Printing systems and methods for operating printing systems

Methods performed by a printing system are described herein. A portion of a print area is located by operating an optical sensor to respond to a color shift. The color shift is from a printed reference color. The color shift is caused by a fixer fluid applied to the portion. Printing systems and tangible machine readable storage mediums are also described herein.

BACKGROUND

Some printing systems form a printed image by ejecting ink from ink printheads. Thereby, ink is applied onto a print medium for printing a pattern of individual dots at particular locations. The printed pattern reproduces an image on the printing medium. At least some of these printing systems are commonly referred to as inkjet printers.

A fixer fluid may be used for improving print quality of a printed pattern. In particular, a fixer fluid may address coalescence, bleed, or similar effects characterized by ink or pigment migration across a printed surface. A printing system may include a treatment printhead configured to eject a fixer fluid over the print medium. The treatment printhead applies the fixer fluid by ejecting the fixer over the particular locations for ink placement. Thereby, the fixer treats ink on the print medium in order to address the above mentioned effects. The fixer fluid may be applied before, after or, quasi-simultaneously to the application of the ink.

Some printing systems implement automatic sensing of fixer fluid applied on a print medium. Automatic sensing of fixer on the print medium may facilitate determining whether: a) a particular treatment printhead ejects, in fact, fixer fluid; b) the treatment printhead applies fixer fluid at selected nominal positions; and/or c) the treatment printhead applies fixer fluid at selected nominal densities or flow volumes.

An optical detector may be configured for sensing fixer fluid applied to a print medium. However, sensing of a fixer fluid applied on a print medium can be challenging. For example, optical detectors may have a too low sensitivity for suitably detecting fixer fluid on the print medium.

DETAILED DESCRIPTION

In the foregoing description, numerous details are set forth to provide an understanding of the examples disclosed herein. However, it will be understood by those skilled in the art that the examples may be practiced without these details. Further, in the following detailed description, reference is made to the accompanying drawings, in which various examples are shown by way of illustration. In this regard, directional terminology, such as “top,” “bottom,” “front,” “back,” “leading,” “trailing,” “left,” “right,” “vertical,”, etc., is used with reference to the orientation of the Figure(s) being described. Because disclosed components can be positioned in a number of different orientations, the directional terminology is used for purposes of illustration and is in no way limiting.

While a limited number of examples have been disclosed, those skilled in the art will appreciate numerous modifications and variations therefrom. It is intended that the appended claims cover such modifications and variations as fall within the true spirit and scope of the examples.

A fixer fluid is a fluid that facilitates reducing mobility of ink on a print medium. Fixer fluids are typically materials that may be applied beneath a colored ink drop (pre-coats or undercoats) and/or materials that may be applied over a colored ink drop (post-coats or overcoats.) Further examples of fixer fluids are detailed below. As set forth above, a fixer fluid is typically used for improving print quality of a printed pattern by addressing at least one of coalescence, bleed, or similar effects characterized by ink or pigment migration across a printed surface.

Methods for locating a treated portion of a print area are described herein. As used herein, a treated portion refers to a portion of a print medium to which a fixer fluid is applied. The fixer fluid applied to the treated portion may cause a color shift from a reference color. The treated portion is located by operating an optical sensor to respond to the color shift. Further, the optical sensor may be operated with a light emission device selected from a plurality of light emission devices. A selected light emitting device (LED) and a reference color may be predetermined for improving the response of the optical sensor to the color shift. In some examples herein, the reference color is predetermined by comparing light emission spectra from a plurality of printed colors. In some examples herein, a selected light emitting device is predetermined by comparing a light emission spectrum from a printed test color and a light emission spectrum from a printed color shifted from the test color by applying the fixer fluid. Moreover, both an LED and a reference color may be predetermined for optimizing the optical sensor response by choosing an LED and a reference color that maximize the optical sensor response to the color shift caused by a fixer fluid.

The diagram ofFIG. 1shows a portion of a printing system1according to an example. Printing system1is for reproducing an image30on a print medium10. Typically, printing system1is an inkjet printer. Printing system1includes a movable carriage12mounted on a carriage rod4. In the illustrated example, carriage12supports four ink printheads16,18,20(which constitute a printhead assembly), a treatment printhead14, and an optical sensor24for locating printed areas on print medium10. Optical sensor24includes a light detection device62and an assembly of Light Emitting Devices (LED). In particular, optical sensor24includes four LEDs58,59,60,61. Further, printing system1includes a print media transport assembly28, on which print medium10is supported and advanced in a media advance direction52. A controller48is operatively connected to a memory device34and the above elements of printing system1.

As used herein, a printhead is a device typically including a nozzle or a nozzle array26through which drops of a fluid (e.g., an ink or a fixer) can be ejected. The particular fluid ejection mechanism within the printhead may take on a variety of different forms such as, but not limited to, those using piezo-electric or thermal printhead technology.

