US 3791271 A
A photographic compositor character storage and optical retriever includes a plurality of axially spaced coaxially discs each having concentric circular groups of characters, as well as outer flash indexing marks and inner code symbols. Parallel flash beams are directed parallel to a face of each disc and are deflected at successive circular groups perpendicular to the disc and the beams transmitted through aligned characters are then deflected parallel to and along the opposite face of the disc. The sets of character beams from the discs are parallel and each group corresponding to a radially aligned set of characters from a respective disc is reflected by a corresponding first mirror to direct the beams along parallel lines colinear with the respective letter beams of the other discs and directed to respective second mirrors corresponding to longitudinally aligned disc characters which reflect the beams along colinear axes to an objective system. Selection of an individual character from the group in the X Y matrix effected by each flash and as reflected by the mirrors is accomplished by retracting or advancing corresponding one or more of the first and second mirrors.
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Description (OCR text may contain errors)
United States Patent n91 Nishikawa I Feb. 12, 1974 PHOTOGRAPHIC COMPOSITOR  Inventor:
 Assignee: Asahi Kogaku Kogyo Kabushiki Kaisha, Tokyo-to, Japan 22 Filed: Dec. 8, 1971 21 Appl. No.: 205,816
Yukiyasu Nishikawa, Saitama, Japan  Foreign Application Priority Data Primary Examiner-John M. l-loran Attorney, Agent, or F irm-Stanley Wolder [5 7] ABSTRACT A photographic compositor character storage and optical retriever includes a plurality of axially spaced coaxially discs each having concentric circular groups of characters, as well as outer flash indexing marks and inner code symbols. Parallel flash beams are directed parallel to a face of each disc and are deflected at successive circular groups perpendicular to the disc and the beams transmitted through aligned characters are then deflected parallel to and along the opposite face of the disc. The sets of character beams from the discs are parallel and each group corresponding to a radially aligned set of characters from a respective disc is reflected by a corresponding first mirror to direct the beams along parallel lines colinear with the respective letter beams of the other discs and directed to respective second mirrors corresponding to longitudinally aligned disc characters which reflect the beams along colinear axes to an objective system. Selection of an individual character from the group in the X Y matrix effected by each flash and as reflected by the mirrors is accomplished by retracting or advancing corresponding one or more of the first and second mirrors.
4 Claims, 10 Drawing Figures PATENIEDFEBIZIQM 3.791.271
SHEET 4 OF 5 INVENTO R lr/msz/ A bw/mna ATTORNEY Mal 1 PHOTOGRAPIIIC COMPOSITOR BACKGROUND OF THE INVENTION The present invention relates generally to improvements in photographic compositors and it relates particularly to an improved character storage device and optical retrieval and projection system.
Automatic photographic or optical compositors have been widely used in Europe and America largely because the compositor of this type is of simple construction by reason of the relatively small number of letters constituting the respective languages. Although the automatic photographic compositor has recently also been employed in Japan, there have been many problems with respect to their manufacture and maintenance since the conventional letter pattern carrying section is generally large-sized and the letter selecting mechanism is necessarily complicated due to various kinds of characters, consequent to the large number of letters used in the Japanese language. The letter patterns are usually carried on rotatable discs or drums (these are generally referred to hereinafter as discs), for example, but when a large number of letter or character patterns are carried on a single disc, a circumferential speed of the disc, which is of large diameter, necessarily increases and a flash lamp of very short ignition period is required. There has been suggested a system to overcome this drawback in which the diameter of the disc is reduced by carrying a large number of letter patterns individually on a plurality of discs which are, in turn, arranged in three-dimensional manner. There are two methods with respect to an optical system for character retrieval or representation to be used in this arrangement. One of these two is a method in which one optical system is provided for each letter disc, and the other is a method employing a single optical system wherein a particular disc on which the desired letter pattern is carried can be brought to an appropriate position in said optical system for representation prior to printing. The former requires, however, a large number of optical elements, the arrangement and adjustment of which are complicated. With the latter method, on the other hand, a relatively long time is necessary for the movement of the letter discs and the printing speed is sharply decreased, especially in case of a copy which requires a high frequency of such movements. There have already been suggested various methods also with respect to letter selection. One of them is a method in which P pieces of letter patterns are simultaneously illuminated and the desired letter is selected by operating movable mirrors such as letter selecting mirrors so as to be focussed into the corresponding image. Such a method, however, has required about P to 2P pieces of movable mirrors and the mechanism is necessarily complicated.
