Printing involves technologies in which material, often including coloured pigments or dyes, is applied to a substrate. Digital printing involves technologies in which a printed image is created directly from digital data, for example using electronic layout and/or desktop publishing programs. Known methods of digital printing include full-colour ink- jet, electrophotographic printing, laser printing, and thermal transfer printing methods.

Colourless or transparent liquid compositions which contain little or no pigment or dye have also been developed to be used in printing and can be used to provide a desired (for example, glossy) appearance to a printed article, and/or to provide a protective layer over the print or substrate.

Detailed Description

This disclosure is not limited to the particular materials and process steps disclosed herein because such materials and process steps may vary somewhat. The

terminology used herein is used for the purpose of describing particular examples only. The terms are not intended to be limiting because the scope of the present disclosure is intended to be limited only by the appended claims and equivalents thereof.

It is noted that, as used in this specification and the appended claims, the singular forms "a," "an," and "the" include plural referents unless the context clearly dictates otherwise.

As used herein, the term "about" is used to provide flexibility to a numerical range endpoint by providing that a given value may be "a little above" or "a little below" the endpoint. The degree of flexibility of this term can be dictated by the particular variable and would be within the knowledge of those skilled in the art to determine based on experience and the associated description herein.

As used herein, the term "substantially" or "substantial" refers to the complete or nearly complete extent or degree of an action, characteristic, property, state, structure, item, or result.

As used herein, a plurality of items, structural elements, compositional elements, and/or materials may be presented in a common list for convenience. However, these lists should be construed as though each member of the list is individually identified as a separate and unique member. Thus, no individual member of such list should be construed as a de facto equivalent of any other member of the same list solely based on their presentation in a common group without indications to the contrary. Numerical data may be expressed or presented herein in a range format. It is to be understood that such a range format is used merely for convenience and brevity and thus should be interpreted flexibly to include not only the numerical values explicitly recited as the limits of the range, but also to include all the individual numerical values or sub-ranges encompassed within that range as if each numerical value and sub- range is explicitly recited. As an illustration, a numerical range of "about 20 to about

200" should be interpreted to include not only the explicitly recited values of about 20 to about 200, but also include individual values and sub-ranges within the indicated range. Thus, included in this numerical range are individual values such as 25, 50, 78 and 167 and sub-ranges such as from 20-50, from 32-84, and from 104-195, etc. This same principle applies to ranges reciting only one numerical value. Furthermore, such an interpretation should apply regardless of the breadth of the range or the

characteristics being described.

Brief Description of the Figures

Figure 1 shows an example of apparatus for measuring the thickness of a transparent material layer printed on a substrate;

Figure 2 shows an example of a method for measuring the thickness of a transparent material layer printed on a substrate;

Figure 3 shows an example of absorption of light by a transparent layer;

Figure 4 shows how the optical density of a transparent layer varies with thickness of the layer;

Figure 5 shows another example of apparatus for measuring the thickness of a transparent material layer printed on a substrate;

Figure 6 shows another example of a method for measuring the thickness of a transparent material layer printed on a substrate; Figure 7 relates to the fluorescence of a substrate comprising fluorescent agents; Figure 8 shows an example of printing apparatus incorporating apparatus for measuring the thickness of a transparent material layer printed on a substrate;

Figure 9 shows another example of a method for measuring the thickness of a transparent material layer printed on a substrate; and Figure 10 shows an example of a processor for apparatus for measuring the thickness of a transparent material layer.

Transparent ink can be used, in some examples, to provide an optical effect (e.g. a gloss or matt finish) and/or to provide a protective coating. In order to provide its intended function, a layer of a certain minimum thickness may be desired. However, an unduly thick film adds cost and may make the printed article more difficult to handle.

Measurement of thickness of printed pigmented inks for quality control and process efficiency purposes is often carried out based on the colour density of the printed ink. Such inks can be illuminated and the received light analysed to determine density based on the attenuation of light by the coating. This is performed by an apparatus known as a 'densitometer', which can be provided separately from, or integrated into, printing apparatus.

