US2882792A - Luminous output transducer - Google Patents

Description

April 21, 1959 s. w. LEVINE 2,832,792

LUMINOUS OUTPUT TRANSDUCER Filed Sept. 10, 1954 2 Sheets-Sheet 1 r43 Screen I Genera *0 I S e r/ize INVENTOR BY WM ATTORNEY April 21, 1959 s. w. LEVINE LUMINOUS OUTPUT TRANSDUCER 2 Sheets-Sheet 2 Filed Sept. 10, 1954 5. W: Levi/1e INVENTOR BY WW W ATTORNEY United States Patent LUMINOUS OUTPUT TRANSDUCER Samuel W. Levine, New York, N.Y., assignor to Fairchild Camera and Instrument Corporation, a corporation of Delaware Application September 10, 1954, Serial No. 455,117

7 Claims. (Cl. 88-61) This invention pertains to transducers, and more particularly to a transducer capable of producing a luminous output in the form of a dot or spot of light having a predetermined shape but whose size is a function of a varying electrical input applied to the transducer.

So-called light valves of a large variety of designs are well known in the optical and electro-optical arts, finding particular application in modulating light beams in response to input signals of electrical or acoustic origin. In general, the varying luminous output of such light valves consists of changes in the intensity of a beam, for example in variable density sound recording or photographic film. However, light valves have also been de-.

vised in which the luminous output is constituted by a focussed line or strip of light, one of whose dimensions is varied in response to the input signal. Such types are employed for variable-area sound recording, and when used for push-pull recording, one dimension of the luminous line or strip is altered by equal amounts on opposite sides of a central ray. The present invention aims to provide an optical system comparable to a light valve, but whose luminous output takes the pattern of a focussed geometrical shape (specifically, a square) whose dimensions as to both length and width will vary simultaneously in accordance with an input signal. In other words, the luminous output may consist of a square dot of light whose size will be controlled by the input signal to the transducer, but whose shape factor will remain a geometrical square regardless of the variations in said signal. However, the dot may equally well take other geometrical shapes, as will appear hereinafter.

The invention has a particular important application in'graphic arts, since it is ideally and peculiarly adapted to the production of successive dots of varying sizes which, when laid down in a regular pattern upon a photographic or other photosensitive material, will produce directly a halftone representation of the information constituting the signal input to the transducer. Of course, such signal information must be of proper form and periodicity so that separate discrete dots are produced in regular order, rather than a continuous line whose width only varies with the picture information.

It is a further important object of the invention to provide a transducer of the above type in which the luminous flux of the output beam is substantially constant, when integrated over the time of exposure of one dot, to produce the efiect of a beam of substantially constant intensity whether the dot size called for is large or small. This is necessary to minimize the density variations in the final product or picture, which would otherwise result because of the finite speed with which the light valve opens and closes.

Without intending to limit the invention thereby, a better understanding of the purposes of the new device may be had from consideration of one successful application thereof. In the making of halft-one printing plates by automatic photoelectric engraving apparatus, it is necessary to engrave the proper dot structure upon a suitable sheet, the dots being represented by indentations or the like in the sheet material. Such a procedure is well disclosed in U. S. Patent No. 2,575,546, issued November 20, 1951, to J. A. Boyajean. In that particular process, the engraving results from the periodic contact of a hot stylus with a plastic blank, the size of the dot or cavity produced at each point on the plate being determined by the depth to which the hot stylus enters such plate. While that procedure, and the apparatus covered by the patent, has gone into wide and successful use, the necessity for utilizing a hot stylus imposes limitations which are undesirable in many cases. Similarly, in other engraving techniques Where the product is a representation by discrete dots, the necessity for physical movement of the cutter or stylus, especially at reasonably high repetition frequencies, requires the precise control of a substantial amount of electromechanical energy.

Considered as a substitute for a mechanically moving tool or stylus, the present invention may be used in connection with a photosensitive plate to produce the desired dot structure in various ways. Thus, if the sheet being engraved is a suitable plastic which can be locally hardened by luminous energy, the same may have the desired pictorial representation developed thereon by known processes familiar to those skilled in the photomechanical aspects of the graphic arts field; e. g., by the use of light-hardenable materials or the like. Likewise, if the sheet being treated is photographically sensitive, a direct halftone picture will be produced, after exposure of the entire surface to the dot pattern produced by the invention, by the ordinary processes of photographic development.