The apparatus or present invention eliminates the drawbacks of the prior art devices as mentioned above, and employs a compact letter pattern carrying section as well as a small number of movable parts in the letter selecting mechanism.
SUMMARY OF THE INVENTION It is thus a principal object of the present invention to provide an improved compositor.
Another object of the'present invention is to provide an improved photographic compositor.
Still another object of the present invention is to provide an improved photographic compositor having a compact high capacity high speed character storage section and a character retrieving and projection mechanism of simple construction and a minimum of moving parts.
A further object of the present invention is to provide a device of the above nature characterized by its reliability, efficiency, versitality, adaptability and superior operation.
The above and other objects of the present invention will become apparent from a reading of the following description taken in conjunction with the accompanying drawings which illustrate a preferred embodiment thereof.
In a sense the present invention contemplates the provision of a compositor character storage and retrieving device comprising a plurality of axially spaced concurrently rotated coaxial discs, each disc having a plurality of concentric circular sets of characters having light transmission properties different from the surrounding areas of the disc, first means for illuminating predetermined areas of the rear faces of the discs, at least the said illuminating means being disposed between successive discs, and second means exposed to the front faces of the discs in said predetermined areas for directing the character intercepted beams passing through the discs out of the storage device, at least some of the second means being disposed between successive discs.
Advantageously, the illuminating beams are directed between and parallel to successive discs and deflected along a disc radial line to be incident on a radial group of characters and the character delineating beams from the opposite sides of the respective discs are deflected in directions parallel to and outwardly of the discs. The character selection mechanism includes a first set of elongated mirrors upon each of which the character beams of a radial group of characters are incident and reflected along parallel axes colinear with the characters of the other radial groups. A second set of mirrors intercept the first beam axes and reflect the beams along a common axis to a projection lens system. A character is selected by moving the mirrors into and out of the character beam paths so that the beam from a preselected character reaches the projection lens.
The improved apparatus is compact, efficient and of high capacity, rugged, reliable and versatile. The projected characters are sharp and adjustable in disposition and dimensions, and the retrieval mechanism is simple with a minimum of moving parts.
BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view illustrating a letter pattern carrying section according to the present invention;
FIG. 2 is a fragmentary plan view of a letter disc;
FIG. 3 is a perspective view showing the construction of a letter selecting mechanism according to the invention;
FIG. 4 is a perspective view showing the arrangement, with respect to a single letter disc, of an optical system for introducing the beam for illumination and of an optical system for withdrawing the letter beams;
FIG. 5 is a fragmentary schematic plan view of the letter disc and the optical system adjacent thereto as DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the accompanying drawings which illustrate a preferred embodiment of the invention, the improved apparatus includes a plurality of identical elements and these are generally identified, for example, as the elements 1 and they are identified individually as the element ll, 1-2, l-n. Referring to FIG. 1, a plurality of information discs 1 on which letter patterns are carried (hereinafter referred to merely as the letter discs) are mounted on and regularly axially spaced along a common rotatable shaft 2 and beams for illuminating the letter patterns 3 are introduced between two adjacent letter discs, for example, between 1-1 and 1-2 in parallel with the planes defined by the two letter discs so that said beams are reflected by a reflecting optical system in the direction normal to said planes and illuminate the letter patterns 3 at predetermined points. The beams having passed through the letter patterns 3 (hereinafter referred to merely as the letter beams) are reflected again by the reflecting optical system in the direction parallel with the planes defined by the letter discs so that said letter beams are taken out from the letter discs through the space between the following pair of letter discs, for example, 1-2 and 1-3. The reflecting optical system is arranged in such a manner that the beams thus obtained from the respective letter discs have their optical axes mutually parallel and the points at which these beams intersect with the plane normal to their optical axes define an X Y matrix. These beams are thereafter introduced to a letter selecting section of an arrangement shown in FIG. 3. There are provided in a common plane'a plurality of objectives 6 each in association with a respective letter beam. The arrangement is such that these objectives 6 have their optical axes in parallel to each other and are equally spaced from respective letter patterns 3. The letter beam having passed through arespective objective 6 is reflected by disc selecting mirror 7 which constitutes a group of letter beam selecting mirrors, and thereby only a group of letter beams coming from the letter disc on which the desired letter pattern is carried is selected. The group of letter beams thus selected are further directed to row selecting mirrors 8 which form another group of letter beam selecting mirrors and thereby only the letter beam passing through the letter row in which the desired letter pattern is contained is selected so as to reach a projector lens 9.
