EP0171273B1

EP0171273B1 - Full tone electrophotographic imaging reproduction

Info

Publication number: EP0171273B1
Application number: EP19850305528
Authority: EP
Inventors: Stephen Po-Ming Cheng
Original assignee: Individual
Current assignee: Individual
Priority date: 1984-08-06
Filing date: 1985-08-02
Publication date: 1991-10-30
Also published as: EP0171273A3; JPS61112171A; EP0171273A2; DE3584550D1

Description

This invention relates to photoreproduction using the system known as xerography.
In the xerographic system a latent electrostatic image is created on a photoconductor surface to which charged toner material is subsequently applied, transforming the electrostatic image into a visual image. The toner is then transferred onto a sheet and fused to it. To create the electrostatic image the subject is first projected onto a photoreceptor which receives the latent image as a charge density varying over its surface according to the light intensity projected by the subject, the area receiving less light having a higher charge density. This charge density pattern is developed by applying charged toner material and the toner material is transferred to a charged dielectric sheet.
A problem of this system is the presence of a transition zone at the boundaries between areas of differing charge densities corresponding to abrupt changes between light and dark areas of the visual image, giving an "edge enhanced" or grey area of reproduction at such boundaries.
It is an object of the present invention to provide a method and apparatus for obtaining a photoreproduction of improved clarity by sharpening abrupt boundary lines between light and dark areas of a visual image.
It is a further object of the invention to provide a method and apparatus for obtaining a photoreproduction having gradation of darkness corresponding more correctly with the subject matter being reproduced.
Another object of the invention is to provide a method and apparatus for obtaining photoreproduction having an electrostatic field of increased strength, allowing the use of toner particles of smaller size and therefore a reproduction of finer grain and resolution.
Essentially the invention consists of a method of producing an electrostatic charge image comprising the sequentially performed steps of:
(1) bringing an electrode into intimate contact with a photoreceptor, the photoreceptor having a dielectric substrate and a photoconductive film intimately bonded to the substrate, the electrode being adapted to be brought into intimate contact with the substrate of the photoreceptor along a length of the electrode, and charging the photoreceptor with an electrostatic charge of one polarity and projecting an image of a photograph on the photoreceptor, (2) charging the photoreceptor with an electrostatic charge of opposite polarity, and (3) projecting a photographic image of the subject again on the photoreceptor, whereby an electrostatic charge image is formed on the photoconductor surface, and wherein the electrode comprises:

(i) a belt of flexible material; and either
(ii) an intermediate conductive film intimately bonded to the belt and grounded, and an overlying layer of dielectric material bonded to the conductive film and constructed and arranged to be brought into intimate contact with the substrate of the photoreceptor along a length of the electrode; or
(iii) an overlying layer of dielectric material bonded to the belt being conductive and grounded, and constructed and arranged to be brought into intimate contact with the substrate of the photoreceptor along a length of the electrode.

According to one aspect of the present invention there is provided an electrostatic image system of photoreproduction comprising:

(a) a photoreceptor comprising a dielectric substrate and a photoconductive film intimately bonded to the substrate; and
(b) an electrode comprising:
- (i) a belt of flexible material; and either
- (ii) an intermediate conductive film intimately bonded to the belt and grounded, and an overlying layer of dielectric material bonded to the conductive film and constructed and arranged to be brought into intimate contact with the substrate of the photoreceptor along a length of the electrode; or
- (iii) an overlying layer of dielectric material bonded to the belt being conductive and grounded, and constructed and arranged to be brought into intimate contact with the substrate of the photoreceptor along a length of the electrode.

One preferred embodiment includes the steps of charging the photoreceptor with an electrostatic charge of one polarity and projecting a preselected off-focus image of the subject prior to charging the photoreceptor with an electrostatic charge of opposite polarity and again projecting a preselected off-focus image of the subject on the photoreceptor after the charging step.
The invention is further described with reference to the accompanying drawings in which:

Figure 1 is a cross-sectional view of a photoreceptor and electrode;
Figure 2a is a schematic diagram of the first step in photoreproduction using the photoreceptor and electrode of Figure 1;
Figure 2b is a schematic diagram showing the migration of negative charge to the interface in the photoreceptor;
Figure 2c is a schematic diagram showing the relative distribution of charge density effected by the step of Figure 2a;
Figure 3a is a schematic diagram showing the second step in photoreproduction using the photoreceptor and electrode of Figure 1;
Figure 3b is a schematic diagram showing the relative distribution of charge density effected by the step of Figure 3a;
Figure 4a is a schematic diagram showing the third step in photoreproduction using the photoreceptor and electrode of Figure 1;
Figure 4b is a schematic diagram showing the relative distribution of charge density effected by the step of 4a;
Figure 5 is a schematic diagram showing the fourth step in photoreproduction using the photoreceptor and electrode of Figure 1;
Figure 6 is a schematic view showing the method of projection of an image onto both sides of a photoreceptor;
Figures 7 and 8 are schematic diagrams showing an alternative embodiment of the invention;
Figures 9 to 12 are schematic diagrams showing the relative distribution of charge density in the alternate embodiment of Figures 7 and 8; and
Figures 13 and 14 are schematic diagrams relating to the theoretical basis for the alternate embodiment of Figures 7 and 8.

The illustrative embodiment shown in Figure 1 of the drawings comprises (a) a photoreceptor 10 having a dielectric substrate 12 and a photoconductive film 14 intimately bonded to the substrate with an interface 15, and (b) an electrode 16 having a lower belt 18 of flexible material, an intermediate conductive film 20 intimately bonded to belt 18 and grounded, and an upper layer 22 of dielectric material bonded to film 20. Photoreceptor 10 and electrode 16 are capable of being brought into intimate contact as shown in Figure 1 and the following material and thicknesses are preferred:

Substrate 12 and photoconductive film 14 are preferably of equal capacitance. If belt 18 is made of a conductive metal such as aluminium, intermediate conductive film 20 may be omitted.
An example embodiment of the method of the invention is shown in Figures 2 to 6 of the drawings. In the first step of the example method photoreceptor 10, together with electrode 16, is passed beneath a corona charge station 24 which is connected to a source of negative electrical potential. An image 26 to be photocopied is projected by a light source 28 by means of a lens 29 onto photoconductive film 14 of photoreceptor 10 through an opening 30 in corona charge station 24 as seen in Figure 2a. Image 26 is scanned at the same rate of speed as the movement of photoreceptor 10, as indicated by arrow 31. The result of this projection is the migration of negative ions, in those areas of photoreceptor 10 subjected to light impingement, through film 14 to interface 15 where the negative charge is trapped, as seen in Figure 2b. In Figure 2c, the relative distribution of the charge density is indicated at the surface of film 14 (negative) by numeral 32, at interface 15 (negative) by numeral 34 and in electrode 16 (positive) by numeral 36, the positive charge distribution in electrode 16 being induced by the negative charge at interface 15 and at surface of film 14.
In the next step photoreceptor 10, together with electrode 16, is passed beneath a corona charge station 38 which is connected to a source of positive electrical potential, as seen in Figure 3a, resulting in a relative distribution of charge density as seen in Figure 3b, which shows a positive charge 40 at the surface of film 14, a negative charge 42 at interface 15 and a negative charge 44 at electrode 16.
In the next step photoreceptor 10 and electrode 16 are passed beneath a transparent high voltage booster station 46 connected to a source of positive electrical potential and image 26 is again projected by a light source 48 and a lens 49 onto photoconductive film 14 of photoreceptor 10, as seen in Figure 4a. The result of this projection is seen in the relative distribution of charge density seen in Figure 4b, which shows a positive charge 50 in the dark area of the surface of film 14 and a negative charge 52 in the light area of the surface of the film, no charge at interface 15, and a negative charge 54 at electrode 16.
After photoreceptor 10 is given its second exposure to the image, as described with respect to Figure 4, toner material is applied in known manner as shown in Figure 5. A developer housing 58 encloses a bucket conveyor 60 which delivers developer 62 consisting of positively charged carrier and negatively charged powdered toner material to a plurality of magnetic brushes 64 which sweep over film 14 of photoreceptor 10, while at the same time electrode 16 is peeled away from the back of the photoreceptor. A grounded electrode 66 is positioned adjacent substrate 12 of photoreceptor 10 at an angle to the photoreceptor whereby the distance between the substrate 12 and the electrode 66 increases from the point of separation of electrode 16 from the photoreceptor. The presence of electrode 66 serves to enhance the contrast of the developed image on the photoreceptor.
As electrode 16 is peeled off from the back of the substrate 12 it is replaced by a solid plastic support 68, which carries conductive electrode 66 at its outer surface. Support 68 is slightly conductive, about 10¹⁵ ohm-cm, so that any static charge accumulated by rubbing against substrate 12 is discharged. As photoreceptor 10 moves down, the charge latent image surface moves further and further away from electrode 66. This tends to increase the electric field intensity inside the development system. However, on the other hand, the deposition of toner particles on the image surface tends to decrease the electric field intensity. By suitably designing the angle of the edge of support 68 it is possible to achieve a condition that the increase in field intensity is exactly balanced by the decrease caused by the deposition of toner particles. As a result the electric field intensity is kept constant inside the development system. This prevents an excessive strong electric field buildup inside the development system which would cause "arcing" between the image charge and brushes 64. At the end of the development procedure the latent image charge is completely neutralized by the deposited toner particles. The developed image can then be transferred and fixed. If the photoreceptor itself is used as a permanent image recipient, such as zinc oxide coated paper, the transfer process can be omitted.
Some photoreceptive materials, for example selenium, conduct positive charges when light activated. Figure 6 shows the arrangement required for light impingement on electrode 16 as well as on photoreceptor 10 to achieve the same result as in the previous embodiment. In this case image 26 is projected by a light source 70 and a lens 72 onto a mirror 74, splitting it into two images which are projected by a mirror 76 and a mirror 78 onto the upper and lower surfaces, respectively, of photoreceptor 10, thus causing the positive ions to migrate to the upper surface of film 14, leaving behind a negative charge density as seen in Figure 2b. This split image procedure is only necessary in the first step shown in Figure 2a. In this case both electrode 16 and substrate 10 are made of transparent material.

