US3633065A

US3633065A - Shaped beam tube

Info

Publication number: US3633065A
Application number: US868055A
Authority: US
Inventors: Robert H Compton; Alexander Bell
Original assignee: Stromberg Datagraphix Inc
Current assignee: ANACOMP Inc 11550 NORTH MERIDAN STREET CARMEL INDIANA 46032 A CORP OF INDIANA
Priority date: 1969-10-21
Filing date: 1969-10-21
Publication date: 1972-01-04
Anticipated expiration: 1989-01-04

Abstract

A shaped beam tube is described wherein the means for shaping the cross section of the beam include a pair of axially spaced electron opaque members, each having a set of apertures therein. Each aperture in one set corresponds to an aperture in the other set for producing a particular desired image. The opaque members are positioned with their corresponding apertures aligned on the path of the electron beam.

Description

Unite States Patent inventors Robert H. Compton a Alexander Bell, Carlsbad, both of Calif. App]. No. 868,055 Filed Oct. 21, 1969 Patented Jan. 4, 1972 Assignee Stromberg DatagraphiX, Inc. San Diego, Calif.

SHAPED BEAM TUBE l 1 Claims, 8 Drawing Figs. 0.8. CI 315/14, 315/21 CH Int. Cl H01] 29/82 FieldofSearch 3l5/14,2l CH, l4, 12 C [56] References Cited j UNITED STATES PATENTS 3,501,673 3/ 1970 Compton 315/31 3,329,858 7/1967 McNaney... 315/21 CH 2,925,526 2/l960 McNaney 3l5/2l Cl-l 3,385,990 5/1968 McNaney 315/21 CH Primary ExaminerRodney D. Bennett, Jr.

Assistant ExaminerN. Moskowitz Attorneys-John R. Duncan and Anderson, Luedeka, Fitch,

Even & Tabin ABSTRACT: A shaped beam tube is described whlrein the I a, I

PATENTED JAN 41972 INVENTORS ROBERT H. COMPTON ALEXANDER ATTORNEYS DM L SHAPED BEAM runs This invention relates to shaped beam tubes and, more particularly, to an improved shaped beam tube, of either the aperture selection or the beam selection type, in which superior image quality is attainable.

In character-generating tubes of the shaped beam type, one or more electron beams are shaped as they pass from an electron gun to a target such that the resulting cross section of each shaped beam is of predetermined configuration. The

area of the target energized by impingement of each beam thereon therefore corresponds in shape to the resulting cross section of the beam.

The desired beam cross section is attained by passing the beam through a shaping aperture in an electron opaque plate. The plate generally contains a plurality of apertures of various shapes. In tubes of the so-called aperture selection type, the apertures are all flooded to produce a plurality of shaped beams. Another electron opaque plate having a single selection aperture therein is positioned downstream from the first plate. The beams are deflected together in a desired manner so that one of the shaped beams is selected by passing through the selection aperture and onto the target screen. The unselected beams are blocked and do not reach the target. In the beam selection type of shaped beam tube, a single beam is deflected to select one of the plurality of apertures, and is subsequently deflected to a desired position on the target screen. This type of tube also employs, typically, another opaque plate having an aperture for trimming the beam prior to impingement on the shaping aperture, the trim aperture passing only those electrons within a rectangular periphery just including the desired shape.

In shaped beam tubes of the aperture selection type, the electron opaque plate having a selection aperture therein is positioned between the target screen and the other apertured plate. In shaped beam tubes of the beam selection type, the opaque plate having a single trim aperture is positioned between the electron gun and the shaping aperture plate. In both the beam selection type of tube and the aperture selection type of tube, the trim or selection aperture plate reduces the amount of stray electrons reaching the screen. Such stray electrons cause a diminution in contrast.

Even though a selection or trim aperture is utilized, a problem is still experienced in prior art shaped beam tubes with respect to stray electrons reaching the screen. In beam selection tubes, the result of such stray electrons is a faint glow diffused over the screen. In aperture selection tubes, the result of such occurrences is a small square or rectangle immediately surrounding the image of each selected character, and this small square or rectangle is an image of the selection aperture made visible by the stray electrons which pass therethrough. This dim image of the selection aperture, often referred to as background illumination," is particularly objectionable in microfilm recorders. This is because the microfilm integrates the light flux, incident on any incremental area, over the elapsed time. In many types of recordings, the areas of background illumination may overlap. These overlapped areas between rows and columns of characters may be exposed to the background illumination of the selection aperture for as much as four times the exposure period of one character. This results in a substantial deterioration of the quality of the image reproduced because of spurious light areas which confuse the appearance of the image.

