US 2583546 A
Description (OCR text may contain errors)
Jan. 29, 1952 C, F CARLSON 2,583,546
ELECTROl-DHOTOGRAPHIC RECORDING Filed May 12, 1948 sas/wu ffm/va 71; 72
y fon/Mmm 0- Patented Jan. 29, 1952 UNIT-ED STATES PATENT OFFICE ELECTBOPHOTOGRAPHIC RECORDING Chester F. Carlson, Woodside, N. Y.
Application May 12, 1948, Serial No. 26,663
8 Claims. l
This invention relates to methods and apparatus for the electric transmission and recording of intelligence.
An object of the invention is to improve such methods and apparatus.
.Another object .is Ato provide methods and apparatus for converting electrical signals and the like into an electric charge pattern on an electrophotographic plate and for converting an electric charge pattern on an electrophotographic plate or other electrostatic record element into electric signals or variations.
A further object is to improve methods and de- -vices for facsimile, telepicture and phototelegraphic purposes, electric image recording and related purposes.
An additional object is to provide a method and apparatus for recording and `reproducing sounds and related variations.
Other objects of the invention will be apparent from the following Adescription and accompany- `ing drawings taken in connection with the appended claims.
The invention accordingly consists in the features of construction, combination of elements, and arrangement of parts as will be exemplified in the structures to be hereinafter described and the scope of the applications of which will be indicated in the following claims.
In the accompanying drawings in which are shown by way of illustration several possible embodiments ofmy invention:
Figure l1 iis a diagrammatic illustration partly in section. of a transmitting system and apparatus embodying features of the preesnt invention, and villustrating features of the method of the invention;
Figure 2 illustrates a receiving system and apparatus embodying other features of the invention and adapted to be associated with the system of Figure 1, or for use in other systems;
Figure 3 shows a modiilcation of part of the transmitting apparatus;
Figure 4 is a diagram of a modified transmitting apparatus illustrating other aspects of the invention; and
Figure 5 is a detail view of a further modified transmitting apparatus.
In Carlson Patent 2,297,691 issued October 6, 1942 is described a process of Electrophotography and an electrophotographic plate for use therein. The plate. in its preferred form comprises a conductive backing, such as a sheet of metal. coated with a layer of photoconductive insulating material. An electrostatic charge pattern or 2 latent image can be produced on kor in the layer by a suitable combination of electric charging and photographic exposure, as described in that patent.
'Carlson Patent 2,357,809 of September 12, 1944 shows in Figure l0 a modified form of electrophoto plate, and in Figure 11 a form of master plate capable of acquiring an electrostatic latent image or charge pattern. Methods of producing charge patterns on such plates are described. for example, on page 6 of that patent.
Another Carlson patent, namely 2,277,013 of March 17, 1942 describes methods and devices for the Electric Recording and Transmissionof Pictures and shows another form of electrophotographic element adapted for such purposes.
The present invention contemplates methods and apparatus for obtaining or transmitting electric signals or like variations from electrophoto- Agraphic plates of the kind described in Carlson Patents 2,297,691 and 2,357,809. It is also contemplated that, in some instances, signals may be transmitted from other electrostatic record elements such as master plates of the kind shown in Figure 11 of Patent 2,357,809. This invention further contemplates certain methods and apparatus for producing an electric charge pattern or electrostatic latent image on an electrophotographic plate from electric current signals or the like.
In a preferred embodiment the invention contemplates a scanning system and method for detecting and transmitting element by element and line by line the electrostatic charge pattern on an electrophotographic plate or other electrostatic charge-carrying element, whereby pictures, in
telligence, sound recordings and the like may be transmitted over facsimile systems or the like, or may be transferred from the plate or element to another plate or element or electro-mechanically engraved in metal, wax, plastic or other material to form printing cuts, sound recordings and the like.
While a preferred embodiment of the invention is described herein, it is contemplated that considerable variation may be made in the method of procedure and the construction of parts without departing from the spirit of the invention. In the following description and in the claims, parts will be identified by specific names for convenience, but they are intended to be as generic in their application to similar parts as the art will permit.
