WO2012172366A1

WO2012172366A1 - Radiation tracking apparatus

Info

Publication number: WO2012172366A1
Application number: PCT/GB2012/051386
Authority: WO
Inventors: John Cridland; Chris Wyres; Lee Holloway; Tristan Phillips
Original assignee: Datalase Limited
Priority date: 2011-06-15
Filing date: 2012-06-15
Publication date: 2012-12-20
Also published as: GB201110025D0

Abstract

A substrate marking apparatus (10) is suitable for irradiating a substrate (5) that moves relative thereto and it comprises; a radiation source (1); a beam steering unit (2); a monitoring means (3); and a processing means (4), The radiation source (1) is operable to output a beam of radiation (11) and the beam steering unit (2) is operable to receive this radiation (11) and transmit it to an output point (21). The output point (21) is operable to move between at least a start position and a stop position. The monitoring means (3) is operable to monitor the position and speed of the substrate (5) which it is desired to irradiate and to transmit this information to the processing means (4). In turn, the processing means (4) is operable to receive this information from the monitoring (means 3) and transmit instructions to: the radiation source (1) so as to control the emission of radiation therefrom; and the beam steering unit (2) so as to control the movement of the output point (21). The apparatus therefore includes a means to steer the radiation emitted by the radiation source (1) such that any image dot distortions resulting from relative motion of the substrate (5) and the radiation source (1) are minimised or eliminated. Such an arrangement therefore results in an image quality that is independent of the relative speed of the substrate (5)and the radiation source (1).

Description

Radiation tracking apparatus

The present invention relates to an apparatus suitable for irradiating a moving substrate with a radiation source and an associated method. In particular, it relates to an apparatus and method which reduce the effect that the movement of the substrate has on the spatial distribution of radiation incident thereon.

It is often desired to irradiate a substrate with a specific type of radiation. For example, in the field of inkless printing, radiation sources are used to irradiate a substrate which changes colour in response to the incident radiation. Typically, one or more arrays of multiple laser sources are used to irradiate a responsive substrate that moves in relation to the array(s). Such arrangements are known in the art and may utilise a first wavelength to activate the substrate and second wavelength to effect the colour change. For example, one such arrangement is described in UK patent application number 1001 110.4 which describes an inkless colour printing apparatus where dots or regions of colour are created within a substrate when it is irradiated by IR and/or UV light.

The responsive substrate changes to one of a range of different colours via the appropriate sequence of exposures to light of different wavelengths and powers and the final colour and its density are dependent upon the spatial energy density received at any given exposed location. Furthermore, for a given exposure time the energy density at any given point on the substrate depends on the beam shape and intensity distribution and the speed of motion of the media in relation to the array of emitters. Typically, each irradiation of the substrate by each laser in the array produces a single pixel of an image and it is desirable to produce regularly shaped pixels of uniform colour. It should be noted that a variation in energy density incident upon the substrate may be due to the intensity distribution of the radiation source and/or due to the relative motion of the substrate and the emitter. The creation of a radiation source with a uniform intensity profile can be achieved using standard techniques known in the art. For example, a Gaussian intensity distribution may be changed to a top hat distribution by using diffractive or refractive optical beam converters. However, correcting for the relative motion of the substrate and the emitter is rather more complicated.

Dot elongation resulting from the relative motion of emitters and responsive substrate is well known in the art and one known solution essentially involves increasing the power of the emitter and reducing the exposure time so that the substrate only moves a very small distance during the exposure time. However, for reasonable substrate speeds this can require a substantial increase in the peak emitter power. Increasing the peak emitter power may not be desirable for a number of reasons. For example, it may be less efficient and therefore more expensive since certain substrates produce denser colours for the same incident energy when irradiated with a lower intensity for a relatively longer exposure time.

Another technique know in the art is to compress the beam profile in one axis so that it at least partially compensates for the dot or pixel elongation that results from the relative motion of substrate and emitter. However, this technique does not correct for variation in the energy density delivered to the substrate that results from the relative motion of substrate and source.

It is therefore an object of the present invention to provide a solution that at least partially overcomes or alleviates the above problems According to a first aspect of the present invention there is provided an apparatus which is suitable for irradiating a substrate which moves relative thereto, the apparatus comprising: a radiation source which is operable to emit a beam of radiation; a beam steering unit which is operable to receive the radiation emitted by the radiation source and transmit it to a movable output point; a monitoring means which is operable to monitor the motion of a substrate which it is desired to irradiate; and a processing means which is operable to receive information from the monitoring means and in response thereto transmit instructions to: the radiation source so as to control the emission of radiation therefrom; and the beam steering unit so as to control the movement of the output point.