It is an object of the present invention to provide an improved shaped beam tube. V

Another object of the invention is to provide a shaped beam tube wherein background illumination is minimized.

A further object of the invention is to provide an improved shaped beam tube wherein secondary emission of electrons is minimized to improve the quality of the image produced.

Other objects of the invention will become apparent to those skilled in the art from the following description, taken in connection with the accompanying drawings wherein:

FIG. 1 is a schematic full section view of a shaped beam tube constructed in accordance with the invention;

FIG. 2 is an enlarged cross-sectional view of a portion of the tube ofFIG. 1;

FIGS. 3-7 are fragmentary views illustrating various combinations of aperture sizes in accordance with the invention;

and

FIG. 8 is a schematic perspective view of a portion of a shaped beam tube constructed in accordance with an alternative embodiment of the invention. Z

Very generally the shaped beam tube of the invention comprises an evacuated envelope 1], an electron beam source 12, an electron-responsive target 13, and electron lens means I4 for focusing the electron beam. Means 16 are also provided for forming at least a portion of the cross section of the beam into a predetermined shape to produce a correspondingly shaped image on the target. The forming means include a pair of axially spaced electron opaque members 17 and I8, each having a set of apertures therein. The sets of apertures in the respective opaque members are arranged identically so that each aperture in one set has a corresponding aperture at the corresponding position in the other. Each pair of corresponding apertures in said sets is shaped to produce a particular desired image. The plates are positioned transversely of the electron beam and with their corresponding apertures aligned with the path of the electron beam.

Referring now more particularly to FIG. I, the schematic diagram of the shaped beam tube illustrates a tube of the aperture selection type. The glass or ceramic envelope ll of the tube is indicated by the dotted line which is formed with a bellshaped section joined to an elongated neck section. One end of the bell-shaped section is closed by a suitable glass faceplate, not illustrated. The target screen 13 is supported on the inner surface of the faceplate. As is common in the art, the target screen may be a coating of a suitable electron sensitive light-emitting material so that a visual display will be produced by impingement of electrons on the coating.

The electron beam originates at a cylindrical cupshaped cathode 12 which surrounds a heater 21. The cathode may be comprised of nickel coated on its'end surface with barium oxide or some other similar material which produces free electrons at elevated temperatures. A suitable electrical connection is made to the heater 21 for passing a heating current therethrough, and the cathode itself is maintained at a suitable potential, such as at ground. The free electrons emitted by the cathode are directed into a beam 22 by accelerating elements 23 and 24 so that the beam will impinge upon the target screen. Between the element 24 and the screen, the beam is very small and therefore only the axis of the beam 22 is indicated, whereas elsewhere the envelope of the beam is also indicated. 2

The accelerating elements 23 and 24 are maintained at a positive potential with respect to the cathode by an accelerating potential source 26. An aquadag coating 25 is provided on the inside of the bell-shaped part of the envelope, as is known in the art. The target screen 13 and the coating 25 are maintained at a substantial positive potential with respect to the cathode by an anode potential source 27. Suitable circuitry (not illustrated) for providing and regulating such potentials at the respective sources 26 and 27 may be provided. In addition, a grid element 28 is provided adjacent the cathode in order to shut off and turn on the electron beam, and to collimate the beam emitted by the cathode.

In order to form the cross section of the beam into the shape of a desired character, the beam-forming means 16 are pro-' vided. The beam-forming means will be described in greater detail subsequently, and include a pair of axially spaced electron opaque members or

plates

17 and 18, each having a plurality of apertures therein shaped to provide desired images. The plates are positioned parallel with each other, perpendicular to the axis of the tube. The apertures in the opaque members are arranged in similar patterns, preferably, in identical matrices. The apertures at corresponding positionsin the respective matrices correspond to a particular image shape.