Each of ink printheads16,18,20,22is configured to eject ink of a different color (referred to as base colors). In particular, ink printheads16,18,20,22are fluidly connected to an ink reservoir (not shown). The ink reservoir includes separated reservoirs for providing different ink types to the ink printheads. Thereby, base colors and secondary colors may be reproduced on print medium10. Base colors are reproduced on print medium10by depositing a drop of a required ink type onto a dot location. Secondary or shaded colors are reproduced by depositing drops of different base colors on adjacent dot locations; the human eye interprets the color mixing as the secondary color or shading. Commonly used ink types include cyan ink, magenta ink, yellow ink, and black ink.

A treatment printhead as used herein is a printhead configured to eject fixer fluid for treating an area of a print medium. A treatment printhead is fluidly connected to a fixer fluid reservoir (not shown) for providing fixer fluid to the treatment printhead.

It will be appreciated that the printing system may include any suitable number of printheads. In some examples, printing system1may include at least one treatment printhead, such as two or more treatment printheads. In a further example, printing system1may include at least one ink printhead, such as two to six ink printheads, or even more ink printheads. Further, a printhead of printing system1may be a disposable printhead or a fixed printhead, which is designed to last for the whole operating life of printing system1.

Controller48is configured to execute methods described herein. Controller48may be implemented, for example, by one or more discrete modules (or data processing components) that are not limited to any particular hardware, firmware, or software (i.e., machine readable instructions) configuration. Controller48may be implemented in any computing or data processing environment, including in digital electronic circuitry, e.g., an application-specific integrated circuit, such as a digital signal processor (DSP) or in computer hardware, firmware, device driver, or software (i.e., machine readable instructions). In some implementations, the functionalities of the modules are combined into a single data processing component. In other versions, the respective functionalities of each of one or more of the modules are performed by a respective set of multiple data processing components.

Memory device34is accessible by controller48. Memory device34stores process instructions (e.g., machine-readable code, such as computer software) for implementing methods executed by controller48, as well as data that controller48generates or processes such as alignment correction data38. Memory device34may include one or more tangible machine-readable storage media. Memory devices suitable for embodying these instructions and data include all forms of computer-readable memory, including, for example, semiconductor memory devices, such as EPROM, EEPROM, and flash memory devices, magnetic disks such as internal hard disks and removable hard disks, magneto-optical disks, and ROM/RAM devices.

Controller48is operatively connected to treatment printhead14, ink printheads16,18,20,22, and the respective reservoirs to control operation thereof, in particular ejection of ink and fixer fluid for printing a pattern on print medium10(such as an image30). Controller48receives print job commands and data from a print job source (not shown), which may be a computer source or other source of print jobs. Controller48acts on the received commands to provide motion control signals to: i) print media transport assembly28to advance print medium10in the media advance direction52; and ii) carriage12to traverse across print medium10. Controller48may generate the motion control signals in consideration of estimation of printhead misalignments, for example by using calibration data stored in memory device34. Further, controller48provides firing signals to nozzle arrays26in the respective printheads in order to eject ink and/or fixer at particular locations on print medium10.

For printing, controller48generates motion signals for engendering a relative movement between the printheads mounted on carriage12and print medium10. In the illustrated example, carriage12traverses over a printing surface of print medium10along an axis54and print medium10is advanced in a media advance direction52. It will be appreciated that printing system1may engender relative displacement between the printing printheads and the print medium in different manners such as, but not limited to, scanning of carriage12over an area of print medium extending along the vertical and horizontal directions.

The length of nozzle arrays26defines a print swath or band53. The width of this band is commonly referred to as the “swath width”, which defines the maximum pattern of ink or fixer fluid which can be laid down in a single transition of carriage12. Print medium10is typically held stationary while the printheads complete a print swath. Typically, after carriage12traverses a print swath53, print medium10is advanced in direction52and carriage12traverses again for printing another portion of print medium10. Advance of print medium10may be performed after a back and forth transition of carriage12over print swath53. Typically, print medium10is advanced a fraction of the swath width. Thereby, ink and/or fixer on a particular spot of print medium10can be deposited from different nozzles of a printhead in successive transitions of carriage12over the particular spot.

Optical sensor24is configured to detect positions of marks printed on print medium10. Optical sensor24may detect printed marks by i) emitting light from LEDs58,59,60,61, ii) illuminating a portion of a printed mark with the emitted light, and iii) detecting light reflected from the illuminated portion by operating light detection device62.

As used herein, an LED refers to a device capable of emitting light in a selected spectral bandwidth. A selected spectral bandwidth corresponds to a particular color. For example, LEDs58,59,60,61of optical sensor24may include a blue LED, a green LED, an orange LED, and red LED. Further, light detection device62(or, more particularly, its spectral bandwidth of detection) is typically chosen for being capable of detecting light reflected from markings printed with the base colors. It will be understood that this configuration of optical sensor24is not limiting and other configurations are contemplated. For example, optical sensor24may include two LEDs and be configured for detecting cyan, magenta, and black ink. Optical sensor24typically includes focusing means for suitably emitting and collecting light.