Such an arrangement allows the motion of the output point to substantially match that of the substrate whilst radiation is emitted therefrom. Advantageously, this allows the spatial intensity distribution of the radiation incident upon the substrate to be substantially the same as the spatial intensity distribution emitted by the radiation source.

The movement of the output point may be of any suitable type so as to allow the beam of radiation emitted therefrom to track with the substrate as it moves. That is, preferably, the output point is operable to move so that the projection of the beam of radiation emitted by the output point on to the substrate is substantially stationary with respect to the substrate. For example, the movement may comprise a translation and/or a rotation.

In one embodiment, the motion is a translation wherein the output point moves parallel to the substrate and at substantially the same speed as the substrate. In an alternative embodiment, the motion is a rotation of the output point so that the angle with which the radiation is incident upon the substrate is altered continuously from a first angle to a second angle. Preferably, for such embodiments the range of angles of incidence is small. Furthermore, preferably, for the entire range of angles the radiation emitted by the radiation means is incident on the substrate in a direction which is substantially perpendicular to its surface. The rotation may be achieved by any suitable means, for example, by rotation of a mirror at the output point.

Furthermore, the above arrangements allows for the output point to return to its start position whilst it is not emitting radiation. This can allow a plurality of regions or pixels of the substrate to be irradiated sequentially as the substrate moves relative to the apparatus whilst ensuring that the spatial intensity distribution of the radiation incident upon the substrate remains substantially the same as the spatial intensity distribution of the beam emitted by the radiation source.

The output point may be operable to move so that the distance that the projection of the beam of radiation onto the substrate moves is substantially the same as, or slightly smaller than, the width of the beam emitted by the radiation source.

The monitoring of the motion of the substrate may involve monitoring the position and/or speed of a region of the substrate upon which the radiation is incident.

The spatial intensity distribution emitted by the radiation source may take any form as is desired and/or required. In one embodiment the spatial intensity distribution emitted by the radiation source is substantially uniform over the beam profile. Alternatively, different spatial intensity distributions may be emitted by the radiation source, for example, to create different coloured patterns within a single dot or pixel.

The spatial intensity distribution emitted by the radiation source may be time dependent so that different distributions are used for subsequent irradiations.

The radiation emitted by the radiation source may be of a suitable wavelength and intensity to effect a colour change to the area of the substrate upon which it is incident. Alternatively, the substrate may be irradiated for any other reason as is desired or required and the radiation chosen to be suitable for this purpose.

The radiation source preferably comprises one or more arrays of multiple sources. Said one or more arrays of multiple sources may comprise a linear array or a two dimensional array.

For embodiments wherein the one or more arrays of sources comprise two dimensional arrays, said arrays may be regular two dimensional arrays. Alternatively, adjacent rows or columns of the two dimensional arrays may be staggered. Preferably, for embodiments wherein adjacent rows or columns of the two dimensional arrays are staggered for each such row or column in the array there is at least one other row or column that is aligned therewith.

For embodiments wherein the radiation source comprises an array of sources, the beam steering unit may comprise a plurality of output points, one for each source. The processing means may be able to move each of these output points independently. Advantageously, this can compensate for local variations in the speed of the substrate speed caused by stretching or slip.

The radiation source may be operable to emit more than one wavelength of radiation. In one preferred embodiment, the radiation source comprises a first array of multiple laser sources which are operable to emit IR or NIR radiation and a second array of multiple laser sources which are operable to emit UV radiation. In another preferred embodiment the radiation source comprises: a first array of multiple laser sources which are operable to emit IR or NIR radiation; a second array of multiple sources which are operable to emit UV radiation; and a third array of array of multiple laser sources which are operable to emit IR, NIR or visible radiation.

The apparatus may comprise a conveying means. The conveying means may be operable to move a substrate relative to the radiation source.

Preferably, the conveying means arranges the substrate so that radiation emitted by the radiation means is incident on the substrate in a direction which is substantially perpendicular to its surface. The conveying means may move the substrate substantially continuously. Alternatively, the conveying means may move the substrate incrementally. The conveying means may move the substrate at a substantially continuous speed.

The output point may be operable to move from its start position to a stop position and vice versa at any speed as is desired and/or required. Preferably, the processing means is operable to ensure that whilst the radiation source is emitting radiation the output point moves so that the direction and speed of the projection of the radiation beam onto the substrate substantially matches that of the region of the substrate upon which the radiation is incident. Preferably, the output point has a start position and the processing means ensures that when the radiation source stops emitting radiation the output point returns to the start position. Preferably, the time taken for the output point to return from a stop position, at which the radiation source stops emitting radiation, to the start position is substantially shorter than the time taken to move from the start position to the stop position.

