BACKGROUND

[0001] Print apparatus may generate an image on a substrate by applying print agents, such as inks, toner, overcoats, fixers, treatment and the like to the substrate, for example by ejecting a liquid print agent from nozzles of a printhead in inkjet printing, or by transferring an image formed on a photoconductive imaging plate to the substrate, or in some other way. A printing apparatus may comprise a sensor to measure light within the printing apparatus, for example when performing color calibration operations.

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 simplified schematic representation of an example apparatus;

[0004] Figure 2 is a graph showing an example of a relationship between print agent quantity and contamination level;

[0005] Figure 3 is a schematic representation of another example apparatus;

[0006] Figure 4 is a flowchart of an example method of determining if a light level exceeds a threshold;

[0007] Figure 5 is a flowchart of another example method of determining if a light level exceeds a threshold;

[0008] Figures 6A and 6B are graphs showing examples of a relationship between ambient light intensity and measured light intensity; and

[0009] Figures 7 and 8 are examples of a machine-readable medium associated with a processor. DETAILED DESCRIPTION

[0010] Over time and during use of a printing apparatus, components of the printing apparatus may become contaminated, or dirty. For example, printing operations can create small drops of print agents, such as ink, which may accumulate on surfaces of components of the printing apparatus. For example, during inkjet printing drops of print agent are ejected from a nozzle. In addition to ejection of a main drop, further smaller drops of ink may also be formed, referred to as satellite drops. Due to their small size, these satellite drops may be especially prone to travelling throughout the interior of the printing apparatus and landing on various surfaces within the printing apparatus. In some examples other contaminants may also accumulate within the printing apparatus, for example toner particles (e.g. in laser printers), dust, fibres (e.g. from print substrates), or the like. Accumulation of contamination within a printing apparatus may interfere with the normal operation of components of the apparatus. However it may not be practically possible to completely avoid such contamination from forming.

[0011] Figure 1 is an example of an apparatus 100, which in some examples may be a 2D or 3D printing apparatus or may be for association with such a printing apparatus. In this example the apparatus 100 comprises processing circuitry 102 and a sensor 104. In this example the sensor 104 is to measure a light intensity. In some examples, sensor 104 may be to detect ambient light. In some examples, the sensor 104 may additionally be used to perform other measurements within a printing apparatus, for example measuring light intensity and, in some examples, spectral properties of light. Such a sensor may be used within a printing apparatus to perform calibration operations, for example by measuring light reflected from or transmitted through a surface, for example by measuring intensity, color, or any other property of the light. For example, the sensor 104 may measure light reflected from a substrate which is to be printed on, a surface within the printing apparatus, and/or print agent deposited on a surface, such as a print substrate. For example, when the surface is a print substrate, the sensor 104 may be used to measure a property of the print substrate, such as its color or reflectivity, and the measurements may be used to vary a printing parameter when the apparatus is printing, for example to adjust the printed image to account for the properties of the substrate. When the surface comprises deposited print agent, the measurements may be used to calibrate the printing apparatus. For example, the measured color of the reflected light may be compared with an expected color and if the measured color differs from the expected color, the relative deposited quantities of different print agents may be varied in subsequent printing operations to compensate such that the printed colors more closely match the expected colors. Performing such corrections may be referred to as ‘color calibration’. In other examples, other aspects of image quality may be determined based on measurements acquired by the sensor 104.

[0012] The sensor 104 may comprise an element or apparatus capable of detecting a property of light, such as an intensity and/or a waveband. For example, the sensor 104 may comprise a photodetector, such as a photodiode, phototransistor or photoresistor which is capable of measuring light intensity. Such a photodetector may be sensitive to light across a broad range of wavelengths, for example infrared, visible and ultraviolet. In other examples the photodetector may sense a subset of wavelengths, such as visible light, or a subset of visible light. In some examples a filter may be provided to further control the range of wavelengths sensed by the photodetector. For example, blue, red or green filters may be placed between the photodetector and the surface it is intended to measure to filter out light except blue, red or green respectively. Similarly any other color of filter, or a different filter type, may be used.

[0013] In some examples the sensor 104 may comprise a plurality of photodetectors. Each photodetector may be intended to detect a different range of wavelengths, for example by placing different filters between each photodiode and the measured surface. In some examples the sensor 104 may comprise four photoreactors, and each photoreactor is associated with a different color of filter, for example blue, red, green, and orange filters. By performing measurements with photodetectors to detect different colors of light, the light color of a surface may be measured by the sensor 104. [0014] In some examples the sensor 104 comprises a spectrophotometer to measure the received light. A spectrophotometer may be able to measure the intensity and spectrum (i.e. color) of received light. In other examples the sensor 104 may comprise a densitometer or colorimeter.

