WO2006114600A2

WO2006114600A2 - Multi-colour printing

Info

Publication number: WO2006114600A2
Application number: PCT/GB2006/001498
Authority: WO
Inventors: David Miller
Original assignee: Datalase Ltd.
Priority date: 2005-04-25
Filing date: 2006-04-25
Publication date: 2006-11-02
Also published as: GB0508359D0; WO2006114600A3

Abstract

A substrate-marking system comprises a substrate-marking apparatus and a substrate. The substrate includes an additive susceptible to changing colour to one of at least two selectable colours upon irradiation, each selectable colour being different from the colour, if any, of the additive prior to irradiation. The apparatus comprises a laser diode for emitting a beam of laser light and means for aligning a desired point on the substrate with the laser beam. In use, the laser beam irradiates the desired point thus causing the additive to change colour at the point. The apparatus further comprises means for controlling a fluence level of the laser beam at the point such that, in use, the resultant colour of the additive can be selected from the selectable colours.

Description

MULTI-COLOUR PRINTING

Field of the Invention

This invention relates to a method of multi-colour printing and a system therefor.

Background to the Invention

Lasers have been widely used to achieve marking, typically by ablation but also by causing material, that can absorb laser energy, to char or change colour. WO97/47397 discloses a method and apparatus for marking a product with identification indicia. A coating layer is formed on a substrate to be marked. The coating layer contains an additive that is darkenable under the action of a CO₂ laser beam. CO₂ lasers have typically been used for this purpose due to their long operating lives, at least 10,000 operating hours. The coating is darkenable upon irradiation with focused energy of the laser source. For identification purposes it is preferable that the contrast between the darkened and non-darkened areas of the coating be as great as possible, particularly when the printing concerns the imaging of bar codes.

Known laser marking techniques have the disadvantage that only a single colour change may be effected upon irradiation with the laser.

Summary of the Invention

In accordance with a first aspect, the present invention is a substrate-marking system comprising a substrate-marking apparatus and a substrate which includes an additive susceptible to changing colour to one of at least two selectable colours upon irradiation, each selectable colour being different from the colour, if any, of the additive prior to irradiation, the apparatus comprising a laser diode for emitting a beam of laser light, means for aligning a desired point on the substrate with the laser beam such that, in use, the laser beam irradiates the desired point thus causing the additive to change colour at said point, and means for controlling a fluence level of the laser beam at said point, in use, to select the resultant colour of the additive at said point from the selectable colours.

The system of the present invention enables substrate marking by a laser to effect multi-colour printing. The term multi-colour is herein intended to mean true multi-colour rather than merely different shades of a single colour formed on the substrate, although multiple shades of multiple colours are intended to be covered by the term multi-colour. The system is suitable for high speed industrial application to which end the laser source has a long operating life, is efficient, reliable, readily controllable, and is capable of very high resolution printing, ultimately into the field of molecular resolution. The system is capable of high speed printing on a wide variety of substrates in a cost efficient manner.

In accordance with a second aspect, the present invention is a method of substrate marking using a substrate-marking system in accordance with the first aspect of the present invention, the method comprising the steps of aligning a desired point on the substrate with a laser beam emitted by the laser diode, irradiating the desired point with the laser beam to cause the additive to change colour at said point, and controlling a f luence level of the laser beam at said point to select a colour of the additive as a result of the irradiation at said point from one of at least two selectable colours, each selectable colour being different from the colour, if any, of the additive prior to irradiation. The method may further comprise repeating the above mentioned steps for a plurality of desired points to produce a multi-tonal colour image on the substrate.

