shadow mask sheet manufactured from an iron-nickel alloy is drape-drawn by means of a drawing process to form a shodow mask 15. Prior to the actual drawing process the shadow mask sheet is subjected to an annealing treatment. After the drawing process the shadow mask is successively subjected to a complementary annealing treatment in a non-oxidizing gas atmosphere at a temperature between 700.degree. C. and 1200.degree. C. to improve the magnetic screening properties of the shadow mask 15, and to an annealing treatment in an oxidizing gas atmosphere. After oxidation the shadow mask 15 is connected on a supporting frame 17 which may be subjected to an annealing treatment in a non-oxidizing gas atmosphere and then to an oxidizing annealing treatment.

Abstract: An improved method of making a color picture tube shadow mask includes first constructing an apertured flat mask which is formed into a domed contoured mask. The mask has a rectangular periphery with two long sides and two short sides. The mask has a major axis, which passes through the center of the mask and parallels the long sides, and a minor axis, which passes through the center of the mask and parallels said short sides. The mask includes slit-shaped apertures aligned in columns that essentially parallel the minor axis. Adjacent apertures in each column are separated by tie bars in the mask, with the spacing between tie bars in a column being the tie bar pitch at a particular location on the mask. The improvement comprises a first step of calculating the desired tie bar shadow locations at several discrete areas, for a given scan line pitch, as viewed from a distance in front of a viewing screen of a tube.

Abstract: A laminated shadow mask assembly for use in a color picture tube including an evacuated envelope having a longitudinal axis and also having a phosphor deposited screen lying generally perpendicular to the longitudinal axis, which assembly comprises a finely perforated, laminated shadow mask having a plurality of apertures defined therein in a predetermined pattern; a support frame for the support of the laminated shadow mask; and expansion compensating couplings disposed around the support frame for connecting the support frame in position within the evacuated envelope with the laminated shadow mask held generally parallel to the phosphor deposited screen.

Abstract: A method of forming an electron reflecting layer on a perforated shadow mask in a color cathode ray tube, which method comprises the steps of preparing an aqueous suspension containing water glass and powdered bismuth oxide, applying the suspension by the use of a spraying technique to one of opposite surfaces of the perforated shadow mask which confronts a color electron gun assembly within the color cathode ray tube, thereby to form an electron reflecting coating, and drying the coating to complete the electron reflecting layer. The aqueous suspension is prepared by adding to water both the powdered bismuth oxide of 0.1 to 1.0 .mu.m in average particle size in a quantity within the range of 20 to 45 percent by weight relative to the total weight of the aqueous suspension and the water glass having a solid component within the range of 25 to 30 percent by weight relative to the total weight of the water glass, the amount of the water glass added to the water being within the range of 37.5 to 62.

Abstract: A slit-type foil tension mask and associated front assembly for a color cathode ray tube comprise a series of parallel strips separated by slits. The strips are loosely coupled by widely spaced ties, the wide tie spacing being such as to produce a strip coupling which promotes handleability of the mask during mask and tube fabrication and facilitates damping of strip vibration when mounted in a tube, but which is insufficient to induce unacceptable Poisson contraction of the mask when uniaxially tensed along the direction of the strips in the plane of the mask, or to permit an unacceptable thermal expansion perpendicular to said strips.

Abstract: The power handling capability of a nickel-iron-based tension foil shadow mask is enhanced by a process which creates a high emissivity layer or region on the surface of the body of the mask. One version of the process includes contacting the mask foil with a strong reducing acid to provide a nickel-enriched, iron-depleted surface layer of the body of the mask. The nickel of the nickel-enriched, iron-depleted surface layer is converted to a nickel phosphide compound by again contacting the foil with a strong reducing acid having an effective amount of hypophosphite ions. In another version of the process, the nickel phosphide compound is formed in a single treatment with a strong reducing acid containing hypophosphite ions.

