US 2277013 A
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March 17, 1942. c. F. CARLSON ELECTRIC RECORDING AND TRANSMISSION OF PICTURES Filed June 27, 1939 2 Sheets-Sheet 1 EXPO-SURE TIME Y\\\\\\\\\\\\\\\\\\\\\\N\\\\\\\\\\\\\ IY\\ gggg i March 17, 1942. CARLSON 2,277,013
ELECTRIC RECORDING AND TRANSMISSION OF PICTURES Filed June 27, 1939 2 Sheets-Sheet 2 W6 I 7 I INVENTOR Patented Mar. 17,1942
UNITED STATES PATENT orrlca ELECTRIC RECORDING AND TRANSMISSION OF PICTURES Chester F. Carlson, Jackson Heights, N. Y.
Application June 27, 1939, Serial No. 281,316
ment of parts, and methods of manufacture and operation referred to above or which will be brought out and exemplified in the disclosure hereinafter set forth, including the illustration in the drawings.
In the drawings: Figure 1 is a perspective view of an electrophotographic plate embodying features of my invention;
Figure 2 is a section on the line 22 of Figure 1;
Figure 3 is a section of a modified plate;
Figure 4 shows a case for holding the plate;
Figure 5 shows a camera arrangement therefor;
Figure 6 is an exposure graph or chart;
Figures 7 and 8 illustrate features of a scanning device for phototelegraphy or facsimile transmission; and
Figure 9 is a diagrammatic illustration of the transmission system.
A feature of the present invention resides in the use of a novel type of photoelectric photographic plate or electrophotographic element whereby an "electrostatic latent image" may be produced by photographic exposure and subsequently used for facsimile transmission of the image or for other purposes as will more fully appear in the following detailed description.
Heretofore in facsimile systems photographs, documentary matter, maps, charts and printed or written material have been transmitted to distant points by wrapping a sheet carrying the original pictorial or documentary matter around a cylinder, rotating the cylinder while scanning the image with a spot of light and picking up the reflected light variations by a photoelectric cell which converted the variations into varying electric currents. The varying currents are then amplified and transmitted by wire or radio to a distant point at which they are reconverted into a visible image by a scanning method in a suitable recorder.
5 Transmission of images from an original in this way has hertofore been encumbered by the necessity of first producing a suitable original of appropriate size and appearance which would fit the transmitter cylinder. This has necessitated 10 special typographical work or the making of transmission could be started.
Another inherent weakness of such systems is the low emciency of the optical system since only photostats or photographs of suitable size before ll a small part of the light of the scanning beam is reflected and received by the photocell.
I have discovered means for avoiding these cumbersome and time consuming steps and for more efficiently utilizing the scanning system,
10 which means are more fully described herein.
While a preferred embodiment of the invention is described herein, it is contemplated that considerable variation may be madein the method of procedure and the construction of parts ll without departing from the spirit of the invention. In the following decription 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 80 the art will permit.
Referring to the drawings Figure 1 is a perspective view of my photoelectric photographic plate or element 20. Figure 2 is a magnified cross-section of the plate and Figure 3 is a II cross section of another form 20a of the plate.
The plate 20 comprises a backing layer 2| formed of a thin layer of metal or other highly conductive material. Thin metal foil such' as aluminum or copper foil is suitable.
40 Covering the front surface of backin'g zif'is'a thin layer 22 of dielectric material of goodii'n'sulating value and uniform in characteristics and thickness throughout the area of the plate. Where plate 2| is of aluminum foil layer 22 may be an integral electrolytically formed dense layer of aluminum oxide. Otherwise the layer may be an insulating lacquer or other solid dielectric material or a thin sheet of dielectric material cemented to the backing. The material is selected to have high insulating value, and the ability to form a uniform dense flexible layer. The material must also be substantially free of any tendency to become a conductor of electricity under ordinary illumination.
A second dielectric layer 23 is disposed directly on top of dielectric layer 22 preferably as a uniform coating thereon. Layer 23, however, differs in composition from layer 22 and comprises a substance which may be described as a photoconductive insulating material. Such materials are good insulators in the dark, but when illuminated with light of suitable wave length become partial conductors of electricity during the period of illumination, returning immedi ately to the insulating state when illumination is shut off.
