US2956487A - Electrostatic printing - Google Patents

Description

f Oct. 18, 1960 E. c. GlAlMo, JR 2,956,487

ELECTROSTATIC PRINTING Filed March 23, 1955 s sheets-sheet 1 BY "Mm Z if-@a 500,962: ,Q 60 dif Oct. 18, 1960 E. c. GIAIMO, JR 2,956,487

ELECTROSTATIC PRINTING Filed March 23, 1955 3 Sheets-Sheet 2 fl @u TT INVENTUR. Zn/,W0 6. bw/w, d?

Y@II e Oct. 18, 1960 E. c. GlAlMo, JR 2,956,487

ELECTROSTATIC PRINTING Filed March 23, 1955 5 Sheets-Sheet 3 f z E. 5

United States Patent @nice 2,956,48'? Patented Oct. 18, 1960 ELECTROSTATIC PRINTING Edward C. Giaimo, Jr., Cranbury, NJ., assignor to Radio Corporation of America, a corporation of Delaware Filed Mar. 23, 1955, Ser. No. 496,159

"` Claims. (Cl. 95-1.7)

This invention relates generally to improved methods and means for electrostatic printing, and particularly, but not necessarily exclusively, to improved methods and means for controlling the steps of electrostatic printing processes and apparatus to produce developed visible images having a desired background color value and image color value.

An electrostatic printing process is that type of process for producing a visible record, reproduction or copy which includes as' an intermediate step, converting a light image or electrical signal to a latent electrostatic charge pattern on an electrically-insulating substrate. The process may also include converting the latent charge pattern into a visible developed image, hereinafter referred to as the print, which may be a substantially faithful reproduction except that it may be different in size, color or contrast.

A typical electrostatic printing process may include, first, producing a uniform electrostatic charge upon the surface of a photoconductive insulating material, such as selenium, anthracene or zinc oxide dispersed in an electrically-insulating, film-forming vehicle. A light image, derived for example, from a photographic transparency, is projected upon the charged surface of the photoconductive material, discharging the portions irradiated by the light rays, while leaving the remainder of the surface in a charged condition, thereby forming a latent electrostatic image. The latent electrostatic image is developed to a visible powder image or print by applying a developer powder which is held electrostatically to selected areas of the surface. The print thus formed may be fixed directly to the photoconductive surface, or it may be transferred to another surface upon which the visible image may be desired and then fixed thereon.

The various process steps are adjusted manually according to the judgment of the operator. Thus, the operator observes the color value of the print and adjusts the printing apparatus to produce subsequent prints with desired characteristics. Because of the physical difficulties involved, the color values of the latent electrostatic image, the projected light image and the photographic transparency image are rarely `observed and hence adjustments are usually not made therein.

By color value of the print is meant the brilliance of color of both the developed and the non-developed areas of the print with respect to some reference value or standard. The background color value is the color value of the non-developed areas of the print, and the image color value is the color value of the developed areas' of the print.

By color value of the latent electrostatic image is meant the magnitude of charge with respect to some reference value or standard of both the areas of the latent electrostatic image which will ultimately be developed areas and background areas of the print. The background color value of the latent electrostatic image is the magnitude of charge, with respect to some reference value, of the areas lof the latent electrostatic image which will ultimately be background areas inthe print. The image color value of the latent electrostatic image is' the magnitude of charge with respect to some reference value of the areas of the latent electrostatic image which will ultimately be the developed areas of the print.

Similarly, by the background color values of the projected light image and the photographic transparency image is meant the respective brightness of color with respect to some reference brightness of the areas of the respective images which correspond to the background areas of the print ultimately produced therefrom. The image color value of the projected light image and the photographic transparency image is the relative brightness with respect to some reference brightness of the areas of the respective images which correspond to the developed areas of the print ultimately produced therefrom.

An object of the invention is to provide improved methods and means of electrostatic printing.

Another object is to provide improved automatic controls upon methods and means for the continuous electrostatic printing of visible images,

A further object is to provide improved methods and means for automatically adjusting one or more features of an electrostatic printing apparatus to produce prints having a desired background color value and a desired image color value, and,

Another object is to provide improved methods and means of electrostatic printing wherein a desired adjustment to an electrostatic printing apparatus is anticipated and the adjustment automatically made.

