US3724941A - Electrophotographic apparatus - Google Patents

Abstract

Electrophotographic reproduction systems in which the contrast may be suitably varied in accordance with the particular type of original image which is to be reproduced by varying the amount or degree of electrostatic charge applied to a photoreceptive surface during uniform charging thereof.

Description

Field of Search ..355/3, 4, ll, 17

0 United States Patent m1 [111 3,724,941 Honjo et al. [451 Apr. 3, 1973 [54] ELECTROPHOTOGRAPHIC APPARATUS [56] References Cited [75] Inventors: Satoru llonjo; Seiji Matsumoto; UNlTED STATES PATENTS Masaakl Takimoto, all of Asaka,

Japanv 1,485,147 2/1924 Patterson ..353/63 I 2,965,483 12/1960 Byrne ..355/17 X [73] Assignee: Xerox Corporation, Stamford,

' Conn. Primary Examiner-John M. Horan [22] Filed: sept- 16, 1971 Attorney-James J. Ralabate et al.

[21] Appl. No.2 181,022 [57] ABSTRACT i Electrophotographic reproduction systems in which [30] Foreign Application Priority Data the contrast may be suitably varied in accordance with l the particular type of original image which is to be July 22, 1970 I Japan .....45/6353l reproduced by varying the amount or degree of elec trostatic charge applied to a photoreceptive surface I 12::-

1324,2521; v I I [58] 4 Claims, 5 Drawing Figuresmmgwm ms 3,724,941

SHEET 1 or 5 LOG EXPOSURE FIG.

' INVENTORS.

TORU 0 0 55m MA SU JOTO BY MASAA2 TAKIMOTO RNEV PATENTEUAPR 3 I975 SHEET 2 0F 5 FIG. 3

BACKGROUND OF-TI-IE INVENTION This invention relates generally to electrophotography and more particularly to electrophotographic reproduction systems in which the contrast may be suitably varied in accordance with the particular type of original image which is to be reproduced.

In general, an original which is to be viewed by reflection, for example a photograph printed on a printing paper, tends to produce increased optical flare and reduced contrast due to surface reflection or for the reason that lighting is conducted from the side where observation is made.

-On the other hand, an original which is to be observed by transmitting light, such as for example a color slide, allows easy light interception and also produces diminished optical flare and sharp contrast. This is due to the fact that no surface reflection takes place and also because lighting is conducted from the side opposite to theme where observation is performed. This is indicative of the fact that little decline of contrast is caused by the optical system.

Therefore in case of replicating or duplicating these two different types of originals, it has been found that indistinct or blurred duplicates with poor contrast tend to be produced when employing reflection exposure, while the transmission type originals tend to give rise to too much contrast, thus making it difficult or even impossible to obtain true reproduction or original tone in highlight or shadowed parts.

SUMMARY OF THE INVENTION Accordingly, it is an object of the present invention to obtain electrophotographic reproductions which closely resemble in contrast the images reproduced.

It is another object of the present invention to provide an improved electrophotographic arrangement which varies the contrast according to the type of original which is to be reproduced.

It is still another object of the present invention to provide an improved electrophotographic arrangement employing a single photoreceptive surface which produces copies true to various types of originals.

It is yet a further object of the present'invention to provide an improved electrophotographic arrangement which produces an increased contrast for original images having a somewhat flat contrast and which produces a reduced contrast for original images which are sharp in contrast.

These and other objects of the invention are attained in an electrophotographic arrangement in which an electrophotographic photoreceptor which may be formed by placing a photoconductive insulating layer on an electroconductive support and uniformly electrostatically charged throughout the photoreceptor body to a degree consistent with the type of original image to be reproduced. According to the invention, exposure to an original image is then performed to form an electrostatic latent image of the original. Thereafter, finely divided charged electroscopic particles hereinafter referred to as toners, are selectively deposited on the electrostatic image to render the latent electrostatic image visible, and the toner image may then be transferred to a supportsurface for fixing thereto or directly fixed onto the photoreceptive surface. The invention takes into consideration the fact that the contrast of the resultant electrostatic image after exposure is related to the amount or quantity of charge applied during uniform charging of the photoreceptive surface. To this end, the amountof electrostatic charge applied during uniform charging is varied in accordance with the type of original image which is to be reproduced.

