JPH0525112B2 - - Google Patents

Description

FIELD OF THE INVENTION This invention relates to an improved method and apparatus for controlling the level of electrostatic charge on a photoconductive surface on which an electrophotographic image is to be made.

Description of the Prior Art In known electrophotographic reproduction devices, such as copiers, an electrostatic charge is placed on an area of the photoconductor as it is moved past a charging location.
The photoconductor is then moved to an exposure location where the area is exposed to imaging radiation to form an electrostatic latent image of the document to be reproduced. This latent image is then developed and, in the case of a plain paper copier, subsequently transferred to a sheet of paper so that a reproduced image appears on the sheet. The photoconductor is then cleaned or otherwise prepared for the next copying process.

In such devices, it is important to provide a substantially uniform charge over the area where the latent image is to be formed. Too little charge in some parts of this area can cause areas on the copy to appear washed out, while too much charge in some parts of this area can cause areas on the copy to appear washed out. It may be too dark compared to other areas. Therefore, copy quality, especially for pictorial materials, can be seriously affected if a non-uniform charge is applied on the photoconductor.

The prior art describes devices for controlling the level of charge deposited on a photoconductor, as exemplified by US Pat. Nos. 4,105,321 and 4,248,519. In this device,
A voltage is applied to the corona wire to create a positive electrostatic charge on the photoconductive belt. The level of charge imparted to the belt is measured by an electrometer placed in close proximity to the belt, and the signal from this electrometer is compared to a reference signal representing the maximum level at which the belt is intended to be charged. . If there is a charge deficiency, the charge level is increased to the extent necessary to bring it up to the reference charge. If the signal obtained from the electrometer indicates that the charge exceeds this maximum level, erase it with the brightness necessary to reduce the charge placed on the belt to the desired level. The light source is illuminated.

It is known in the electrophotographic art to charge a photoconductor with a selected electrostatic charge, either positive or negative, based on the type of photoconductor used. Preferably, this charge is placed by a corona charging device consisting of one or more corona wires transverse to the direction of travel of the photoconductor through the charging station. If positive corona charging is used, it is expected that known charging techniques can be used to impart a nearly uniform charge on the photoconductor. Negative corona charging is more difficult to control and can result in significant charge non-uniformity. A glow may be observed from the positively charged corona wire and a uniform glow may be observed surrounding the entire length of the corona wire, indicating uniform current flow from the corona wire to the photoconductor.
On the other hand, discrete glow points often occur along negatively charged corona wires. This glow point is related to the non-uniformity that occurs in the charging of the photoconductor. The charge non-uniformity changes over time, as glow points appear at different locations along the corona wire during the day due to changes in humidity within the copier or other factors. “Electrophotography” by M. Shafuart
1975 edition, pages 466-472 (RM Schaffert,
Electrophotography, 1975 edition, pages 466
-472). ) Small amounts of non-uniformity can be tolerated, but as previously shown, large changes in non-uniformity will have a significant impact on image quality. The extent of this problem depends on the nature of the equipment and the material to be copied. Clearly, continuous tone or halftone originals pose more problems than text reproductions. Color copiers require more uniformity in charging than monochrome copiers.

It is an object of the present invention to ensure that large changes in charge level occur due to the charging source used or other operating conditions such that charge is deposited on a surface in a direction transverse to the direction of movement of the surface that would otherwise pass through the charging source. The objective is to provide an imaged area of the photoconductor that is generally uniformly charged, especially if there is a tendency for this to occur. As used herein, the term "generally uniform charge" means that changes in charge level may occur from area to area, but the various parts that make up the area may vary within a desired narrow range. It has a level of charge that enters the value.

Another object of the invention is to control the imaging exposure of a charged photoconductor having a charge level different from a desired reference level such that the imaging exposure compensates for the change in charge from the reference level. It is to be.

SUMMARY OF THE INVENTION It is an object of this invention to provide an improved electrophotographic method for controlling the charge on a movable photoconductive member that is electrostatically charged by a charging device to form a developable electrostatic image. This is accomplished by a copying method and apparatus. The invention relates to an apparatus for measuring the level of charge on a photoconductive member at each of a plurality of positions transverse to the direction of movement of the member and generating a signal representative of the level; The present invention is directed to an adjustment device that can responsively differentially adjust the charge level at lateral positions having a different level of charge than a reference level.