PHOTOCONDUCTORS

Bipolar photoconductors 14 are most suitable for this invention. The common bipolar photoconductors are amorphous silicon (a - Si:H), ZnO treated with urazole or H₂S, or its resin containing Mn or other additives, various organic photoconductors containing certain substituted cycloheptenyl compounds and organic photoconductors comprising a halogen - ketone - formaldehyde resin. Single-polar photoconductors such as amorphous selenium (as menioned above) and most organic photoconductors can also be used in this invention. Two techniques can be used to solve the single-polar conducting problem. One is a transparent base electrode 16 which permits rear exposure. The second technique is adding a layer of lower-energy-gap material at interface 15. The lower-energy-gap material can be crystal selenium or the like in the form of small insulated dots of 10-20 um in size and spaced 5 um apart. Then use red or other low energy light in the on-focus and off-focus negative charge injection process. The red light or other low energy light can penetrate the photoconductor layer and reaches the lower-energy-gap layer. Carriers will be produced on absorption of red light photons by the lower-energy-gap layer. Carriers produced at the interface region migrate back through the photoconductor layer to the surface.
It will be appreciated that the latent image formed by the method of this invention will have a varying degree of charge density in exact proportion to the opacity pattern of the actual image. Thus either line images of only black and white or images being varying degree of greyness between these two extremes may be reproduced faithfully. Also because of the strong electric field inside the development system extremely high resolution can be achieved.
Of course the method of the invention may be carried out using a positive charge in the step of Figure 2a followed by a negative charge in the steps of Figures 3a and 4a.
In the charge process because light area has a negative charge trapped at interface 15 the charge density on the surface of film 14 will be higher in the light area than in the dark area (see Figure 3b). At the boundary between light and dark areas there is a transition zone about 1.5 mm (1/16 of an inch) in which the charge density changes gradually. There is a higher charge density at the image edge and consequently this causes an "edge enhanced" copy (see Figure 4b). This is not desirable in many imaging applications where solid area development is desired, such as a picture. The use of an off-focus lens minimizes this undesirable "edge enhanced" effect.
To explain the off-focus process reference is made to Figures 13 and 14. In Figure 13 two electrodes A and C are separated by two dielectrics D1 and D2. B is the interface between the two dielectrics. For the sake of simplicity let the electrical capacitance between AB and the capacitance between BC have the same value and let them be named C1 and C2 respectively. A D.C. voltage source is connected to electrode A while electrode C is grounded. A uniform, positive charge e_c of charge density density Rc appears on electrode A and a uniform negative charge e_c' appears on electrode C. Now place a small point charge p at interface B which is negative and whose charge density Rp is equal to Rc. Thus Rp and Rc are equal but opposite in polarity. Because of the introduction of negative charge p, induced positive charges e_p and e_p' will appear in electrodes A and C respectively. These induced charges e_p and e_p' tend to distribute in such a way that there is more concentration at a location close to p than further away from p. A mathematical formula can be produced which can calculate the exact charge distribution. Because the capacitance C1 and C2 are equal, then the relationship e_p = e_p' = 1/2 p exists. Now place another charge q₁ at the interface B directly underneath e_p. Charge q₁ is equal and opposite in polarity to e_p. The charge distribution of q₁ is exactly the same as e_p. Again there will be induced positive charges

and

on A and C respectively. The same mathematical formula can calculate the exact charge distribution of