Examples of such materials are sulfur, anthracene and anthraquinone individually or in melted mixtures or compositions. Sulfur, for example, is an excellent dielectric material in the dark, it being one of the best insulators known. However, it increases in conductivity up to a millionfold under suitable illumination. Ihe peak of the photoconductivity response curve for sulfur is in the blue-violet part of the spectrum (around 4700 Angstrom units) and hence light in the neighborhood of that wave length is most effective where a sulfur layer is used, although it has considerable response throughout the greater part of the visible spectrum.
Anthracene is also a good insulator in the dark. It has its peak of photoelectric response in the neighborhood of the green part of the spectrum. Its spectral response curve is fairly comparable to photographic films and to phototubes now in use. Anthraquinone is also responsive to visible light.
While the above materials are given by way of example other photoconductive insulators may also be used. However, as will appear more fully hereafter, the well-known photoconductive semi-conductors such as metallic selenium and cuprous oxide are unsuitable since these materials are semi-conductors even in the dark and hence are incapable of holding 'an electrostatic charge under any conditions. They are never insulating but remain partial conductors at all times, although they are somewhat more conductive when illuminated.
An extremely thin transparent layer 24 of metal or other conductive material is intimately in contact with the front face of photoconductive insulating layer 23. Layer 24 may preferably comprise a noble metal such as gold deposited as a thin transparent layer by cathode sputtering or by vapor deposition in a well known manner. Instead of a continuous transparent layer the layer 24' may, less desirably, comprise a very fine wire screen or a grating of non-transparent conductive material.
Layer 24 terminates a short distance from the edges of the sheet where it is provided with a border 25 of conductive material of greater thickness than the transparent part of the layer. This may be formed, for example, of silver or other conductive paint. Border 25 defines the picture area of the plate.
The plate 20, comprising the layers above referred to is preferably sufliciently thin to be readily flexible and will not ordinarily exceed a few thousandths of an inch in thickness. The area of the surface inside border 25 will ordinarily be several square inches 'the size being selected to suit the apparatus subsequently to be described.
One edge of the plate is provided with a somewhat wider border than the others and a series of holes 26 are punched near the edge.
Plate 20a of Figure 3 is of somewhat modified construction and may be made as follows: For
a backing, a sheet 21 of paper, unplasticized Cellophane, cellulose acetate or Celluloid or other sheet material is used which has a metallic coating 2|a on its surface. Commercially available metallized papers are suitable, or the metal layer may be deposited by sputtering, metal spraying or by painting with a conductive paint.
Insulating layer 22a is deposited on or affixed to the metal layer and may be, for example, an insulating resin such as polystyrene deposited from a solution of polystyrene in a vaporizable solvent, shellac or rosin, various dielectric synthetic resins, rubber, or wax deposited from solution or otherwise applied.
The front layers of the element are formed separately and then attached to the layer 22a, as follows: A transparent sheet 28 of unplasticized Cellophane, cellulose acetate or Celluloid or the like is coated with a transparent layer 24a of metal such as gold deposited by sputtering or vapor deposition. Layer 23a of photoconductive insulating material is then applied to the metal layer by dissolving the material in a solvent (carbon disulfide is suitable for sulfur, anthracene or anthraquinone) and coating the layer with the solution and allowing the solvent to evaporate. This leaves a layer of fine crystals uniformly distributed on the metal surface.
Another method is to sprinkle the surface with the finely powdered photoelectric material and then melt it onto the surface by carefully controlled heating.
A third method, suitable for anthracene (and anthraquinone) comprises placing the anthracene in a shallow dish, covering the dish with the metal coated sheet and then heating carefully to sublime the anthracene and condense it on the metal surface in a thin adherent layer.
The front layers and backing layers are then united by placing the surfaces of layers 22a and 23a in face-to-face contact applying uniform pressure on the two outside faces of the assembly and heating carefully until the layers stick together. This may result'from the softening or partial melting of one of layers 22a or 23a.
Before assembly of the. two parts of the plate layer 24a is provided with a conductive paint border 25a and a conductive paint strip 30 on the front surface of sheet 28 is connected to border 25a.
In Figures 2 and 3, where layer 22 (or 22a) is of transparent insulating material the relative position of layers 22 (or 2211) and 23 (or 23a) may, if desired, be reversed so the layer 22 (or 22a) is adjacent the transparent metal layer 2l-as will appear more fully hereafter.
Having described my novel photoelectric plate element I will now describe the use of the plate for recording images and subsequently transmitting the images by facsimile and similar systems. (Where plate 20 is referred to it will be under stood that plate 20a may likewise be used.)
