GB2577368A

GB2577368A - Improvements in or relating to laser marking

Info

Publication number: GB2577368A
Application number: GB1910585.7A
Authority: GB
Inventors: L Phillips Tristan; Cridland John; Montes Gomez Lidia; Al-Waidh Aos
Original assignee: DataLase Ltd
Current assignee: DataLase Ltd
Priority date: 2018-07-24
Filing date: 2019-07-24
Publication date: 2020-03-25
Also published as: WO2020021260A1; GB201910585D0; GB201812067D0

Abstract

A method of calibrating a laser marking system 1 comprising an array of emitters (21 - 26). The method comprises activating the emitters to mark a series of block images as a test pattern on to the substrate 2 using a series of different marking settings. capturing an image of each marked block. processing the captured images to determine the optical density of the areas associated with each individual emitter. Determining an optical density value for each emitter for each of the utilised marking settings and performing an interpolation to determine how the marking settings corresponding to optical density values which is then stored in a look up table. Also disclosed is a method of calibrating a laser marking system of the above type, comprising extracting an intensity profile through the image in a direction substantially perpendicular to the relative motion between the substrate and the emitters.

Description

IMPROVEMENTS IN OR RELATING TO LASER MARKING

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

Laser marking and imaging systems are well known. Such systems are used in conjunction with substrates, for example labels, that comprise colour change material. Upon controlled exposure to laser light from the marking system, 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.

Many marking systems comprise multi-source laser arrays, which direct the laser output to the substrate via a marking head. The substrate is typically scanned past the marking head but in some arrangements, the marking head scanned past a stationary substrate. The individual laser diodes are modulated based on the image requirements to generate an array of dots or pixels in the substrate. The benefit of this approach is that the imaging speed is independent of image content and multisource laser diode optic fibre array systems have been developed which are capable of recording image information on colour change substrates moving at speeds up to and above 2m/s.

The individual laser diodes in such systems are typically coupled to an input end of a marking optical fibre, the other end of the marking fibre being an emitting end. The emitting ends of the marking fibres are typically arranged in a 1 D or 2D array within the marking head.

The modulation of individual laser diodes to achieve a given optical density typically comprises modulation of the input current so as to control the output power emitted by the diode. Additionally or alternatively, the modulation may include varying the pulse duration of the diode, the duty cycle of the diode or the relative velocity of the substrate relative to the diode. Any or each of these inputs together define a marking setting which relates to the target optical density in the marked image.

There is natural variation between the output power of the individual laser diodes and between the coupling efficiencies of individual laser diodes and the corresponding marking fibres. As a result, the output power emitted from the emitting end of each fibre may differ for a given input current. Such differences in output power may be measured during calibration and the driving currents of individual laser diodes may be adjusted consequently to equalise output power levels.

Whilst this does provide some compensation so as to reduce banding, it is not sufficient to eliminate banding in the marked image. This is because there is also variation in the emitted beam intensity distribution and spot size for each laser diode. Furthermore, some marking substrates will have relatively significant differences in response to variations in laser power, intensity distribution or spot size. It is possible to measure such beam parameters for each emitter, for example using a suitable camera. Nevertheless, this is time consuming and it is still very challenging to predict the resulting optical density for any given marking substrate.

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

Summary of the Invention

According to a fourth aspect of the present invention there is provided a method of calibrating a laser marking system of the type comprising a plurality of emitters, each emitter operable to emit light on to a substrate for marking, the output light energy of each emitter being modulated by varying a marking setting for each emitter, the method comprising the steps of activating the emitters to mark a series of block images on to the substrate using a series of different marking settings; capturing an image of each marked block; processing the captured images to determine the optical density of the areas of the marked block associated with each individual emitter and thereby determining an optical density value for each emitter for each of the utilised marking settings; and performing an interpolation to determine marking settings corresponding to optical density values for intermediate marking settings between the utilised marking settings wherein the determined marking settings for each optical density value for each emitter are stored in a look up table and if there are marking settings having different parameter values that correspond to the same optical density value, the method involves selecting one marking setting only for storage in

the look up table.

This method provides for ready calibration of the emitters in an image marking system. Furthermore, such calibration can be obtained directly from a single procedure. Additionally, look up tables are a convenient and efficient means of managing calibration in systems equipped with means for high speed digital electronic computation.