Other objects of the invention will become readily apparent to those skilled in the art in view of the following detailed disclosure and description thereof, especially when read in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a graphical illustration of the tonal reproduction characteristics in electrophotography wherein the abscissa represents the logarithm of exposure and the ordinate represents the optical density of the obtained toner image.

FIG. 2 is a schematic illustration of one embodiment as contemplated by the present invention.

FIG. 3 is a schematic illustration of another embodiment as contemplated by the present invention.

FIG. 4 is a schematic illustration of one embodiment for implementing a change between optical systems for transmittive original images and reflective original images as contemplated by the present invention.

FIG. 5 isa schematic illustration of another embodiment for implementing a change between optical systems for transmittive original images and reflective original images as contemplated by the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The photoreceptive surface employed in the present invention may comprise any photoconductive member suitable for use in electrophotography. Suitable photoconductive members include for example, photoreceptors prepared by laying vacuum evaporated coatings of selenium on electroconductive supports, photoreceptors formed by coating on electroconductive support members layers of film-forming insulating resins having dispersed therein pulverized photoconductive materials such as cadmium sulfide, zinc oxide or titanium dioxide, and photoreceptors formed by coating organic photoconductive films on electroconductive supports. Such photoreceptors may also contain various sorts of additives either singly or in combination so as to increase photosensitivity or to give optimum spectral sensitivity and such photoreceptors are contemplated within the scope of the present invention.

As will be appreciated, it is convenient to selectively use various photoreceptive members according to the object or purpose of use as is well known. For instance, in the case of selenium vacuum evaporated films, it is preferable to transfer the images to other image receiving members, while in the case of organic photoconductors, it may be preferred to use the photoreceptor for the purpose of obtaining transparent images by making use of its optical transparency. On the other hand, in the case of resin layers dispersed with zinc oxide, titanium dioxide or the like, when applied on suitable supports, such as paper, there may be no need of transferring the images whichmay ,be directly fixed and preserved as arecord. This may also be'true in the case where an organic photoconductor is layed on a paper sheet. lt'will also be appreciated that in case of using a combination of photoeonductive powder-resin layers tration of the tonal reproduction characteristics in electrophotography, wherein the abscissa represents the logarithm of exposure and the ordinate represents the optical density of the obtained toner image. This example applies to a case where the electrical charge on or v in the photoreceptor differs in polarity from the charge on or in the toner. lntthe. graph, curve A shows the,

characteristicobserved when theelectrical charge is heavy, and curve B illustrates the characteristic observed when the electrical charge is slight. The characteristic represented by curve A is suited for duplication of original images having flat contrast. That is, this pattern is suited for reproduction from reflective original images. It provides excellent reproduction where the optical density of the image reproduced is widely varied with increased contrast as compared with limited change of exposure. The characteristic represented by curve B is suited for duplication of originalimages having sharp contrast. That is, this pattern is suited for reproduction from transmittive original images. it allows excellent reproduction of tones over a wide variation of exposure.

in this manner, it ispossible to obtain a replicated image having always excellent contrast either from a reflective type or transmittive type original, by suitably varing the amount of electrical charge. Thus, by suitably changing the lighting system, with concomitant change of the amount ofelectrical charge, according to the type of the original tob'e replicated, whether it may be a reflective type or a transmittive type, it is possible to always obtain ahigh-quality image by using a single electrophotographicdevice.