Abstract: A method for preparing a shadow mask with a plurality of apertures for a color picture tube from a metal shadow mask sheet includes the step of exposing the entire surface of the photosensitive layers provided on the both surfaces of the metal sheet to an unpatterned light before or after the step of exposing the photosensitive layers to patterned light in order to form resist films.

Abstract: For fine-(for example post-)deflection of electron beams is a display tube, particularly of the flat, thin type, electrically insulated deflector plate portions are placed on either side of each beam prior to its reaching the phosphor screen. By choosing a special mask aperture geometry, a deflector is produced after a plate portion is bent out of the mask plane. By combining two masks (plates) with bent plate portions, an electrode structure is obtained having two separate flat deflector aperture electrode pairs per electron beam.

Abstract: The power handling capacity of non iron-based flat tensioned foil shadow masks for flat faceplate cathode ray tubes (CRTs) is enhanced by providing the flat tensioned mask (FTM) with a thin outer layer of iron, which is converted to iron oxide by blackening, or heating. The iron layer, which is preferably at least 0.04 mil thick, is deposited on the FTM by electroplating in a ferrous ammonium sulfate bath either before or after chemical etching of the FTM. The thin iron layer increases FTM emissivity and reduces FTM doming at high electron beam energies. The iron coating may be blackened in the frit-like cycle during CRT assembly or in a separate heating operation. A blackened outer layer of cobalt may also be used to increase FTM emissivity.

Abstract: A front assembly for a tension mask color cathode ray tube comprises a faceplate having an inner surface thereon for receiving a pattern of luminescent phosphor elements. A foil mask is supported in tension by the faceplate and has a pattern of apertures registered with the pattern of phosphor elements. The assembly is characterized by the faceplate having registration affording means disposed on the external periphery of the faceplate outside the tube vacuum enclosure constructed and arranged to permit repeatable precise registration between said foil mask and said faceplate to facilitate screening the pattern of phosphor elements upon the faceplate inner surface. By having the registration affording means located externally of the tube enclosure, the viewable area of the inner surface is maximized and the faceplate size for a given screen area is reduced.

Abstract: The mask of our invention can be used in image forming units, for example in ion projection microlithography. The mask comprises a mask foil clamped into a retaining frame. The mask foil has a larger thermal expansion coefficient than the retaining frame. To make this mask the mask foil and retaining frame are heatead to a higher temperature than room temperature and clamped in position at this temperature.

Abstract: An ingot of an Invar alloy is rolled in one direction into a plate, a number of slot holes are formed in this plate, and the plate is pressed into a curved form, thus providing a shadow mask of a color CRT. The slot holes of this shadow mask have longitudinal axes extending substantially at right angles to the rolling direction of the plate. The holes maintain their shapes after the plate has been pressed.

Abstract: A method of manufacturing a color display tube having a magnetic quadrupole post-focusing mask, in which the mask is magnetized by magnetic field shaping means of soft iron, or the magnetic equivalent, having raised portions, for example strips placed against opposite sides of the mask sheet and subjected to an external strong magnetic field by an electromagnet to induce magnetic poles through the thickness of the mask sheet. The geometric arrangement of the raised portions of the field shaping means is such that cyclically a north pole, a south pole, a north pole and a south pole are induced into the sheet material surrounding each aperture. Using this technique, the entire area of a mask sheet can be magnetized in one operation. Also masks for small tubes and high definition tubes can be made.

Abstract: A shadow mask (13) in accordance with the present invention comprises: an iron plate (14) having many apertures (15) for electron beams, wherein each of the apertures comprises a relatively medium diameter portion (15c), a relatively smaller diameter portion (15b), a relatively larger diameter portion (15a) in order from one side of the plate in which said one side is to be irradiated by the electron beam; a first surface-treated layer of Fe.sub.3 O.sub.4 which covers the allover surface of the plate; and a second layer (5) for preventing thermal strain of the plate caused by heat due to energy of the electron beams, wherein said second layer is applied over said first layer only on said one side of the plate.