Optical sensor24may be used for estimating a misalignment of an ink printhead. For estimating misalignment of an ink printhead, printing system1may print a set of test patterns (not shown) by activation of selected nozzles in selected ink printheads. Printing system1operates carriage12for scanning optical sensor24over the printed test patterns. During scanning, optical sensor24emits light on particular portions of print medium10and detects light reflected from these portions. Thereby, optical sensor24registers areas of the test markings and provides corresponding electrical signals to controller48. Optical sensor24may also be used to test whether an ink printhead ejects, in fact, ink. Further, optical sensor24may additionally be used to determine whether an ink printhead applies ink at the nominal densities or flow volume.

As set forth above, optical sensor24can be used to locate a portion of a print area treated with fixer fluid by operating optical sensor to respond to a color shift from a reference color. The color shift is caused by the fixer fluid, as illustrated inFIG. 2. Method described herein may be performed for a fixer fluid which is transparent to detection by the optical sensor. As used herein, transparent to detection indicates that a printing system cannot directly detect, in a reliable manner, a fixer fluid deposited on the print medium using the optical sensor.

FIG. 2is a simplified diagram of a printing pattern printed by printing system1. As shown in the upper part of the diagram, controller48may operate one or more of ink printheads16,18,20,22for applying an ink selection42onto a portion63of print medium10. Thereby, a color68is printed on portion63. Color68may be a base color or a secondary color depending on the ink selection42used.

As shown in the lower part of the diagram, controller48may operate treatment printhead14and one or more of printheads16,18,20,22for applying a fixer fluid40and ink selection42onto a portion64of print medium10. Thereby a color70, which is shifted from color68, is reproduced on portion64. That is, the interaction of applied fixer fluid40with applied ink causes a color shift74relative to the color that would be reproduced without fixer, i.e., color68.

Color shift74depends on various factors. Firstly, color shift74depends on the type of applied fixer. Further, color shift74depends on the quantity of fixer fluid applied to portion64. Typically, a fixer fluid according to examples herein causes a color shift in the L* coordinate of the color space. Further, a fixer fluid according to examples herein may cause a color shift in other coordinates of the color space, such as the a* or b* coordinates. A DeltaE (dE) parameter of the CIE(L*,a*, b*) 1976 may be used as a measure of the color shift. DeltaE is defined as the geometric distance between two colors in the CIELab colorspace:
dE=√{square root over ((L1−L2)2+(a1−a2)2+(b1−b2)2)}{square root over ((L1−L2)2+(a1−a2)2+(b1−b2)2)}{square root over ((L1−L2)2+(a1−a2)2+(b1−b2)2)}

According to examples herein, fixer fluid40may be applied such as to cause a color shift of at least 1 dE or, more specifically, a color shift between 0 and 20 dE.

FIG. 3is a process flow diagram illustrating a method performed by printing system1according to an example. The depicted process flow200may be carried out by execution of sequences of executable instructions. In an example, the executable instructions are stored in a tangible machine readable storage medium such as, but not limited to, memory device34. Process flow200may be carried out by controller48or any other suitable element of a printing system.

Process flow200is executed for locating a portion of a print area treated with a fixer fluid. The process flow is typically executed for determining whether a) treatment printhead14ejects fixer fluid, b) treatment printhead14applies fixer fluid at selected nominal positions, and/or c) the treatment printhead applies fixer fluid at nominal densities or nominal flow volumes. In the following, process flow200is described with reference to elements depicted inFIG. 4.FIG. 4schematically shows an arrangement for operation of an optical sensor according to an example herein.

Process flow200typically includes a pre-processing step210. Pre-processing step210may include printing reference color68(illustrated by a hatched pattern) on a print area66. In particular, controller48may control the ink printheads so as to print a selected reference color68by ejecting an ink selection over print area66. Reference color68may be any color reproducible by printing system1, such as one of the base colors or any secondary color that may be derived from the base colors.

Pre-processing step210may include applying a fixer fluid over a portion64of print area66. In particular, controller48may control treatment printhead14so as to treat portion64by ejecting fixer fluid over that portion. Typically, the fixer fluid is applied before or quasi-simultaneously to the application of the ink selection for reproducing reference color68. The fixer fluid may also be applied after the ink selection is applied. It will be understood that the portion of print area66reproducing reference color68do not necessarily have to completely surround the treated portion as illustrated in the figure. Portions of print area66reproducing reference color68may be disposed in the neighborhood of treated portion64.

As set forth above, treatment of portion64with the fixer fluid may cause a color shift from reference color68. In particular, the interaction of fixer fluid with ink on print area66may produce a physical or chemical reaction that results in a color shift relative to the color that would be reproduced without application of the fixer fluid (i.e., reference color68). Therefore, application of fixer fluid to portion64may result in shifted color70(illustrated by a cross-hatched pattern).