The matrix of apertures in thefirst'plate 17 is flooded by the 'electron beam and the electron beam passing through the apertures is thus segmented into a bundled individually shaped beams. For simplicity, the bundle of shaped beams is indicated. as the single beam 22 in the drawings. As will be described, each of the beams in the bundle passes througha respective aperture in the'second plate 18. I I

' A focusing electrode 29, connected to a variable potential source 31, in conjunction with element 23 'and'the'shaping 7 means 16, forms an electron lens which converges the bundle '22 of beams and produces an electron image of the apertures I of the shaping means 16 in the plane of a further apertured' I plate 32. I I I I I The beam or bundle of shaped beams 22 is converged to a minimum cross section at a point 33 and then diverges to its original size or (preferably) a somewhat greatersize in the.

plane of the aperture plate 32. The plate 32 is supported in the acceleratingelement 24 and defines a selection aperture 34. The aperture 34 is small enough to permit'only'one of the,

shaped beams in the bundle 22 to pass therethrough, but is conditions, any electron which passes through the aperture 34 large enough to pass the one beam without affecting its shape.

The other shaped beams are blocked by the plate 32.

Between the forming means 16 and the aperture plate 32,

the entire bundle 22 is deflected to select the particular shaped beam in the bundle which is to be passed through'the' the shaping means 16 will appear at the aperture 34.

The problem of background illumination 7 in previously known tubes is'a result of secondary emission of electrons and the passage of such stray electrons through the selection aperture 34. A major portion of the stray electrons reaching the selection aperture '34 are'generated by secondary emission the cathode 21 impinges on the cathode side of the aperture plate 17. The secondary electrons have random velocities and 7 directions and some pass through the shaped apertures in the aperture plate 17, generally in a direction not parallel to the path of the electrons in the shaped beams. The electron lens formed by the elements 29 and 23, which produces an electron image of the apertures in the plane of the selection aperture plate 32, is designed to cause every electron which comes through an aperture to be directed to pass through a corresponding point in the plane of the plate 32. Primary electrons in the shaped beam travel parallel to the axis of the tube, or in a direction not far from axial, and such electrons are perfectly imaged into the shape of a character in aperture 34, as intended. Secondary electrons travel at random angles which may deviate greatly from the axial direction, and the lens 29, 23 images them imperfectly onto points in the vicinity of the character in aperture 34. Thus, in previously known tubes many of the random velocity secondary electrons which pass through the selected aperture in the first aperture plate 17 reach the selection aperture 34 and pass through it.

The shaped beam tube of the instant invention differs from previously known tubes in that two electron

opaque plates

17, 18 with shaping apertures are provided in the instant tube, while previous tubes were provided with only one such shaping plate. The addition of the second plate and the application of suitable potentials to the two shaping plates greatly reduce the number of secondary electrons reaching the selection aperture 34, as will be further described and explained hereinafter.

A second electron lens follows the aperture 34 and images the electrons at the aperture 34 on the target screen 13. The second lens consists of a pair of

lens elements

41 and 42 connected to a potential supply 43 which provides suitable potentials on the elements for focusing the shaped beam image at the aperture 34 on the target screen, and for providing a desired degree of magnification. A yoke 40, driven by a suitaof the other, and each matrix is disposed perpendicular to the ble driving circuit 45, provides the'final deflection of the beam to the desired position on the screen. Since, under imaging will impinge on the target screen, the stray electrons as well as the beam electrons energize the target screen. Thus, a small square or rectangle of weak illumination is produced surrounding the image of the selected character. This small square or rectangle is an image of stray electrons at the aperture 34and constitutes the background illumination that is particularly objectionable in microfilm recorders for the reasons previously explained. I I I The beam-forming means 16 of the instant invention may be more clearly seen in FIG. 2. The forming meansinclude two support members 44 and 46,'each.of which consists of a short cup shaped element having a central opening therein. Axial separation is provided by an insulating washer 50. The elec tron opaque members'17 and '18 each comprise a metallic plate, the plate 17 being mounted within the support member 44 and the plate 18 being mounted within the'support member I 46. Each plate is provided'with a matrix'of apertures 49 consisting of the variousdesired shapes to be utilized in the display. The two matrices are arranged identically so'that each I aperture in one matrix corresponds to the apertureat a similar position in the othermatrix. The corresponding apertures may 'beidentical or merely similar in shape, as explained below. I I

The matrices are precisely aligned with each other so thatthe rows and columns of one areparallel to the rows and columns axis of the tube and parallel with the other. I I I I support memberismaintained by a pair of holes in the plate when the initial large area high-current beam emanating from which fit over a corresponding pair of aligning posts '47 and 48 on the associated support members. A pair of

discs

51 and 52,

each having a central opening therein to avoid covering the apertures in the matrices, fit against the respective plates 44 and 46. Each disc has two holes for cooperating with the posts 47, 48. The holes are slightly smaller than the posts so that the ends of the holes grasp the posts when the discs are pressed into place. The discs then hold their associated plates flat against the respective supports.

