US3686675A

US3686675A - Apparatus and method for drawing with a spot of radiant energy

Info

Publication number: US3686675A
Application number: US21218A
Authority: US
Inventors: Thomas L Faul; Richard G Young
Original assignee: FAUL CORADI Inc
Current assignee: FAUL CORADI Inc
Priority date: 1970-03-19
Filing date: 1970-03-19
Publication date: 1972-08-22
Anticipated expiration: 1989-08-22

Abstract

A line is traced on a photosensitive surface by a moving spot of light which has a uniform and constant intensity, and variable outside dimensions to change the width of a line produced thereby. Within the spot, there is a darkened area which changes in size according to the velocity of the spot over the photosensitive surface in order to expose the photosensitive surface uniformly along the length of the line. A cathode ray tube presents a picture which is projected on the photosensitive surface to create the spot of light.

Description

United States Patent Faul et al. [451 Aug. 22, 1972 s41 APPARATUS AND METHOD FOR 3,600.5 l3 '8/1971 Pendred ..95/12 x DRAWING WITH A SPOT OF 3,610,119 10/1971 Gerber et a1 ..95/1 ENERGY 2,415,880 2/1947 Hassler ..346/ 109 [721 Inventors h M skaneawles. NY; 31322333 31323 $3257.if.fl::::::::::::::::::::3i

, Richard G. Young, Fairfax, Va.

Assignee: Faul-Coradi, Inc. Primary Examiner-Joseph W. l-lartary 16 March 19 1970 Attorney-Beveridge & De Grandi [2!] Appl. No.: 21,218 ABSTRACT A line is traced on a photosensitive surface by a mov- 52 us. Cl .;..346/l, 95/1, 178/6.7 R, t! Spot of light which has a uniform and constant g 346/110 tensity, and variable outside dimensions to change the 51 Int. Cl. ..G03b 27/00 Width line Pmduced therebywithin the Field of Search "346/110 108 95/12. there is a darkened area which changes in size accord- 1378/6 i 7 ing to the velocity of the spot over the photosensitive surface in order to expose the photosensitive surface uniformly along the length of the line. A cathode ray [56] References Cited tube presents a picture which is projected on the UNITED STATES PATENTS photosensitive surface to create the spot of light. 3,548,713 l2/l970 Webster ..95/l 9 Claims, 12 Drawing Figures 54 zoom LENS /82 g CONTROL 52 I se 60 9 TELEVlSlON DISCRIMINATOR fi CAMERA 48 SIGNAL ADDER 44 T TELEVISION DISCRIMINATOR 42 CAMERA 40 LIGHT MEASURING cmcun l l l comv msou cmcun I SPOT vuocnv l CIRCUIT Patented Aug. 22, 1972.

.2 Sheets-Sheet 1 I. M N mWD S 8 5 C 6 2 R 5 00 0 w I A W W m T R A N S w I NIL I M E D m LR T R MN I 0,0 D 00 Z 4 0 N 4 0A D 3 .r VM IN E 0 IL I E B T VE EM A I. C I 2 w I 4 5. 4 G 4 l 4 LIGHT MEASURING CIRCUIT COMPARISON CIRCUIT I SPOT VELOCITY CIRCUIT I INVENTORS THOMAS L, FAUL RICHARD G. YOUNG BY [5% J Temwu ATTORNEYS Patented Aug. 22, 1972 3,686,675

.2 Sheets-Sheet 2 FIG. 3a t FIG. 3b

V; V HG 9 Wm HG. ll

APPARATUS AND METHOD FOR DRAWING WITH A SPOT F RADIANT ENERGY This invention relates to apparatus for drawing with a beam of light or otherradiant energy on a surface which is sensitive to the particular form of radiant energy presented by the beam.