Referring to the drawings, Figure 1 shows the essential features of an apparatus for scanning esame an electrophotographic plate carrying a charge pattern or electrostatic latent image and for converting the charge variations obtained by scanning in sequence individual areas of the plate into signal currents. The apparatus comprises a drum II), shown in cross-section mounted on a driven shaft II for rotation for scanning purposes. The electrophotographic plate I2 comprising a nexible metal foil or sheet I3 having a coating I4 of photoconductive insulating material is wrapped around drum I and secured at its ends in a V-shaped groove I5 extending along the face of drum I0. The groove carries a row of pins I6 along one of its faces for anchoring one end of the foil I3, which is suitably perforated to fit over the pins. The other end of the foil is clamped rmly against the other side of the V groove by a clamp bar I1, suitably arranged to be moved into and out of clamping position. Reference is made to Figures '1 and 8 of Carlson Patent 2,277,013 for further details of construction.
A carriage I8 Vis slidably supported on a pair of guides I9 for movement along the side of the drum and a motor-driven advance screw extending parallel to shaft I I meshes with a thread- `ed lug 2| on carriage I8 to advance the carriage slowly along the side of the drum.
The carriage comprises a grounded metal frame 22 slidable in guides I9 and integral upstanding wall 23 concavely rounded to follow the contour of the vcylindrical surface of drum Il) and thereby provide a concave wall uniformly spaced from the surface ofA the drum throughout its area. The spacing is sulicient to leave a narrow air gap between the surface of coating I4 of the electrophotographic plate and the concave surface of wall 23.
A small aperture 24 is formed in the middle of wall 23, the sides of the aperture being tapered or flared outward on the back face of the wall. 'The Wall of aperture 24 which intersects vwith inner face 25 of wall 23 is brought sharply to an angular edge defining the smallest diameter of the aperture. This diameter depends upon the width of the line it is desired to scan on the plate I2. Thus, if it is desired to scan with a resolution of 100 lines per inch the diameter of aperture 24 is preferably in the order of 0.01. This diameter may be somewhat greater than the width of the line to bescanned, however, and useful results are obtainable even with diameters twice that oi' the line width. The spacing between face 25 and the surface of coating I4 is made as small as possible and preferably is not greater than the line width, such as 0.01 inch.
A scanning electrode 26 comprising a mounting screw portion 21 and pin-like electrode portion 28 having a flat or-slightly rounded end 32 is mounted with end 32 disposed in the center of aperture 24 and substantially flushfwith inner face 25- of wall 23. The electrode is supported in an insulating bushing member 29 which is clamped on the back of wall 23 by a metal clamp ring 30 which is screwed on to the back of wall 23. Bushing 29 hasa metal sleeve insert 3I into which screw 21 is threaded. The passage 33 throughbushing 29 is spaced from electrode 28 to reduce the capacity between the electrode and grounded frame 22.
Shaft II carries a cam arm 34 at one end of drum I0. A cam-actuated switch 35 is mounted adjacent the end of the drum and comprises contact arm 36 and a cam-actuated movable spring arm 31, having its end 38 in the path of travel of cam arm 34, so as to close contact 39 supported on spring 31 against contact 40 supported on arm 36 momentarily once during each revolution of drum IU. The end of cam arm 34 is positioned diametrically opposite to clamp bar I1 and spring end 38 is diametrically opposite to electrode 26 so that contacts 39, 40 are closed momentarily at the instant clamp bar I1 passes the end 32 of the electrode. Metal clamp bar I1 is continually grounded through shaft II and the supporting framework so that as it passes electrode 26 it will not induce any electric charge on the tip 32 of the electrode.
