BACKGROUND

[0001] A printing system may include a pen or a printhead with a plurality of nozzles that deliver or deposit print agent onto a printing substrate. Printing systems can be two-dimensional (2D) printing systems or three-dimensional (3D) printing systems. Printing processes may comprise heating. Heat may be applied by a heating device to the delivered or deposited print agent. Examples of heating in printing processes may be drying, curing, dye-sublimating or fixing.

BRIEF DESCRIPTION OF THE DRAWINGS [0002] Various example features will be apparent from the detailed description which follows, taken in conjunction with the accompanying drawings, wherein: [0003] FIG. 1 is a block diagram of a method according to an example.

[0004] FIG. 2 represents a color chart according to an example.

[0005] FIG. 3 is a block diagram of a method according to an example.

[0006] FIG. 4 schematically illustrates a system according to an example.

[0007] FIG. 5 schematically illustrates a system according to an example.

[0008] FIG. 6 represents a controller including a processor and a non-transitory machine-readable storage medium according to an example.

[0009] FIG. 7 is a block diagram of the instructions included in the non- transitory machine-readable storage medium of FIG. 6.

DETAILED DESCRIPTION

[0010] The present disclosure presents examples of methods and apparatuses for adjusting a heating parameter of a heating device used in a printing process. Heating devices may be used to apply heat onto a print agent delivered and deposited on a printing substrate. The print agent on the printing substrate may be heated by conduction, convection or radiation or any mix thereof. For example, a heating device having a radiation emitter may heat or radiate a print agent on the printing substrate. Examples of heating in a printing process may be drying, curing, dye-sublimating or, more generally, fixing the print agent to the substrate. [0011] Some printing processes may include curing the print agent ejected onto the printing substrate. Some print agents may comprise a solvent liquid, a pigment and latex particles. The solvent liquid, e.g. water-based solvent, may be evaporated in a drying process. Then, in a curing process, the print agent may be heated and the latex particles may be cross-linked to form a film that encapsulates the pigment. In curing processes, print agent may be heated with a heating device to a temperature between 90 °C and 150 °C.

[0012] Sublimation printing (also known as dye-sublimation printing or “dye- sub”) may be used to directly or indirectly print an image on textile printing substrates, e.g. garment fabrics. Textile materials may comprise cotton or polymers, e.g. polyester. In some examples, the image may be printed onto a textile printing substrate provided in a roll or sheet format, whereas in further examples, the image may be printed directly onto a garment, i.e. direct-to- garment printing. Moreover, some dye-sub methods may involve printing an image onto a sublimation transfer printable medium, e.g. paper, with a printing system and transferring this image to final substrate, e.g. to a polyester fabric or to a polymer-coated substrate fabric.

[0013] In a sublimation printing process, a dye within a print agent, e.g. ink, may be converted from a solid to a gaseous state to penetrate into a printing substrate and may then be fixed on the printing substrate to form an image. The print agent may be heated at a sublimation temperature or above to convert it to gas which permeates fibers of some printing substrates, e.g. fibers of a textile printing substrate. The gas may be again converted to a solid state when the temperature drops and the print agent may thus be absorbed and integrated into the printing substrate, e.g. integrated into the fibers of a fabric printing substrate. An image is thus printed and fixed into the printing substrate. In some examples, a sublimation temperature of the print agent may be between 150°C and 250°C. In some examples, a sublimation temperature of the print agent may be between 170°C and 220°C. In some examples, the print agent may be heated to a temperature at or above the sublimation temperature of the print agent. [0014] Color response of an image generated onto a printing substrate may depend on the heating profile applied to the printing substrate. For example, in dye sublimation printing processes, lightness of the image may depend on the temperature reached by the print agent deposited on the printing substrate and its duration. Lower heating temperatures may involve lightness greater than expected. For example, higher heating temperatures may damage the printing substrate, e.g. a coating of the printing substrate, or modify its expected color response, e.g. obtaining a color response different to the expected color response. Color parameters may thus be affected by the heating temperature. [0015] The heating profile may depend on the energy or heat generated by a heating device or how this energy is delivered by the heating device. A heating parameter may be regulated to heat the print agent on the printing substrate at a predetermined heating profile.

[0016] In this disclosure, “heating profile” refers to the heating parameter or heating parameters that define energy or heat transferred from the heating device to a portion of the printing substrate and how this energy is transferred. In one example, a heating profile may be defined by a heating temperature applied during a heating time. In one example, the heating profile may be defined by the radiation emitted by the heating device and the frequency of this radiation. In this disclosure, “heating parameter” refers to a parameter that characterize the energy generated and applied by the heating device towards the printing substrate. Examples of a heating parameter may be heating temperature, heating rate, heat or energy generated by the heating device, heating time, radiation and radiation frequency. Modifying the heating parameter implies modifying the energy transferred to printing substrate. The heating profile may be modified by adjusting the heating parameter to a specific heating parameter value. A heating parameter value is an amount or a magnitude of the heating parameter.

[0017] FIG. 1 is a block diagram of a method according to an example. The method 300 comprises heating 310, with a heating device, a plurality of color charts on a printing substrate using a heating parameter, wherein each of the color charts is heated using a different heating parameter value. A different heating parameter value may be set to heat each color charts. The heating device may be controlled to apply a different heat profile to each of the color charts. In one example, the color charts may thus be heated at different heating temperatures. In some examples, the color chart may be heated at different heating time, at different radiance or a different radiance frequency.