The interpolation may be based on any suitable function. Suitable interpolation functions may include linear functions, quadratic functions, polynomials functions or the like. Linear interpolation may be utilised where interpolation is between two or more values. Quadratic interpolation may be utilised where interpolation is between three or more values.

The optical density values may be determined by averaging measured optical densities for two or more dot images associated with each emitter for each block.

Alternatively, the optical density values may be determined by averaging measured optical density along a line associated with each emitter for each block.

The series of block images may correspond to a series of target optical densities. The series of target optical densities may be substantially evenly spaced across the range of possible optical densities. In one embodiment, block images at least three different target optical densities are used. In further embodiments, other numbers of different target optical densities may be used, including but not limited to five, six or seven different target optical densities.

In some embodiments, the marking settings utilised may correspond to a series of marking settings substantially evenly spaced across the range of possible marking settings. Varying the marking settings may comprise varying a single parameter of the marking setting. In such cases, the series of values of the selected parameter may be substantially evenly spaced across the range of possible values. In other embodiments, varying the marking setting may comprise varying two or more of the parameters of the marking setting. In such cases, the series of values of the selected parameters may be substantially evenly spaced across the range of possible values of said parameter. In one embodiment, block images at least three different values for the varied parameters are utilised. In further embodiments, other numbers of different values for the selected parameters may be used, including but not limited to five, six or seven different values.

In one embodiment, varying the marking setting may involve varying the pulse duration and the power parameter.

In the event that there are marking settings having different parameter values that correspond to the same optical density value, the marking setting selected for storage may be the marking setting using the lowest power parameter value. Alternatively, in other embodiments, the marking setting selected for storage may be the marking setting where the varied parameter values are closest to the middle of their allowed range.

The target optical density may be a single value for the whole block. In such embodiments, the initial marking setting may be equal for each emitter. The target optical density may be a range having an upper threshold and a lower threshold. The range may have a common upper threshold and a common lower threshold for the whole block. In some embodiments, the reference may have a different upper threshold or lower threshold for each emitter. In some embodiments, the target optical density value may vary for each emitter or the upper and or lower thresholds may vary for each emitter. In one example, this variation may be related to the response of an image capture device. This can provide compensation for non-uniformity of image capture.

The method may be repeated using block images with different target optical densities. This can allow adjusted marking settings to be obtained for each emitter at multiple different target optical densities. In one embodiment, block images at least three different target optical densities are used. In further embodiments, other numbers of different target optical densities may be used, including but not limited to five, six or seven different target optical densities. In such embodiments, the target optical densities may be substantially evenly spaced between a maximum optical density and a minimum optical density. For instance, in the case of where three different target optical densities (minT, midT and maxT) are used: minT may be towards the lower end of the range of the possible optical densities; midT may be towards the centre of the range of possible optical densities; and maxT may be towards the upper end of the range of the possible optical densities.

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. Increment or decrement of the marking setting may involve incrementing or decrementing a single one of the above quantities. In this manner operation of the system can be calibrated for particular fixed values of the remaining quantities. In further embodiments, the method may be carried out by incrementing or decrementing each of the quantities in turn whilst the other quantities retain fixed values. In this manner, operation of the system can be calibrated for variation in more than one quantity.

The power parameter for each emitter may be related to the power input signal applied to the emitter. 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. Using the input power parameter significantly reduces the amount of work and speeds up the calibration process.

Measurement of the marked image may be carried out by use of any suitable measuring device. In preferred embodiments, the marked image is measured by an imaging device, such as a camera. The optical density may be measured with respect to the greyscale value in the captured image. The greyscale value measured may be a peak greyscale value for each emitter or may be an average greyscale value for each emitter.

The measurement of optical density in the captured image may involve extracting an intensity profile through the image in a direction perpendicular to the relative motion between the substrate and the emitters. Such a profile will have ripples or peaks at a pitch corresponding to the separation of the emitters or the separation of the emitter beams as focused on to the substrate. The method may include the step of identifying successive peaks in the intensity profile. The method may also include the step of associating successive peaks in the intensity profile with successive emitters.