For the purpose of recording an electrostatic image on the plate 20 it is inserted in a fiat metal case 3| (Figure 4). Plate 20 slides into a suitable slot at the end of the case 3| so that its back is against the back wall of the case and layer 2| is in metallic contact with the case. The front of case 3| is cut away to provide an opening slightly larger than the area defined by border 25 on plate 20. A closure slide 32 is inserted in a suitable slot in the case 3| in front of plate 20 and spaced from it. Suitable black plush linings are provided around the ends of the slots in case 3| to exclude light when the slide 32 is in place. Thus case 3| provides not only protection from injury but also excludes light from the surface of plate 20 when the slide 32 is in place.
Figure shows a camera arrangement suitable for forming an electrostatic image on plate 20. This comprises a camera box 33 having a bellows 34, lens system 35 and shutter 33 all of wellknown type. One side of the camera is provided with a plush-lined slot 31 adjacent the back wall of the camera to'receive case 3|.
Case 3| is introduced into the camera through slot 31 as shown and rests against the back wall of the camera. Slide 32 is then drawn out of the case part way (as shown) so that the surface of the plate is ready for exposure in the camera.
A contact spring 38 is set in a recess in the back wall of the camera and engages metal case 3| conductively. Spring 33 is connected to terminal 39 on the outside of the camera. Terminal 39 is considered a ground terminal and may or may not be actually grounded during the use of the camera, as indicated by the dotted line. Terminal 39 is connected to one pole of battery 43 and also to the top contact 4| of a single-pole, double-throw switch. The other pole of battery 40 is connected through resistance 42 with the bottom contact 43 of the switch.
On the side of the camera opposite the slot 31 is an opening closed with an insulating bushing 45 through which passes a terminal screw 45. A contact spring 41 is secured by the inner end of terminal screw 46 and has its free end bent to extend toward the rear wall of the camera at an angle and then away from the rear wall. When case 3| is inserted in the camera spring 4! slides over the edge of the case andwhen slide 32 is drawn out spring 41 springs down into contact with border on the face of plate 20. Terminal screw 46 is connected to the center double-throw key contact 44 of the switch.
It will be observed that when contacts 4| and 44 are in engagement they afford a direct connecting circuit between transparent metal layer 24 and metal backing 2 I. These layers are otherwise insulated from each other by insulating layer 22 and photoconductive insulating layer 23.
Photographic exposure is preferably made in the following manner: Key 44 is first depressed to close contacts 43, 44 and thereby connect the free terminal of battery to transparent metal layer 24, through resistance 42.
Battery 4|] may be a or 90 volt radio B battery. However, for highest efficiency of operation and shortest exposure time one may use batteries of several hundred volts, the practical limit being that voltage which the layers 22 and 23 will safely withstand without breaking down. Resistance 42 prevents a fortuitous direct shortcircuiting of the battery.
It is apparent that plate 20 forms a sort of ccndenser with electrodes 2| and 24 separated by thin insulating layers 22 and 23. Hence when key 44 is actuated the electrode 24 will be charged to a potential above or below the potential of electrode v2| corresponding to the voltage of the battery. In the following discussion we will assume the electrode 24 to be negative with respect to electrode 2 l, although it will be apparent that the opposite battery orientation may be used with similar practical results.
With layer 24 connected to the battery and the camera being properly focussed upon the object to be photographed the shutter 33 is opened for a predetermined time interval, thus allowing the image of the object to be projected onto plate 23.
The light passes through transparent metal electrode layer 24 and, at least to some extent. through photoconductive insulating layer 23. The material of layer 23, as previously mentioned, is a good insulator in the dark but becomes partially conductive while exposed to light. Moreover, the conductivity is proportional to the intensity of the light.
The potential difference between electrodes 24 and 2| produces a strong electric field through the layers 23 and 22 urging electrons to leave electrode 24 and travel toward electrode 2|. They are normally stopped by the two insulating layers. However, when layer 23 is illuminated and becomes temporarily a partial conductor electrons are allowed to migrate through layer 23 to the interface I00 between layers 23 and 22 where they are prevented from traveling further due to the permanent insulating character of layer 22. I
If we select for consideration an infinitesimal area of the picture surface which is brightly illuminated it will be evident that if exposure is continued for a suflicient time the electrons migrating through layer 23 will accumulate at interface I00 and build up a charge there which eventually reaches a maximum determined by the battery voltage. When the maximum is reached no further charge will accumulate at that spot.