However, where a printed coating is transparent, the attenuation of light is reduced. An accurate layer thickness cannot be obtained using established methods as outlined above as the weak attenuation of light by a transparent layer is easily masked by noise in the electronic apparatus used to acquire the sample. In addition, attenuation of light incident on a transparent material layer on a substrate is complicated by any variation of optical properties (i.e. optical inhomogenities) in the substrate. Considering, for example, a substrate such as paper, even if a transparent layer of consistent thickness is applied, the light intensity returned from a uniformly illuminated substrate through the printed film or coating may vary from place to place due to optical inhomogenities such as variations in colour, surface texture, transparency, surface sheen and the like.

The present disclosure provides examples of apparatus and methods to measure the thickness of a transparent material printed in a layer on a substrate.

As shown in example apparatus in Figure 1 , an apparatus 100 comprising a light source 102 and a detector 104 is provided. The apparatus 100 further comprises a processor 106, and examples of the functioning of the processor 106 are set out below. As can be seen in Figure 1 , in one example, the apparatus 100 is arranged such that light from the light source 102 may be incident on first 108a, and second 108b portions of a transparent material which have been printed on a substrate 1 10.

As set out in Figure 2, in one example, a method for measuring the thickness of a layer of transparent material printed on a substrate comprises illuminating the transparent material (block 202), and measuring the intensity of light received from a first portion of the material (block 204) and from a second portion of the material (block 208). An average is calculated from these measurements (block 208) and used to determine layer thickness (block 210).

This average may be, for example, an average relating to intensity, or directly relating to attenuation of light by the material. The regions 108a, 108b may be, for example, illuminated at the same time, in overlapping time frames, or one after the other.

Transparent inks do, in general, follow the Beer-Lambert law, i.e. the intensity of the light decreases when propagating through the ink film and the intensity reduction increases as a function of the thickness of the layer. Thus, by measuring light attenuation, the thickness can be calculated so long as sufficiently accurate measuring techniques are employed. By taking at least two readings and calculating an average, the optical density of even transparent layers can be determined, as taking multiple readings effectively increases the signal-to-noise ratio. Taking readings from different portions of material decreases the inaccuracy originating in the optical inhomogeneity of the substrate. In some examples, more than two readings are taken and used to determine an average value. Taking further readings will further reduce the signal-to- noise ratio. In other examples, in the region of about 10 to about 200 readings may be taken.

The term 'layer' as used herein is intended to cover print and coatings. The term 'transparent' means at least substantially transparent to the naked human eye, including colourless and non-pigmented layers.

In the example of Figure 1 , a first 108a and a second 108b portion of transparent material are provided as discrete regions. Although these are shown as separate, they may be contiguous, or part of a substantially continuous transparent layer. However, in order to sample the transparent layer, in some examples, the sampled portions are spatially separated. This means that any localised quality of the substrate, such as a patch of enhanced reflectance, is unlikely to be seen in all samples taken. In addition, providing separate patches may facilitate the gathering of signals from non-coated regions, as is discussed below in regard to the example of Figure 5. Further, in some examples, regions may be arranged linearly on the substrate 1 10. As the

substrate 1 10 may be moved linearly relative to the apparatus 100, providing a linear arrangement of printed portions of transparent material may facilitate the sampling of such portions 108a, b. Again, this is further discussed below.

The substrate 1 10 may for example comprise any substrate. For example the substrate may comprise paper, plastic, metal, ceramics, packaging (for example, wrappers, bottles, cartons, etc), articles of manufacture (such as Compact Discs and Digital Video Discs), or indeed any substrate capable of being printed upon. The substrate may therefore be relatively planar, or may be a shaped substrate.