Peaks within the pattern may bedent fied by position. In such cases, a peak at an edge of the marked pattern may be determined to correspond to an emitter at an edge of an emitter array. The method may include the step of counting peaks. This can enable peaks in the interior of the pattern to be associated with emitters in the interior of an emitter array by relative position within the pattern. The absence of an expected number of peaks can be used to infer the failure of an emitter. The method may include monitoring the relative separation of peaks within the pattern. A significantly increased separation between peaks may infer the failure of an emitter in the interior of the emitter array. The absence of an expected number of peaks without a significantly increased separation between peaks or taking into account a significantly increased separation implying failure of an interior emitter may be used to infer the failure of an edge emitter. In such cases, the method may include the step of remarking the block with one edge emitter deactivated. If the number of peaks is unchanged this may infer that the deactivated edge emitter has failed. If the number of peaks decreases by one, this may infer that the other edge emitter has failed.

The marking of each block image may include the marking of one or more identification codes adjacent to the block image. The identification code may be associated with an emitter or group of emitters. The identification code may be aligned with the section of the block image marked by the emitter. This can allow the identification code to be marked by the same emitter as the related section of the block image. The identification code may comprise a characteristic sequence of dots or the like. In particular, the identification code may comprise a binary encoded series of dots.

The method may include the step of detecting and reading the identification code. This step may include the comparing the identification code to adjacent identification codes. This may further include the step of inferring one or more additional features in the code in the light of the comparison with adjacent identification codes. This can help ensure correct identification of the emitter where marking of the identification code is weak or otherwise unclear.

The emitters may comprise lasers or laser diodes. In some embodiments, 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 marking head may be mounted so as to allow relative motion between the marking head and a substrate to be marked.

The emitter may be operable to emit light with any suitable wavelength, including but not limited to visible or near infrared (MR) 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 1100nm and 1400 to 1600nm.

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 (AGM).

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 sal i cyl ate.

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 NW 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 (WO,), doped WON, non-stochiometric doped WO, and organic N1R absorbing molecules such as copper pthalocyan nes or the like.

According to a second aspect of the present invention, there is provided a calibration apparatus for a laser marking system comprising a plurality of emitters, each emitter operable to emit light on to a substrate for marking, wherein the laser marking system is operable to mark a block image of a target optical density, wherein the calibration apparatus comprises means for capturing an image of the marked block; and a calibration processor operable to process the captured image to determine whether to increment or decrement the marking settings for each emitter according to the method of the first aspect of the present invention.

According to a second aspect of the present invention, there is provided a calibration apparatus for a laser marking system comprising a plurality of emitters, I1 each emitter operable to emit light on to a substrate for marking, wherein the laser marking system is operable to mark a block image of a target optical density, wherein the calibration apparatus comprises means for capturing an image of the marked block; and a calibration processor operable to process the captured image to determine and store in a look up table an association between marking settings and optical density for each emitter according to the method of the first aspect of the present invention.

According to a third aspect of the present invention, there is provided a laser marking system comprising a plurality of emitters, each emitter operable to emit light on to a substrate for marking, a control unit operable to modulate the output of each emitter by varying a marking setting wherein the control unit is operable to look up a marking setting associated with a target optical density in a look up table and wherein the look up table is populated using the method of the first aspect of the present invention or the apparatus of the second aspect of the present invention.

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

According to a fourth aspect of the present invention, there is provided a method of calibrating a laser marking system of the type comprising a plurality of emitters, each emitter operable to emit light on to a substrate for marking, the output light energy of each emitter being modulated by varying a marking setting for each emitter, the method comprising the steps of: activating the emitters to mark a series of block images on to the substrate using a series of different marking settings; capturing an image of each marked block; processing the captured images to determine the optical density of the areas of the marked block associated with each individual emitter; comparing the determined optical density values to a target optical density so as to determine an optical density value for each emitter for each of the utilised marking settings; and thereby calculating an association between marking setting and optical density wherein the measurement of optical density in the captured image involves extracting an profile through the image in a direction substantially perpendicular to the relative motion between the substrate and the emitters.

The method of the fourth aspect of the present invention may incorporate any or all features of the method of the first aspect of the present invention or the apparatus or system of the second and third aspects of the present invention. In particular, the association between marking setting and optical density may be calculated according to the method of the first aspect of the present invention.