Switch 35 is connected in a Series circuit comprising a battery 4I and relay 42 so that the relay is actuated each time contacts 33, 40 are closed by cam 34. Relay 42, in operating, closes its front contacts 43 to connect a source of electric potential to electrode 2B through conductor 44. The source of potential may comprise a grid bias arrangement for a vacuum tube circuit, for example, battery 45 shown in Figure l, a positive terminal of which is grounded, and the negative terminal Vof which is connected through a high resistance potentiometer 46 to ground. The sliding contact 41 of the potentiometer is connected through conductor 48 to the armature of relay 42 so as to be connected through contact 43 to conductor 44 when the relay is actuated. A small condenser 49 may be connected between potentiometer contact 41 and ground if desired.
A detector, amplifier and signal transmission circuit is connected to conductor 44 to receive signal variations from electrode 26 in any suitable manner. Any one of several known circuits may be used. In the circuit illustrated the grid terminal 5I of triode 50 is connected to conductor 44 and the output terminals 52, 53 extend to a suitable amplifier 54 in which the signal is amplied and modulated if desired, the resulting signal being used for any desired purpose or sent over a signal channel 55 which may comprise a wire circuit, a recording system, a radio transmission system, a sound transmission system or a telephone system.
The operation of the system shown may be as follows: Electrophotographic plate I2, carrying an electrostatic charge image on the surface of coating I4 is wrapped around drum I0 and clamped into position by clamp bar I1. Carriage I8 is moved along its track until pin end 32 is adjacent to one end of drum I0 and the edge of plate I2. The drive mechanism is then started to rotate drum I0 and the plate I2 at a uniform rate and to rotate screw 20 to advance carriage I8 slowly along the side of the drum so that end 32 of electrode 26 Will scan the surface of coating I4 in a spiral path of the desired pitch. Thus, if plate I2 carries a picture which it is desired to transmit over a facsimile system at lines per inch screw 20 will advance carriage I8 uniformly at the rate of one inch for each 100 revolutions of drum I0. As cam 34 passes contact arm Y31 it colses contacts 39, 40 momentarily at each revolution of the drum. This serves to operate relay 42 for an instant to connect conductor 44 and grid terminal 5I to the grid bias circuit to apply a predetermined negative grid bias potential to the grid and conductor. This potential is adjusted, prior to operation, by moving potentiometer contact 41 to apply a negative bias to the grid of triode 50 suflcient to prevent any substantial capture of electrons by the grid. Hence, when contacts 43 are again opened, while grounded clamp bar yI1 is still adjacent to electrode 26,'the grid potential will remain sub'- stantially constant.
As rotation of the drum carries the surface of electroph'otographic plate I2 past the end 32 of the electrode 26 this same potential is maintained on the grid of the tube as long as the surface of plate I2 which passes end 32 is not an electrically charged area, since the grid and connecting conductor 64 are insulated from any adjoining conductors. When an electrically charged elemental area of the coating I4 of plate I2 passes tip 32, however, a charge is induced on tip 32 by the proximity of the charged surface of the coating. Assuming that the coating carries an electrostatic latent image in the form of a negative charge, it will induce a positive charge on end 32 of the scanning electrode. Since the grid and conductor 44 constitute an isolated conductive body the positive charges induced on the tip 32 are drawn in part from the grid, thereby increasing the negative bias of the grid and conse- ;quently reducing the plate current in the tube.
It will thus be apparent that as negatively charged elemental areas corresponding to parts of the electrostatic image pass the scanning electrode the plate current of triode 50 will be decreased as long as a charged area is passing the electrode and that the plate current will again return to normal due to return of positive charges from tip 32 to the grid as soon as the charged area has passed by. The signal obtained ln the plate circuit will accordingly vary in accordance with the variation in charge on the plate along the spiral scanned by the electrode 26.
The signal, after amplification by amplifier 54, which may if desired include means to modulate a standard carrier signal frequency with the plate current signal, is fed over a signal channel 55.
The nature of the signal channel will depend on the desired use of the signal and may comprise a wire circuit leading to a facsimile receiving station of conventional design or such as that shown in Figure 2,'or a radio broadcast channel,
or the circuit of a local recording device such as a facsimile recorder, or an electro-mechanical engraving device such as a printing plate engraver.
