US3850142A

US3850142A - Image development amplification

Info

Publication number: US3850142A
Application number: US00161228A
Authority: US
Inventors: A Jvirblis; W Roth
Original assignee: Diagnostic Instruments Inc
Current assignee: Diagnostic Instruments Inc
Priority date: 1971-07-09
Filing date: 1971-07-09
Publication date: 1974-11-26
Anticipated expiration: 1991-11-26
Also published as: DE2233538A1; DE2233538C3; BE785976A; NL7209376A; GB1396553A; FR2145967A5; IT958478B; CA967064A; DE2233538B2

Abstract

In the development of latent electrostatic images formed on one side of an electrically insulative substrate mounted on a conductive surface, a capacitance is interconnected between the conductive surface and a potential of a level different from that of the conductive surface. The capacitance accepts charges from the other side of the substrate thereby enhancing the external electric field of the latent image.

Description

United States Patent Jvirblis et al.

[ IMAGE DEVELOPMENT AMPLIFICATION [75] Inventors: Alex E. ,Jvirblis; Walter Roth, both of La Jolla, Calif.

[73] Assignee: Diagnostic Instruments Inc., St.

Louis, Mo.

[22] Filed: July 9, 1971 [21] Appl. No.: 161,228

[52] US. Cl 118/637, 117/934, 118/629 [51] Int. Cl G03g 13/00 [58] Field of Search 118/637, 621, 629, 49.1, 118/49.5, 638; 117/931, 93.3, 93.4; 250/495, 65; 96/1.4

[56] References Cited UNITED STATES PATENTS 2,692,948 10/1954 Lion 1. 250/65 2,802,948 8/1957 Vyverberg 250/65 2,817,598 12/1957 Hayford 117/17.5

2,817,765 12/1957 I-layford et al 250/65 2,824,813 2/1958 Fauser et al. 117/l7.5

2,825,814 3/1958 Walkup I. 250/49.5 2,914,221 11/1959 Rosenthal 118/637 X 2,952,241 9/1960 Clark et al. 118/637 51 Nov. 26, 1974 3,185,051 5/1965 Goffe 95/1.7 3,256,197 6/1966 Fauser et a1. 117/17.5 X 3,441,437 4/1969 Epstein et a1 118/637 X 3,599,605 8/1971 Ralston et al 118/637 OTHER PUBLICATIONS K. S. Lion, A Method of Increasing Photographic Sensitivity by Electrical Discharges," Vol. 24, No. 3, 367-368, Mar. 1953, Journal of Applied Physics.

Primary ExaminerW. C. Reynolds Assistant Examiner-Leo Millstein Attorney, Agent, or FirmKoenig, Senniger, Powers and Leavitt [5 7 ABSTRACT 16 Claims, 2 Drawing Figures PATENTEL, i-LUV 26 I974 5am? I INVENTORS ALEX JVIRBLIS WALTER ROTH SDKOLSKI 8 WOHLGEMUTH ATTORNEYS I IMAGE DEVELOPMENT AMPLIFICATION BACKGROUND OF THE INVENTION The general process known as ionography involves making X-ray images without the utilization of silver halide film. The basic process was disclosed by E. L. Criscuolo, in NAVORD Report 4033 of July 6, 1955, U.S. Pat. No. 2,900,515, in an article by R. A. Youshaw and J. A. Holloway in Non-Destructive Testing, September October 1959, and by K. H. Reiss in Z. Angew. Physik, Vol. 19, p. l (1965). This process utilizes two parallel plate electrodes, across which or between which a dc. voltage is applied whereby one electrode is a positive electrode and the other is a negative one. When the positive electrode is nearest the X-ray source it must not absorb much of the X-ray beam. It has affixed to it an image receiving sheet or receptor which may be transparent or opaque but must be an electrical insulator such as a thin sheet of a plastic film or the like. The negative electrode has a thin film or layer of a material which is an efficient absorber of X-rays. In the aforementioned Reiss reference a. heavy metal, such as lead, tungsten or molybdenum was utilized as an absorber of the X-rays and was, in effect, a photoemitter. The image receiving insulator on the anode and the photoemitter layer face, each other across the gap between the electrodes with the object being examined disposed on either the outer side of the anode or cathode; preferably on the outer side of the anode. A quenching gas is flowed, or in cases may be stationary, inthe gap between the electrodes.