According to a fifth aspect of the present invention, there is provided a calibration apparatus for a laser marking system comprising a plurality of emitters, each emitter operable to emit light on to a substrate for marking, wherein the laser marking system is operable to mark a block image of a target optical density, wherein the calibration apparatus comprises means for capturing an image of the marked block; and a calibration processor operable to process the captured image to determine and store in a look up table an association between marking settings and optical density for each emitter wherein the measurement of optical density in the captured image involves extracting an through the image in a direction substantially perpendicular to the relative motion between the substrate and the emitters according to the method of the first aspect of the present invention.

According to a sixth aspect of the present invention, there is provided a laser marking system comprising a plurality of emitters, each emitter operable to emit light on to a substrate for marking, a control unit operable to modulate the output of each emitter by varying a marking setting wherein the control unit is operable to look up a marking setting associated with a target optical density in a look up table and wherein the look up table is populated using the method of the fourth aspect of the present invention or the apparatus of the fifth aspect of the present invention.

The apparatus of the fifth and the system of the sixth aspects of the present invention may incorporate any or all features of any of the preceding aspects of the present invention as required or as 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 2a is a schematic illustration of a calibration block and line identifying codes according to the method of the present invention; and Figure 2b is a schematic illustration of a calibration block and an associated intensity profile along line X-X across the block according to the method of the present invention.

Figure 3 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.

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 to 1100nm and 1400 to 1600nm may be utilised. I5

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 3, 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 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.

The system further comprises a control unit 3. The control unit 3 is operable to control the output of each diode 21-26 in response to a marking setting. In this context the marking setting is defined by reference to 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. The power parameter for each emitter 21-26 is related to the power input signal applied to the emitter. In some embodiments, the control unit 3 is operable to control the relative velocity Vx between the substrate 2 and the marking head 10. In other embodiments, Vx is controlled by a production line controller or motion system controller and the control is operable to receive signals from said controller indicate of Vx.

For calibration, the control unit 3 is operable to control the diodes 21-26 so as to mark a series of calibration blocks 101-103 on the substrate 2. The calibration blocks 101-103 are block images of marked using variation of the marking setting. In the examples shown in figure 2, for simplicity, there are three separate block images 101-103, at different marking settings across the range of possible marking settings. In this example, block 101 is towards the lower end of the marking setting range and hence the lower end of the optical density range; block 102 is towards the centre of marking setting range and hence around the centre of the optical density range; and block 103 is toward the upper end of the marking setting range and hence the upper end of the optical density range. The skilled man will however appreciate that more block images (say five, six, seven or more) may be used for calibration if desired.

In such embodiments, the calibration method can be carried out utilising variation of the marking setting in relation to both power parameter and pulse duration across their allowed ranges to produce a series of block images of different optical densities. In further embodiments, the skilled man will appreciate that other marking setting parameters can be varied, if desired or appropriate.

One or more images of the calibration blocks 101-103 are captured by an imaging device 4. The captured images are processed by a calibration unit 5 so as to determine the optical density of the areas of the calibration blocks 101-103 associated with each individual emitter 21-26. Typically, the optical density can be determined with respect to a greyscale value of the imaging device 4.

The relation between the optical density marked by each emitter as defined by marking setting (whether by variation in power parameter and pulse duration only or otherwise) can be determined directly. The relationship for intermediate marking settings can then be determined by interpolation. All the determined settings can then be stored in a look up table.

In one such example, with \Tx of 2m/s and a system resolution of 200dpi, the allowed range of pulse durations may be from, say, 20gs to, say, 63us and the maximum laser power may be of the order of SW, corresponding to a hexadecimal power parameter of 530. A series of calibration blocks are then marked using different power parameters and pulse durations. Typically, there would be at least three different, evenly spaced, power parameters and at least three different, evenly spaced, pulse durations, making for a total of nine calibration blocks. For better results additional different power parameters and pulse durations can be used, as required. In the case of using five different power parameters and five different pulse durations in the present example, the pulse durations may be 20its, 30gs, 40its, 52gs and 63gs and the respective power parameters (in hexadecimal) may be 190, 28A, 315, 422, and 5:30.

Each marked calibration block is captured using camera 4 and processed to determine a relationship between the power parameter, pulse duration and the measured greyscale value. The measured greyscale values for each block may be an average of multiple selected dot images or an average along a line in the direction of relative motion.