At another spot on the picture surface which is only dimly illuminated by light from the object the rate at which the charge accumulates at interface I00 will be slower due to the lower conductivity produced in layer 23 at that point. However, if the exposure were continued long enough this spot would also reach the same charge as the first spot. This condition would correspond to that experienced when a photographic film is grossly overexposed and the film is uniformly blackened.
In the present case, therefore, proper exposure requires that the shutter be closed before the dimly illuminated area becomes fully charged. This will be seen more clearly by referring to Figure 6 which shows the degree of charge vs. exposure time for a brightly illuminated area (curve a) and a dimly area (curve b). It will be apparent that the greatest charge contrast is ob ained if the exposure is stopped at time t, where the spread between the two curves is greatest.
Assuming that exposure of plate 20 is continued for time t and'the shutter is then closed, layer 23 immediately becomes insulating again and the charges are held trapped at the interface I00 or within layer 23. Key 44 may now be released and when it engages contact 4| electrodes 2| and 24 will be short circuited and discharged. Resistance 48 protects the contacts. The charges at interface ||I|| or in layer 23, however, remain trapped due to the now insulating character of layer 23. It willthus be evident that an "electrostatic latent image has been produced in layer 23 in which the charge density varies over the picture area in a manner corresponding to the light variations over the area of the object photographed.
Slide 32 may now be closed and case 3| removed from the camera.
Should an error have been made in the exposure or a retake be desired case 3| and plate ing a spot of light upon the 20 may be left in the camera with slide 32 open. Mounted inside the camera is a small incandescent lamp 49 connected to a battery 50 through switch If the electrodes 2| and 24 of plate 20 are short circuited by contacts 4|, 44 and the switch 5| is closed to light lamp 49 layer 23 will be made uniformly conductive and its trapped charges will be freed and allowed to discharge. The plate 20 will thus be prepared for a new exposure. It is advisable to discharge the plate in this manner before each picture exposure to be certain of the condition of the plate at the start of exposure.
Assuming that an electrostatic latent image has been impressed on the plate 20 as previously described and case 3| is closed and removed from the camera, the exposed plate may be kept for a considerable period of time prior to use for facsimile transmission or related purposes. I have successfully retained such an electrostatic latent image for at least an hour and it is believed that a transmissable electric image may be retained at least for several hours without difficulty where layers of the highest dark insulating value are used.
When it is desired to transmit the electric image by a facsimile or related system it is merely necessary to connect the electrodes 2| and 24 to the input terminals of a conventional facsimile amplifier-transmitter system and scan the picture area of the plate with a narrow beam of light. By this operation the elemental areas of the plate are discharged in sequence through the input circuit of the amplifier thus producing the facsimile signal variations.
Figures 7, 8 and 9 show features of a scanning device suitable for scanning plate 20 for the facsimile transmission of an image recorded thereon. For the most part the scanning and transmission equipment may be of any well-known standard type including the usual synchronizing and framing arrangements. Various other refinements may be added which are of advantage but are not essential to the understanding of the present invention.
The scanner comprises essentially a drum 60 about which the plate 20 is wrapped, a lamp 6| and a lens and diaphragm system 62 for focussurface of the plate. A screw 53 drives the lamp and lens system along the side of the drum. The entire assembly is enclosed in a light tight cover 64, having an aperture 65 at the top.
The drum is provided with a longitudinal V- shaped groove 66 along its side lined with felt G1, and having a row of projecting pins 69 on one face of the groove. In loading the drum the plate 20 is pulled part way out of its case 3| so that holes 26 are exposed. The case and plate are then inserted through aperture 65 in the cover of the scanner and holes 26 hooked over pins 68. A light impervious thin flexible sack 69 of fabric, rubber or thin leather is then drawn over case 2| and its mouth fastened over the neck of aperture 65. If the apparatus is used in a darkroom this precaution is not necessary.
A clamping bar 19 of insulating material of such shape as to fit into groove 66.is supported on cam controlled levers 1| at its ends. Manually controlled levers 13 are adapted to be set in one of two positions where they are held by a locking key 14. In one setting of the levers rollers 15 at the end of the levers are in a position to engage the cam surfaces 12 of levers 1| and thereby raise clamping bar 19 out of groove 66 when the drum 60 reaches a point in its rotation where the bar and groove are at the top of the drum. This position is shown in Figure 8.
If now the drum is rotated a little in a clockwise direction the bar 10 falls back into groove 66 clamping the edge of plate 20. The drum may now be rotated through one revolution while case 3| is drawn away from the drum and laid on top of the case in sack 69.
