Imaging method for providing images of uniform print density

Abstract: Imaging apparatus and method for providing images of uniform print density. The apparatus (10) includes a print head (45) having a plurality of nozzles (120) containing ink. Each nozzle has an image forming characteristic, such as print density, associated therewith. A heater (150) associated with each nozzle is in heat transfer communication with the ink for heating the ink, so that, as the ink is heated, its surface tension relaxes. As surface tension relaxes, static back-pressure acting on the ink ejects the ink from the nozzle. A voltage supply unit (160) is provided for supplying a voltage pulse to each of the heaters for activating the heaters and a controller (140) interconnects the heaters and the voltage supply unit for controlling the voltage pulse. Controlling the voltage pulse causes the image forming characteristic for each nozzle to be altered to the extent that the image forming characteristics for all the heaters will become uniform. In this regard, the controller includes a memory unit (220) capable of informing the controller of the voltage pulse duration to be applied to each heater for obtaining uniform image forming characteristics. Alternatively, the memory unit may inform the controller of the pulse amplitude to be applied to each heater for obtaining uniform image forming characteristics. Therefore, either the voltage pulse amplitude or the voltage pulse duration applied to each heater is controlled such that the image forming characteristics (e.g., print densities) of all nozzles are uniform.

Description: BACKGROUND OF THE INVENTION The present invention relates generally to imaging apparatus and methods and, more particularly, to an imaging apparatus and method for providing images of uniform print density, so that printing non-uniformities, such as banding, are avoided. In a typical ink jet printer using a multi-nozzle head, digital signals as to each of four colors (i.e., red, green, blue and black) regarding an image are processed in a manner so that the multi-nozzle head forms a printed color image on a recorder medium, such as paper or transparency. Indeed, ink jet printing has become recognized as a prominent contender in the digitally controlled, electronic printing arena because of its non-impact, low-noise characteristics, its use of plain paper and its avoidance of toner transfers and fixing. U.S. Pat. No. 4,275,290, which issued to Peolo Cielo et al. on June 23, 1981, discloses a liquid ink printing system in which ink is supplied to a reservoir at a predetermined pressure and retained in orifices by surface tension until the surface tension is reduced by heat from an electrically energized resistive heater. The heater causes ink to issue from the orifice and to thereby contact a paper receiver. This system requires that ink be designed so as to exhibit a change, preferably large, in surface tension with temperature. The paper receiver must also be in close proximity to the orifice in order to separate the drop from the orifice. These features increase the complexity and cost of the printing system. However, ink jet printers may produce non-uniform print density with respect to the image deposited on the recorder medium. Such non-uniform print density may be visible as so-called "banding". "Banding" is evinced, for example, by repeated variations in the print density caused by delineations in individual dot rows comprising the output image. Thus, "banding" can appear as light or dark streaks or lines within a printed area. "Banding" is influenced by factors such as ink drop volume variations, print head carriage motion anomalies, electrical resistance variation of the heaters, and/or the presence of damaged nozzles. One important factor producing "banding" is variability in the nozzle orifice diameter caused by variations in the manufacturing process used to make the nozzles constituting the print head. Even small variations between nozzles of a print head may lead to visible "banding". More specifically, when the ink droplet is pushed outwardly during ejection from the nozzle, the moving ink droplet must overcome flow resistance caused by the nozzle's flow channel and also flow resistance caused by the nozzle's orifice. Therefore, the ejection speed of the droplet is strongly dependent on the flow resistance or drag force exerted by the nozzle's flow channel and the nozzle's orifice. Nozzle diameter affects flow resistance or drag force and therefore affects the amount of ink ejected from the nozzles. Moreover, nozzle diameter also affects the meniscus shape of the ink at the nozzle's orifice, which in turn affects droplet volume and ejection rate. In addition, heater electrical resistance can vary among nozzles due to slight variations in the composition of the material comprising the electric resistance heaters disposed in the nozzles. Variations in electrical resistance among nozzles causes variations in the amount and ejection speed of the ink thereby leading to variations in print density. All the afore mentioned factors negatively affect print density and invite "banding". Therefore, a problem in the art is non-uniform print density due to the presence of physical variations among the print nozzles, such as variations in nozzle diameter and electrical resistance. Techniques specifically addressing the problem of non-uniform print density are known. One such technique is disclosed in U.S. Patent No. 5,038,208 titled "Image Forming Apparatus With A Function For Correcting Recording Density Unevenness" issued August 6, 1991 in the name of Hiroyuki Ichikawa. This patent discloses memory means for storing data corresponding to image forming characteristics (i.e., print density) of each nozzle of multi-nozzle print heads, and a corrector means for correcting the image forming signals based on the data stored in the memory