The substrate 1 10 may be moved relative to a substantially stationary apparatus 100. Such an arrangement may be readily incorporated into existing printer and/or densitometer designs which are usually constructed to convey a substrate 1 10 through or past an apparatus 100. However, alternative designs are possible. As one alternative example, the apparatus 100, or parts thereof, may move relative to a stationary substrate 1 10. As another alternative, more than one detector 104 and/or more than one light source 102 may be provided, such that light received from the portions 108a, 108b can be measured at the same time by different detectors 104. As a further alternative, the light source 102 could be arranged to flood the substrate 1 10, such that the portions 108a, 108b receive light from the light source 102 at the same time. The received light may be spatially filtered. For example, the detector 104 may have an angle of reception which can be changed to ensure that the light from the first portion 108a can be distinguished from light from the second portion 108b. As a further alternative, light received from the first 108a and second 108b portion may be received at one time, and distinguished for example using polarising or colour filters depending on the angle at which light is received, or through use of a detector 104 comprising a detector array wherein individual detector pixels may receive light from at least substantially just one of the portions 108a, 108b. The angle of light emitted by the light source 102 may be changed to illuminate different regions of the substrate 1 10.

Indeed, any arrangement which allows the light received from different regions of the substrate 1 10 to be distinguished may be employed.

Figure 3 shows an example of the absorption of a transparent layer illuminated with various wavelengths of illuminating light. In the example of Figure 3, the layer is Indigo Transparent Ink 076, printed as a 1 micron layer on a transparent polyethylene terephthalate (PET) substrate. The value for absorption is determined, as is standard practice, as a dimensionless number equal to the logarithm of the ratio of intensity of the illuminating light to the intensity of the reflected light.

The absorption for transparent layers and films will be expected to be low (at least when considering visible light), perhaps typically around or below about 0.1 . For comparison, typical colors exhibit absorption between 1.5 and 3. However, it will be appreciated that there is no defined or established attenuation level below which a material would be deemed 'transparent'.

As can be seen in the example of Figure 3, light in one region of the wavelength spectrum undergoes greater attenuation than light in another region of the wavelength spectrum. Such behaviors is seen in a range of materials. In light of this, the light source 102 and/or detector 104 may be chosen or configured with regard for wavelengths so that relatively high attenuation is seen for a given material used to form the transparent layer. The thickness of the transparent material may for example be determined by calculating the average of the ratio of the intensity of light emitted from the light source 102 to the intensity of the received light as it is detected by the detector 104. A given material will have a given attenuation. Figure 4 shows an example for the relationship between optical density and thickness of a layer of Indigo transparent ink 076 printed on Condat 135 gsm paper.

As has been mentioned above, optical density (and equivalently absorption) depends on wavelength. For measuring the optical density of yellow, the wavelength of blue colour is used where the absorption of yellow is the highest. Thus setting the densitometer to "Yellow" (as shown in Figure 4), indicates that the wavelength of blue light is being monitored.

While, as a matter of practice, the term "absorption" is preferred in optical physics and "optical density" is preferred in the printing industry, it will be appreciated that they relate to the same physical attribute. In the printing industry, therefore, typically apparatus for measuring optical density is employed (hence the name of the apparatus: a densitometer). While the chart of Figure 3 was obtained with an optical spectrometer (and therefore shows 'absorption'), the graph of Figure 4 has been produced by such a densitometer. Typically, a printed transparent layer may be in the order of about 0 to about 5 microns thick in order to perform its function of providing a particular finish and/or act as a protective layer. In one example, the typical ink thickness of an ink layer printed by Hewlett Packard Indigo printing presses may be about 1 micron.

Therefore, the methods and apparatus described above provide a simple method for determining layer thickness, even when the layer is formed of transparent material. Indeed, the method may be performed as part of an in-line process and/or may make use of a standard densitometer, controlled to carry out thickness measurement as described in this disclosure. In some printers, an in-line densitometer, comprising both a light source and detector, may already be provided. Such a densitometer could be repurposed, or provided with additional functionality (for example, this may comprise reprogramming the densitometer) to perform the method of Figure 2.