[0015] In some examples the sensor 104 comprises a light source. In some examples the light source may provide a broad range of wavelengths of light e.g. white light. In other examples the light source may provide relatively narrow bands of light, for example the light source may be a colored LED. In other examples the light source may provide a narrow band of light, for example the light source may be a laser. When the light source provides a broad range of wavelengths of light, a filter or filters may be arranged such that a narrower band of light may be provided by the light source. For example a red filter may be placed in front of the light source to provide red light, a green filter placed in front of the light source to provide green light and so on. In some examples, a plurality of filters may be to allow one light source to provide different bands of color, for example the filters may be moveable to allow different filters to be placed in front of the light source or the filters may be provided with shutters which can be selectively closed and opened to allow a subset of filters to allow light through at any one time.

[0016] In order to measure different colors of light, either the sensor 104 may comprise a single photoreceptor and a light source capable of providing different bands of wavelengths of light or the sensor 104 capable of detecting different wavelengths of light (e.g. it comprises multiple photoreceptors or a spectrophotometer) and a single light source.

[0017] In this example the processing circuitry 102 comprises a contamination level module 106 and an assessment module 108.

[0018] In this example, in use of the apparatus 100, the contamination level module 106 determines a contamination level of the sensor 104. The ability of the sensor 104 to accurately measure light may be reduced due to contamination present on the sensor 104, which may accumulate on the sensor 104 as described above. For example contamination, such as print agent or substrate fibres, may block light from entering the sensor 104, thereby resulting in a measured light level which is less than the actual light level of light incident on the sensor. As used herein, a light level may refer to a measure of the amount of incident radiation on the sensor, for example luminous flux, luminous intensity, radiant flux or radiant power incident on the sensor 104 (e.g. measured in lumens, candela, watts per hertz or watts respectively). The ‘actual light level’ as the term is used herein refers to the level of light incident on the sensor 104 i.e. the measured light level plus the amount of light blocked or absorbed by the contamination on the sensor 104.

[0019] The contamination level may be a measure of the quantity of contamination present on the sensor 104. For example, the contamination level may correspond to, or comprise an estimate of, a quantity of print agent present on the sensor 104. In other examples the contamination level may correspond to a measure of a reduction in light measured compared with the actual light incident on the sensor 104, for example the contamination level may be expressed as a ratio of the measured light intensity to the actual light intensity, for example the contamination level may be the ratio of the measured light intensity of a sensor with contamination thereon to the measured light intensity of a clean sensor at the same actual light level. In some examples the contamination level may be based on a property which is correlated with the quantity of contamination on the sensor 104.

[0020] In some examples, the contamination level may comprise, or be based on, a measure of usage of the printing apparatus. For example, the contamination level may comprise, or be based on, any or any combination of: a quantity of print agent consumed, deposited or printed, a quantity of sheets of substrate printed; a duration of time the print apparatus (or the sensor apparatus thereof) has been operational; a duration since the print apparatus/sensor apparatus thereof was last cleaned; an area of substrate printed upon (e.g. in square meters); energy used by a printhead; number of cycles performed by a scan axis of a printing apparatus; or the like. A quantity of print agent may be measured in volume of print agent, mass of print agent or number of drops of print agent (in some examples in combination with drop size). In some examples the contamination level may be defined in arbitrary units. [0021] In this example, in use of the apparatus 100, the assessment module 108 determines, based on the measured light intensity and the determined contamination level, whether an actual light intensity incident on the sensor exceeds a light intensity threshold.

[0022] A printing apparatus may allow light from outside of the printing apparatus, referred to herein as ambient light, to enter the printing apparatus. Ambient light may enter the printing apparatus via an opening, such as a transparent or semi-transparent window. For example, a printing apparatus may be provided with such a window to allow a user to inspect the progress of a printing operation, for example to allow early detection of issues during the printing operation. This light may interfere with the operation of components of the printing apparatus. For example, when the ambient light level is above a threshold, the measurements performed by the sensor 104, or by another light sensitive sensor, may be inaccurate. For example, the sensor 104 (or another light sensitive sensor) may be used to perform a calibration operation of the printing apparatus. However, if the sensor inadvertently detects ambient light, then errors may occur in the calibration operation. Therefore, when the actual light intensity incident on the sensor is determined to be above the light intensity threshold, an action may be taken to improve the accuracy of the measurements performed by the sensor, for example to reduce the amount of ambient light incident on the sensor or to notify a user of the printing apparatus.

[0023] In some examples ambient light may enter a printing apparatus through other openings, for example openings for substrate transport and/or spaces between panels of the printing apparatus.

[0024] As the contamination level on the sensor 104 increases the proportion of light incident on the sensor 104 which is detected by the sensor 104 may decrease because a fraction of the incident light may be obscured by the contamination. For example, the contamination may comprise drops of print agent which absorb light which is incident on the sensor 104 preventing the sensor 104 from detecting all of the incident light. [0025] The proportion of light obscured by the contamination may vary in a predictable manner as a function of the contamination level. For example, when the contamination level is low e.g. when the printing apparatus is new or has recently been cleaned, the proportion of light obscured by contamination may be low. However, when the printing apparatus has been used for a long period of time, without cleaning, the proportion of light obscured by the contamination may be relatively high. Therefore, the assessment module 108 can determine if the actual light intensity exceeds the light intensity threshold based on the measured light intensity and the contamination level. For example, for a given contamination level, there may be a threshold light level reading of the sensor 104 which indicates whether the ambient light exceeds a threshold, wherein the threshold is determined based on the contamination level. In such examples, there may be a predetermined relationship between contamination thresholds and threshold measured light levels. In other examples, an indication of the actual light intensity may be determined to determine if the light intensity exceeds the threshold.