In order to obtain uniform width and density of lines which are traced on a sensitive surface by a moving spot, it is important that the sensitive surface receive a substantially constant amount of radiation along the length of the lines. Prior to this invention, others have obtained uniform exposure to radiation by changing the intensity of the irradiated spot in accordance with its velocity, with the intensity being equal throughout the extent of the spot. Changes in the width of a line may be effected either by maintaining a constant dimension measured along its path, changing its width and turning the spot so it remains at a constant orientation with respect to its path across the photosensitive surface or by varying its sirnensions in all directions while changing its intensity. It is also known to utilize a flashing light source, with its frequency controlled in accordance with the speed and size of the spot.

This invention, rather than changing the intensity of the spot of light involves a spot which has a constant intensity and undergoes dimensional changes to provide for proper exposure of the line. In order uniformly to expose a sensitive surface as a spot moves thereacross, the spot dimension measured perpendicular to the spot path are held constant and the dimensions measured parallel to the spot path are varied either internally or externally in accordance with the velocity of the spot. The spot may be in the shape of an annulus with its inside diameter varied in accordance with the spot velocity, and its outside diameter varied in order to change the width of the lines drawn. I

We have also provided for the continuous analysis of a portion of the light beam which creates the spot. A signal indicative of the spot velocity is compared to a signal generated by a portion of the light beam being analyzed, and this comparison creates an error signal which produces changes in the dimensions of the darkened area.

Another concept utilized in the disclosed apparatus involves the use of a display means such as a cathode ray tube for presenting a picture in the form of thedesired spot.

A further understanding of the invention is facilitated by reference to the following description an the accompanying drawings wherein.

FIG. 1 is a diagrammatic view showing a spot projecting head which moves relative to a photosensitive surface.

FIG. 2 shows one manner of constructing an apparatus for drawing on a photosensitive surface accordthen transmitted by the fiber optic member 24 to the In FIG.-1, a light projecting head-2 is movable along perpendicular axes in directions indicated by the arrows 4 and 6. The light head includes a projecting lens system 8 which emits a beam of light focused on a photosensitive sheet 10. The light is preferably but not essentially in the visible light range. The spot 12 created by the beam from the light head is moved along either of its coordinate axes. Movement of the spot along the path'indicated by the broken line 14 is preferably initiated in a known manner by signals generated by a computer having storage or memory components which contain the desired data for programming the light head movement will create an exposed line having a width w on the photosensitive sur face 10.

If the spot 12 is directed on the photosensitive surface 10 as the velocity of the spot along the line 14 varies, there must be some compensation in the spot 12 to prevent difi'erent amounts of exposure along the line created by the moving spot. As previously mentioned, others have provided this uniform exposure by changing the intensity of the light spot or by rapidly flashing the spot at frequencies which vary according to the velocity of the light head- I In contrast to these prior practices, the spot 12 in the practice of this invention has a uniform intensity and it undergoes dimensional variations. The spot of FIG. 1 is annular so that it surrounds a centrally located void area of a lesser intensity which suitably is dark so that its intensity is zero. I

The outside dimensions of the spot 12 govern the width w the line, and the dimensions of the darkened area 16 change in order to control the amount of expof sure of the photosensitive surface 10. When the velocity of the light head 2 parallel to the path 14 is low as when the light head is under-going its initial ac celeration, the size of the darkened area 16 is relatively large and the thickness 2 of the spot is small; however, with an increase in the velocity of the light head, the-darkened area 16 decreases in diameter in order to The control of the outside dimensions of the spot 12 and the dimensions of its darkened area 16 may be produced using various electronic, optical or mechanical elements associated with the light head. One system is shown in FIG. 2 where a stationary cathode ray tube 18 provides a picture having the desired proportions of the spot.

The picture on the cathode ray tube 18 may be created in various manners. In the disclosed embodiment, television cameras having conventional photomultiplier tubes are used. Other suitable apparatus may utilize signal generators which do not rely upon the use of a television camera The face plate or screen of the cathode ray tube 18 lies in the object plane of a projection lens system 20. The beam of light from the projection lens system passes through a plane which is selectively obstructed by a shutter 28 and then is reflected by a partially reflective mirror 26 before coming into focus on one end of a fiber optic member 24.