For each of the five calibration power parameters, the greyscale values between those measured at the five calibration pulse durations are determined by interpolation. Similarly, for each of the five calibration pulse durations, the greyscale values between those measured at the five calibration power parameters are determined by interpolation. The interpolation may be linear for two or more adjacent data points or it may quadratic or other function interpolation for three or more adjacent data points. The outcomes can be stored in look up tables in calibration store The result of the above procedure for each emitter is a calibrated relation between power parameter, pulse duration and greyscale optical density value. In some instances, there may be two different pairs of power parameter and pulse duration that correspond to the same greyscale value. In such instances one calibration relation need be stored in the look up table. The selected calibration relation may be based on criteria such as: the calibration relation including the lowest power setting value; or the calibration relation where both power parameter and pulse duration are closest to the centre of the allowed range.

This approach has the advantage that repeated measurements are not required.

In some such embodiments, the selected marking settings for calibration may calibrate to one or more target optical densities. In the above context, the target optical density can be a single value or can be a range having upper and lower thresholds. The range is typically fairly narrow and centred on a target value.

Identification of the area marked by each emitter 21-26 may be by use of a series of unique identifier codes 105 for each emitter marked adjacent to the block 101-103. As is shown in figure 2a, this can be in the form of a binary sequence of dots (or other code) representing the line number. If dots in the code 105 for one emitter 21-26 are weak and difficult to read then the codes on either side may be identified and used to infer the actual line number so that line numbering is consistent.

Whilst identification codes 131-136 are beneficial, the skilled man will appreciate that line identity can be determined by other methods. In figure 2b, an intensity profile 107 transverse (along line X-X) to the relative motion between substrate 2 and marking head 10 is taken for each block 101-103. Such a profile 107 has a rippled form including peaks and troughs at a regular pitch. Counting peaks from the edge of the profile 107 can enable particular emitters 21-26 to be identified, provided the edge emitters 21,26 have not failed. Failed internal emitters 22-25 leave a gap that can be accounted for by checking the pitch of the centre of the lines. The absence of an expected number of peaks taking into account any failed internal emitters 22-25 implies failure of an edge emitter 21, 26. In such cases, the failed edge emitter may be identified by remarking the block with one edge emitter 21 deactivated. If the number of peaks is unchanged this indicates that the deactivated edge emitter 21 has failed. If the number of peaks decreases by one, this indicates that the other edge emitter 26 has failed.

Turning to a first example, a series of calibration blocks 101-103 are produced at a marking setting defined by a fixed speed V,, a fixed pulse duration is with all emitters 21-26 for each block 101-103 set to the same initial power setting P. In one typical example, the duration is corresponds to the longest pulse duration for the expected maximum speed V mx The value of t, may be the same for all emitters for all target greyscales or the value of is may vary from emitter to emitter but not for different target greyscale levels.

In this example, calibration is carried out by varying the power parameter within the marking setting. The skilled man will however appreciate that calibration could equally be carried out by varying the pulse duration and/or relative velocity.

The calibration blocks 101-103 are marked at initial power settings Ps for each emitter 21-26 that are relatively evenly spaced across the possible range of marking opt cal densities. In th s example, block 101 may be marked at an initial low setting minP, block 102 can be marked at an initial intermediate midP, and block 103 can be 5 marked at an initial high setting maxP. The power parameters minP, midP and maxP may satisfy the relations: [Ed(0.1).V,..do] 5 minP < [Ed(0.3).V,..do] (la) [Ed(0.5). V,. do] < midP < [Ed(0.7). V,. do] ( 1 b) [Ed(0.9). V -do] < maxP < [Ed(1.1).Vx. do] ( 1 c) Where Ed(OD) represents the energy density required to achieve the optical density quoted inside the bracket and do is the beam size parameter for the emitter 2126, related to the beam diameter. The inequalities stated are appropriate for materials that have optical density range for 0.2 to 1.2. The skilled man will appreciate that if materials that produce different maximum optical densities are used then the power parameters minP, midP and maxP should be adjusted accordingly. For example, such an adjustment may take place using a material that provides a maximum optical density of around 2.