Typically, reference color68is selected for improving the response of optical sensor24to a color shift produced by a particular fixer fluid. For example, reference color68may be selected such that the contrast between an area with reference color68and an area with shifted color70is sufficiently high to be detected by optical sensor24. Reference color68is selected from a color gamut available to printing system1during execution of process flow200. A color gamut refers to a partial subset or a complete set of colors reproducible by printing system1.

In order to select reference color68, controller48may calculate which reference color is particularly suitable for locating a treated portion in view of a particularly used fixer fluid. Alternatively or additionally thereto, controller48may use color values stored in memory device34for selecting a reference color. The stored color values may be associated to respective types of fixer fluid so that controller48selects a reference color in consideration of the fixer fluid used. Furthermore, reference color68may be predetermined according to, for example, methods exemplified below with reference toFIGS. 5,6A and6B.

It will be understood that process flow200can be performed without requiring execution of a pre-processing step210as described above. For example, according to an example, process flow200is executed using a pre-printed medium. The pre-printed medium typically includes a background region printed with a reference color or colors. The background region includes an area or areas treated with a fixer fluid applied by a printhead, such that a shifted color is reproduced thereon. The shifted color is such that an optical detector of the printing system can distinguish treated areas from non-treated areas. Such a pre-printed medium may be loaded into printing system1.

Process flow200includes a step220of locating a treated portion of a print area by operating optical sensor24to respond to a color shift from reference color68. Step220is subject to condition225, namely that the color shift is caused by a fixer fluid applied to the treated portion. For example, controller48may operate carriage12for scanning optical sensor24over the printed test patterns along scanning lines25. During scanning, optical sensor24emits light on particular areas of print area60by operating one or more of the LEDs. Further, optical sensor24senses light reflected from the illuminated areas using light detection device62. The color shift between reference color68and shifted color70produces a contrast detectable by optical sensor24. That is, the detected light reflection values respectively corresponding to areas with shifted color70and areas with reference color68are substantially different (i.e., distinguishable from each other).

Optical sensor24, or any other suitable element of printing system1such as controller48, registers the detected light reflection values for locating treated portion64. Further, controller48may process the acquired data for correlating light reflection values with corresponding positions. Treated portion64can then be located by comparing light reflection values, identifying treated areas in view of the contrast produced by the color shift, and determining the positions of the identified areas. It is noted that locating treated portion64may merely include determining that a particular area is treated with fixer fluid. Locating treated portion64may further include determining a location65of the treated portion.

In step220, the optical sensor may be operated with an LED selected from the LEDs mounted on the sensor. Typically, the selected LED is selected for facilitating the response of optical sensor24to a color shift caused by a particular fixer fluid. In particular, contrast between reference color68and shifted color70typically depends on the light used for illuminating print area66in step220. Therefore, the response of light detection device62to the color shift depends on which LEDs are used for illumination.

The response to the color shift can be improved by selecting a particular LED for illumination. Some particular LEDs may facilitate a better response to color shift than other LEDs in the optical sensor, as further illustrated below with respect toFIGS. 6A and 6B. Further, using LEDs which do not efficiently contribute to the photodetector response may decrease the capability of the optical sensor for detecting a contrast between reference color68and shifted color70as compared to using only a selected LED or a group of selected LEDs which efficiently contribute to the photodetector response. Therefore, if no particular LED is selected, and all LEDs are used for illumination during location of portion64, the response of optical sensor24to the color shift may be not so efficient as compared to a selection of one or some of the LEDs as described herein.

Accordingly, an LED may be selected during execution of process flow200. For example, controller48may calculate which LED is particularly suitable for locating a treated portion in view of the fixer fluid and the reference color particularly used. Alternatively or additionally thereto, controller48may use values stored in memory device34for selecting an LED to execute step220. The stored values may be associated to particular conditions for printing such as which fixer fluid is being used or the particular spectral response of light detection device62. A LED may be predetermined according to the methods exemplified below with reference toFIGS. 5,6A and6B.

Printing system1may use the results from step220in a post-processing step230for performing different tasks. Post-processing step230may include determining whether treatment printhead14ejects fixer fluid. For example, controller48may execute step220; if no treated portion is located in print area66, controller48may determine that treatment printhead14does not eject fixer fluid. Thereby, printing system1may determine whether any problem, such as clogging, affects treatment printhead14.

Post-processing step230may include estimating a misalignment of a treatment printhead using a determined position of a treated portion as set forth below with respect toFIGS. 8 and 9. Further, a misalignment of treatment printhead14may be compensated during subsequent printing based on the result of the estimation.

Post-processing step230may include determining whether treatment printhead14applies fixer fluid at selected nominal densities or flow volumes. As set forth above, a color shift caused by a fixer fluid depends, among other factors, on the quantity of applied fluid. Further, the response to the color shift depends on the color shift, in particular on the contrast between reference color68and shifted color70. Color shift response and quantity of applied fluid may be correlated using semi-empirical data. Controller48may analyze data acquired by optical sensor24for determining the color shift produced by the applied fixer fluid. From this analysis, controller48may infer the quantity of fixer fluid applied and determine whether ejection of fixer fluid by treatment printhead14deviates from particular nominal conditions.