The beam emitted by the radiation source may be any shape as is desired or required. For example, as is common in the art, a circular beam shape may be used. However, with inkless printing techniques it can be undesirable to overlap pixels and therefore the maximum fill factor which can be achieved is or the order of 78%, which will result in less than the maximum colour saturation. Therefore, in order to increase the fill factor closer to 100% square or rectangular beam shape may be used.

Said square or rectangular beam shapes may be achieved by any suitable means. This may include any or all of the following: square or rectangular apertures; diffraction beam shaping, for example a diffraction phase mask etched into the end of an optical fibre; a short length of square or rectangular section optical fibre connected to the end of an optical fibre and mounted into beam optics; square or rectangular waveguides including monolithic blocks with refractive index changes written into the block by direct femto-second lasers, proton exchange, metal in diffusion; epitaxial grown waveguides; tapered waveguides; or liquid waveguides. Preferably, a uniform square beam is achieved using a square optical fibre waveguide core with a square cladding or, alternatively, a circular cladding.

To minimise beam size at subsequent optical elements, particularly the tracking or scanning elements, some form of micro-optics will be required to minimise divergence and hence beam size. A larger beam will increase the voltage requirements for an electro-optic scanner and will generally increase the fly back time for acousto-optic or mechanical scanners. The apparatus may further comprise micro-optical elements. For embodiments wherein the radiation source comprises an array of emitters, these may be employed to correct for positional errors in the array, commonly referred to as smile. Additionally or alternatively, said micro-optics may be used to minimise divergence of the beam and hence beam size.

The apparatus may also include a processing means that adjusts the timing of the individual radiation sources such that the smile of the elements in the array can be compensated in such a way that the imaged pixels form a straight line across the width of the array.

The apparatus may further comprise a pixel mask. The pixel mask may transform the shape and spatial intensity distribution of the beam as desired or required. For example, the pixel mask may transform a circular beam with a Gaussian distribution into a square or rectangular beam with a top hat intensity distribution. Alternatively, the pixel mask may be employed to create individual intensity distributions within each pixel so that an increased gamut of perceived colours may be produced. Preferably, the pixel mask would enable such intensity profiles to change on a dot to dot basis in real time.

Such an individually addressable phase or amplitude pixel mask may comprise a tilting mirror MEMS device to generate the required pattern. Additionally or alternatively, the pixel mask may comprise patterned transparent electrode structure deposited onto an electro-optic crystal to create mico-pixels. When a voltage is applied to particular micro-pixel a local refractive index change is induced that adjusts the local speed of light and thus introduces a phase difference on the incoming wave front. The application of different voltages to different micro-pixels thus creates a phase mask. The emerging wave front from the device may be transformed by an optical system such that the required intensity distribution results in the image plane of said system. Alternatively or additionally, the pixel mask may comprise any or all of the following: optically addressed spatial light modulators (OASLM), magneto- optics, liquid crystals.

The beam steering unit may comprise one of a range of different technologies. The technology chosen may depend upon the following factors: the speed of 'fly back' required, cost, technical feasibility in terms of voltage, drive current, RP power etc. The speed of 'fly back' should be understood to mean the time within which the output point is required to return from a stop position to the start position. If very high 'fly back' speeds (<= ~500ns) are required electro-optic techniques are applicable. For fly back times of the order > ~500ns the acousto-optic or electro-optics devices can be used. For fly back times > ~30us micro mechanical, acousto-optic or electro- optics devices may be used; and for fly back time > ~100us mechanical tilting mirror or other mechanical, micro mechanical, acousto-optic or electro-optics devices may be used.

The speed requirement for fly back time may be relaxed if the laser power of the radiation source is increased. For example, for an embodiment wherein the fly back time is 1% of the total scan time, by increasing the power of the radiation source by around 0% the required fly back time may increase by a factor of 10.

The beam steering unit may be operable to specifically create coloured fringe patterns as required or desired by deliberately mismatching the speed of the output point(s) and the substrate. An arrangement wherein the one or more arrays of sources comprise two dimensional arrays increases the time allowed for the output point to return from its stop position to its start position whilst ensuring that there is substantially no gap between adjacent pixels, as will be explained further below.

For an embodiment comprising a linear array of radiation sources, and assuming that the substrate moves continuously, each source must track and irradiate a single pixel and then return to its start position in the time that it takes the substrate to move a distance equal to the width of a single pixel. Therefore, for such one dimensional arrays the speed of return from the stop position to the start position for the tracking system needs to be fast, in relation to the speed of the tracking from the start to stop positions.

However, for a two dimensional array with n rows of radiation sources, n rows of pixels can be irradiated simultaneously. Therefore, in order to ensure no gaps between pixels, and assuming again that the substrate moves continuously, each source must track and irradiate a single pixel and then return to its start position in the time that it takes the substrate to move a distance equal to the width of n pixels. Therefore the time available for the tracking system to return from the stop to the start position is greater than or equal to the time required to scan one dot or pixel dimension. In this case therefore there is no requirement for the tracking device to have the return speed to be much greater than the scan speed. Such a configuration therefore allows a larger range of optical scanning technologies to be used.