[0026] Therefore, the apparatus 100 may account for contamination present on the sensor 104 to provide a more accurate measurement of whether the actual light level exceeds the light level threshold. The apparatus 100 may therefore provide an indication or notification to a user, or perform some corrective action, when it is determined the ambient light levels are too high for the sensor (or another light sensitive sensor within the apparatus) to perform accurate measurements.

[0027] For example, a corrective action to reduce the ambient light entering the printing apparatus may comprise reducing the light levels in the environment of the printing apparatus (e.g. closing window coverings, turning off artificial lights, moving the printing apparatus to a darker area or the like) or closing a shutter of a window of the printing apparatus. In some examples the printing apparatus may take an action to reduce the ambient light entering the printing apparatus, for example by closing an opaque shutter on a window of the printing apparatus or covering the printing apparatus with a light protection device (e.g. an opaque cover). [0028] Figure 2 is a graph which shows an example of a relationship 202 between a contamination level of the sensor 104 and a level of usage of the sensor/printing apparatus, which in this example is a quantity of print agent consumed, deposited or printed by a printing apparatus, but may be a different measure of usage in other examples. For example, this relationship may be predetermined for a particular print apparatus, or print apparatus type. In some examples, the relationship may be determined by monitoring a contamination level of a sensor over time, for example by directly measuring the dirtiness of the sensor components. In some examples, a sensor may be deliberately ‘dirtied’ in order to characterise its behaviour. For example, in a characterisation operation, a ‘life test’ may be performed once or on multiple occasions which comprises printing in a similar manner to a user (e.g. ratio of print agents and/or ratio of images and text that a user may be expected to print) and recording a measured light level by the sensor and the quantity (e.g. volume) of print agent consumed. In such examples the ambient light level may be controlled while sensor readings are acquired. The relationship between a measured light level to quantity of print agent may be determined to provide the contamination level for that particular type of printing apparatus, type of printhead and/or type of sensor. In other examples, the relationship may be determined based on theory.

[0029] The graph shows that generally as the print agent quantity increases, the contamination level also increases. In this example the relationship 202 between print agent quantity and contamination level is non-linear and generally the gradient decreases as the print agent quantity increases. This can be understood because a relatively small amount of contamination on the sensor may cause a significant decrease in the amount of light entering the sensor 104, whereas as the sensor 104 becomes increasingly obscured, the same amount of additional contamination may have less effect. This trend may continue until the sensor becomes completely obscured and further contamination on the sensor 104 may not have any effect as the sensor becomes completely obscured. However, while a particular curve is shown here, the relationship may be different in other examples. [0030] Once the relationship has been determined, determining the contamination level may comprise determining the level of usage. In other words, the contamination level module 106 described in relation to Figure 1 may determine the contamination level of the sensor 104 based on a quantity of print agent deposited by a printing apparatus, for example based on the relationship 202 shown in Figure 2. The print apparatus may track the quantity of print agent deposited. In some examples, this may serve as a proxy for the contamination level, while in other examples, the contamination level module 106 may use the relationship 202 to calculate the contamination level, for example when the print agent quantity is Qo the contamination level may be determined to be Co based on the relationship 202. In some examples the quantity of print agent may be the quantity of print agent deposited, consumed or printed since an event occurred. The event may be a cleaning operation, for example the print agent quantity may be the amount of print agent deposited since the sensor 104 was last cleaned. In other examples, the event may comprise the first use of the printing apparatus such that the print agent quantity may be the amount of print agent deposited since the apparatus was first used. The event may also be first use of the sensor 104, for example if the sensor 104 was replaced. The apparatus 100 may record, for example in a memory, the print agent quantity and this value may be reset when certain events occur (e.g. cleaning operations, replacement of a sensor or the like). As mentioned above, other factors such as the amount of substrate printed, energy used by a printhead; number of cycles performed by a scan axis of a printing apparatus or the time since an event may also or additionally be used to determine the contamination level.

[0031] Figure 3 is an example of an apparatus 300, which may be a printing apparatus or an apparatus 300 for association with a printing apparatus and may be an example of the apparatus 100 described in relation to Figure 1.

[0032] The apparatus 300 comprises processing circuitry 302, wherein the processing circuitry 302 comprises a contamination level module 106 and an assessment module 108 as described in relation to Figure 1 . The apparatus 300 also comprises a sensor 104 as described in relation to Figure 1 . However in this example, in addition to determining if an ambient light level exceeds a threshold, the sensor 104 is also used in color calibration. Using the same sensor 104 for both these tasks may reduce the complexity of the apparatus 300.