A rubber or felt roller 16 is held against the drum surface by spring held levers 11 and serves to guide plate 20 against the drum surface during the first revolution. As the drum completes its first revolution cams 12 again engage rollers 15 lifting clamp bar 19 out of groove 66. As the second revolution of drum 60 begins clamp bar 10 again falls into groove 69 clamping both the leading and trailing edges of plate 20 against felt lining 61.
Levers 11 are then manually actuated to lift roller 16 away from the face of the drum. Levers 13 are also adjusted to withdraw rollers 15 from the path of cam surfaces 12.
Clamp bar 10 has metal strips 18 extending along its clamping faces, strips 18 engaging the border 25 of plate 20. On the end of drum 60 (Fig. 7) is mounted a slip ring 19 having spring fingers which are positioned for contact with strips 18 when the clamp bar is in clamping position. Thus the transparent electrode 24 is connected to slip ring 19. A bifurcated sliding contact spring 8| rides on slip ring 19, and affords a means for connecting electrode 24 to the facsimile circuits. Backing electrode 2| is in direct contact with the metal facing on drum ill which in turn is grounded to the shaft 82 of the drum.
When the plate 20 has been mounted on the drum 60 as described and rollers 15 and 16 have been retracted the scanning may be begun by rotating drum 60 at a suitable speed and simultaneously rotating screw 63 to slowly advance the scanning beam projector along the side of the drum in a well known manner.
As the light spot scans the surface the elemental areas of layer 23 are each in turn rendered conductive by the incident light and their trapped charges thereby allowed to discharge through the input circuit of the facsimile amplifier (see Fig. 9). Since the elemental areas are charged in proportion to the light intensity of the original electrodes ample, if insulating layers of the same thickness and 2|. and 24. For ex- 22 and 23 are both one line width. However, where desired, the
line width and screw pitch may be adjusted to space apart successive turns of the scanning beam by one or more line widths so that alternate strips remain unscanned. This leaves a reserve image on the plate which may later be scanned for another transmission of the image by shifting the scanning projector one line width on its supporting slide.
As another alternative the original scanning may be effected with a lower intensity of scanning beam so as to discharge only part of the trapped charge on the first scanning. The retained charge image may then be released on a subsequent scanning of the plate.
The present invention aifords a means for rapid recording and transmission of pictorial or documentary material without the delay and expense incident to chemical photography, photostating, special typographical work and the like. It also affords a ready means to reduce all material to be transmitted to a standard size and form regardless of the character of the original. It enables an enlargement or reduction in the size of a picture or page to be transmitted. Where reduction is resorted to it enables a considerable reduction in the size of the entire scanning apparatus, bringing about economies and greater portability.
In the case of news pictures and the like it will be possible for the cameraman to take a photograph on the electrophoto plate and immediately transmit it by facsimile to the central ofllce where it can be recorded as a visible picture or as an engraved half tone within a few seconds after the picture was taken.
It is also contemplated that the transmitting and recording equipment may both be located in one place or even incorporated intoa single machine to afford a rapid means of producing finished copies of pictorial or documentary matter for general purposes such as for records and for identification papers, or for the direct engraving of cuts.
The photoelectric photographic plate is most economical to use since it may be reused an indefinite number of times without deterioration.
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:
1. An electrophotographic element comprising two layers of conductive material separated by two intervening layers, one of said intervening layers being formed of insulating material and the other of said intervening layers being formed of photoconductive insulating material, at least one of said conductive layers being light pervious.
2. An electrophotographic element comprising two layers of conductive material separated by 76 two intervening layers, one of said intervening layers being formed of insulating material and the other of said intervening layers being formeu of photocondu'ctive insulating material, at least one of said conductive layers being light pervious, said layers :being bonded together in a unitary sheet-like element.
3. An electrophotographic element comprising two layers'of conductive material separated by two intervening layers, one of said intervening layers being formed of insulating material and the other of said intervening layers being formed of photoconductive insulating material, at least one of said conductive layers being light pervious, and a current collecting border of conductive material on said light pervious layer.
4. An electrophotographic element comprising a conductive backing layer, a thin layer of highly insulating material adhering thereto, a thin layer of photoconductive insulating material secured to the surface of said insulating layer and a thin transparent metal layer secured to the face of said photoconductive insulating layer.