means. However, this patent does not appear to disclose an efficient and cost effective solution to the problem of non-uniform print density or "banding". For example, the Ichikawa patent discloses that image processing is required for correcting density non-uniformities for each input image file. That is, image processing is required for each and every input image for which output density correction is desired. Correcting density non-uniformities for each input image file is undesirable because it is time consuming. Also, this patent discloses that modulation in the output code value is made at a relatively limited number of discrete levels for halftoned images at a typical printing resolution (i.e., 600 dots per inch). However, printing at discrete levels may not eliminate visual printing defects, such as "banding". W0-A-96 32289 discloses an imaging method involving use of a printhead that includes a plurality of nozzles each defining a fluid cavity containing print fluid under pressure. The nozzles each terminate in an outlet wherein a meniscus of the print fluid has a predetermined surface tension that is changed in response to heat. Heater elements in heat transfer communication with the print fluid menisci heat the print fluid menisci so that the surface tension of the print fluid menisci relax as the heater elements heat the print fluid menisci. Each of the heater elements is adapted to receive a predetermined voltage pulse. A voltage supply unit is associated with the heater elements and supplies the respective predetermined voltage pulses to the heater elements to generate heat sufficient to reduce surface tension of each respective meniscus so that the print fluid in the meniscus extends out further but such extension is insufficient to over overcome the surface tension to separate from the body of ink. The formation of a droplet is accomplished through close proximity of the receiver medium to the nozzle opening which effectively wicks or otherwise attracts a droplet onto the receiver medium. There is no description in D1 of free drops being created. EP-A-0 709 213 is directed to a method for correcting printhead non-uniformity involving print heads that employ bubble-jet action to eject a drop. Bubble-jet printing is characterized by creation of a bubble or significant void in the ink, the generation of which and the collapse of which causes a drop to be ejected from the nozzle. A known problem with bubble jet printing is that occasionally nozzles may be blocked through coalescence of bubbles thus creating imaging artifacts. EPA-0 609 997 is directed to an inkjet printer that includes a print head having firing thermal resistors responsive to groups of firing pulses. Rapid heating of the resistors causes the ink directly adjacent a thermal resistor to vaporize to form a bubble. Thus, this reference is also directed to a bubble-jet type of printer that is characterized by excessive heating and significant void formation which increases the likelihood of blockage of the nozzles and resultant imaging artifacts. The object of the present invention is to provide a suitable imaging method for providing images of uniform print density produced by print nozzles, so that printing non-uniformities, such as banding, are avoided, even when the print nozzles have different physical attributes resulting in different printing characteristics. DISCLOSURE OF THE INVENTION The invention resides in an imaging method according to claim 1. Embodiments of the present invention are defined in claims 2 to 4. An advantage of the present invention is that images of uniform print density are provided even in the presence of variations in such factors as electrical resistance of the heater and/or diameter of the nozzle orifice. Another advantage of the present invention is that images of uniform print density are produced in a more time efficient manner compared to prior art techniques. A further advantage of the present invention is that use thereof eliminates visual printing defects, such as "banding". These and other objects, features and advantages of the present invention will become apparent to those skilled in the art upon a reading of the following detailed description when taken in conjunction with the drawings wherein there is shown and described illustrative embodiments of the invention. BRIEF DESCRIPTION OF THE DRAWINGS In the detailed description of the preferred embodiments of the invention presented hereinbelow, reference is made to the accompanying drawings, in which: FIG. 1 is a view in partial vertical section, with parts removed for clarity, of an imaging apparatus showing an ink-jet print head printing an image onto a recorder medium, this view also showing a controller connected to the print head for controlling image forming characteristics associated with the print head; FIG. 2 is a view in horizontal section of a portion of the print head, this view also showing a plurality of nozzles and associated cavities filled with ink, each of the nozzles having an electric resistance heater in heat transfer communication therewith; FIG. 3 is a detail view in horizontal section of one of the nozzles; FIG. 4 is a view in vertical section of the nozzle showing the ink being restrained by surface tension from emerging from the nozzle; FIG. 5 is a view in vertical section of the nozzle showing an ink droplet emerging from the nozzle as the surface tension begins to relax; FIG. 6 is a view in vertical section of the nozzle showing the ink droplet emerging further from the nozzle as the surface tension further relaxes; FIG. 7 is a view in vertical section of the nozzle showing the ink droplet having emerged from the nozzle and propelled toward the recorder medium by back-pressure; FIG. 8 is a graph illustrating print density as a function of pulse voltage amplitude; FIG. 9 is a graph illustrating print density as a function of pulse width or