When an object disposed adjacent the anode isirradiatedby X-rays or gamma rays, this-electromagnetic radiation is differentially absorbed by the object and passesthrough the transmissive anode and insulator layeraffixed thereto and across the gap and strikes the photoemitter where it is strongly absorbed by the photoemitter which, as a consequence, ejects electrons having energies up to many kilo-electron volts. The number of electronsemitted from any element or portion of the photoemitter is dependenton the number of X-ray photons absorbed in thatportiomthe depth of the absorption and'the photon energy. On leaving the photoemitter surface, the electrons find themselves in a.d.c. field and travel toward the positive electrode. The quenching gas serves to slow down the electrons so that they will not scatter when reaching the insulator, and to increase their number by secondary ionization. Upon arrivingat the insulator surface, the electrons and any negative ions which may have been formed by attachment to components of the quenching gas are collected in an image configuration forming. a'latent electrostatic image consisting of negative charges corresponding to elements or portions of the object which are relatively transparent to X-rays and no charges or fewer charges corresponding to portions or elements of the object which are opaque or relatively opaque to X- rays. This latent image is then made visible by development or by cathode .tube display ray techniques.

For every charge formed on or produced on the latent image side of the insulative receptive layer or receptor, there is induced a mirror image charge of opposite polarity on the opposite side of the receptor. Thus, if the latent image is comprised of a plurality of negative charges on the surface of the insulative receptor, like positive charges are induced on the opposite side of the receptor adjacent the conductive plate backing the receptor. In the development of the latent electrostatic image the image is exposed, for example, to a gas eous cloud of a development powder having a charge of opposite polarity to the latent image charge. For example, where the latent image comprises negative charges, positively charged particles of powder are supplied and are attracted to the negative charges constituting the latent images. In order for the developer powder to be attracted, the latent image must provide or form an external or protruding electrical field. It has been previously found that the external field of each latent image charge is substantially affected and reduced by the induced charges of opposite polarity formed on the other side of the image receptor. In other words, the lines of force or the field lines are directed inwardly through the receptor itself toward the induced charges of opposite polarity and are sufficiently not directed outwardly from the latent image so as to properly attract the developer powder.

This problem of induced charges on a receptor diminishing the lines of force of the fields protruding from the latent image charges was previously recognized and discussed in, for example, Walkup US. Pat. No. 2,784,109 relative to the field of xerography. In xerography a latent electrostatic image is produced on a photoconductor element and since the photoconductor is an insulator when not illuminated, charges of induced opposite polarity are produced on the opposite side of the photoconductor in xerography in the same manner as described herein. In order to draw the fields of force outwardly from the receptor and away from the induced charges of opposite polarity formed on the opposite side of the receptor from the latent image, the Walkup patent, as well as several other subsequent patents in the field of xerography, disclose what is known as a developing or toning electrode spatially disposed near the surface bearing the latent image. The developing electrode has an induced charge of opposite polarity from that of the image. By controlling the potential of the developing electrode relative to a backing plate for the receptor, the field lines emanating from the latent image charge areaffected. For example, where the latent image is comprised of positive charges, as shown in the Walkup patent, the developing electrode can have a dc. potential applied thereto which is negative with respect to the conductive backing plate of the receptor. This will tend to reduce the effect of the induced negative charges and draw field lines emanating from the positive charges outwardly into the powder cloud to produce an overall heavier powder deposit on the receptor. The dc. voltage or potential applied to the developing electrode can be further varied relative to the backing plate of the receptor which'is at a ground potential to make it positive with respect thereto. By controlling-the potential from positive to negative with respect to the potentialof the backingplate, the fields emanating from latent images can be controlled to affect the amount of powder attracted "to and deposited on the latent image; Such prior art toning electrode, which is suitable for xerographic applications, where the latent image is formed or produced in one location and development takes place in a subse quent location, is not always feasible in an ionographic.

process particularly where it is desired to develop'the latent image in place in the same location in whichit is formed. Further, the utilization of the developing,- electrode may require an additional power source -to' control the potential thereof. Additionally, its placement relative to the latent image surface as well as the amount of potential on the electrode are relatively critical. Also, the use of the developing electrode introduces a costly element into the overall system, and increases the physical and mechanical complexity of the developing system.