In general, electrostatic printing processes as hereinbefore described and apparatus therefore may be improved by the invention which comprises producing an electrical signal a characteristic of which is a function of the departure from a standard of the color value of one of the images of the printing process and then utilizing said electrical signal to control atleast one step or feature in the printing process. According to typical aspects of the invention, one or more of the characteristics of the photographic image, the projected light image, the electrostatic image or the developed image may be detected to provide a control effect to adjust or regulate any one or more of the apparatus elements which contribute to said characteristics, in order to automatically provide the desired image sensitivity, density, contrast or brilliance.

The foregoing objects and other advantages will be more fully described in the following detailed description when read in conjunction with the accompanying drawings in which:

Figure l is .a partially sectional, partially schematic view of a composite electrostatic printing system including embodiments of improved apparatus in accordance with various features of the invention,

Figure 2 is a partially sectional, partially schematic view of an improved control means in accordance with one feature of the present invention,

Figure 3 is a partially sectional, partially schematic view of an improved means for an electrostatic printing process in accordance with another feature of the present invention,

Figure 4 is a partially sectional, partially schematic view of an improved means for an electrostatic printing process in accordance with another feature of the present invention,

Figure 5 is a partially sectional, partially schematic View of an improved means for an electrostatic printing process in accordance with another feature of the present invention.

According to the invention, a control signal may be derived, for example, from a photographic transparency image from which a light image is projected, from the projected light image, from the latent electrostatic image deseas? or from a visible developed image derived from a latent electrostatic image. rEhe control signal may be utilized, for example, to control the magnitude of a uniform electrostatic charge produced on the surface of a photoconductive material, the brightness of a projected light image, the magnitude and direction of a unidirectional electric field applied to the developer power during development of the latent electrosta ic image and the proportion of developer powder particles introduced into a developer mix. Each of these features is described in the composite electrostatic printing system described generally in 'Figure 1.

Referring to Figure l, a composite electrostatic printing apparatus may include a continuous web passing over two spaced rollers or pulleys 25. The continuous web comprises a backing 2i, for example paper, on the inner surface thereof in contact with the rollers 2S and a photoconductive coating 23, for example photoconductive zinc oxide dispersed in a silicone resin, on the outer surface thereof. The web passes along a predetermined path between the rollers 2S along which are located stations having apparatus for carrying out the steps of the process. The web first passes a station having apparatus for providing uniform electrostatic charge upon the photoconductive coating 23. A suitable apparatus is a corona apparatus comprising a plurality of wires 27 spaced from the photoconductive coating 23 and connected to a voltage source 26. A backing electrode 29 is contacted with or closely spaced from the paper backing 2t on the opposite side from the wires 27. When a sufficiently high voltage is applied to the wires 27, a corona discharge issues from the wires 27 and deposits charged particles upon the photoconductive coating 23. The deposited charge, hereinafter referred to as uniform electrostatic charge, may be positive or negative depending upon the polarity of the voltage applied to the corona discharge wires 27, and may also vary in magnitude as will hereinafter be described.

The web, having a uniform electrostatic charge on the coating 23 thereof, now progresses to the next station where a light image is projected upon the charged photoconductive coating 23 discharging the charge in the illuminated areas of the coating 23 to produce an electrostatic image thereon. The light image may be projected from a projector 33 having a photographic transparency or film 35 in the aperture thereof, a projection bulb or other source of illumination 39 connected to a voltage source 37, and an optical system il including a lens and an iris.

The continuous web bearing the latent electrostatic image on the photoconductive coating 23 now progresses to a station where the latent electrostatic image is developed to a visible image. For this purpose there is provided a magnetic brush comprising a rotary magnetic system including a magnetic shaft 49, inclined elliptical magnetic discs i thereon and means for conducting a magnetic field to a counter magnet 55 such that the magnetic field is concentrated in a gap between the discs Sll and the counter magnet S5 and across the web. The shaft 49 rotates in a clockwise direction in Figure 1 carrying with it on the ends of the discs Si a quantity of developer mix 47 comprising magnetic carrier particles and developer powder particles.