[0018] The heating profile provided by the heating device may be defined by a heating parameter value of the heating device. The heating parameter value is the amount or a magnitude of the heating parameter. In some examples, the heating parameter value may be the heat or thermal energy transferred from the heating device towards the printing substrate. The heating parameter value may be set at a predetermined level to adjust the heating profile of each of the color charts of the plurality of color charts. Variations of the heating parameter value thus imply variations of the heating profile of the color charts, e.g. the heating temperature.

[0019] The method 300 further comprises comparing the comparing two or more color charts of the plurality of color charts as represented at block 310. The color or the color response of the two or more color charts is compared. For example, lightness of the two or more color charts is compared. The influence of the heating profile may thus be determined. In some examples, a user may manually compare the two or more color charts. In some examples, a color measuring device may be used to compare color charts.

[0020] As represented at block 330, the method 300 comprises selecting, based on the comparison, a color chart of the two or more color charts. A color chart from the two or more color charts may be selected based on the color response. For example, the color chart of the two or more color charts providing a better color response may be selected.

[0021] The method 300 additionally comprises adjusting, based on the selection, the heating parameter of the heating device, as represented at block 340. The method 300 may thus be used for calibrating the heating parameter of the heating device based on a color response of the compared color charts. The heating parameter value of the heating device may thus be adjusted to provide an optimum color response. The heating parameter value for sublimating or fixing print agent on a printing substrate may thus be precisely determined. [0022] The two or more color charts are compared to determine the influence of the heating profile, and consequently of the heating parameter value, on the color response of each of these color charts. The adjusted heating parameter value, e.g. amount of energy to be transferred from the heating device to an image, may be set in the heating device for printing methods involving heating. [0023] Comparing different color charts, instead of comparing a printed image on the printing substrate with a calibration image printed on a different printing substrate, allows increasing the precision of the calibration. Real conditions of the print agent, of the printing substrate and of the heating device may thus be taken into account for optimizing the heating parameter value, e.g. the amount of energy transferred by the heating device. The heating parameter may thus be adjusted for a given printing substrate and a given print agent. Variations between different batches of printing substrate or print agent may thus be considered for adjusting the heating parameter of the heating device. Image quality may consequently be increased.

[0024] In some examples, selecting the color chart of the two or more color charts is manual. After comparing two or more color charts heated at different heating profile, a user may select one color chart of the two or more color charts. The selection may be based on the color response of the color charts. In some examples, a user interface device may be used for selecting one color chart from the two or more color charts.

[0025] In some examples, a user may compare the two or more color charts of the plurality of color charts. The user may compare the color response of the two or more color charts and then select the color chart having the best color response.

[0026] In some examples, comparing 320 the two or more color charts may comprises using a color measuring device to scan the two or more color charts. In some examples, a controller may obtain a color parameter value for each scanned color chart and compared the obtained color parameter values. In some examples, a user interface device may receive the color parameter value for each scanned color chart and the user may compare the color parameter values to select one color chart from the two or more color charts, each of them heated following a different heating profile.

[0027] In some examples, comparing 320 the two or more color charts of the plurality of color charts comprises scanning, with a color measuring device, the two or more color charts; obtaining a color parameter value for each scanned color chart; and comparing the obtained color parameter values. Accordingly, the color measuring device scans two or more color charts heated using different heating parameter values.

[0028] A color parameter is a parameter indicative of the color or of the color response of the two or more color charts. The color measuring device, e.g. a spectrophotometer, may scan and analyze the two or more color charts. Color parameter values for the heated color charts may be determined by the color measuring device.

[0029] In some examples, the color measuring device may output a signal based on the result of scanning these two or more color charts towards a controller. The controller may then determine the color parameter value of these two or more color charts.

[0030] In some examples, the color parameter values are determined by the color measuring device and the controller receives the color parameter values. [0031] The color parameter value is indicative of the color response of a printed image. The color parameter values of the scanned color charts are compared to determine the influence of the heating profile on the color response of each of the scanned color charts. The adjusted heating parameter value, e.g. amount of energy to be transferred from the heating device to an image, may be set in the heating device for printing methods involving heating.

[0032] In some examples, the controller compares the obtained color parameter values. Based on the comparison, the controller may adjust the heating parameter of the heating device. Consequently, methods involving a controller to compare the two or more color charts may reduce the user- interaction. These methods involve an automatic calibration. Manual operations involved in adjusting the heating profile to be applied in sublimating or fixing processes may be avoided. Productivity of calibrating a heating device used in printing processes may consequently be increased.

[0033] In some examples, adjusting the heating parameter comprises determining a heating parameter value. The determined heating parameter value may be stored in a storage medium. The heating parameter value may be used in sublimating or fixing print agent on the printing substrate. For example, the heating parameter may be re-adjusted by selecting a different heating parameter value for a different heating process or for a different printing substrate or for a different print agent.

[0034] The color sensor device, e.g. spectrophotometer, may measure a reflection or transmission property of a material as a function of wavelength to measure color. In some examples, the color sensor device may convert the measured data to color space to describe a color. Examples of color spaces may be Munsell color space, CIELAB color space and CIEXYZ color space. [0035] In the Munsell color space, a given color is defined by hue, value or lightness and chroma. Lightness in the Munsell color space varies from “0” (corresponding to black) to “10” (corresponding to white).