SUMMARY OF THE INVENTION Among the several objects of this invention may be noted the provision of an image development system in which the image density is desirably enhanced or amplified and the developed image has excellent contrast and resolution; and the provision of such a system which avoids the disadvantages of systems utilizing toning or developing electrodes and is relatively simple and economical in construction. Other objects and features will be in part apparent and in part pointed out hereinafter.

In accordance with the present invention, it has been found that by electrically interconnecting a capacitance between the electrically conductive backing plate, on which is mounted an insulative layer having a latent image thereon, and a potential of a level different from that of the backing plate, the capacitance accepts charges from the other side of the insulative layer during the development of the image. Thus the capacitance is charged by the induced charges on the insulative layer thereby reducing or diminishing the induced charges so that the fields of force from the latent image charges are directed outwardly or protrude into the development area and have an increased attraction for the toner particles.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic representation of an ionographic apparatus for forming latent images; and

FIG. 2 is a schematic view of an insulative layer having a latent image thereon and an electrically conductive backing plate mounted on a powder cloud developing apparatus and illustrating this invention.

Corresponding reference characters indicate corresponding parts throughout the drawings.

DESCRIPTION OF PREFERRED EMBODIMENTS FIG. 1 schematically depicts apparatus for performing an ionographic process. An object 111 is shown as a step wedge which simulates the effect of differential thicknesses of a given body upon the produced image. The ionographic device is comprised of a positive flat electrode plate 13 which may be formed, for example, of aluminum or beryllium or films of these electrically conductive surfaces disposed on nonmetallie supports. Plate 13 should be transmissive to X-rays from an X-ray source 15. Thus it is preferred that the meta] be in as thin a layer as possible, and may take the form of, a thin film coating on a resinous or other suitable flat substrate. In this embodiment, the object is shown disposed adjacent the transmissive electrode 13 between that electrode and the X-ray source 15.

Disposed on and affixed to positive electrode 13 is an image receiving sheet or receptor 17 which can be transparent or opaque but must be an electrical insulator. It, thus, may be any insulative substrate, e.g., paper or thin films or sheets of resinous materials, such as a film of polyester material, such as that sold under the trade designation Mylar, or like material. A negative plate electrode 19 is spatially disposed from the positive electrode 13. Negative electrode 19 must be extremely flat. An economical way of providing this at low cost is to utilize a glass plate with a film coating of conductive metal, such as aluminum or the like. Applied to negative plate 19 is a thin film 21 of a material which is an efficient absorber of X-rays or gamma rays. This film may, for example, be a film of lead, etc., as disclosed in the above'cited Reiss publication. The image receiving insulative substrate or layer 17 and the X-ray absorbing film or photoemitter 21 face each other across a gap 23. A suitable power supply source 25 maintains the potential between the electrodes and across the gap 23 during formation of electrostatic latent images on the surface of receptor 17.