Now referring to FIG. 2 which illustrates a preferred embodiment of the letter disc according to the present invention, the letter patterns 3 are arranged in a plurality of concentric circles at regular circumferential intervals in such a manner that these letter patterns 3 are radially aligned. There are provided on the outermost circumferential border marks 4 for flash timing at circumferentially equal intervals the number corresponding to the number of the letter patterns contained in each circumferential letter row, and inside the letter rows identifying letter conversion codes 5 are provided.
It is assumed for convenience of explanation that the letter pattern carrying section is formed of four letter discs each carrying the letter patterns on four concentric circles. An opticafsystem 10-11 for introducing an illumination beam (such as when a light guide is used) introduces the illumination beam parallel to the plane defined by the letter disc 11 and the beam is then deflected by a prism 11-1. Each prism 11 is of rightangled isosceles triangle in transverse section and has an oblique reflector face which is at an angle of 45 to the plane defined by each letter disc 1. The illumination beam incident upon the prism 11-1 is deflected by said reflector face so as to be normal to the plane of the letter disc 1-1 and illuminates the letter pattern 3-11. The letter beam having passed through said letter pattern 3-11 is reflected by the rear surface of the following prism 11-2 so as to be directed between the letter discs 1-1 and 1-2 in parallel with the planes of these letter discs and is reflected on a mirror 12-1. This mirror 12-1 is disposed at an angle of 45 to the axis of the letter beam. The letter beam incident upon this mirror 12-1 is reflected, again in the plane parallel with the disc plane, by 90 so that the beam is directed from the letter discs and to the objective 6-11 which is disposed on the optical axis of beam. Similarly, the illumination beams led through the light guides 10-12, 10-13 and 10-14, respectively, illuminate the respective letter patterns 3-12, 3-13 and 3-14 after having beam deflected by the prism 11-1. The letter beams having passed through the respective letter patterns are deflected by the prism 11-2, then reflected again by the mirror 12-1 falling on the respective objectives 6-12, 6-13 and 6-14. The letter beams introduced through the light guides of the. following stage 10-21, 10-22, 10-23 and 10-24 pass between the letter discs 1l and 1-2 and are reflected by the prism 11-2 so as to illuminate the letter patterns 3-21, 3-22, 3-23 and 3-24, respectively. The letter beams having passed through these letter patterns are reflected by the rear surface of the prism 11-3, guided between the letter discs 1-2 and l3, and reflected'by the mirror 12-2 and finally fall on the respective objectives 6-21, 6-22, 6-23 and 6-24. Identical optical systems as described above are disposed in the following stages:
When the light guides 10, the prisms 11 and the mirrors 12 are located in a relationship which is common to all the stages with respect to the rotatable shaft, it is possible to arrange 16 letter beams in such a manner that these 16 beams have their optical axes parallel with one another and the points at which these 16 beams intersect the plane normal to said axes define an X Y matrix. Since the distances between the letter patterns and said points at which sixteen letter beams intersect the plane normal to their optical axes are equal, the objectives 6 may be disposed on such points with their optical axes corresponding to those of the respective letter beams so that the objectives are also arranged in an X Y matrix establishing a common positional relationship between these objectives 6 and the respective letter patterns 3.