duration; FIG. 10 shows a test image printed on recorder medium for a density uniformity calibration of the print head nozzles; FIG. 11 shows a density patch belonging to the test image, this density patch having a marginal area of insufficient print density; FIG. 12 is a graph illustrating a voltage pulse with a predetermined constant amplitude and a predetermined duration, the voltage pulse being provided to the electric resistance heater in the nozzle for heating the ink in order to relax the surface tension of the ink; FIG. 13 is a graph illustrating electrical resistance as a function of nozzle number; FIG. 14 provides a look-up table showing print density as a function of voltage pulse amplitude supplied to each nozzle; FIG. 15 provides a look-up table showing print density as a function of voltage pulse duration supplied to each nozzle; FIG. 16 is a graph illustrating print density as a function of number of scanned pixels of the test image; FIG. 17 is a flow-chart illustrating certain steps belonging to the method of the invention; and FIG. 18 is a graph illustrating a voltage pulse with a constant voltage amplitude portion and a logrithmically varying voltage amplitude portion. DETAILED DESCRIPTION OF THE INVENTION Referring to Fig. 1, there is shown an imaging apparatus, generally referred to as 10, having a uniform image forming characteristic for producing an output image lacking printing defects such as "banding". In the preferred embodiment of the invention, the image forming characteristic is print density. However, it will be appreciated that the image forming characteristic may be any suitable image forming characteristic related to image quality. Imaging apparatus 10 comprises a printer, generally referred to as 20, electrically connected to an input source 30 for reasons disclosed hereinbelow. Input source 30 may provide raster image data from a scanner or computer, outline image data in the form of a page description language, or other form of digital image data. The output signal generated by input source 30 is received by a controller 40, for reasons disclosed in detail hereinbelow. Referring to Figs. 1 and 2, controller 40 processes the output signal generated by input source 30 and generates a controller output signal that is received by a print head 45 capable of printing on a recorder medium 50. In some printers recorder medium 50 may be fed past print head 45 at a predetermined feed rate by a plurality of rollers 60 (only some of which are shown). That is, recorder medium 50 may be fed, by rollers 60, from an input supply tray 70 containing a supply of recorder medium 50. Each line of image information from input source 30 is printed on recorder medium 50 as that line of image information is communicated from input source 30 to controller 40. Controller 40 in turn communicates that line of image information to print head 45 as recorder medium 50 is fed past print head 45. When a completely printed image is formed on recorder medium 50, recorder medium 50 exits the interior of printer 20 to be deposited in an output tray 80 for retrieval by an operator of imaging apparatus 10. Although the terminology referring to "print head 45" is used in the singular, it is appreciated by the person of ordinary skill in the art that the terminology "print head 45" is intended to also include its plural form because there may be, for example, four print heads 110, each one of the print heads 110 being respectively dedicated to printing one of four colors (i.e., red, green, blue and black). Turning now to Figs. 1, 2, 3, and 4, print head 45, which belongs to printer 20, is there shown in operative condition for printing an image on recorder medium 50. Print head 45 comprises a plurality of ink fluid cavities 90 for holding print fluid, such as a body of ink 100. Each cavity 90 is in communication with a print fluid reservoir 110 for supplying ink 100 into cavity 90. Moreover, associated with each cavity 90 is a nozzle 120 for allowing ink 100 to exit cavity 90. In this regard, each nozzle 120 includes a flow channel 130 and a generally circular orifice portion 140 in communication with flow channel 130. Orifice portion 140, which is disposed proximate recorder medium 50, opens toward recorder medium 50 for depositing ink 100 onto recorder medium 50. Moreover, lining orifice portion 140 and flow channel 130 is a generally annular electrothermal actuator (i.e., an electrical resistance heater element) 150 for heating ink 100, heater 150 having a predetermined electrical resistance. Thus, each heater 150 is in heat transfer communication with ink 100. A voltage supply unit 160 is electrically connected to print head 45 for supplying a voltage pulse to each heater 150. Each nozzle 120 has an image forming characteristic (e.g., print density) associated therewith. As described more fully hereinbelow, the voltage pulse is capable of altering the image forming characteristic to define an altered image forming characteristic. Controller 40 controls the voltage pulse so that the altered image forming characteristics for all nozzles 120 are uniform. As best seen in Figs. 5 and 6, an ink bulge, meniscus or droplet 170 outwardly emerges from orifice region 140 as resistance heater 150 increases temperature in order to heat ink 100. This heating of ink 100 results in a localized decrease in surface tension of droplet 170. As the surface tension of droplet 170 decreases, it assumes a substantially cylindrical form due to a surface tension gradient from the tip of orifice region 140 to the center of droplet 170, and due to viscous drag or flow resistance along the surface of flow channel 130 and orifice region 140. Fig. 7 shows droplet 170 separated from ink body 100 and ejected from orifice region 140 as it is propelled outwardly toward recorder medium 50 to establish an ink mark upon recorder medium 50. Droplet 170 will eventually be