Abstract: A transparent panel for a display device of cathode-ray tube type comprising, an electromagnetic shielding web defined by the wires made of an electromagnetic shielding material, and the transparent panel members sustaining said electromagnetic shielding web to shield the radiated electromagnetic waves so as to protect the observer from the effects of the waves.

Abstract: A color display tube comprises a shadow mask (12) placed in front of a display screen (8). The shadow mask (12), at least on the side remote from the display screen, is coated with a glass layer (14) comprising a heavy metal.

Abstract: Color television tube provided with a perforated mask and three aligned electron guns, the openings of the mask far from the center of the mask having a major axis inclined with respect to the vertical, the inclination .alpha. increasing with the distance from the center and this inclination being in the direction opposite to that of the image of a non-inclined slot of the mask that would be produced on the screen, by an optical projection system simulating the tube deflector.

Abstract: A cast ingot of an invar alloy is forged, hot-and cold-rolled, annealed and subjected to a controlled rolling to provide a shadow mask plate. An X-ray diffraction pattern is formed in an electron-beam hole-formation surface of the shadow mask plate, and a draft in a controlled rolling step is so controlled that the "g" value is 2 or more. The "g" value is given asg=(I.sub.1 +I.sub.2)/I.sub.3whereI.sub.1 =the X-ray diffraction integrated intensity at the {200} crystal faces;I.sub.2 =the X-ray diffraction integrated intensity at the {111} crystal faces; andI.sub.3 =the X-ray diffraction integrated intensity at the {220} crystal faces.The shadow mask plate is etched to provide shadow masks each having electron-beam holes formed therein, noting that one hole surface side which has greater {100} texture is used as a larger-diameter hole surface side.

Abstract: A shadow mask blank is disclosed for use in the manufacture of an ultra-high resolution color cathode tube. The mask blank is adapted to be expanded and secured under tension in spaced adjacency to the screen of the tube. The blank comprises a central field of electron-beam-passing apertures, an outer band adapted to be gripped during the expanding, and at least one field of selectively weakened metal between the central field and the outer band. The selectively weakened metal is effective to equalize stress and prevent wrinkling of the blank under electron beam heating when the blank is expanded and secured under tension. An inner band peripheral to the central field may be provided for mounting the blank on a support structure and securing it thereto. Means may also be provided for linking the inner band with the outer band to selectively strengthen weakened areas.

Abstract: A process for pre-treating a metal foil shadow mask is disclosed for use in a cathode ray tube wherein the shadow mask is maintained under high tension within said cathode ray tube, and wherein the mask is subjected to predetermined relatively high temperatures during tube manufacture. The process according to the invention comprises pre-heating the shadow mask in a predetermined cycle of temperature and time effective to minimize subsequent permanent dimensional changes in the mask that occur when it is subjected to the predetermined relatively high temperatures, but ineffective to significantly reduce the tensile strength of the mask by annealing.

Abstract: A color cathode ray tube contains a shadow mask having a layer consisting essentially of a ceramic material, such as lead borate glass, on at least one side of the major face of the mask. Thermal expansion of the shadow mask is limited by bonding the ceramic layer to the shadow mask at a temperature at least as high as the normal operating temperature of the shadow mask. When the mask cools to room temperature, it does not fully contract but retains a residual tensile stress because of the bonded ceramic layer.

Abstract: A color picture tube comprises a shadow mask which is formed to, oppose at a small gap therefrom, a phosphor screen formed on the inner surface of a panel. The shadow mask comprises a sheet of a nickel-containing iron alloy which contains 0.1% by weight or less of manganese and having an austenite grain number of 7 or less both within and at surfaces of the sheet, the austenite grain number being defined by JIS G 0551 of the Japanese Industrial Standards. The iron alloy sheet is annealed in a vacuum of 10.sup.-1 Torr or less at a temperature of 1,000.degree. C. or higher prior to formation into the shadow mask.

Abstract: In production of a shadow mask for a color cathode ray tube from spinodal decomposition type magnet alloys, application of two staged agings before shaping successfully avoids thermal deformation of the product.