Considering the letter selecting mechanism operation in reference to the embodiment of FIGS. 3, 7 and 8, there is provided a disc selecting mirror 7 for each of the letter discs, and the arrangement is such that the reflecting surface of each mirror 7 is at an angle of 45 to the plane including the optical axes of the four letter beams coming from each letter disc and all the optical axes of sixteen letter beams after reflected by the disc selecting mirrors 7 are included in a common plane. The row selecting mirrors 8, for each of the letter beams from respective letter rows, are disposed at the positions upon which the letter beams reflected by the disc selecting mirrors 7 are incident i.e., four mirrors are provided so that all the four optical axes of the letter beams after reflected by these four row selecting mirrors are in a straight line. The projector lens 9 is fixedly located at the position upon which the letter beams reflected by row selecting mirrors 8 are incident and the optical axis of projector lens 9 coincides with that of the letter beams reflected by the row selecting mirrors 8. Now, the manner in which only the letter beam coming from the letter pattern 3-11 is selected so as to be'incident upon the projector lens 9 will be described with reference to FIG. 7. In this, neither operation of the group of the disc selecting mirrors 7 nor that of the group of the row selecting mirrors 8 is needed, because only four letter beams coming from the letter disc 1-l are incident upon the group of the row selecting mirrors 8 after having been reflected by the disc selecting mirror 7-1 and only the letter beam coming from the letter pattern 3-11 is incident upon the projector lens 9 after having been reflected by the row selecting mirror 8-1, while the letter beams coming from the other letter discs 1-2, 1-3 and 1-4 are reflected by the respective disc selecting mirrors 7-2, 7-3 and 7-4 but they are prevented from reaching the row selecting mirrors 8 by the rear surfaces of the disc selecting mirrors 7-1, 7-2 and 7-3 which are ahead of the respective paths of light. Furthermore, the letter beams coming from the letter patterns 3-12, 3-13 and 3-14 which are incident upon the group of the row selecting mirrors 8 are also prevented from reaching the projector lens 9 by the rear surfaces of the row selecting mirrors 8-1, 8-2 and 8-3 which are ahead of the respective paths of light, as in the case of said disc selecting mirrors. Now the case where only the letter beam coming from the letter pattern 3-22 is selected is described by reference to FIG. 8. This is accomplished by removing the disc selecting mirror 7-1 and the row selecting mirror 8-1 from the paths of light. These selecting mirrors occupy the positions as shown by two-dotted broken lines in FIG. 3, respectively, after being removed. When these selecting mirrors are in the moved position, the letter beams coming from the letter disc 11 can not be directed to the row selecting mirrors and only four letter beams coming from the letter disc 1-2 fall on the row selecting mirrors 8. Then only the letter beam coming from the letter pattern 3-22 is selected through the row selecting mirror 8-2 to be incident upon the projector lens 9. Similarly, the group of the disc selecting mirrors 7 and the group of the row selecting mirrors 8 may be operatively connected so that any one of 16 letter beams is selected to be guided to the projector lens 9. In any case, as it is not necessary to actuate either disc selecting mirror 7-4 or the row selecting mirror 8-4 these two mirrors may be fixedly located and, accordingly, letter selection is accomplished merely by operating six mirrors. As may be seen from the drawings, although the length of the light path from the objectives 6 to the projector lens 9 are different with particular letter beams, this fact causes no trouble when appropriate optical system as will be described later, is used. In contrast to the operation of the selecting mirrors as mentioned above in connection with the letter selecting mechanism, letter selection can be accomplished by locating the selecting mirrors usually off the respective paths of light and inserting only the selecting mirror which is in the path of light of the desired letter beam into the desired position. Letter selection may be accomplished also by arranging the selecting mirrors in such a manner that the letter rows are first selected and then the letter discs are selected.
FIG. 9 shows the path oflight from the letter patterns 3 to the primary focusing point 13 which is developed in a straight line having the optical elements as mentioned hereinbefore positioned thereon. FIG. 9 (a) particularly shows the path of light from the letter pattern 3-11 to said primary focusing point 13 which is the shortest of the 16 letter beams, while FIG. 9 (b) shows the path oflight from the letter pattern 3-44 to said primary focusing point 13 which is the longest of the 16 letter beams. When each objective 6 is disposed as shown in such a manner that the position of its front focus coincides with that of the respective letter pattern 3 with its centre on the optical axis of said objective, the light coming from a point on the letter pattern 3 exits the objective 6 in the form of a beam, or a bundle of parallel rays of light. With such an arrangement, the letter patterns can always be focused into the respective images at the position of the rear focus of the projector lens 9 (this position corresponds to said primary focusing point 13) independently of the length of the light path from the objective 6 to the projector lens 9. Thus, the desired letter pattern, wherever it may be carried, can always be focussed into the respective image on the primary focusing point 13 according to such arrangement in which the particular letter disc and the letter row containing the letter pattern to be printed are detected on the basis of the information on the letter to be printed, which is read out by such means as a paper tape reading out device, while the letter discs 1 are rotated around the rotatable shaft 2, the letter selecting mirrors having been moved to the operating position in advance, the letter conversion code 5 and the timing mark 4 are photoelectrically read out to detect the time point at which the desired letter pattern passes the optical axis of the objective, and the flash lamp is then ignited synchronously with this time point.