The image of the face of the cathode ray tube 18 is photosensitive surface 38, where the light spot created thereby is moved along the path established by moveprovide a uniform exposure along the length of the line ment of the light head 30. The end-of the fiber optic member 24 which confronts the photosensitive surface 38 may ride extremely close to the photosensitive surface by means of air beatings or it may be associated with a projection lens system on the light head 30.

The light head 30 is supported on a conventional beam and tranu'nel arrangement where the beam 32 moves in the direction of the X-axis along the

trackways

34 and 36. The carriage 30 moves on beam 32 in the Y-axis direction. Preferably, movement of the light head 30 is produced by electric motors which rotate lead screws in a known manner to move the carriage 30 in the Y-axis direction and the beam 32 in the X-axis direction.

In FIG. 2, two

television cameras

40 and 48 are used to generate the signal which is displayed on the screen of the cathode ray tube 18. The first of these cameras 40 has a zoom lens 42 and is directed toward a picture which has a light spot 44 on a dark background 46. Changes in the setting of the zoom lens '42 will of course change the outside diameter of the spot-creating image 44 as it appears on the face of the cathode ray tube 18. An operator or a computer may change the setting of the zoom lens 42 in order to change the width of the line projected on the photosensitive surface 38.

The second television camera 48 has zoomlens 50 which focuses ona picture having a dark spot 52 on a light background 54. Changes in the setting of the zoomlens 50 will produce changes in the inside dimensions of the spot image 44 on the cathode ray tube 18. When the image of spot 52 seen by the television camera 48 is greater than the size of the light spot 44 seen by the television camera 40, the face of the cathode ray tube will be darkened. Reduction in the size of the image of the dark spot 52 will result in an annular spot-creating image such as the one shown in FIG. 2.

In order to provide a distinctive definition between light and dark areas, the signals from the

television cameras

40 and 48 pass through

discriminators

56 and 58 which may be of the pulse height type to eliminate any background noise or interference in the signals. The signals from both of the television cameras are added at 60, preferably in exact time coincidence, and the combined signals are carried to the cathode ray tube by the conductor 62. The cathode ray tube may utilize phosphors which provide a very bright picture.

As mentioned previously, the setting of zoom lens 42 establishes the outside diameter of the picture of light spot appearing on the face of cathode ray tube 18 in order to produce a line of a desired width w on the photosensitive surface 38. The appropriate internal dimension of the spot on the face of the cathode ray tube 18 is dependent both upon the outside diameter of the picture of the spot on cathode ray tube 18 and the velocity of the light head 30 with respect to the photosensitive surface 38.

One method of controlling the size of the darkened interior portion of a spot is shown in FIG. 2 and is based upon the principle that the thickness of the illuminated area through the center thereof will be the same for any given velocity regardless of the outside dimensions of the light spot. This principle may be understood by referring to FIGS. 3a and 3b which show spots having different outside dimensions. Although their sizes are greatly. exagerated, it will be assumed for purposes of this explanation that they are shown in the dimensions which are projected on the photosensitive surface.

As a starting point, it will be assumed that the spot shown in FIG. 3a is moving in any direction at a given velocity which is conveniently referred to as v It also will be assumed that for this velocity, the photosensitive surface is properly exposed by using a spot with the thickness If the velocity of the spot doubled to become v then the thickness of the spot, i.e. the difference between the outside and inside radii of the spot must increase to become t,. I

Since the amount of light striking any area on the photosensitive surface is dependent upon both the velocity of the spot and its dimensions measured along its path of movement, it will be appreciated that the spot shown in FIG. 3b, even though it has a smaller outside diameter, must also have the thickness when the velocity is v; and the thickness t, when the velocity-is Since the areas of the illuminated portions of these spots in FIGS. 30 and 3b are dependent in part upon the outside diameter of the spot, the light of the entire spot does not provide an accurate analysis of the amount of light striking the photosensitive surface as the spot moves along its path. However, if only that portion of the illuminated spot within the zone lying between the parallel

broken lines

64 and 65 is analyzed, the effect of the differing diameters of the spots is avoided. The

lines

64 and 65 extend through the darkened void area. The area which is illuminated between the

lines

64 and 65 in FIG. 3a to provide proper exposure for one suitable spot velocity is equal in size to the area which should be illuminated between the

lines

64 and 65 in FIG. 3b for that same velocity.

