WO2023027718A1

WO2023027718A1 - Spectrographic measurements

Info

Publication number: WO2023027718A1
Application number: PCT/US2021/047877
Authority: WO
Inventors: Alessandro Beltrami
Original assignee: Hewlett-Packard Development Company, L.P.
Priority date: 2021-08-27
Filing date: 2021-08-27
Publication date: 2023-03-02

Abstract

In an example, a method includes performing a plurality of spectrographic measurements of a sample of a substrate using UV light. A measure of variability of the measurements may be calculated using processing circuitry and, based on the measure of variability, it may be determined whether to use UV light for subsequent spectrographic measurements of a printed output.

Description

SPECTROGRAPHIC MEASUREMENTS

BACKGROUND

[0001] Printed outputs may be reviewed to determine a measure of print quality. For example, spectrophotometers may be used for measuring colors on printed substrates. Such measurements may be used to determine if printed colors meet predetermined standards and/or if the colors printed remain consistent within and/or between printing runs.

BRIEF DESCRIPTION OF DRAWINGS

[0002] Non-limiting examples will now be described with reference to the accompanying drawings, in which:

[0003] Figure 1 is a flowchart of an example method of determining whether UV light is to be used in measurements of a printed substrate;

[0004] Figure 2 is a flowchart of another example method of determining whether UV light is to be used in measurements of a printed substrate;

[0005] Figure 3 is a schematic drawing of a device comprising a spectrophotometer; and

[0006] Figure 4 is a schematic drawing of an example machine-readable medium associated with a processor. DETAILED DESCRIPTION

[0007] Spectrophotometers are used for measuring colors, which may include colors printed on a substrate. While the term “spectrographic measurements” is used to refer to such measurements herein, other terms, such as “spectrophotometric measurements” and “spectral measurements” may be used. Substrates may in principle comprise any material, for example comprising paper, card, plastics, fabrics or the like. Moreover, while examples herein are generally described in terms of ‘2D’ printing, in some examples, a substrate may for example comprise a layer of a build material (such as a granular or liquid build material) which is to form a layer of an object in additive manufacturing, also referred to as ‘3D printing’. Moreover, substrates may themselves be 3D objects having surface colors printed directly thereon.

[0008] Spectrographic measurement modes for measuring printed outputs may make use of different standardized illumination conditions, for example using ‘white light’, but with different definitions of such light. It may be appreciated that printed outputs can appear different depending on the illumination condition. For example, a printed output may appear different under natural light (i.e., daylight), which includes a significant amount of UV light, than under an artificial light which does not. The type of substrate used may also have an impact. For example, some substrates include optical brightener agents, which can absorb ultraviolet (UV) radiation, which is invisible to the human eye, and fluoresce to emit light at the blue end of the visible spectrum. Such substrates can appear to a human observer to be a bright white under illumination conditions which include UV light, and the perceived color of a colorant printed thereon can differ depending on whether the light source used to view the printed output contains a significant portion of UV radiation or not.

[0009] Examples of spectrographic measurement modes include “M series” modes as defined by ISO 13655:2017, wherein different M series modes use different lighting conditions. For example, an M0 measurement mode may use a light source(s) which provide light around Commission Internationale de I’Eclariage (CIE) Standard I Hum inant A with a color temperature of approximately 2856 K. This has the appearance of a tungsten incandescent light (i.e., a ‘traditional’ incandescent light bulb), although the light may be provided in some other way, for example using halogen or LEDs to mimic the ‘white light’ color profile of such a light source. The spectrum of light output by such a light source is generally between 300nm and at least 780nm. [0010] The M1 measurement mode includes a specification of a significant UV content of the light source and specifies that the spectral power distribution of the light source used to measure the specimen should match, or be similarto, CIE llluminant D50. The CIE llluminant D series relates to light which provides a closer approximation of daylight, and in the context of printed substrates, use of the M1 mode can introduce visual effects due to fluorescence, caused either by optical brightener agents in substrates or fluorescence of the printed materials (e.g., inks, toners orthe like, generally referred to as colorants herein). The spectrum of light output by such D50 source(s) includes wavelengths between 300 and 830 nm, and the color temperature is around 5003 K.

[0011] Therefore, M1 measurements utilize, alongside light sources which are similar to those used for an M0 measurement, a UV source. For example, there may be an array of LEDs comprising at least one UV LED, wherein the UV LED is used to carry out an M1 measurement. In some examples, as well as changing the amount of UV radiation used, the color spectrum of the visible light sources may also be changed between measurement modes, for example to more closely match the standard llluminants mentioned above.

[0012] For completeness, there are other “M-series” measurement modes with more specialized applications, such as the UV free (or UV-cut) M2 mode .which is intended to mimic a UV-free environment (as for example may be the case in a museum where an art piece is displayed in a UV free environment to preserve it), and M3 mode, which uses polarizers to remove the effect of gloss, for example to predict a color of a dry substrate while that substrate is still wet.