[0036] In the CIELAB color space, a given color is defined by L*a*b*. L* defines the lightness of a color and it ranges from “0” (corresponding to black) to “100” (corresponding to white). The combination of a* and b* defines the hue and the chroma of a given color. The term a* varies from green to red and the term b* from blue to yellow. In the CIELAB color space, a difference between two colors may be quantified by delta E (DE). Delta E or DE may be calculated by determining the Euclidean distance from the difference between L*, a* and b* of two colors according to the following expression: wherein L*i and L*2 are respectively the lightness (L*) of the first and of the second color, a*i and a*2 are respectively the redness-greenness (a*) of the first and of the second color and b*i and b*2 are respectively the yellow-blueness (b*) of the first and of the second color. Delta E or DE greater than 1 may indicate that the two colors are different. [0037] The color measuring device may thus quantify the color difference between two color test charts.

[0038] In some examples, the method to calibrate the heating parameter comprises scanning, with the color measuring device, an additional color chart of the plurality of color charts and repeating, the scanning of the additional color chart and comparing the obtained color parameter values until a comparison result value between two color parameter values is equal to or less than a reference differential color value. Additional color charts are scanned while the color response of these additional color charts improve the color response of previous color charts. This iterative process may be stopped when the difference between two color parameters of any of the scanned color charts is less than a reference differential color value. This may indicate that the color response of the latest scanned color chart substantially does not imply a significant color response improvement over at least one of the other scanned color charts. Or in other words, the method may iteratively scan and compare the color parameter values while the color parameter values are substantially different.

[0039] In some examples, the color parameter values may be compared in pairs. The color parameter values may be selected depending on the heating profile. For example, in heating profiles comprising a heating temperature, the color parameter values may be selected in ascending order of the heating temperature of the corresponding color chart. Identifying that a color response of two consecutive heating temperatures does not improve may indicate that the highest heating temperature of these two consecutive heating temperatures does not substantially improve the color accuracy. The heating parameter value used by the heating device to apply energy to the color chart at the lowest heating temperature of these two consecutive heating temperatures may be stored. This stored heating parameter value may be used in subsequent printing and heating operations.

[0040] The reference differential color value may be adjusted for a given application. Applications requiring a higher color accuracy may require a lower reference differential color value than other applications. For example, the reference differential color value may be delta E equal to 1. In other examples, the reference differential color value may be delta E equal to 0.5. When the reference differential color value is delta E equal to 0.5, the method iterates until the color difference between two color charts is below 0.5. Color accuracy may thus be improved.

[0041] In some examples, the color parameter is lightness, and the reference differential color value may be, for example, 0.25 measured in the CIELAB color space. In this example, the method iterates until the lightness difference between two color charts is below 0.25, measured in the CIELAB color space. [0042] Iterating the scanning of the additional color charts and comparing the obtained color parameters may allow to automatically determine an optimum color parameter value. The heating parameter value may be determined to provide a heating profile corresponding to the heating profile of the color chart having the optimum color parameter value. The method may further comprise storing the determined heating parameter value for subsequent heating operations. The heating parameter for subsequent heating operations may thus be precisely adjusted.

[0043] In some examples wherein, the heating profile comprises a heating temperature, the color charts of the plurality of color charts are scanned in ascending order of the heating temperature. The color chart heated at the lowest heating temperature is scanned before the other color charts. Then, the color charts heated at a consecutive heating temperature are scanned. In these examples, the color charts heated at a consecutive heating temperature are compared. This allows comparing the color parameter values of the two last scanned color charts, rather than comparing one color chart with all the other color charts.

[0044] In other examples, the color charts of the plurality of color charts are scanned in a random order of heating temperature or of the heating profile. In these examples, the color parameter value of the last scanned color chart is compared with the color parameter value of the previously scanned color charts. [0045] In some examples, the method may comprise storing the color parameter values obtained for each of the color charts. Different color parameter values may thus be compared.

[0046] In some examples, heating 310 the plurality of color charts on the printing substrate using a heating parameter comprises heating, with the heating device, a plurality of non-printed zones. Each of the non-printed zones is associated with one color chart of the plurality of color charts. The method 300 may further comprises comparing the non-printed zones associated with the two or more color charts with a reference substrate color. Influence of the heating profile or of the heating parameter value on the non-printed zones may thus be determined.

[0047] In some examples, the user may compare the non-printed zones associated with the two or more color charts, with the reference substrate color. [0048] In some examples, the color measuring device may scan the two or more color charts of the plurality of color and the non-printed zones associated with the two or more color charts. A target zone may be defined as the color chart and the non-printed zone associated with this color chart. A color chart and its corresponding non-printed zone may thus be concurrently heated.

[0049] The method may further comprise obtaining a color parameter value for each of the scanned non-printed zones and comparing the obtained color parameter values of the scanned non-printed zones with a reference substrate color value. The reference substrate color value may be indicative of the expected color parameter value of a non-heated zone of the printing substrate. The color parameter values of the non-printed zones, each of them heated at different heating profile, i.e. using a different heating parameter value, is compared with the color parameter value of the non-heated zone of the printing substrate. Influence of the heating profile, and consequently on the heating parameter value, on the non-printed zones may thus be determined.