intercepted by recorder medium 50 to "soak into" and be absorbed by recorder medium 50. Moreover, each resistance heater 150 may be selectively energized many times by voltage supply unit 160 to deposit a multiplicity of ink marks upon recorder medium 50 in a predetermined pattern according to the image file residing in input source 30. Of course, the image printed onto recorder medium 50 should possess a uniform print density to avoid "banding". However, it is known that "banding" is a recurring problem in the printing arts. Often "banding" (i.e., print density non-uniformity) results from variability in the print head fabrication process. For example, banding can be caused by variability in the diameter of orifice region 140 due to variations in the manufacturing process used to make nozzle 120 or by variability in electrical resistance among resistance heaters 150 due to slight variations in the chemical composition comprising heaters 150. Even small variations in diameter and electrical resistance can lead to visible "banding". Therefore, a long-standing problem experienced in the art is banding, which may be caused by the presence of physical variations among individual print nozzles 120. To solve this problem, the present invention controls the voltage pulse amplitude or, alternatively, the voltage pulse duration supplied to each heater 150 to compensate for physical anomalies (e.g., variations in the diameter of orifice region 140, and/or variations in electrical resistance of heaters 150) associated with individual nozzles 120. Controlling the voltage pulse in this manner obtains uniform print density on recorder medium 150. This result is attainable because controlling the voltage pulse amplitude and/or voltage pulse duration supplied to each nozzle 120 controls the surface tension of ink droplet 170, which in turn controls the rate and the volume of ink released from each nozzle 120. Controlling the rate and volume of ink released from each nozzle 120 controls the print density provided by each nozzle 120. As described more fully hereinbelow, nozzles 120 are calibrated such that each nozzle 120 will selectively receive a predetermined pulse voltage amplitude or pulse voltage duration as print head 45 is operated in order that print densities for all nozzles 120 are substantially the same (i.e., uniform), even though physical attributes among nozzles 120 may vary. However, to fully appreciate the present invention, it is instructive first to briefly discuss the relationship between print density, voltage pulse amplitude, voltage pulse duration, and heater resistance. Therefore, the volume of ink 100 ejected by print head 45 is a function of the amplitude and duration of the voltage pulse supplied to print head 45. Larger droplets 170 with larger volumes of ink will cause higher density images on recorder medium 50. Conversely, smaller droplets 170 with smaller volumes of ink will cause lower density images on recorder medium 50. Thus, print density is a function of the amplitude and the duration of the electric pulse received by print head 45 because the volume of ink released is a function of the amplitude and duration of the voltage pulse. In other words, the dependence of print density of print head 45, as a whole, on voltage amplitude and voltage duration can be expressed by the following functional relationship: D = f (V p , T) where, D = print density of print head 45; V p = voltage pulse amplitude supplied to print head 45; and T = voltage pulse duration supplied to print head 45. Equation (1) provides print density for print head 45, taken as a whole, and is illustrated graphically for print head 45 in Figs. 8 and 9. In Fig. 8, print density D is shown as a function of voltage pulse amplitude V p while holding the voltage pulse duration T constant. In Fig. 9, print density D is shown as a function of voltage pulse duration T while holding the voltage pulse amplitude V p constant. The precise functional dependence of print density D upon voltage pulse amplitude V p and voltage pulse duration T as illustrated by Figs. 8 and 9, respectively, is obtainable by a process that includes measuring print density D of a uniform test image printed by the relatively large number of nozzles 120 of print head 45, as described more fully hereinbelow. Therefore, referring to Fig. 10, there is shown a representative test image 180 used in a process for calibrating nozzles 120, so that nozzles 120 will print with uniform print density regardless of physical anomalies among individual nozzles 120. Test image 180 includes a plurality of "density patches" 190 having print densities D varying from a minimum print density D l (i.e., near white or light halftone) to a maximum print density D w . The print densities D for each of the density patches 190 is preferably measured by use of a densitometer (not shown) which scans a generally circular print area (e.g., approximately 0.20 square centimeters) of each density patch 190. Preferably, the densitometer is used to scan many different areas of each density patch 190. These multiple densitometer readings are averaged to provide an averaged density value for each density patch 190. A separate test image 180 is produced at each of a plurality of voltage pulse amplitudes while keeping the voltage pulse duration constant. Also, a separate test image 180 is produced at each of a plurality of voltage pulse durations while keeping the voltage pulse amplitude constant. This process results in a multiplicity of print density measurements because measurement of print density using the densitometer is repeated for each density patch 190 of each test image 180. Moreover, the foregoing process is repeated for each of the print heads 110 (e.g., for each of the print heads corresponding to each of the colors red, green, blue and black). Referring to Fig. 11, it was observed that more valid densitometer readings