The invention further provides for exposing the photoconductive member with a narrow beam of radiation modulated with imaging information to form an electrostatic latent image, and in doing so, generating one or more data regarding the distribution of charge on the photoconductor. A method and apparatus for converting a signal and also modulating said narrow beam in an electrostatic latent image forming area during exposure to compensate for charge levels on a photoconductor that differ from a reference level. There is.

[Brief explanation of drawings]

The following detailed description of selected embodiments of the invention is made with reference to the accompanying drawings.
In the drawings, FIG. 1 is a schematic side elevational view of a copying machine equipped with an apparatus according to the invention, and FIG. 2 is oriented toward the location of a generally uniform charge on a photoconductor. 3 is a schematic diagram of the portion of the copier shown in FIG. 1 shown in FIG. Figures 4a-c and e are flowcharts illustrating hypothetical examples of charge level readings across a photoconductor to facilitate understanding of the apparatus and method of the present invention. 4d is a diagram representing the illumination of lamps according to a hypothetical example, and FIG. 5 is a schematic diagram showing an arrangement of lamp groups that may be used in accordance with the invention. and FIG. 6 is a diagram similar to that of FIG. 2 showing another embodiment of the invention, and FIG. 7 is a control for controlling the parts of the copier shown in FIG. 9 is a flow diagram illustrating the sequence of operations used by the apparatus; FIG. 8 is a schematic side elevational view of a further embodiment of the invention; and FIG. 2 is a schematic diagram showing additional details of elements forming a charge control device for an embodiment; FIG.

DESCRIPTION OF SELECTED EMBODIMENTS As devices of the type described herein are well known, the following description will be directed specifically to those elements forming part of or cooperating more directly with this invention.

For a general understanding of the web electrophotographic reproduction apparatus 10 with which this invention is useful, reference will now be made to FIG. As shown, a photoconductor member in the form of a photoconductive web 16 is movably adjusted about rollers 4-9 in the direction indicated by arrow A. As shown in FIG. Roller 4 is driven by a drive mechanism 18, shown for simplicity as consisting of a motor-pulley arrangement. Insulating layer or side 1 of web 16
6a is charged at a corona charging station (charger) 20. Charger 20 consists of one or more corona generating wires 20a, a shield 20b, and a grid 20c for regulating the flow of negative corona current from the wires to the photoconductor member. At an appropriate time thereafter, an information medium 13, such as a document, is irradiated with radiation from a flash lamp 14 at an image exposure station. This radiation is reflected by the information medium and projected onto the charged insulating surface 16a of the web 16 by the lens 15, selectively dissipating the charge and directing the medium 13 to specific areas of the web.
forms an electrostatic latent image. For a more detailed disclosure of Webb, see commonly assigned US Pat. Nos. 3,615,406 and 3,615,414, issued October 26, 1971. A plurality of light sources 56 and an electrometer are arranged between the exposure station and the charger 20, as will be explained in detail later.

The apparatus 10 further includes a moving electrostatic image that is brought into contact with finely divided toner particles that adhere to the charged web surface in a configuration defined by the electrostatic image to form a visible toner image. a developer station 22 for forming, a transfer station 25 where the toner image is transferred to and subsequently permanently fused to the receiving surface of the copy sheet 26, and a cleaning station 31 where residual toner particles are removed from the web 16. There is.

At the development station, the insulating surface 16 of the web 16
The electrostatic image on a is passed through two magnetic brushes or rollers 24a and 24b mounted within the housing 27 of the developer station 22. Housing 27 holds a supply of developer containing a mixture of toner and carrier particles. Brush 24
a and 24b may be constructed according to any one of a variety of designs known in the art. For a more complete description of the general construction of a similar reproduction device, reference may be made to commonly assigned U.S. Pat. No. 4,025,186.

Although a web-type copier is shown, it will be appreciated that the present invention is also particularly suitable for reproduction apparatus using drums and thin film photoconductors. In either case, as will be understood by those skilled in the art, a microcomputer with a built-in program can be effectively used as a logic and control device to control the operation of the copier.