Now the optical system for performing the functions essential to the photographic compositor, such as letter sizing and conversion of magnification, will be described in reference to the embodiment as shown by FIG. 10. A magnification coverting lens 14 is fixedly disposed in such a manner that the position of the front focus thereof coincides with the primary focusing point 13 and the optical axis thereof coincides with that of the projector lens 9. A movable lens 16 and a movable mirror 17, as a unit are so arranged to be movable together along the optical axis of the magnification converting lens 14.
Photosensitive film 18 is set in such a manner that the position of rear focus of the movable lens 16 (corresponding to the secondary focusing point 19) is on the emulsion surface of the film. With the optical system arranged as mentioned above, the image in the air focused at the primary focusing point 13 may be effectively relayed so as to be focused into an image on the photosensitive film. The letter sizing is accomplished by moving the movable lens 16 and the movable mirror 17 according to the signal ofletter sizing. Thus, the secondary focusing point 19' as shown in FIG. 10, for example, corresponds to the position at which another letter pattern is to be printed.
Composite magnification M of the image focused at the secondary focusing point 19 is obtained from the relation:
fz/fi) fl/f3 where f,,f ,f and f, designate focal lengths of the objective 6, the projector lens 9, the magnification converting lens 14 and the movable lens 16. It will be seen from the above relation that the conversion of magnification may easily be accomplished by selecting the desired lens out of several lens is of different focal lengths and bringing said desired lens on the optical axis. Functions of turning and transforming the image may easily be added by inserting an optical system for image turning or an optical system for image transformation for obtaining a transformed letter such as a flat letter into the position in the drawing (this position corresponds to the beam portion between the magnification converting lens 14 and the movable lens 16). The present invention has been described by way of example with the letter pattern carrying section comprising four letter discs each containing four letter rows. When the letter pattern carrying section comprises n pieces of letter discs each containing m rows of letters, (m X n) pieces of information objectives, n pieces of disc selecting mirrors and m pieces of row selecting mirrors are necessary. The number of mirrors which should be arranged movably for letter selection is the total of letter beam selecting mirrors, (m+n) minus 2, that is (m n 2).
The present system has the following advantages. First, the efficiency of letter carrying is improved and a number of letter patterns may be storedin a relatively limited space since the letters or characters are distributed in a three-dimensional volume or space in comparision with the conventional manner in which the letters are two-dimensionally carried on a rotatable drum or disc. The reduced space occupied by the letter patterns not only results in a small sized compositor but also advantageously influences the ignition duration of a flash lamp, and the speed and the quality of printing.
The quality of printing means the quality which may be affected by the flow of an image due to the movement of the corresponding letter pattern during the ignition time of the flash lamp. With such a small-sized letter disc as provided by the invention, the circumferential speed at a given rate of rotation may be lowered and therefore a constant quality of printing may be obtained by use of a lamp having its ignition duration prolonged to such an extent as corresponding to that of the decrease of the circumferential speed. When the flash lamp of the same ignition duration is used, on the other hand, the quality of printing is improved to the'extent corresponding to that of the decrease of the circumferential speed. Although the rate of rotation of the letter disc has often been limited from the view point of printing quality, the rate of rotation may be increased, according to this invention, until the same circumferen- I 8 tial speed that is critical for the desired quality of printing in the conventional device. The period within which the letter pattern to be printed appears on the optical axis of the objective is thereby shortened and, accordingly, the printing speed is increased. The compositor according to the present invention may be easily adapted for different numbers of letters by increasing or decreasing the number of stages comprising identical parts since each letter pattern carrying section, the associated optical system and the letter selecting mechanism is constituted by identical parts assembled in the desired number of stages. The optical system for illumination and the optical system for taking out the letter beams may be disposed adjacent to each other, and for this reason these two optical systems can be con structed integrally with each other. According to such a construction, the part of the compositor occupied by the optical system for illumination and the optical system for taking out the letter beams may be retracted out of the circumference of each letter disc without causing any displacement of the relative position of these two optical systems, enabling the letter disc to be easily exchanged.