A preferred carrier material for the developer mix consists of alcoholized iron, that is, iron particles free from grease and other `impurities soluble in alcohol. These iron particles are preferably relatively small in size, being in their largest dimension about 0.002 to 0.008 inch. Satisfactory results are also obtained using a carrier consisting of iron particles of a somewhat wider range of sizes up to about 0.001 to 0.020 inch.

A preferred developer powder may be prepared as follows: a mixture comprising 20G grams of 200 mesh Piccolastic resin 4358 (an elastic thermoplastic resin cornposed of polymers of styrene, substituted styrene and its homologs) marketed by the Pennsylvania Industrial Company, Cliarton, Pa. and 12 grams of Carbon Black G marketed by the Eimer and Amend Co., New York, N.Y. are thoroughly mixed in a stainless steel beaker at about 200 C. The mixing and heating should be done in as short a time as possible. The melt is poured upon a brass tray and allowed to cool and harden. The hardened mix is then broken up and ball milled for about 2t) hours. The powder is screened through a 200 mesh screen and is then ready for use as a developer powder. This powder takes on a positive electrostatic charge when mixed with iron powder. It therefore will develop the negatively charged areas of an electrostatic image. About 2 to 10 grams, but preferably 5 grams, of the developer powder and about grams of the magnetic carrier material are blended together giving the preferred developer mix. Other ratios of developer powder to magnetic carrier material may be used.

The developer mix 47, carried by the magnet, is contacted across the photoconductive coating 23 bearing the latent electrostatic image depositing developer powder particles upon the desired areas of the latent electrostatic image producing a visible developed image. The shaft 49 is electrically-grounded and a brush electrode 57 in contact with or closely spaced from the paper backing 21 opposite the quantity of developer mix 47 is connected to a voltage source 58 for applying a desired voltage to the brush electrode 57. By applying such voltage, to the brush electrode S7, a unidirectional electric eld in addition to the electric field of the latent electrostatic image is applied across the web between the brush and the brush electrode and particularly between the photoconductive coating 23 and the developer mix d?.

There is also provided a trough 53 for containing a quantity of developer mix from which the magnetic discs S1 may pick up fresh portions as they rotate, and a reservoir 59 for holding developer powder particles to add to the depleted developer mix contained in the trough 53. The developer powder particles from the reservoir 59 are fed into the trough 53 by vibrating the reservoir 59 with a mechanically connected vibrator 61 which is operated by a voltage source 63.

The web bearing developed areas 65 of the latent electrostatic image upon the photoconductive coating 23 is now conducted to a station where the developed image may be transferred to a second substrate and fixed thereon or the developed image may be fixed directly to the photoconductive coating 23. As shown in Figure 1, a series of resistance wires 67 are substantially operated to apply heat to the developed image which heat fuses either the developer powder particles comprising the developed image or the photoconductive coating 23 upon which the developed image resides causing the developed image to adhere to the surface of the photoconductive coating 23. For this purpose either the developer powder, preferably, or the photoconductive coating comprises a `thermoplastic material.

According to the invention any of the steps of the process may be automatically controlled by first sensing the color value of the photographic transparency, the projected image, the latent electrostatic image, or the developed image or any combination of these images to produce a control signal, which control signal may then be used to vary the operation of one of the steps of the process. For exampie, the color value of the photographic transparency image may be sensed before projection. Before a light image is projected from the transparency 35 a light source may project a beam of light through the transparency which transparency transmits a proportion of the light according to its contrast characteristic upon a photocell 7i. The photocell 71 generates a sensing signal which is a function of the brightness of the light striking it. The sensing signal is then conducted to a comparator' 73 that produces a comparison signal derived from the sensing signal. The comparison igual is a function of the departure of the color value of the sensed image from a standard or reference color value. The comparison signal is now amplified to a desired value producing a control signal A which is available to control a step in the process. The control signal A therefore is derived from the transparency 35 before it is projected upon the photoconductive coating 23.

By a similar system a controlled signalv C may be derived from the transparency 35 after it has been projected upon the photoconducti-ve coating 23. accomplished by means of a light source 75 which projects a beam of light through the transparency 35 and a photocell 77 which receives the transmitted light producing a sensing signal. The sensing signal is a function of the density of the sensed area of the transparency 35. The sensing signal is fed to a comparator and amplifier 79 which produces a comparison signal and amplies the comparison signal to produce a control signal C.

