Technical Field of the Invention

The present invention relates to improvements in or relating to laser marking. In particular, the present invention relates to calibration of a laser marking system comprising a multi-fibre array.

Background to the Invention

Traditionally, labels for products and/or packaging have been pre-printed in bulk and applied to the products/packaging as required. The drawback to such actions is that the information on pre-printed labels is fixed. This does not provide flexibility to customise labels or to adapt labels readily at short notice.

One solution to this issue is to digitally print labels on demand using inkjet printers. This requires the supply of liquid inks, solvents and the like at the point of printing. This can be undesirable in many sectors, such as food and beverages, particularly if the inks/solvents provide a contamination risk.

As an alternative, labels may be produced using laser marking. Various forms of laser marking apparatus are known and are used in conjunction with label substrates that comprise colour change material. Upon controlled exposure to laser light from the marking apparatus, portions of the substrate change colour forming a desired image. The image may be monotone or coloured depending on the material and/or the nature of the exposure. The image may comprise text, numbers, codes or the like as well as pictographic elements.

In some implementations, the substrate is provided in the form of an extended tape on a supply reel. The tape is unwound from the supply reel via one or more rollers and passes a laser marking head. This enables successive like or custom labels to be marked on the substrate. Subsequently, the tape can be wound on to a storage reel via one or more further rollers for later application to products/packaging or individual labels may be cut from the tape and applied directly to products/packaging. In other implementations a suitable substrate is provided directly on the product/packaging and this is marked by laser exposure as the product/packaging passes the laser making head. The laser making head may comprise a single laser or a multisource laser array. The benefit of the multi-emitter array is that the imaging speed is independent of image content. In a particular configuration the array comprises a ID or 2D array of fibres coupled to a plurality of laser diodes illuminates the target as it moves in front of the imaging head. The or each laser emitter is modulated based on the image requirements by varying a marking setting defined by any one or more of: power parameter of the emitter; pulse duration of the emitter; duty cycle of the emitter; and the relative velocity between the emitters and the substrate. This results in colour change where the spot emitted by the or each emitter is incident upon the substrate, thereby forming an image.

To induce a desired colour change reaction on a marked area of the substrate, the marked area must be exposed to a particular energy density. Furthermore, this energy must be deposited in the marked area within a particular time period. Accordingly, there is also an associated power requirement. Energy deposited in the marked area of the substrate is absorbed which results in the temperature of the substrate increasing as a result of marking activity. The substrate temperature may have an impact on the colour change reaction. In some instances, the absorbed energy is required to raise the substrate temperature sufficiently to enable a colour change reaction to take place. Additionally or alternatively, the substrate temperature may have an impact on the colour change reaction. For instance, the optical density of the marked area of the substrate may vary.

Ultimately, the temperature of the marking area of a substrate is determined by the energy supplied to the marking area and the initial substrate temperature. Depending on the storage and handling conditions, the initial substrate temperature may vary considerably, for instance it is conceivable that the temperature may vary from less than IOC to 40C or more in different worldwide environments. As a result, a supplying a fixed amount of energy to substrates with differing initial temperatures will result the marked areas having varying optical densities.

It is therefore an object of the present invention to provide a method and apparatus for compensating or calibrating a laser marking system that at least partially alleviates or overcomes the above problems.

Summary of the Invention According to a first aspect of the present invention there is provided a laser marking system comprising: one or more laser emitters operable to emit laser light on to a substrate for marking; a substrate temperature sensor operable to detect an initial substrate temperature prior to marking; and a control unit operable to modulate the output of said emitters by varying a marking setting, wherein the control unit is operable to vary the marking setting in response to the initial substrate temperature.

According to a second aspect of the present invention there is provided a method of operating a laser marking system of the type comprising one or more laser emitters operable to emit laser light on to a substrate for marking, a substrate temperature sensor operable to detect an initial substrate temperature prior to marking, and a control unit operable to modulate the output of said emitters by varying a marking setting, the method comprising the steps of: detecting the initial substrate temperature; and varying the marking setting in response to the initial substrate temperature.

The present invention thus provides compensation for substrate temperature when carrying out laser marking. This can ensure consistent marking outcomes if substrate temperature varies.

The substrate temperature sensor may be a non-contact sensor. Such a sensor would have a sensing element that is not in physical contact with the substrate. This is beneficial as this avoids potential difficulties in locating the sensing element in contact with the substrate. It further avoids potential difficulties should contact between the sensing element and the substrate generate additional heat through friction or hinder relative motion between the substrate and the one or more emitters. These advantages outweigh any reduction in sensitivity that may result from use of a non-contact temperature sensor.

In some embodiments, the non-contact temperature sensor may comprise a radiation thermometer. Such devices may comprise one or more radiation sensing elements suitable to detect radiation emissions from the substrate and thereby determine substrate temperature. Suitable examples of non-contact temperature sensors include but are not limited to: infrared thermometers/pyrometers, two wavelength ratio-metric radiation thermometers/pyrometers, multi- wavelength ratio-metric thermometers and optical pyrometers. The temperature sensor may have one or more sensitivity settings. This can enable the temperature sensor to be calibrated or adjusted to compensate for differing substrate materials.