[0013] A user may make a selection between an M0 and an M1 measurement mode based on an intended viewing condition and/or the amount of fluorescence associated with a substrate being used in a print run. Generally, a higher fluorescence (either as measured, or as indicated in some other way such as on a data sheet associated with the substrate) indicates that an M1 measurement mode may be more suitable. Therefore, as the use of optical brightener agents has increased, so too has the use of M1 measurement modes. However, the apparatus used to obtain robust M1 measurements is more costly than that used to obtain M0 measurements, due to either or both of providing the UV light and the sensor used to sense light in this range. Where less costly apparatus is used, it may be the case that, for a given sample of substrate (albeit that the substrate may contain optical brightener agents), use of non-UV measurement apparatus may provide a more robust result. [0014] Figure 1 shows an example of a method, which may be a method for selecting a spectrographic measurement mode for a substrate. The method may therefore provide part of a set-up, or calibration, procedure for a spectrophotometer (which apparatus may also be referred to as a spectrometer). The selection may be a selection between a measurement mode which includes use of a UV light source (e.g., an M1 measurement mode) or one which does not (e.g., an MO measurement mode).

[0015] Block 102 comprises performing a plurality of spectrographic measurements (or spectrophotometric measurements) of a sample of a substrate using UV light. This may for example comprise performing at least 3, or at least 5 or at least 10 spectrographic measurements. The measurements may be taken in succession, for example with a time delay therebetween which is representative of an expected time delay when measuring a printed substrate (noting that, in such a circumstance, the spectrophotometer may be repositioned relative to the substrate between measurements in order to measure a plurality of patches, for example relating to different colors, and/or to make measurements on different substrate sheets). For example, the time delay between the measurements may be between 200ms and 2 seconds.

[0016] In some examples, the measurements may be mapped into a color space, for example using a CIE standard illuminant. For example, performing the spectrographic measurement may comprise calculating a CIELAB coordinate using the D50/2‘ standard illuminant. In order to calculate such a coordinate, color data collected by a sensor may be processed (e.g., multiplied) by a curve indicative of a D50 ‘standard’ light source. The designation of “2”’ is indicative of a two-degree field of view. Specification of the field of view reflects the fact that the size of a color patch can have an impact on its perceived color and is one example of an established standard for such measurements. Other mappings, for example based on other illuminants and/or fields of view may be used in principle. Generally, however, stating a standard may be useful in comparing results between different apparatus.

[0017] CIELAB space provides a three-dimensional model based on human color perception, i.e., models the human visible ‘color gamut’ in three dimensions, wherein the three dimensions are L* (L-star, often referred to as ightness), a* (a green-red color axis) and b* (a blue-yellow color axis). A CIELAB coordinate relates to a point in color space with L* a* and b*values. However, in principle, other color spaces such as CIEXYZ, RGB, HCL, Hue-Saturation-Value (HSV), Hue-Saturation-Lightness (HSL), Yule-Nielsen-corrected XYZ, or the like may be used to characterize the measurement of the sample. In particular examples, a color space may be selected which is perceptually linear (such as CIELAB).

[0018] In some examples, the measurements made in block 102 are made using an M1 measurement mode of a spectrophotometer, which may comprise a spectrophotometer which is operable in an M0 mode and an M1 mode. For example, a spectrophotometer may comprise at least one light source which is operable to provide illumination without a substantial contribution in the UV wavelength range, and at least one light source which is operable to provide illumination with a significant contribution in the UV wavelength range, wherein the UV wavelength range may be defined as being between about 100nm to around 400nm. In some examples, the illumination may be filtered to provide illumination in the intended range. For example, the light source may comprise an array of individually controllable light sources, comprising at least one UV light source and at least one light source which emits light in the visible range.

[0019] For example, the sample may be a sample of a substrate which is intended to be used in a print run, but may be unprinted. In other examples, an unprinted portion of a sample of the substrate which is printed may be measured. In some examples, each of the measurements is made of the same ‘patch’ of substrate, whereas in other examples, measurements may be taken of different patches. In examples, a measured ‘patch’ may have a rectangular or circular area with a diameter which is slightly larger (e.g., about 4mm larger) than an aperture of the spectrophotometer, so a 4mm aperture may collect light from a region of the substrate which is a square or circle having a height and width of around 8mm.

[0020] Block 104 comprises calculating, using processing circuitry, a measure of variability of the measurements. This may be any measure or indication of the variation, i.e., how extent to which the measurements differ from one another. In one example, this may comprise a ‘pairwise’ comparison of around ten, 20 or 30 measurements. In such an example, a color difference between pairs of samples (which may be referred to as a Delta-E, or AE difference) may be determined. For example, in the CIELAB color space discussed above, the color difference may be the Euclidian distance between the coordinates of each measurement.