Further examples of apparatus and methods for determining thickness of printed transparent layers are now described.

In particular, in an example of apparatus 500 now described with reference to Figure 5 (in which parts in common with Figure 1 are labelled with like numbers), the

detector 104 is provided with a filter 502. In this example, the filter 502 is arranged to act as a bandpass filter intended to limit the wavelength(s) of light incident on the detector 104. As noted above, the absorbance of a layer of a given material may be greater for one particular wavelength band than for another. Therefore, to enhance sensitivity, the light reaching the detector 104 may be restricted to just such wavelength band(s). For example, the optical density may be measured in a mode, in which only wavelengths of light in a band corresponding to, for example, yellow light, or blue light, or some other range, are passed by the filter 502. Alternatively or additionally, the filter 502 may be a spatial filter arranged to ensure that, at least substantially, only light from a predetermined region of the substrate 1 10 is incident on the detector 104. The filter 502 may for example limit the angle of acceptance for light reaching the detector 104. The apparatus in this example further comprises a filter 504 in association with the light source 102. This filter 504 may act to spatially filter the output of the light source 102 (or any other light in the vicinity of the light source 102) for example to limit the spot size of light incident on the substrate, and/or act as a waveband filter, and/or may time modulate the output of the light source 102, as further explained below. The apparatus of Figure 5 further comprises a substrate conveyer 508, arranged to move the substrate 1 10 past the light source 102 and detector 104. This may be, for example, in the form of a belt, a roller, a series of rollers, or the like.

As will be noted, in Figure 5, a plurality of portions of transparent material 108a-d are provided, and the attenuation of light by all such portions may be measured by the apparatus 500. In this example, the regions 108a-d are printed in with a linear configuration at a regular, known, spacing.

In addition, in this example, the substrate 1 10 is moved relative to the apparatus 100 at a substantially constant speed by the substrate conveyer 508. The light source 108 is arranged to illuminate a region of the substrate and samples are gathered by the detector 106 from the illuminated region. Each of the portions 108a-d, which in this example are linearly arranged in the direction of travel of the substrate 1 10, moves through the illumination/sampling region. As they are regularly spaced and moved at constant speed, the illumination/sampling region is occupied by a portion 108a-d at regular intervals. This therefore allows readings relating to the portions 108a-d to be easily identified based on the time at which they are acquired.

Such examples therefore employ a form of 'phase locking', in that samples relating to light from the portions 108a-d are gathered at a time based on the periodicity of the layout of the portions 108a-d of the transparent material intended to be measured. So called 'time gating' of the signals could be carried out at the time the signals are collected (for example, providing a shutter for the detector 104, or switching the detector 104 off when a measurement is not desired) but in this example is carried out by a signal conditioning module 506, which only passes signals obtained in

predetermined time periods to the processor 106. The signal conditioning module 506 therefore effectively applies a time-gate to the signal detected by the detector 104. By reducing the unwanted signals considered for processing, the noise within the signals is reduced. It will also be noted, that, by applying the portions 108a-d as discrete patches, and arranging the patches in a line which corresponds to the direction of travel of the substrate relative to the apparatus 500, a linear set of readings can readily be obtained from relatively well spaced portions. Providing well spaced portions contributes to increasing the diversity of the samples in that localised optical inhomogenities are unlikely to be seen in all samples.

Additionally, in some examples, samples may be taken from background

portions 510a-c of the substrate 1 10 (i.e. regions which do not carry a transparent coating layer). In the example of Figure 5, the sampled background portions 510a-c lie along the line of the portions 108a-d of transparent material.

Such samples can be obtained in a simple manner with a relatively small beam of light, and, in this example, moving the substrate 1 10 relative to the apparatus 500 in a linear manner. This in turn allows a plurality of distinct regions of the substrate 1 10 (whether over-printed with a transparent layer, unprinted, or both) to be sampled, and thus means that any inhomogenities within the substrate 1 10 e.g. a localised area of relatively high reflectance, or a localised colour abnormality, may be averaged out as long as a sufficiently large sample set is obtained.