Alternatively, the color value of the projected light image may be sensed. This may be accomplished by placing a photocell 81 within the projected light image from the projector 33. The photocell 81 produces a sensing signal which is a function of the color Ivalue of the projected light image, which sensing signal is fed to a comparator and amplifier 83 which produces a comparison signal and a control signal in a manner similar to that described under control signal A.

Alternatively the color value of the latent electrostatic image may be sensed. This may be accomplished by means of a vibrating probe 85 on one side of the web having a probe backing electrode 87 associated therewith in contact with or closely spaced from the other side of the web. The operation of the vibrating probe, which will subsequently be described in greater detail, produces a sensing signal which is a function of the color value of the latent electrostatic image by reading the voltage of areas of the electrostatic image. The sensing signal from the vibrating probe 35 is fed to an amplifier 89 which produces a control signal D in a manner similar to that described under contro-l signal A.

Alternatively, the color value of the developed image may be sensed. For this purpose, a light source 91 projects a narrow beam. of light upon the surface bearing the developed image. The surface reects an amount of light which is a function of the color value of the area. The reflected light is directed to a photocell 93 which produces a sensing signal which sensing signal is fed to a comparator and amplifier 95 `which produces a comparison signal and a control signal E in a manner similar to that described to control signal A.

By sensing the color value of the developed areas or some combination thereof of one of the images of an electrostatic printing process, such as hereinbefore described, a sensing signal may be produced which may be used to produce a comparison signal which is a function of the departure of the sensed area from the desired color value in corresponding areas of the developed visible image. The area sensed may be, for example, in the margin which is generally of a single color value, usually all black or all white. The area sensed may also be other areas of the image. In the latter case the sensing signal may include integrating circuits or may include circuits which sense areas of the image above or below a predetermined value, for example, only the white areas or only the black areas of the transparency or of the developed image, or only the areas above or lbelow a predetermined potential in the case of the latent electrostatic image.

A control signal may be used to control one of the steps of an electrostatic printing process. For example, any oneof the control signals A, B, C, D, or E may be to control themagnitude-of electrostatic charge produced on the photoconductive coating. For example, a

voltage source 31` is varied thereby varying the voltage i "applied to a` grid 23 closely spaced between the wires This is 6 2,7 of the corona discharge apparatus and the photoconductive coating 23. When this voltage is varied between 0 and 1000 volts preferably of the same polarity as the voltage applied to the wires 27 of the corona discharge apparatus, the amount of charge deposited upon the photoconductive coating 23 may be varied from 0 to a maximum in a manner similar to the operation 0f a biasing voltage upon the grid of a radio tube. The control signal applied to the voltage source 31 may control the magnitude of charge actually deposited upon the photoconductive coating 23, thus providing a limited variation of voltage in the latent electrostatic image subsequently produced during projection.

Alternatively, the brightness of the projected light image may be controlled by a control signal. For this purpose,v any one of the control signals A, B, C, D, or E may be applied to a voltage source 37 to vary the voltage applied to the filament of a projection bulb 39 thereby varying the brightness of the light source in the projector 33. In so doing, the brightness of the projected image is varied thus changing the rate of discharge of the charged photoconductive coating 23 during the production of the latent electrostatic image.

Alternatively, any of the control signals A, B, C, D, or E may be utilized to control a voltage source 45 which in turn controls a motor 43 which may open or close an iris in the optical system 41. In so doing, the brightness of the image projected from the projector 43 may be varied, thereby varying the rate of discharge of the charged photoconductive coating 23 during the production of the latent electrostatic image.

A control signal may be used to control the devel-opment of the latent electrostatic image. For this purpose, any of the control signals A, B, C, D, or E may be utilized to control a voltage source 58 which varies the polarity and voltage applied to the backing electrode 57 for the developer brush. By changing the voltage applied to the backing electrode 57, the electric field between the developer mix 47 and the photoconductive coating 23 is varied changing the attraction of the developer powder particles for the latent electrostatic image.