The principles discussed in connection with FIGS. 30 and 3b are employed in the apparatus of FIG. 2. A portion of the light from the projection lens system 20 passes through the partially reflective mirror 26 and strikes an opaque baffle 68 which has an area 70 which is transparent to the radiant energy. This area lies on the center of the light beam passing through the mirror 26 and it permits the passage of light in the area which corresponds to the zone between

lines

64 and 65 in FIGS. 3a and 3b. The light passing through the baffle 68 strikes a photosensitive element 72 which may be a photoelectric cell or a photoresistive cell. This cell fonns at least a portion of a light measuring circuit 74 which generates a signal indicative of the amount of light passing through the transparent areas 70 of the bafile 68.

The speed of the spot with respect to the photosensitive surface 38 may be ascertained in any of several ways such as by electronically resolving the signals from the motors which drive the light head 30 along the X and Y axes. Such a circuit is shown in block form at 76 and it generates a signal indicative of the spot speed. For any given spot velocity, the amount of light striking the photosensitive element 72 should be the same regardless of the outside dimensions of the spot. Therefore, the signal from the light measuring circuit and the spot velocity circuit 76 are balanced in a comparison circuit 78 which may be a conventional bridge circuit. An error signal indicative of the difference between the signals from the

circuits

74 and 76 is carried by a conductor 80 to the zoom lens control 82 which automatically positions the zoom lens 50 to vary the thickness of the spot-creating image on the face of cathode ray tube The operation of the circuitry shown in FIG. 2 is easily explained by first assuming that the spot is moving at a velocity of v to produce an output signal of a certain value from the spot velocity circuit 76. At a given instant, if the light striking the photosensitive elements 72 is insufficient because the darkened area within the spot is too large, then the signal from the light measuring circuit 74 will beless than the signal from the spot velocity circuit 76. This will create an error signal which is carried by the conductor 80 to the zoom lens control in order to decrease the area of darkened area 52 on the face of cathode ray tube 18. This will increase the signal from the light measuring circuit 74 until it becomes balanced with the signal from the spot velocity circuit 76. When the spot velocity increases to v then again the signal from the light measuring circuit 74 is insufficient and will result in anerror signal from comparison circuit 78 to reposition the zoom lens 50. When the velocity of the spot is reducedto zero, the darkened interior area will enlarge until it obstructs the entire area which previously was illuminated. As precautionary measure, the shutter 28 may also be closed at this point.

Several advantages accrue from the use of a cathode ray tube to create the picture which is to be projected onto the photosensitive surface. One advantage is that it provides a relatively cool light source, both in terms of heat generated during operation and the wave-length of light striking thephotosensitive surface. A cathode ray tube easily provides light spots of varying configuration. Movable spots for drawing lines may be of various other shapes selected according to the type of work being done. For example, when pronounced comers are desired where a line turns abruptly, a spot having a square periphery may be used.

A cathode ray tube also permits the display of alphanumeric characters and other symbols which may be projected onto the photosensitive surface when the light head is stationary. In this fashion, the apparatus may be used to write words or place symbols on the photosensitive surface. The symbols may be created on a conventional cathode ray tube by viewing pictures thereof with a television camera or by using known wave shaping circuits which generate voltages to approximate the component lines of each symbol. Alternatively, it is possible to use known special-purpose cathode ray tubes which employ an internal matrix plate which shapes the electron beam into the form of selected symbols. Such tubes are described on pages 5-36 and 5-37 of the Handbook of Automation Camputation and Control, 1959, published by John Wiley & Sons, Inc.; on pages 142 and 143 of Digital Computer Basics, 19,68, Navpers 10088, published by the US. Government Printing Oflice; and, on pages 109 and l of The Encylopedia of Electronics, 1962, published by Reinhold Publishing Corporation, all of which are incorporated herein by reference.