[0021] This may provide a number P of AE values, where P =

|_n other words, if there are 10 samples acquired in block 102, then (10*9)/2, or 45 AE values may be determined.

[0022] In some examples a measure of variability may be determined therefrom. For example, the value of the 95th percentile may be determined as a measure of variability (i.e., the highest value left when the top 5% of the set of P AE values, when numerically sorted, is discarded). The use of the 95^th percentile removes outliers from the data set, and thus may avoid errors from being included in the analyzed data, and another cut-off, or no cut-off may be appropriate in other examples, for example depending on the accuracy of the apparatus.

[0023] In other examples, a different measure or indication of the variability of the measurements may be determined, for example a range of the measurements, an interquartile range, a standard deviation, a variance or the like.

[0024] Block 106 comprises, determining, based on the measure of variability, whether to use the UV light for subsequent spectrographic measurements of a printed output. The printed output may comprise an image or text printed on a substrate of the same type as the substrate measured in block 102, or may comprise the same substrate. The subsequent spectrographic measurements of a substrate when that substrate is printed may for example be to identify or analyze a color printed thereon.

[0025] In some examples, block 106 may comprise determining whether to operate a spectrophotometer (which may be the same spectrophotometer as that used to acquire the measurements in block 102) in an M0 or an M1 measurement mode. In some examples herein, acquiring measurements without UV light may mean acquiring measurements in an M0 measurement mode and acquiring measurements with UV light may mean acquiring measurements in an M1 measurement mode.

[0026] For example, the measure of variability determined in block 104 may be compared to a threshold and, if the measure of variability exceeds the threshold, then it may be implied that the variability under the UV light source is too high to be reliable, and non-UV light source(s) are to be used in subsequent color measurements, for example of colors of an image printed on the substrate. Thus, in one example, if the variability is below a threshold, then the spectrophotometer may be set to operate using UV illumination (e.g., in an M1 measurement mode) and if the variability is above the threshold, then the spectrophotometer may be set to operate at least substantially without UV illumination (e.g., in an M0 measurement mode). In some examples, block 106 may comprise selecting or setting the mode. The mode may for example be set automatically by the processing circuitry, which may comprise processing circuitry of the spectrophotometer or of a printer (for example, a flag may be set within a memory indicative of the selected mode), and/or a user may be notified of the advised mode, for example, via a display of the spectrophotometer and/or of a printer (noting that the spectrophotometer may be integral to a printer in some examples). [0027] In some examples, the method of Figure 1 may be carried out using machine-readable instructions stored on a machine-readable medium which are executed by at least one processor. The processor may control the apparatus to perform the measurements and/or process the resulting data. The processor may comprise a processor, or processing circuitry of a spectrophotometer and/or of a printer. The method of Figure 1 may for example be carried out following user input or may be carried out automatically at the start of a print run. In some examples, the selected mode may be used for the duration of a print run. In other examples, the selected mode may be selected as the operational mode until the method is re-run. In some examples, the method may be re-run when a new type of substrate is to be used for printing, or as prompted by a user.

[0028] As noted above, some spectrophotometers (for example, relatively inexpensive spectrophotometers) may have a relatively high degree of variability in particular associated with UV measurement modes and thus for a given spectrophotometer and/or for a given combination of a spectrophotometer and substrate, the variability may offset the benefits of measuring under UV light. Without being bound by theory, this may be because UV illumination can be less stable, and/or stabilization of the illumination may not have occurred before the measurement is taken. Therefore, the method of Figure 1 may allow the variability to be taken into account when selecting a measurement mode.

[0029] In some examples, additional factors may be taken into account when selecting the measurement mode. For example, the degree of fluorescence and/or the comparative variability of spectrographic measurements taken without use of a UV light source (for example, in an M0 mode) may be used to determine whether subsequent spectrographic measurements of the substrate are to be taken using UV light (for example, whether the spectrophotometer is to operate in an M0 or an M1 mode).

[0030] Such an example is now described in relation to Figure 2, which is another example of a method for determining a measurement mode for a spectrophotometer. The example of Figure 2 further comprises a method for using the spectrophotometer in the mode. The method may be carried out using one ore more processors or processors, for example using processing circuitry of a spectrophotometer and/or a printer.

[0031] Block 202 comprises performing a first set of spectrographic measurements of the sample of a substrate using UV light. In this example, the same patch of substrate is measured multiple times. Block 202 may be carried out as described for block 102 above. In this example, block 202 comprises operating the spectrophotometer in an M1 measurement mode.