[0033] The processing circuitry 302 of apparatus 300 further comprises a modification module 304, a light threshold module 306, a contamination threshold module 308 and a color calibration module 310 and the apparatus 300 further comprises a memory 312.

[0034] In this example, the memory 312 stores a first predetermined relationship between the quantity of print agent deposited and the contamination level of the sensor 104. The first predetermined relationship between the quantity of print agent deposited and the contamination level may for example be the relationship 202 shown in Figure 2, or a different relationship or mapping. In some examples the first predetermined relationship is stored as a look up table, such that for a particular quantity of print agent a contamination level may be looked up in the look up table. In other examples, the relationship may be stored as a function or the like. The first predetermined relationship may be obtained during a characterising procedure performed on an example of the printing apparatus. For example, a printing apparatus of the same type as the printing apparatus may deposit print agent and the contamination level of the sensor may be measured as a function of the quantity of print agent deposited. These measurements may be stored in the look up table. This characterising procedure may be performed for a single printing apparatus of a particular type (e.g. a particular module) and the measurements may be used to form the first predetermined relationship for each printing apparatus of that particular type. For example, the first predetermined relationship may be stored in the memory 312 when the apparatus 300 is manufactured.

[0035] In this example the contamination level module 106 determines the contamination level of the sensor 104 based on the first predetermined relationship. Determining the contamination level of the sensor 104 may comprise obtaining the print agent quantity deposited by the printer, for example the quantity of print agent deposited since a particular event such as a cleaning operation, and using the first predetermined relationship to determine the contamination level, as described in relation to Figure 2.

[0036] In this example the memory 312 stores a second predetermined relationship between the measured light intensity and an actual light intensity. The second predetermined relationship may describe the relationship between the measured light intensity and the actual light intensity at a particular contamination level. In some examples the memory 312 may store multiple predetermined relationships between measured light intensity and actual light intensity, each at different contamination levels. The predetermined relationships between measured light intensity and actual light intensity may be obtained in a similar manner to the first predetermined relationship, for example by performing a characterising operation by measuring the light intensity as a function of the actual light intensity. When there are multiple predetermined relationships between measured light intensity and actual light intensity at different contamination levels stored in the memory 312, they may be obtained by measuring the light intensity using the sensor 104 as a function of the contamination level and the actual light intensity. For example, the measured light intensity may be recorded as a function of actual light intensity as the printing apparatus (or a test printing apparatus during a characterisation operation to characterise the relationship) is used and the contamination level increases. As described in relation to the first relationship, the second relationship may be stored as a look up table, function or the like.

[0037] In this example, the modification module 304 determines a modified light intensity based on the measured light intensity, the contamination level and the second predetermined relationship. The modified light intensity may be an estimate of the actual light intensity. In other words, the sensor 104 measures the light intensity and the modification module 304 determines an estimate of the actual light intensity by looking up the measured light intensity in the second relationship to obtain the modified, or estimate of the actual, light intensity.

[0038] The modification module 304 may further take account of the contamination level to determine the modified light intensity, where the modified light intensity is an estimate of the actual light intensity. For example, as mentioned above, the memory 312 may store multiple predetermined relationships between measured light intensity and the actual light intensity and the modification module 304 may select a predetermined relationship between measured light intensity and the actual light intensity that corresponds to the contamination level. In some examples the predetermined relationship which most closely corresponds to the determined contamination level is selected, whereas in other examples an interpolation may be performed between predetermined relationships to obtain a new predetermined relationship. The interpolation may comprise performing a linear interpolation between the predetermined relationships, as will be described in greater detail in relation to Figure 6B.

[0039] In this example the assessment module 108 is used to determine if the modified light intensity (i.e. the estimate of the actual light intensity) is greater than a light intensity threshold and if the ambient light level is greater than the light intensity threshold, then the light threshold module 306 may perform an action, for example to reduce the amount of ambient light entering the printing apparatus or to notify a user that the ambient light level is too high. When a user is notified of the high ambient light level, they may take an action to reduce the ambient light entering the printing apparatus, for example by reducing the light levels in the environment of the printing apparatus (e.g. changing the location of the printing apparatus, reducing artificial or natural lighting incident on the printing apparatus, or the like), closing an opaque shutter of a window of the printing apparatus or covering the printing apparatus, as described above. In some examples the printing apparatus may take an action to reduce the ambient light entering the printing apparatus, for example by automatically closing an opaque shutter on a window of the printing apparatus.

[0040] In this example, therefore, the assessment module 108 compares the modified light intensity determined by the modification module 304 to a threshold light intensity to determine if the threshold is exceeded. However, in other examples, the assessment module 108 may use the sensor reading directly by comparing a measured light level to a threshold, wherein the threshold is determined based on the contamination level.