With reference to Fl G -Z there is illustrated a schematic arrangement'e'mbodying the above-noted principle, in which AC designates an alternating current source generally in the range of 100 to 200 volts and 1 designates a boosting transformer. On the primary side of the transformer there are provided two taps T and T, which are connected to the electric source AC through a change-over switch 2. The number of turns in the secondary, windings of the transformer 1 are in a ratio with the number of turns in the primary windings so as to induce a voltage in the secondary windings in the range of about 4 to 8 KV. One end of the secondary,

windings is connected to a commutating diode 3 while the other end isconnected to a smoothing condenser 4. The numeral 5 designates a tine conductor wire for producing a corona discharge hereinafter referred to as a corona wire. Usually, a wire having a diameter of about 30 to 50 t is fottnd suited for this purpose. A cable'designated by reference numeral 6 is passed between the secondary side of the transformer l and the corona wire 5. A high'voltage is applied through this cable 6 to the corona wire 5 to give rise to corona discharge, and a photoreceptor 8 passing nearby may base plate 9 made of an electroconductive material such as metal and is moved in the direction of the arrow while maintaining a spaced distance of about 10 to 40 mm from the corona wire 5. The amount of charge imparted to the photoreceptor during this operation may be varied either by changing the distance between the corona wire 5 and the photoreceptor 8, or by changing the traveling velocity of the photoreceptor 8, or by changing the voltage applied to the corona wire 5. In the present embodiment, the last-mentioned method is employed. A change-over switch 2 is arranged to be changed over in response to change-over of the optical system according to the type (reflective or transmittive) of the original. The reference numeral 10 designates a timer for determining the exposure time, and 11 designates a switch for changing over the light sources to be lighted according to the type (reflective or transmittive) of the original. As will be seen, a light source 12 is connected to the contact 11a side of the" switch 11. The light source 12, which usually comprises a tungsten filament bulb or the like, is used for effecting exposure for replication of a transmittive type of original 13. Reference numeral 14 designates a projection lens. It will also be seen that a pair of exposure light sources 15 and 15 adapted for use in replication of a reflective type original 16 are connected to the contact 11b side of the switch 11. Usually a tungsten filament bulb can be used for each of the light sources 15 and ISZReference numeral 17 designates a projection lens and :18 designatesa mirror. The above elements 10 to 18 in combination constitute an optical system. Reference numeral 19 indicates a charged photoreceptor which is placed on a conductive block 20 and is so arranged that the original isimaged on the upper surface thereof. Of course, the light sources 15 and 15', lens 17 and mirror 18 are used for conducting duplication of a reflective original. In the case of a transmittive type original, these are replaced by the light source 12 and lens 14, and during this operation, the optical system within enclosure 21 including the mirror 18 and lens 17 is moved aside. Of course, any other parts that will hinder projection of the transmittiveor reflective originals are removed away as occasion demands. The shifting mechanism of enclosure 21 may be arranged in interlocked relation with the switches 11 and 2 so that shifting of the mechanism may be conducted automatically by simple changeover operation of the switches.

In operation, when it is desiredto make a reproduction of for example a reflective original 16, the corresponding optical system comprising the lens 17, mirpassed beside the corona wire 5 and heavily charged during passage beside the wire 5, and then subjected to exposure to projection of the reflective original to produce an electrostatic image, which is then developed by a toner.

Any known developing method, such as the cascade method, magnetic brush method, aerosol method or liquid developing method, may be used. But, in the case of the present invention, where tonal reproduction is envisaged, a high-quality method is preferred. For this reason, the aerosol method in which the electrostatic latent image is contacted with air in which the toner is floated or the liquid developing method in which the electrostatic latent image is contacted with an insulating liquid in which the toner is dispersed, may be used, as these methods are particularly suited for the present invention. In particular the latter method is preferred.

In the case of the transmittive original, the shifting mechanism of enclosure 21 is shifted and at the same time the switch 2 is connected to the tap T, on the primary side of the transformer 1 so as to lower the boosting ratio to accordingly reduce the voltage applied to the corona wire 5. This results in a reduced amount of electrical charge on the photoreceptor 8. Simultaneously, the switch 11 is changed over to the contact 11a to energize the light source for the transmittive original, whereby to project the image of the original 13. In this manner, it is possible to prevent the contrast from becoming too flat in duplication of a reflective original, or to prevent the contrast of a transmittive original from becoming too sharp. Moreover, this is accomplished while using the same photoreceptor which results inobtaining'reproductions having excellent contrast.