[0050] In some examples, the reference substrate color value may be a predetermined color parameter value for a given printing substrate. The reference substrate color value may be obtained from a user interface device or from a storage medium. [0051] In some examples, the reference substrate color value may be obtained from scanning a non-heated zone of the printing substrate. A color parameter value of the non-heated zone of the printing substrate may be compared with the heated non-printed zones.

[0052] The method may further comprise adjusting the heating parameter of the heating device, further based on the comparison of color parameters of the non-printed zones with the reference substrate color. For example, the heating parameter may be adjusted also taking into account the comparison of color parameter values obtained by the color measuring device with the reference substrate color value. The heating parameter may thus be adjusted considering both the color response of the color charts and the color response of the non- printed zones. Taking into account the effect of the heating profile on the printing substrate may prevent a deterioration of the printing substrate. For example, higher heating temperatures may deteriorate the printing substrate. This may involve a change in the color response. For example, lightness of an overheated zone may be different to lightness of a zone of the printing substrate which has not been heated. Accordingly, an excessive heating time or an excessive heating time of the printing substrate may be prevented.

[0053] Comparing the non-printed zones with the reference substrate color chart may also be used to prevent ghosting or vapor mark defects. Vapor mark defects may occur when an excess of print agent has been delivered onto the printing substrate. Evaporation rates of the print agent vapor may influence the vapor mark defects. Low evaporation rates of the print agent vapor may reduce the ghosting or vapor mark defects.

[0054] In some examples, the color parameter comprises lightness. In some examples, the color parameter comprises hue. In some examples, the color parameter comprises chroma. In some examples, a color parameter may be a combination of lightness, hue and chrome. In some examples, a color parameter comprises redness-greenness. In some examples, a color parameter comprises yellow-blueness. In some examples, the color parameter comprises a combination of lightness, redness-greenness and yellow-blueness. [0055] In some examples, the method comprises printing the plurality of color charts on the printing substrate. Print agent may be delivered by a printhead of a printing apparatus onto the printing substrate.

[0056] During printing, an advancing system of the printing apparatus may move the printing substrate along a printing substrate advance direction. The plurality of color charts may be moved towards the heating device along the printing substrate advance direction.

[0057] In some examples, the plurality of color charts may be distributed in a direction perpendicular to the printing substrate advance direction. This may allow heating the plurality of color charts within a same swath. Printing substrate waste and printing time may thus be reduced.

[0058] FIG. 2 illustrates a color chart according to an example. The color chart 220 of FIG. 2 is an example of a color chart printed on the printing substrate. The color chart 220 comprises a plurality of rows and a plurality of columns. Each row represents a print agent composition. The color chart 220 of this figure comprises five rows, which corresponds to five different print agent compositions. In this example, the plurality of rows comprises a row printed from C (cyan), a row printed from M (magenta), a row printed from Y (yellow), a row printed from K (key) and row printed from CMYK (a combination of C, M, Y and K). In some examples, other combinations of print agent may be used. In some examples, the color chart may comprise less than 5 rows, e.g. two rows, or more than 5 rows, e.g. ten rows. The color chart may comprise a suitable number of rows.

[0059] Each column of this color chart 220 represents a different amount of print agent. The amount of print agent increases from left to right. This color chart 220 comprises five columns. These five columns represent an amount of print agent that increases from 20% to 100% in intervals of 20%. An amount of 100% represents the maximum amount of specific print agent composition than the printhead can deliver on the printing substrate. The maximum amount may vary from one print agent composition to another print agent composition.

[0060] The rows and columns of the color chart 220 defines a plurality of color patches. In this example, the color chart 220 comprises 25 color patches. Each of the color patches is defined by an amount of print agent and by the print agent composition.

[0061] Print agent compositions and the amount of print agent to generate a color chart may depend on the type of printing substrate and on the type of image to be printed and heated in e.g. a sublimation printing.

[0062] Printing a color chart may comprise printing a plurality of color patches with a first print agent composition, wherein each of the color patches of the first plurality of color patches have a different amount of the first print agent composition. For example, the first print agent composition may be cyan and 5 color patches may be printed, with an amount of cyan of 20%, 40%, 60%, 80% and 100%, respectively. In addition, a second plurality of color patches may be printed with a second print agent composition, wherein each of the color patches of the second plurality of color patches has a different amount of the second print agent composition. For example, the second plurality of color patches may correspond to the five color patches generated with an amount of magenta of 20%, 40%, 60%, 80% and 100%.

[0063] The color measuring device may obtain a color parameter value for each color patch of the first plurality of color patches and of the second plurality of color patches. Color response for each print agent composition heated at different heating profiles may thus be compared. For example, the color patch of 20% of cyan of a color chart heated at a first heating profile may be compared with the color patch of 20% of cyan of a color chart heated at a second heating profiles. Influence of the heating profile, and consequently of the heating parameter value, may thus be determined for each print agent composition. [0064] In some examples, the color chart may comprise a plurality of color patches and a non-printed zone surrounding each of the color patches. These color patches may be generated by using different print agent compositions, e.g. different colors. The non-printed zone is thus associated with the color chart.

[0065] FIG. 3 is a block diagram of a method according to an example. The method 400 of FIG. 3 may be used to determine a heating parameter of a heating parameter that generates a heating profile to be applied from the heating device towards the printing substrate in printing processes involving heating. For example, in sublimation printing processes.