[0041] The light intensity threshold may be a predetermined threshold and may be determined by performing measurements with the sensor 104 at different ambient light levels and determining a maximum light level at which accurate measurements can be made. In some examples, this relationship may be predetermined for a given sensor, or a given class or type of sensors. In some examples the threshold may be set at this maximum light level or in other examples a guard band may be applied such that the light intensity threshold is less than this maximum light level.

[0042] In this example the contamination threshold module 308 determines if the contamination level is greater than a predetermined contamination level threshold. If the contamination level is high, the accuracy of measurements performed by the sensor 104 may be reduced, and/or the performance of other print apparatus components may be reduced. Therefore, when the predetermined contamination level threshold is exceeded, an action may be taken. For example, the contamination threshold module 308 may perform an action when it is determined the contamination level determined by the contamination level module 106 is greater than the predetermined contamination level threshold, such as initiating a cleaning operation to reduce the amount of contamination on the sensor 104 and/or providing a notification to a user to notify them that the contamination level has been exceeded. The user may then initiate a cleaning operation or may arrange for a technician to repair or clean the printing apparatus or sensor. A cleaning operation may comprise wiping a surface of the sensor 104 to remove contamination therefrom, for example by wiping the surface with a cloth.

[0043] In this example the color calibration module 310 performs a color calibration of a printing apparatus when it is determined the actual light intensity is less than the light level threshold. As described previously, the assessment module 108 may determine, based on the modified light level determined by the modification, if it appears that the actual light intensity incident on the sensor 104 exceeds the light intensity threshold, and if the actual light intensity exceeds this threshold, this may indicate the ambient light level is too high to perform an accurate color calibration. Therefore, when the actual light level is below the light intensity threshold the color calibration may be accurately performed. A color calibration operation may comprise performing a measurement with a sensor, which in this example is the sensor 104 but may in principle comprise a separate sensor, and altering a printing parameter based on the measurement. For example, the sensor 104 may be used to measure a color, for example a color of a substrate or a color of a print agent deposited on a substrate. For example, a test pattern comprising at least one print agent may be deposited and may be measured.

[0044] In some examples, the sensor 104 may comprise a light source to illuminate a surface being measured. For example, the sensor 104 may illuminate a surface and measure the intensity and/or spectrum (i.e. color) of light reflected from the surface, for example as described in relation to Figure 1 . A parameter relating to a printing operation may then be varied based on this measurement. For example, the relative quantities of different print agents to be used in a printing operation may be varied. For example, the test pattern may be formed by printing colored regions on a substrate using colored print agents (e.g. ink or toner) and each colored region may be measured. If it is identified that the colors differ from their intended colors then a parameter defining a color transformation may be varied accordingly. In a simplified example, if a green region is printed and measured, and the measurement indicates that the printed color is “more yellow” than anticipated, then a parameter may be varied such that when subsequently printing a similar green color, relatively less yellow ink and relatively more cyan ink is used to print the color green.

[0045] In more detail, in some examples a test pattern comprising a pattern of printed colors (e.g. printed with print agents) is printed and measured with the sensor 104. Each printed color may correspond to one print agent or may comprise a combination of print agents. The measurements may describe the measured color in a color space, such as a CIELAB color space. The measurement of the printed colors in the pattern may provide a level of saturation of the printing apparatus (e.g. chroma, L*, b*, reflectance, or the like), The measured level of saturation may be compared with a reference level of saturation. If the measured saturation is higher than the reference saturation, then a printing parameter may be varied to specify less of a particular print agent is to be printed. Conversely if the measured saturation is lower than the reference saturation, then the printing parameter may be varied to specify more of the print agent is to be printed. The printing parameter may specify more or less of the print agent is to be printed by specifying a size or number of drops of that print agent. The color calibration may be performed for each print agent of the printing apparatus, for example for each color of ink. In some examples further color calibration operations may be performed based on combinations of print agents (e.g. ICC profiles). In some examples color calibration may comprise calculating color consistency using the CIEDE2000 metric.

[0046] Figure 4 is an example of a method, which may be a method of determining if a light level in a printing apparatus exceeds a threshold.

[0047] In this example, block 402 comprises measuring a light level within a printing apparatus using a sensor. The sensor may be a sensor as described in relation to Figures 1 or 3 and the light level may correspond to the light intensity. The printing apparatus may comprise the sensor or the sensor may be associated with the printing apparatus, for example it may be part of a separate sensing apparatus. The printing apparatus may for example comprise the apparatus 100, 300 of Figures 1 or 3.

[0048] In this example, block 404 comprises determining a measure of usage of the printing apparatus. The measure of usage may be a measure of usage since the sensor was installed or cleaned. The measure of usage may be a quantity of print agent consumed, deposited or printed by the printing apparatus. In some examples the measure of usage may be some other quantity, such as a duration the printing apparatus has been operational, energy used by a printhead; number of cycles performed by a scan axis of a printing apparatus or a number of sheets of substrate or substrate area printed by the apparatus. In some examples the measure of usage may be a measure of usage since an event occurred. In this example the event may be a cleaning operation, however in other examples the event may be the first operation of the sensor or the printing apparatus. A cleaning operation may comprise removing accumulated contamination (e.g. print agent (e.g. inks or toner), dust, fibres and the like) from a sensor used to measure the light level within the printing apparatus.