These relations may alternatively be restated in terms of greyscale values. In the case of 8 bit greyscale resolution (0-255), equations la-lc can be restated as [Ed(25).Vx.do] < minP < [Ed(70) Vx.do] (2a) [Ed(116).Vx.do] <midP < [Ed(162).Vx.do] (2b) [Ed(208).Vx.do] maxP [Ed(255).Vx.do] (2c) In implementations using more than three calibration blocks, then the blocks should be evenly spaced across the optical density range in a similar manner to that detailed above.

Following the marking of the calibration blocks 101-103, images of the blocks 101-103 are captured by camera 4. A transverse profile 107 is then extracted from the captured images, in the form of a plot of greyscale value across the width of the image. The profile 107 has ripples with a pitch that correspond to the effective pitch of the emitters in the marking head 10, as a focused on to the substrate 2. The peaks can each be associated with one of the emitters 21-26 and thus an optical density (measured in terms of greyscale) corresponding to each emitter 21-26 is determined.

The optical density can be measured based on the greyscale value at peak centre or on the average value across the width of the peak.

The determined greyscale values are then used to determine a relation between the optical density marked by each emitter, as defined by marking setting (variation in power parameter). The relationship for intermediate marking settings can then be determined by interpolation. All the determined settings can then be stored in a look up table.

The skilled man will also realise that in implementations where the relationship between optical density and power parameter is determined as discussed above, the pulse duration for ntermediate target greyscales may be calculated as a fraction of the maximum pulse duration. In such examples, the relationship between greyscale value and pulse duration may be linear or some other functional form. The advantage of calculating the pulse duration is that fewer measurements are required, albeit at the potential cost of accuracy in calibration. In such examples, the calculated fractional pulse durations may be stored in a look up table in the calibration store.

In the above example, it is possible to also include some measure of calibration with respect to other parameters making up the marking setting including, for instance, the pulse duration ts. This may be the case where the values of ts differ between emitters. In this context the different is values may be determined by selecting an initial pulse duration ts in the region of say 70%-90% of the maximum allowed tmaxt. A calibration block is marked using a selected power setting, typically in the range of 50%-80% of the maximum allowed power setting.

Following the marking of the calibration blocks 101-103, images of the blocks 101-103 are captured by camera 4. As in the previous examples, greyscale values for each emitter are extracted from the images of the blocks 101-103. The determined greyscale values are then used to determine a relation between the optical density marked by each emitter, as defined by marking setting (variation in pulse duration ts).

The relationship for intermediate marking settings can then be determined by interpolation. All the determined settings can then be stored in a look up table.

As another example, it is possible to use pulse duration to calibrate for greyscale values across the full possible range. In such cases, a fixed power parameter P, may be selected for all target greyscales. The value of P, may be the same for all emitters for all target greyscales or the value of Ps may vary from emitter to emitter but not for different target greyscale levels.

A series of calibration blocks 101-103 are marked at selected target greyscale values (for example 0, 64, 128, 192, 255 in an 8-bit system) with associated pulse durations in the allowed range. Maximum allowed pulse duration tax may be related to the beam diameter at the image plane and the velocity V" of relative motion between imaging head 10 and substrate 2. In the case where V, is 2m/s and the system has a resolution of 200dpi tma. may be -63ps. For pulse durations exceeding 63its, in such cases, the emitters remain on constantly for marking adjacent dots.

Minimum pulse duration is selected to be close to the largest value that does just not produce a visible mark. Typically, with V, of 2m/s and a resolution of 200dpi, this would be of the order of 20ps.

As the pulse duration is reduced gaps may appear between consecutive dots in the direction of travel as well as in the direction of the plane of the array. This makes simply scanning across the block image at any location not practical. Where this is the case, the method may involve identifying dots within the captured image and measuring the optical density at or around the centre of each dot in a line substantially perpendicular to the travel direction across the width of the image. The dots selected correspond to those produced by the array of emitters at the same time. In such cases, it is preferable that the measurements are made for a number of dots and the average value taken. Preferably the average should be over at least three dots.

As in the previous examples, greyscale values for each emitter are extracted from images of the blocks 101-103 are captured by camera 4. The determined greyscale values are then used to determine a relation between the optical density marked by each emitter, as defined by marking setting (variation in pulse duration ts). The relationship for intermediate marking settings can then be determined by interpolation. All the determined settings can then be stored in a look up table.