The temperature sensor may be operable to detect initial substrate temperatures in the range 0°C to 50°C. The temperature resolution of the temperature sensor may be, say, 2°C, preferably, say, 1C or most preferably less than 1°C. In one embodiment, the temperature resolution of the temperature sensor may be, say, 0.2°C.

The temperature sensor may be provided in or adjacent to a marking zone where the substrate is marked.

The control unit may be operable to vary the marking setting by direct reference to a look up table or interpolation between values in a look up table. The interpolation may be linear interpolation or interpolation based on a polynomial function. In one example, the polynomial function may be a quadratic function. Alternatively, the control unit may be operable to vary the marking setting by applying a scale factor to the marking setting. The scale factor may be determined by direct reference to a look up table or interpolation between values in a look up table. The interpolation may be linear interpolation or interpolation based on a polynomial function. In one example, the polynomial function may be a quadratic function. The values in the look up table or scale factor may be determined by a calibration process or may be calculated from a model of marking setting variation in response to temperature. The model may be a theoretical model.

In some embodiments, the system may be provided with a calibration unit. The calibration unit may be operable to store one or more look up tables or scale factors for use in calibration. In some embodiments, the calibration unit may be operable to calculate an interpolation between values stored in a look up table or between stored scale factors. In some embodiments, the calibration unit may comprise a calibration data store and a calibration processing unit.

The marking setting may be defined by any one or more of: power parameter of the emitter; pulse duration of the emitter; duty cycle of the emitter; and the relative velocity between the emitters and the substrate. Variation of the marking setting may involve varying any one or more of the above parameters.

The power parameter for each emitter may be related to the power input signal applied to the emitter and/or to an expected greyscale value. Using the power parameter enables a direct relation to be calculated between the input to each emitter and the optical density of the consequent mark. This beneficially means that it is not necessary to measure the actual laser power output for each emitter nor to derive the power output power versus demand curves.

In some embodiments, the system may be provided with a temperature conditioning unit operable to cool and/or heat the substrate. In some embodiments, the temperature conditioning unit may be operable to mitigate or overcome adverse effects that can occur when the substrate temperature is too high. These effects can include substrate deformation such as rippling and folding. In some embodiments, the temperature conditioning unit may be operable to heat or cool the substrate to mitigate temperature compensation based on marking setting variation. In some embodiments, the temperature conditioning unit may be operable to heat or cool the substrate so as to enable calibration test measurements.

The temperature conditioning unit may be provided in or adjacent to a marking zone where the substrate is marked. The temperature conditioning unit may comprise a roller. The roller may be provided with an active temperature conditioning system. Suitable active temperature conditioning systems may comprise a network of internal conduits through which a temperature conditioning fluid may be circulated and/or one or more thermoelectric temperature conditioning devices.

The roller surface may be adapted to absorb incident light. This may be achieved by the provision of an absorbing layer such as a chemically blackened layer or an anodised layer. The roller surface may additionally or alternatively comprise a transparent coating. In some embodiments, the roller may be provided with a fluid layer between an outer transparent shell and the roller surface. The fluid may be transparent or may comprise a solution or suspension containing light absorbing materials. In one embodiment, the temperature conditioning unit comprises a block comprising a facing surface wherein the substrate runs over the facing surface of the block. Temperature conditioning may thus be achieved by contact between the substrate and the block. The block may be provided with an active temperature conditioning system. In some embodiments, the active temperature conditioning system may comprise a network of internal conduits through which a temperature conditioning fluid may be circulated and/or one or more thermoelectric temperature conditioning devices. The facing surface may be provided with one or more slits in the marking zone.

In some embodiments, the temperature conditioning unit may comprise one or more fluid blowers. Such fluid blowers may be operable to output a flow of temperature conditioned fluid, which is subsequently incident upon the substrate. The temperature conditioned fluid may be air, water or the like.

In some embodiments, the temperature conditioning unit may comprise a preheat source operable to heat the substrate prior to and/or during marking. A suitable preheat source may comprise one or more laser diodes. The laser diodes may be operable to illuminate the substrate with NIR radiation. In one embodiment the preheat source may comprise a one dimensional array of laser diodes or a diode bar. In another embodiment the preheat source may comprise a radiant heater emitting IR radiation. The preheat source may be provided with an optical arrangement operable to homogenise the intensity distribution of emitted light. In particular, the optical arrangement may be operable to homogenise intensity distribution perpendicular to the direction of relative motion of the substrate.

In such embodiments, the temperature conditioning unit may be operable in response to the control unit. This can allow the temperature of the substrate to be controlled before marking. In this manner, the substrate temperature may be maintained at a substantially constant level or a desired level minimising the requirement to vary marking settings.