As set forth above, a selected reference color and a selected LED may be predetermined for performing location of a treated portion.FIG. 5is a process flow diagram of a method for predetermining operating conditions of a printing system according to an example herein. The illustrated process flow500can be used for predetermining an LED from the LEDs of an optical sensor used for location of a treated portion as described herein. For example, a predetermined LED can be selected for operating optical sensor24in process flow200described above. Further, the illustrated process flow500can be used for predetermining a color from a color gamut available to a particular printing system. For example, a predetermined color can be selected as reference color68in process flow200described above.

The depicted process flow200may be carried out by execution of sequences of executable instructions. In an example, the executable instructions are stored in a tangible machine readable storage medium.FIG. 7schematically shows a system700for carrying out process flow500. System700includes a system controller748operatively connected to a memory734and a spectrophotometer702. System700may be a dedicated system for executing process flow500, so that the process can be executed independently from printing system1. Alternatively, system700may form part of printing system1. It should be noted that process flow500may be executed by simulation of the process steps in an appropriate computer system using semi-empirical data.

System controller748and memory734may be constituted analogously as controller48and memory device34described above. Memory734may store operating conditions of printing system1predetermined by executing process flow500. Predetermined values by system700may be provided to printing system1in different manners such us, but not limited to, by operatively communicating system700to printing system1or by manually storing the predetermined values in memory device734.

Spectrophotometer702is a photometer that can measure light intensity as a function of the light source wavelength. In particular, spectrophotometer702can acquire an emission spectrum of a sample surface704printed with a color706. For example, an emission spectrum can be acquired using a D50 standard illuminant and an XRite Eye-One spectrophotometer (X-Rite, Incorporated USA, Mi)

In the following, process flow500is described with reference to elements depicted inFIGS. 6A and 6B, which are simplified diagrams of emission spectra corresponding to printed colors.

At step510, a plurality of emission spectra is obtained for one or more printed colors. For each of the printed colors, a spectral pair is acquired using spectrophotometer702. In particular, at step512, a first emission spectrum is acquired for the printed color without being treated with a fixer fluid. Further, at step514, a second emission spectrum is obtained from the color treated with the fixer fluid. Step510may be executed by operating printing system1for printing a print medium with one or more colors at selected regions with and without fixer. That is, two areas may be printed for each color; one area is treated with fixer fluid, and another area remains untreated.

It will be understood that step510may be executed in different manners. For example, printing system1may print different print media with different colors with and without treatment. Further, the printed colors may be reproduced by a printing system different than printing system1. Further, the printed colors may be colors not included in a color gamut of printing system1. Using colors similar to those printed by printing system1may be sufficient for predetermining operating conditions of printing system1; the color in the color gamut of printing system1most similar to the predetermined color may be selected. As set forth above, the emission spectra can also be obtained using simulation based on semi-empirical values.

FIGS. 6A and 6Bshow emission spectra according to step510. Emission spectrum92and emission spectrum94(seeFIG. 6A) correspond, respectively, to the emission spectrum of color112untreated and emission spectrum of a color shifted from color112by treatment with fixer fluid. Emission spectrum96and emission spectrum98(seeFIG. 6B) correspond, respectively, to the emission spectrum of color114untreated and emission spectrum of a color shifted from color114by treatment with fixer fluid. As illustrated, treatment with a fixer may cause a color shift which can be detected through differences between spectra corresponding to a color and spectra corresponding to the color shifted by fixer treatment. These differences may be quantified by calculating the area between both spectra.

After emission spectra are obtained, emission differences between the first emission spectrum and the second emission spectrum at different frequencies are determined at step520. Typically, the determined emission differences indicate how optical sensor24responds to a color shift caused by a fixer fluid applied to a particular color. Step520may be performed for each of the printed colors used in step510. In the shown examples, fixer treatment causes a color shift noticeable when the first and second spectra for a particular color are compared. The determined emission differences at step520correspond to differences resulting from treatment of a color with fixer fluid.

In some non-limiting examples, the emission differences are determined at selected frequencies104,106,108,110corresponding to light emitted by the LEDs of optical sensor24. The selected frequencies may correspond to peak frequencies of each of the emission spectra of the LEDs (i.e., the frequencies at which a color spectrum reaches a maximum).FIGS. 6A and 6Billustrate LED light spectra84,86,88,90corresponding to light emitted by LEDs of optical sensor24. In the example, LED light spectrum84corresponds to a blue LED having a peak frequency104; LED light spectrum86corresponds to a green LED having a peak frequency106; LED light spectrum88corresponds to an orange LED having a peak frequency108; and LED light spectrum90corresponds to a red LED having a peak frequency110.

A comparison between emission differences at different frequencies indicates which LED light facilitates an adequate response of optical sensor24to a color shift corresponding to treatment of a particular color. Therefore, an LED may be predetermined according to the determined emission differences. The predetermined LED may be selected in order to operate printing system1for performing step220of process flow200. In particular, a predetermined LED as described herein may be selected during operation of printing system1for facilitating location of a treated portion. Further, the other LEDs may be turned off during the locating step such that sensitivity of light detection device62to a color shift caused by a fixer fluid is improved relative to operating all the LEDs of light detection device62.