For greyscale target values outside those established using the marking of calibration blocks 101-103, interpolation can be used to determine appropriate pulse durations for each emitter. Further look up tables can be generated and stored in the calibration store 6. The skilled man will also realise that in implementations where the pulse durations are calibrated as discussed above, the power parameter for intermediate target greyscales may be calculated as a fraction of the maximum power parameter. In such examples, the relationship between greyscale value and power parameter may be linear or some other functional form. The advantage of calculating the power parameter is that fewer measurements are required, albeit at the potential cost of accuracy in calibration. In such examples, the calculated fractional power parameter may be stored in a look up table in the calibration store.

In the above example, it is possible to also include some measure of calibration with respect to other parameters making up the marking setting including, for instance, the power parameter Ps. This may be the case where the values of Ps differ between emitters. In this context the different Ps values may be determined by selecting an initial power parameter Ps in the region of say 90% of the maximum allowed Pmaxt. In this context, the initial pulse duration ts may be selected to be in the region of say 70%-90% of the maximum allowed ts. Alternatively, a lower power parameter may be selected, say 60%-90% of P. and the pulse duration ts selected such that a continuous line is produced by each emitter. In such cases, pulse duration may satisfy the relation ts > do/V, A calibration block is marked using a selected pulse duration ts, typically in the range of 50%-80% of the maximum allowed pulse duration ts. In response to determined greyscale values differing from the target greyscale, a relationship between the individual power parameters Ps for each emitter is determined and then stored in a look up table in the calibration store 6.

The skilled man will also realise that in implementations where the power parameters are calibrated as discussed above, the pulse duration for intermediate target greyscales may be calculated as a fraction of the maximum pulse duration. In such examples, the relationship between greyscale value and pulse duration may be linear or some other functional form. The advantage of calculating the pulse duration is that fewer measurements are required, albeit at the potential cost of accuracy in calibration. In such examples, the calculated fractional pulse durations may be stored in a look up table in the calibration store.

The above embodiments are described by way of example only. Many variations are possible without departing from the scope of the invention as defined in the appended claims.