The system may be calibrated by activating the one or more emitters to mark a series of block images at a range of different test marking settings and/or test initial substrate temperatures; determining the average optical density of each marked block; and thereby determining a calibration relationship between marking setting, substrate temperature and marked optical density. The calibration process may be carried out in response to the control unit and/or in response to the calibration unit.

In one embodiment, determination of optical density of the marked blocks may be achieved by use of a spectrophotometer or an optical densitometer. In an alternative embodiment, determination of optical density of the marked blocks may be achieved by: capturing an image of each marked block; and processing the captured images to determine the average optical density of each marked block.

In some embodiments, the calibration relationship may be determined by marking a series of block images at each allowed marking setting. Alternatively, the calibration relationship may be determined by marking a series of block images at a subset of allowed marking settings. In such instances, the selected allowed marking settings may be provided across the full range of allowed marking settings or may be provided across a selected subrange of allowed marking settings. The subrange may be toward the centre of the full allowed range. The subset may comprise at least four marking settings, preferably six marking settings or more and most preferably eight marking settings or more. Alternatively, it is possible to carry out single point or three- point calibration using a single test marking setting (or three test marking settings). This requires less testing but may not be as accurate.

In some embodiments, the calibration relationship may be determined by marking a series of block images at a series of test substrate temperatures. The series of test substrate temperatures may be provided across the full range of operating temperatures or may be provided across a selected subrange of operating temperatures. The subrange may be toward the centre of the full allowed range. The series of test substrate temperatures may be separated by constant temperature intervals. In some embodiments, the temperature intervals may be, say, 1°C. In other embodiments, the temperature intervals may be, say, 5°C or more.

In some embodiments, the temperature conditioning unit is operable to control initial substrate temperatures for carrying out calibration. In such embodiments, the temperature conditioning unit may be operable in response to the control unit or the calibration unit. Determining a calibration relationship may include the steps of populating a look up table with marking settings corresponding to particular optical densities at particular substrate temperatures. The look up table may be stored in the calibration unit or the control unit. Where block images are marked at a sufficiently wide range of different marking settings and substrate temperatures the marking settings stored in the look up table can be used directly to vary marking settings in order to compensate for substrate temperature across the full expected operating range of the system. In cases where a marking setting or detected substrate temperature fall within the intervals between the test marking settings and/or the test substrate temperatures, intermediate marking settings may be determined by interpolation. The interpolated marking settings may be stored in the look up table. Storing interpolated marking settings for future use is usually preferable to calculating such settings on the fly, particularly if processing speed is an issue. In this manner compensation for substrate temperature can be achieved across the full expected operating range of the system without prior testing at each possible marking setting or substrate temperature. The interpolation may be linear interpolation. In other embodiments, the interpolation may be based on a polynomial function. In some such embodiments, the polynomial function is a quadratic function. The interpolation may be carried out by the calibration unit or the control unit.

Determining a calibration relationship may include the steps of determining a compensatory scale factor to apply to marking settings for each test substrate temperature. The scale factor may be used to scale the marking setting in response to the detected substrate temperature. The determined scale factors may be stored in the calibration unit or the control unit. Where the detected substrate temperature falls between test substrate temperatures, an intermediate scale factor may be determined by interpolation. The interpolation may be linear interpolation. In other embodiments, the interpolation may be based on a quadratic function. The interpolation may be carried out by the calibration unit or the control unit.

The one or more emitters may be provided in a marking head. The marking head may additionally comprise suitable optics for focussing and/or directing the emitted light. The optics may comprise lenses, mirrors or a combination thereof. The marking head may be adapted to move whilst the substrate remains in a fixed position. Alternatively, the substrate may be adapted to move past the marking head.

The one or more emitters may comprise one or more lasers or laser diodes or laser diode bars. A laser diode bar comprises a plurality of co-mounted laser diodes each diode subject to a common marking setting. Accordingly, whilst a diode bar comprises multiple diodes, it effectively operates like a single emitter for the purposes of the present invention.

In a system having a single emitter, the marking head may be provided with a scanning unit. The beam scanning unit may be operable to scan a beam of light emitter by the emitter relative to the substrate the scanning unit may be operable to scan the beam in a vector manner or in a raster pattern. The scanning unit may comprise any one or more of: tilting mirror galvanometer scanners, acousto-optics scanners, electo-optic scanners and the like together with a focusing lens.

In an alternative system comprising a single emitter, the marking head is provided with a spatial light modulator (SLM) operable to receive the emitted light from the emitter and output a modulated beam where beam intensity may vary spatially. In such embodiments, the emitter may be operable to generate a uniform beam for receipt by the SLM. In such embodiments, the uniform beam may have a substantially constant intensity distribution in the plane parallel to propagation. The uniform beam may be formed by provision of a suitable optical arrangement. The optical arrangement may comprise any one or more of: a lens array, a lenslet array or diffraction or holographic elements.

