US3321307A

US3321307A - Exposure control in xerographic printing

Info

Publication number: US3321307A
Application number: US295143A
Authority: US
Inventors: Urbach Franz
Original assignee: Eastman Kodak Co
Current assignee: Eastman Kodak Co
Priority date: 1963-07-15
Filing date: 1963-07-15
Publication date: 1967-05-23
Anticipated expiration: 1984-05-23
Also published as: GB1055696A; BE650516A

Description

May 23, 1967 F. URBACH 3,321,307

EXPOSURE CONTROL IN XEROGRAPHIC PRINTING Filed July 15, 1963 FIELD METER AMPLIFIER 1 FIG FRANZ URBACH 70 n7, V 8 f2 INVENTOR.

7/ ELECTRO I METER BY ATTOR/VE Y5 United States Patent 3,321,307 EXPOSURE CONTROL IN XEROGRAPHIC PRINTING Franz Urbach, Rochester, N.Y., assignor to Eastman Kodak Company, Rochester, N.Y., a corporation of New Jersey Filed July 15, 1963, Ser. No. 295,143 Claims. (Cl. 96-4) The present invention relates to xerography and particularly to a method and apparatus for controlling the exposure of a charged photoconductor sheet.

It is the object of the invention to provide apparatus for automatically terminating the exposure of a xerographic plate when the electrostatic image being formed by the exposure has reached approximate optimum quality. Optimum quality of an electrostatic image is that distribution of charges which, when developed with a suitable toner, produces prints whose quality is the optimum obtainable with the particular photoconductor and toner being used. As is well known, the quality in a print refers to the minimum and maximum densities and to the contrast of the print. High quality prints have extremely low minimum density (clean highlights), fairly high maximum density and a contrast which approximates or slightly exceeds that of the original subject being reproduced.

The present invention is also particularly useful in the copying of documents.

A further object of the invention is to provide method and apparatus for controlling the termination of the exposure time. It is a particular object of the invention to provide such control independent of the intensity of the exposure over a wide range of intensities, including all or practically all intensities found in light transmitted through commonly over-exposed and under-exposed transparencies (negatives or positives). It is also a particular object of the invention to provide a system which corrects for variations in sensitivity of the recording material at the same time as it corrects for the density of the transparency being printed.

The present invention is applicable to the various forms of xerography including those in which the toner adheres to the charged areas and those in which the toner adheres to the discharged areas of the electrostatic image. It is equally applicable to systems in which the toner image remains on the photoconductor (zinc oxide in resin on paper base is sufiiciently inexpensive) and those in which the image is transferred from a reusable photoconductor (which may be relatively expensive) to a separate receiving sheet. The invention utilizes the change in an average electric field adjacent to the photoconductor as the charge on the photoconductor changes from a uniform, relatively high-density charge to an imagewise distribution of charge with a lower average density. The proper average electric field is attained by an exposure time which depends on the intensity of the exposing image and the sensitivity of the photoconductor.

While it is true that different subjects will have slightly different average fields associated with the optimumquality electrostatic images thereof, it turns out in practice that a statistically very high yield of optimum-quality prints are obtained when variations in subject matter are neglected or ignored. The present invention for any one subject gives optimum-quality prints from negatives which range from two stops under-exposed to four stops over-exposed. Furthermore, setting the system to give such optimum results for an average subject of the type to be reproduced gives acceptable quality for practically all subjects of that same general type. The setting is, of course, somewhat different when used for document copying, for example from microfilm, than when used for continuous-tone printing; and the setting is diiferent for different types of toners, but over a wide range of types and sensitivities of photoconductors the setting is unaflected.

According to the present invention, the electric field, or more exactly the change in the electric field, adjacent to a substantially uniformly charged photoconductor is measured during the exposure to an image. The exposure is started either manually or by the measuring system. The exposure efiectively removes charges in the exposed areas, causing the field to change, and the exposing is terminated when the measured field or the change in field reaches a predetermined value. As pointed out above, excellent quality prints are obtained from a wide range of negatives and a wide range of photoconductor sensitivities when such a control system is used.

The term effectively removes charges is used herein to include movements of charges from the surface, move ments of opposing charges to or nearer the surface and what is sometimes referred to as a disorientation of dipoles. The present invention is not concerned with the theory or convention adopted, but uses the term in its commonly accepted sense.