A gas is maintained in the gap. For example, an inert gas or a halogenated hydrocarbon, such as sold under one of the Freon trade designations mixed with a hydrocarbon, may be used. When the foregoing apparatus is irradiated by X-rays from source 15, the X-rays are first differentially absorbed by the step wedge object 11 and then pass relatively unattenuated through the posi tive electrode 13, the insulative substrate 17 and the gas. The X-rays are then efficiently absorbed by the photoemitter layer 21 which, as a consequence thereof, ejects electrons having may kilo-electron volts of energy. The number of electrons emitted from any element of the photoemitter 21 is dependent upon the number of X-ray photons absorbed in that element, the depth of absorption and the X-ray photon energy. On leaving the photoemitter, the electrons find themselves in a dc. field within gap 23 and are attracted to and travel toward the positive electrode 113. One of the main functions of the gas is to slow down the electrons so that they will not scatter when reaching the insulative layer 17. The electrons lose their energy by exciting, dissociating and, in some cases, ionizing or attaching to the molecules of the particular gas used. Upon arriving at the insulating layer surface 18, the electrons or negative ions are deposited and collected in an image configuration forming a latent electrostatic image 20 constituted by areas of negative charges of magnitude corresponding to the relative X-ray transparency of the respective portions of object 11. That is, under thicker elements or portions of the object which are relatively opaque to X-rays there is much less charge deposited or collected than under object portions which are less opaque to X-rays. A corresponding image of induced positive charges 22 is formed on the opposite side 24 (FIG. 2) of the insulating receptor layer 17.

The latent electrostatic image 20 formed on the insulating layer or substrate surface 18 may then be scanned with an electron beam and, by known video techniques, displayed on a cathode ray tube. Altematively, the latent image may be made visible and fixed permanently to the insulator by any one of a number of other techniques. One such technique is exposure of the electrostatic image-bearing insulative layer to a cloud of powdered toner particles charged either positively or negatively, which adhere in image configuration on surface 18 and are then fixed by heating to the melting point of the toner particles.

The above-described ionographic system thus produces latent electrostatic images which may be either negatively charged as shown in the drawings, or positively charged, depending upon the polarity of connection of the electrodes to power supply 25. That is, it will be understood that the polarity of the above process and the resulting image may be reversed by disposing the image receiving sheet on the negative electrode and the photoemitter layer on the positive electrode. In this latter instance the positive ions formed in the gap produce a positive latent image on the surface 18 of the insulative substrate.

Referring more particularly to FIG. 2, the backing plate 13 together with the insulating receptor layer 17 is shown mounted on a powder development chamber or enclosure 27 which is schematically shown and which has an electrically conductive interior surface which is spaced away from and opposed to said insulative substrate. While the margins of surface 18 rest on the edge of the development chamber opening, insulative substrate 17 is effectively spaced from the conductive interior surface thereof and more particularly is the interface 24, on which are present the induced charges, spaced from this electrically conductive surface.

Within chamber 27 a cloud of small particles 28 of developer such as graphite, charcoal, or colored or pigmented resin particles are utilized in a form of a fine cloud or aerosol dispersion. The charged particles may either have the same charge as the latent image and thus would be negatively charged in which case they would be directed to the noncharged areas on the receptor 17 forming a negative image, or they may be of opposite charge to the latent image thus being directly pulled to the latent image forming a positive image or reproduction. Prior to developing the latent image in accordance with this invention, a capacitance 29 is interconnected between electrode or plate 13 and the conductive interior surface of chamber 27.

Assume, for example, that negatively charged images are to be developed with positively charged toner. FIG.

2 represents the initial condition of the development process. The dielectric surface is given a charge density of (0 00 The field inside the dielectric film 17 is very strong. The field protruding from the surface over the charged area is very weak and contributes to a small induced charge 0);, on the inside surface of chamber 27. This weak protruding field along with the stronger fringing fields which occur near the boundary of the charged areas are important to the development process since they exert attractive forces on the toner particles.

Before or as a cloud of toner particles is introduced in the chamber, a capacitor discharging or shunting switch 31 is opened to allow the capacitor 29 to accept charges from the reverse surface of receptor 17. The external fields of the latent images are enhanced by the presence of positive toner particles 28, and the dielectric-substrate interface charges 22 are liberated and allowed to charge the capacitance or capacitor 29 which was previously discharged by using switch 31. In other words, when charged toner particles are near the insuber walls by deposition of powder from the cloud compete with like charges from the substrate and hence limit the development density.