The diversity of the sample set is thus increased: Measuring signals from areas of the substrate bearing a transparent coating and background (non-coated) areas and taking the difference between the two sets of readings can reduce the impact of optical inhomogenities when assessing the thickness of the transparent layer.

Further, as explained above, providing a regular array of background portions 510a-c, as well as a regular array of portions 108a-d of transparent material readily allows a signal to be gathered in as a periodical function of time, for example the average value(s) determined may be determined on only those signals received in

predetermined time periods. This may allow samples to be easily cleaned from stochastic disturbances, further improving the signal-to-noise ratio.

As noted above, the filter 504 provided in association with the light source 102 may time modulate the light reaching the substrate 1 10. Such time modulation may be arranged, for example, such that the substrate is only illuminated when a region 108a-d or, in some examples, a background region 508a-c between/adjacent to a region 108a- d, is substantially in a position to be measured (for example, at least substantially centrally aligned with relation to the point of incidence of a beam of light from the light source 102, and/or wholly within the illumination/sampling region mentioned above). The light source 102 and/or filter 504 may be arranged such that the spot size is wholly contained within a transparent material portion 108a-d, or in a background

portion 508a-c between the transparent portions 108a-d so as to ensure that the samples relate purely to either background or transparent layer portions.

In an example, now described with reference to Figure 6, at least one background portion 508a-c of the substrate 1 10 is also illuminated and the light received therefrom is measured. In the example of Figure 6, the number of transparent material portions 108a-d which are to be measured is designated as m. m is any integer greater than 1. The number of uncoated background substrate portions 508a-c which are to be measured is designated as t. t is any integer. In practice, m or t may be, for example, in the range of about 1 to about 500. In one example, m and t are in the range of about 20 to about 200, in another example, m and t are in in the range of about 50 to about 200. m and t may be the same or may be different. The more measurements that are taken, the greater the improvement in signal to noise ratio. However, taking and processing measurements takes time and processing resources and therefore there is trade-off between acquiring enough measurements to ensure a satisfactory signal-to-noise ratio to in turn ensure an accurate result is obtained, and the time and/or resources required to gather the result. The values of m and t may therefore vary and/or be selected accordingly. Therefore, as set out in Figure 6, in one example, a method includes illuminating the material (block 602), setting a counter n to 1 (block 604) and measuring the intensity of light received from a first portion of the layer (block 606). In block 608, it is determined whether the number of portions which have been measured is equal to the number of portions required (m). If not, in block 610, the counter is incremented and further measurement(s) performed until the desired number of measurements has been gathered, at which point an average is calculated from these measurements (block 612).

In addition, the method comprises illuminating an uncoated portion of substrate

(block 602), setting a counter s to 1 (block 616) and measuring the intensity of light received from a first background portion of the substrate (block 618). In block 620, it is determined whether the number of background portions of substrate which has been measured is equal to the number of portions required (t). If not, in block 622, the counter is incremented and further measurement(s) performed until the desired number of measurements has been gathered, at which point an average is calculated from these measurements (block 624). It will of course be appreciated that Figure 6 illustrates the general case: if there is only one sample (t=1 ), block 624 is irrelevant. Of course, many other methods of counting the number of samples taken could be used.

The difference between these two signals is indicative of the absorption due to the layer, and may be used to determine layer thickness (block 626).

The measurements taken in blocks 606 and 618 could be used to calculate a ratio between the intensity of light incident on each of the portions 108a-d, 508a-c (as determined from knowledge of the output of the light source 102, which could be predetermined or measured) and the intensity of the light received therefrom. Block 626 may therefore comprise determining a signal indicative of the layer thickness comprising the difference between the average ratio calculated from the light received from coated portions 108a-d and the average ratios seen from the uncoated

background portions 508a-c. Alternatively, the intensity may be considered without determining a ratio.