[0066] During a sublimation process, print agent deposited on the printing substrate is heated following a heating profile. Print agent may be heated to a temperature above the sublimation temperature of the printing substrate during a heating time. Heating temperature may thus be a heating parameter of the heating device. In one example, the heating temperature applied during a predetermined period of time for sublimating an image on a given printing substrate may be set at To. In this example, To is 228°C. This heating temperature for sublimating an image on this printing substrate may be adjusted to obtain an optimum color response according to any of the examples herein. This To may be obtained from a storage medium or from a user-interface device. In this example, the heating temperature is a heating parameter of the heating device. In other examples, heating time, heating time, radiance, radiance frequency may be a heating parameter of the heating device. In some examples, a plurality of heating parameter may be adjusted. For example, heating time and heating temperature may be adjusted.

[0067] The method 400 comprises printing 410, with a printhead of a printing apparatus, a plurality of color charts on a printing substrate. A printhead or a plurality of printheads may be used to deliver print agent on the printing substrate. The color charts may be generated according to any of the examples herein. In some examples, the plurality of color charts is distributed along a width direction of the printing substrate. In this example, five color charts are printed on the printing substrate. However, in other examples, a different number of color charts may be printed.

[0068] A plurality of target zones is heated at block 420. One target zone comprises a color chart and a non-printed zone. Each color chart is associated with a corresponding non-printed zone to define the target zone. The non- printed zone may be adjacent to the color chart or integrated within the color chart. In this example, the plurality of target zones comprises five target zones. [0069] In this example, the target zones are heated at different heating profiles using different heating parameter values. In this example, the heating profiles comprises a heating time and a heating temperature. The heating time of these heating profiles is fixed to a predetermined heating time, while the heating temperature varies from one heating profile to another one. A first target zone is heated at a first heating temperature T-i, a second target zone is heated at a second heating temperature T2, a third target zone is heated at a third heating temperature T3, a fourth target zone is heated at a fourth heating temperature T4 and a fifth target zone is heated at a fifth heating temperature T5. In this example, the heating temperatures T 1 , T2, T3, T4 and T5 respectively corresponds to 210°C, 215°C; 220°C, 225°C and 230°C.

[0070] The method 400 further comprises scanning two or more target zones of the plurality of target zones as represented at block 430. In this example, the target zones are scanned in ascending order of heating temperature. The first and the second target zones are thus scanned with a color measuring device, e.g. an embedded spectrophotometer. The first target zone comprises a first color chart and a first non-printed zone and the second target zone comprises a second color chart and a second non-printed zone.

[0071] A color parameter value of the color charts and a color parameter value of the corresponding non-printed zones is obtained as represented at block 440. In this example, the color parameter is lightness. The color measuring device may thus obtain the lightness of the color charts and of the non-printed zones. Lightness in the CIELAB color space of the first color chart L*i _c and of the second color chart L*2c may be obtained. Additionally, lightness of the first non- printed zone L*i _n and of the second non-printed zone L*2n may be obtained. [0072] In this example, L*i _c corresponds to 3.0, L*2c corresponds to 2.5, L*i _n corresponds to 90.2 and L*2n corresponds to 90.2. Color response of the non- printed zones remains substantially constant.

[0073] At block 450, comparing the obtained color parameter values of the scanned color charts and comparing the obtained color parameter values of the corresponding non-printed zones with a reference substrate color value is represented. In this example, L*i _c is compared to L*2c. L*i _n and L*2n are compared with the reference substrate color value. In this example, the reference substrate color value is the desired lightness of the printing substrate. The reference substrate color value for this example may be 90.

[0074] If a comparison result value between the obtained color parameter values of the scanned color charts is greater than a reference differential color value and a comparison result value between the obtained color parameter values of the scanned non-printed zones with the reference substrate color value is less than a threshold, as respectively represented at blocks 460 and 470, then scanning, with the color measuring device, an additional color chart of the plurality of color charts and a non-printed zone associated with the additional color chart as represented at block 480.

[0075] The method further includes repeating, the scanning of the additional color chart and the non-printed zone associated with the additional color chart and comparing the obtained color parameter values of the scanned non-printed zones with the reference substrate color value and comparing the obtained color parameters.

[0076] If the comparison result value between the obtained color parameter of the scanned non-printed zones and the reference substrate color value is not less than the threshold or the comparison result value between the obtained color parameter values of the scanned color charts is not greater than the reference differential color value, then the heating parameter of the heating device is adjusted, based on comparing the obtained color parameter values of the scanned non-printed zones with the reference substrate value and on comparing the obtained color parameter values of the color charts as represented at block 490.

[0077] The reference differential color value and the threshold may be any suitable value. In some examples, the reference differential color value and the threshold may have the same value. In this example, the reference differential color value is 0.25 and the threshold is 0.3. As the difference between l_*2c (2.5 in this example) L*i _c (3.0 in this example) is greater than the reference differential color value (0.25 in this example) and the difference between L*i _n (90.2 in this example) and l_*2n (90.2 in this example) and the reference substrate color value (90.0 in this example) is less than the threshold (0.3 in this example), a third target zone is scanned. The third target zone comprise a third color chart and a third non-printed zone.

