COPY CONTRAST AND DENSITY CONTROL

Abstract: This is a method and apparatus for controlling the contrast, density and solid area development of copies produced by an electrophotographic copier. The problem addressed is in making a straightforward selection of a set of interrelated values of the photoconductor charge level voltage V<uo>u, the exposure setting E<uo>u and the development electrode bias voltage V<uB>u to obtain desired Density in/Density out response curves for the copier. A matrix of sets of values are stored in a memory (36), a particular set is addressed by manual selectors (70, 72) and those values are inputed to a logic and control unit (31) which controls the various machine characteristics to obtain the desired response curve.

Description: __ _τ COPY CONTRAST AMD DENSITY -CONTROL, The present invention relates to electro¬ photographic copiers and, more particularly, to a method and apparatus for optimizing the contrast, 5 deήέity and solid area development of copies produced by such copiers. The electrophotographic reproduction process is well known. Briefly, this process comprises the steps of (1) uniformly charging a photoconduetive ^0 recording element to an initial voltage level V , (2) imagewise exposing the charged recording element to an image of a document to be reproduced, such exposure E being sufficient to selectively dissi¬ pate the uniform charge on the recording element to ] _5 leave behind a latent electrostatic image, and (3) developing the latent image by ' applying electroscopic toner particles thereto. Typically, the toner par¬ ticles are applied in the presence of a development electrode (e.g. a biased magnetic brush) which is 20 biased to a voltage level VT, so that it assrs-ts in the development of "solid" areas of the latent elec¬ trostatic image. See, for example*, U.S. -Patent No*. 2,573,881 issued to alkup. It is also well known that : -the contrast and 25 dens ity of image's produced by the e-l^o^ropbotttgra-pbic process can be adj usted by controlling the -lev : el (Si uniform charge V initially appli&d ' to the r-fectfϊii- ing elenient and /or the exposure level ~Ε . -So, too , is it " known that -the -extent ' of sol-id are-a de lopment 30 can be controlled by adjusting ■• he -bf-ss -v ' lt a ' ge At***, applied to the development "el ctrb-d-e . Varying the bias voltage also provides soϊή ' e -c ntrol ov r i Hpy contras t and density , especial ly -thfe -m ni ' mύm copy dens i ty . 35 ' S ince the parame-ir'er-s V- , E -arid V g - are interrelated -a d ch-ahges in th-e * val- -e ^o ' f ' " one " c-a ' n ' have -Z- a relatively dramatic effect on copy quality, copier maη.y : f' ' act/ύrers ' "-'h v-i^he etofore elected to provide ϊϊ-Be copier operator "' «rϊ" h only limited control over "the - settings of these "parameters. Usually, the manϋf * a ' c- 5 turer presets these parameters at the factory ' o that, for a given recording element and tone , -the copier produces copies having nominally acceptable contrast, density and solid area development 'tor "normal" originals. To provide the operator with 10 some means for accommodating originals of unusually high or low contrast or density, most copiers include a single selector knob or push button control to "lighten" or "darken" copies. While this relatively crude control suffices for most office uses, electro- 15 photographic copiers are being found in ever- increasing numbers in print shops where extremely high quality reproductions are expected. It would be desirable, in such applications, to provide the operator with a relatively simple means for "fine 20 tuning" (i.e. optimizing) the copy quality for a wide O P *3- ,.,.-. ' ..levels of V . E Λ and Vr, in .accordance ,wittκ r ,- ..-..three values of the designated, set.- _. , . .-..-,- _ _- . - , The apparatus of the ..invention includes .memory means having stored thereon -the above- 5 ~ -_mentioned matrix selector means for addressing the c _. - ..matrix to designate a desired set o three values, . and logic and control means respons.ive to the desig¬ nated set to adjust Vo, Eo and V- D-, in accordance with the values of the designated set. The apparatus 10 . of the invention can accommodate a wide variety of sets which, from time to time, can be adjusted, cor¬ rected or updated to insure consistent line and solid area development, regardless of the age of the toner and recording element, and the attendant changes in 15 the properties of these components. DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic showing a side eleva- tional view of an electrophotographic copier in accordance with the invention; 20 Fig. 2 is a block diagram of the logic and control unit shown in Fig. 1; Figs. 3-9 set forth graphs which illustrate typical D TN /D 0UT response