The charging of the surface must be terminated before or at the moment exposure starts. In practice, the surface is charged uniformly and brought to the position for exposure before the measuring electrometer or field meter is rendered operative. For example, the electrometer may have its field-sensing electrode shorted to ground or shielded from the charged photoconductor until the photoconductor is in place and ready for exposure. When the shield is used and then removed, the sensing electrode of the electrometer or field meter is immediately subjected to a high electric field. When the grounded electrode is used and then ungrounded, it remains at ground potential until the field of the charged photoconductor surface changes either through spontaneous decay or exposure. The exposing light is then turned on or the shutter in the projection printer is opened either manually or by a relay operated by the output of the electrometer or field meter. Then, as the field falls (causing, in the case of the removed shield, a decrease of potential on the sensing electrode and in the case of an unground electrode an increase of the potential thereof) the measuring is used to terminate the exposure at a predetermined value of the average electric field or at a predetermined change in the field, namely that corresponding approximately to an optimum quality electrostatic image. It should be noted, at this point, that one of the advantages of the present invention arises from the fact that it takes into account any reasonable amount of natural leaking away of the electrostatic charges as well as the discharging due to the image exposure. While excessive natural leakage of charge will tend to degrade the image, the present invention still assures the optimum available among such degraded images.

When the present invention is used for document copying, the type of development may determine whether the meter should be set to terminate the exposure at a predetermined value or at a predetermined change from the initial value. The latter is particularly useful for any type of development which takes place with no grounded electrode near the recording surface at the time of development; this includes fringe development by powder cloud, liquid development or cascade development with insulating carrier for the toner. The exposure is terminated substantially at a fixed field value when the development is to be in the presence of a grounded electrode as for example with magnetic brush development or some forms of liquid or powder cloud development with a grounded electrode adjacent the image surface.

Meters of two general types are well-known and com mercially available from various manufacturers. Each of the two types has its advantages when used with the present invention.

One type is superior to the other when the sensing electrode used therewith is a transparent one held immediately in front of the charged surface of the photoconductor being measured. One form of electrometer useful with this type of sensing electrode is the General Radio D.C. Amplifier and Electrometer, Type 1230A. The sensing electrode being transparent does not obscure the printing beam.

A second type of meter commonly used employs a mechanical chopper between the source of the field and the sensing electrode. The chopper and sensor constitute two sectored discs or vanes, one of which is stationary and the other rotating behind or preferably in front of the stationary one. A steady field creates an alternating signal in the sensing electrode. The front vane (either the chopper or the fixed one) is grounded, or biased to a fixed potential, and the other one acts as the sensing electrode supplying an AC. signal to the field meter amplifier. Since such a sensing electrode with chopper is normally opaque, it is positioned, in the present invention, to one side of the image-forming beam so as not to obscure the beam. The sensing electrode in this case faces the surface to be measured, obliquely.

I have found that such oblique measurements of the average field are satisfactory for the purposes of the present invention.

Electrometers or field meters of the rotating electrode type are described in many publications. An elementary description appears, for example, in Measurements of Electrical Polarization in Thin Dielectric Materials by Tyler, Webb and York, Journal of Applied Physics, vol. 26, pp. 61-68, January 1955. Also, the five references listed in a footnote on page 56 of this Tyler et al. article describe useful forms of such electrometers. The various known forms of electrometers have, of course, different ranges of current values in their output or measuring circuits. If these values are not sufficient to operate the relays or the equivalent involved in initiating and terminating the exposure, various degrees or stages of amplification are introduced.

Other objects and advantages of the invention will be fully understood from the following description when read in connection with the accompanying drawing in which:

FIG. 1 schematically illustrates a preferred embodiment of the invention.

FIG. 2 similarly illustrates the field meter or electrometer employed in FIG. 1.

FIG. 3 illustrates a modification of one part of the arrangement shown in FIG. 1 to incorporate a different embodiment of the invention.

In FIG. 1, light from a lamp illuminates a transparency 11 and an image thereof is focused by a lens 12 on the uniformly charged surface of a photoconductor 15 carried on a conducting support 16. A shutter for the optical system is indicated schematically at 13. The lamp is energized directly from a 110-volt A.C. source 17 when switch 18 and relay contact 19 are closed. The relay contact 19 is normally open and is closed by a relay solenoid 20 when the latter is energized. If the switch 18 is already closed, closing of the relay contact 19 initiates the exposure; alternatively the exposure can be started manually by closing the switch 18 after the relay contact 19 is closed by the solenoid.