Description of the Invention The substrate-marking system in accordance with the first aspect of the present invention may be used with a wide variety of substrate materials, for example, metals, alloys, glasses, ceramics, plastics, fabrics, wood, paper, card, resins, rubbers, foams, composites, stone and edibles, although virtually any material may be suitable. The colour change additive is preferably provided in a composition, or matrix, which may be applied in the form of a liquid as a coating on the substrate. It is highly desirable that the composition containing the additive be transparent, or at least translucent, and colourless so that the composition may be covertly applied. It is further desirable that the additive be non-toxic so that the composition may be ingested, for example, for printing on pills or fruit. The additive may alternatively be provided within the substrate itself, where the substrate allows, for example, where the substrate is made of plastics material then the additive may be incorporated into the substrate during manufacture of the substrate. Alternatively, where the substrate is made of fabric material, the additive, or a composition containing the additive, may be applied in liquid form to the fabric and absorbed therein. In a yet further example, the additive may be applied between two layers of a substrate thus sandwiching the additive, or a composition containing the additive, therebetween. Various other examples for incorporating the additive on or in the substrate will be apparent from the following description or will be readily appreciated by those skilled in the art. In developing suitable additives for use in the substrate marking system of the present invention, examples of which are provided herein in the following description, the inventors sought additives which are susceptible to changing colour to one of at least two selectable colours upon irradiation, each selectable colour being different from the colour, if any, of the additive priorto irradiation. It is clearly desirable, in some situations, that the additive, and in fact the substrate, have no colour priorto irradiation with the laser. Such a combination of additive and substrate is particularly desirable for printing on transparencies for use with overhead projectors and the like. The inventors have further sought additives which are susceptible to changing colour under low fluence levels. The term fluence refers to the total amount of energy applied by the laser per unit area of the substrate. It is clearly desirable, to increase the energy efficiency of the system and the speed at which the system may operate, to provide an additive or additives which are susceptible to changing colour at low fluence levels. As described above, CO₂ lasers, and to some extent YAG lasers, have been used previously due to the high fluence levels required by current laser printing techniques. Prior art printing techniques require fluence levels of the order of 1J/cm² for around 10 seconds to achieve marking by burning or ablation.

By providing additives susceptible to changing colour under low fluence levels the present inventors have utilized highly efficient laser diodes as the laser light source, rather than a conventional CO₂ or YAG laser. Fluence levels of less than 500mJ/cm² are preferably provided by the laser beam. Until now, laser diodes have not been considered suitable laser sources for substrate marking systems, mainly due to their low power and poor beam quality. However, laser diodes have many advantages over conventional CO₂ lasers. Conventional CO₂ lasers may be pulsed such that the laser output consists of a series of intense energy pulses. These energy pulses are typically pumped at a frequency of approximately 4.5 kHz but frequencies in the range of 20-30 kHz are achievable. Due to the lead in and lead out time of each pulse, the frequency at which conventional CO₂ lasers can be pumped is limited to prevent the pulses from overlapping thus forming a, so called, continuous wave output. Laser diodes have the distinct advantage in that the semiconductor therein may be switched virtually instantaneously and so laser diodes can be operated well into the MHz region. High switching speeds are particularly desirable in substrate marking systems to increase the speed of the system. In addition, the lack of overlap between successive pulses greatly improves the potential control of each laser pulse. Typical laser diodes can currently be switched in approximately 100 nanoseconds, although this is likely to decrease in the future.

Laser diodes further have the advantage in that they are relatively cheap and their cost is decreasing by approximately 20% year on year. Infrared and near infrared laser diodes are readily utilized in the telecommunications industry for their low cost. Laser diodes operating in the UV spectrum are significantly more expensive at present, but again their cost is decreasing year on year. In general, the lower the wavelength of the laser light the smaller the spot size that can be created and so UV diodes are particularly suitable for the type of very high resolution printing enabled by the present invention. Laser diodes can have a problem, however, in that they have a relatively high beam divergence and poor beam quality. For low resolution printing this drawback, when compared with CO₂ lasers, is not necessarily a prohibitive problem. However, for the very high resolution printing envisaged for the present invention it is preferable that the substrate-marking system further comprises means for shaping the laser beam. In a preferred example, the laser beam is shaped by coupling a fibre optic cable to the laser diode for homogenizing the laser beam. Further preferably, collimating and/or objective lenses are provided between the laser diode and the substrate. These may be provided in any suitable number and may be disposed before of after any beam steering means that may be provided as the alignment means for aligning the laser beam with the substrate. The print resolution may be altered by providing a movable objective lens which may be moved to alter the focal length, and therefore the spot size, of the laser beam lasing the substrate. A motorized zoom lens is provided as a suitable example.

It is envisaged that the substrate-marking system may comprise either a single laser diode as the laser light source or a plurality of laser diodes in a linear or matrix array. Where a plurality of laser diodes are provided each may be provided with respective optics for controlling their respective beams. Where a plurality of laser diodes are provided the printing speed of the system may be dramatically improved. The conventional CO₂ lasers used in current substrate-marking systems have associated size, cost and power consumption drawbacks and so the provision of a plurality of laser sources until now has been prohibitive.