We also have the relationship

Here again we can place a negative charge q₂ at interface B underneath

which is equal and opposite to

and has exactly the same charge distribution. The process can be repeated many many times until the induced charge

is so small that it can be negligible. Let

Figure 14 shows the curves of q, e_q and e_p with the Z axis equal to zero. We have the relationship

at any point x₀,z₀ on the plane X,Z. Mathematically we have the relationship:

${q=p(1/x +1/x² +1/x³ + - - - +1/x}^{n}) for X>1$

In this example since C1 equals C2, x equals 2.
Since

We have
In the case of the present invention D1 is photoconductive layer 14. D2 is a dielectric substrate 12. C1 is the capacitance of photoconductive layer 14. C2 is the capacitance of dielectric substrate 12. A is the surface of photoconductor 14. B is interface 15 between the photoconductor and the substrate. C is intermediate conductive film 20. Charge p is the injected negative charge at interface 15. Charge q is the off-focus injected negative charge at interface 15. Charges e_c, e_p and e_q are placed on the surface of photoconductor 14 by positive charging station 38. e_c is caused by the potential applied to the charging station 38. Charges e_p and e_q are caused by the grounding effect of charging station 38. In the off-focus exposure process e_p and e_q will move down to cancel q. In the subsequent exposure process that part of the e_c charge above p will move down to cancel p. Thus a point is discharged on the surface of the photoconductor 14. A latent image is formed by summing up all the points.
An off-focus lens can be defined as a lens which has a special light diffusion such that when it is applied to this electrophotographic imaging system, the light from any one point of the original image can be diffused to the photoconductor surface in such a way that the light intensity distribution on the photoconductor is in the same shape as the charge distribution of the function F(q_(x,z)) calculated above. As a result we can achieve the desired condition that

at any point x₀,z₀ of the photoconductor surface. The preselected off-focus image is formed by projecting an image through this off-focus lens.
The on-off focus ratio is a measure of ratio of the amount of light photons directed to the photoconductor surface during the two processes (on focus and off focus). For complete elimination of the "edge enhanced" effect the ratio is equal to p/q, which in turn equals C2/C1, as proved above. In some copying requirements a certain amount of "edge enhanced" effect is desirable such as in art work. In this case the on/off focus ratio can be adjusted to be greater than p/q to achieve the desired amount of "edge enhanced" effect.
Referring now to Figures 7 and 8 of the drawings, an off-focus lens 29a is added to the apparatus of Figure 2a as seen in Figure 7, and image 26 is projected onto photoconductor 10 as an added step between the step of Figure 2a and the step of Figure 3a. Subsequently, an off-focus lens 49a is added to the apparatus of Figure 4a, as seen in Figure 8, and image 26 is projected onto photoconductor 10 as an added step between the step of Figure 3a and the step of Figure 4a. The relative densities resulting from each of the sequential steps of Figures 2a, 7, 3a, 8 and 4a are shown in Figures 9a, 9b, 10, 11 and 12 respectively.

Claims

A method of producing an electrostatic charge image comprising the sequentially performed steps of:
(1) bringing an electrode into intimate contact with a photoreceptor, the photoreceptor having a dielectric substrate and a photoconductive film intimately bonded to the substrate, the electrode being adapted to be brought into intimate contact with the substrate of the photoreceptor along a length of the electrode, and charging the photoreceptor with an electrostatic charge of one polarity and projecting a photographic image of a subject on the photoreceptor, (2) charging the photoreceptor with an electrostatic charge of opposite polarity, and (3) projecting a photographic image of the subject again on the photoreceptor, whereby an electrostatic charge image is formed on the photoconductor surface, and wherein the electrode comprises:
(i) a belt of flexible material; and either

(ii) an intermediate conductive film intimately bonded to the belt and grounded, and an overlying layer of dielectric material bonded to the conductive film and constructed and arranged to be brought into intimate contact with the substrate of the photoreceptor along a length of the electrode; or