[0049] In this example, block 406 comprises determining, based on the determined level of usage and the measured light level, if an actual light level incident on the sensor exceeds a light level threshold. In some examples, this may comprise comparing the measured light level to a threshold, wherein the threshold is determined based on the level of usage. In some examples, this may comprise determining an adjusted light level, wherein the adjusted light level is an estimate of the actual light level incident on the sensor light level and comparing this to the threshold. In other examples, the measured light level may be compared to a threshold, wherein the threshold varies based on the determined level of usage. The light level threshold may be a light intensity threshold as described in relation to Figures 1 and 3. When the ambient light incident on the sensor is below the light level threshold, light sensitive sensor(s) within the print apparatus may be capable of accurately performing measurements however when the ambient light incident on the sensor is above the light level threshold the ability of such sensor(s) to perform accurate measurements may be impaired.

[0050] In some examples a contamination level may be determined based on the level of usage and the adjusted light level may be determined based on the contamination level and the measured light level, as described in relation to Figures 1 to 3. In some examples a contamination level may not be explicitly determined, and instead the adjusted light level may be determined based on the level of usage and the measured light level without intermediately explicitly determining a contamination level. In such examples, the level of usage acts as a proxy for the contamination level.

[0051] Figure 5 provides an example of the method of Figure 4. As discussed in greater detail below, blocks 502, 504 and 510 may provide an example of the method of blocks 402, 404 and 406 respectively described in relation to Figure 4.

[0052] In this example, block 502 comprises measuring a light level within a printing apparatus using a sensor, as described in relation to Figure 4. The sensor may comprise the sensor 104 described in relation to Figures 1 and 3.

[0053] In this example, block 504 comprises determining a measure of usage of the printing apparatus since the sensor was installed or cleaned by determining a quantity of print agent consumed by the printing apparatus since installation or cleaning of the sensor. A printing apparatus may measure or track the amount of print agent it consumes. For example, a printing apparatus may record the volume or mass of print agent deposited, for example the volume of ink in millilitres may be recorded. In other examples a printing apparatus may record the number of drops of print agent and in examples wherein the printing apparatus deposits different sizes of drops, it may also record the sizes of the drops. In other examples the mass of print agent remaining in the printing apparatus may be measured, from which the quantity of print agent deposited may be inferred. The measure of usage may be indicative of the contamination level on the sensor because as more print agent is consumed by the printing apparatus more contamination, such as drops of print agent, may accumulate on the sensor.

[0054] In this example, block 506 comprises obtaining (for example from a memory or the like) a relationship between an actual light level and the measured light level based on the measure of usage. As the printing apparatus is used, the sensor may become increasingly contaminated and therefore the amount of light incident on the sensor blocked by the contamination may increase. Therefore, measured light level may be lower than the measured light level. The difference in the measured light level and the actual light level may follow a predictable relationship. Therefore, this relationship may be predetermined and can be used to determine the actual light level based on the measured light level. The relationship between the actual light level and the measured light level may correspond to the second predetermined relationship described in relation to Figure 3.

[0055] The relationship between the actual light level and measured light level may describe the relationship between the actual light level and measured light level at a particular level of usage. For example, obtaining the relationship may comprise obtaining the relationship based on the level of usage. For example, a memory may store a plurality of relationships between the actual light level and measured light level, each relationship corresponding to a particular level of usage. Obtaining the relationship may comprise selecting the relationship which corresponds to the same, or closest, level of usage as the determined level of usage. In some examples obtaining the relationship may comprise performing an interpolation between two relationships. For example, a relationship corresponding to a level of usage less than the determined level of usage may be selected and a relationship corresponding to a level of usage greater than the level of usage may be selected. An interpolation may then be performed on the selected relationships to determine a new relationship for the particular determined level of usage.

[0056] In this example, block 508 comprises determining an adjusted light level based on the obtained relationship. The adjusted light level may be an estimate of the actual light level incident on the sensor, and may be determined based on the determined level of usage, the measured light level and the obtained relationship between the actual light level and the measured light level. The adjusted light level may be an estimate of the actual light level incident on the sensor. As described above, for example in relation to block 406 of Figure 4, the obtained relationship may relate the measured light level to the actual light level at a particular level of usage. Therefore, the adjusted light level may be determined by inputting the measured light level to the relationship. In some examples, the relationship may be in the form of a look up table, and the measured light level, and some examples the level of usage, may be looked up to determine the adjusted light level. In other examples, the relationship may be embodied as a transformation function or the like. [0057] In this example, block 510 comprises determining if the actual light level is greater than a light level threshold by determining if the adjusted light level is greater than a threshold. When the actual light level is determined to be greater than the light level threshold, block 512 comprises providing a notification to a user. The notification may comprise a visual notification or an audible notification. For example, the notification may be displayed on a screen, may comprise illumination of light or may comprise generating a sound. The notification may inform a user that the actual light level is greater than a predetermined amount, and therefore the ambient light incident on the sensor is sufficiently high to adversely affect measurements performed by the sensor, for example as described in relation to Figure 3.