Turning now to FIG. 2, there is shown a schematic representation of a device constructed in accordance with the present invention.
The electrometer or electrostatic voltmeter 50 is conventional and includes a power supply, amplifier and output circuitry, and a probe 50b mounted on a rotatable lead screw 52.
The module 50a includes a module 50a. When the step motor 54 is energized, the lead screw 5
2, the lead screw translates probe 50b in the transverse direction indicated by arrow X across the width of web 16. Charger 20 and electrometer probe 50b
A plurality of light sources 56 are arranged between the two. Each of these light sources emits light of an appropriate spectral frequency, which is sensitive to the photoconductor, causing the exposed portion of the photoconductor to become partially conductive such that the charge level of the exposed portion is at least partially reduced. reduce
These light sources may consist of a plurality of variable intensity lamps, each of which transmits light to a respective light tube that transmits light to a respective lateral position on the web 16. Other light sources that can be used include plasma displays, LED arrays, digitally controlled electroluminescent plates, lasers or halogen lamps or neon lamps with holes in a PLZT crystal, and There is a long arc lamp. The selected embodiment will be described with respect to a panel display. An example of such a panel display is the Burroughs Self-Scanning Panel Display, model SSD0124-0039 (Burroughs SELF-
SCAN Panel Disploy, Model SSD0124−
[0039] which will be explained in more detail below.

The apparatus shown in FIG. 2 will be described with respect to the hypothetical example shown in FIGS. 4a-e. In FIG. 4a, the nominal or desired charge level Vo for the photoconductor, for example -550 volts, is shown as a straight line. In the lateral direction "X" the actual Vo in this example is shown by the curve "C", which has some variability around this nominal level due to charge non-uniformity for the reasons mentioned above. I have it.
A certain amount of variation from Vo, for example ±10 volts, is acceptable in this example, so this range of variation is designated as the uniformity band. For simplicity, we will discuss charge levels in terms of absolute values or magnitudes, so a charge level of -550 volts is considered higher than a charge level of -540 volts in this case. Depending on the situation, this band may be wider or narrower, or perhaps even have no width, but the higher and lower levels are the same with respect to the nominal Vo.

It will be noted that some portions of the photoconductor have charge levels outside this band.
In accordance with the present invention, charger 20 and light source 56 are adjusted to vary the charge level of photoconductor 16 so that it falls within a selected uniformity band.
See Figure 4e.

1 per day before the copier is placed in copy mode.
Once or twice, or more often if necessary, a calibration operation is performed to ensure that the actual charge level V ₀ is within the uniformity band. During this calibration,
Adjustments are made to the charger 20 and the light source 56 using the method described below. After proofreading, the reproduction apparatus can be operated in a copy mode to perform the process of producing a copy. It will be appreciated that when the operator presses switch 45 to begin calibration, an interrupt signal is provided to microcomputer 47, which also controls the operation of copying machine 10. Prior to pressing switch 45, signal V ₁ was applied to summing port 48 during normal machine operation initiation, thereby causing low voltage control grid power supply 49
is set to condition the charger 20 so that the charger produces a nominal charge level V ₀ of, for example, −550 volts. As shown in Figure 4a,
The actual V ₀ placed on the photoconductor will differ from this nominal level. Grid-controlled programmable power supplies are well known in the art and are described, for example, in U.S. Pat.
No. 4,166,690 and US Pat. No. 4,294,536.