The SLM may be operable to image a single pixel or row of pixels within a marked image at one time or may be operable to image multiple pixels or arrays of pixels within a marked image simultaneously. The SLM may be operable to scan the modulated beam relative to the substrate additionally, or alternatively, the SLM may be provided with a scanning unit operable to scan the modulated beam relative to the substrate. The scanning unit may comprise any one or more of: tilting mirror galvanometer scanners, acousto-optics scanners, electo-optic scanners and the like together with a focusing lens. The SLM may comprise any one or more of: Digital Micro-mirror Devices (DMD), liquid crystal devices, grating light valves, planar light valves, and the like. Examples of suitable DMDs include but are not limited to: DLP™ range of products from Texas Instruments, for example DLP4500NIR. Examples of suitable liquid crystal devices include but are not limited to: Liquid Crystal On Silicon LCOS SLM X10468-04WL supplied by Hamamatsu or E512-A-DVI supplied by Meadowlark_or model R12.288-l-RT supplied by Meadowlark. Examples of phase only liquid crystal devices include but are not limited to: the SLM-200 supplied by Santee Corporation; the PLUT02 range from Holoeye, in particular the PLUT02-NIR-080 or PLUT02- NIR-049 for higher power use. Examples of suitable grating light valves include but are not limited to: FI 088 or G1088 supplied by Silicon Light Machines. Examples of suitable planar light valves include but are not limited to: the PLV from Silicon Light Machines.

In a system having multiple emitters, the emitters may comprise individually addressable laser diode arrays (IALDA) or individually addressable laser arrays (IALA). In some embodiments, the emitters may comprise an emitting end of a marking fibre wherein an input end of the marking fibre is coupled to a laser diode. The emitters may be configured as a one dimensional or two dimensional array. Where the array is a one dimensional array, it may be a simple linear array or may be a staggered array. In particular, the emitters may collectively form a marking head.

The emitters may be operable to emit light with any suitable wavelength, including but not limited to visible or near infrared (NIR) wavelengths. Generally, for marking applications, wavelengths in the range 200nm to 20000nm might be suitable. In some embodiments, the emitters are operable to emit light with wavelengths in the ranges: 390 to 460nm, 500 to 550nm, 620 to 660nm, 900nm to l lOOnm and 1400 to 1600nm.

The substrate may comprise a label. The label may be mounted on a product or packaging before marking. Alternatively, the label may be provided on a reel of label substrate for marking before subsequent application to a product or packaging. The substrate may comprise an extended tape. In such embodiments, the system may additionally comprise transport means operable to transport the substrate from the reel past the marking head.

The substrate may comprise a colour change material operable to change colour in response to illumination by the emitters. The colour change material may comprise substances including but not limited to any of: a metal oxyanion, a leuco dye, a diacetylene, a charge transfer agent or the like. The metal oxyanion may be a molybdate. In particular, the molybdate may be ammonium octamolybdate (AOM). The colour change material may further comprise an acid generating agent. The acid generating agent may comprise thermal acid generators (TAG) or photo-acid generators (PAG). In one embodiment, the acid generating agent may be an amine salt of an organoboron or an organosilicon complex. In particular, the amine salt of an organoboron or an organosilicon complex may be tributyl ammonium borodi salicylate.

The substrate may comprise an NIR (near infrared) absorber material. The NIR absorber material may be operable to facilitate the transfer of energy from an NIR laser illumination means to the colour change material. The NIR absorber material may comprise substances including but not limited to any of: Indium Tin Oxide (ITO), non- stoichiometric reduced ITO, Copper Hydroxy Phosphate (CHP), Tungsten Oxides (WOx), doped WOx, non-stochiometric doped WOx and organic NIR absorbing molecules such as copper pthalocyanines or the like.

According to a third aspect of the present invention there is provided a method of calibrating a laser marking apparatus of the type comprising one or more laser emitters operable to emit laser light on to a substrate for marking, a substrate temperature sensor operable to detect an initial substrate temperature prior to marking, and a control unit operable to modulate the output of said emitters by varying a marking setting, the method comprising the steps of: marking a series of block images at a range of different test marking settings and/or test initial substrate temperatures; determining the average optical density of each marked block; and thereby determining a calibration relationship between marking setting, substrate temperature and marked optical density.

According to a fourth aspect of the present invention, there is provided a calibration apparatus for a laser marking system of the type comprising one or more laser emitters operable to emit laser light on to a substrate for marking, a substrate temperature sensor operable to detect an initial substrate temperature prior to marking, and a control unit operable to modulate the output of said emitters by varying a marking setting, the calibration apparatus comprising: a sensor operable to determine the average optical density of a series of block images marked at a range of different test marking settings and/or test initial substrate temperatures; and a calibration unit operable to thereby determine a calibration relationship between marking setting, substrate temperature and marked optical density.

The method of the third and apparatus of the fourth aspects of the present invention may incorporate any or all features of the first and second aspects of the present invention as required or desired.