In general, then, the invention provides a transducer which produces a recurrent dot pattern as a luminous output, rather than as the result of the physical action of a tool. Inasmuch as halftone reproduction techniques require the use of discrete dots whose order of size is quite small (individual dots being not normally resolvable at the intended viewing distance), the ability to produce dots in a size range Whose lower limit may be of the order of 0.001 inch is desired. On one hand, the solution of the problem is complicated by diffraction effects which result when it is attempted to define a small light beam by physical apertures. Also, the design is made difiicult by the fact that successive dots, even though of dilferent sizes, must have their centers spaced with great precision; in other words, changes in size of the dot produced must not involve changes in the centered positions of successive dots, nor changes in the geometrical shape of the dots, nor noticeable changes in the time integral of luminous flux comprising each dot when divided by the area of said dot. The invention satisfactorily accomplishes all of these aims, and does so by devices which are relatively inexpensive to manufacture and sufiiciently compact so that they may be substituted directly, in many cases, for purely electromechanical transducers formerly used.

The invention will best be understood by referring now to the following detailed specification of certain preferred embodiments thereof, given by Way of example, and taken in connection with the appended drawings, in which:

Fig. l is a schematic View, principally in side elevation, of one preferred form of transducer incorporating the invention,

Fig. 2 is a view in plan showing the shape of an opaque aperture mask used in Fig. 1,

Fig. 3 is a diagram illustrating the shape of the luminous output beam of the Fig. 1 device,

Fig. 4 is a view similar to Fig. 2 of a modified aperture mask,

Fig. 5 is a diagram sirnilar'to Fig. 3 illustrating the instantaneous output when using the Fig. 4 mask,

Fig. 6 is a view similar to Fig. l but showing a modified arrangement of the parts, and

Fig. 7 is a view in perspective of still another modified form of transducer.

For reasons which have in part been discussed above, the use of a physical aperture of variable size is prohibited; such components, including iris diaphragm struc tures and the like, would produce substantial diffraction effects especially when at their lower limits of size, and in addition require multiple moving parts whose masses must be accelerated at high rates if signals of frequencies ordinarily encountered are to be reproduced. The mass and friction effects both involve high power requirements which shouldbe avoided. The arrangement shown in Fig. 1 of the drawings requires only asimple stationary aperture mask, and a single moving part which takes the form of a tiltable mirror, but nevertheless satisfies the requirements for variable dot size and precise centering of all dots. As shown in that figure, a light source offsuit: able and constant intensity is indicated by numeral I0, together with a reflector 12 and a condensing lens s ystem 14 toproduce a concentrated beam of light directed towards the aperture mask 16, and of suflicient size to cover completely the aperture therein. This aperture, as better shown at 18 in Fig. 2, is of isoscles triangular shape, with an apex angle A of 90 if square dots are tobe produced.

An objective lens 20 is positioned to image theaperture 18 at a knife edge 22 whose sharp edge protrudes partly into the beam representing the aperture, image shape. However, between lens 20 and knife edge 22, a: reflector 24 is positioned, preferably constituted by a. front-surface reflectormounted for tilting througha limited angle about an axis perpendicular to the plane of the drawing and lying in the reflecting surface of the element 24; the'tilt axis should pass through the point of the reflecting surface at which the central ray of the impinging beam st'rikes'the same, so that tilting movements of the reflector 24 will'p'erm'it more or less of the apertureis image at knife edge22 to'be occluded thereby, without: any sidewise shifting of the shadow edge produced by the knife edge.

Suitable means are provided for tilting of reflector 24 in response to electrical control or input signals. As shown, such means may include a coil 26 energized by the sig{ nals and operating to move an armature 28 or the like attachedsecurely to the reflector 24.. The reflector may; be'freel'y pivoted and provided with restraining springs to; return it to its proper zero-deflection position, orthe Jtilt may result from torsion or bendingof the reflectorsupm port. Such mechanical details may be widely variedwithw out departing from the spirit, of the invention. Actually, it is quite feasible to mount the reflector 24 upon the. stylus-carryingpart of the electromechanical transducer shown in'the Boy ajean patent, although the powerfcapa bilitles of the lattenare 'greater than necessary for the purpose of tilting the reflector, 'which maybe anjex -l tremely light structure. i

From the above, it' follows that the beamshape leav ing the knife edge22 constitutes a triangle whose base is. defined by the knife edge, and whose altitude. line bisects. the 90 degree angle opposite to the base. The base (knife edge) being stationary, the position of the center. of the base line is also stationary. The desiIedsquareoHtput dot shape is obtained by combining, along their common. base lines, two such triangles, asbest indicated in Fig. 3, wh ch d agrams the shape of the final outputbeain. In.

the latter figure, the vertical dot and dash line indicates the maged position of" the knifeedge with reference .to

the beam'fand 'th'e'triangle to the left or that line.repre-..

sen tsthe beam. shape leaving the physical knifeedgq 22.