In the calibration mode, microcomputer 47 provides commands in the form of a series of pulses to motor 54. Motor 54 is then actuated to move electrometer probe 50b across web 16. For a particular example, as shown in Figure 4b,
For example, there are eight transverse positions for an image area approximately 35 cm wide. At each position (determined by the number of pulses sent to the motor), the microprocessor operates an analog-to-digital converter 57, which converts the specified It provides a digital representation of the charge level at the transverse position, which is stored in the memory at a predetermined one of eight positions. Preferably, the electrometer remains in each position long enough for the photoconductor to make one complete revolution past the electrometer. This allows the electrometer to give an average reading for that particular narrow strip of photoconductor. After the first specific transverse position charge level is detected and stored in memory, measurements are made for each of the next seven positions. FIG. 4b shows the digitized signal level. After all eight positions have been measured and stored, the microcomputer 47 takes these digitized readings by direct measurement of V ₀ or from a low grid power supply having a known relationship to the nominal V ₀ . Compare with the nominal level V ₀ detected by using the voltage V ₁ . This can be done by converting the analog voltage V ₁ to digital form stored in the computer's memory by an analog-to-digital converter 62.
A nominal V ₀ value is then calculated or otherwise derived, and this value is compared to each of the eight electrometer measurements of the charge level at each traversal position. If the charge level at any traversal position is measured to be lower than the lower limit of the uniformity band, the computer increases the charge level at the traverse position with the lowest charge level to bring this charge level below the uniformity band. Determine the minimum additional charge required to bring it to the lowest value for . The signal V ₂ representing the required additional charge is then digitally
It is provided by analog converter 64 which responsively provides an offset signal to summing port 48 to adjust the voltage applied to the grid of corona charger 20. Conveniently, the computer operates the electrometer probe 50b until the charger is adjusted as shown in Figure 4c so that the lowest measured charge level is just within the lower limit of the uniformity band. One or more additional intersections may be programmed. After the last crossing determines that the charger is properly adjusted, the computer also has data as to whether the charge level at any crossing position exceeds the upper limit of the uniformity band. . If the charge level at any position is measured to be greater than the upper limit of the uniformity band, the computer determines the difference between the charge levels at each position above, for example, the midpoint of the band or the nominal V ₀ ; A signal proportional to this difference is provided to lamp controller 70. A lamp controller represents a control module configured to turn on a particular number of lamps associated with a particular traversal position immediately above it. FIG. 5 shows an example of a possible arrangement of lamp groups which, when brought together and offset as shown with respect to the direction of movement of the photoconductor indicated by arrow A, will illuminate the active area. It consists of three panel display devices mounted so as to span the width of the conductor 16. Each of these displays has 17 rows with 192 gas plasma display lamps 56a in each row. Of the 17 rows of indicator lamps in each column, the lamps are selected to be illuminated to erase charge from each transverse location on the photoconductor where that column resides. For example, the first
One or more lamps in the first row and column may be illuminated to remove any excess charge (above the nominal V ₀ ) from the first transverse location on the photoconductor. If more overcharge (beyond the nominal V ₀ ) is present at this location, the charge can be reduced to the nominal V ₀ by illuminating each of the lamps in one or more rows up to 17 in the first column. good. Since each transverse location on the photoconductor where a charge level measurement is to be made has a group of lamps associated with a designated column, one or more of these lamps is illuminated row by row to illuminate each column's lamps. The charge at the transverse photoconductor location may be reduced. Since the illumination from the lamps tends to spread out, it is desirable to avoid illuminating lamps close to the ends of each row to avoid light from lamps in one row affecting adjacent locations.
It is therefore desirable to use only those lamps near the center of the row, using the tendency of their light to spread out to substantially cover the width of each location.
The level of illumination emitted by each lamp is preferably preset to provide fine control over the charge reduction effectiveness of each lamp. Therefore, filters may be placed in front of the lamps and/or the current to the lamps may be adjusted so that each lamp erases only a small amount of charge.

Returning now to the example shown in FIG. 4, lamp controller 70 responds to signals from the computer by:
Illuminate the number of rows of lamps in each column needed to reduce the charge at each location to a nominal V ₀ . The charge at each overcharged location is therefore differentially reduced according to the difference between the charge level measured at that location and the reference value, in this case the nominal value V ₀ . This reference value may also be the upper limit of the uniformity band.

As shown in FIG. 4d, only the lamps corresponding to transverse positions on the photoconductor that are higher than the nominal level V ₀ are illuminated, and the charge at the corresponding transverse photoconductor position is brought to the nominal V ₀ level. Only a decreasing number of rows are illuminated. After the corresponding lamp has been illuminated, additional measurements are taken by the electrometer at each transverse position on the photoconductor, each measurement being at the nominal V ₀ at any position.
Compare again to the nominal V ₀ to ensure that no larger charge level was measured. If any location still had a larger charge level, the computer would tell the lamp controller that the number of lamp rows illuminated for each traversal location should be increased based on this new measurement. Inform. By adjusting the charger so that the lowest measured charge level is just within the lower limit of the uniformity band and turning on the appropriate erase lamp, as shown in Figure 4e, All traversal positions of fall within the uniformity band. Now that the charging device is calibrated,
This can be used in copying mode by illuminating the appropriate charge erase lamp and adjusting the charger appropriately. As noted from FIG. 3, the process of adjusting the charger or charge reduction lamp after taking a reading from the electrometer is an iterative process. That is, the cyclic process of taking measurements and adjusting the charger or lamp is repeated based on the final measurements until the desired result is obtained.