Alternatively, any of the control signals A, B, C, D, or E may be utilized to control a voltage source 63 which operates a vibrator 61 to feed developer powder particles into the trough 53. When the proportion of developer powder particles to magnetic carrier particles in the developer mix is high, the developed areas of the print are more dense. Thus, the control signal applied to the voltage source 63 may be utilized to increase the color value of the developed areas of the print by increasing the proportion of developer powder added to the developer mix. The control signal applied to the voltage source 63 may be utilized to decrease the density of the developed image by withholding developer powder from the depleted developer mix.

By choosing a proper combination of control signals and controlled steps, one may anticipate the need for adjustment for any particular print. For example, control signal A may be derived from a position on the photographic transparency 35 which corresponds geometrically to the areas of the photoconductive coating which will ultimately carry the print derived therefrom and which at the same time is being charged by the corona discharge wires 27. By applying control signal A to the voltage source 31, the desired uniform electrostatic charge may be applied to the photoconductive coating 23. The control signal A will change for each successive frame of the transparency 35. According to the invention the magnitude of uniform electrostatic charge required is anticipated and automatically applied.

There will now be described in more 'complete detail a process and apparatus for sensing the color value of the latent electrostatic image to produce a sensing signal which is utilized to control the unidirectional electric field applied to developer powder particles during the development of a latent electrostatic image.

Example 1.--Referring to Figures 1 and 2, the web bearing the latent electrostatic image upon the photoconductive coating 23 passes from the projection station to the developing station. Between these stations a vibrating electric probe 85 is positioned to read the voltage of the surface of the photoconductive coating 23 along the margin of the latent electrostatic image. The edge of the latent electrostatic image is chosen because most printed matter contains a margin of a uniform color value usually white or black. A signal derived from the margin of the latent electrostatic image is a convenient datum for producing the desired background color value in the developed image.

The electric probe consists of an insulated metal disc 85 mounted on an insulated, magnetically-responsive shaft 86 which is vibrated magnetically on a spring suspension 88 which is mechanically tuned to the frequency of a driving voltage. The driving voltage is impressed across a coil 90 inducing a magnetic eld in an insulating core 92 coaxially mounted around the shaft 86. The disc 85 is closely spaced from the photoconductive coating 23 and a grounded probe backing electrode 87 is contacted with or closely spaced from the paper backing 21 of the web.

When an electrically charged insulator such as the photoconductive coating 23 is placed between the probe 85 and the probe backing electrode 87, an electric charge is induced on the probe which charge is of equal and opposite magnitude to that of the charge on the insulator. The voltage developed between the probe and ground due to the induced charge is dened by the equation: V=q/c, where q is the charge and c is the capacity between the probe and ground. The voltage will change if the capacity is changed, for example, by the motion of the probe. Thus, if c=c sin Zwft due to the driving force, then V=Q/c0 sin 21rft, yielding a sinusoidal voltage. The peak magnitude of the voltage for a given probe spacing for a given driving force will be determined by the magnitude of the charge. In effect, the vibrating probe is a sinusoidal voltage generator whose output voltage is determined by the charge on the surface of the insulator being read.

lf an electric charge is present under the electric probe 85, a signal is developed by the probe. The developed voltage is converted into an alternating voltage by the vibrating action of the probe which alternating voltage is amplified by the amplifier tubes 121, 123, and 125. The ampliiied signal is then fed to a triode 127 in a cathode follower circuit which provides isolation between the high and low impedance sections of the circuit. The output of the triode 127 is rectied by a signal rectiiier tube 129 and the D.C. voltage thus produced is applied to the grid of the series tube 131 having a circuit associated therewith wherein the effective resistance between the cathode and the plate may be varied by changes in grid voltage. A resistance 134 and a capacitor 133 in the circuit between the signal rectifier tube 129 and the series tube 131 provide delay action of the signal into the series tube 131 by preventing the grid voltage from changing instantaneously. This circuit allows for delay in the control signal to provide anticipation for a subsequent step in the electrostatic printing process. The changes in the grid voltage of the series tube 131 cause the current through the resistor 132 to change, developing a varying voltage across this resistor. The voltage thus developed across the resistor 132 is connected to backing electrode 57 providing a biasing action to the magnetic brush. Rectifier tube 136 provides rectified biasing voltage from the line voltage for the anodes of each triode tube.