curves for the copier and a recirculating document feeder. Whenever the term "document" is used, it refers to a sheet having an image to be copied. The term "copy" refers to the output of the copier such as a copy sheet having a fixed toner image. Referring now to Fig. 1, a recirculating document feeder 50 is positioned on top of an expo¬ sure platen 2 of a copier 1. The feeder 50 includes feed rollers 51 which transport a document S across the exposure platen* 2. The platen 2.is constructed of transparent glass. When energized, two xenon flashlamps 3 and 4 flash illuminate the document S. For a specific disclosure of a typical exposure station, see commonly assigned U.S. Patent No. 3,998,541, issued December 31, 1976. By means of an object mirror , 6, lens system 7, and an image mirror 8, an image of the illuminated document is optically stopped on discrete -image areas of a.moving recording element, shown in the form of an endless photoconductive web 5. The photoconductive web 5 includes a photo- conductive layer with a conductive backing on a poly¬ ester support. The photoconductive layer may be formed from, for instance, a heterogeneous photocon¬ ductive composition, such as disclosed in commonly assigned U.S. Patent No. 3,615,414, issued October 24, 1971. The endless web 5 is trained about six transport rollers 10, 11, 12, 13, 14, and 15. Roller 10 is coupled to a drive motor M in a conven¬ tional manner. Motor M is connected to a. source of potential when a switch SW is closed by a logic and control: unit (LCU) 31. When the switch SW is closed, the roller 10 is driven by the motor M and moves the web 5 in a clockwise direction as indicated by arrow 16. This movement causes successive image areas of the web 5 to sequentially pass a series of electro¬ photographic work stations of the copier. For the purpose of the instant disclosure, several copier work stations are shown along the web's path. These stations will be briefly described. For more complete disclosures of them, see commonly assigned U.S. Patent No. 3,914,047. First, a charging station 17 is provided at which the photoconductive surface 8 of the web 5 is sensitized by applying to such surface an electro¬ static charge of a predetermined voltage. The station 17 includes an A.C. charger shown as a three wire A.C. charger. The output of the charger is con¬ trolled by a grid 17A connected to a programmable power supply 17B. The supply 17B is in turn con¬ trolled by the LCU 31 to adjust the voltage V applied onto the surface 9 by the .charger 17 in accordance with a selected set-point number as will be described later. For an example of digital regu¬ lation of a corona charger,*- see U.S. Patent No. 4,166,690. In a specific embodiment of the invention, the grid voltage was adjusted about a nominal value of -500 volts with a 600 hertz A.C. square signal applied to the corona wires. At exposure station 18, the inverse image of the document S is projected onto the photoconductive surface 9 of the web 5. The image dissipates the electrostatic charge at the exposed areas of the photoconductive surface 9 and forms a latent electro¬ static image. A programmable power supply 18A, under the supervision of the LCU 31, controls the intensity or duration of light incident upon the web 5 to adjust the exposure E by the lamps 3 and 4 in accordance with a selected set-point number as will be described later. For a specific example of such an exposure station and programmable power supply, see commonly assigned U.S. Patent No. 4,150,324, issued August 8, 1978 to Seil. A dual magnetic brush developing station 19 includes developer, having iron carrier particles and electroscopic toner particles with an electrostatic charge opposite to that of the latent electrostatic image. The developer is brushed over the photocon¬ ductive surface 9 of the web 5 and toner particles to adhere to the latent electrostatic image to form a visible toner particle, .transferrable image. The dual-magnetic brush station 19 includes two rollers, a transport- roller 19A, and a developer roller 19B. As is well understood in the art, each of the rollers 19A and 19B include a conductive applicator cylinder which may be made of aluminum. In the disclosed embodiment, conductive portions, such as the drive shaft and applicator cylinder of the transport roller 19A, acts as an electrode and are electrically connected to a source of fixed D.C.