Abstract: The structure is material such as a ceramic in the form of a bundle of open cells, a "honeycomb", with the shape of the cells corresponding to the shape of the openings desired in the grid. The cathode is formed in the cells, rather than adding the grid to a ready-made cathode. The grid is formed by coating the end of the cell walls with a conducting material (metal and/or carbon). The surface of the cathode is suitably recessed from the grid.

Abstract: A method of manufacturing a focusing shadow mask for a color cathode ray tube is disclosed in which two conductive plates or surfaces, forming the electrode structure, are secured together by insulating material so that they do not contact each other. Strips or columns of insulating material are accurately located with respect to the matrix of apertures formed in the focus mask by etching cavities which may be in the form of grooves within at least one of the plates forming the electrode structure between adjacent aperture areas, and filling such accurately positioned cavities with a dielectric frit material, and then after firing such frit material etching away the surface of the plate so as to provide raised strips or pillars of insulator material firmly bonded and embedded in the mask structure.

Abstract: The novel method comprises (a) producing on one surface of a metal plate an array of substantially-parallel ridges, at least the tops of the ridges being electrically insulating, (b) producing an array of generally rectangular-shaped apertures through the plate in columns between the ridges and (c) producing electrically-conducting strips on the tops of the ridges.

Abstract: The novel CRT includes a color-selection structure which comprises a first electrode having a plurality of column and row elements forming a plurality of apertures, and a second electrode including an array of conductors electrically insulated from the first electrode. The plurality of apertures in the first electrode comprise alternate columns of first and second apertures. The first apertures have a width less than the width of the second apertures which are intersected by the conductors of the second electrodes to form first and second windows. The CRT also includes a viewing screen comprising an array of substantially parallel phosphor stripes of three different emission colors arranged in cyclic order in adjacent triads. Each of the triads of phosphor stripes is associated with a column of first and second windows and a column of first apertures.

Abstract: A color picture tube is disclosed, which comprises a phosphor screen, a shadow mask disposed near and facing the screen and having a main surface with a number of apertures, and electron guns for emitting electron beams passing through said apertures to bombard said screen and thereby selectively illuminating phosphors on the screen. The main surface of the shadow mask on the side of the electron guns is provided with a coating layer consisting of a material having a thermal expansion coefficient lower than those of the material of the shadow mask, and relatively low electrical conductivity, whereby doming of the shadow mask and miss-landing of electron beams can be suppressed.

Abstract: A cathode-ray tube of the post-focusing type comprising color selection means in the form of a ferromagnetic plate having a large number of apertures. The plate is magnetized so that a magnetic quadrupole lens is formed in each aperture. The ferromagnetic plate consists of an Fe-Co-Cr alloy having 25-27% by weight of Co and the remainder essentially Fe.

Abstract: In a method of manufacturing a color cathode-ray tube comprising a magnetic quadrupole post-focusing mask (7) the mask (7) is formed by a plate of permanent magnetizable material in which apertures (9) are provided arranged according to rows. Sections of the plate are successively magnetized by positioning parallel conductors (21, 22, 23) on at least one side of the plate in such manner that each time two conductors through which an electric current flows in opposite directions are situated between two rows of apertures (9) and a magnetic quadrupole is formed along the circumference of each of the apertures (9). A number of outermost conductors (30, 31, 32, 33) determines an edge area of the section. In order to prevent the occurrence of disturbing magnetization errors in the edge area, the edge area is made to coincide with a part of a preceding section already magnetized according to the desired strength.

Abstract: A method of manufacturing a picture tube shadow mask wherein a thin metal plate containing iron and nickel as major components is first etched to form a plurality of mask apertures, then, annealing of the metal plate is performed and, after being cooled in a reducing atmosphere, a darkened oxide layer is formed on the surface of the annealed metal plate by subjecting the metal plate at first to a relatively weak oxidizing steam atmosphere and then to a relatively strong oxidizing steam atmosphere.