At step530, an LED of optical sensor24is predetermined according to the determined emission difference. For a particular color, the highest emission difference indicates which LED light results in a higher response of optical sensor to a corresponding color shift. Typically, an LED is predetermined for improving the response of the optical sensor to a particular color shift. For example, the LED may be predetermined for maximizing the difference between photodetector response to untreated areas and photodetector response to treated areas.

For execution of step530, controller748may first determine the emission differences100from data provided by spectrophotometer702. Further, controller748may compare the determined emission differences and select an LED corresponding to the higher emission difference. In the illustrated example ofFIG. 6A, the maximal emission difference for color112is at peak frequency104, which corresponds to LED light spectrum84emitted by a blue LED. Accordingly, controller748may predetermine a blue LED for being selected during execution of step220(i.e., locating a treated portion) when reference color68corresponds to color112. In the illustrated example ofFIG. 6B, the maximal emission difference for color114is at peak frequency106, which corresponds to LED light spectrum86emitted by a green LED. Accordingly, controller748may predetermine a green LED for being selected during execution of step220when reference color68corresponds to color114.

A color may be predetermined from the printed colors referred to in step510. Further, the predetermined color may be selected as reference color68during execution of step220in process flow200(i.e., locating a treated portion). In particular, emission differences determined in step520may be compared for different colors in order to assess which color is associated with a color shift caused by a particular fixer fluid that facilitates an adequate response of optical sensor24. Accordingly, at step540, a color is predetermined from the plurality of printed colors according to emission differences determined at step520. Typically, the predetermined color corresponds to a color with the highest determined emission difference. For example, after executing step520, controller548may compare emission differences for color112and color114. Controller548may then determine that the highest emission difference corresponds to color112. Accordingly, controller748may predetermine color112for being selected as reference color68in process flow200.

Typically, the printed colors used in process flow500correspond to a color gamut of printing system1. In particular, process flow500may be executed for a complete gamut of printing system1. Executing process flow500for the complete gamut facilitates predetermining a reference color that improves sensitivity for locating a treated portion. If a predetermined color is not included in a color gamut of printing system1, process flow200may be executed using the most similar color available to printing system1. Both an LED and a reference color may be predetermined for improving the optical sensor response by choosing an LED and a reference color that maximize the optical sensor response to the color shift caused by a fixer fluid. In particular, the predetermination may be performed by testing color spectra corresponding to part of or the whole color gamut of printing system1and choosing the LED and the reference color that lead to the highest emission differences.

For locating a treated portion, e.g., by execution of process flow200, at least one of a selected LED or a selected reference color may be predetermined for improving the response of optical sensor24to a color shift caused by a particular fixer fluid. In particular, both the LED and the reference color may be selected according to predetermined values such that the response of optical sensor24to a color shift caused by a particular fixer fluid is improved. For example, controller548may predetermine color112and the blue LED for being selected during execution of step220(i.e., locating a treated portion). Selection of a LED and a reference color predetermined as described herein facilitates maximizing the response of optical sensor24to a color shift caused by a particular fixer fluid.

As set forth above, an LED of the optical sensor or a reference color may be predetermined for improving the response of the optical sensor to a color shift, i.e., for facilitating the response of the optical sensor. In particular, a selected LED and a selected reference color may be predetermined such that the contrast between an untreated area (e.g., an area with reference color68) and a treated area (e.g., an area with shifted color70) can be detected by the optical sensor. Typically, the selected LED and the selected reference color are predetermined for maximizing this contrast.

As set forth above, a misalignment of a treatment printhead may be estimated using a determined position of a treated portion.FIG. 1illustrates a misalignment of treatment printhead14caused by skew from a nominal position14′ (illustrated by a dashed line). Misalignment of treatment printhead14results in an incorrect placement of fixer fluid40on print medium10. In the illustrated example, printing system1operates treatment printhead14for applying fixer fluid40over a vertical line49. However, due to misalignment, fixer fluid40is applied on print medium10along a line49′, which is rotated relative to vertical line49an angle50. Vertical line49corresponds to the theoretical positions where fixer fluid would be applied without misalignment, or with an accurate correction thereof.

It will be understood that misalignment of a treatment printhead may have other sources such as an incorrect placement of treatment printhead14in the vertical direction. Further, incorrect positioning of other elements of printing system1, such as carriage12or carriage rod4, may also cause misalignment of treatment printhead14. A combination of different sources may also originate misalignment of treatment printhead14.