If desired, the charger is first adjusted so that this is never likely to overcharge the photoconductor and thus have any transverse position below the minimum of the uniformity band. Good too. In such a case, a controller to provide an offset would not be needed, and only a controller directed to canceling the charge would be useful.

Although a single movable electrometer is shown in the discussion of the selected embodiment, it is possible to provide a plurality of stationary electrometers and place them across the width of the photoconductor and read each of the electrometers. It will be appreciated that suitable circuitry may be provided for taking the time order.

Reference will now be made to a second embodiment of the invention shown in FIGS. 6 and 7. In this embodiment, the same elements as described with respect to the first embodiment of the invention are identified with the same reference numerals. Since these same elements function in the same manner as previously described, discussion of this embodiment need only be directed to the differences between the operation of this embodiment and the first embodiment. In the second embodiment, a needle charging station 3 is used instead of the charge erasing lamp in the embodiment of FIG.
5 is used. In the charging station 35, a plurality of rigid needle-like charging elements A-H are arranged across the direction of movement A of the photoconductor 16. During the calibration phase of the device, both the primary charger 20 and the needle chargers A-H are turned on so that these charging elements emit a corona current. In this embodiment, both charging stations emit a negative corona current onto the photoconductor. Electrometer probe 50a is now moved in the X direction across the photoconductor to measure the charge level at each of eight positions across the width of the photoconductor. The electrometer module 50b then supplies these analog signals to an A-to-D converter 57, which in turn supplies the digitized values of these signals to the microcomputer 47. Besides storing these values, the microcomputer also stores in memory the grid supply reference voltage V ₁ and calculates the limit values of the desired uniformity band. Now with reference to FIG. 7 and the discussion that immediately follows, the terms shown in parentheses are intended to describe alternatives. This alternative is illustrated by substituting the word in parentheses in each case for the word immediately preceding it. If the electrometer 50 readings indicate that all locations have charge levels lower (higher) than the lower (upper) level of the uniformity band, then the charge output of the primary charging location 20 is Apply a signal to summing port 48 and combine it with signal V ₁ to determine the grid voltage.
It can be increased (decreased) by increasing (decreasing) V ₀ . The electrometer again takes measurements at each of the eight locations to determine whether all locations have a charge level lower (higher) than the lower (upper) level of the uniformity band. This process is repeated iteratively until this condition is met. Once this condition is met, the microcomputer then determines how much additional charge is needed from the needle charger to ensure that all traverse locations are at charge levels that fall within the desired uniformity band. Determine whether Each of the charged needles is adjusted differentially with respect to each lateral reading by the electrometer. Repeatedly adjusting the voltage at needle charger 35 after successive additional readings by the electrometer brings the charge level at each location under the needle charger to a level within the desired uniformity band. It will be done. A needle charger controller 36 connected to the microcomputer stores the determined voltage setting for each charging needle A through H and maintains this setting during the copy mode of the copier. Although this embodiment has been described for eight charged needles, one for each measurement transverse position, more needles can be placed across the width of the photoconductor and grouped together, e.g. It will be appreciated that the four needles may be at the same electrical potential and used to adjust the charge at each of the eight transverse positions. This is the case if the effect of each needle is limited to adding charge along a narrow band below this needle, and if these bands, say four, are equal in width to each band making up the transverse position. may occur, so this would be desirable.

The effect of the charge conditioning device, sections 56 and 35, described in the first and second embodiments is to differentially adjust the non-uniform charging by the corona charging station 20 to produce a generally uniform composite on the photoconductor. The idea is to give it a charge. That is, in the embodiments described above, a generally uniform electrostatic charge was formed over the area where the electrophotographic image was to be formed by selective enablement of a charge reduction device or a charge increase device. This invention further relates to electrophotography technology,
Since both the charge reduction device and the image exposure device described herein involve the use of light, it is contemplated that the equivalent would be the use of the charge reduction device during the exposure operation. For example, if the imaging exposure is done by using a narrow beam,
The intensity of the beam or the time spent at any "point" may be adjusted according to the information obtained from the electrometer and the source of the image information.