[0078] A lightness of the third color chart l_*3c and a lightness of the third non- printed zone l_*3n is obtained. In this example, the value of l_*3c is 2.2 and the value of l_*3n is 90.1. As the difference between l_*3c (2.2 in this example) and L*2C (2.5 in this example) is greater than the reference differential color value (0.25 in this example) and the difference between l_*3n (90.1 in this example) and the reference substrate color value (90.0 in this example) is less than the threshold (0.3 in this example), a fourth target zone is scanned. The fourth target zone comprises a fourth color chart and a fourth non-printed zone.

[0079] The value of the lightness of the fourth color chart l_*4c is 2.0 and the value of the lightness of the fourth non-printed zone l_*4c is 90.0. The difference between l_*4c (2.0 in this example) and l_*3c (2.2 in this example) is less than the reference differential color value (0.25 in this example). As the difference of the lightness between the fourth color chart and the third color chart is less than the reference differential color value, the color response of the fourth color chart is similar to the color response of the third color chart.

[0080] Heating the color chart at the heating temperature T3 of 220°C (as the third color chart) provides a similar color response than heating the color chart at the heating temperature T4 of 225°C (as the fourth color chart). The heating parameter of the heating device is then adjusted to provide the printing substrate with the heating temperature corresponding to the penultimate scanned color chart. In this example, the heating parameter may be adjusted to heat the printing substrate at the heating temperature T3 (corresponding to 220°C. The heating parameter value, in this example the heating temperature, to heat the printing substrate at the heating temperature T3 may be determined. The heat device may be powered at a power level to adjust the heating parameter at the determined heating parameter value. The heating device may thus be adjusted to provide a heating temperature of T3 (corresponding to 220°C in this example) during the predetermined period of time.

[0081] The heating temperature for sublimating an image on this printing substrate may thus be lowered from To (in this example 228°C) to T3 (in this example 220°C). This may save energy and a good color accuracy is maintained.

[0082] FIG. 4 schematically illustrates a system according to an example. The system 100 comprises a heating device 10 to apply heat to the printing substrate 200 and a controller 130. The method of calibrating a heating parameter according to any of the examples herein may be implemented by the system 100 of this figure.

[0083] The heating device 10 may operate at different heating parameter values. The printing substrate 200 may thus be heated at different heating profiles.

[0084] The controller 130 of this figure is to operate the heating device 10 to heat a plurality of color charts, each of the color charts to be heated at a different heating profile. For example, a first color chart may be heated at a first heating profile and a second color chart may be heated at a second heating profile.

[0085] The controller 130 may regulate the heating parameter of the heating device 10. In some examples, the heating parameter is set at a first heating parameter value for heating the first color chart at the first heating profile and at a second heating parameter value for heating the second color chart at the second heating profile.

[0086] In addition, the controller 130 of this figure is to obtain a selection of a color chart from the plurality of color charts. In some examples, the controller may receive the selection of the color chart from a user, e.g. from a user interface device. In some examples, the controller may select the color chart. [0087] The controller 130 may thus be used to calibrate the heating parameter of the heating device 10 based on the selection of the color charts. The color chart may be selected based on its color response.

[0088] Real conditions of the print agent, of the printing substrate 200 and of the heating device 10 may thus be taking into account for optimizing the heating parameter value, e.g. the amount of energy transferred by the heating device 10. The heating parameter may thus be adjusted for a given printing substrate 200 and a given print agent. Variations between different batches of printing substrate 200 or print agent may thus be considered for adjusting the heating parameter of the heating device 10. Image quality may be consequently increased.

[0089] In some examples, the heating device 10 comprises an electrical resistance to produce heat. Electricity may pass through the electrical resistance which is heated up.

[0090] In some examples, the heating device 10 comprises a radiation emitter. The radiation emitter may emit radiation in a relatively narrow band or in a relatively wide band, e.g. in the whole infrared spectrum. Light-emitting diodes (LED’s) or laser diodes are examples of radiation emitters with a relatively narrow band.

[0091] In some examples, the heating device 10 comprises an infrared heater element. The infrared heater element may be an electrical infrared heater element. An infrared heater element may emit electromagnetic radiation with wavelength from 700 nm to 1 mm.

[0092] In some examples, the infrared heater element may comprise a wire that generates radiant heat energy when electrical current is conducted by the wire, causing it to become heated. The wire may comprise, for example, tungsten, alloys of iron or aluminum. In some examples, the wire may be coiled to increase the radiation surface and to increase the uniformity of the radiation. [0093] In some examples, the infrared heater element may comprise infrared heater lamps. In some examples, an infrared heater lamp may comprise an incandescent lamp comprising a quartz tube filled with pressurized inert gas. The wire, e.g. a tungsten wire, may be surrounded by the quartz tube.

[0094] In some examples, the infrared heater elements may comprise ceramic infrared heating elements.

[0095] In some examples, the heater element may comprise an ultraviolet heater element. An ultraviolet heater element may emit electromagnetic radiation with wavelength from 10 nm to 400 nm.

[0096] In some examples, the ultraviolet heater element may comprise an ultraviolet lamp. In some examples, the ultraviolet heater element may comprise an ultraviolet LED. In some examples, the ultraviolet heater element may comprise an ultraviolet laser.

[0097] In some examples, the heating element may comprise a diode laser which generates laser radiation through a semiconductor. In some examples, laser diodes may emit electromagnetic radiation in specific wavelength ranges comprised in the infrared band. For example, a Nd: YAG laser is an example of a diode laser emitting at a wavelength of 808 nm.