Abstract: A shadow mask for a color picture tube, which has a plurality of regularly aligned apertures, an oxide film having good adhesion and anticorrosion characteristics which is made of a metal alloy consisting of as the major constituents, iron and nickel, wherein an iron content of at least a surface layer of a portion of the shadow mask which has the apertures is higher than that of the substrate of the metal alloy.

Abstract: A flat panel display for video and/or information display including a geometric array of individually energizable, low energy electron emitters in combination with an array of continuous channel electron multipliers for amplifying and directing the electron outputs of the emitters onto a phosphor screen to emit visible light.

Abstract: A method of manufacturing a shadow mask comprises the steps of: forming photoresist films on two major surfaces of a sheet made of a shadow mask material; forming latent images by selectively exposing the photoresist films, respectively; developing the latent images to form photoresist film patterns, respectively; and spraying an etching solution for etching the shadow mask sheet having the photoresist film patterns onto the two major surfaces thereof to etch bared portions of the shadow mask sheet, wherein the photoresist film formed on one of the two major surfaces of the shadow mask sheet is thicker than that formed on the other of the two major surfaces. The step of spraying the etching solution comprises a first step and a subsequent second step, wherein an impact of the etching solution against the shadow mask sheet in the first step is greater than that in the second step.

Abstract: A color picture tube provided with a plurality of shadow masks, each of which is formed of a main part having a curved surface with a large number of apertures and a skirt section supporting the peripheral portion of said main part, and the curved surfaces of said plural shadow masks are spaced from each other at a prescribed distance. The parts extending substantially perpendicularly to the curved surfaces of the skirt sections of at least two of the plural shadow masks is comprised of alternately arranged support portions and intervening cutouts. The support portions of the skirt section of one shadow mask face the corresponding alternate cutouts of the skirt section of the other shadow mask and the cutouts of the skirt section of said one shadow mask face the support portions of the skirt section of the other shadow mask so as not to overlap each other.

Abstract: The method of making a display panel 10 comprising forming an assembly of a glass base plate having a set of longitudinal first slots in which anode electrodes are secured and having an array of cathode electrodes seated on the top surface of the base plate and oriented transverse to the anodes. An apertured electrode plate is disposed adjacent to the base plate and its electrodes, and a face plate assembly is prepared by providing a glass plate and forming on one surface, in order, a large-area electrode and one or more insulating layers, after which an apertured insulating layer is formed thereon by a photo-etching process. The face plate assembly carrying the apertured insulating layer is assembled with the other parts of the panel, and the panel is processed to completion.

Abstract: A storage target is disclosed for use in a scan converter storage tube, direct view storage tube, etc. Included are a storage substrate in the form of a single crystal of sapphire, and a collector electrode in the shape of a directional pattern on a storage surface of the storage substrate. In order to afford a high writing speed the directional pattern of the collector electrode is oriented at an angle ranging from -45 degrees to +45 degrees with respect to the projection of the c axis of the single sapphire crystal on the storage surface of the substrate. The collector electrode is preferably in the form of electrically interconnected parallel stripes extending in the direction of the projected c axis.

Abstract: A modulator assembly for a flat panel display device is fabricated with a single piece substantially U-shaped isolation member. The modulation electrode pairs are formed as bifilar strips configured singularly to and insulatively affixed to the isolation member.

Abstract: In a first method, a meander-shaped coil (20,21) is provided on each side of the mask (7) having apertures (9). The coil (21) is oriented perpendicularly relative to the coil (21). If a current flows through the coils (20,21) in the correct direction, a magnetic quadrupole is formed along the circumference of each aperture (9). In a second method, a holder (30,31) with permanent magnetic strips (32) is provided on each side of the mask (7). The strips (32) in the one holder (30) are oriented perpendicularly relative to the strips in the other holder (31).A coil 40 is provided around the holders 30 and 31 with which a decaying magnetic alternating field, is generated which initially drives the material of the mask (7) on both sides of the hysteresis curve into saturation. After the decay of said alternating field a magnetic quadrupole is present along the circumference of each aperture (9).