According to the invention, a sensing electrode is positioned to one side of the printing beam and is arranged to face the charged surface of the photoconductor 15 obliquely. The sensing electrode 25 is stationary behind a rotating sector blade or chopper 26 of an electrometer of the above-discussed type. The chopper 26 is biased slightly above or below ground by a battery,

shown schematically at 27, in order to permit a zero adjustment to be made on the recording means. The rotating chopper alternately shields and exposes the stationary elect-rode 25. This mechanical chopping of the field produces an AC. potential across a high resistance 28.

In the operation of the device, a metal shield 31 is placed across the illuminating beam and in front of the sensing electrode 25 of the field meter, until a fully charged photoconductor 15 is in place, as shown, and ready for exposure. This shield 31 maintains the field meter inoperative, i.e. prevents the field meter from measuring the average field adjacent to the photoconductor 15 until the photoconductor sheet is ready to receive the exposure. The shield 31 is then removed manually by grasping the extension 32 and withdrawing the shield from the metal housing 33 which is grounded and which shields the whole unit. Instead of the simple extension 32, mechanical means can be provided for removing the shield or for moving it to an inoperative position, rendering it ineffective. When the shield is removed or rendered ineffective, the electrometer sensing electrode becomes operative, and a signal acnoss the resistor 28 is amplified in the field meter amplifier 36 and impressed across a resistance 37 which in this case is a 500-0hm resistance.

Standard field meter amplifiers such as 36 are provided with output connectors, one of which is grounded and across which the variable resistance 37 is connected. With a field meter of the type described below having a final rectifier (such as 57, FIG. 2) which may be connected with either polarity, the output can be a negative, negative-going signal for an increasing positive surface potential, or with a reverse arrangement, the output can be a negative, negative-going signal for an increasing negative surface potential. With a SOD-ohm resistor at 37 the output of the field meter 36 produces for example a potential acnoss this resistor 37 between zero and approximately 0.5 volt as the magnitude of the surface potential under measurement is increased.

The following example is given merely for clarity; it describes the operation of the amplifier for one group of settings of the controls. With no input signal, the 6 BH6 tube 40 has its control grid at +0.04 volt from ground. The cathode is also above ground by the potential drop across the cathode resistor. In this condition the tube 40 is conducting fairly heavily. The potential at the plate is +62 volts which is dropped to +35 volts by the Zener diode (IN205). This potential drop plus the potential drop across a portion of the resistor 44 puts the grid of the 6C4 tube 43 at 16 volts which is sufficient to keep the tube 43 cut off and no current flowing through the relay 20. As mentioned above, a negative, negative-going output current from the field meter amplifier is produced as a sensed field strength from the photoconductor 15 increases. As increasing negative current passes through the resistor 37, the potential on the grid of tube 40 becomes more negative. I-ts plate current then decreases, causing the plate potential to rise. When the grid of tube 40 reaches -0.5 volt from ground, the plate of this tube 40 is at +84- volts. The diode 42 drops this to +56 volts and tube 43 grid is then at 2.0 volts, as determined by the ad justment of the contact on resistor 44. This grid voltage permits sufficient current to flow in the plate circuit to actuate the relay 20, closing the relay contact 19 and starting the exposure if switch 18 is already closed. This energized condition of relay 20 is reached immediately after the shield 32 is removed, thus exposing the

sensing electrode

25, 26 to the field adjacent to the charged photoconductor 15.

Immediately upon starting the exposure, the surface potential on the photoconductor 15 starts to decrease, which causes the potential on the grid of the tube 40 to rise, increasing the plate current and causing the plate potential to drop, causing the grid on tube 43 to be come more negative reducing the plate cur-rent through relay 20 until it releases. This occurs when the grid potential on tube 40 has risen to -0.2 volt from ground, at which time the plate potential is 70 volts. This is dropped to +42 volts by the Zener diode 42, and the grid of the tube 43 is at -11 volts at the point selected on the resistor 44. This is just suflicient to permit the relay 20 to release, terminating the exposure.

Instead of using the shield 31, the sensing electrode 25 may be grounded until the sheet 15 is ready for exposure. Opening the ground connection renders the

sensing electrode

25, 26 operative, starting the exposure which, as described above, terminates when the potential on the grid of the tube 43 reaches -2.0 volts. It is customary to use a vacuum plate of the type commonly used in process photography, to hold the plate 15 flat on the shielded easel of the projection printer illustrated schematically in FIG. 1. After the exposure is terminated, the photoconductor sheet 15 is developed or toned by any standard xerographic method; the toner image may be fused to the photoconductor or transferred to a receiving sheet.