are obtained when the densitometer avoids a marginal region 200 of density patch 190. This is so because the print density in marginal region 200 may not be representative of the print density of density patch 190 as a whole. This assumes, of course, that printing of the test image is begun in marginal region 200 of density patch 260 and moves vertically downwardly. It was further observed that the source of the problem of non-representative printing in marginal region 200 may be due, for example, to the halftoning algorithm used to generate test image 180. With this densitometer data, the precise function shown in Equation (1) for print head 45 is obtained by mathematical means well known in the art, such as by means of statistical curve-fitting procedures. Using this precise function provides print density D as a function of V p , which is plotted in Fig. 8. Also using this precise function provides print density D as a function of T, which is plotted in Fig. 9. However, it should be appreciated that Figs. 8 and 9 show print density D of print head 45 taken as a whole and does not provide print density of individual nozzles 120. In other words, Equation (1), from which Figs. 8 and 9 are plotted, provides a functional relationship defining an aggregate print density for print head 45, as whole. However, as stated hereinabove, print density among nozzles 120 may vary due, for example, to variations in nozzle orifice diameter and/or electrical resistance of heaters 150. It is therefore desirable to calibrate nozzles 120, so that all nozzles 120 of print head 45 print with uniform print density, even though physical attributes among nozzles 120 may vary. Therefore, according to the present invention, either of two techniques may be used to provide uniform print density of individual nozzles 120 in view of the unique physical attributes associated with each nozzle 120. These two techniques are defined herein as the "Resistance Calibration Technique" and the "Density Calibration Technique" and are described in detail hereinbelow. Resistance Calibration Technique: The Resistance Calibration Technique may be used to determine the print density D of each nozzle 120 in view of the inherent electrical resistance of each resistance heater element 150 associated with each nozzle 120. Electrical resistance among heater elements 150 may vary due to slight variations in the chemical composition of individual heater elements 150. However, print density D of each nozzle 120 can be controlled by controlling the electric heating pulse applied to each heater element 150 (i.e., to each nozzle 120), even though the electrical resistance among heater elements 150 may vary. As previously mentioned, print density D of print head 45 as a whole is provided by Equation (1); however, it is desirable to determine the print density D for each nozzle 120 within print head 45. In this regard, print density D for each nozzle 120 is provided by an approximation to Equation (1) as follows: D ≈ f (E) = f ((V p ) 2 T/R) where, E = average heat energy applied to each heater element 150 (i.e., each nozzle 120); and R = electrical resistance inherent in each heater element 150 (i.e., each nozzle 120). Referring to Figs. 12 and 13, a square wave voltage pulse 210 of constant voltage amplitude V pi is sequentially applied to each heater 150 associated with each nozzle 120. That is, constant voltage pulse 210 is sequentially applied to each heater 150 from the first heater 150 to the last heater 150 in print head 45. The last heater 150 is represented as heater number "N" in Fig. 13. As square wave voltage pulse 210 is input to each heater 150, the output voltage is measured at each heater 150 and a resistance R i is calculated for each heater 150. Using these calculated values of heater electrical resistances R i , the average resistance R for all heaters 150 in print head 45 is then calculated as follows: R = (ΣR i ) / N where, R = calculated average electrical resistance of all heaters 150 (i.e., all nozzles 120); R i = calculated electrical resistance of the "i th " heater 150 (i.e., the "i th " nozzle 120); N = total number of heaters 150 (i.e., nozzles 120); and i = 1 to N. In this manner, the average electrical resistance R is calculated. Next, the corrected voltage pulse amplitude V pi or the corrected voltage pulse duration T i to be applied to each nozzle 110 is calculated. In this regard, Equation (2) can be rewritten as follows: (V pi ) 2 / R i = (V p) 2 / R = E / T which, in turn, can be rewritten as V pi = (E R i / T) 1/2 = V p (R i / R ) 1/2 where, V pi = voltage pulse amplitude to be applied to the "i th " nozzle to obtain the desired heating energy E for each heating voltage pulse. In other words, V pi is the voltage pulse amplitude to be applied to the "i th " nozzle 120 in order for the print density of the "i th " nozzle 120 to be equal to the print density D of print head 45. Thus, voltage amplitude V pi for each nozzle 120 is selected such that print density of each nozzle 120 matches the desired aggregate print density value for print head 45, as a whole. In this manner, nozzles 120 will print with uniform print density because each nozzle 120 will print with the print density D of print head 45. Alternatively, the voltage pulse duration of the square wave voltage pulse 210 may be used to calibrate each heater 150 in order to provide uniform print density. In this regard, the voltage pulse duration T i applied to each heater 150 (i.e., each nozzle 110) is calculated by first rearranging Equation (4) as follows: T i / R i = T / R = E / (V p ) 2 where, T i = voltage pulse duration to be applied to the "i th " nozzle to obtain the desired heating energy E for each heating voltage pulse. Equation (6) can be rewritten as follows: T i = R i E / (V p ) 2 = T R i / R . Thus, Equation (5) provides the voltage pulse amplitude V pi or alternatively Equation (7) provides the voltage pulse duration T i to be applied to each nozzle 110 in order to calibrate each heater 150 ( i.e., each nozzle 120) so that all nozzles 120 provide uniform print density even though electrical resistances among heaters 150 may vary. However, it should be recalled that calibration of each heater 150 (i.e., each nozzle 120) using the Resistance Calibration Technique compensates for variabilities only in electrical resistance among individual heaters 150 (i.e., among individual nozzles 120). Referring to Figs. 1, 2, 3, 14 and 15, once the pulse voltage amplitudes V pi and/or the pulse voltage durations T i are obtained by the steps recited hereinabove, these values of V pi and T i and the print density D of print head 45 are stored electronically in a memory unit, such as a Read-Only-Memory (ROM) semiconductor computer chip 220 connected to controller 40. As best seen in Figs. 14 and 15, the values of D, V pi , and T i stored in chip 220 are represented herein as first and second look-up tables, generally referred to as 230 and 240, respectively. The values of D, V pi , and T i stored in chip 220 are used as parameters for each nozzle 120 during normal operation of apparatus 10, as described in more detail hereinbelow. More specifically, during normal operation of apparatus 10, the desired print density D is selected, such as by means of input source 30, and is then communicated to controller 40. Once controller 40 accepts density value D to be printed by print head 45, controller 40 is informed by first lookup table 230 in chip 220 as to the correct voltage amplitude V pi to apply to each nozzle 120 in order to obtain uniform print density D from each nozzle 120. In this case, only first look-up table 230 is stored in chip 220. This is so because pulse voltage duration T is held at a constant value by controller 40 and, therefore, there is no need to store second look-up table 240 in chip 220. Alternatively, once controller 40 accepts a density value D to be printed by print head 45, controller 40 is informed by second lookup table 240 stored in chip 220 as to the correct voltage pulse duration T i to apply to each nozzle 120 in order to obtain uniform print density D from each nozzle 120. In this case, only second look-up table 240 is stored in chip 220. This is so because the pulse voltage amplitude V p is held at a constant value by controller 40 and, therefore, there is no need to store first look-up table 230 in chip 220. Although the Resistance Calibration Technique only calibrates heaters 150 to compensate for variabilities in electrical resistance, an advantage of using the Resistance Calibration Technique is its simplicity. That is, each heater 150 (i.e., nozzle 120) belonging to print head 45 is calibrated merely by supplying the square wave voltage pulse 210 illustrated by Fig. 12 and measuring the resulting electrical resistance R i of each heater 150, as illustrated by Fig. 13. In this manner, each nozzle 120 can be conveniently calibrated during manufacture of print head 45. In addition, each nozzle 120 can be recalibrated, if necessary, "in the field" at a customer site to accommodate print head 45 to the specific environmental conditions (e.g., humidity, dust, temperature, e.t.c.) present at the customer's site. Such environmental conditions may have altered the original calibration of print head 45 performed during manufacture of print head 45. However, print density depends on other physical characteristics of nozzles 120 in addition to electrical resistance. Therefore, if desired, nozzles 120 may be calibrated to compensate for physical characteristics in addition to electrical resistance. To achieve this result, the present invention provides a technique, defined herein as the Density Calibration Technique, which compensates for variability in substantially all physical characteristics in addition to electrical resistance. Density Calibration Technique: The Density Calibration Technique calibrates nozzles 120 to compensate for substantially all variabilities among nozzles 120, including variabilities caused by different amounts of electrical resistance, in order to obtain uniform print density. This technique is described in detail hereinbelow. Returning to Figs. 10, 11, 12, 13 and 16, print head 45 to be calibrated is used to print the previously mentioned test image 180 in the manner described hereinabove. This produces print density patches D l to D w . The previously mentioned densitometer is then used to measure the resulting print densities in two directions (i.e., vertically and horizontally), preferably at a resolution of at least 300 dpi. The density values are integrated vertically down each density patch in order to obtain the one-dimensional density profile of Fig. 16. Thus, Fig. 16 characterizes print density non-uniformity due to physical variabilities among nozzles 120. It is understood that print density measurements are not taken in marginal region 200 for the reasons provided hereinabove. These print density values may be fit, by means well known in the art, to an analytical function so that the print density value for each nozzle 120 is conveniently obtained by reference merely to the analytical function. After the print densities are obtained, the required voltage pulse amplitude and voltage pulse duration are calculated, as described in detailed hereinbelow. In this regard, print density D i at the "i th " nozzle 120 for a specific density patch 220 is provided by modifying Equation (1) as follows: D i = f i (V pi , T) where, D i = print density for "i th " nozzle 120; V pi = the corrected pulse voltage amplitude for "i th " nozzle 120; T = pulse voltage duration for "i th " nozzle 120; and i = 1 to total number of nozzles "N". It is appreciated that Equation (1) and Equation (8) differ to the extent that Equation (8) provides print density D i for each nozzle 120 (in order