The two support members 44 and 46 may be held in coaxial and rotational alignment in any suitable manner, depending upon the construction of the particular tube. For example, when utilized in a tube wherein the internal elements are supported on several long thin ceramic rods, the support members may be held in coaxial and rotational alignment by welding small metal strips to the outer surface of the support members to strap the support members to the ceramic rods. By way of further example, when the invention is used in a tube having a construction as shown in US. Pat No. 3,354,339, the support members may be maintained in coaxial alignment by the inner surface of a ceramic tube and in rotational alignment by the cooperation of a radially extending pin, on each of the supports, with a slot in the ceramic tube.

The material of which the two

plates

17 and 18 are made should have a high enough melting point to prevent melting of areas heated by the electron beam. The material should also have a low enough vapor pressure to prevent the accumulation of excess gas within the tube envelope. The material should also have a low thermal coefficient of expansion in order to avoid excessive warping when heated by the electron beam. The material should also be etchable, since the apertures are most easily formed therein by an etching process. The preferred material of which the plates are comprised is beryllium copper. Other materials which may be satisfactory are oxygen-free copper, and nickel. Such materials are convenient from the standpoint that they may be made very thin and manufactured in continuous strips.

The thickness of the

plates

17 and 18 is preferably between about one-half mil to about 3 mils. Thicknesses less than about one-half mil are susceptible to burning out or buckling due to excessive heating or thermal gradients. Nevertheless, it is a general rule that the thinner the plate is, the better the character definition obtainable. If the plate is too thick, exceeding about 3 mils in thickness, the etching step effects an undercutting of the aperture with a consequent deterioration in the quality of the character definition.

The

plates

17 and 18 should be mounted axially spaced from each other a distance which avoids touching of the plates, even through some distortion of the plates will occur due to thermal gradients. Generally, the closest possible spacing is typically about equal to the thickness of one of the plates. Usually, a greater spacing is utilized since the closer the plates are together, the less effective the suppression of background illumination. The maximum spacing is about times the width of the narrowest character line in the apertured plate 18, the plate closest to the target screen. Any greater spacing generally presents extreme difficulty in achieving the proper alignment of the plates.

As previously mentioned, the corresponding apertures in the respective matrices may be either identical or merely similar. If the lines forming the character and comprising the aperture in the plate 17 closest to the cathode are wider than the lines in the other plate 18, greater brightness of the character image may be achieved over that possible where the apertures are identical (FIG. 3). The increase in line width may be slight, such as may be accomplished by over etching the plate (FIG. 4), may be moderate as by doubling the line width on the photomaster from which the matrix of apertures is etched (FIG. 5); or may be extreme wherein the characters on the first matrix are merely blobs having an outline generally like the character to be reproduced (FIG. 6). The more identical the apertures in the first matrix are to the corresponding apertures in the second matrix the lower the background illumination but, in addition, lower brightness is available. On the other hand, although a higher background illumination results with larger line widths in the apertures of the first matrix, a corresponding increase in brightness does occur. As a further alternative, the line widths in the second matrix may be made larger than those in the first matrix (FIG. 7).

Several modes of operation of the invention are possible. Each matrix plate may be operated at the same potential or with one plate more positive than the other. In any case, the matrix plate closest to the cathode serves as a heat shield to reduce radiant heat transfer from the cathode to the second matrix plate. This permits the second matrix plate to remain cool, flat, unbuckled and nearly the intended size and shape. Moreover, the first matrix plate intercepts a major portion of the incident beam from the cathode and, therefore, reduces the current impinging on the second matrix plate. Since the second matrix plate is thereby required to dissipate less electrical power, it remains cool and undistorted. Moreover, because the first matrix plate intercepts the major portion of the incident beam from the cathode, secondary electrons generated by impingement of the electron beam on the second matrix plate are reduced. A major portion of the unavoidable secondary electron emissions occur on the cathode side of the first matrix plate, and these secondary electrons are less likely to produce a background illumination because most of them will not succeed in passing through the apertures in both the first matrix plate and the second matrix plate to thereby be accelerated to the screen. Although some secondary electron emissions may be produced at the cathode side of the second matrix plate, they are few in number, since the beam current impinging upon the second matrix plate is small. Therefore, few secondary electrons generated at the second plate reach the target screen.