AIf the plate I2 carries an electrostatic record which it is desired to convert to sound the signal channel may comprise a telephone or loud speaker system, or an electro-mechanical engraving or embossing device for engraving or embossing a sound record on wax, metal, cellulose acetate or the like or an electro-magnetic recording device for recording on wire or ferromagnetic discs. Other applications of the signal will readily be apparent.
During scanning of plate I2 the grounded conductive wall 23 functions as a shield for electrode 26 so that it will be unaffected by charged areas of plate coating I4 which are not substantially directly under tip 32 of the electrode. The spacing between electrode tip 32 and the surface of coating I4 can be varied by screwing the electrode 26 in or out to obtain the desired relation between sensitivity of the scanning system and the shielding effect of wall 23. During use it is necessary to prevent light from striking the charged coating I4 of the electrophotographic plate by operating in a dark room or providing a suitable housing for the drum.
Once each revolution, when bar I1 passes the electrode, the grid bias is brought to normal by the momentary closing of contacts 43 to correct for any slight drift in bias resulting from insula- B tion leakage or capture of electrons by the grid.
The capacity between electrode 26 and ground is kept at a minimum and conductor 44 is made as short as possible to insure transmission of the maximum available signal to the grid. Since the charge on coating I4 is of relatively high density, an adequate Isignal is obtainable at the grid.
The system will function similarly in scanning a plate carrying a positive charge image or both negatively and positively charged elemental areas. When a positively charged area passes the end 32 the negative grid bias will be decreased to allow more plate current to flow.
Instead of using electrophotographic plate I2 the system and method can also be used for scanning and transmitting signals from a master plate, such as that shown in Figure 11 of my Patent 2,357,809, comprising a sheet metal or other conductive backing carrying an insulating image which is electrostatically charged. Plate I2 is simply replaced by the master plate.
Other than as described and shown, the scanning and transmission equipment may be of any well known type including the usual synchronizing and framing arrangements where required. Various other refinements can be added but are not essential to an understanding of the present invention.
Referring to another aspect of my invention, Figure 2 illustrates a facsimile signal receiver which may, if desired, be used in conjunction with the transmitter of Figure 1 or independently to receive and record other signals. It may also be used to record sound-modulated electrical variations. The receiver may comprise an amplifier if required, such as amplifier 56, for amplifying and demodulating signals received over signal channel 55, as shown, or from other sources. The demodulated signal is fed to a light beam modulator or light valve 51 of any known form for modulating a beam of light from a light source such as lamp 58 behind a diaphragm 61 provided with a light aperture. The modulated light beam is focussed by the lens system in tube 59 through a small aperture 13 onto the surface of a coating 6I on an electrophotographic plate 60.
Plate 60 is mounted on a drum 62 for rotation on shaft 63 inside a cylindrical metal housing 64 closely spaced to the surface of coating 6I. The scanning member comprising light source, light valve, lens tube and housing are all mounted on a sliding carriage 65 arranged to be advanced parallel to shaft 63 by a motor driven advance screw 66 in the same manner as the carriage of Figure l.
rlhe walls of housing 64 are turned outward along one side to provide a pair of spaced outwardly-extending walls 68 forming an aperture or slot along the side of the housing. A row of spaced needles 69 is mounted in the slot by a supporting block 10 of insulating material, the needles extending through the block toward drum 62 in spaced parallel relation and terminating in points spaced slightly from coating 6I of the electrophotographic plate.
A 6000 volt D. C. source 1I, such as a transformer-rectifier system, has its high voltage D. C. output terminal A connected by conductor 11 and switch 18 to needles 69. The grounded terminal B is connected through rheostat 16 to shaft 63 and hence to drum 62 and plate 60. A control switch 12 is provided in a supply conductor of the high voltage source.
In operation, coating 6I on eleotrophotographic plate 6-0 is first given a uniform electrostatic charge over its surface. This may be done by closing switches 12 and 18 and rotating drum 62 a few times. A corona discharge takes place from needles 69 to coating 6I thereby applying a charge to layer 6l. When switches 12 and 18 are again opened to de-energze the needles a uniform charge is retained over the surface of coating 6I.