Detailed Description of the Invention

In order that the invention may be more clearly understood one or more embodiments thereof will now be described, by way of example only, with reference to the accompanying drawings, of which:

Figure 1 is a schematic block diagram of a laser marking system according to the present invention;

Figure 2 is a schematic block diagram of a laser marking system according to the present inventions where the emitters are an individually addressed laser diode array;

Figure 3 is a schematic block diagram of a laser marking system according to the present invention comprising a single laser emitter;

Figure 4 is a schematic block diagram of a laser marking system according to the present invention comprising a single laser emitter; Figure 5 is a schematic illustration of a series of calibration blocks marked in the method of the present invention;

Figure 6 is a plot illustrating variation in marked optical density with variation in marking setting for three different temperatures; Figure 7 is a plot illustrating the effectiveness of single point calibration in temperature compensation according to the present invention;

Figure 8 is a plot illustrating the effectiveness of three point calibration in temperature compensation according to the present invention; and

Figure 9 is a plot illustrating the effectiveness of multipoint calibration in temperature compensation according to the present invention.

Turning now to figure 1, a multisource laser diode fibre array imaging system 1 comprises a marking head 10 in the form of a linear fibre array and a plurality of laser diodes 21-26. Each laser diode 21-26 is provided with a corresponding emission element 31-36. The emission elements 31-36 provide a coupling between the output of the laser diodes 21-26 and the input ends of corresponding fibres 11-16 making up the array. The emitting ends of the fibres 11-16 are maintained in position within the emitting head 10 by suitable formations. The skilled man will appreciate that the particular layout of figure 1 is a simple example for the purposes of explanation. In practice, the system 1 may have many more than six laser diodes and the emitting ends of the corresponding fibres may be arranged in formations other than a linear one dimensional array, including but not limited to two dimensional arrays and skewed or staggered arrays. In use, a substrate 2 is positioned in front of the marking head 10, the substrate 2 comprising a colour change material that changes colour in response to exposure to laser light. Suitable modulation of the output of the laser diodes 21-26 in combination with relative motion between the marking head 10 and the substrate 2 enables a pattern, image, text or the like to be marked on the substrate.

The laser diodes 21-26 are selected to have an output wavelength that is effective for initiating colour change in the substrate 2. Examples of suitable wavelengths range form 200-20000nm. More typically wavelengths in the range 390 to 460nm, 500 to 550nm, 620 to 660nm, 900-1 lOOnm and 1400 to 1600nm may be utilised.

The emission elements 31-36 may be formed integrally with the laser diodes 21-26 or may be a separate component. Typically, the emission elements 31-36 comprise optical fibres, ferrules or optical fibre connectors. In an alternative embodiment as illustrated in figure 2, the marking head 10 may comprise one or more individually addressable laser diode arrays (IALDA) 7. As in this example, the IALDA 7 may comprise an array of laser diodes 21-26. In addition, in such embodiments, the marking head 10 comprises a beam conditioning optic 8. Typically, the beam conditioning optic 8 may be a fast axis correction optical element. The marking head 10 may additionally comprise an imaging lens assembly 9 to further correct and focus the emitters 21-26 onto the substrate 2.

In the further alternative embodiment of figure 3, the marking head 10 comprises a single laser diode 27. In addition, in such embodiments, the marking head 10 comprises a beam focussing optic 28 and a scanning unit 29. The focussing unit 28 and scanning unit 29 are operable to direct the scan beam on to the substrate 2 as necessary for marking. The beam focussing optic 28 and scanning unit 29 positions may be interchanged. In many instances, it is preferred that the beam is scanned before entering the focusing optic 28, since the focussing optic 28 can be used to flatten the imaging surface.

In the further alternative embodiment of figure 4, the marking head 10 comprises a single laser diode 37. In addition, the marking head 10 comprises a beam conditioning optic 38. The beam conditioning optic 28 is operable to generate a beam with a substantially uniform intensity distribution. In this embodiment, the marking head further comprises a spatial light modular (SLM) 39 and projection optics 40 to relay the required intensity pastern to the substrate 2. The SLM 39 is operable to receive a beam with a uniform intensity distribution and output a modulated beam having a spatially modulated intensity distribution. Controlling the spatial modulation of the modulated beam and the direction of this beam to the substrate allows the desired marking performance.

The system further comprises a control unit 3. The control unit 3 is operable to control the output of each diode 21-26, 27, 37 in response to a marking setting. In this context the marking setting is defined by reference to one or more of: emitter power; emitter pulse duration (tp); emitter spot diameter (d ₀ ); emitter duty cycle, or the like. The power parameter for each emitter 21-26 is related to the power input signal applied to the emitter and may be defined as target image greyscale value (GS). Where the substrate 2 is in motion and the marking head is fixed in position, the system 1 may additionally comprise a substrate motion sensor operable to detect motion of substrate 2 and/or the control unit 3 may be operable to vary the substrate motion. In such embodiments, the substrate speed (V) relative to the marking head may be incorporated into the marking setting. In embodiments where the system comprises a scanning unit 29 or SLM 39, the control unit may be operable to control the scanning unit and/or SLM as appropriate.