Capacitor 29 should be large enough to be capable of accepting and storing all the charges released by the electrostatic image. Its maximum effective size depends on the quantity of surface charge available on the latent image. The particular capacitance value will depend upon the dimensions of the developing chamber and the image receptor. It is known, for example, that a charge density of approximately 2X10 coul./micron results in good images. The total charge on a 200 cm image would then be:

To store this charge at a potential of 100 v. the capacitance must be:

In this example, the capacitor should be at least 0.04uF. Larger capacitors would perform equally well but will not add to the effectiveness of the process. In

the practiceof this invention, capacitor 29 may be connected to plate electrode 13 merely by providing a clip or providing a plug-in receptacle in plate 13. Various other means can be thus provided for mechanically and electrically connecting the capacitor to the plate 13 prior to the development of the latent image. For example, capacitor 29 may be affixed to and part of the development chamber 27 so that plate 13 can be plugged into the development chamber, automatically interconnecting the capacitor between it and the developwere developed with toner (such as available under the lativesubstrate 17 they serve to weaken the field of trade designation Xerox type 22) in a powder cloud development chamber of 8 Xl2 6 inches in dimension, and incorporating corona charging of the toner to a charge opposite from that of the latent image produced.

First it was demonstrated, by making several images, that the densities of images were repeatable when made successively on the same day and under the same conditions. Thereafter, a set of images were made and developed with all parameters kept constant, except that the aluminized bakelite substrate constituting plate electrode 13 was interconnected to the development chamber by means of various individual respective capacitors, each being first discharged.

The table below lists the experimental conditions and the transmittance of the second most transmissive step of an aluminum stepwedge image obtained which was measured by comparison of the image with a series of neutral density filters on a light box. The corresponding densities were calculated by using D (density) =logT (transmittance). The normalized values of D with respect to a floating plate condition where the receptor plate is not electrically connected into the system are also listed.

The density of the images was improved from both the floating and grounded or shorted configurations by the connection of a capacitor between the aluminized bakelite backing plate and the electrically conductive interior surface of the development chamber. Increasing the capacitance resulted in an increased density.

Normalized Density In the particular embodiment shown in FIG. 2, the capacitance 29 was connected to the conductive interior surface of chamber 27. It should be understood that the capacitance can be connected between the electrode or plate 13 and any suitable potential of a level different from that of the plate 13. For example, the capacitance can readily be connected to a source of zero potential, i.e., ground.

Though the above invention has been described relative to use of a powder cloud of toner particles for development of the latent image, any other technique using charged developer particles would be applicable to the process of this invention. Thus, for example, liquid developers could be used where the charged toner particles are dispersed in a liquid medium.

Thus, it can be seen that the utilization of a capacitor during the development of a latent electrostatic image accomplishes the desired end results of this invention in a simple and effective manner.

In view of the above, it will be seen that the several objects of the invention are achieved and other advantageous results attained.

As various changes could be made in the above apparatus and methods without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. Apparatus for developing a latent electrostatic image comprising:

an electrically conductive plate;

an electrically insulative substrate mounted on said plate, said insulative substrate adapted to have a latent electrostatic image formed on an exposed surface thereof and an opposite polarity mirror image charge induced on the other surface thereof which mirror image charge tends to degrade and interfere with development of the latent image;

a capacitance; and

means for connecting said capacitance between said conductive plate and a potential different from that of said conductive plate during development of said image thereby to accept and dissipate the mirror image charge from said other surface of said substrate and reduce said induced charge whereby the development of the latent image is enhanced.

2. The apparatus of claim 1 wherein said capacitance is connected between said conductive plate and ground.

3. The apparatus of claim I wherein said capacitance is connected to an electrically conductive surface spaced from said conductive plate.

4-. The apparatus of claim 3 which further includes means for selectively discharging said capacitance.

5. The apparatus of claim 4 which further comprises means for distributing a cloud of electrostatically charged powder particles between the insulative substrate and said electrically conductive surface.