Where portions of material 108a-d and background regions 508a-c are arranged periodically, as discussed in relation to Figure 5, the measurements taking in blocks 606 and 618 may for example be taken periodically (and possibly alternately), and/or identified based on time gating applied to a signal after collection.

These blocks may be carried out by a processor 106, for example under the control of appropriate machine readable instructions. In still further examples, the substrate 1 10 may contain fluorescent substances and the light source 102 is chosen or filtered to cause fluorescence thereof. The substrate 1 10 may be treated to provide such substances or the substrate may in any case contain such substances. For example, a substrate 1 10 such as paper may contain optical brighteners which are fluorescent pigments or dyes that absorb UV light and emit light in the visible spectrum, typically in the blue range. To illustrate this, Figure 7 shows data relating to the fluorescence of Condat 135gsm paper. Two curves are shown, the solid line 702 showing the excitation against the wavelength of incident light in nanometers, and the dotted line 704 showing the characteristic emission curve in arbitrary units.

As the thickness of the transparent layer increases, more fluorescent light is absorbed thereby, and the intensity of the light emitted by the fluorescent substances which reaches the detector 104 decreases. This results in an apparent increase of the optical density of the layer.

In such an example, the intensity of the received light in the visible region of the spectrum may be measured. In particular, the light at the waveband of fluorescence may be considered. If, for example, the fluorescence is blue, in such an example, light may be measured in the blue range, for example being filtered such that only the waveband corresponding to blue light reaches the detector 104.

Given that, as the thickness of the transparent material increases, more fluorescent light gets absorbed thereby, the intensity of the light emitted from a subtrate by fluorescing substances therein which is received by the detector 104 decreases with thickness. In the case where the fluorescence is blue, this will result in an apparent increase of the optical density of the print in yellow (because yellow is the

complementary colour to blue), which could be measured.

In examples where the substrate 1 10 fluoresces under UV light, the light source 102 may be, in some examples, a UV source or at least comprise some UV content in the illuminating beam. It will therefore be appreciated that the light received by the detector 104 and used to determine layer thickness may comprise light other than that emitted by the light source 102.

The methods and apparatus described above allow monitoring of thickness of transparent printed layers via measuring light attenuation without modification of existing transparent inks.

However, the transparent material may comprises a substance such as UV absorbent agents. Such a layer could still be transparent in the optical range, but the amount of attenuation of UV light caused by the layer of material will increase thus being more readily discernible. Again, in such examples, the light source 102 may be, in some examples, a UV source or at least comprise some UV content in the illuminating beam.

Similarly, the material may comprises a substance including fluorescent agents and the light source 102 chosen to cause florescence thereof. This may mean that the apparent optical density of the transparent material layer will reduce as the thickness of the layer increases.

In such examples, the signal acquired may be larger than for typical transparent layers and therefore fewer readings may be required to obtain a satisfactory signal-to-noise ratio. Figure 8 is a schematic representation of a printing press 800 comprising a substrate supply 802, a printing apparatus 804 and an apparatus for determining optical density of a printed layer, in this example a densitometer 806. The densitometer 806 may comprise an apparatus 100, 500 as shown in Figure 1 or Figure 5. The substrate supply 802 may comprise means for conveying a substrate 1 10 to the printing apparatus 804, and/or a source of the substrate 1 10 itself. The printing apparatus 804 may comprise any printing apparatus capable of applying a transparent layer to a substrate 1 10. For example, the printing apparatus 804 may comprise a liquid electrophotographic printing apparatus.