With the present mechanism, combined operation of (m+n 2) movable mirrors need be only placed under control of instructions from the outside in order to select, at a given time point, the desired one letter out of (m X n) letter patterns which are present on optical axes of the group of objectives for printing this selected letter. This means that a radical reduction of movable elements as contrasted with the conventional method in which substantially 2 P pieces of movable elements are usually required for selection of P pieces of letters. Such a reduction of movable elements enables an improvement of printing quality especially in precision in setting of letters. When the movable mirror is so arranged that said mirror is moved in the plane defined by the mirror and the angle of the mirror is accurately maintained, the printing quality is in no way affected by any residual oscillation of the movable mirror in the direction of movement thereof. Each group of the disc selecting mirrors and the row selecting mirrors includes one mirror which need not be actuated and this mirror is so made as to be fixed with a sufficient precision as a standard of adjustment, on the basis of which, therefore, adjustment may be easily accomplished in assembling and maintaining each group of said disc selecting mirrors and of row selecting mirrors.
When the letter carrying section and theletter selecting mechanism according to the present invention are used in combination, an increase in the number of letter discs, and accordingly of letters carried thereon, does not lower the printing speed at all, as is usually present in conventional structures. Since the printing speed, when such combination is utilized, largely depends upon the total of the time which the actuation of the mirror requires for selection of letters and the duration until the desired letter pattern appears on the optical axis of the respective objective, the printing speed is not at all affected even when the number of letter discs is increased to increase the number of letters to be carried.
While there has been described and illustrated a preferred embodiment of the present invention it is apparent that numerous alterations, omissions and additions may be made without departing from the spirit thereof.
carrying section comprising a plurality of letter pattern carrying discs which are rotatably mounted on a common rotatable shaft substantially at regular intervals and optical systems each disposed on rear side of a respective disc wherein beams for illuminating letter patterns carried on said discs are introduced between a pair of adjacent letter pattern carrying discs and the beams having passed through the letter patterns are taken out by said optical systems through the space between a pair of letter pattern carrying discs of the following stage, said optical system including a letter selecting mechanism comprising a plurality of objectives having their optical axes parallel with one another ar-' ranged in the form of an X Y matrix, each letter beam coming from the letter pattern carrying discs being reflected to the objective with its optical axis coinciding with the optical axis of said letter beam, and a group of letter beam selecting mirrors each disposed on the path of respective letter beam so as to select the desired letter beam out of a plurality of letter beams by controlling the operation of said letter beam selecting mirrors according to external instructions, the selected letter beam being always on the same optical axis.
2. A photographic compositor according to claim 1, and wherein said optical system illuminates a plurality of letter patterns at the same time and the desired letter beam is optically selected out of letter patterns coming from these letter patterns so as to be focused into the corresponding image on a photosensitive material, said optical system being combined with an optical system for adjusting the projected letter image.
3. A photographic compositor comprising a plurality of axially spaced concurrently rotatable coaxial discs each having a plurality of concentric circular sets of characters, first means confronting a first face of each of said discs for simultaneously deflecting the axes of a character delineating beam from each of said sets 1 from a direction perpendicular to said discs to a direction extending outwardly between successive discs, and means for selecting and focussing one of said deflected character beams to form an image of the selected character on a predetermined focal plane, and means comprising a plurality of first reflectors deflecting the character beams from each of said discs along parallel first axes colinear with the character beams from said other discs and a plurality of second reflectors intercepting each of said first axes and reflecting beams along said first axes along a common second axis, at least some of said reflectors being movable between positions in and out of registry with said first axes and said second axis.
4. A photographic compositor comprising a plurality of axially spaced concurrently rotatable coaxial discs separated by a plurality of successive interdisc spaces delineated by the confronting faces of successive leading and trailing discs, each of said discs having a concentric circular set of characters, and means including an optical system disposed in each of said interdisc spaces, for directing an illuminating beam toward a respective trailing disc and for directing a corresponding beam directed by a respective optical system in the next leading interdisc space through said disc into the respective trailing interdisc space transversely outwardly from said respective trailing interdisc space, and means for selecting and focussing said beams emerging from said interdisc spaces to form an image of a selected character in a predetermined focal plane.