* potential, shown as a battery 19C. Conductive portions of development roller 19B also act as an electrode and are electrically connected to a programmable supply 19D controlled by the LCU 31 for adjusting V β in accordance with a selected set-point number as will be described later. For a specific disclosure of a dual magnetic brush which can be used in accordance with the invention, see commonly assigned U.S. Patent No. 3,543,720. See commonly assigned U.S. Patent Nos. 3,575,505, 3,654,893, and 3,674,532 for disclo¬ sures of biasing development station rollers. The copier 1 also includes a transfer station shown as a corona charger 21 at which the toner image on web 5 is transferred to a copy sheet S 1 ; and a cleaning station 25, at which the photoconductive surface 9 of the web 5 is cleaned of any residual toner particles remaining thereon after the electroscopic images have been transferred and is o:.:?ι " discharged of any residual electrostatic charge remaining thereon. As shown in Fig. 1, a copy sheet S' is fed from a supply 23 to continuously driven rollers 14, (only one of which is shown) which then urge the sheet against a rotating registration finger 32 of a copy sheet registration mechanism 22. When the finger rotates free of the sheet, the driving action of the rollers 14 and sheet buckle release cause the sheet to move forward onto the photoconductor in alignment with a toner image at the transfer station 21. After transfer of the unfixed electroscopic images to a copy sheet S', such sheet is transported to fuser 27 where the image is fixed to it. To coordinate operation of the various work stations 17, 18, 19, 21, and 25 with movement of the image areas on the web 5 past these stations, the web has a plurality of perforations along one of its edges. These perforations generally are spaced equi- distantly along the edge of the web member 16. For example, the web member 5 may be divided into six image areas by F perforations; and each image area may be subdivided into 51 sections by C- perfora- tions. The relationship of the F and C perforatons to the image areas is disclosed in detail in commonly assigned U.S. Patent No. 3,914,047. At a fixed loca¬ tion along the path of web movement, there is provided suitable means 30 for sensing F and C web perforations. This sensing produces input signals into the LCU 31 which has a digital computer, preferably a microprocessor. The microprocessor has a stored program responsive to the input signals for sequentially actuating then de-actuating the work stations as well as for controlling the operation of ' many other machine functions as disclosed in U.S. Patent No. 3,914,047. Programming of a number of commercially available microprocessors such as in INTEL model 8080 or model 8085 microprocessor (which along with others can be used in accordance with the invention) , is a conventional skill well understood in the art. The following disclosure is written to enable a program¬ mer having ordinary skill in the art to produce an appropriate contrast and exposure control program for the microprocessor. The particular details of any such program would, of course, depend on the archi¬ tecture of the selected microprocessor. Turning now to Fig. 2, a block diagram of a typical logic and control unit (LCU) 31 is shown which interfaces with the copier 1 and the feeder 50. Leads 144 (see Fig. 1) from feeder 50 provide inputs to and receive outputs from LCU 31 to synchro¬ nize the operation of the feeder. The LCU 31 con- sists of temporary data storage memory 32, central processing unit 33, timing and cycle control unit 34 and stored program control 36. Data input and output is performed sequentially under program control. Input data are applied either through input signal buffer 40 to a multiplexer 42 or to interrupt signal processor 44. The input signals are derived from various switches, sensors, and analog-to-digital con¬ verters. The output data and control signals are applied to storage latches 46 which provide inputs to suitable output drivers 48, directly coupled to leads. These leads are connected to the work stations and to a copy sheet registration feeding mechanism 22. As shown, interrupt signals are provided by copy buttons 76, 78, 80, and 74 shown in detail in Fig. 10, and information representing a particular set-point of the matrix shown in Fig. 11 is selected by exposure knob 70 and contrast knob 72 which provide inputs to buffers 40 via their respec¬ tive analog/digital coverters (not shown). Returning now to the microprocessor, the contrast and exposure control program includes the matrix shown in Fig. 11, which is in a digitized format, located in stored program control 36, provided by one or more conventional Read Only Memories (ROM). The ROM contains operational programs in the form of binary words corresponding to instructions and numbers. These programs are perma¬ nently stored in the ROM and cannot be altered by the computer operation. The temporary storage memory 32 may be con- veniently provided by a conventional, Read/Write memory or Random Access Memory (RAM) . For ' a ' detailed explanation of