Another advantage of using a cathode ray tube for creating an image of the spot is that, through proper electrical circuitry, it can provide a great variety in spot shapes, sizes and orientations. Properly selected and controlled spots can provide uniform exposure along the length of a line and also across its thickness. For ex ample, the spot may be of a rectangular shape which is continuously reoriented so that it has a longitudinal axis which always extends along or is tangent to the path of the spot. Reorientation of the spot is preferably performed electronically, but it may also be accomplished by mechanically rotating the cathode ray tube or only its deflection coils or plates. This longitudinal axis is usually also an axis of symmetry of the spot. Such a spot is shown at in FIG. 4. With this arrangement, the width w of the spot i.e. the dimension measured transverse to the spot path 132, remains constant when drawing a line of a given width; however, the length I of the spot which is its dimension measured parallel to the path of movement varies in dependence upon thespeed of the light head over the photosensitive surface.

' The type' of spot described in the preceeding paragraph has certain drawbacks when the'spot has a substantial longitudinal dimension as it is moving along a curve path since the leading and trailing corners of the spot which lie towardthe outside of the arcuate path will project a distance greater than one-half the desired line width beyond the center of the spots path. This cause some fuzziness on the edges of the line. To avoid this difliculty, it is proposed to provide a spot with convex side edges which, in effect, will eliminate the leading and trailing right-angle comers of the spot. This may be done, for example, by a spot of the type shown in FIG. 5 which may be described as a truncated circular spot having its side edges 134 and 136 arcuate with their centers of curvature located at the center 138 of the spot. The leading and trailing

edges

140 and 142 of the spot are defined by straight lines which lie substantially perpendicular to the path 144 of the spot.

Of course, as a spot such as this moves along its programmed path 144, its longitudinal axis of symmetry is continuously reoriented in order to lie parallel or tangential to the desired path. The same desirable effects attributable to the spot of FIG. 5 may also be achieved by oval, polygonal or elliptical spots which are provided with convex side edges.

The velocity-dependent changes in the longitudinal dimensions of the spots of FIG. 4 and 5 may also be made without changing their maximum longitudinal dimensions. For example, the spots may be formed of series of longitudinally spaced-apart bands which extend transversely to the path of the spot. As velocity changes occur, the width or number of bands may change to change the longitudinal dimension of the spot (the total length of the illuminated segments) while the distance between the forward and rearward edges of the spot remains constant. I

Various other spot shapes may be utilized to produce satisfactory results.

For example, in FIG. 6, the light spot is in the shape of a bullseye target with a circular central portion 146 and one or more annular portions 148 concentric therewith. In this particular type of spot, the velocityresponsive changes are accomplished by varying the radial dimensions of the darkened interior area 150, either by reducing the inside diameter of the annular area 148 or increasing the outside diameter of the circular area 46.

A square spot 152 which surrounds a darkened interior area is shown in FIG. 7 and it has the capability of producing a square comer when the path of the light head shifts from exclusive movement in X-axis direction to exclusive movement in the Y-axis direction. The interior dimensions of the spot are varied in response to changes in the spot velocity in order to provide proper exposure along the length of a line.

The scanning of the face of the cathode ray tube by the electron beam may be produced by-conventional horizontal and vertical sweep circuits which produce a scanning raster as in ordinary television sets. In lieu of this, it is possible to produce images on the face of a cathode ray tube simple by imposing certain signals on the horizontal and vertical deflection plates or coils associated with the cathode ray tube. It is well known, for example, that the electron beam willform a circular image on the face of a cathode ray tube if the mutually perpendicular pairs of deflection plates or coils are subjected to sine wave voltages which are 90 out of phase with each other. A further refinement of this concept as shown in FIG. 8 where the voltage V I on one pair of the deflection plates or coils follows a degenerating sine wave, i.e. a sine wave with a progressively decreasing amplitude for each cycle. The other pair of deflection plates or coils receives the voltage V, which identical to but 90 out of phase with V The effect of this voltage pattern is to cause the electron beam to move in a substantially circular path which has a decreasing radius. This may also be described as a very tight spiral. The maximum amplitude will determine the outside diameter of the annular image on the cathode ray tube, and the minimum amplitude will determine the inside diameter of the annular image. Appropriate circuitry may generate a repetitive signal of this type in order to maintain the annular image on the face of the cathode ray tube.