In this regard, reference will now be made to FIGS. 8 and 9, in which a scanning exposure type electrophotographic apparatus is shown. Parts designated with a prime (') in these figures correspond to like-numbered parts shown in FIGS. 1 and 2 having similar functions. In this embodiment, a photoconductor 16', exemplarily shown mounted on a drum, is provided with a charging station 20' consisting of a negative corona current discharge device and a grid suitably spaced from the photoconductor.
It rotates through the air, and is electrically charged. The exposure station, which will be described in more detail below, has an exposure beam 80 reflected from a rotating polygon mirror 81. A suitably image-modulated beam traverses the scan width of the photoconductor in synchronization with the drum to form an electrostatic image on the photoconductor. The photoconductor then passes through a conventional development station 84, which may be of the cascade type shown or of the type shown in FIG. At this station, toner particles are deposited on those portions of the photoconductor that have not been sufficiently discharged by the exposure beam to develop the latent image. The developed image is then transferred to a web of copy paper that is fed into contact with the photoconductor. The copy sheet receives a charge from the electrostatic discharger 86, resulting in transfer of the developed image from the photoconductor to the copy sheet. Copy sheets are supplied from supply reel 88 and advanced by drive rollers 90 around guide rollers 90 to receiving box 94 . Fuser 93 fuses the image to the copy sheet as it is fed to box 94.

A usable image is provided because the information content of the scan point is represented by a modulated or varying intensity of light with respect to its position within the scan width. The scanning point dissipates static charge according to its light intensity as it traverses the charged surface of the photoconductor at a given scanning angle. The electrostatic charge pattern thus generated is developed and then transferred to a copy sheet as described above. The photoconductor is then cleaned by cleaning device 98 and then transferred to charging device 2 in preparation for the next copying process.
Recharged by 0'. The polygon mirror 81 is the motor 1
00 and is rotationally synchronized with a synchronization signal representing the scanning speed used to obtain the image signal of the document, which signal is referred to as the "page storage device" in FIG. The device 95 displayed as
It can be stored in binary form in a binary storage device denoted by .

The beam also has "charge uniformity vs. beam intensity correction"
It is modulated by a signal representing charge control information stored in binary form in a storage device 91 labeled . This information causes probe 50b' to move across the width of the photoconductor and provide a reading of the voltage level of the photoconductor charged by charger 20' at a particular traverse position of the photoconductor. This is obtained by means of an electrometer 50' provided therein. As shown for the embodiments described in FIGS. 1 and 2, the microcomputer 47' can be programmed to take these readings by means of an electrometer during preparation for operation of the copier or when the copier is in its original state. If so, you can create them at certain times during the day when you are not in copy mode. The reading of probe 50b' is detected by electrometer module 50a' and transducer 5
7' converts the analog signal into a digital signal. The microcomputer 47' then
Determine whether there are any locations that have a charge level below the lower limit of the uniformity band as previously discussed for the embodiments of FIGS. 1 and 2. If additional charging is required, the grid voltage source 49' is adjusted until repeated electrometer measurements determine that all traversal positions are charged at least to the lower limit of the uniformity band. . The final set of measurements used to calibrate the charger is then used by the computer to determine which of the eight positions require compensation due to the presence of excess charge. An adjustment value for the intensity of the exposure beam is calculated for. These adjustment values are stored in the storage device 91.
stored in digital format. As can be noted in FIG. 9, the signal representing the optical information from page store 95 is combined in multiplier 96 with the signal representing the charge adjustment value. The resulting signal is then converted to an analog signal by a digital-to-analog converter 97, and the analog signal is coupled to an acousto-optic modulator 99 by beam forming optics 105. Modulate the beam focused on. The source of this beam is, for example, a helium-neon laser source 1
06. The zero-order undiffracted beam is absorbed by a beam stopper, and the first-order diffracted beam modulated with optical information and charge control information passes through a beam expander 101 and a filter 102 and is reflected by a polygon mirror 81. Visual field flattening optical system 10
7 and onto the photoconductor 16'. Optical sensor 1
08 is placed near one edge of the photoconductor to detect the light reflected from the polygon mirror 81 at the beginning of each page row. Signals from this sensor and from the shaft encoder 103 which detects the rotational position of the drum are fed to a microcomputer 47' and are used to synchronize the movement of the drum with the scanning exposure.