the theory of copier contrast and exposure control, reference may be made to the following article: Paxton, Electro- photographic Systems Solid Area Response Model, 22 Photographic Science and Engineering 150 (May/June 1978) . It is believed helpful to use this theory in explaining the present invention. One way to explain copier contrast and exposure control theory is to examine the four-quadrant plots or graphs shown in Figs. 3-9, which show how changes in V , E and Vg effect the D™/D ouτ response curve Quadrant I. D IN refers to original document reflective den¬ sity, and D QU _ refers to copy reflective density. The term contrast, as used herein, refers to the rate of change (i.e. slope) of the D TM D 0UT curve * To facilitate understanding these graphs, the following terms are again defined: v., = Developer roller bias. V = Initial voltage (relative to ground) on the photo- conductor just after the charger 17. Vp = Photoconductor voltage (relative to ground) just after exposure by flash lamps. E = Actual exposure of photo- conductor. In accordance with this invention, the image quality of copies produced by copier 1 can be optimized by the p r rop r er selection of VO. EO, and V Ώ B. In Fig. 3, we will assume that these parameters have already been determined for a copier, and thus it has a particular E > ,- N /D 0 JJ T response curve (as shown in quadrant I). At its lower end, the E, N /D 0UT response curve terminates at a point, called the breakpoint D_. -When the input document density D T „ is at or below a density which corresponds to* the breakpoint D_, no toning takes place and the' output copy density is that of plain white copy paper D p . In Fig. 3, the D β point corresponds to a D, N of approximately 0.3. In selecting the appro¬ priate D γN /D 0 r jT response curve, it is important to select the appropriate D β point. For example, if a copier is adjusted to have the response curve of Fig. 3, and if a document contained information with a D γN of 0.2, then this information would be lost. On the other hand, if the lowest density of informa¬ tion in the document had a Ω „ of 0.4, then a copy may contain objectionable background if the Dg point is, say, 0.3. Thus, it is desirable to set the D B of a response curve, at a position which corres- ponds to the lowest D, N level of information on a document. The present invention permits an operator to select a desired D, N /D QUT response curve and to position such curve in Quadrant I so it has a desired D β breakpoint. O The effects on the D TM D 0UT res P onse curve by changing E , V , and V Ώ will now be o O -J described. Changes in exposure parameter E , as shown in Quadrant II of Fig. 4, change the • D TN/ D 0UT response curve and there is a breakpoint (D β ) shift in the -D TM /D Q TJ-**-- response curve. Increasing exposure will translate the curve to the right and the D„ point moves to correspond to an increased D TN value. Changes to parameter V , as shown in Quadrant III of Fig. 5, cause both a breakpoint D-Q and contrast shift curve translation and rotation) . Increasing V lowers the breakpoint and increases copy contrast. The p r rop r er combination of * V " o and Eo parameters can result in the conditions shown in Fig. 6 where the breakpoint remains fixed, but the copy contrast (i.e., slope of the response curve) increases with increasing E and V . Simulta- ° o o neous changes to E and V parameters constitute the basis for contrast control. The contrast and density control apparatus, in accordance with the invention, performs two func- tions. It provides convenient means for maintaining a predetermined D TM D Q UT re ** * - at: -- or - s hip (process control) and provides the operator with specific con¬ trols over contrast and density to compensate for a range of input document contrasts and densities. Toning contrast γ is the constant of proportionality between toner mass deposited on a photoconductor and photoconductor voltage Vp. Viewed differently, it is the slope of the D QUT /V F curve, (Fig. 7), and is a function of changing environmental conditions, toner age, and toner concentration in the developer mixture. As the toner age or life increases, the toning contrast decreases. Changes in toning contrast can be offset by a corresponding change in parameters V and E . Thus, by increasing V and E (Fig. 8) as toning contrast decreases, a stable response can be maintained. Up to this point, we have shown how V and E affect the ■D TM /- 0 OΠ* T < response curve. Changes in these parameters affect copy contrast of both lines and solids. The third process control para¬ meter in accordance with the invention is development roller bias voltage parameter V β . It has been determined that a predetermined bias level of the transport roller 19A can