5. An electrophotographic element comprising a conductive backing layer, a thin layer of highly insulating material adhering hereto, a thin layer of photoconductive insulating material secured to the surface of said insulating layer and a thin transparent metal layer secured to the face of said photoconductive insulating layer and a current collecting border of conductive material on said thin metal layer,
6. An electrophotographic element comprising a conductive backing layer, a thin layer of highly insulating material adhering thereto, a thin layer of sulfur secured to the surface of said insulating layer and a thin transparent metal layer secured to the face of said sulfur layer.
7. An electrophotographic element comprising a conductive backing layer, a thin layer of highly insulating material adhering thereto, a thin layer of photoconductive material selected from the group consisting of anthracene and anthraquinone secured to the surface of said insulating layer and a thin transparent metal layer secured to the face of said photoconductive layer.
8. An electrophotographic element comprising two layers of conductive material separated by two intervening layers, one of said intervening layers being formed of insulating material and the other of said intervening layers being formed of photoconductive insulating material, at least one of said conductive layers being light pervious, protective layers of non-conductive material over the front and back surfaces of said element, the protective layer over said light pervious layer being light pervious, and conductive contacts on said protective layers connected respectively to said conductive layers.
9. An electrophotographic element comprising a conductive backing layer, a thin layer of highly insulating material adhering thereto, a thin layer of photoconductive insulating material secured to the surface of said insulating'layer and a thin transparent metal layer secured to the face of said photoconductive insulating layer, protective layers of non-conducting material over the outer faces of said conductive backing layer and said transparent metal layer, the protective layer over said transparent metal layer being transparent, and contact on the outer faces of said protective layers connected respectively to the underlying conductive backing and the transparent metal layer.
10. The method of electrophotographically recording an electrostatic latent image which comprises exposing a layer of photoconductive insulating material to a light image while simultaneously engaging one face thereof with a conductive contacting layer and spacing a second conductive layer from the other face thereof by an intervening layer ofinsulating material, and simultaneously imposing an electric potential difference between the two conductive layers, thereby to build up an electrostatic latent image in said layer of photoconductive insulating material, and then shutting ofi the illumination of said layer to trap said image thereon.
11. The method of recording and transmitting an electric picture which comprises exposing a layer of photoconductive insulating material to a light image while simultaneously engaging one face thereof with a conductive contacting layer and spacing a second conductivelayer from the other face thereof by an intervening layer of insulating material, and simultaneously imposing an electric potential difference between the two conductive layers thereby to build up an electrostatic latent image in said layer of photoconductive insulating material, and then shutting off the illumination of said layer to trap said image thereon, and subsequently transmitting said image by similarly disposing said photoconductive insulating layer in relation to two conductive layers, scanning said photoconductive insulating layer with a light beam and electrically detecting and amplifying the attendant electrical variations produced between said conducting layers, and transmitting the amplified variations.
12. The method of transmitting an electrostatic latent image contained in. a layer of photoconductive insulating material which comprises engaging one surface of said layer with a coextensive contacting layer of conductive material, spacing a second layer of conductive material from the opposite surface of said photoconductive layer by an intervening insulating layer, scanning said photoconductive layer with a light beam and simultaneously detecting and amplifying the attendant electrical variations between said conductive layers during scanning, and transmitting the amplified variations.
13. An electrophotographic element comprising two layers of conductive material separated by two intervening layers, one of said intervening layers being form-ed of insulating material and the other of said intervening layers being formed of photoconductive insulating material, at least one of said conductive layers being light pervious, said element-being flexible and being exposed to the air at atmospheric pressure.
14. An electrophotographic element comprising two layers of conductive material separated by two intervening layers, one of said intervening layers being formed of insulating material and the other of said intervening layers being formed of photoconductive insulating material, at least one of said conductive layers being light pervious, said layers being bonded together in a unitary sheet-like element, said element being flexible whereby it is capable of being rolled into cylindrical form without injury thereto.
15. The method of electric picture recording and-transmission with the use of an electrophotographic plate of the type comprising a pair of conductive layers separated by a layer of photoconductive insulating material in contact with one of said conductive layers and an insulating layer between said photoconductive layer and the other of said conductive layers and in which all the layers on one side of the photoconductive layer are light transmitting, which method comprises exposing said photoconductive layer to a light image through the light transmitting layers while simultaneously applying an electric potential difference between said conductive layers, and subsequently connecting said conductive layers to the input terminals of an electric pic ture transmission system and scanning the plate with a beam of light.
CHESTER F. CARLSON.