If the areas of the latent electrostatic image that are to be developed are negative, then the background areas are positive with respect to the areas to be developed and the developer powder particles are preferably charged positively to be more attractive to the negatively charged areas. By applying a positive potential to the backing electrode 57, the positively charged developer powder particles see a more positively-charged background and are thereby more strongly repelled away. Thus, by adjusting the apparatus of Figure 2 to sense the background color value of the latent electrostatic image, the developing step may be automatically adjusted such that the electric field applied to the developer powder in addition to the image field provides the desired attraction to the developer powder particles and produces the desired background color value. The apparatus may also be adjusted to make the background areas of the latent electrostatic image more attractive to the developer powder particles and so produce a different color value. Similarly, the background color value of the print may be adjusted where the background areas of the electrostatic image is negatively charged. The electric Probe may also sense the image areas of the latent electrostatic image and', by similar systems, control the developing step to produce image areas of a desired color value.

Example 2.-Referring to Figures l and 3, a photocell 71 is used to view the margin of the photographic transparency or film 35 about to be fed through the projector 33. Simultaneously a photocell 101 views a standard 36 having a predetermined color value. Identical light sources 69 and 102 are used to illuminate the photocells 71 and 101 to control the color value of the background areas of the print, in a manner similar to that described under Example 1.

As an alternative to photocell viewing a standard, a positive voltage may be applied to the grid of the tube 105 by throwing a switch 114 and adjusting the resistor to a desired value.

The operation of the system shown in Figure 3 is initiated by the production of signals from the two photocells 71 and 101. The photocell signals are amplified by amplifier tubes 103 and 105 respectively. The circuits of the amplifier tubes 103 and 105 are so adjusted that when both photocells receive the same amount of light, the plate voltage of each tube with respect to ground is the same. When an unequal light intensity occurs, the plate voltages with respect to ground change. The difference in the two voltages is compared in a tube 108 by feeding the signal of one photocell into the cathode and the signal of the other photocell into the grid. When the light intensity reaching the tube 71 decreases, the grid of the tube 108 becomes positive with respect to the cathode causing the grid of the tube 109 to swing negative. The voltage at the plate of tube 109 increases and applies a more positive voltage to a series tube 111. When the grid of series tube 111 becomes positive, it conducts more strongly, raising the voltage across its cathode resistor 113. Electrode 57 behind the magnetic brush 47 is connected to the cathode of series tube 111 and becomes positive when the cathode of series tube 111 becomes positive. The positive voltage applied to the electrode 57 changes the electric iield existing between the developer mix 47 and the photoconductive coating 23 thereby changing the sensed signal to an alternating voltage, amplifying the alternating voltage, rectifying the alternating voltage to produce a control signal and then applying the control signal to a backing electrode to vary the additional field applied to a magnetic brush. The biasing voltage is preferably adjusted so as to effect a reduction to the background color value of the developed image, although it may be used for other purposes, for example, reversing the developed image or reducing the contrast characteristic of the developed image.

As an alternative to photocell viewing a standard, a positive voltage may be applied to the grid of the tube 105 by throwing a switch 114 and adjusting the resistor 115 to a desired value.

Example 3.-Referring again to Figures 1 and 3, a

A,-photoczell 77 is used to view the margin of the lm that has been fed through the projector 33 in the same manner as the photocell 71 was utilized in Example 2. The circuitry and operation are the same as in Example 2.

Example 4.-Referring again to Figures 1 and 3, a photocell 93 is used to view the margin of the developed image after leaving the developing station by light reection of the illumination from a light source 91. The signal from the photocell 93 is then treated in the same manner and with similar circuitry as the signal from photocell 71 of Example 2.

There will now be described in more complete detail a process and apparatus for sensing the color value of a photographic transparency image to produce a sensing signal which is utilized to control the brightness of the projected light image by varying the iris size of the optical system of a projector.

Example 5.-The control signal derived from the photocells in Examples 2, 3 and 4 may be used to produce a control signal which is then used to eiect a change in the brightness of the projected light image from the projector 33. The control signal may be utilized, for example, by changing the aperture of the iris in the optical system 41 interposed in the light path from the projector 33.