The signals shown in FIG. 9 operate much in the same way as those of FIG. 8, except that-they will have the effect of producing a square picture of the type shown in FIG. 7. The pairs of deflection plates or coils will lie at the opposite comers of the image shown in FIG. 7.

The apparatus of FIG. 2 is shown for illustrative purposes, but it is susceptable to many modifications within the spirit of the inventive concepts disclosed in this specification. In some situations, the cathode ray tube may be mounted directly on the light-projecting head for concurrent movement with the light spot created thereby. Light spots may also be created by apparatus which does not employ cathode ray tubes, as will be seen from an inspection of the modified embodiments shown diagrammatically in FIGS. 10 and 11.

In FIG. 10, there is a light source 84 in the form of an incandescent filament associated with a reflective mirror 86 which directs light into a condenser or collimating lens 88. The light emanating-from the lens 88 is directed on the upper surface of a baffle 90 which has a central aperture 92, the diameter of which establishes the width of a line drawn by this apparatus. The light from the lens system 88 is a beam which is in an unfocused or confusedstate in the plane of the baflle 90 in order to illuminate the baffle 90 uniformly.

Located. centrally within the aperture 92, there is a flexible and hollow body 94 which is inflated by a liquid or gaseous fluid in order to vary the difference between the inside and outside radii of the annular light-transmitting area established by the bafile and the body 94. The size of the body 94 is varied according to the speed of the light head by a motor 96 connected to the rod of a piston and cylinder arrangement 98 which, in turn, moves fluid through the conduit 100 to expand and contract the body 94. A projection lens system 102 has its object plane coincident with the plane of the baffle 90, and it is focused so that its image plane is coincidental with the photosensitive surface 104.

Of course, the apparatus of FIG. 10 may be modified to provide some of the advantages of the system shown in FIG- 2. For example, the outside diameter of the light spot may be. varied either by replacing the battle it 90 with a conventional iris diaphragm or by using a zoom lens system in placeof the fixed focused projection lens system 102. Automatic control of the thickness of the spot may be provided by control means of the type shown in FIG. 2.

' In FIG. 11, there is a light source 106, a reflector 108 and a lens system 110 which are identical to their counterparts in the FIG. 10 embodiment. Light from the lens system 1 10 is in a convergent beam which strikes a baffle 112 having an aperture 114. An internal baffle 116 is used to create and vary the internal dimensions of a light spot. Variations in such dimensions is produced by motor 118 which moves the baffle-supporting arm 120 about a pivot pin 122 so that upward movement of the baffle 116 will increase the thickness of the light spot. A projection lens system 124 projects the image established by the baffles 112 and 116 onto the photosensitive surface 126.

Numerous other forms of apparatus may be devised to employ the principles utilized in the disclosed embodiments. Accordingly, the breadth of our invention is not intended to be limited only to the embodiments shown, but is intended to encompass other methods and apparatus falling within the scope and spirit of the claims which follow.

I claim: l. A method of drawing on a sensitive surface with a spot of radiant energy directed thereagainst comprising the steps of projecting on the sensitive surface a spot of radiant energy having constant outside dimensions and a given intensity, said spot surrounding a centrally located void area which has a lesser intensity than said given intensiy,

moving the spot relative to the sensitive surface to irradiate a line on the sensitive surface,

changing the dimensions of the void area in response to changes in the velocity of the spot across the sensitive surface, with the size of the void area decreasing with increases in the velocity of the spot so that the spot will produce a line which has a constant density along its length.