Misalignment of treatment printhead14can be estimated by automatically determining the position of areas treated with a fixer fluid. In particular, treatment printhead14may apply a treatment fluid to multiple portions of print area66. The treated portions may form a calibrating pattern. Further, the positions of the treated portions may be determined as set forth above. The determined positions may be compared to nominal positions in order to estimate misalignment of treatment printhead14. As used herein, a nominal position refers to positioning data estimated by printing system1according to stored alignment data. The nominal positions correspond to positions where treated portions should be located if alignment data of treatment printhead14is accurate, e.g.: treatment printhead is not affected by misalignment, or misalignment is accurately corrected by printing system1.

FIG. 8is a process flow diagram for automatically aligning a treatment printhead according to an example herein. The depicted process flow600may be carried out by execution of sequences of executable instructions. In an example, the executable instructions are stored in a tangible machine readable storage medium such as, but not limited to, memory device34. Process flow600may be carried out by controller48or any other suitable element of a printing system.

Process flow600facilitates improving print quality of printing system1. Process flow600may be performed at predetermined servicing intervals as part of routine maintenance of a printing system. Additionally or alternatively thereto, process flow600may be performed after events that may compromise alignment of a treatment printhead. For example, process flow600may be performed when a new treatment printhead is mounted on carriage12or after servicing of elements coupled to carriage12such as carriage rod4. Additionally or alternatively thereto, a user may prompt a printer system, through a user terminal, to execute process flow600, in particular when a user has indicia that a treatment printhead is misaligned (e.g., after noticing poor print quality.) A user terminal (not shown) may be configured to receive a user prompt to execute process flow600and send a suitable signal to controller48for executing the process.

In the following, process flow600is described with reference to elements depicted inFIG. 9, which schematically shows an arrangement for operation of an optical sensor according to an example herein.

At step610, a fractional pattern67is printed on a print area66. Typically, step610includes a step612of applying an ink selection to print area66for reproducing a reference color68. For example, controller48may control the ink printheads so as to apply one or more inks for reproducing reference color68on print area66, which thereby constitutes a background region. Typically, reference color68is predetermined using a method as illustrated in process flow500described above with regard toFIG. 5.

Step610may include a step614of applying a fixer fluid to particular portions of a print area so as to reproduce a shifted color70. Shifted color70corresponds to a color shifted from reference color68by the interaction between ink applied at step612and fixer fluid. For example, controller48may control treatment printhead14so as to apply fixer fluid over portions64ato64i, on which ink is applied before or after the fixer fluid application. Typically, portions64ato64iare selected for composing a calibration pattern, i.e., a set of positions adequate for estimating misalignment of treatment printhead14. The portion of print area66reproducing reference color68does not necessarily have to completely surround the treated portions64ato64ias illustrated in the figure: portions of print area66reproducing reference color68may be at the neighborhood of the treated portion64and/or may not completely fill print area66.

Controller68controls positioning of treatment printhead14according to a set of alignment correction data38. The correction data takes into account misalignments of treatment printhead estimated in a previous printhead alignment process. The previous printhead alignment process may be analogous to process flow500. Typically, the set of alignment correction data38is stored at memory device34. Further, controller48associates a nominal position to each of the treated portions.

Typically, the fixer fluid is applied before or quasi-simultaneously to the application of the ink selection for reproducing reference color68. The fixer fluid may also be applied after the ink selection is applied. Similarly as set forth above, the applied fixer fluid reacts616with ink of the ink selection applied for reproducing reference color68. Thereby, a color70shifted from reference color68(i.e., a shifted color70) is reproduced in treated portions64ato64i.

The positions of treated portions64ato64iare determined at step620. For example, controller48may operate optical sensor24for responding to a color shift caused by the fixer fluid in portions64ato64i. Optical sensor24is typically operated with a selected LED while turning off the other LEDs so as to increase sensitivity of the sensor to the color shift as set forth above. Such a sensitivity increase facilitates that optical sensor24detects a contrast between reference color68and shifted color70for accurately determining the positions of treated portions64ato64i. The selected LED may be predetermined using a method as illustrated in process flow500described above with regard toFIG. 5.

Typically, process flow600is performed as part of an alignment procedure for estimating misalignment of ink and treatment printheads. Therefore, further calibration patterns can be provided adjacent to print area66such as a print area78. Print area78includes a calibration pattern69composed of a pattern of calibration dots82ato82i. Calibration dots82ato82iare printed with a color80reproduced by applying ink from one ink printhead, i.e., a base color. In the example, color80can be directly detected by optical sensor24.

Optical sensor24is scanned over patterns67,69following scanning lines25for detecting the positions of treated portions64ato64iand calibration dots82ato82i. A reference dot76is an indication for optical sensor24of the position of patterns67,69. During scanning, optical sensor24generates a signal corresponding to light projected from the LEDs and reflected from print medium10so that the position of treated portions64ato64iand calibration dots82ato82ican be determined.

At step630, a misalignment of a treatment printhead is estimated using the determined position at step620. For example, controller48may compare the determined positions of treated portions64ato64iwith associated nominal positions. If the determined positions do not coincide with the nominal position, controller48determines that treatment printhead14is misaligned. Further, from a difference between determined positions and nominal positions, controller48can quantify the misalignment.