Claims

CLAIMS1. A method of calibrating a laser marking system of the type comprising a plurality of emitters, each emitter operable to emit light on to a substrate for marking, the output light energy of each emitter being modulated by varying a marking setting for each emitter, the method comprising the steps of: activating the emitters to mark a series of block images on to the substrate using a series of different marking settings; capturing an image of each marked block; processing the captured images to determine the optical density of the areas of the marked block associated with each individual emitter and thereby determining an optical density value for each emitter for each of the utilised marking settings; and performing an interpolation to determine marking settings corresponding to optical density values for intermediate marking settings between the utilised marking settings wherein the determined marking settings for each optical density value for each emitter are stored in a look up table and if there are marking settings having different parameter values that correspond to the same optical density value, the method involves selecting one marking setting only for storage in the look up table.
2. The method of claim 1 wherein the interpolation is based on a linear functions, quadratic function or other polynomial function.
3. The method of claims 1 or claim 2 wherein the optical density values are determined by averaging measured optical densities for two or more dot images associated with each emitter for each block.
The method of any one of claims 1 to 3 wherein the optical density values are determined by averaging measured optical density along a line associated with each emitter for each block.
The method of any one of claims 1 to 4 wherein varying the marking setting comprises varying two or more of the parameters of the marking setting.
The method of claim 5 wherein the series of values of the selected parameters may be substantially evenly spaced across the range of possible values of said parameters.
The method of any one of claims 1 to 6 wherein varying the marking setting involves varying the pulse duration and the power parameter.
A method as claimed in any preceding claim wherein the marking setting is 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.
9. A method as claimed in any preceding claim wherein measurement of the marked image is carried out by use of an imaging device and optical density is measured with respect to the greyscale value in the captured image.
10. A method as claimed in claim 8 or claim 9 wherein the measurement of optical density in the captured image involves extracting an intensity profile through the image in a direction substantially perpendicular to the relative motion between the substrate and the emitters.
11. A method as claimed in claim 10 wherein the method includes the steps of identifying successive peaks in the intensity profile; and associating successive peaks in the intensity profile with successive emitters.
12. A method as claimed in any preceding claim wherein the method includes the marking of one or more identification codes adjacent to the block image.
13. A method as claimed in claim 12 wherein the identification codes are associated with an emitter or group of emitters and are aligned with the section of the block image marked by the associated emitter or emitters.
14. A method as claimed in claim 12 or claim 13 wherein the method includes the step of detecting and reading the identification code.
15. A method as claimed in claim 14 wherein the method includes comparing the identification code to adjacent identification codes.
16. A method as claimed in any preceding claim wherein the emitters comprise lasers or laser diodes.
17. A method as claimed in any preceding claim wherein the emitters comprise individually addressable laser diode arrays (IALDA) or individually addressable laser arrays (1ALA).
18. A method as claimed in any preceding claim wherein the emitters comprise an emitting end of a marking fibre wherein an input end of the marking fibre is coupled to a laser diode.
19. A method as claimed in any preceding claim wherein the colour change material comprises substances including but not limited to any of: a metal oxyanion, a leuco dye, a diacetylene, a charge transfer agent or the like.
20. A method as claimed in claim 19 wherein the colour change material further comprises an acid generating agent.
2k A method as claimed in any preceding claim wherein the substrate comprises an NIR (near infrared) absorber material.
22. A calibration apparatus for a laser marking system comprising a plurality of emitters, each emitter operable to emit light on to a substrate for marking, wherein the laser marking system is operable to mark a block image of a target optical density, wherein the calibration apparatus comprises means for capturing an image of the marked block; and a calibration processor operable to process the captured image to determine and store in a look up table an association between marking settings and optical density for each emitter according to the method of any one of claims 1 to 21.
23. A laser marking system comprising a plurality of emitters, each emitter operable to emit light on to a substrate for marking, a control unit operable to modulate the output of each emitter by varying a marking setting wherein the control unit is operable to look up a marking setting associated with a target optical density in a look up table and wherein the look up table is populated using the method of any one of claims 1 to 21 or according to the apparatus of claim 22.
24. A method of calibrating a laser marking system of the type comprising a plurality of emitters, each emitter operable to emit light on to a substrate for marking, the output light energy of each emitter being modulated by varying a marking setting for each emitter, the method comprising the steps of: activating the emitters to mark a series of block images on to the substrate using a series of different marking settings; capturing an image of each marked block; processing the captured images to determine the optical density of the areas of the marked block associated with each individual emitter; comparing the determined optical density values to a target optical density so as to determine an optical density value for each emitter for each of the utilised marking settings; and thereby calculating an association between marking setting and optical density wherein the measurement of optical density in the captured image involves extracting an intensity profile through the image in a direction substantially perpendicular to the relative motion between the substrate and the emitters.
25. A method as claimed in claim 24 wherein the method includes the steps of identifying successive peaks in the intensity profile; and associating successive peaks in the intensity profile with successive emitters.
26. A method as claimed in claim 24 or claim 25 wherein the method includes the marking of one or more identification codes adjacent to the block image.
27. A method as claimed in claim 26 wherein the identification codes are associated with an emitter or group of emitters and are aligned with the section of the block image marked by the associated emitter or emitters.
28. A method as claimed in claim 26 or claim 27 wherein the method includes the step of detecting and reading the identification code.
29. A method as claimed in claim 28 wherein the method includes comparing the identification code to adjacent identification codes.
30. A calibration apparatus for a laser marking system comprising a plurality of emitters, each emitter operable to emit light on to a substrate for marking, wherein the laser marking system is operable to mark a block image of a target optical density, wherein the calibration apparatus comprises means for capturing an image of the marked block; and a calibration processor operable to process the captured image to determine and store in a look up table an association between marking settings and optical density for each emitter wherein the measurement of optical density in the captured image involves extracting an intensity profile through the image in a direction substantially perpendicular to the relative motion between the substrate and the emitters according to any one of claims 24 to 29.
31. A laser marking system comprising a plurality of emitters, each emitter operable to emit light on to a substrate for marking, a control unit operable to modulate the output of each emitter by varying a marking setting wherein the control unit is operable to look up a marking setting associated with a target optical density in a look up table and wherein the look up table is populated using the method of any one of claims 24 to 29 or according to the apparatus of claim 30.