In the present invention, the system 1 comprises a substrate temperature sensor 41 operable to detect an initial substrate temperature prior to marking activity. The substrate temperature sensor 41 typically has a sensing element that is not in physical contact with the substrate. Such non-contact sensors are beneficial as this avoids the potential for frictional heating or damage from contact between the substrate 2 and the temperature sensor 41. Such benefits outweigh any potential improvement in sensitivity that may result from use of a direct contact temperature sensor. Suitable examples of non-contact temperature sensors 41 include but are not limited to: infrared thermometers/pyrometers, two wavelength ratio-metric radiation thermometers/pyrometers, multi- wavelength ratio-metric thermometers and optical pyrometers.

The temperature sensor 41 is either selected or configured such that it has a sensitivity range covering the expect operational temperature range of the system 1. In general, this may incorporate temperatures from say, 0°C to 50°C. The temperature resolution of the temperature sensor 41 may be selected such that adequate compensation for temperature variations can be achieved. To some extent, the necessary resolution can vary depend upon the temperature sensitivity of the substrate 2 material. In one example, the temperature sensor 41 sensitivity may be, say, 2°C, preferably, say, 1°C or most preferably less than 1°C, for example 0.2°C.

In use, the control unit 3 is operable in response to the temperature sensor 41 to vary the marking setting for emitters 21-26, 27, 37 in response to the initial substrate temperature as detected by the temperature sensor 41. This variation of the marking setting provides compensation for substrate temperature variation. It is therefore possible to provide a marked image on the substrate 2 with a substantially consistent optical density despite variation in the substrate temperature.

Optionally, the system may additionally be provided with a temperature conditioning unit 42 operable to cool and/or heat the substrate 2. In some embodiments, the temperature conditioning unit 42 may be operable in response to the control unit 3 to help compensate for variations in the substrate temperature. In such cases, the temperature conditioning unit may be operable to heat and/or cool the substrate 2 when the ambient temperature in the vicinity of the system is close to or outside the normal operating temperature range or to heat/cool the substrate 2 to desired test temperatures for the purpose of calibration as will be discussed in more detail below. Additionally or alternatively, the temperature conditioning unit may be provided to mitigate or overcome adverse effects that can occur when the substrate 2 is too warm. These effects can include substrate deformation such as rippling and folding. A further possibility is that the temperature conditioning unit 42 may comprise a preheat source operable to heat the substrate 2 before marking to help activate colour change material. A preheat source may comprise an infra red source such as one or more laser diodes.

A number of types of temperature conditioning units 42, particularly in the context of cooling the substrate but readily adaptable to heating the substrate 2 are discussed in our prior UK patent application no 1816020.0. Such temperature conditioning units 42 include: rollers, rollers provided with a network of internal conduits through which a temperature conditioning fluid may be circulated and/or one or more thermoelectric temperature conditioning devices; rollers with surfaces adapted to absorb or diffuse incident light; rollers provided with a fluid layer between an outer transparent shell and the roller surface; blocks comprising a facing surface wherein the substrate runs over the facing surface of the block; blocks comprising a facing surface and a network of internal conduits through which a temperature conditioning fluid may be circulated and/or one or more thermoelectric temperature conditioning devices; and/or one or more temperature conditioned fluid blowers.

For calibration, the control unit 3 is operable to control the diodes 21-26 or 27, 37 so as to mark a series of calibration blocks 101-106 on the substrate 2, as shown on figure 3. The calibration blocks 101-106 are uniform block images marked using a different test marking setting and/or a test substrate temperature. In the examples shown in figure 5, for simplicity, there are six separate block images 101-106, each illustrating a different optical density. In this particular example, as will be discussed in more detail below, each block image 101-106 is marked using a different combination of marking setting and substrate temperature. In many embodiments, a series of blocks 101-106 are imaged at a common substrate temperature and subsequent series of blocks 101-106 are imaged using the same marking settings at different substrate temperatures. Typically, the marking setting is varied by varying the emitter power parameter, typically expressed as a greyscale (GS) value. The skilled man will however appreciate that alternative marking setting variations and/or alternative bock image shapes can be used if desired.

The optical density of block images 101-106 is measured by a sensor 4. As the block images 101-106 are intended to be uniform, the sensor 4 is generally operable to measure the average optical density across the block 101-106 or at least an average of two or more samples of different areas of the block 101-106. Typically, the sensor 4 may comprise a spectrophotometer or an optical densitometer. Example of suitable sensors 4 include but are not limited to the SpectroEye™ or eXact™ available from Xrite™.

The determined optical density values for each block 101-106 are then output to a calibration processing unit 5. The optical density values can then be processed alongside marking setting and substrate temperature to determine a calibration relationship between the optical density resulting from a marking setting and substrate temperature combination. The calibration relationship can then be stored in a calibration data store 6. The control unit 3 can then apply the calibration relation in response to signals indicative of the substrate temperature received from substrate temperature sensor 41 when marking images.