The"complementary ,triangle to the rightof the center.

line is produced optically in a mannernowto be, described.

Returning. now .to. .lig. 1, the beam leaving knife edge 22 proceeds to a beam-splitter or semi-reflector 30 disposed at an angle (preferably at 45) to the beam direction. One half of the beam proceeds by transmission through 30 to a full reflector 32 and is reflected through a focussing lens 34 onto the surface 36, which may be the photosensitive surface to be engraved or upon which the succession of dots is to be imaged. This surface is shown in perspective for clarity, but it will normally be perpendicular to the plane of the drawing. The other half of the beam proceeds from beam splitter 30, by reflection, to a further full reflector 38 and is focussed by lens 40 upon surface 36 in a position adjacent to the spot produced by the. first half of the beam. Since the common base line of the two triangular spots is fixed with respect to the optical axis at the knife edge, and hence with respect to the two axes arriving at the surface 36, it follows that tilting mirror 24 will produce the variable size symmetrical square shape desired, and that the center of the square will always fall on the same spot with reference to the fixed parts of the optical system. This results from the fact that one half of the beam arriving at surface 36 undergoes a single reflection (at reflector 32 after leaving the mirror 24), while the other half undergoes two such reflections, one at beam splitter 30 and one atreflector 38. Each reflection reverses thesenseor direction of effective beam deflection due to tilting of; 24, whence the use of a number of reflections differing by one, for the two halves, produces opposite movement ofthe two triangularhalves of the outputdot.

The desired succession of such dots is,- of course, produced-by proper relative movement betweenthe surface 36 and 'the" body of the transducer, in the usual manner exemplified for instance by the Boyajean patent. The, productionof-individual discrete dots in the desiredregular succession will require someon-ofli'modulation ofthe lightbeam producing the dots, e.g., bymodulation of the lightsource 10 or by chopping the beam itself'at a regular'rate. in the electrical signals applied to coil 26, if the. zeroinput position of the reflector is made to correspond to a dot of zero size or less, i.e., to the existence of no light output whatever or to a finitesignal for zero light.

Since the principal purpose of the invention is ultimately to provide screen plates made up of discrete dots of predeterminedgshape and variable size, it is desirable toin-- sure that the exposure which produces each such dot is fairly brief as compared with the rate of motion of'surface 36 relative to the exposure beam; otherwise, blurring of the dot shape will result. Thus, the light source ;10 may be of a type which can be flashed briefly by peaks or. pulses derived from the energizing ,voltage source 41, in;

Fig. 1. Alternatively, a steady light source may beemploy'ed, and the on-off modulation obtained by a lightchop'per-or sector disc to give a brief exposure which will the on-ofi modulation will be controlled from such genr The screen generator is indicated schematically, in Fig. l at 43, controlling the peaked voltage source 41 as erator.

to frequency,but it will be understood that such a screen generator would also provide the necessary frequency control if on-oif modulation is accomplished by a lightchopperor by modulating the signal energy applied to coil 26.

If" the beam arriving at the aperture maskis homo-, geneous as to intensity, the mask shown in Fig. ;2 will allpwthe centralportionpf a large dot to be lovers,

The on-ofi modulation mayalso be incorporatedirposed as com ared with a small dot. 1 The reason for this is that in forming a dot, the device commences with the aperture in effect closed, and then allows the aperture to open as reflector 24 is tilted, the maximum tilt corresponding to the size of the dot being produced. Since the reflector 24 has a finite speed of movement, a longer time will be required for the effective aperture to open to a large size than is required for it to open to a smaller size. Thus in forming a large dot, the central portion will be exposed for a longer time than the total exposure time involved in making a small dot. To prevent this, it is possible to provide a decrease in the central light flux which occurs progressively as the dot size increases, by employing a special shape of mask shown at 42 in Fig. 4. The left profile 44 there corresponds to the aperture 18 of mask 16, but the right-hand profile, instead of being a straight line, is the edge of a smaller triangle indicated at 46. Obviously, for dot sizes above a certain value, the projecting point of this opaque triangle will obtrude upon the beam passing through the mask, and more will obtrude as the dot size increases. However, below said value, the full intensity of the original beam will be passed. In effeet, the instantaneous shape of the dot when of moderate size will then be as shown in Fig. 5, in which the light is passed only in a marginal strip around the square beam cross-section at the surface on which it is focussed.