Means for aligning a desired point on the substrate with a laser beam for irradiating the desired point to cause the additive to change colour at that point may take many forms. Various alignment means are envisaged within the scope of the present invention. In a typical substrate marking system, such as that described in WO97/47397, a galvanometer is provided. Where a galvanometer is used as the alignment means, the laser source is typically spatially fixed and a beam of laser light emitted therefrom is directed towards the galvanometer. The galvanometer may either comprise a pair of mirrors for scanning in the X and Y directions, respectively, or the galvanometer may comprise a single mirror for scanning in a single axis (i.e. X or Y). In the case of a dual galvanometer system (i.e. having two mirrors), the substrate- marking apparatus, including the laser source and the dual galvanometer system, is spatially fixed relative to the substrate to be marked. In the case of a single galvanometer system (i.e. having one mirror), the substrate-marking apparatus, including the laser source and the galvanometer, and the substrate are moved relative to one another along an axis substantially perpendicular to the scanning axis of the galvanometer mirror. Either of these galvanometers are suitable for use as the aligning means in the substrate-marking system of the present invention. The galvanometer may be driven by a control system operating in a vectoring or progressive scan mode. In a vectoring mode, the laser beam follows only the areas of the substrate to be marked. In a progressive scan mode, the laser beam tracks the substrate in successive lines, marking the substrate where necessary. Instead of the single mirror galvanometer described above, a rotating polygon of known type may be used as a cheaper, higher speed alternative for progressive scanning.

Alternatively, the alignment means may take the form of a scanning head for causing the laser beam to traverse in at least one direction (X or Y) relative to the substrate. The scanning head may include the laser diode, or alternatively, the laser diode may be provided separately from the scanning head. In the case where the laser diode is coupled to a fibre optic cable, the flexible fibre optic cable can allow the laser diode to be disposed separately from the scanning head, the scanning head including an end of the fibre optic cable furthest from the laser diode. The scanning head may be adapted to move in two directions (X and Y) relative to a spatially fixed substrate, or the scanning head may scan in a single direction (X or Y) relative to the substrate which may be moved along an axis perpendicular to the scanning axis of the scanning head. It is further envisaged that the substrate-marking apparatus may be spatially fixed and that the substrate itself may be moved scanwise in at least one direction (X or Y) relative to the spatially fixed substrate marking apparatus. In a further example of the present invention, the alignment means may comprise a scanning head for causing the laser beam to traverse in one direction relative to the substrate whilst the substrate is rotated. The substrate may be rotated by any suitable drive means, such as an electric motor, disposed on one side of the substrate, whilst the scanning head is adapted to move between an axis of rotation of the substrate and a periphery of the substrate.

The alignment means of the substrate-marking system of the present invention may comprise any combination of means described above or any other means suitable for ensuring that either a portion or the entirety of a surface of the substrate to be irradiated by the beam(s) of laser light emitted by the laser diode(s) is exposed to the beam(s).

The laser diode laser may be pulsed at approximately 25 nanoseconds pulses. Pulse duration of between approximately 10 nanoseconds and 50 nanoseconds is envisaged to be suitable for use with the colour change additives to which the system and method of the present invention are specifically intended. Other pulse durations may be equally suitable depending on the additive, the maximum power output of the laser diode, the intended speed of operation of the substrate-marking system of the present invention, or the intended printing resolution. The laser diode may alternatively have a continuous wave output and the beam emitted therefrom may be gated. Low power laser diode sources necessarily require longer pulse durations. The additive included in or on the substrate is adapted to change colour to one of at least two selectable colours upon irradiation with the laser light according to a fluence level of the laser beam at the point under irradiation. Since the fluence level is a measure of the total amount of energy applied per unit area, the colour change which the additive undergoes at the desired point is a function of the laser beam power, the area of the substrate under irradiation and the dwell time of the laser beam at that power on that area. Accordingly, it becomes possible to select the resultant colour of the additive at the desired point as a result of the irradiation by the laser beam according to a number of different factors. In some instances the colour change may also be dependent on the substrate material. In one example, the substrate marking system may be controlled such that the laser beam spot size on the substrate is substantially constant throughout the marking operation. In addition, the marking speed is also substantially constant such that the dwell time of the laser beam directed towards a desired point on the substrate having an area equivalent to the spot size of the laser beam is substantially constant for each similar desired point on the substrate. In this manner, the colour change of the additive to produce a resultant colour at each desired point on the substrate from the selectable colours may be effected by modulating the power output of each laser beam irradiating the desired point for the dwell time by a suitable laser beam power level modulating means. Alternatively, during the fixed dwell time, a different number of pulses may be controlled, or the pulse duration at a constant power during the dwell time may be controlled to effect the colour change. This mode of operation is particularly suitable for bitmap imaging whereby an image to be formed on the substrate is converted into a bitmap image comprising a matrix of pixels which, by the system of the present invention, is produced on the substrate. The pixel resolution of this image may be readily altered by changing the focus spot size of the laser beam. This may be effected by moving the movable objective lens. The switching between high laser power and low laser power may be effected almost instantaneously between successive pulses of the laser to cause the additive to change to different colours for each subsequent pixel in the pulsing sequence. Where it is desired that no colour change of the additive is required for a plurality of adjacent pixels then the system may be configured to jump from one area of the substrate to another area of the substrate to improve the marking speed.