[0032] In this example, a set of measurements which is between three measurements and 30 measurements may be acquired, although more measurements may be acquired in other examples. As described above, these measurements may be carried out at predetermined intervals, for example being separated by milliseconds or seconds, for example to mimic the expected measurement cycle of the apparatus when measuring a printed output.

[0033] Block 204 comprises performing a second set of spectrographic measurements of the same patch of the substrate, in this case without use of UV light. In this example, block 202 comprises operating the spectrophotometer in an M0 measurement mode. In some examples, this may comprise turning off a specific source of UV light. Therefore, the second set of measurements in this example is at least substantially, and can be entirely, free of illumination by UV light and/or without use of the specific UV light source. In other examples, a UV filter may be used to prevent UV light from reaching the substrate sample when acquiring the second set of measurements. Other light sources, e.g., visible light sources, may also be controlled to change their emitted spectrum when switching between modes. While in this example, the same patch of substrate is measured in blocks 202 and 204, in other example it may be a different patch (or patches) of the same substrate and/or a different sample of the substrate (for example a different sample of a substrate of the same substrate type).

[0034] In addition, in some examples, some parts of a spectrum may be excluded from the analysis of one or both sets of measurements. For example, some white LEDs have very low energy emission for wavelengths around 400-410 nm, and this can add uncertainty during calibration and/or during measurement against some industry standard(s). Since the methods herein concern assessment of variability (ratherthan, for example, accuracy), such sources of uncertainty may be excluded in some examples.

[0035] These measurements are then processed according to two separate processes, each of which is described in turn.

[0036] As part of the first process, block 206 comprises selecting a first colorimetric standard. As an example, the first colorimetric standard is D50/2’.

[0037] In block 208, each of the acquired spectrographic measurements of the first and second set is processed based on the first colorimetric standard (for example as described in ISO 13655) to provide a first set of CIELAB coordinates for the first set of measurements and a second set of CIELAB coordinates for the second set of measurements.

[0038] In block 210, a measure of variability of the first (M1) set of coordinates is determined. For example, this may be determined by finding the difference between the coordinates in a pairwise manner, to calculate AE for each pair, and determining the measure of variability as the 95^th percentile as outlined above.

[0039] In block212, a measure of variability of the second (M0) set of coordinates is determined. Again, each of the CIELAB coordinates of the second (M0) set of measurements may be compared, pairwise, to calculate AE for each pair, and the 95^th percentile determined to provide a measure of variability. This provides two measures of variability, one in relation to the first set of measurements (i.e., the set of measurements determined using UV light), and a second measure of variability in relation to the second set of measurements (i.e., the set of measurements acquired without using UV light). As noted above, the measure of variability may be provided in some other way, but generally the same method may be used in block 210 and 212.

[0040] In this example, in block 214, a relationship between these two measures of variability is determined. The determined relationship may be any relationship which indicates whether the variability is greater under UV light than without the use of UV light. In particular, in this example, the ratio of the first set of measurements to the second set of measurements is determined. This may

¹ be exp rressed as.Ratio = ^{Var M1} This value Var(MO) may be expected to be greater than 1 when the variability of measurements is greater under UV light, and the value of the ratio is indicative of how much more variable measurements are under UV light than absent UV light.

[0041] As part of the second process, an indication of the fluorescence of the substrate is determined. Block 216 comprises selecting a second colorimetric standard. As an example, the second colorimetric standard is D65/10’, where 10’ indicates a 10- degree field of view.

[0042] Block 218 comprises averaging the measurements of the first set of measurements to provide a first (M1) spectrographic average measurement and averaging the second (M0) set of measurements to provide a second spectrographic average measurement. The averages in this example are mean values.

[0043] In block 220, the first and second spectrographic average measurements are used to calculate two CIELAB coordinates based on the second colorimetric standard. [0044] The difference in the amount of blue light detected is then determined in block 222. In particular, in one example, the value of the b* coordinate of the CIELAB coordinates of the two average values is determined, Ab*. It may be recalled that the b* axis relates to a measure of blue light on a blue-yellow scale, and therefore specifically to the type of light which is likely to be increased under UV light when fluorescence is present. Thus, the difference in this value may be indicative of a degree of fluorescence.

[0045] In some examples, for example in order to compensate for the possibility that some UV light is present in the apparatus when measuring in M0 mode, a correction factor may be used. It may be noted that a ‘standard’ Ab* evaluation is made using a M2 measurement mode, and in this example an M0 and M1 measurement modes are used.