. While the arrangement of Fig. 1, with or without the modification as to aperture shape of Fig. 3, performs quite well, and permits the production of square output dots as small as a thousandth of an inch on the side, a preferred embodiment shown in Fig. 6 permits simplification of the optical parts, is generally easier to maintain in proper adjustment, and has a much greater depth of focus. Like reference characters in this figure denote the same or equivalent parts as those shown in Fig. 1. Thus, numeral again denotes a light source, here shown as a ribbon filament lamp, and a condenser 44 concentrates the light upon an aperture disc or mask 16, opaque except for the aperture which may be of the shapes as shown in Fig. 2 or Fig. 4. An objective lens 20 images the light from the aperture, after reflection from tiltable mirror 24, in the plane of the knife edge 22. Tiltable mirror 24 is provided with drive means as in the previous form of the invention.

From knife edge 22, a collimating lens 48 converts the rays to a parallel beam, which beam enters a prismatic beam splitter generally designated by numeral 50. This is formed of two right- angle prisms 52 and 54, contacting along one common face as at 56, which interface is provided with a half-silvered or other beam-splitting layer. The layer may be coated on one or both prism faces or otherwise provided in ways well known to those skilled in optics. The prism 50 is disposed so that the beam enters it normally to one face (of prism 52), and since the beam is col-limated, aberrations that would otherwise be introduced by the prism are avoided. One half of the entering beam is thus reflected at layer 56, then totally internally reflected at the outer surface of prism 52, and passes to the focussing lens 58, which in this case may be a single lens receiving both components of the spot beam. The other half beam, transmitted by layer 56, is reflected only once, namely by total internal reflection at the outer surface of prism 54, and is also focussed by lens 58 upon the receiving surface 36. Inasmuch as prism assembly 50 is in effect a single integral structure, the alignment of the various reflectors, including the beam splitting surface, is readily maintained. Also, the manufacture of this component involves less cost than the separate manufacture and assembly of the three reflectors of Fig. l, and the arrangement gives a much greater effective depth of focus for such a system.

operationally, the embodiment of Fig. 6 is no dilferent from that earlier described, so that further elaboration as to this form is believed to be unnecessary. The screen generator 43 is here shown applying ori-ofi modulation to the input signals applied to the drive coil of mirro 24, as already described.

Both of the forms of the invention described above have employed a common illuminating system, a common mask and knife-edge and output beams which suffer respectively a single reflection, and a pair of reflections, after leaving the knife edge. A form of the invention will now be described in which separate illuminating systems are employed, a double knife edge element, and output arrangements in which only a single reflector is needed. Bearing in mind that the symmetrical dot shape results from combining beams which have suffered a difference in number of reflections of one it is apparent that the result can be obtained by reflecting one component beam once only, and the other not at all.

Referring now to Fig. 7 of the drawings, a pair of light sources are indicated at 10 and 11, these being identical with the sources previously described. Each source has the condensing system 14, 15, and each has an aperture mask 16, 16. These masks are oriented so that their triangular apertures have the bases parallel to one another. Separate lenses 20 and 21 focus the beams at knife edge device 22', which dilfers from that previously described in having its opposite ends both sharpened to intersect partly each of the two beams from tilting reflector 24. The beam from aperture 16 is focussed at one edge of knife edge 22', and the resulting image is projected to the photosensitive surface 36 by lens 64. The beam from aperture 16 is focussed at the other edge of knife edge 22 after reflection from the front: surface mirror 60 to reverse the angular sense of rota-- tion of the beam passing from mirror 60 through lens. 62 with respect to the angular rotation of the beam pass-- ing through beam 64. Thus, for example, as the mirror rotates counter-clockwise, a greater portion of the pro jected aperture 16 is intercepted by the knife edge and one-half of the resulting square is decreased in size. This same counter-clockwise rotation of mirror 24 decreases the eifective size of the projected aperture 16' because of the reversal in angular direction produced by the reflection by mirror 60.