produce lines on copies having satisfactory contrast and density assuming an appropriate combination of V and E is selected. In an embodiment of the invention, the ' transport roller bias was fixed at -200V. The devel¬ opment roller bias V β primarily affected the break- point of the solid area response and their relative position in the E TM /E T J- T . curve, Quadrant I. Dual biasing makes it possible to have independent control of the line and solid area breakpoints. Although it has been found satisfactory to use a fixed transport roller bias, it will be understood that line copy response can be further adjusted by making the trans¬ port roller bias adjustable. The operator controls consist of the two rotary knobs, exposure knob 70 and contrast knob 72, and the special print copy button 74 (see Fig. 10). These controls are in addition to the normal, darken and lighten copy buttons 76, 78, and 80 usually found on copiers. Both knobs have nine discrete positions. The first knob 70 functions as an exposure (i.e. density) control and translates the breakpoint of the D-, N /D QUT curve (Fig. 4). When the knob 72 is turned, any one of nine different copy contrasts can be selected. The posi¬ tion of the knob 72 defines the shape (i.e. slope) of a particular D, N /D 0 „ response curve, and the position of knob 70 defines its location in Quadrant I and positions the D β point. To obtain a copy representative of the con¬ ditions selected by the exposure and contrast knobs, the special print copy button 74 must be depressed. If one of the normal, darken or lighten copy buttons is depressed, the computer ignores positions of the knobs 70 and 72, and a res P onSe curve corresponding to the normal, darken or lighten copy button selected will be produced. By means of this arrangement, a casual operator can choose to make copies by the conventional normal, darken or lighten copy button selection method. The two control knobs 70 and 72 (nine posi¬ tions each) correspond to eighty-one set-points which in turn correspond to different D TN /D n .> T response curves. A normal copy can also be obtained by depressing the special print copy button 74 when the exposure knob 70 is in position 5 and the contrast knob 72 is in position 6. Darken and lighten copies also have their own set-points number, but they are not part of the eighty-one set-points. As shown in Fig. 11, there is a 9 x 9 matrix, which will be understood to be located in an ROM of stored program control 36. The matrix is an array of quantities arranged in nine rows and nine columns. There are eighty-one positions in the matrix. At the inter- section of each column and row there is a set-point having three set-point numbers which from top to bottom represent parameters V O, EO. and V B JD res- pectively. These numbers provide adjustments for copier parameters V , E , and V β . The particu- lar numbers shown in Fig. 11 are for a specific copier which used a specific type of photoconductor and are given for illustrative purposes only. The eighty-one set-points can accommodate a wide range of parameter adjustments so that a copy having a desired contrast and density can be produced regardless of line and solid area contrast and density, of input documents, toning contrast, and toner age or other conditions of the copier. The matrix numbers that are actually stored in memory are in a digital format and correspond to values of specific parameters. The microprocessor converts these numbers into adjustments of corres¬ ponding programmable power supplies. An operator, by selecting a particular row (knob 70) and column (knob 72), selects a particular one of the 81 set-points with its numbers. The contrast knob selects the column of the matrix, and the exposure knob selects the row. At the intersection of the column and row is the desired set-point. For a specific example using the numbers shown in Fig. 11, at matrix posi¬ tion (5,6), the V and E numbers are both 0. There is no adjustment of the power supply 17B, and V ideally should be at a predetermined voltage level of say 476 volts. Also, E is at the normal exposure level without adjustment. V- B is at 80 volts. At matrix position (2,8), the number 60 corresponds to an increase of 60 volts to provide a Vo of 336 volts, the number .01 indicates Eo Λ is increased by .01 log E and the number 60 indicates V B is 60 volts. As illustrated in Fig. 11, for any given exposure (row), changing the column position changes V , E , and V-g. However, for any column, a change in the exposure knob (row) changes Vo Λ and E Λ o while V-rD, remains constant, cr In operation, let us assume an operator believes an output copy having contrast which corres¬ ponds to position 8 of exposure knob 70 would be desirable. In this example, let us further