Referring to Figure 4, the control signal is fed to a series tube 159 at a connection 141 changing the cathode voltage of the tube. The voltage change at the cathode of tube 159 is amplified by amplifier tubes 143 and 145. The ampliiied cathode voltage of the amplifier tube 145 is fed into the armature 147 of a D.C. motor which Aarmature is in a bridge-type circuit. The D.C. motor has its eld coils 149 connected to a voltage source 155 through a potentiometer 151. The shaft of the armature 147 is mechanically connected to a potentiometer 152 on one side ofthe armature bridge circuit such that the armature 147 seeks a balancing position in the bridge circuit. vWhen the signal output from the amplifier tube 145 is removed from the armature 147, the armature 147 returns to its original balance position. The shaft of the armature 147 is also mechanically coupled to the control mechanism of an iris 41 of the projector 33 and changes the brightness of the projected image so as to cause the control signal to return to O or to a predetermined value. The equilibrium position of the armature 147, that is the armature position when no control signal is applied, is determined by manually setting a rheostat 153.

As the iris 41 is opened, a brighter light image causes a more rapid discharge of the illuminated areas of the charged photoconductive surface 23. As the iris 41 is closed, a dimmer light image is projected causing less rapid discharge of the illuminated areas of the charged photoconductive surface 23. Such adjustment of the iris 41 permits automatic adjustment to any optimum rate of discharge for the particular print being produced.

There will now be described in more complete detail a process and apparatus for sensing the color value of the photographic transparency image to produce a sensing signal which is utilized to control the proportion of developer powder added to the developer mix.

Example 6.T he proportion of developer powder particles to magnetic carrier particles in the developer mix 47 may be increased by additions of developer powder from the container 59. This operation may be accomplished automatically by utilizing any of the control signals A, B, C, D, or E. For example, referring to Figures l, 4 and 5, the shaft of the armature 147 may have connected thereto a rst armature cam 161 or other projection which causes a switch 163 which is ordinarily opened to close depending upon its position. The switch 163 is in series with a cam 165 operating a cam switch 167 which permits a predetermined quantity of developer powder to be introduced into the container 59 at regular intervals as the recording machine operates. The rate of developer powder injection is ordinarily determined by a variable speed motor 169 driving a cam 165 which in turn operates a switch 167 which controls a vibrator 61 coupled to the container 59. When the switch 163 is opened, the vibrator is not operated allowing the portion of developer powder in the developer mix to become depleted.

A switch 173, ordinarily opened, is connected in parallel with the cam switch 167 operated by a second armature cam 171. When the armature 147 rotates in a counter-clockwise direction as viewed in Figure 5, the switch 173 is closed operating the vibrator 61 to add more developer powder to the developer mix. The apparatus is adjusted such that when the switch 173 is closed, the developer powder is added at a rate greater than the rate of depletion from the developer mix. By tying the developer powder feed to an automatic control signal derived from one of the images produced in the process, it is possible to lower the developer powder concentration of the developer mix by withholding the addition of developer powder as it is depleted from the developer mix to produce prints with lower color values, or by increasing the proportion of developer powder in the developer mix by -adding developer powder in a proportion greater than its depletion during its operation to produce prints of higher color values. Thus, the proportion of developer powder in the developer mix may be varied within a wide range of operating proportions to produce prints having a desired color value. As the armature operates both the iris and the vibrator 59, the circuit of Figure 5 prevents the system from running yaway because the iris 41 tries to compensate for background density because of too high a proportion of developer mix.

There have been described improved methods of electrostatic printing wherein a desired adjustment to a step in the process is anticipated and the adjustment automatically made. There have lalso been described improved methods and means for automatically adjusting one 0r more means of an electrostatic printing apparatus so as to produce developed visible images having desired background color value and a desired image color value.