[0058] When the actual light level is not greater than the light level threshold, block 514 comprises determining an amount of contamination on the sensor and comparing the determined amount of contamination to a contamination level threshold. When the amount of contamination is greater than the second threshold, block 516 comprises performing an action. When the amount of contamination is greater than the contamination level threshold, the contamination may be sufficient to cause the measurements performed by light sensitive sensor(s) of the apparatus to be inaccurate. Therefore, the action may be a notification presented to a user to notify the user that the measurements may be inaccurate and/or that the sensor(s) should be cleaned. The notification may be displayed on a screen, may comprise illumination of light or may comprise generating a sound. In some examples the action may comprise performing a cleaning operation, for example the action may comprise cleaning the sensor. For example, the action may comprise wiping the sensor to remove contamination, such as print agent, from a surface of the sensor. In some examples, the action may be performed automatically, for example by under the control of a controller of the print apparatus.

[0059] If at block 514 it is determined that the usage level is not greater than the contamination level threshold, in some examples the method may continue to perform further measurements with the sensor and/or continue to perform a calibration operation, such as a color calibration operation, as described in relation to Figure 3.

[0060] Although in this example the adjusted light level is compared to the light level threshold prior to comparing the contamination level to the contamination level threshold, in some examples the usage level may be compared to the usage level threshold prior to comparing the adjusted light level to the light level threshold. In still other examples, an adjusted light level may not be determined, and the light level may be compared directly to a threshold, wherein the threshold is determined based on the level of usage.

[0061] Figure 6A is a graph which shows example relationships between ambient light intensity and measured light intensity. Moreover, it shows how a threshold measured light intensity may be determined for a given usage level or level of contamination. In this example three relationships are shown, each corresponding to three different levels of contamination of the sensor (e.g. three different amounts of usage of a print apparatus in which the sensor is installed). The relationship Co is the relationship between ambient light intensity and measured light intensity when the sensor is clean, for example prior to use of the printing apparatus or after cleaning of the sensor and prior to re-use of the printing apparatus. As can be seen from the graph, as the ambient light intensity increases, the measured light intensity also increases. In this example the relationship between ambient light intensity and measured light intensity is nonlinear, however in some examples the relationship may be different to that shown, and may for example be approximately linear. For example, the output signal (i.e. voltage) of a photodiode may be approximately linear as a function of illuminance. However some light sensors may not have a linear relationship, for example when a sensor becomes saturated with incident illumination, the measured light level may remain constant as ambient light level increases. A particular relationship for a given sensor may be characterised by testing or based on theory, as discussed above. At a particular ambient light intensity, A, when the sensor is clean, the measured light intensity may be determined to be Mo based on the relationship C- o. [0062] The graph also shows a relationship, Ci, between ambient light intensity and measured light intensity at a first contamination level and a relationship, C2, at a second contamination level, wherein the second contamination level is greater than the first contamination level. In other words, at the second contamination level there is a larger amount of contamination (e.g. print agent) on the sensor. Conversely the ambient light intensity may be estimated for each contamination level for a particular measured light intensity by selecting the appropriate relationship (e.g. Co, Ci or C2) based on a measure of usage of the printing apparatus, such as an amount of print agent used. For example at the first contamination level, the measured light intensity is Mi at the ambient light intensity, A, and at the second contamination level the measured light intensity is M2, wherein M2 is less than Mi. As can be seen from the graph as the contamination level increases, the measured light intensity which corresponds to a particular ambient light intensity decreases. This is because as the contamination level increases, the amount of incident light blocked by the contamination increases so the sensor measures a lower light level for a particular level of incident light.

[0063] Moreover, if A is the threshold maximum level of ambient light at which the light sensitive components of the print apparatus function adequately, then the threshold sensor reading (i.e. the threshold light intensity, or light level) associated with A depends on the contamination level. At the first contamination level, an ambient light measurement of greater than Mi indicates that the ambient light threshold is exceeded whereas a reading between Mi and M2 is indicates that ambient light levels are acceptable. However, at the second contamination level, any reading above M2 indicates that the ambient light level is too high. Thus, block 406 of Figure 4 may comprise determining, based on the contamination level, a threshold measured intensity M _x corresponding to the threshold measurement for that contamination level, i.e. corresponding to a level of ambient light of A, then determining if the acquired measurement exceeds that measurement threshold. In other examples, the assessment module 108 may determine the threshold measured intensity for the contamination level or usage level and determine if the acquired measurement exceeds that measurement threshold.

[0064] The light level threshold may be determined by selecting the appropriate relationship, inputting the ambient light intensity which corresponds to the maximum light intensity at which the sensor is capable of providing accurate measurements and from the relationship determining the measured light intensity which corresponds to the input ambient light level.