[0046] Such a correction factor may be determined for a spectrophotometer, or a class of spectrophotometers for example during product development, testing or calibration, based on light sources therein, and in particular the energy level of any UV light given off by nominally non-UV sources (e.g., white LEDs). In one example, the Ab* value is multiplied by a correction factor of k. For example, in a case where some UV light is emitted by nominally non-UV sources, the correction factor may have a value of around 3 to 5. If however the nominally non-UV light sources of a spectrophotometer have a very low, or no, energy profile in the UV range, or such light may be removed in some other way (e.g., filtered out), then correction may not be indicated. In other examples, where an M2 measurement mode is available, this may be used to determine Ab*.

[0047] The fluorescence level of the substrate is determined by comparing the value indicative of the amount of blue light (for example, the value of Ab*, or the value of the product of Ab* and k) with at least one threshold in block 224.

[0048] For example, if the value is less than a first threshold, the fluorescence level may be determined to be faint. If the value is above the first threshold but less than a second threshold, then the fluorescence level may be determined to be low. If the value is above the second threshold but below a third threshold, then the fluorescence level may be determined to be moderate. If the value is above the third threshold, then the fluorescence level may be determined to be high.

[0049] It will be appreciated that, while in some examples, a correction factor is described, in other example the correction factor may effectively be included in setting the thresholds, or the apparatus may be configured such that a correction factor is not used as described above. [0050] While in this example the same set of MO and M1 measurements was used in both the first and second processes, in principle different measurements may be used in the different processes. Moreover, in this example, the fluorescence of the sample of substrate is determined in blocks 216 to 224. However, in other examples, this information may be provided in some other manner, for example as data in a data sheet provided with the substrate or electronically, or based on user knowledge or the like. In addition, in other examples, the fluorescence of the sample of substrate may be determined by measurement in some other way.

[0051] Block 226 uses the results of the first and second processes to determine a measurement mode for use in measuring colors in a printed substrate. In particular, in this example, the selection of the mode may be determined according to the table set out below:

[0052] Generally, in some examples, a higher degree of florescence may be associated with an increased likelihood that a measurement mode using UV light is indicated. Viewed another way, a higher degree of florescence may be associated with an increased tolerance of variability in a measurement mode using UV light.

[0053] Block 228 comprises selecting the mode of operation of the spectrophotometer which was used to obtain the measurements in block 202 and 204 based on the determination and configuring the spectrophotometer to operate in that mode. In examples, the mode of operation is selected from a first mode, in which UV light is not provided and a second mode in which UV light is provided. In this example, the mode is selected from an M0 mode and an M1 mode. The selected mode may be stored in a memory of the spectrophotometer/printer. [0054] As can be seen from the table above, in this example, certain fluorescence levels are always associated with a particular mode. Thus, in some examples, the processes of block 216 to 224 may be carried out first and the method may terminate if the fluorescence is determined to be faint or high. In other examples, the method may be carried out when the fluorescence is not known (for example, based on provided data or user knowledge) to be faint or high.

[0055] Block 230 comprises printing a substrate of the same type as the sample of substrate (which includes the same substrate as the sample substrate) to provide a printed output. In some examples the substrate may be printed with one or a plurality of colors. In particular examples, the substrate may comprise a ‘test patch’ which may be printed with one or more colors which are expected to have predetermined qualities. In other examples, the colors to be measured may comprise part of the image being printed.

[0056] Block 232 comprises using the spectrophotometer in the selected mode to perform spectrographic measurement(s) of the printed substrate, and more particularly in examples, of at least one color printed thereon, for example the color(s) of a test patch. The measurements may then be compared, for example to predetermined standards and/or target colors, to determine if the print apparatus is producing colors which meet intended print quality standards.

[0057] In some examples, the measurements of block 232 may be repeated on each printed output of a print run, or periodically throughout a print run, to determine if the printer is continuing to operate in an intended manner. As the methods set out above select a measurement mode based at least in part on the variability of measurements, the repeatability of the measurements may be increased. Thus, by using the methods set out herein, the risk that a detected ‘change’ in a printed color output is in fact due to the variability of the performance of the spectrophotometer is reduced, as are the possible delays and/or wasted materials associated with a user acting on a notification of such a change. In some examples, an indication of the measurement mode may be associated (e.g., stored) with the measurements.

[0058] Figure 3 shows an example of a device 300 comprising a spectrophotometer 302 (which may also be referred to as a spectrometer). The spectrophotometer 302 in this example comprises a light source 304, wherein the amount of UV light output by the light source is controllable. For example, the light source 304 may comprise at least one UV light emitter and at least one other, at least substantively non-UV, light emitter. The light emitter(s) may for example comprise any or any combination of LED(s), halogen bulb(s), incandescent bulb(s), xenon lamp(s) or some other light sources.