(iii) an overlying layer of dielectric material bonded to the belt being conductive and grounded, and constructed and arranged to be brought into intimate contact with the substrate of the photoreceptor along a length of the electrode.
A method according to claim 1, characterised by including the step of:
(4) moving the electrode away from the photoreceptor, whereby to increase the electric field above the surface of the photoconductive film.
A method according to claim 1, characterised by including the steps of moving the electrode away from the photoreceptor and applying particulate toner material carrying a charge of said one polarity to the photoconductive film of the photoreceptor, (5) charging a sheet of material with an electrostatic charge of said opposite polarity and applying the sheet to the photoconductive film of the photoreceptor, (6) removing the sheet from the photoreceptor, and (7) fusing the toner material on the sheet whereby the reproduction of the photographic image is fixed thereon.
A method according to Claim 1, 2 or 3 including the steps of charging the photoreceptor with an electrostatic charge of one polarity and projecting a preselected off-focus image of the subject prior to charging the photoreceptor with an electrostatic charge of opposite polarity and again projecting a preselected off-focus image of the subject on the photoreceptor after the charging step.
A method according to Claim 1, 2, 3 or 4, characterised in that said one polarity is negative and said opposite polarity is positive.
A method according to Claim 1, 2, 3 or 4, characterised in that said one polarity is positive and said opposite polarity is negative.
A method according to Claim 1, characterised in that both the substrate of the photoreceptor and the electrode are transparent and including the step of projecting the image both on the photoreceptor and on the electrode.
An electrostatic image system of photoreproduction comprising:
(a) a photoreceptor (10) comprising a dielectric substrate (12) and a photoconductive film (14) intimately bonded to the substrate; and

(b) an electrode (16) comprising:
(i) a belt of flexible material; and either

(ii) an intermediate conductive film (20) intimately bonded to the belt and grounded, and an overlying layer (22) of dielectric material bonded to the conductive film and constructed and arranged to be brought into intimate contact with the substrate (12) of the photoreceptor along a length of the electrode; or

(iii) an overlying layer of dielectric material bonded to the belt being conductive and grounded, and constructed and arranged to be brought into intimate contact with the substrate (12) of the photoreceptor along a length of the electrode.
A system according to Claim 8, characterised in that the photoconductive film (14) is amorphous silicon and the substrate (12) is a polyester resin.
A system according to Claim 9, characterised in that the belt is a polyester resin, the conductive film (20) is copper iodide and the overlying layer (22) silicon nitride.
A system according to Claim 10, characterised in that the thickness of the photoconductive film (14) of the photoreceptor is 25 - 150 µm, the thickness of the conductive film (20) of the electrode is 10 - 50 nm, and the thickness of the overlying layer of the electrode is 100 nm - 5 µm.
A system according to Claim 8, characterised in that the electrode is transparent and the substrate of the photoreceptor is transparent.
A system according to any of Claims 8 to 12, characterised by means sequentially (1) to bring the electrode into intimate contact with the photoreceptor and to charge the photoreceptor with an electrostatic charge of one polarity and to project a photographic image of a subject on the photoreceptor whereby a charge is injected on the interface between the photoconductive film and the substrate, (2) to charge the photoreceptor with an electrostatic charge of opposite polarity, (3) to project a photographic image of the subject again on the photoreceptor, whereby an electrostatic charge image is formed on the surface of the photoconductive film.
A system according to Claim 13, characterised by means to charge the photoreceptor with an electrostatic charge of one polarity and to project a preselected off-focus image of the subject prior to charging the photoreceptor with an electrostatic charge of opposite polarity and again projecting a preselected off-focus image of the subject on the photoreceptor after the charging step.
A system according to Claim 13, characterised in that said one polarity is negative and said opposite polarity is positive.
A system according to Claim 13, characterised in that said one polarity is positive and said opposite polarity is negative.
A system according to Claim 13, or Claim 14, characterised in that the substrate of the photoreceptor and the electrode are transparent, and including means to project the image both on the photoreceptor and on the electrode.
A system according to Claim 14, characterised by means to charge the photoreceptor with an electrostatic charge of one polarity and to project a preselected off-focus image of the subject on the receptor before the first projection of the image and again subsequent to the second projection of the image.
A system according to Claim 14, characterised by means to charge the photoreceptor with an electrostatic charge of one polarity and to project a preselected off-focus image of the subject on the receptor during, before or after the first projection of the image and again during, before or after the second projection of the image.
A system according to Claim 13, characterised by means for moving the electrode away from the photoreceptor whereby to increase the electric field above the photoconductor.
A system according to Claim 20, characterised by means for moving the electrode away from the photoreceptor and applying particulate toner material carrying a charge of said one polarity to the photoconductive film of the photoreceptor whereby the image on the photoconductive film becomes visible.