When the matrix plate closest to the cathode has less width in the lines of its apertures than the second matrix plate (FIG. 7), or when the two matrix plates are formed with apertures which are identical (FIG. 3), satisfactory operation may be achieved by maintaining the first matrix plate at about +80 to about +110 volts with respect to the cathode. The second matrix plate is then operated at a potential which is at ground or slightly negative with respect to the cathode, for example, 0 to about -20 volts. Under these conditions, low-velocity stray electrons in the region between the two matrix plates and which are not within the shaped beams themselves are repelled by the negative voltage on the second matrix plate and return to the first plate. Accordingly, background illumination is minimized.

when the matrix plate closest to the cathode is formed with lines comprising the shaped apertures which are substantially wider than the corresponding lines in the apertures in the second matrix plate (FIGS. 4-6), the first matrix plate may be operated at volts with respect to the cathode and the second matrix plate operated at a somewhat more positive potential, e.g., to +350 volts. The primary electrons striking the first matrix plate in the region between the aper tures are intercepted by the plate. Many secondary electrons are produced, some of which return to the surface of the first matrix plate while others pass through the apertures therein, generally at random angles and not parallel to the tube axis. The greater portion of the secondary electrons produced by the first matrix plate and which enter the space between the matrix plates are attracted to and captured by the second matrix plate. At the same time, some primary electrons which pass through the wider apertures in the first matrix plate impinge on the second matrix plate and generate secondary electrons. The secondary electrons generated by the second matrix plate, however, are relatively few because most of the excess beam current (that is, that current not needed for defining each character itself) is removed by the first matrix plate before the beam strikes the second matrix plate. Some of the secondary electrons generated at the second matrix plate are attracted back to it, while relatively few penetrate the character-shaped apertures in the second matrix plate at random angles to contribute to background illumination.

Other factors contribute to improved quality images in ac cordance with the invention. With a given voltage applied to the second matrix plate, the first matrix plate may be adjusted for optimum character clarity and minimum background illumination. Accordingly, the first matrix plate acts as a further focusing electrode to adjust the electron trajectory approaching the second matrix. In addition to this, both the matrix plates physically trim off uncollimated electrons which contribute to stray electrons around the desired matrix character.

Another factor affecting character definition is the absolute value of both voltages maintained on the matrix plates as they affect the spreading of the electron beam due to the space charge effect. Thus, relatively high operating voltages of +80 volts on the first matrix plate and +350 volts on the second matrix plate produce what are probably the best results.

Although the invention has been described herein in connection with a shaped beam tube of the aperture selection type, the invention is applicable to use in a shaped beam tube of the beam selection type as well. In the latter type of tube, it is still desirable to minimize the presence of stray secondary electrons.

Referring now to FIG. 8, an alternative embodiment of the invention is illustrated schematically. The embodiment of FIG. 8 is a shaped beam tube of the beam selection type. It is shown in simplified schematic form for clarity. Parts having similar construction and operation to parts in the embodiment of FIGS. 1 and 2 have been given identical reference numerals preceded by a I. A narrow electron beam 122 is produced by a cathode 112, controlled by a grid 128 and accelerated by an accelerating element 151 maintained at a suitable potential. The beam 122 then impinges on an apertured plate 132 having a single rectangular trim aperture 134. The trimming plate 132 stops electrons in the fringes of the beam, and passes only the central portion of the beam in which the most uniform distribution of electron densities and electron velocities are found. The trimmed beam is deflected from the tube axis by two pairs of

deflection plates

152 and 153, to select a desired pair of corresponding apertures in a pair of matrix plates I17 and 118. The trimmed beam impinging on matrix plate I17 is just large enough to cover one character-shaped aperture of matrix plate 117. The beam is directed in a path parallel with the tube axis as it passes through the apertures by an electrostatic convergence lens 154. The arrangement of the matrix plates and supporting members may be similar to that in the previous embodiment.

After passing through aligned and corresponding apertures in the plates 117 and 118, the beam is deflected back once again toward the axis of the tube by an electrostatic convergence lens 155. Two pairs of deflecting

plates

156 and 157 redirect the beam along the tube axis. Final focusing of the beam is provided by a lens 142 to produce an image of the shaping apertures in the plane of the target screen 13 and final positioning of the image on the target screen is determined by a deflection yoke 140.