In another preferred example of the present invention the laser diode may be controlled so as to operate in a binary fashion between an off state between pulses and an on state at a preselected power level for each pulse. By coupling this control to the alignment means the dwell time of each pulse at each desired point on the substrate may be controlled to select the desired resultant colour at said point. Under this mode of operation, the scanning speed will be non-uniform in the creation of a multi-colour image on the substrate. Stepper motors may be utilized for driving a scan head or a substrate table upon which the substrate is mounted.

In yet another preferred example of the present invention, the power level of the laser beam may be modulated according to both the desired colour change of the additive to be effected at each point and also according to the spot size for a constant scanning speed. In this manner, both high resolution and low resolution printing may be effected in different regions of the substrate during a single printing operation.

A control means for controlling the alignment means, and for controlling the fluence level of the laser beam may be a single control means or a plurality of co-operating control means. The control means is preferably a computer control means which uses a look up table containing such variables as the substrate material, the additive material, the concentration of the additive material in the substrate, the laser power from a feedback device, a position of the objective lens for controlling the spot size of the laser beam and the desired colour to be achieved at each point on the substrate to ensure correct reproduction of the desired image on the substrate.

In accordance with the system of the present invention it is further envisaged that a plurality of laser diodes may be provided in a linear or matrix array. Said plurality of laser diodes may be incorporated into any one of the examples of the present invention described above or any combination thereof. Where a plurality of laser diodes are provided, they may either each be provided without any optics where low resolution, high speed printing is desired, or may each be provided with a respective optical element or elements. The optical elements may include an optical fibre coupled to the respective laser diode, a collimating lens, an objective lens, or other suitable optics. Where a plurality of laser diodes are provided in a linear array, the array may be moved scanwise relative to the substrate in a direction perpendicular to the array such that the system according to the present invention may mark imagewise a region of the substrate, or even the entire substrate.

Where the plurality of laser diodes are provided in a matrix array, the array may be of a sufficient size so as to cover an entire region of the substrate to be marked. In this instance, the alignment means may comprise any means suitable for maintaining a spatial relationship between the diode array and the substrate. In this instance, there is no requirement for the system to comprise any form of scanning means. Where the array of laser diodes is provided having an array size smaller than that of the region of the substrate to be image wise marked, the scanning means as described above may be incorporated into the system. Where an array of laser diodes is provided, and each diode is coupled to a respective fibre optic cable, the free ends of the fibre optic cable may be bundled together such that the resolution of the laser beams output by the fibre optic cables may be higher than that output by the laser diodes to which they are connected. In any of the above described examples of the present invention where a laser diode is coupled to a fibre optic cable, the end of the fibre cable furthest from the laser diode may be shaped so as to function as a lens to focus the emergent laser beam.

The substrate-marking system according to the present invention operating with an array of laser diodes can print at speeds of up to approximately 100m to 300m per minute depending on the laser diode source, although it is envisaged that this speed may be improved as colour change additives, laser diodes and the alignment and fluence level control means are improved. The present invention is therefore equally suitable for high speed low resolution printing, as it is for very high resolution printing.

In a yet further example of the present invention, a single laser diode laser light source is aligned with desired points on the substrate in a vectoring format rather than a progressive scan format. In this mode of operation, only desired points in the substrate where colour change is to be effected are irradiated with the laser beam. Preferably, the laser beam is switched in a binary fashion between an off mode and an on mode at a preselected power level for printing a single colour on the substrate. Subsequently, the power level of the laser diode in the on mode is set to a second predetermined power level for printing a second colour on the substrate. This sequence may be repeated as required to achieve the desired number of colours and shades to be imagewise printed on the substrate from the number of selectable colours achievable from the substrate and additive combination under irradiation.