6. In an apparatus for the development of latent electrostatic images on a prescribed imaging surface of an electrically insulative substrate and including a prescribed development chamber adapted to contain a prescribed array of charged developer particles, this chamber including at least one electrically conductive developer surface disposed operatively adjacent said array of developer particles, said image comprising a pattern of prescribed maximum total image charge and said developer particles being charged to a polarity opposite that of this image, the improvement therein comprising, in combination:

substrate conductor means disposed adjacent the insulative substrate, in electrical ohmic contact with the side thereof opposite said imaging surface so as to accommodate the transfer thereto of a mirror charge pattern induced by the original image charge pattern;

capacitance means electrically coupled between said substrate conductor means and at least one of said conductive developer surfaces and adapted to re ceive and store a total charge at least equal to said maximum total image charge contemplated at the operating potential; and

capacitance discharged means adapted to selectively shunt said capacitance means thereby to remove the charge stored therein.

7. Apparatus for developing a latent electrostatic image comprising:

an electrically insulative substrate mounted on a conductive plate, said insulative substrate having a latent electrostatic image of a predetermined total charge formed on an exposed surface thereof and an opposite polarity induced charge on the other surface thereof;

a development chamber having an interior conductive portion and adapted to have said substrate positioned thereon with the image bearing surface opposed to and spaced from said conductive portion;

means for distributing a dispersion of electrostatically charged powder particles within said chamber between the insulative substrate and the conductive portion for development of said image; and means for enhancing the external field of said latent image, said means including a capacitance adapted to be interconnected between said conductive plate and said conductive portion of said chamber during development of said image thereby to accept charges from said other surface of said substrate and reduce said induced charge, whereby the density of the developed image is increased. 8. Apparatus as set forth in claim 7 which further includes means for selectively discharging said capacitance.

9. Apparatus as set'forth in claim 8 in which said casaid capacitance is connected between said conducpacitance is adapted to receive and store charges of a tive plate and ground. magnitude at least equal to the predetermined total 12. The apparatus of claim 10 wherein: image g said capacitance is connected to an electrically con- In an apparatus for developing a latent electfo- 5 ductive surface spaced from said conductive plate.

static image on an electrically insulative substrate mounted on an electrically conductive plate wherein development tends to be degraded and interfered with by a mirror image charge induced on the back surface of said substrate adjacent said plate, this substrate being adapted to have a latent electrostatic image formed on an exposed front surface thereof and an opposite polarity induced charge on the opposite back surface thereof, the improvement comprising, in combination:

dissipation means adapted to dissipate the mirror 13. The apparatus of claim 10 which further includes means for selectively discharging said capacitance.

14. The apparatus of claim 13 which further com- 10 prises means for distributing a cloud of electrostatically strate and said electrically conductive surface.

15. The apparatus of claim 10 as arranged to form part of a powder cloud development chamber;

this chamber including electrically conductive surimage charges so induced on Said back Surface of faces spaced from the conductive plate, and means the substrate to thereby enhance any electrostatic for dlsmblmng a cloud of el ectros taucauy Charged development field projected to the Side of toner particles between the insulative substrate and said substrate adapted to receive the said image for at least a P of the conductwe Surfaces of the development; this dissipation means including cadevelopment f i pacitive storage means and means for coupling said and Whereln 531d CaRaCltWe means Coupled P iti storage means between i plate and a tween said conductive plate and said conductive potential different from that of said plate, at least Chamber during development of aid i a to th b 16. The combination as recited in claim 10 wherein cept the charges from said back su f f said bis also included switch means for selectively coupling strate and reduce the induced charge there, thus said capacitive means to said potential source when enhancing said development. said mirror image charge is to be removed therefrom. 11. The apparatus of claim 10 wherein:

charged powder particles between the insulative sub-

Claims

1. Apparatus for developing a latent electrostatic image comprising: an electrically conductive plate; an electrically insulative substrate mounted on said plate, said insulative substrate adapted to have a latent electrostatic image formed on an exposed surface thereof and an opposite polarity mirror image charge induced on the other surface thereof which mirror image charge tends to degrade and interfere with development of the latent image; a capacitance; and means for connecting said capacitance between said conductive plate and a potential different from that of said conductive plate during development of said image thereby to accept and dissipate the mirror image charge from said other surface of said substrate and reduce said induced charge whereby the development of the latent image is enhanced.