Electrophotographic printing techniques involve the formation of a latent image on a photoconductor surface mounted on an imaging plate. The photoconductor is first sensitized to light, usually by charging with a corona discharge, and then exposed to light projected through a positive film of the document to be reproduced, resulting in dissipation of the charge in the areas exposed to light. The latent image is

subsequently developed into a full image by the attraction of oppositely charged toner particles to the charge remaining on the unexposed areas. The developed image is transferred from the photoconductor to a rubber offset blanket, from which it is transferred to a substrate, such as paper, plastic or other suitable material, by heat or pressure or a combination of both to produce the printed final image.

The latent image is developed using either a dry toner (a colorant mixed with a powder carrier) or a liquid ink (a suspension of a colorant in a liquid carrier). The toner or ink generally adheres to the substrate surface with little penetration into the substrate. The quality of the final image is largely related to the size of the particles, with higher resolution provided by smaller particles. Dry toners used in solid electrophotography are fine powders with a relatively narrow particle size distribution that are expelled from fine apertures in an application device. Coloured liquid inks used in liquid

electrophotography are generally comprised of pigment- or dye-based thermoplastic resin particles suspended in a non-conducting liquid carrier, generally a saturated hydrocarbon.

A transparent layer may also be provided by such apparatus, often as a coating on the printed substrate.

Such a printing press 800 may operate as set out in Figure 9. In block 902, a layer of transparent material is printed on a substrate 1 10 to provide a plurality of transparent layer regions. As discussed above, this layer may be applied as a contiguous layer or in a plurality of discrete regions. Where the layer is applied as discrete regions, these may be regularly spaced. Indeed, the discrete regions may be interspersed with background regions, which are not printed with the transparent layer. The regions may be printed so as to form a linear pattern in some examples. In block 904, regions of the substrate are illuminated and in block 906 light received from these regions is measured. These regions comprise portions which are printed with a transparent layer. In some examples, the regions may also comprise portions which are not printed with a transparent layer, e.g. background regions of the substrate. In block 908, one or more averages are determined from the measured light. This may comprise calculating a first average for portions bearing the coating. In some examples, a second average for background portions is also determined. In block 910, the layer thickness is determined from consideration of the determined average(s).

Block 902, for example, may be carried out by the printing apparatus 804. Blocks 904- 910 may, for example, be carried out by a densitometer 806.

In some examples, such measurements may comprise part of a control loop, i.e. once the layer thickness has been determined, it may be compared to a desired value or range of values. If this desired value/range is not matched, the printing press 800 may be controlled so as to change the conditions by which the layer is applied, to make it correspondingly thicker or thinner.

Figure 10 shows an example of a processor 950 capable of functioning as a processor 106 of Figure 1 . The processor comprises a signal receiving module 952 to receive a plurality of signals, wherein the signals are indicative of the attenuation of light by portions of a transparent material layer printed on a substrate, an averaging module 954 to determine an average value from signals received by the signal receiving module 952 and a layer thickness determination module 956 to determine, from the average value, the thickness of the transparent material layer. The processor 950 further comprises a memory 958. The memory 958 is arranged to store machine readable instructions which, when executed, cause the

modules 952, 954, 956 to act as described above.

The examples of 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 is not limited to disc storage, CD- ROM, optical storage, etc.) having computer readable program codes therein or thereon. 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. It shall be understood that each flow and/or block in the flow charts and/or block diagrams, as well as combinations of the flows and/or diagrams in the flow charts and/or block diagrams can be realized by machine readable instructions. 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 106, 950 as described in relation to Figures 1 , 5 and 10 may execute the machine readable instructions. Thus the functional modules or functional units 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 processers. 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.

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 operation steps to produce computer- implemented processing, thus the instructions executed on the computer or other programmable devices provide a step for realizing functions specified by flow(s) in the flow charts and/or block(s) in the block diagrams. 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 (e.g. a

processor 106) implement the methods recited in the 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.

While reference to certain examples has been made herein, various modifications, changes, omissions, and substitutions can be made without departing from the spirit of the disclosure. It is intended, therefore, that the disclosure be limited only by the scope of the following claims. The features of any dependent claim can be combined with the features of any of the other dependent claims, and any independent claim.