[0059] Although not shown, the spectrophotometer 302 may further comprise at least one sensor, capable of detecting light in at least the visible range. For example, the sensor may be able to detect light in a range of around 380nm to around 700nm. In some examples, the spectrophotometer 302 may comprise a refractive element, such as a grating or prism, which spatially separates light depending on its wavelength. Light from the light source may be reflected by a surface (for example a printed or unprinted substrate), wherein the reflected wavelengths depend on the color of the surface. This light may be refracted by the refractive element, and a ‘rainbow’ produced thereby may be incident on an array of sensor elements (e.g., charge coupled devices, or CCDs) providing the sensor. The amount of light detected at each sensor provides an indication of the color of the surface from which the light was reflected. While this provides one example, many other spectrophotometer designs exist. For example, a ‘rainbow’ may be scanned across a single sensor by rotating the refractive element, or chromatic filters may be used to sample different portions of reflected light. In other examples, a transmitted rather than a reflected spectrum may be measured.

[0060] The device 300 further comprises processing circuitry 306. In this example, in use of the device 300, the processing circuitry 306 calculates an indication of a variability of a plurality of spectral measurements (or spectrographic, or spectrophotometric measurements) of a sample of a substrate of a predetermined substrate type acquired by the spectrophotometer 302 (e.g., a sensor thereof) under UV illumination. For example, this may be carried out as described in relation to block 102 or 202 above. Moreover, the processing circuitry 306 selects a mode for a subsequent spectral measurement of a substrate of the substrate type based on the variability, where the selection comprises selection of one of a first mode, in which the light source is controlled so as to minimise UV light (i.e. the UV light emitted thereby is significantly reduced, or prevented), and a second mode, in which the light source is controlled to emit UV light (i.e. a significant portion of UV light). For example, operating in the first mode may comprise operating in the M0 mode, and operating in the second mode may comprise operating in the M1 mode. The first mode may be selected when the variability is above a threshold, and the second mode may be selected when the variability is below a threshold.

[0061] In some examples, the processing circuitry 306 may further control the light source. For example, when operating in the second mode, the processing circuitry 306 may control a UV emitter (e.g., a UV LED) of the light source to emit light, whereas when operating in the first mode, the processing circuitry 306 may control the light source 304 such that the UV emitter(s) of the light source is/are in an off state.

[0062] In some examples, the processing circuitry 306 may, in use of the device 306, calculate an indication of a variability of a plurality of spectral measurements of the sample acquired by the spectrophotometer (e.g., a sensor thereof) without using UV illumination, or with the UV illumination minimised (e.g., in an M0 measurement mode), for example operating as described in relation to block 204 and/or 212 above. The processing circuitry 306 may characterize a relationship (e.g., a ratio) between the variability of the measurements acquired using UV illumination and the variability of the measurements acquired without using the UV illumination, or with the UV illumination minimised, for example as described in relation to block 214 above. The processing circuitry 306 may select the mode, at least in part, based on the relationship.

[0063] In some examples, the processing circuitry 306 may determine an indication of a degree of fluorescence of the substrate, and select the mode based on the degree of fluorescence and the indication of variability. For example, this may be determined by user input, or by data transferred to the processing circuitry 306. In other examples, the processing circuitry 306 may for example determine the indication of the degree of fluorescence by comparing measurements taken using a UV light source with measurements taken without using the UV light source (or with the UV illumination minimised). In some examples, the processing circuitry 306 may for example carry out any of block 218 to 224.

[0064] In some examples, the processing circuitry 306 may for example process data collected by the spectrophotometer according to at least two colorimetric standards, which may for example comprise standard llluminants, for example CIE illuminants. For example, collected data may be processed using a colorimetric standard of D50 (and 2- degree field of view) to determine the variability between measurements, and using a colorimetric standard of D65 (and a 10-degree field of view) to determine the fluorescence. These choices reflect standards in the industry, and in principle other choices may be made, albeit that adhering to predetermined standards may make, for example, measurements from different apparatus easier to compare against each other.

[0065] In some examples, the device 300 is a print apparatus comprising an ‘in line’ spectrophotometer 302. In such examples, the print apparatus may print a further substrate (for example, a further sample of the substrate measured during the selection of the measurement mode) with at least one colorant, and the spectrophotometer 306 may make a plurality of measurements of the printed output (for example, a measurement of a colorant printed on each of a plurality of substrate sheets) using the selected mode.

[0066] Figure 4 shows an example of a machine-readable medium 402 in association with a processor 404. The machine-readable medium 402 stores instructions 406 which, when executed by the processor 404, cause the processor 404 to carry out tasks. In this example, the instructions 406 comprise instructions 408 to cause the processor 404 to calculate a variability of a set of spectrographic (or spectrophotometric) measurements made using an M1 measurement mode. For example, this may comprise carrying out processes as described in relation to any or any combination of blocks 102, 104, 202, 206, 208 and 210 above.