The percentage variation in the number of turns between the taps of the boosting transformer 1 should preferably be within the range of 5 to 30 percent when the secondary side voltage is on the order of 5 KV and within the range of 7 to 40 percent when the secondary side voltage is on the order of 7KV. The amount of exposure necessary for obtaining an intermediate density C at larger electrostatic charge amounts may be from 1.3 to 1.5 times or in some cases nearly 3 times as much as in the case of lesser electrostatic charge amounts as may be seen by D and E in FIG. 1. It is generally desirable to conform the brightness of the projected images to the necessitated exposure by adjusting the light source for projection of the original or the iris of lens. It is of course possible to adjust the exposure time by use of a timer or other means.

In FIG. 3 there is illustrated another embodiment of the present invention, in which like reference numerals are used to designate the same or similar parts as those described in connection with the embodiment'of FIG. 2.

In this embodiment, as will be seen, a plurality of corona wires 5 are used, and a shielder plate 22 is inserted between the photoreceptor 8 to be charged and the corona wires 5. The shielder plate which is arranged to be movable in the manner as indicated by the arrow, is inserted between corona wires 5 and photoreceptor 8 to produce a greater shielding effect when it is desired to lessen the amount of electrostatic charging and when it is desired to increase the amount of electrostatic charging the shielder plate may be either positioned to provide a smaller shielding effect or may be completely removed. Movement of the shielder plate 22 is associated with change-over of the light sources in the optical system for projection of a transmittive original. For a reflective original, although the same operation as described in connection with FIG. 2 may be followed, the optical system and the light source for the transmittive original are provided separately as shown by enclosure 24. Reference numeral 23 designates a screen made of ground glass or the like, and the transmittive original 13 is projected onto the screen 23 through a lens 14.

In case of reproducing the transmittive original 13,

the assembly enclosed by enclosure 24 is moved as indicated by the arrow such that the screen 23 will coincide with the position where the reflective original 16 is shown placed. In this position the image of the screen 23, instead of the reflective original 16, will be projected onto the photoreceptor 19 by utilizing the optical system 17 and 18 for the original. Shift of the assembly 24, operation of the switch 11 for change-over of the light sources, and movement of the shielder plate 22, are all associated with each other. Development of the photoreceptor after exposure may then be conducted in the same manner as described in connection with the embodiment of FIG. 2.

In FIG. 4 there is illustrated an embodiment for change-over of the optical systems for the reflective type originals and the transmittive type originals. In this figure, reference numeral 30 denotes the optical system for the transmittive type original, including the parts corresponding to the light source 12 and lens 14 of FIG. 2. Reference numeral 31 represents the optical system for the reflective type original, including the parts corresponding to the lens 17, mirror 18, etc., as shown in FIG. 2.

In case of performing reproduction from the transmittive type original, the optical system 31 for the reflective original, is turned about a fulcrum 33 in the manner as shown by the arrow until the system comes to take the position 32 shown by dotted lines. In this position, the optical system 31 will not interfere with the optical system 30 used for the transmittive type original. Rotation of this optical system 31 also causes rotation of a shaft 34 and hence the arms 35 and 36 secured thereto are accordingly rotated into the respective positions 35" and 36 as shown by the dotted lines. With rotation of the arms 35 and 36, the movable electrodes 37 and 38 which are mounted at the ends of arms 35 and 36, respectively, are separated away from the respective fixed electrodes 39 and 40, with which they have theretofore been contacted. In the rotated position electrodes 37 and 38 are then contacted with the separately provided fixed electrodes 41 and 42 respectively.

Thus, if the lead wires 43 through 48 are suitably connected to the respective electrodes and other elements, for example connecting wire 43 to a suitable power source for energizing the respective light sources, connecting wire 44 to a light source for the reflective optical system, connecting wire 45 to a light source for the transmittive optical system, connecting wire 46 to a suitable power source foren ergizing boosting transformer 1, connecting wire 47 to a terminal in the charging high-voltage source for reflective originals and connecting wire 48 to a terminal in the charging high voltage source for transmittive originals, it is possible to automatically perform change-over of the charging conditions simultaneously with change-over of the optical systems.