2. Apparatus for drawing on a photosensitive surface with a spot of radiant energy comprising,

a base member for supporting a sheet of sensitive material which undergoes chemical or physical changes upon exposure to the radiant energy,

rectangular.

g a housing supportedfor movement along coordinate means for automatically changing the spot dimension which lies parallel to the spot path in response to changes in the spot velocity without changing the maximum dimension across the spot in a direction perpendicular to the spot path, with the magnitude of the dimension which lies parallel to the spot path increasing with increases in the velocity of the spot. 3. Apparatus according to claim 2 wherein the spot has a circular periphery and the changes in the. spot dimension are produced by changing the radial dimensions of the spot while holding its outside diameter constant.

4. Apparatus according to claim 2 wherein the spot is annular.

5. Apparatus according to claim 2 wherein the spot is 6. Apparatus according to claim 2 wherein the spot is in the shape of a square which surrounds a darkened interior area.

7. Apparatus according to claim 2 having means for generating a first signal indicative of the amount of radiant energy in an area corresponding I to an area of the spot bounded on its sides by a pair of lines which lie substantially parallel to the path of the spot, means for generating a second signal indicative of the velocity of the spot across the sensitive surface,

means for comparing said first and second signals and'generating an error signal indicative of thecomparison, said means for changing the dimension of the spot which lies parallel to the spot path being responsive to the error signal. Y 8. Apparatus according to claim 2 including display means providing a picture having the shape and prodesired in the spot,

said means for projecting the spot being located and constructed to receive light from the display means and to direct it on the sheet of sensitive material. '9. Apparatus according to claim,8 wherein the display means is a cathode ray tube.

Claims

1. A method of drawing on a sensitive surface with a spot of radiant energy directed thereagainst comprising the steps of projecting on the sensitive surface a spot of radiant energy having constant outside dimensions and a given intensity, said spot surrounding a centrally located void area which has a lesser intensity than said given intensity, moving the spot relative to the sensitive surface to irradiate a line on the sensitive surface, changing the dimensions of the void area in response to changes in the velocity of the spot across the sensitive surface, with the size of the void area decreasing with increases in the velocity of the spot so that the spot will produce a line which has a cOnstant density along its length.

2. Apparatus for drawing on a photosensitive surface with a spot of radiant energy comprising, a base member for supporting a sheet of sensitive material which undergoes chemical or physical changes upon exposure to the radiant energy, a housing supported for movement along coordinate axes which lie substantially parallel to a sheet of sensitive material on said base member, means for projecting a spot of radiant energy which moves with the housing and is of constant intensity on the sensitive sheet, means for moving the housing along said coordinate axes so that the spot will follow a path to produce a line on the sensitive sheet, means for automatically changing the spot dimension which lies parallel to the spot path in response to changes in the spot velocity without changing the maximum dimension across the spot in a direction perpendicular to the spot path, with the magnitude of the dimension which lies parallel to the spot path increasing with increases in the velocity of the spot.

3. Apparatus according to claim 2 wherein the spot has a circular periphery and the changes in the spot dimension are produced by changing the radial dimensions of the spot while holding its outside diameter constant.

4. Apparatus according to claim 2 wherein the spot is annular.

5. Apparatus according to claim 2 wherein the spot is rectangular.

6. Apparatus according to claim 2 wherein the spot is in the shape of a square which surrounds a darkened interior area.

7. Apparatus according to claim 2 having means for generating a first signal indicative of the amount of radiant energy in an area corresponding to an area of the spot bounded on its sides by a pair of lines which lie substantially parallel to the path of the spot, means for generating a second signal indicative of the velocity of the spot across the sensitive surface, means for comparing said first and second signals and generating an error signal indicative of the comparison, said means for changing the dimension of the spot which lies parallel to the spot path being responsive to the error signal.

8. Apparatus according to claim 2 including display means providing a picture having the shape and proportions desired in the spot, said means for projecting the spot being located and constructed to receive light from the display means and to direct it on the sheet of sensitive material.

9. Apparatus according to claim 8 wherein the display means is a cathode ray tube.