Abstract: A method of making a mask arrangement including two (2) or more of masks for a mask-focusing color picture tube and the mask arrangement formed by the method. A plurality of apertured flat masks, each mask comprising an effective portion having apertures and a non-effective portion surrounding the effective portion, are prepared. The flat masks are aligned and stacked, then filling material is poured into the apertures. The filling material is solidified thereby fixing the flat masks together. The fixed masks are pressed into a predetermined curved shape. Then, the filling material is removed from the apertures and the masks are separated.

Abstract: A method of manufacturing a camera tube having structure suitable for the HN system comprising a glass faceplate covered by an n-type transparent electrode layer consisting of, for instance, Nesa glass on which a thin p.sup.+ -type layer, a p-type layer and an n-type layer are deposited in succession to form a photoconductive layer on which a block layer is deposited to form a protected photoconductive target. A metal mesh covered by an insulation material and a collector electrode for collecting secondary electrons emitted from the target are arranged on the electron beam scanning side of the target.

Abstract: The novel method comprises (1) producing in one major surface of a metal masking plate an array of substantially-parallel ridges separated by valleys or grooves, (2) removing metal from the other of said major surfaces in generally rectangular-shaped areas opposite the valleys and extending through the plate, (3) covering the tops of the ridges with an electrically-insulating first coating and (4) covering the first coating with an electrically-conducting second coating.

Abstract: The invention relates to a color display tube having a shadow mask (12) positioned in front of a display screen (8). The shadow mask (12) is coated at least on the side remote from the display screen with a heavy metal layer (14) of selectively varied thickness. The heavy metal has an atomic number exceeding 70 and has a high electron reflection coefficient, thus minimizing energy absorbed by the shadow mask from electrons impinging on the mask. Because of the selectively-varied thickness of the layer, reflection of electrons intercepted by the mask toward the screen is minimized without sacrificing the reduction in thermal expansion of the mask afforded by such a reflective layer.

Abstract: The photographic method utilized for fabricating slit-type shadow masks is improved by utilizing specially-shaped elements in a pattern on one of the photographic masters. In particular, the photographic master includes trapezoidal-shaped elements at locations away from the vicinity of the pattern minor axis, with the larger bases of the trapezoids facing away from the pattern minor axis.

Abstract: In a first method, a meander-shaped coil (20,21) is provided on each side of the mask (7) having apertures (9). The coil (21) is oriented perpendicularly relative to the coil (21). If a current flows through the coils (20,21) in the correct direction, a magnetic quadrupole is formed along circumference of each aperture (9). In a second method, a holder (30,31) with permanent magnetic strips (32) is provided on each side of the mask (7). The strips (32) in the one holder (30) are oriented perpendicularly relative to the strips in the other holder (31) A coil 40 is provided around the holders 30 and 31 with which a decaying magnetic alternating field is generated which initially drives the material of the mask (7) on both sides of the hysteresis curve into saturation. After the decay of said alternating field, a magnetic quadrupole is present along the circumference of each aperture (9).

Abstract: Method of manufacturing a deep drawn CRT mask in which the mask blanks are cut from the steel foil, each mask blank having a pattern of apertures. To facilitate the subsequent deep drawing and in order to obtain a grain size for good magnetic screening by the shadow mask an annealing process is carried out. An optimum grain size of the mask material between 0.015 and 0.040 mm with an average grain size between 0.020 and 0.030 mm is obtained by annealing at a temperature from 600.degree. C. to 850.degree. C. which lies above the temperature at which the mask blanks may adhere together by thermomolecular welding. In order to prevent rejects as a result of said welding, a stack of mask blanks is laid on a curved substrate prior to annealing (FIGS. 3f, 3g), which substrate preferably has the form of a part of a cylinder the radius of which is equal to the radius of the mask after deep drawing. After annealing in a furnace (FIG. 3h) the mask blanks are placed on a flat substratum (FIGS.