In FIG, there is illustrated still another embodiment of the invention in which the transmittive original is projected onto a screen 50 and the image is the projected onto a photoreceptor by using the reflective image optical system. In case of reproducing a transmittive original, the optical system 51 for the transmittive original is moved in the direction of the arrow, whereby the screen 50 will be positioned in the location 52 at which the reflective original would normally be placed. In this position, the image of the screen is projected onto the photoreceptor by mirror 18 and lens 17. Movement of this optical system is transmitted through a cable 53 via roller 54 to arm 56, causing an end of arm 56 to move in the direction of the arrow pivoted at 57. Movement of this arm 56, now reduced into a suitable momentum, is conveyed to arm 58 whence it is further transmitted through cable 59 via rollers 60 to a shielder plate 61. The shielder plate 61 moves in the direction of the arrow with a point 62 working as a fulcrum, but it is possible to let the shielder plate make a substantially parallel movement by sufficiently elongating arm 63 supporting the plate 61. I

The amount of charge received byphotoreceptor 8 from corona discharge 5 may be varied in accordance with the amount of movement of the shielder plate. Such amount of movement, that is to say, the amount of effective shielding'work done by the shielder plate may be controlled by adjusting the length of the arm 58. Reference numeral 64 designates a spring which is adapted to facilitate movement of the shielder plate 61. Reference numeral 65 designates a microswitch adapted to effect change-over of the electric sources for the light sources, for the reflective original and for the transmittive original in conformity to movement of the arm 56.

Thus, according to the present invention, the amount of electric charge may be suitably adjusted and also the optical systems and other associated elements are suitably changed over according to the type of the original to be reproduced, so that the image reproduced from an original, whether it is a reflective type or a transmittive type, has always excellent contrast.

While the invention has been described with reference to preferred arrangements and embodiments,

member, means for pro ecting an image of a light reflective original onto the surface of said photoconductive member, and

means for varying the amount of electrostatic charge applied to said photoconductive member in response to the selective activation of said image projecting means.

2. In an electrophotographic apparatus,

a photoconductive member adapted to receive a uniform electrostatic charge,

means for uniformly electrostatically charging said photoconductive member,

means for projecting an image of a light transmitting original to be reproduced onto the surface of said photoconductive member,

means for projecting an image of a light reflective original to be reproduced onto the surface of said photoconductive member,

means selectively rendering one of said image projecting means operable, and

means responsive to said selective rendering means for varying the amount of electrostatic charge applied to said photoconductive member by said means for uniformly electrostatically charging said photoconductive member.

3. Structure according to claim 2 wherein said means for varying the amount of electrostatic charge applied to said photoconductive member comprises means for varying the voltage supplied to said means for uniformly electrostatically charging said photoconduc' tive member. I

4. Structure according to claim 2 wherein said means for varying the amount of electrostatic charge applied to said photoconductive member comprises means positionable intermediate said photoconductive member and said means for uniformly electrostatically charging said photoconductive member.

Claims (4)

1. An electrophotographic apparatus comprising in combination, a photoconductive member adapted to receive a uniform electrostatic charge, means for uniformly electrostatically charging said photoconductive member, means for projecting an image of a light transmitting original onto the surface of sAid photoconductive member, means for projecting an image of a light reflective original onto the surface of said photoconductive member, and means for varying the amount of electrostatic charge applied to said photoconductive member in response to the selective activation of said image projecting means.

2. In an electrophotographic apparatus, a photoconductive member adapted to receive a uniform electrostatic charge, means for uniformly electrostatically charging said photoconductive member, means for projecting an image of a light transmitting original to be reproduced onto the surface of said photoconductive member, means for projecting an image of a light reflective original to be reproduced onto the surface of said photoconductive member, means selectively rendering one of said image projecting means operable, and means responsive to said selective rendering means for varying the amount of electrostatic charge applied to said photoconductive member by said means for uniformly electrostatically charging said photoconductive member.

3. Structure according to claim 2 wherein said means for varying the amount of electrostatic charge applied to said photoconductive member comprises means for varying the voltage supplied to said means for uniformly electrostatically charging said photoconductive member.

4. Structure according to claim 2 wherein said means for varying the amount of electrostatic charge applied to said photoconductive member comprises means positionable intermediate said photoconductive member and said means for uniformly electrostatically charging said photoconductive member.

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