It is also possible, of course to charge the plate rby frictional methods before it is placed on drum 62.
With the uniformly charged plate on drum 62 ready to use, carriage 65 is moved to align aperture 13 with one end of the drum. Lamp 58 is energized and the drive motor for shaft 83 and screw 66 is started to rotate the drum and slowly move the aperture and associated light projecting system along the side of the drum. Any signals received over channel 55 are amplified and demodulated by ampliiier-demodulator 56 and fed to light valve 51 which thereby varies the quantity of light projected from lamp 58 onto plate coating 6I, by varying the width or intensity of the light beam.
The controlled light beam, as it scans the photo-conductive insulating coating 6| in a spiral path, discharges a greater or lesser proportion of the charge held on the elemental areas scanned. Elemental areas which receive a greater quantity of light, due to transmission of more light through the light valve, will be more fully discharged than areas receiving less light. The result is an electrostatic image on plate coating 6I which corresponds to the signal variations received. When the lsignal is a facsimile picture signal, coming, for example, from the transmitter of Figure 1, or from a conventional facsimile transmitter, and the rotations o drum 62 have been synchronized with the sending drum (such as drum I) an electrostatic latent image corresponding to the image transmitted, will be formed on coating 6l. This can be used for retransmission or can be developed to render it visible in the manner described in my aforementioned Patents 2,291,691 and 2,357,809, for example, by removing the plate and dusting the coating with an electroscopic powder.
For best results it is preferred that adjacent widths of the path traversed by the light beam during scanning be contiguous or very slightly overlapping in order to avoid leaving a fully charged strip between adjacent turns of the spiral scanned path.
A sound signal may be recorded in the same way and used subsequently to reproduce the sound in the apparatus of Figure 1 as long as the charge remains on the plate. I have found that such plates can retain their charge image for several days without difficulty if kept in the dark. The sound record can also be developed with powder and transferred and affixed to paper or lrn surface. Recordings of other types of signal variations can be made in 'the same ways.
It will be noted that light aperture 13 in Figure 2 comprises a small hole in a metal cup 14 fitted into insulating sleeve so as to be insulated from the frame. This cup is connected by conductor 19 to the slide of a potentiometer 80 which can be bridged across high voltage output terminals A and B by closing a switch 8|.
Another method of scanning may be carried out as follows: A potential is applied to cup electrode 14 by closing switches 12 and 8l. Potentiometer 88 is adjusted so that no corona discharge takes place from the bottom wall of the Cil cup to the coating. The plate is scanned with the coating in uncharged condition. When an elemental area of the plate passes the aperture the light beam renders that area momentarily conductive. The electric eld created by the cup electrode through the layer attracts charges to the surface of the coating where they become trapped by the return of the coating to insulating condition as it passes out of the range of the light beam. The amount of charge thus built up on an elemental area depends on the area of the spot of light or the intensity of the light, according to the type of light valve used. Hence, the plate is charged in accordance with the signal variations. The sign of the charge on the plate produced by this method will be the opposite to that obtained with the preceding method. However, either polarity of image can be obtained by reversing the connections to terminals A and B.
According to a modiiied method oi scanning the charged plate, after a uniform charge has been applied by needles 69, switch 18 is opened to disconnect the needles but switch 12 is left closed to maintain the high voltage supply in operation. Switch 8l is then closed during scanning. This applies a high voltage to cup electrode 14 of the same sign as the charge on coating 6 l. The potential on cup electrode 14 creates an intense field through layer 6I, in the vicinity of the aperture 13, reinforcing or accentuating the electrical eld tending to drive the charge through coating 6I to the grounded backing of plate B8. The eflciency of scanning is thereby improved.
By applying a small ch-arge density to the plate coating prior to scanning and creating an intense field by cup electrode 14 during scanning it is possible to obtain an actual reversal of charge on the coating in the areas traversed by the light beam when the light valve permits the maximum light to reach the plate. There is thus built up an image of one polarity on a background of opposite polarity.