Preferably, the additive is a thermally sensitive additive which changes colour upon application of thermal energy by the laser beam. In this instance, the additive may include a charge-delocalising compound and a photoacid, the photoacid, in use, generating an acid upon irradiation by the laser thereby forming a charge transfer complex with the compound. The charge-delocalised compound may include a heteroatom selected from N, O and S, and an aromatic group conjugated thereto. The charge-delocalising compound may be an amine, for example carbozole.

Alternatively, the additive may be susceptible to colour change from application of light energy in the form of laser light. With this type of additive the laser energy is not converted into heat but instead it is thought that a quantisation effect is responsible for polymerising the additive to give a colour dependent upon the conjugation length. One example of such an additive is diacetylene which may further be combined with a photoacid or photobase for tuning the quantisation effect to particular wavelengths corresponding to commercially available laser diodes. This is particularly advantageous since it becomes possible to tune the additive to relatively cheap laser diodes.

Whilst vector format imaging may be advantageous where only limited areas of the substrate are to be imagewise marked, where a full colour image is to be produced over the entire substrate area, bitmap imaging is equally fast and bitmap imaging is more commercially acceptable and so bitmap imaging is the preferred mode of operation.

It is preferable that the additive be readily formulated in a solvent or water based ink as a coating composition which may be applied to any suitable substrate. One particularly commercially important example of the present invention is a substrate-marking system in which the substrate is paper and the additive is coated as a liquid thereon and subsequently cured such that the substrate having the additive may be marked by the substrate marking apparatus functioning as a desktop printer. In such an example, the only consumables will be the electricity required for the substrate-marking apparatus and the coated paper. There will then be no requirement for replenishing liquid ink or toner in the printing system.

Brief Description of the Drawings Figure 1 is a flow diagram illustrating the interaction between components of the substrate-marking system of the present invention;

Figure 2 is a schematic view of a galvanometer system used as the alignment means for the system of the present invention;

Figure 3 is a schematic view of an alternative alignment means for the system of the present invention;

Figure 4 is a schematic view of a further alternative alignment means for the system of the present invention;

Figure 5 is a schematic view of an exemplary lasing head for use with the system of the present invention; Figure 6 is a schematic view of an alternative lasing head for use with the system of the present invention;

Figure 7 is a view of the underside of the lasing head according to Figures 5 or 6; and, Figure 8 is a view of the underside of an alternative lasing head for use with the system of the present invention.

Detailed Description Figure 1 shows a block diagram of an exemplary embodiment of the system of the present invention. The substrate marking system 1 comprises a laser diode 2 which may be cooled by a cooling device 3. The laser diode 2 outputs a beam of laser light which is transferred through a first optical system 4 and into an optical fibre 5 for homogenizing the laser beam. Upon exiting the optical fibre 5, the beam enters a further optical system 6 which may typically include a collimating lens and/or an objective lens. The laser beam then enters a galvanometer beam deflection system 7 such as that shown in detail in Figure 2. The galvanometer beam deflection system 7 comprises an X direction galvanometer 8 and a Y direction galvanometer 9. The X and Y direction galvanometers 8, 9 have respective mirrors 10, 11 for steering the coherent laser beam 12 departing the laser source 13 which comprises the laser diode 2, the cooling device 3 and the optical systems 4, 5, 6. The focussing optical system 6 may be provided for focussing the coherent beam 12 before entering the galvanometer beam deflection system 7, or after the beam has departed the galvanometer beam deflection system 7. The former is shown in the embodiment of Figure 2 in which the focussing optical system 6 is constituted by a movable objective lens, for example a zoom lens, movable to alter the focussing, and therefore the spot size, of the emergent coherent laser beam 12.

The galvanometer beam deflection system 7 is controlled by a scanner control electronics module 14 which receives input from a central control system 15. The central control system 15 receives input from a human machine interface 16 which may be a keyboard, personal computer, or the like; signals from product sensors 17, such as manually operated pulse generators or switches; or external control systems. The central control system 15 may also receive input from a substrate motion system 18 for moving the substrate 19 itself. The substrate motion system 18 may take the form of a device for rotating the substrate 19 as shown in Figure 3, or a device for controlling movement of a substrate table, as shown in Figure 4, or a conveyor. The alternative systems of Figures 3 and 4 will be described in greater detail hereafter. Returning to Figures 1 and 2 a power supply unit 20 supplies power to the laser diode 2, the control system 15, and the scanner control electronics module 14 for controlling the galvanometer beam deflection system 7. The dual axis galvanometer beam deflector system 7 may be used in conjunction with the substrate motion system 18, configured such that movement in one or both axes of the galvanometer is controlled to compensate for movement of the substrate motion system 18. Operation of the exemplary system of Figures 1 and 2 will now be described.