In the embodiment just described, the laser beam is modulated at a number of discrete points across the width of the photoconductor for each scan line. For each such point, modulation requires input consisting of optical information and charge control information. If necessary, the charge control information may be changed to compensate for other factors, such as variations in the surface reflectance of polygon mirror 81. When copying a blank area of an original document, the modulated beam is at an intensity, defined here as the total exposure intensity, which reduces the charge level in the corresponding area of the photoconductor below which the photoconductor Reduce toner to such a level that it does not adhere to such areas of the body. The area of the photoconductor corresponding to the image information is not properly discharged or its charge is changed according to the signals from the "page storage" device and the "charge uniformity versus beam intensity correction" device. , the charge in this area forms an electrostatic latent image that is developed as it passes through development station 84. The signal from the ``Charge Uniformity vs. Beam Intensity Correction'' device is the charge level detected by the electrometer 50' at a particular location and the value is stored in the memory of the microcomputer. the desired charge level, as exemplified by "bandwidth".

Generally, exposure of the photoconductor with a laser beam is accomplished during step-by-step scanning of the photoconductor to provide contrast in the finished print. Bright areas in the print are created by giving a complete exposure of many or all points in the corresponding area of the photoconductor, and gray areas are created by giving a complete exposure of only some points in the corresponding area of the photoconductor. The black areas can be created by applying a light exposure or, more preferably, by applying an appropriate amount of exposure to each point proportional to the desired density in the final print, and the black areas are Created by exposing points in an area to light. Complete exposure of any point on the photoconductor by scanning the laser beam can reduce the charge level at that point to a level below which toner will not adhere, so that the initial charge at that point is nominally V Whether it is higher or lower than ₀ is usually not important. What is important is that points in the image areas that receive no or little image light are charged to V ₀ so that they are properly developed. It is also important to determine the charge level of points in the image area that correspond to the gray areas of the print and should be charged to a voltage level lower than V ₀ according to the desired density for this area using density-image information and charge correction information. and shall be changed accordingly. With this in mind, at each point where full exposure is required by the information stored in the page store, the multiplier provides a signal representative of this information to converter 96 and AOM 99 and suppresses the charge correction information. The multiplier can be configured as follows.
At each point where exposure is required by the page storage device, a multiplier provides a charge correction information signal to modulate the laser beam to a level below its full exposure level, increasing the charge at such point on the photoconductor. Adjust the level to a level within the uniformity band. To the extent that some exposure, but less than a full exposure, is required by the page storage device, the multiplier combines the page storage device information signal and the charge correction device information signal to modulate the laser beam in such a manner. to the appropriate level for the desired density.

Until now, Figs. 8 and 9 regarding laser beams have been used.
Having described the illustrated embodiment, it is recognized that other apparatus are known for forming a narrow beam of light modulated with image information for exposing an image area on a photoconductor. It will be done. These devices, such as light emitting diode (LED) arrays that extend across the width of the photoconductor,
The LED can be modulated with image information and charge control information to compensate for variations in charge level from a reference level during imaging exposure operations.

Many sources of charge non-uniformity are time dependent. Therefore, it may certainly happen that the illumination characteristics determined at the beginning of the day are not suitable for the desired uniformity at the end of the day. Many control algorithms can be used to automatically cause the copier to enter calibration mode from copy mode. For example, depending on the amount of non-uniformity that the copier can tolerate, it can be calibrated once a day, twice a day, etc. without requiring the operator to initiate the calibration mode. mode. For example, the calibration mode may be entered automatically during preparation of the copying machine or between the start of a job and the first copy.

Claims (1)

Claims: 1. A photoconductive member having an area on which a developable electrostatic image is to be formed; means for moving the photoconductive member; charging means for electrostatically charging said region of the photoconductive member; said charging means charging at a plurality of locations in said region transverse to the direction of movement of said photoconductive member; means for detecting different charge levels on said photoconductive member and generating a signal indicative of the detected charge level; at a plurality of locations in the region transverse to the direction of movement of the photoconductive member to provide a charge level;
control means for adjusting the charge level to form an electrostatic image on the area of the photoconductive member in response to a signal indicative of an image to be formed on the photoconductive member; An electrophotographic copying apparatus comprising: an exposure means for exposing the photoconductive member having the photoconductive member; and a means for developing the exposed electrostatic image.