Although the first matrix plate 117 is not flooded by the electron beam as in the previous embodiment, nevertheless, some secondary electron emissions are produced which contribute to background illumination. Thus, the problem of background illumination is present in a tube of the type shown, and construction and operation of the beam-forming means 116 as previously described aids in the suppression of background illumination.

It may therefore be seen that the invention provides an improved shaped beam tube wherein background illumination is minimized for superior image quality and contrast. A variety of operational modes are possible, depending upon the desired degree of contrast, desired level of background illumination, and desired level of brightness. A particular advantage flowing from use of the invention is the capability of producing superior microfilm images from cathode ray tube displays.

Various modifications of the invention, in addition to those shown and described herein, will become apparent from the foregoing description and the accompanying drawings. Such modifications are intended to fall within the scope of the appendant claims.

What is claimed is:

l. A shaped beam tube comprising an evacuated envelope, an electron beam source within said envelope an electron responsive target within said envelope and spaced from said source, electron lens means between said source and said target for focusing the electron beam means between said source and said target for forming at least a portion of the cross section of the beam into a predetermined shape to produce a correspondingly shaped image on the target, and means for directing said electron beam through said forming means along a selected path substantially parallel to an axis between said source and said target, said forming means including a pair of axially spaced electron opaque members, each having a set of apertures therein, said sets of apertures in said respective opaque members being arranged similarly so that each aperture in one set has a corresponding aperture at the corresponding position in the other and with each pair of corresponding apertures in said sets being shaped to produce a particular predetermined image, said plates being positioned transversely of the electron beam and with their corresponding apertures aligned with the path of the electron beam.

2. A shaped beam tube according to claim 1 wherein each of said electron opaque members is a metallic plate.

3. A shaped beam tube according to claim 2 wherein each plate has the apertures therein arranged in a matrix, and wherein means are provided for supporting said plates in axial and rotational alignment on a common axis.

4. A shaped beam tube according to claim 1 wherein the apertures define characters made up of lines, wherein the lines forming the apertures in the electron opaque member closest to said source are substantially wider than in the other electron opaque member.

5. A shaped beam tube according to claim 4 wherein the lines forming the apertures in the electron opaque member closest to said source are approximately double the width of the lines forming the apertures in the other electron opaque member.

6. A shaped beam tube according to claim 4 wherein the lines forming the apertures in the electron opaque member closest to said source are substantially greater than double the width of the lines forming the apertures in the other electron opaque member.

7. A shaped beam tube according to claim 1 wherein the apertures define characters made up of lines, wherein the lines forming the apertures in the electron opaque member closest to said target are at least as wide as in the other electron opaque member.

8. A shaped beam tube according to claim 1 wherein the apertures define characters made up of lines, wherein each of said electron opaque members comprise plates of a thickness between about one-half mil to about 3 mils, and wherein said plates are spaced apart along the axis of the tube a distance which is not less than about the thickness of said plates and which is not greater than about 20 times the width of the narrowest character line in the plate closest to said target.

9. A shaped beam tube according to claim 8 wherein the material of which said plates are comprised is a metal selected from the group consisting of beryllium copper, oxygen-free copper, and nickel.

10. A shaped beam tube according to claim 4 including means for operating said shaped beam tube with a potential on said electron opaque member closest to said source which is positive with respect to said source, and with said other opaque member substantially more positive.

11. A shaped beam tube according to claim 7 including means for operating said shaped beam tube with a potential on the electron opaque member closest to said target which is negative with respect to said electron beam source, and with a potential on the other electron opaque member which is positive with respect to said electron beam source.

Claims

1. A shaped beam tube comprising an evacuated envelope, an electron beam source within said envelope, an electron responsive target within said envelope and spaced from said source, electron lens means between said source and said target for focusing the electron beam, means between said source and said target for forming at least a portion of the cross section of the beam into a predetermined shape to produce a correspondingly shaped image on the target, and means for directing said electron beam through said forming means along a selected path substantially parallel to an axis between said source and said target, said forming means including a pair of axially spaced electron opaque members, each having a set of apertures therein, said sets of apertures in said respective opaque members being arranged similarly so that each aperture in one set has a corresponding aperture at the corresponding position in the other and with each pair of corresponding apertures in said sets being shaped to produce a particular predetermined image, said plates being positioned transversely of the electron beam and with their corresponding apertures aligned with the path of the electron beam.