[0098] In some examples, the heating device 10 comprises inductive heating elements, and/or a circulating heating medium, e.g. a hot fluid. In some examples, the heating device 10 may burn a heating substance, e.g. natural gas.

[0099] In some examples, the heating device 10 comprises of heating cylinders or calenders. These heating cylinders may contact the printing substrate to apply pressure and transfer heat to the printing substrate.

[0100] In some examples, the system comprises a plurality of heating devices 10. In some examples, the heating device 10 comprises a plurality of heaters. For example, a plurality of heaters may comprise infrared heater elements. The heating parameter of each of these heaters may be set to a predetermined value. A first heater of the heating device 10 may heat the first color chart at the first heating profile and a second heater of the heating device 10 may heat the second color chart at the second heating profile.

[0101] FIG. 5 schematically illustrates a system according to an example. The system 100 comprises a heating device 10 to apply heat to the printing substrate 200, a color measuring device 20 to obtain a color parameter value, a printhead 30 to deliver print agent onto the printing substrate 200 and a controller 130. In this disclosure, delivering includes firing, ejecting, spitting or otherwise depositing print agent or ink.

[0102] The controller 130 of this figure is to instruct the color measuring device 20 to provide a scan of two or more color charts of the plurality of color charts. [0103] In addition, the controller 130 of this figure is to compare the scan of the two or more color charts. For example, the controller may compare the scan of a first color chart with the scan of a second color chart. In some examples, the controller 130 may be to obtain color parameter value of the two or more scans. For example, the controller may obtain a first color parameter value corresponding to the first color chart and a second color parameter value corresponding to the second color chart. In some examples, the color measuring device 30, e.g. a spectrophotometer, may scan and analyze the two or more color charts, e.g. the first and the second color charts. Color parameter values for each color charts may be determined by the color measuring device 30. The color parameter may be according to any of the examples herein, e.g. the color parameter may be lightness.

[0104] The controller may be to determine a heating parameter value of the heating device 10 based on a comparison between the scans of the two or more color charts.

[0105] The controller of this figure allows an automatic calibration of the heating parameter. A user-interaction may be reduced as the methods according to examples of this figure relates to an automatic calibration. Manual operations involved in adjusting the amount of heat to be applied in sublimating or fixing processes may be avoided. Productivity of calibrating a heating device used in printing processes may consequently be increased.

[0106] In some examples, a printhead may comprise a plurality of print agent ejection assemblies. A print agent ejection assembly may eject or deliver print agent from a nozzle by activating a drop actuator associated with the nozzle, e.g. in fluid communication with the nozzle.

[0107] In some examples, the drop actuator may be a heating element, e.g. a thermal resistor element. An electrical current may pass through the heating element to generate heat. This heat may cause a rapid vaporization of print agent in a print agent chamber or firing chamber, increasing an internal pressure inside this print agent chamber. This increase in pressure makes a drop of print agent exit from the print agent chamber to the printing substrate through a nozzle. These printing systems may be called thermal inkjet printing systems.

[0108] In some examples, the drop actuator may be a piezoelectric element. A piezoelectric element may be used to force a drop of print agent to be delivered from a print agent chamber or reservoir onto the printing substrate through a nozzle. A voltage may be applied to the piezoelectric element, which may change its shape. This change of shape may force a drop of print agent to exit through the nozzle. These printing systems may be called piezoelectric printing systems.

[0109] The method of calibrating a heating parameter according to any of the examples herein may be implemented by the system 100 of this figure.

[0110] As in FIG. 4, the controller 130 controls the operation of the heater device 10 and adjust the heating parameter of the heater device 10. In addition, the controller 130 of this example is to operate the color measuring device 20 and the printhead 30 to print a plurality of color charts.

[0111] In this example, the printhead 30 is mounted on a printhead support 40. In this figure, the color measuring device 20 comprises a spectrophotometer mounted at the printhead support 40. This spectrophotometer may be used for scanning the print agent delivered on the printing substrate in printing operations.

[0112] The printing substrate may advance or move along a printing substrate advance direction 210. The printing substrate may be moved by an advancer (not shown in FIG. 5). An advancer may include a roller and/or a wheel. The printing substrate 200 may be of any shape or size to be used in the printing system.

[0113] The heater device 10 of FIG. 5 is downstream the printhead 30. The advancer may move the color charts towards the heater device 10 along the printing substrate advance direction 210. Printing and heating operations may be performed within the same system.

[0114] In some examples, the printhead support 40 comprises a carriage. The printhead 30 may thus be mounted on a carriage for moving across a scan axis. The printhead 30 may travel repeatedly across a scan axis for delivering print agent onto a printing substrate 200, which may advance along printing substrate advance direction 210. In some examples, the color measuring device 20 may also travel across the scan axis. The scan axis may be substantially perpendicular to the printing substrate advance direction 210. This type of printing systems is known as scanning-axis printing systems. In some examples, several printheads 30 may be mounted on the carriage. In some examples, four printheads 30 may be mounted on a single carriage. In some examples, eight printheads 30 may be mounted on a single carriage.

[0115] In some examples, the printhead support 40 may be static. The printhead 30 may extend along a width of a printing substrate 200. The plurality of nozzles may be distributed within the printhead 30 along the width of the printing substrate 200. Such an arrangement may allow most of the width of the printing substrate 200 to be printed simultaneously. These printer systems may be called page-wide array (PWA) printer systems.