' The optics are arranged so that the base lines of the variable size triangles projected on to surface 36 are coincident, so that these two images combine to form a single dot or spot of square shape and variable size just as in the previous forms of the invention.

One advantage of this form of the invention lies in the fact that since no splitting of the original beam occurs, a given light source intensity and optical components permits about twice as much light to be directed into the resulting spot, because light is transmitted throughout the system from each of the two sources. Fundamentally, however, the same optical principle (differential number of reflections) is employed. While the double knife edge element 22' is shown as an integral element, it is obvious that separate knife edges could be used so long as they are located in fixed positions corresponding to the knife edges at the opposite ends of the single element 22, or in other fixed positions which will produce the same optical effect.

While the invention has been described in connection with certain preferred examples in accordance with the patent statutes, the details can be varied in many respects without departing from the spirit of the invention. Thus, whereas the aperture masks shown herein comprise opaque elements with triangular apertures or transparent parts, combined to provide a truly square output shape, other output shapes of symmetrical configuration can be obtained by using different masks. A slight deviation from true square shape may be desired, for example, where relative movement between the transducer output axis and the surface 36 would tend to elongate the dot in the direction of surface movement, or more especially where the projection angle onto surface 36 is other than 90?; 'I he square dots may also-be intentionally oriented-- with'respectto the. dimensional directions of surface 36 so as to look like diamonds or lozenges rather than' squares. Suchalterations and substitutions, and changes astoother details,,are intended to be covered by the invention as deflnediin the appended claims.

. Whatis claimed is:

1. A- luminous output transducer for the preparation of photographic reproductions consisting of discrete half'- tone dot patterns corresponding to-electricalsignals, com prising means forming two beams of light defininggeometricallyidenticalimagesof a predetermined shape each of saidimages having substantial dimensional extension intwo orthogonal directions ina plane perpendicular to periodically during movement of said medium to cause the discrete sequential exposure of spaced points thereof to the varying size pattern of: light to produce thedesired half-tone dot representation of said signals.

2; The invention in accordance with claim 1, in which said means for projecting comprises means defining separate optical paths for said beams, and a reflector in at least one ofsaid paths,- andin which the number of reflectors in said' respective paths differs by an odd integer.

3. The invention in accordance with claim 1, in which the means forming saidtwo beams of light comprises at least one element having a triangular beam-defining? aperture.

4. The invention in'accordance with claim 1, in which the means forming said two beams of light comprises at least one mask having a chevron-shaped aperture.

5. The invention in accordance with claim 1, in which said signal-responsive means comprises a mirror galvanometer.

6. The-invention in accordance with claim 1, in which saidmeans-for projecting comprises anoccluding element common to said two beams oflight, and in which. said means for alteringthe sizes of saidimages comprisesmeans for moving the direction of the rays formingsaid images with respect to said occluding element.

7. A luminous output transducer for" the preparation of photographic reproductions consisting of discrete halftone dot patterns corresponding to continuous electrical signals, comprising: a light source, means defining an aperture of chevron shape, said light source being disposed to illuminate said aperture, a ray deflector posi-' tioned to receive rays passing through said aperture from said light source, means for rotating said deflector to 7 vary' the direction of the rays reflected thereby in accordance with said electrical signals, alight occluding, elementhaving one straight edge positioned with respect to said deflector so as to intercept more or less of the beam of rays leaving said deflector, on a line connecting the legs of said chevron, optical means for dividing said beam into two'components each having rays imaging the shape of said aperture as modified by said occluding element,

means for focussing said components upon a moving light-sensitive medium in contiguous relation at the image of said lineto form a succession of illuminating spots shaped substantially as hollow rectangles of external sizes determined by the instantaneous position of said deflector, and means for periodically interrupting said beam com'' ponents during motion of said medium to image upon the latter a discrete series of spaced dots of varyingsizes,

and of which the dots corresponding to larger external. size receive at their central region a reduced total inten-- sity of applied illumination.

References Cited inthe file of this patent UNITED STATES PATENTS 2,311,159 Dimmick Feb. 16, 1943 2,436,148 Maurer Feb. 17, 1948 2,518,743 Bach Aug. 15, 1950 OTHER REFERENCES Half-tone Stops (Mertl'e), The American Photoengraver, vol. 21, No. 10, September 1929, pages 937-- 951, pages 938, 941, 944, 947 relied on.

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