The skilled man will appreciate that the calibration method may involve marking block images 101-106 at as many (or as few) marking setting and substrate temperature combinations as required to provide sufficiently accurate temperature compensation across a useful operating range of both temperature and marked image optical density. Where a particular substrate material is relatively insensitive to temperature variation or where a smaller operating range of temperature variation or optical density variation is expected, the number of marking setting and substrate temperature combinations and hence the number of block images 101-106 can be reduced. Similarly, if a lower accuracy compensation is considered acceptable, the number of marking setting and substrate temperature combinations and hence the number of block images 101-106 can be reduced.

In one example, the calibration relationship may be defined by storing details of marking setting and temperature combinations with the resultant optical density value in a look up table in calibration data store 6. In the event that it is desired to mark a substrate with a particular optical density, the control unit 3 may be operable in response to the desired optical density value and the substrate temperature as measured by temperature sensor 41 to look up the marking setting corresponding to the desired optical density in calibration data store 6. The control unit 3 can then vary the marking setting used in marking to correspond to the looked up marking setting. If the look up table does not contain an exact match to a particular optical density, marking setting or temperature, the control unit 3 may simply apply the closest value stored in the look up table or may be operable to interpolate between the stored values.

In another example, the calibration relationship may be defined by a temperature dependent scale factor applied to a predefined relationship between marking setting and optical density. In such examples, the scale factor may be calculated by the calibration processing unit 5 by analysis of the output of optical sensor 4 and temperature sensor 41. The scale factors resulting from this calculation are then stored in calibration data store 6. In the event that it is desired to mark a substrate with a particular optical density, the control unit 3 may be operable in response to the desired optical density value and the substrate temperature as measured by temperature sensor 41 to look up the scale factor corresponding to the substrate temperature and apply that scale factor to the marking setting. The scaled marking setting may then be used in marking operation.

In order that temperature compensation calibration and the methods for determining such calibration relationships may be more clearly understood, some examples are discussed below. Turning to figure 6, this shows a series of best fit curves (61, 62, 63) based on measured data points illustrating variation of optical density of blocks 101-106 marked at different marking settings at three different substrate temperatures T1 (curve 61), T2 (curve 62), T3 (curve 63) wherein T3 > T2 > Tl . The curves may be determined by making measurements at every possible marking setting or at selected marking settings in the operational range. The best fit curves 61, 62, 63 in figure 6 are derived from fitting a quadratic function to the measured data points. For clarity four measured data points are illustrated only for the T2 plot. Nevertheless, for more accurate fitting, six, eight or more data points for each temperature would be preferred.

At T2, a greyscale value of 150 corresponds to an optical density value of -0.7. as is illustrated by the additional lines provided on figure 6, a greyscale value of -115 at T3 or a greyscale value of -193 at Tl will result in the same optical density value as a greyscale value of 150 at T2. Accordingly, the optical density value of 0.7 could be associated in a look up tables with the greyscale values indicated above for temperatures Tl, T2 and T3. A similar exercise may be used to determine marking setting and temperature combinations corresponding to other optical density values. In this manner a look up table could be generated for each possible combination.

It is however more realistic to measure a relatively small number of marking setting, substrate temperature and optical density relationships and use interpolation to populate a look up table for intermediate values. Additionally or alternatively, if the desired optical density or measured substrate temperature does not correspond precisely to a stored relationship, it is possible to compute interpolations on the fly, but this may require significant additional processing resources. In this context, the interpolation may be linear but may be more accurate if based on a quadratic function of the type below:

OD = a. GS ² + b. GS + c (1) where OD is optical density, GS is greyscale value and a, b and c are constants.

In this case if we let the subscript Ό’ and‘N’ refer to the original (or reference) temperature and new temperature respectively then since we want the same optical density at different temperatures we can write: where GS ₀ and GS _N are the initial and new greyscale values. Solving for GS _N we have:

Equation (3) may be used to calculate the new GS _N values for the new temperature for all GS ₀ values. For each original (or reference) greyscale value, new GS _N values may be calculated for each test temperature. At temperatures intermediate between the test temperatures the GS _N value may be determined by linear interpolation between adjacent data pairs. Alternatively, a polynomial function (in this example a quadratic function) may be fit to the test GS values and the function used to calculate the new GS _N value at intermediate temperatures.

As a further alternative, an expression of the type:

OD = OD _m [ 1 - exp -k(GS + B(T— T _A )) (4) may be fit to the measured data. Where OD _m is the maximum optical density for the particular coating, k and B are factors, T ₀ and T _N are the initial and final temperatures and T _A is an arbitrary offset temperature factor. Adopting similar notation to above we can write

OD _m [ 1 - exp -k{GS ₀ - B (T ₀ - T _A ))] = OD _m [1 - exp k _N {GS _N - B(T _N - T _A ))] (5)

Solving for GS _N leaves

The skilled man will appreciate that the above expression could equally be written in terms of other marking setting parameters rather than greyscale. In this manner, a look up table for the required GS values may be created for each temperature.