[0067] In this example, the instructions 406 further comprise instructions 410 to cause the processor 404 to select use of either an M0 or an M1 measurement mode for subsequent spectrographic measurements based on the variability of the set of measurements, for example as described in relation to block 106 and/or block 226 and 228 above. The subsequent spectrographic measurements may be spectrographic measurements of colorants printed on the same substrate, or same type of substrate, as measured to acquire the set of spectrographic measurements used to calculate the variability. In examples, the set of spectrographic measurements and the subsequent spectrographic measurements may be carried out using the same apparatus, for example the same spectrophotometer.

[0068] In some examples, the instructions 406 further comprise instructions to cause the processor 404 to calculate a variability of a set of spectrographic measurements made using an M0 measurement mode (e.g. as described in relation to any or any combination of blocks 204, 206, 208, 212 and 214 above), and to select the use of either the M0 or the M1 measurement mode for subsequent spectrographic measurements (for example, measurements of colors printed on that substrate or substrate type) based on the variability of both the sets of measurements (for example, based on a ratio of the measure of variability of the measurements as described above). Generally, in some examples, a higher degree ef florescence may be associated with an increased likelihood that the M1 measurement mode is indicated. Viewed another way, a higher degree of florescence may be associated with an increased tolerance of variability when using the M1 measurement mode.

[0069] In some examples, the instructions 406 further comprise instructions to cause the processor 404 to determine a measure of fluorescence of a substrate to be printed and select use of either the MO or an M1 measurement mode for subsequent spectrographic measurements based on the variability of the set of measurements and the measure of fluorescence. The fluorescence may for example be determined by user input, from data provided to or stored on the machine-readable medium 402 and or the processor 440, and/or by measurement, for example using the measurement processing as set out in blocks 216 to 224 above.

[0070] In some examples, the machine-readable medium 402 may be provided as part of a printer, as part of a spectrophotometer or as part of the control circuitry of a printer comprising a spectrophotometer. The machine-readable medium 402 may further comprise instructions to control other parts of the printer and/or the spectrophotometer to carry out printing and/or measurement tasks. The machine-readable medium 402 may for example comprise instructions to control an amount of UV light emitted by a light source of a spectrophotometer.

[0071] Examples in the present disclosure can be provided as methods, systems or machine-readable instructions, such as any combination of software, hardware, firmware or the like. Such machine-readable instructions may be included on a computer readable storage medium (including but not limited to disc storage, CD-ROM, optical storage, etc.) having computer readable program codes therein or thereon.

[0072] The present disclosure is described with reference to flow charts and/or block diagrams of the method, devices and systems according to examples of the present disclosure. Although the flow charts described above show a specific order of execution, the order of execution may differ from that which is depicted. Blocks described in relation to one flow chart may be combined with those of another flow chart. It shall be understood that each block in the flow charts and/or block diagrams, as well as combinations of the blocks in the flow charts and/or block diagrams can be realized by machine readable instructions.

[0073] The machine-readable instructions may, for example, be executed by a general-purpose computer, a special purpose computer, an embedded processor or processors of other programmable data processing devices to realize the functions described in the description and diagrams. In particular, a processor or processing apparatus may execute the machine-readable instructions. Thus, functional modules of the apparatus and devices may be implemented by a processor executing machine readable instructions stored in a memory, or a processor operating in accordance with instructions embedded in logic circuitry. The term ‘processor’ is to be interpreted broadly to include a CPU, processing unit, ASIC, logic unit, or programmable gate array etc. The methods and functional modules may all be performed by a single processor or divided amongst several processors.

[0074] Such machine-readable instructions may also be stored in a computer readable storage that can guide the computer or other programmable data processing devices to operate in a specific mode.

[0075] Such machine-readable instructions may also be loaded onto a computer or other programmable data processing devices, so that the computer or other programmable data processing devices perform a series of operations to produce computer-implemented processing, thus the instructions executed on the computer or other programmable devices realize functions specified by block(s) in the flow charts and/or block diagrams.

[0076] Further, the teachings herein may be implemented in the form of a computer software product, the computer software product being stored in a storage medium and comprising a plurality of instructions for making a computer device implement the methods recited in the examples of the present disclosure.

[0077] While the method, apparatus and related aspects have been described with reference to certain examples, various modifications, changes, omissions, and substitutions can be made without departing from the spirit of the present disclosure. It is intended, therefore, that the method, apparatus and related aspects be limited only by the scope of the following claims and their equivalents. It should be noted that the above- mentioned examples illustrate rather than limit what is described herein, and that those skilled in the art will be able to design many alternative implementations without departing from the scope of the appended claims.

[0078] The word “comprising” does not exclude the presence of elements other than those listed in a claim, “a” or “an” does not exclude a plurality, and a single processor or other unit may fulfil the functions of several units recited in the claims.

[0079] The features of any dependent claim may be combined with the features of any of the independent claims or other dependent claims.