3. The apparatus of claim 1 wherein said capacitance is connected to an electrically conductive surface spaced from said conductive plate.

4. The apparatus of claim 3 which further includes means for selectively discharging said capacitance.

6. In an apparatus for the development of latent electrostatic images on a prescribed imaging surface of an electrically insulative substrate and including a prescribed development chamber adapted to contain a prescribed array of charged developer particles, this chamber including at least one electrically conductive developer surface disposed operatively adjacent said array of developer particles, said image comprising a pattern of prescribed maximum total image charge and said developer particles being charged to a polarity opposite that of this image, the improvement therein comprising, in combination: substrate conductor means disposed adjacent the insulative substrate, in electrical ohmic contact with the side thereof opposite said imaging surface so as to accommodate the transfer thereto of a ''''mirror charge pattern'''' induced by the original image charge pattern; capacitance means electrically coupled between said substrate conductor means and at least one of said conductive developer surfaces and adapted to receive and store a total charge at least equal to said maximum total image charge contemplated at the operating potential; and capacitance discharged means adapted to selectively shunt said capacitance means thereby to remove the charge stored therein.

7. Apparatus for developing a latent electrostatic image comprising: an electrically insulative substrate mounted on a conductive plate, said insulative substrate having a latent electrostatic image of a predetermined total charge formed on an exposed surface thereof and an opposite polarity induced charge on the other surface thereof; a development chamber having an interior conductive portion and adapted to have said substrate positioned thereon with the image bearing surface opposed to and spaced from said conductive portion; means for distributing a dispersion of electrostatically charged powder particles within said chamber between the insulative substrate and the conductive portion for development of said image; and means for enhancing the external field of said latent image, said means including a capacitance adapted to be interconnected between said conductive plate and said conductive portion of said chamber during development of said image thereby to accept charges from said other surface of said substrate and reduce said induced charge, whereby the density of the developed image is increased.

8. Apparatus as set forth in claim 7 which further includes means for selectively discharging said capacitance.

9. Apparatus as set forth in claim 8 in which said capacitance is adapted to receive and store charges of a magnitude at least equal to the predetermined total image charge.

10. In an apparatus for developing a latent electrostatic image on an electrically insulative substrate mounted on an electrically conductive plate wherein development tends to be degraded and interfered with by a mirror image charge induced on the ''''back'''' surface of said substrate adjacent said plate, this substrate being adapted to have a latent electrostatic image formed on an exposed front surface thereof and an opposite polarity induced charge on the opposite back surface thereof, the improvement comprising, in combination: dissipation means adapted to dissipate the mirror image charges so induced on said back surface of the substrate to thereby enhance any electrostatic development field projected to the ''''front'''' side of said substrate adapted to receive the said image for development; this dissipation means including capacitive storage means and means for coupling said capacitive storage means between said plate and a potential different from that of said plate, at least during development of said image, to thereby accept the charges from said back surface of said substrate and reduce the induced charge there, thus enhancing said development.

11. The apparatus of claim 10 wherein: said capacitance is connected between said conductive plate and ground.

12. The apparatus of claim 10 wherein: said capacitance is connected to an electrically conductive surface spaced from said conductive plate.

13. The apparatus of claim 10 which further includes means for selectively discharging said capacitance.

14. The apparatus of claim 13 which further comprises means for distributing a cloud of electrostatically charged powder particles between the insulative substrate and said electrically conductive surface.

15. The apparatus of claim 10 as arranged to form part of a powder cloud development chamber; this chamber including electrically conductive surfaces spaced from the conductive plate, and means for distributing a cloud of electrostatically charged toner particles between the insulative substrate and at least a portion of the conductive surfaces of the development chamber; and wherein said capacitive means is coupled between said conductive plate and said conductive chamber surfaces.

16. The combination as recited in claim 10 wherein is also included switch means for selectively coupling said capacitive means to said potential source when said mirror image charge is to be removed therefrom.