A scale factor S _f may be calculated by dividing the new GS _N value by the original or reference value. In the case of equation (6) the scale factor may be determined from The scale factor can thus be used to scale the greyscale value applied to achieve a particular optical density at a particular temperature. Calculated scale factors can be stored in the calibration data store 6. The skilled man will of course realise that equivalent functions which use alternative marking setting parameters can also be derived.

Whilst the above derivation is based on using test temperatures, similar to the situation above, scale factors at intermediate temperatures may be determined by linear interpolation between adjacent S _f values at the test temperatures (see equation (8) below). Alternatively, intermediate S _f values may be calculated by fitting a polynomial function (in this example a quadratic function) to the test data see equation (9) below).

For example, the scale factor S _f for the laser source at temperature between say Ti and T2 may satisfies an equation of the type

Sf = m. T + c (8) where coefficients m and c may be found by linear regression and T represents the output from the temperature measuring device. Where T lays between the temperatures Ti and T2 corresponding to the calibration curves.

Alternatively, S _f may be of the form

Sf = a. T ² + b. T + e (9)

The skilled man will nevertheless appreciate that a higher order polynomial may be used, if desired or appropriate.

In many instances, OD _m , B and TA will be substantially constant for a particular combination of substrate 2 and marking head 10 whilst the parameter k may vary with temperature. By plotting the value k versus temperature it is possible to fit a mathematical function to the data, this function may be a linear, quadratic, logarithmic or a power function of T, or any other suitable function. It is then possible to use this function to calculate k for all temperatures and use equation (6), equation (7) or other suitable equation to calculate the required GS value or scale factor.

In the special case where k is constant over the range of operating temperatures, the calculation of scale factors is simplified as equation (7) simplifies to S _f = l + B(T ₀ - T _N )/GS ₀ (10)

Depending on the imaging system, the substrate, the colour change technology and the application the level of temperature calibration may be more or less complicated. For example, in the case of black and white imaging a single point calibration may be acceptable. For a system that has to reproduce a limited range of greyscale a three point calibration may be required and for systems that require good greyscale reproduction or colour or multi-colour reproduction then full range or many point correction is required.

If single point calibration is used, the calibration point should be toward the middle of the required range of optical densities of the marked image. Additionally, the range of temperatures for reasonable correction should be limited to say 5C either side of the single point test temperature. Nevertheless, if marking is limited to substantially black and white images only, then a calibration point closer to the maximum optical density should be chosen. Typically, the calibration may provide acceptable results for a wider temperature range, say from 10°C to 50°C. In such cases temperature compensation below the test temperature may require the emitters to operate beyond their normal power setting range. Nevertheless, such compensation can still be implemented if an additional marking setting parameter is also varied, for instance if the power required is above its normal range, the emitter pulse duration may be increased.

Turning now to figure 7, this illustrates equation (4) plotted at temperatures of 20C (curve 71) and 32C (curve 72). The parameters k, B, OD _m and T ₀ were adjusted to fit the measured data at 20°C. The curves indicate that if the temperature of the substrate is increased from 20°C to 32°C the marked optical density resulting from a particular GS value will be higher. To maintain a particular OD when the substrate temperature changes from 20°C to 32°C, a compensated GS value must be used, for example by using equation (6) or equation (7). In the example of figure 7, a single greyscale value of 240 was selected for compensation, as might be suitable in the case where black and white images are to be marked. The compensation derived from applying a scale factor calculated from equation (7) based on this greyscale value is illustrated by curve 73. It is clear that the compensation is effective over the limited greyscale range of 230 to 255. Below this value the calculated new GS values illustrated by curve 73 result in OD values that deviate significantly from curve 72. Whilst single point calibration is accurate over a limited range, it is readily applied by use of a single scale factor. This provides for relatively simple calculation and implementation.

Where single point calibration is insufficiently accurate but simplified implementation and calculation is still of interest, three point calibration may be used. Here an improved correction is calculated by using three selected GS values. This is illustrated in figure 8 which illustrates equation (4) plotted at temperatures of 20°C (curve 81) and 12C (curve 82). Also shown is corrected curve 83 for a temperature TN of 12°C where the initial reference temperature T ₀ was 20°C. Curve 83 was generated by applying a scale factor calculated from equation (7) using test GS values of 135, 195 and 240. As can be seen in figure 8, the correction of curve 83 is adequate down to a temperature of 14°C.

In the event that complex images or colour images are to be marked then calibration at a wider range for reference points is required.

In the case where a good range of greyscale or colour or multi-colour images are required then calibration at many points is required. This is illustrated in figure 9 which illustrates curves 71 and 72 fitted to the same data as in figure 7. Application of a multipoint correction (curve 74) in this instance provides correction at a much wider range of values, such that curve 74 substantially overlies the whole of curve 71. In such examples, any suitable number of calibration points may be selected depending on the application.

The one or more embodiments are described above by way of example only. Many variations are possible without departing from the scope of protection afforded by the appended claims.