to consider differences in physical characteristics among nozzles 120) and Equation (1) provides a print density D for print head 45 as a whole (and thus does not consider differences among nozzles 120). Thus, Equation (1) demonstrates that print head 45 will print with the ideal print density D only if each nozzle 120 prints with this same print density D. However, in practice each nozzle 120 will not necessarily print with the same print density D due to variabilities found, for example, in the diameter of nozzle orifice portion 140 and/or the electrical resistance in heaters 150. Therefore, Equation (2) determines the print density D i for each "i th " nozzle 120. Thus, for a constant voltage pulse duration T, the print density D i which is produced by the "i th " nozzle 120 is obtained by first noting the following equation: D i = f i (V pi , T). Subtracting Equation (9) from Equation (1) leads to the following mathematical expression: D - D i = f (V p , T) - f i (V pi , T). However, it is understood that the differences among nozzles 120 are assumed to be small so that the derivatives of f i and f are the same to a first order approximation, as follows: D - D i = f (V p , T) - f i (V pi , T) ≈ (∂ f / ∂V p ) (V pi - V p ) where, ∂ f i / ∂V p = partial derivative of the function " f i " with respect to voltage amplitude V p . When solved for V pi , Equation (11) becomes: V pi = V p + (D - D i ) / (∂ f (V pi , T) / ∂V pi ). Therefore, Equation (12) provides the voltage pulse amplitude V pi which should be applied to nozzle "i" to obtain a required print density D, which is the print density for print head 45 as a whole. Print density D is selected by the operator of apparatus 10, such as by means of input source 30. Moreover, using an analogous derivation, the voltage pulse duration T i which can be applied to nozzle "i" to obtain print density D is found as follows: T i = T + (D - D i ) / (∂ f (V p , T) / ∂T). As disclosed more fully hereinbelow, the first and second look-up tables 230/240 described hereinabove for the Resistance Calibration Technique are also constructed for the Density Calibration Technique. Therefore, referring to Figs. 1, 2, 3, 14 and 15, once the pulse voltage amplitudes V pi and/or the pulse voltage durations T i are obtained by the steps recited hereinabove for the Density Calibration Technique, these values of V pi and T i and the corresponding print densities D i are stored electronically in chip 220, which is connected to controller 40. The values of D i , V pi , and T i stored in chip 220 are used as parameters for each nozzle 120 during normal operation of nozzles 120. That is, the desired print density D is selected, such as by means of input source 30, and is then communicated to controller 40. Once controller 40 accepts a density value D to be printed by print head 45, controller 40 is informed by first lookup table 230 in chip 220 as to the correct voltage amplitude V pi to apply to each nozzle 120 in order to obtain uniform print density D among nozzles 120. In this case, only first look-up table 230, which contains the V pi values as a function of density D i , is stored in chip 220. Also, pulse voltage duration T is held at a constant value by controller 40 and therefore, in this case, there is no need to store second look-up table 240 in chip 220. Alternatively, once controller 40 accepts a density value D to be printed by print head 45, controller 40 is informed by second lookup table 240 stored in chip 220 as to the correct voltage pulse duration T i to apply to each nozzle 120 in order to obtain uniform print density D among nozzles 120. In this case, only second look-up table 240, which contains the T i values as a function of density D i , is stored in chip 220. Also, the pulse voltage amplitude V p is held at a constant value by controller 40 and therefore, in this case, there is no need to store first look-up table 230 in chip 220. Moreover, efficacy of both the Resistance Calibration Technique and Density Calibration Technique are enhanced when print line times are compatible with the calibration technique selected. The terminology "print line time" is defined herein to mean the time spent on marking each row of ink pixels on recorder medium 180. That is, when voltage pulse amplitude V pi is varied, the voltage pulse duration T is held constant among all nozzles 120 in print head 45 and the printing line time is set equal to or greater than the constant voltage pulse duration T. Alternatively, when voltage pulse duration T i is varied, the voltage pulse amplitude V p is held constant among all nozzles 120 in print head 45 and the printing line time is set equal to or greater than the maximum pulse duration allowable for nozzles 120. Fig. 17 presents a flow chart 250 summarizing selected steps in the method of the invention. More specifically, flow chart 250 illustrates steps for arriving at Equations (5), (7), (12) and (13). The steps of the Density Calibration technique described hereinabove calibrates nozzles 120 in such a manner that effectively all physical variations among nozzles 120 will be compensated for, in order to obtain uniform print density from nozzles 120. Returning briefly to Fig. 12, the square wave form of voltage pulse 210 is preferably used in those cases where control of print head 45 is provided digitally. That is, square wave voltage pulse 210 is preferable in those cases where the digital signal supplied to print head 45 is either "1" (e.g., for "on") or "0" (e.g., for "off"). However, one constraint or limitation on the amount of heat energy "E" supplied to ink droplet 170 is that the temperature of ink droplet 170 is preferably kept below its boiling temperature, so that nozzles 120 will not be blocked by coalescence of bubbles. As described more fully hereinbelow, a different pulse wave form is substituted for the square wave form of Fig. 12 in order to