A substrate 19 including an additive susceptible to changing colour is provided on a fixed, or movable, support structure. Where a galvanometer system 7 comprising a single mirror is provided for scanning the laser beam in one direction (X or Y) then the substrate 19 is provided on a movable support for traversing the substrate 19 in a direction perpendicularto the scanning direction of the galvanometer 7. The substrate motion system 18 controls movement of the substrate. Where a dual galvanometer system comprising two galvanometer mirrors is provided, then the substrate 19 may be spatially fixed by a suitable support structure, as shown in Figure 2. Alternatively, the substrate may move under the substrate motion system 18 as described above. The substrate 19, prior to introduction to the substrate-marking system 1 , is coated or otherwise provided with the additive susceptible to changing colour upon irradiation. Exemplary additives suitable for use with the substrate-marking system 1 of the present invention will be described hereafter.

An image to be marked on the substrate 19 is input via the human machine interface 16, or external control system 17. The image is converted into an image signal for input to the control system 15. The control system 15 uses the image signal to control the power supply to the laser diode 2 and the scanner control electronics module 14 for controlling the galvanometer beam deflection system 7. In this manner the laser diode 2 is controlled by the control system 15 and the power supply unit 20 to emit a pulse of laser light through the optical systems 4, 5, 6 to the galvanometer beam deflection system 7 which directs the coherent laser beam pulse 12 towards a desired point on the substrate 19. Depending on the fluence level of the incident laser beam at the desired point on the substrate 19, the additive at that point changes colour to one of a plurality of colours, each different from a colour of the additive prior to the irradiation. Subsequently, a further laser beam pulse, which may be of the same or a different power level, is directed by the galvanometer beam deflection system 7 to a second desired point on the substrate 19 to effect another colour change at the second desired point. This process may be repeated for a plurality of desired points on the substrate in order to create an intended image on the substrate 19. The relationship between consecutive pulses and the desired positions on the substrate is dependent on whether the system 1 operates in a progressive scan mode or vector format. In vector format imaging the "pulses" may be relatively long such that the laser is turned on at the start of the vector and off at the end of the vector, or potentially change the power level mid-way through to change the line being marked to a different shade or colour.

Turning next to Figure 3 there is shown an alternative beam alignment system comprising a galvanometer beam deflection system 7 including a single galvanometer mirror 21. The galvanometer mirror 21 directs the coherent laser beam pulse 12 in the X direction of the substrate 19 to be marked. The substrate 19 is rotatable about an axis Z driven by a motor 22. By rotating the substrate 19 and deflecting the beam 12 using the galvanometer mirror 21 , the entire surface of the substrate 19 may be irradiated as desired to form an image thereon.

In an alternative embodiment of the substrate-marking system shown in Figure 3, the laser source 13 may be constrained to move, such as by a track, in one direction such that the emergent laser beam 12 traverses the substrate 19 in the X direction as the laser source 13 moves. Such a system is already contemplated in the art as it is provided in most optical disc drives, for example a compact disc drive.

Turning next to Figure 4 there is shown a further embodiment of the substrate-marking system 1 in accordance with the present invention in which the substrate 19 is mounted on a substrate table 23 movable in respective X and Y directions by substrate table control means 24 and 25, respectively. In the embodiment of Figure 4, the optical system 13 is held spatially fixed whilst the substrate is moved in the X and Y directions by controlling the means 24 and 25, respectively. Control of the laser source 13 and the means 24, 25 is effected by the central control system 15.

As an alternative to the embodiment shown in Figure 4, the optical system may be configured to move in only one of the X or Y directions such that movement of the table 23 or other support structure such as a conveyor need be effected in only the direction perpendicular to a scan direction of the laser source 13.

In Figure 5 there is shown a schematic diagram of an embodiment of a lasing head 26 comprising a linear array of laser sources 13 stacked together and configured to move under the action of the central control system 15 in the Y scan direction. In this manner the lasing head 26 may be moved in the Y scan direction to irradiate the substrate 19 to form an image thereon.

Figure 6 shows a schematic diagram of another embodiment of a lasing head 27. The lasing head 27 comprises laser sources 13 each having an optical fibre 5 connected to a respective laser diode 2 and beam shaping/focussing optics 4. The free ends of the optical fibres 5, that is those furthest from the laser diodes 2, are bundled together. The embodiment of Figure 6 is advantageous over the embodiment of Figure 5 in that higher resolution printing may be achieved in certain circumstances. The lasing head 27 is again controlled by the central control unit 15 to control movement of the lasing head 27 relative to the substrate 19.