assume he sets exposure knob at position 7. Position 7 defined a particular D„ point. He now makes a copy, and let us assume the copy contrast is ' indeed at the desired level, but the copy has some objectionable background. He now would move the D_ point by selecting exposure position 6. The new D TN /D QUT response curve is substantially identical to the previous one, except that the curve has been shifted to the left in Quadrant I, and a new D-g point is defined. The operator would then make another copy to see if the background was eliminated. Assuming it was, then he would produce the desired number of copies. Thus, when an operator makes a change in contrast or exposure, the logic and control will automatically select the appropriate V , V β , and E O Λ parameters values. i The set-points shown in Fig. 11 represent nominal set-points for a copier which could be manu¬ factured in q- i uantity ■* -. Thus, the Vo and Eo numbers are for a "standard copier". Due to manu- facturing variances in corresponding copier parts and toner, these numbers may not produce a copy having the desired contrast and density. To overcome this problem, a larger set of value for Vo, Eo, and Vor, can be stored in ROM. If, in such a scheme, the desired copy result at the normal copy position (nominally selected to be at 5,6) is achieved by finding its actual set-point location within the larger array (say 15 x 15) that achieves the closest Do and contrast for a normal copy. Thus, the normal copy position may, for example, be at set-point (5,7). The contiguous set of 9 x 9 values are then used until a recalibration is performed. The invention has been described with particular reference to a preferred embodiment thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention. _c

Claims: What is claimed is: 1. A method for optimizing the contrast, density and solid area development of copies produced by an electrophotographic copier of the type having an electrostatically chargeable recording element, adjustable charging means for varying the voltage V applied to the recording element, adjustable exposure means for varying the imagewise exposure E of the charged recording element, a development station having at least one development electrode and adjustable bias means for varying the bias voltage V β applied to such electrode, said method being characterized by the steps of: (a) storing in a memory a matrix array of sets of representations of interrelated electrical values, with each set having values which respectivel ^ correspond to specific levels of the voltage V applied onto the copier photoconductor, the copier exposure E , and the bias V-~ applied to the developer roller to establish a voltage between the devel¬ opment roller and the photoconductor respectively; (b) addressing the matrix to designate a particular set; and (c) adjusting the voltage V , exposure E , and the bias voltage V β in accordance with the values of said des- ignated set to effect a change in the solid area and contrast density of « copies. 2. In an electrophotographic copier having: a chargeable photoconductor; adjustable charging means for varying the voltage parameter V , applied onto the photoconductor; an adjustable C ' >Γ I exposure means for varying the exposure parameter E of the photoconductor; a development station having at least one electrode and adjustable bias means for applying an adjustable bias voltage parameter V β to-said electrode for establishing a bias voltage between the photoconductor and said station and means for adjusting said adjustable charging, exposure and bias means to control the contrast, density and solid area development of copies produced by such copier, characterized in that said adjusting means comprises: (a) memory means having stored therein a matrix of sets of representations of interrelated values, each such set having three values which correspond to sp r ecific levels of VO', EO and V D D that determine a predetermined copier D, N /D QUT response curve with a predetermined breakpoint; (b) means for addressing said matrix to designate a particular set; and (c) logic and control means responsive to the values of said designated set to adjust said adjustable charging, expo- sure and bias means in accordance with the D γ * N /D 0UT response curve defined by such designated set values. 3. The apparatus as defined in claim 1 further characterized in that said stored sets are arranged in a plurality of intersecting columns and rows, there being a different set at each intersection, and in that said addressing means includes means for designating the row and column of a desired set.

Inventors: FISKE J, VELZEN R
Applicants: EASTMAN KODAK CO
Application year: 1981
Main classification: G03G15
IPC codes: G03G15, G03G21