Claims

1. A method comprising: performing a plurality of spectrographic measurements of a sample of a substrate using UV light; calculating, using processing circuitry, a measure of variability of the measurements; determining, using processing circuitry and based on the measure of variability, whether to use UV light for subsequent spectrographic measurements of a printed output.

2. The method of claim 1 wherein the plurality of spectrographic measurements comprises a first plurality of spectrographic measurements and the measure of variability of the measurements comprises a first measure of variability of the first plurality of spectrographic measurements, the method further comprising: performing a second plurality of spectrographic measurements of a sample of a substrate without using UV light; calculating, using processing circuitry, a second measure of variability of the second plurality of spectrographic measurements; and comparing the first and second measures of variability and determining whether to use UV light for the subsequent spectrographic measurements of the printed output based on the comparison.

3. The method of claim 2 wherein the comparison comprises a ratio, and wherein determining whether to use UV light for the subsequent spectrographic measurements of the printed output based on the comparison comprises determining if the ratio falls within a predetermined value range.

4. The method of claim 1 wherein the method further comprises determining a degree of fluorescence; wherein determining whether to use UV light for the subsequent spectrographic measurements of the printed output is based on the measure of variability and the measure of fluorescence.

5. The method of claim 4 wherein the plurality of spectrographic measurements comprises a first plurality of spectrographic measurements, the method further comprising: performing a second plurality of spectrographic measurements of a sample of a substrate without using UV light; comparing the first plurality of spectrographic measurements with the second plurality of spectrographic measurements; and determining the degree of fluorescence of the substrate based on the comparison.

6. The method of claim 5 wherein comparing the first plurality of spectrographic measurements with the second plurality of spectrographic measurements comprises: determining an average of the first plurality of spectrographic measurements; determining an average of the second plurality of spectrographic measurements; and calculating a difference indicative of an amount of blue light in the samples based on the averages; and wherein determining the measure of fluorescence of the substrate comprises comparing the calculated difference to at least one threshold.

7. The method of claim 1 further comprising: selecting a mode of operation of a spectrophotometer based on the determination, wherein the mode of operation is selected from a first mode, in which UV light is not provided and a second mode in which UV light is provided; printing a substrate of the same type as the sample of substrate to provide a printed output; and using the selected mode to perform a spectrographic measurement of a printed output, wherein the printed output is printed on a substrate of the same type as the sample of substrate.

8. A device comprising: a spectrophotometer to make a plurality of measurements of a sample, the spectrophotometer comprising a light source wherein the amount of UV illumination output by the light source is controllable; and processing circuitry to: calculate an indication of a variability of a plurality of spectral measurements of a sample of a substrate of a predetermined substrate type acquired by the spectrophotometer using UV illumination; and select a mode for a subsequent spectral measurement of a substrate of the substrate type based on the indication of variability, where the selection comprises selection of one of a first mode, in which the light source is controlled to minimize an amount of UV illumination, and a second mode, in which the light source is controlled to provide UV illumination.

9. The device of claim 8, wherein the processing circuitry is further to: determine an indication of a degree of fluorescence of the substrate; and select the mode based on the degree of fluorescence and the indication of variability.

10. The device of claim 9 wherein the processing circuitry is further to: determine the indication of the degree of fluorescence by comparing measurements taken using UV illumination with measurements taken without using the UV illumination.

11 . The device of claim 9 wherein the processing circuitry is further to: calculate an indication of a variability of a plurality of spectral measurements of the sample acquired by the spectrophotometer without UV illumination; characterize a relationship between the variability of the measurements acquired under UV illumination and the variability of the measurements acquired without UV illumination; and select the mode based on the relationship and the indication of fluorescence.

12. The device of claim 8 further comprising print apparatus, wherein the print apparatus is to print a further sample of a substrate of the predetermined type with at least one colorant; and the spectrophotometer is to make a plurality of measurements of the printed output using the selected mode.

13. A machine-readable medium storing instructions which, when executed by a processor, cause the processor to: calculate a variability of a set of spectrographic measurements made using an M1 measurement mode; select use of either an MO or an M1 measurement mode for subsequent spectrographic measurements based on the variability of the set of measurements.

14. The machine-readable medium of claim 13 further storing instructions which, when executed by a processor, cause the processor to: calculate a variability of a set of spectrographic measurements made using an MO measurement mode; select the use of either the MO or the M1 measurement mode for subsequent spectrographic measurements based on the variability of both the sets of measurements.

15. The machine-readable medium of claim 13 further storing instructions which, when executed by a processor, cause the processor to: determine a measure of fluorescence of a substrate to be printed; and select use of either the MO or the M1 measurement mode for subsequent spectrographic measurements based on the variability of the set of measurements and the measure of fluorescence.

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