Figure 7 illustrates the lasing heads 26 or 27 viewed from the substrate 19. The emergent laser beams 12 are indicated. Figure 8 differs from Figure 7 in that a matrix array of laser diodes 2 is provided in the lasing heads 26 or 27. Otherwise the lasing heads 26, 27 are identical. Again, the emergent laser beams 12 are indicated. Using an array of laser diodes in the head 26 or 27 an image may be created on the substrate 19 without the requirement for scanwise movement of the head 26, 27 relative to the substrate 19.

Combinations of each of the embodiments descried above are readily determinable within the scope of the present invention. Next will be described an example of a coating formulation to be applied to the substrate 19 prior to marking with the substrate-marking apparatus of the substrate-marking system 1 of the present invention. The coating formulation comprises a solution of 10,12 pentacosadiynoic acid, Cyracure 6974 (photoacid generator), Elvacite 2028 (acrylic binder) and methyl ethyl ketone (MEK). This mixture is applied onto paper using a wire bar coater to provide an even coating of the mixture. This coating formulation is susceptible to colour change upon application of light energy in the form of laser light. A UV laser diode 2 emitting in the 400-500 nanometre range is suitable for use in the system 1 of the present invention with the above-mentioned formulation. This coating formulation is transparent and clear and when coated on paper provides a similar reflectance spectrum to that of the bare paper. The reflectance of the coated paper remains substantially unchanged after irradiation with the laser diode 2 to form an image on the substrate 19. This is particularly advantageous in that the problems of differential gloss apparent in many toner or ink based printing systems is overcome. The coating formulation described above is suitable for use with the substrate marking apparatus of the present invention operating at a constant marking speed of approximately 250 mm/s, the f luence control at each desired point on the substrate 19 being controlled by alteration of the output power of the laser diode 2. The above described formulation typically undergoes colour change from colourless to blue, to red, and finally to yellow by respective increases in the fluence level of the incident laser beam 12.

Next will be described a second exemplary coating formulation suitable for applying to the substrate 19 for the substrate marking system 1 of the present invention. The coating formulation comprises a solution of N-ethylcarbazole and photoacid generator Cyracure 6974 (a solution of triarylsulphonium hexafluoroantimonate in tropylene carbonate) in methyl ethyl ketone (MEK). The coating formulation is then applied to the substrate 19, e.g. paper, using a K-bar and allowed to dry thoroughly resulting in a transparent, colourless coating. The coating formulation develops blue and green colours, respectively, with increasing fluence levels, upon irradiation.

The above described formulations are provided as non-limiting examples of formulations to be used on or in the substrate 19 of the substrate-marking system 1 in accordance with the present invention. Further examples are provided in Applicant's co-pending International Patent Applications Nos. PCT/GB2005/004355 and PCT/GB2005/003222. Derivations therefrom and suitable alternatives will be readily appreciated by those skilled in the art.

The system of the present invention is readily applicable to the field of covert imaging, where an image not visible to the naked eye can be written into a film and subsequently revealed on demand. This process involves imaging with a sufficiently low fluence level of UV light to ensure heating is minimised and colouration does not develop. The sample can then be heated, revealing the image when required. This process may find application in any area requiring hidden/covert marking, promotional messages, process indicators and the like.

The system is also readily applicable to negative imaging where a negative image can be written into a film. The imaged area remains uncoloured and the remainder develops colour under blanket-exposure to appropriate thermal or photo stimuli. The process involves writing an image to the sample by imagewise exposure to a suitable low fluence level radiation, followed by exposure to ammonia vapours. The image can then be revealed by simultaneous blanket-exposure to a suitable light source and heating.

Claims

1. A substrate-marking system comprising a substrate-marking apparatus and a substrate which includes an additive susceptible to changing colour to one of at least two selectable colours upon irradiation, each selectable colour being different from the colour, if any, of the additive prior to irradiation, the apparatus comprising: a laser diode for emitting a beam of laser light; means for aligning a desired point on the substrate with the laser beam such that, in use, the laser beam irradiates the desired point thus causing the additive to change colour at said point; and, means for controlling a f luence level of the laser beam at said point, in use, to select the resultant colour of the additive at said point from the selectable colours.

2. A system according to claim 1 , wherein the fluence level control means includes means for modulating a power level of the laser beam irradiating said point.