mitigate formation of voids or bubbles. Therefore, referring to Fig. 18, in order to mitigate formation of bubbles, an analog wave form 260 may be used. Analog wave form 260 has a low voltage preheat region to warm-up ink droplet 170, a peak voltage, and then a logrithmically decreasing voltage region. Analog wave form 260 will allow ink droplet 170 to be released from nozzle 120 without excessive heating, so that significant void formation is precluded. It is understood that analog wave form 260 may be substituted for the square wave form 210, if desired. It is appreciated from the teachings herein, that an advantage of the present invention is that images of uniform print density are provided even in the presence of variations in such factors as electrical resistance of the heaters and/or diameter of the nozzle orifice. This is so because each nozzle 120 is calibrated by means of either the Resistance Calibration Technique or by means of the Density Calibration Technique to compensate for such variability among nozzles 120. Another advantage of the present invention is that use thereof saves time because correcting print density non-uniformities for each input image file is not required. That is, image processing is not required for each and every input image for which output density correction is desired. This is so because print head 45 is preferably calibrated once, such as at manufacture, rather than each time an image file is acquired by input source 30. A further advantage of the present invention is that it eliminates visual printing defects, such as "banding". Of course, this is so because the print nozzles print with uniform density. The invention is described with reference to a scanner or computer being used to provide the input image. However, any suitable input imaging device may be used to provide the input image. As another example, the invention is described with reference to an ink-jet printer. The image forming characteristic is print density in the preferred embodiment of the invention. However, any applicable image forming characteristic may be selected, such as ink droplet volume. Therefore, what is provided is an imaging method for providing images of uniform print density, so that printing non-uniformities, such as banding, are avoided. PARTS LIST 10 imaging apparatus 20 printer 30 input device 40 controller 45 print head 50 recorder medium 60 rollers 70 input supply tray 80 output tray 90 ink fluid cavities 100 body of ink 110 ink fluid reservoir 120 nozzle 130 flow channel 140 orifice portion 150 heater 160 voltage supply unit 170 droplet 180 test image 190 density patches 200 marginal region 210 square wave voltage pulse 220 memory unit/chip 230 first look-up table 240 second look-up table 250 flow chart 260 analog wave form

Claims: An imaging method comprising: providing a plurality of nozzles (120), each of said nozzles defining a fluid cavity containing print fluid therein under pressure, the nozzles each terminating at an outlet wherein a meniscus of the print fluid has a predetermined surface tension that is changed in response to heat; providing a plurality of heater elements (150) in heat transfer communication with the print fluid menisci and heating the print fluid menisci so that the surface tension of the print fluid menisci relax as said heater elements heat the print fluid menisci, each of said heater elements being adapted to receive a predetermined voltage pulse; providing a voltage supply unit (160) associated with said heater elements and supplying a respective predetermined voltage pulse to each of said heater elements, the voltage pulse to a respective heater element generating heat sufficient to reduce surface tension of the respective meniscus so that back pressure of the print fluid in the fluid cavity propels the print fluid as a drop from the fluid cavity so that a free drop is created between the nozzle and the receiver medium without significant void formation in the nozzles; and providing a controller (40) connecting said heater elements and said voltage supply unit and the controller controlling the voltage pulse supplied to each of said heater elements, so that the voltage pulses supplied to some of said heater elements selected for generation of drops to be printed at the same density and color are provided with different amplitudes and/or pulse durations to correct for nonuniformities in operation of respective nozzles, the controller including a memory storing calibration data associated with each nozzle and the controller operating in response to image data and the calibration data to generate voltage pulses to the respective heating elements for producing respective drops on the receiver medium that are uniform when the image data for the respective nozzles have the same density value. The imaging method of claim 1, wherein the memory stores data related to pulse amplitude values and communicates to said controller data related to a pulse amplitude for a print density requested for printing by a nozzle, and data related to amplitude values communicated to said controller for at least some of said nozzles, in response to data requests representing the same density value to the memory unit, are different to provide instructions to the controller for different amplitude values to compensate for nonuniformities in the nozzles. The imaging method of claims 1 or 2 and wherein a voltage pulse to a heater element for generating a drop has an amplitude portion logarithmically varying with respect to time. The imaging method of claim 1 and wherein the print density of each nozzle is determined by the duration of the voltage pulse respectively supplied to each heater element associated with each nozzle.

Inventors: WEN XIN, WEN, XIN, WEN, XIN, C/O EASTMAN KODAK COMPANY
Applicants: EASTMAN KODAK CO, EASTMAN KODAK COMPANY
Application year: 1998
Main classification: B41J2
IPC codes: B41J2