What is claimed:

1. In an electrostatic apparatus lfor producing visible images including conveyor means for transporting along a predetermined path a photoconductive insulating surface sensitive t-o electromagnetic radiation of a given type, and means .disposed along said path in the following order: means for providing a substantially uniform electrostatic charge upon said surface, means for exposing said charged surface to an electromagnetic radiation image of said given type for producing a latent electrostatic image upon said surface substantially corresponding to said electromagnetic radiation image and means for developing a visible image from said latent electrostatic image with a finely-divided developer substance; the improvement for controlling the contrast of the developed image comprising means for sensing the density of at least one of said images to produce an electrical sensing signal, means for simultaneously producing an electrical standard reference signal, electrical means for comparing said sensing signal and said reference signal to produce a control signal, and means for applying said control signal to said means for developing a visible image to control the amount of deposition of said finely-divided developer substance during development of said image in proportion to the magnitude of said control signal.

2. In an electrostatic apparatus for producing visible images including conveyor means for transporting along a predetermined path a photoconductive insulating surface sensitive to electromagnetic radiation of a particular type land means disposed along said path in the following order: means for producing a substantially uniform electrostatic charge on said surface, means for projecting an image of electromagnetic radiation of said particular type from a photographic transparency image upon said charged surface to produce thereon a latent electrostatic image substantially corresponding to said photographic transparency image and means for applying across said surface a mass of developer mix comprising developer powder particles and magnetic carrier particles held together in a loose mass by a magnetic field to produce a visible developed image of developer particles thereon substantially corresponding to said latent electrostatic image; the improvement for automatically controlling the contrast of said visible developed image comprising, means for sensing the background density and image density of said photographic transparency image to produce a sensing signal, means for simultaneously producing an electrical standard reference signal, electrical means for comparing said sensing signal with said reference signal to produce `a comparison signal, and means for applying said comparison signal to said means for applying said developer mix to control the amount of deposition of developer powder particles on said surface in proportion to the magnitude of said comparison signal.

3. In an electrostatic yapparatus for producing visible images including conveyor means for transporting along a predetermined path a photoconductive insulating surface sensitive to electromagnetic radiation of a given type and means disposed along said path in the following order: means for providing a substantially uniform electrostatic charge upon said surface, means for projecting an image of electromagnetic radiation of said given type from a photographic transparency image upon said charged surface to produce a latent electrostatic image upon said surface and means `for applying a mass of developer mix comprising developer powder particles and magnetic carrier particles held together by a magnetic field across the surface bearing said latent electrostatic image to produce a visible developed image of developer particles thereon substantially corresponding to said latent electrostatic image; the improvement for automatically controlling the contrast of said visible developed image comprising means for sensing the background density and image density of said electrostatic image to produce an electrical sensing signal, means for simultaneously producing an electrical standard reference signal, means for electrically comparing said sensing signal and said reference signal to produce a cornparison signal, means for applying said comparison signal as a bias potential to said mass of developer mix to produce a unidirectional electric iield having an intensity proportional to the magnitude of said comparison signal, said unidirectional eld being in addition to the field emanating from said latent electrostatic image,

and means for applying said comparison signal to said 5 means for applying said developer mix to control the quantity of developer powder particles introduced in said developer mix in proportion to the magnitude of said comparison signal whereby the amount of deposition of developer powder on said surface is controlled in proportion to the magnitude of said comparison signal.

4. The apparatus of claim l wherein said means for developing said visible image includes means for contacting across said surface a mass of developer mix comprising developer powder particles and magnetic carrier particles held together in a loose mass by a magnetic eld and wherein said means for applying said control signal includes an electrode disposed adjacent to said mass of developer mix but on the opposite side of said surface with respect to said developer mix and means for applying said control signal between said electrode and said developer mix to produce therebetween a unidirectional electric iield in addition to the electric field emanating from said latent electrostatic image on said surface thereby controlling the amount of deposition of developer powder particles during development of said image in proportion to the magnitude of said control signal.

5. The apparatus of claim 1 wherein said means for developing said visible image includes means for contacting across said surface a mass of developer mix comprising developer powder particles and magnetic carrier particles held together in a loose mass by a magnetic field and wherein said means for applying said control signal includes means for controlling the ratio of developer powder particles to carrier particles in said mix in proportion to the magnitude of said control signal to control the amount of deposition of developer powder particles during development in proportion to the magnitude of said control signal.

References Cited in the tile of this patent UNITED STATES PATENTS (1956), pages 162-163 and 18S-189.

Young et al.: RCA Review, vol. XV, #4, pages 468-

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