[0116] The heating device 10 may comprise a plurality of heaters. The plurality of heaters may comprise infrared heater elements, e.g. infrared lamps. The infrared heater elements may extend along a direction perpendicular to the printing substrate advance direction 210. The controller 130 may control the irradiance emitted from the infrared heater elements towards the printing substrate 200. Each of the infrared heater elements may be independently controlled. In some examples, each of the color charts or target zones may be heated by a specific infrared heater element. This may allow heating a plurality of color charts or target zones at the same time.

[0117] FIG. 6 represents the controller 130 including a processor 131 and a non-transitory machine-readable storage medium 132 coupled to the processor. The processor 131 performs operations on data, for example, operations for calibrating the heating parameter of the heating device 10. The processor may also be a central processing unit for controlling the operation of the system, e.g. of a printing and sublimating system.

[0118] The non-transitory machine-readable storage medium 132 is encoded with instructions 133 which, when executed by the processor 131, cause the processor 131 to adjust a heating parameter of the heating device.

[0119] The non-transitory machine-readable storage medium 132 may include any electronic, magnetic, optical, or other physical storage device that stores executable instructions. The non-transitory machine-readable storage medium may be, for example, Random Access Memory (RAM), an Electrically-Erasable Programmable Read-Only Memory (EEPROM), a storage drive, an optical disk, and the like.

[0120] FIG. 7 is a block diagram of the instructions included in the non- transitory machine-readable storage medium of FIG. 6.

[0121] The non-transitory machine-readable storage medium 132 is encoded with instructions 133 which, when executed by the processor 131, cause the processor 131 to operate a heating device to heat a plurality of target zones of a printing substrate, wherein each target zone of the plurality of target zones comprises a color chart and wherein each of the target zones is to be heated at a different heating profile as represented at block 710; obtain a selection of a color chart of the plurality of color charts as represented at block 720; and adjust, based on the heating profile of the selection, a heating parameter of the heating device as represented at block 730.

[0122] The instructions encoded in the non-transitory machine-readable storage medium represented at blocks 710, 720 and 730 may participate in calibrating or adjusting heating parameters of printing device according to any of the examples herein.

[0123] The processor 131 may control the power settings of the heating device to heat the color charts at different heating profiles by using different heating parameter values. In some examples, the heating device comprises a plurality of heaters. The processor 130 may control the plurality of heaters to apply a heat energy onto the color charts so as to heat a first color chart following a first heating profile and a second color chart following a second heating profile.

[0124] In some examples, the processor 131 may receive power settings or heating profiles or heating parameter values to operate the heating device. In some examples, the power settings or the heating profiles or heating parameter values may be received from a user interface device or from a storage medium, e.g. from a look-up-table.

[0125] In some examples, the processor 131 may operate the heating device to heat additional color charts and/or non-printed zones associated with the color charts. [0126] In some examples, the processor 131 may receive a selection of the color chart from a user interface device.

[0127] In some examples, the processor 131 may instruct a color measuring device to scan a first color chart of a first target zone and a second color chart of a second target zone of the plurality of target zones compare the color charts to be scanned by the color measuring device and select, based on compare the color charts to be scanned by the color measuring device, the target zone from the first and the second target zones. The processor may thus automatically adjust the heating parameter of the heater.

[0128] In some examples, the first color target zones and the second target zones may be selected by the processor 131 to be scanned in ascending order of the heating temperature. For example, the first and the second target zones may be the target zones of the plurality of target being heated at the two lowest heating temperature. Additional color charts may also be selected in ascending order of the heating temperature.

[0129] The processor 131 may obtain a color parameter value of the first and the second color chart. The processor 131 may instruct a storage medium to store a first color parameter value obtained from the first color chart and a second color parameter from the second color chart. The processor 131 may associate the first color parameter value with the first heating profile and the second color parameter with the second heating profile. A look-up-table may be used for storing the first color parameter value associated with the first heating profile and the second color parameter value associated with the second heating profile. Further color parameter values associated with the corresponding heating profiles may also be stored in the look-up-table.

[0130] In this example, based on the color response of the first color chart and the second color chart, the processor 131 may adjust the heating parameter of the heating device. For example, if the second heating profile provides a better color response than the first heating profile, the second heating profile may be stored in the storage medium, e.g. in the look-up-table. The processor 131 may set the heating parameter value corresponding to the second heating profile for heating an image in printing and heating processes, e.g. in printing and sublimating processes. A suitable power setting may be selected by the processor 131 to heat this image at the second heating profile. The processor 131 may thus adjust the heating parameter of the heating device for subsequent printing and heating processes.

[0131] In addition, the processor 131 may control the operation of a printhead or a plurality of printheads. Operating this printhead or this plurality of printheads may be used to print the first color chart and the second color chart during the method to calibrate the heating parameter or to print an image on the printing substrate during e.g. printing and sublimating processes.

[0132] The preceding description has been presented to illustrate and describe certain examples. Different sets of examples have been described; these may be applied individually or in combination, sometimes with a synergetic effect. This description is not intended to be exhaustive or to limit these principles to any precise form disclosed. Many modifications and variations are possible in light of the above teaching. It is to be understood that any feature described in relation to any one example may be used alone, or in combination with other features described, and may also be used in combination with any features of any other of the examples, or any combination of any other of the examples.