3. A system according to claim 1 or claim 2, wherein the alignment means co-operates with the fluence level control means such that a predetermined colour change is effected at each of a plurality of desired points on the substrate.

4. A system according to claim 3, wherein said plurality of colour changes produces a multi-tonal colour image on said substrate.

5. A system according to any one of the preceding claims, further comprising means for shaping the laser beam.

6. A system according to claim 5, wherein the beam-shaping means includes a fibre optic cable coupled to the laser diode for homogenizing the laser beam.

7. A system according claim 5 or claim 6, wherein the beam-shaping means includes a collimating lens.

8. A system according to any one of claims 5 to 7, wherein the beam-shaping means includes an objective, or zoom, lens.

9. A system according to claim 8, wherein the objective lens is movable to alter a focal length or spot size of the laser beam.

10. A system according to any one of the preceding claims, wherein the alignment means is a galvanometer.

11. A system according to claim 10, wherein the galvanometer is driven by a control system operating in a vectoring mode.

12. A system according to claim 10, wherein the galvanometer is driven by a control system operating in progressive scan mode.

13. A system according to any one of the preceding claims, wherein the alignment means is a scanning head for causing the laser beam to traverse in at least one direction relative to the substrate.

14. A system according to claim 13, wherein the alignment means is a scanning head for causing the laser beam to traverse in two orthogonal directions relative to the substrate.

15. A system according to claim 13, wherein the alignment means further comprises means for causing the substrate to rotate.

16. A system according to any one of the preceding claims, wherein the alignment means is a mechanism for causing the substrate to traverse in at least one direction relative to the laser beam.

17. A system according to claim 16, wherein the alignment means is a mechanism for causing the substrate to traverse in two orthogonal directions relative to the laser beam.

18. A system according to any one of the preceding claims, wherein the laser diode is an IR, VIS or UV laser diode.

19. A system according to any one of the preceding claims, wherein the laser diode operates at a wavelength between approximately 10nm and 1mm.

20. A system according to any one of the preceding claims, wherein the laser diode is pulsed and operates at a frequency between approximately 1 Hz and 1GHz.

21. A system according to any one of claims 1 to 20, wherein the laser has a continuous wave output and the beam of laser light emitted therefrom is gated.

22. A system according to any one of the preceding claims, wherein the laser beam provides a fluence level of less than 500m J/cm².

23. A system according to any one of the preceding claims, wherein a plurality of said laser diodes are provided for emitting laser beams.

24. A system according to claim 23, wherein said plurality of laser diodes are provided in a linear array.

25. A system according to claim 23, wherein said plurality of laser diodes are provided in a matrix array.

26. A system according to any one of claims 23 to 25, further comprising means for modulating power levels of each laser beam.

27. A system according to any one of the preceding claims, wherein the additive is a thermally-sensitive additive which changes colour upon application of thermal energy by the laser beam.

28. A system according to claim 27, wherein the additive includes a charge-delocalising compound and a photoacid, the photoacid, in use, generating an acid upon irradiation by the laser thereby forming a charge-transfer complex with said compound.

29. A system according to claim 28, wherein the charge-delocalising compound includes a heteroatom selected from N, O and S, and an aromatic group conjugated thereto.

30. A system according to claim 28 or 29, wherein the charge-delocalising compound is an amine.

31. A system according to claim 30, wherein the amine is a carbazole.

32. A system according to any one claims 1 to 26, wherein the additive is susceptible to colour change upon application of light energy in the form of laser light.

33. A system according to claim 32, wherein the additive includes a diacetylene which, in use, is polymerisable upon application of the laser light.

34. A system according to claim 33, wherein the additive further includes a photoacid or photobase.

35. A system according to any one of the preceding claims, wherein the substrate material is selected from metals, alloys, glasses, ceramics, plastics, fabrics, wood, paper, card, resins, rubbers, foams, composites, stone and edibles.

36. A method of substrate-marking using a substrate-marking system in accordance with any one of the preceding claims, the method comprising the steps of: a) aligning a desired point on the substrate with a laser beam emitted by the laser diode; b) irradiating the desired point with the laser beam to cause the additive to change colour at said point; and, c) controlling a fluence level of the laser beam at said point to select a colour of the additive as a result of the irradiation at said point from one of at least two selectable colours, each selectable colour being different from the colour, if any, of the additive prior to irradiation.

37. A method according to claim 36, further comprising repeating steps a) to c) for a plurality of desired points to produce a multi-tonal colour image on said substrate.