As shown by broken lines in FIG. 1, the lamp may remain on, and the exposure may be controlled by opening and closing the shutter 13 by means of a shutter control actuated by the closing and opening of the switch 19.

It should be noted that this instrument according to the present invention not only corrects for various densities of the transparency 11 or intensities of the lamp 10, but also corrects for sensitivities in the layer 15. In one experiment, photoconduc-tors whose sensitivities vary by as much as 7 to l were used. Each coating was charged to a surface potential of -400 volts under a --9 kv. corona, before exposure. The optimum exposure time was determined for one of the coatings. A series of prints were run on the various coatings and another series were run with a 1.0 neutral density filter inserted in the printer beam. Without interference on the part of the operator, the apparatus was allowed to monitor each of the exposures. When the same average surface potential had been reached, the exposure was automatically terminated. The resulting prints with all of these variations were essentially identical and for practical purposes were indistinguishable. They were all developed with a standard magnetic brush toner, by magnetic :brush development.

As a second experiment, a color transparency was exposed through primary red, green and blue filters to make separations. The automatic control illustrated in FIGS. 1 and 2 properly corrected the exposure times and in the particular test being described, the exposure times turned out to be red, 5.3 seconds; green, 11.4 sec oncls; and blue, 29.5 seconds. Except for the differences due to the colors, the separation prints, after toning with the same developer, appeared to have about the same contrast and average density.

FIGS. 1 and 2 are merely for illustration of a suitable circuit, since any field meter may be used, and any of the many known circuits for operating a relay from the output of a field meter may be used without departing from the spirit of this invention.

In FIG. 2 essential features of one particular field meter 36 are illustrated. The input potential from a point or tap on the l-megohm resistor 28 is fed to two stages of amplification in tubes 50 and 51 (l2AU7) and the output of the last stage is impressed across a 110,000-ohm resistor 52. A voltage signal tapped off this resistor 52 is then fed to the grid of tube 54 (6C4) which is tuned by the tuned circuit 53, the output of which is fed to the control grid of 6AK6 pentode 55 Whose output current is sufficient through a transformer 56 and a rectifier 57 to give a substantial signal across a SOD-ohm resistor 37. From this point on, as shown in FIG. 1, there is further amplification to obtain sufficient current to operate the relay 20. The value of the impedance of the load resistor 37 is selected to match the particular electrometer or field meter being used.

In FIG. 3, the easel of the projection printer holding the charged photoconductive surface 15 is shielded by a shield 70. In this case, the sensing electrode 71 of the electrometer 72 is transparent. A different type of field meter is used, and hence the output is across a resistor 74 which is equivalent to resistor 37 of FIG. 1 but is selected to have proper impedance value. The potential at a selected point on the resistor 74 is fed to an amplifier tube such as 40 in FIG. 1. In FIG. 3, the sensing electrode 71 is placed within a fraction of a millimeter of the photooonductor surface and uniformly covers the whole surface. The field meter is maintained inoperative by closing the grounding switch 73 until the charged plate 15 is in position ready for exposure. Due to the charge on the photoconductor surface, charges of opposite polarity are induced on the sensing electrode while it is maintained at ground potential. When the switch 73 is opened the sensing electrode is ungrounded; the electrometer still indicates ground potential. The exposure is then started causing the charge on the photoconduc-tor to be effectively removed leaving an excess of charges on the sensing electrode so that its potential rises and this rise is indicated on the electrometer. When the potential changes a predetermined amount or reaches a predetermined value, the exposure is terminated.

It should be noted that the present invention requires the average of the field over a substantial area of image to be measured. Accordingly, the preferred embodiments of the invention expose the whole of the exposure areas of the photoconductor, at one time. Scanning exposure of this area complicates the operation of the present invention and is therefore to be avoided.

It should be further noted that the closing and opening of the relay contact 19 can be manual or relay contact 19 can be omitted. That is, the exposure is initiated manually and when the operator (reading the field value on the miliiammeter which is a part of commercially available field meters, in series with the load impedance 37) notes that the field has dropped to the optimum image value, he opens the

switch

19 or 18, terminating the exposure. The preferred embodiments are the automatic ones illustrated, however.