Figure 3 illustrates a modification of the transmitter -apparatus of Figure 1. In this embodiment a scanning electrode 82 replaces electrode 26. Electrode 82 is mounted for longitudinal vibration at ultrasonic frequencies on a vibrator, such as piezo-electric crystal Vibrator 83. Piezo crystal 84 is supported rigidly at one end in a mounting 85 secured to carriage I8 and electrode 82 is secured by mounting 88 to the free end of the crystal to extend parallel to the direction of vibration. A pair of electrodes 81 applied to the faces 0f the crystal are supplied with alternating current of a voltage and frequency suflicient to vibrate the scanning tip 88 of the electrode toward and away from the coating I4 of plate I2 carrying a charge image. Electrode 82 is connected by conductor 89 to the grid of triode 58. high resistance 90 to the grid. During scanning the vibration of tip 88 varies the capacity between the tip and the coating I4 at the frequency of vibration so that an alternating voltage is superimposed on the grid bias whenever a. charged area of the coating passes the tip. The amplitude of the variations will depend on the density of charge on the elemental area being scanned. The output of tube 50 will comprise an alternating signal superimposed on the plate current. With this arrangement the cam-actul ated contacts and relay are not used.
Grid bias battery 45 is connected through' scanning. lLight from lamp 9I passing through a slit in diaphragm 92 is focussed by the lens system in tube 93 through a small aperture 94 in the wall of cylindrical metal housing 95, which is spaced from the coating I4 of electrophotographic plate I2. The rotating assembly comprising shaft I I, drum II! and plate I 2 is insulated from housing 95. The spacing between coating I4 and the inside wall c! cylindrical housing 95 is kept as small as practicable, while still assuring clearance between the plate coating and the housing, and may be in the order of 20 to 50 mils.
In the circuit of Figure 4 the grid of vacuum tube 59 is connected, by conductor 96, to metal housing 95. 'I'he rotating assembly is connected, by sliding contact 91 on shaft II and conductor 98, to the negative terminal of grid bias battery 45, and through resistance 99 to the grid of tube 50.
It will be evident that the electrostatic latent image on coating I4 will induce an opposite charge on the insider wall of cylindrical housing 95. As soon as equilibrium is established, by current through resistance 99, or by momentarily grounding the rotating assembly to the housing, this static charge will have no effect on the grid potential.
In operation of Figure 4, the light beam system and cylinder 95 are slowly advanced along the side of drum I as the drum is rotated in the manner previously described. Plate I2, carrying an electrostatic latent image on coating I4, is scanned in a spiral path by the light beam. As each elemental area of the coating is traversed by the light beam the charge (or part of it) entrapped on that elemental area is released and flows through the coating to the metal backing layer I3. The small quantity of charge which has been induced on housing 95 by the elemental area Vof the plate is now released and flows through conductor 96, resistance 99 and conductor 98 to the metal backing I3 of the plate. During the brief interval of this current flow the potential on the grid of tube 50 is made less negative (if the current is positive) permitting a if momentary increase in tube output current and hence sendingr a signal impulse over the line. The strength of the signal during scanning is proportional to the charge on the plate area being scanned.
In some cases it may be desirable to superimpose a predetermined frequency on the signal by interposing a rotating toothed wheel light chopper |00 in the light beam as shown in Figure 4.
Figure is a fragmentary View illustrating a modification of the transmitter of Figure 1 or Figure 3. In this embodiment wall 23 is replaced by a tubular metal scanning head |23 surrounding electrode pin 28 and terminating in a reduced-diameter rim |25 around end 32 of the nin and spaced from the end. It is evident from this that the shield need not be a wall of eX- tensive area but can be simply a narrow wall or edge close to the end of the electrode, and the term wall as used in the claims, is intended to include such a construction.