At step640, misalignment estimation may be employed to modify correction data. For example, controller48may use a quantification of misalignment data for determining how a set of alignment correction data38should be modified in order to compensate misalignment of treatment printhead14during subsequent printing. In particular, controller48may determine that treatment printhead14is misaligned an angle50(shown inFIG. 1). Controller48may accordingly modify a set of alignment correction data38stored in memory device34. During subsequent printing controller48generates motion signals for carriage12and firing signals for nozzle array such that misalignment is compensated. Thereby, it is facilitated that actual positions for applying fixer fluid coincide with the nominal positions. It will be understood that different methods can be employed for misalignment correction using misalignment estimation as described herein.

In principle, any suitable ink and fixer fluid may be used for implementing the examples described herein. In examples herein, ink and fixer fluid conditions (e.g., type and quantity) are chosen such that the fixer fluid causes a color shift that is detectable as described above. In some examples, the fixer fluid may consist of a cationic polymer for reducing colorant mobility or “fix” ink on a print medium. The ink and fixer compositions may comprise standard dye-based or pigment based inkjet ink and fixer solutions. As a non-limiting example, the fixer may include a water-based solution including acids, salts and organic counter ions and polyelectrolytes. The fixer may include other components such as biocides that inhibit growth of microorganisms, chelating agents (e.g., EDTA) that eliminate deleterious effects of heavy metal impurities, buffers, ultraviolet absorbers, corrosion inhibitors, and viscosity modifiers, which may be added to improve various properties of the ink and fixer compositions. In another example, the fixer may include a component that reacts with the ink. The component may have a charge opposite to the charge of the ink. For instance, if the ink is anionic, the fixer may include a cationic component. In addition, the fixer may be substantially devoid of a colorant or may include a colorant that does not absorb visible light

The fixer fluid may also include a precipitating agent, such as a salt or an acid. The salt may include cations, such as calcium, magnesium, aluminum, or combinations thereof. The salt may include, but is not limited to, calcium nitrate, magnesium nitrate, or ammonium nitrate. The acid may be any mineral acid or an organic acid, such as succinic acid or glutaric acid. The precipitating agent may be used to change the conductivity or the pH of the ink, causing the pigment in the ink to precipitate on the surface of the print medium. The fixer may be over-printed and/or under-printed on the print medium relative to the ink.

Examples may be realized using water based latex-ink and fixer fluid suitable for fixing the latex-ink on the print medium. Thereby, quality of printing with latex-ink may be particularly improved, since latex-ink solutions may be more prone to color bleeding due to fluids in the ink solution. Other examples include solvent inks, water based inks, dye inks, or UV inks as well as fixer fluids appropriated thereto.

The print medium upon which the inkjet ink and/or fixer may be deposited may be any desired print medium. In examples, the print media may be a plain print medium or a commercially coated brochure print medium. Plain print media may include, but are not limited to, Hammermill(R) Fore DP paper, produced by International Paper Co. (Stamford, Conn.), HP Multi-Purpose paper, produced by Hewlett-Packard Inc. (Palo Alto, Calif.), uncoated polyester fabrics, polyester films, or vinyl banners. Commercially coated brochure print media, such as the type used to print brochures or business flyers, are typically hydrophobic and non-porous or less porous than plain paper, including “Lustro Laser”, produced by SD Warren Company (Muskegon, Mich.) Other examples include, among others, self-adhesive vinyls, any PVC banners, Polyproline media, polyethylene media, PET media, or polyester fabrics. The print medium may include a raw material. The print medium may be pre-treated or coated materials.

The examples described above provide methods and systems for locating a portion of a print area, to which a fixer fluid is applied. As discussed above, the examples may be successfully deployed in case that the fixer fluid is transparent to detection by an optical sensor implemented in a particular printing system. However, the examples may also be used for any fixer fluid causing a color shift detectable, e.g., by analyzing color spectra differences as described herein or any other suitable method.

It will be appreciated that examples can be realized in the form of hardware, software module or a combination of hardware and the software module. Any such software module, which includes machine-readable instructions, may be stored in the form of volatile or non-volatile storage such as, for example, a storage device like a ROM, whether erasable or rewritable or not, or in the form of memory such as, for example, RAM, memory chips, device or integrated circuits or on an optically or magnetically readable medium such as, for example, a CD, DVD, magnetic disk or magnetic tape. It will be appreciated that the storage devices and storage media are examples of a non-transitory computer-readable storage medium that are suitable for storing a program or programs that, when executed, for example by a processor, implement examples. Accordingly, examples provide a program comprising code for implementing a system or method as claimed in any of the accompanying claims and a non-transitory computer readable storage medium storing such a program.

In the foregoing description, numerous details are set forth to provide an understanding of the examples disclosed herein. However, it will be understood by those skilled in the art that the examples may be practiced without these details. While a limited number of examples have been disclosed, those skilled in the art will appreciate numerous modifications and variations therefrom. It is intended that the appended claims cover such modifications and variations as fall within the true spirit and scope of the disclosed examples.