Having thus described the preferred embodiments of my invention, it will be understood that variations and modifications can be effected within the spirit and scope of the invention as described hereinabove and as defined in the appended claims.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. In a xerographic process for controlling the exposure of a substantially uniformly charged photoconductor, the steps which comprise: exposing said photoconductor image'wise while simultaneously measuring the changes in the electric field as charges are effectively removed in exposed areas, and terminating said exposing when the measured change in the electric field reaches a predetermined value.

2. In a xerographic process, the steps of uniformly charging to a fixed field value, in the absence of actinic radiation, the surface of a photoconductor, exposing said surface imagewise while simultaneously measuring the change in the electrical field as charges are effectively removed in exposed areas, and terminating said exposing when the measured field falls to a predetermined value.

3. In a xerographic process, the steps of uniformly charging, in the absence of actinic radiation, the surface of a photoconductor, placing a grounded electrode adjacent said photoconductor, ungrounding said electrode, exposing said surface imagewise while simultaneously meas- 7 uring the potential of said electrode as charges are effectively removed in exposed areas, and terminating said exposing when said potential has changed by a predetermined amount.

4. A projection printer for imagewise exposing a substantially uniformly charged surface of a photoconductor sheet comprising means for projecting a light beam onto said surface with an image in focus,

a field meter with its sensing electrode facing said surface but not obscuring said beam for measuring the average electric field adjacent to the surface, and means controlled by said field meter for terminating said projecting means when the field decreases to a value equal to that of an electrostatic image of approximately optimum quality.

5. A printer according to claim 4 including means for maintaining said field meter inoperative to measure said average field until said photoconductor sheet is ready to receive said exposure, and means for rendering said maintaining means ineffective and hence said field meter operative.

6. A printer according to claim 5 in which said maintaining means is an electric ground connected to said sensing electrode and disconnectable therefrom by said rendering means.

7. A printer according to claim 5 in which said maintaining means is a shield removably located between the sensing electrode and said surface and removable by said rendering means.

8. A printer according to claim 4 in which said sensing electrode is transparent and uniformly spaced from said surface.

9. A printer according to claim 4 in Which said sensing electrode is to one side of said beam and faces said surface obliquely.

10. A projection printer for imagewise exposing a substantially uniformly charged surface of a photoconductor sheet comprising means for projecting a light beam onto said surface with an image in focus,

an electrode adjacent to said surface,

means for grounding and ungrounding said electrode,

means for measuring the electric potential of said electrode, and

means controlled by said measuring means for terminating said projecting means when said potential has changed from ground a predetermined amount.

References Cited by the Examiner UNITED STATES PATENTS 2,297,691 10/1942 Carlson 96--1 2,781,705 2/1957 Crumrine et al -1.7 3,013,203 12/1961 Allen et al 324-32 3,251,685 5/1966 Bick'more 96--1 NORMAN G. TORCHIN, Primary Examiner.

J. TRAVIS BROWN, Examiner.

C. E. VAN HORN, Assistant Examiner.

Claims

1. IN A XEROGRAPHIC PROCESS FOR CONTROLLING THE EXPOSURE OF A SUBSTANTIALLY UNIFORMLY CHARGED PHOTOCONDUCTOR, THE STEPS WHICH COMPRISE; EXPOSING SAID PHOTOCONDUCTOR IMAGEWISE WHILE SIMULTANEOUSLY MEASURING THE CHANGES IN THE ELECTRIC FIELD AS CHARGES ARE EFFECTIVELY REMOVED IN EXPOSED AREAS, AND TERMINATING SAID EXPOSING WHEN THE MEASURED CHANGE IN THE ELECTRIC FIELD REACHES A PREDETERMINED VALUE.

4. A PROJECTION PRINTER FOR IMAGEWISE EXPOSING A SUBSTANTIALLY UNIFORMLY CHARGED SURFACE OF A PHOTOCONDUCTOR SHEET COMPRISING MEANS FOR PROJECTING A LIGHT BEAM ONTO SAID SURFACE WITH AN IMAGE IN FOCUS, A FIELD METER WITH ITS SENSING ELECTRODE FACING SAID SURFACE BUT NOT OBSCURING SAID BEAM FOR MEASURING THE AVERAGE ELECTRIC FIELD ADJACENT TO THE SURFACE, AND MEANS CONTROLLED BY SAID FIELD METER FOR TERMINATING SAID PROJECTING MEANS WHEN THE FIELD DECREASES TO A VALUE EQUAL TO THAT OF AN ELECTRSTATIC IMAGE OF APPROXIMATELY OPTIMUM QUALITY.