It is obvious that the mechanical arrangements may be varied in many known ways to facilitate the scanning operation. For example, the parts here illustrated as. mounted on a carriage for movement along the side of the drum may be stationary and the drum itself may be advanced on a screw to effectuate the scanning. Also, it is apparent that the electrostatic record element can vary in form and that the signals may be reproduced from, or recorded on, a ilat plate electrostatic record element or a tape electrostatic record element, such as a metal tape coated with photoconductive insulating material in which electrostatic record comprises a single path or multiple paths lengthwise of the tape.
While the present invention, as to its objects and advantages, has been described herein as carried out in specific embodiments thereof, it is not desired to be limited thereby but it is intended to cover the invention broadly within the spirit and scope of the appended claims.
What is claimed is:
l. In combination, a record member comprising an electrophotographic plate comprising a conductive backing and a coating of photoconductive insulating material thereon for carrying an electric charge; a scanning member for scanning said surface, said scanning member comprising a light source and a projector for projecting a spot of light onto said coating from said source, a movable support for at least one of said members to move said spot of light over the surface of said record member in a predetermined scanning path, and a light modulator for modulating said light beam in accordance with signal currents fed to said modulator.
2. The combination of claim 1, in which a corona discharge electrode is mounted in spaced relation to said coating and a high voltage source has its output terminals arranged to be connected to said electrode and the backing of said record element, said electrode and record element being relatively movable to bring said electrode successively over all parts of said coating to apply a uniform electrostatic charge to said coating preparatory to scanning.
3. In combination, a record element comprising an electrophotographic plate consisting of a conductive backing and a coating of photoconductive insulating material thereon for carrying an electric charge image, a scanning element for scanning said surface, said scanning element comprising a conductive wall uniformly spaced from said coating throughout its area and provided with a small aperture and a light source and light projector for projecting a spot of light from said source through said aperture onto said coating, a movable support for at least one of said elements to move said spot of light over the surface of said record element coating in a predetermined scanning path, and an amplifier, the backing of said element and said conductive wall being connected respectively to the input terminals of said amplier.
4. In combination, an electrophotographic plate comprising a conductive backing and a layer of photoconductive insulating material overlying said backing in contact therewith, charging means for applying an electric charge uniformly over said layer, a light beam projector and means for effecting relative movement between said projector and said layer to scan said layer with said light beam, and a modulator for varying the intensity of said light beam during scanning.
5. The combination as claimed in claim 4, in which means are provided for selectively energizing said charging means or said light beam projector.
6. In combination, an electrophotographic plate comprising a conductive backing and a surface layer of photoconductive insulating material overlying said backing in contact therewith, a scanning member and means to eiect relative movement between said plate and said member to scan the surface of said plate with said member, said member comprising a conductive electrode overlying the surface of said plate and spaced therefrom and having an aperture therein and a light lbeam projector for projecting a light beam onto said plate through said aperture.
7. The combination as claimed in claim 6 in which a potential source is connected between said conductive backing and said electrode.
8. In combination, a conductive cylinder coated with a surface layer of photoconductive insulating material, a scanning member comprising a light beam projector for projecting a spot of light onto said surface layer, and a modulator for modulating the intensity of said light beam in accordance with a signal, a charging electrode for applying an electric charge to said surface layer, means for selectively energizing said scanning member and said charging electrode, means fory rotating said cylinder with respect to said scanning member and charging electrode and means for moving said scanning member axially with respect to said cylinder.
CHESTER. F. CARLSON.
REFERENCES CITED The following references are of record in the file of this patent:
UNITED STATES PATENTS Number Name Date 1,897,796 Fulton Feb. 14, 1933 2,052,383 Cooley Aug. 25, 1936 2,143,214 Selenyi Jan. 10, 1939 2,197,050 Kellogg Apr. 16, 1940 2,200,741 Gray :May 14, 1940 2,277,013 Carlson Mar. 17, 1942 2,283,148 Bruve May 12, 1942 2,297,398 Freis Sept. 27, 1942 FOREIGN PATENTS Number Country Date 456,810 Great Britain Nov. 16, 1936