[0065] Figure 6B is a graph which shows the example relationships Ci and C2 of Figure 6A between ambient light intensity and measured light intensity at first and second usage/contamination levels. As described in relation to Figure 6A, a relationship may be selected based on measure of usage/contamination of the printing apparatus, such as an amount of print agent used. However, a discrete number of relationships may be stored which means that the measure of usage/contamination of the printing apparatus may lie between the values of the stored relationships. In such examples a relationship may be selected which has the closest level of usage to the actual level of usage, however, this may provide a light intensity threshold which is not an accurate estimate of the measured light intensity which corresponds to the ambient light intensity below which accurate measurements may be performed by the sensor. Therefore, in some examples, as described in relation to Figure 3 and in relation to block 508 of Figure 5 the obtained relationship may be an interpolated relationship. Figure 6B also shows such an interpolated relationship, Ci,2, at a contamination level which is between the first and second contamination levels. The interpolation may for example be weighted, depending on whether the actual usage level is closer to the usage level associated with Ci or to the usage level associated with C2.

[0066] Determining the interpolated relationship Ci, 2 may for example comprise obtaining a first relationship, Ci , between the ambient light level and the measured light level at a first level of usage (i.e. first contamination level) less than the determined level of usage and obtaining a second relationship, C2, between the ambient light level and the measured light level at a second level of usage (i.e. second contamination level) greater than the determined level of usage. An interpolated relationship, Ci,2, shown as a dotted line in Figure 6B, may then be determined by interpolating between the first relationship C2 and the second relationship C2. For example, a linear interpolation may be performed between the first and second relationships.

[0067] The interpolated relationship Ci,2 may then be used to determine a modified light intensity MI ,2 in a similar manner described in relation to Figure 6A.

[0068] Figure 7 shows an example of a tangible machine readable medium 702 in association with a processor 704. The machine readable medium 702 stores instructions 706 which, when executed by the processor 704 cause the processor to carry out actions.

[0069] In this example, the instructions 706 comprise instructions 708 to cause the processor 704 to obtain a measured light intensity. The light intensity may be obtained from measurements performed by a sensor, for example as described in relation to Figures 1 and 3 or as described in relation to block 402 of Figure 4 and block 502 of Figure 5.

[0070] In this example, the instructions 706 comprise instructions 710 to cause the processor 704 to obtain an estimate of an amount of accumulated contamination on a sensor used to measure the light intensity. The estimate of accumulated contamination may be based on a measure of usage of the printing apparatus, for example based on a duration of use or a quantity of print agent used, or any other estimate of usage described above. The measure of usage may be the amount the printing apparatus has been used since an earlier ‘clean’ state of the sensor, for example following a previous cleaning operation or since the first operation of the printing apparatus. Estimating the amount of accumulated contamination may be performed as described in relation to the contamination level module 106 of Figures 1 and 3, block 404 of Figure 4 or block 504 of Figure 5.

[0071] In this example, the instructions 706 comprise instructions 712 to cause the processor 704 to determine, based on the measured light intensity and the estimate of the accumulated contamination, if an actual light intensity incident on the sensor is greater than a threshold light intensity. The threshold light intensity may correspond to the light intensity threshold or the light level threshold described in relation to Figures 1 , 3, 4 and 5. In some examples this may comprise executing instructions to obtain a threshold measured light intensity for the estimate of the accumulated contamination, In such examples, determining if the actual light intensity incident on the sensor is greater than the threshold comprises comparing the measured light intensity to the threshold measured light intensity. In other examples, this may comprise determining an estimate of the actual light intensity incident on the sensor based on the estimate of the amount of accumulated contamination

[0072] Figure 8 shows a machine readable medium 802 associated with the processor 804. The machine readable medium 802 comprises instructions which, when executed by the processor 804, cause the processor 804 to carry out instructions 806. The instructions 806 comprise instructions 708 and 710 as described in relation to Figure 7.

[0073] The instructions 806 further comprise instructions 808 to cause the processor 804 to obtain a relationship between the actual light intensity and the measured light intensity. The relationship may be the relationship described in relation to Figures 3, 5, 6A or 6B.

[0074] The instructions 806 further comprise instructions 810 to cause the processor 804 to determine, based on the measured light intensity, the estimate of the accumulated contamination and the obtained relationship, if an actual light intensity incident on the sensor is greater than a threshold light intensity. Determining if the actual light intensity incident on the sensor is greater than the threshold light intensity may be performed as described in relation to the contamination level module 106 of Figure 1 or 3 or as described in blocks 406 or 510 of Figures 4 and 5.

[0075] The machine readable medium 702, 802 may further comprise instructions to perform any of the functions of apparatus 100 or 300 of Figures 1 and 3 or blocks of Figures 4 and 5. [0076] 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.

[0077] 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 diagrams 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.

[0078] 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.

[0079] 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.

[0080] 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.

[0081] 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.

[0082] 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.

[0083] 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.

[0084] The features of any dependent claim may be combined with the features of any of the independent claims or other dependent claims.