The beam steering unit may be wavelength dependent. For such embodiments, if the radiation source comprises a plurality of wavelengths then a beam steering unit may be provided for each wavelength. Alternatively, the beam steering unit may be independent of the wavelength of the radiation and, as such, a single beam steering unit may serve a plurality of different wavelengths.

The beam steering unit may employ any suitable technology. In particular these may include any or all of the following:

(a) Electro-optic technology employing Pockels effect, Kerr effect, or total internal reflection (TIR) in materials such as:- Lithium Niobate, SBN60 (Sr 0.6 Bao.4 Nb₂0₅), SBN-61, SBN-75, SBN50, KTN (KTa0₃ & KNb0₃), Silicon Carbide, KDP, KD*P (larger r63 coeff.), ADP, Potassium Niobate-3, BBO beta barium borate Zinc Sulphide, Zinc Selenide, BSO, Zinc Telluride, CsH2AS04, Barium Titanate, Barium Calcium Titanate, Barium Strontium Titanate, Lithium Tantalate, Strontium Titanate, Silver Arsenic Sulfide, Potassium Tantalum Niobium Oxide, Gallium Arsenide, Copper Chloride, Quartz Lithium Iodate, Polymers, Chalcogenides, BSKNN (Ba_2-X Sr_x ^._yNa_y Nb₅0i₅), PBN60 (Pbi_-x Ba_x Nb₂O_s), KN,SNN;

(b) Acousto-optic devices employing Tellurium Dioxide, Quartz, fused quartz, Lead Molybdate (PbM0₄), extra-dense flint glass, SPS, KRS-5 carbon disulphide, nitro-benzene, nitro-toluene, Lithium Niobate including Lithium Niobate doped with Fe^2÷ and Fe^3"*^" ions, Lithium tantalite, Lithium fluoride, Rutile (T1O₂), sapphire, alpha iodic acid (IIIO );

(c) Mechanical scanning devices such as tilting mirrors driven by galvanometers, piezo-electric or magneto-strictive transducers, rotating polygons or prisms. Micro Electro Mechanical Systems (MEMS) mirror or diffraction systems;

(d) Nematic liquid crystals, Smectic liquid crystals, Blue phase liquid crystals, Cholesteric liquid crystals; (e) Magneto-optics steering via refractive index control;

(f) Photonic crystal 3D gratings that are distorted by electrical/mechanical forces thus altering the beam deflection angle. Photonic crystal 3D gratings that are made using electro-optic or magneto-optic materials that change deflection angle when the refractive index changes as a result of an induced electric or magnetic field;

(g) Fluidic optics that change shape in response to electric field via electro- wetting or electrophoresis or magnetic fields; and/or

(h) Deflection using meta-materials that can have negative or zero refractive index.

According to a second aspect of the present invention there is provided a method of irradiating a substrate comprising the steps of:

(i) providing an apparatus according to the first aspect of the present invention;

(ii) monitoring the motion of the substrate with the monitoring means;

(iii) operating the radiation source to emit a beam of radiation; and

(iv) whilst the radiation source is emitting radiation operating the beam steering unit in response to the monitoring means so that the motion of the output point is such that the motion of the projection of the beam of radiation emitted by the output point on to the substrate substantially matches the monitored motion of the substrate.

The method of the second aspect of the present invention may incorporate any or all of the features of the apparatus of the first aspect of the present invention as discussed above. By ensuring that the output point moves so that the projection of the beam of radiation on to the substrate moves with a direction and speed substantially matching that of the region of the substrate upon which the radiation is incident there is substantially no relative motion between the radiation source and the substrate. Advantageously, the spatial intensity distribution of the radiation source is mapped onto the substrate in such a way that unwanted distortions such as coloured zones, patterns or fringes are sufficiently small that they cannot be perceived with the unaided eye.

The method may further comprise the steps of:

(v) storing the start position of the output point; and

(vi) returning the output point to the start position when the emission of radiation stops.

The method may further comprise the step of repeating sub-steps (iii) to (vi) a plurality of times. This allows a plurality of adjacent regions, or pixels, of the substrate to be irradiated sequentially.

The distance through which the projection of the beam of radiation onto the substrate moves is preferably substantially the same as, or slightly smaller than, the width of the beam emitted by the radiation source. Preferably, the time taken for the output point to return from a stop position, at which the radiation source stops emitting radiation, to the start position at step (vi) is substantially shorter than the time taken to move the output point from the start position to the stop position at step (iv).

The method may further comprise the step of controlling the movement of the substrate relative to the apparatus. The substrate may be controlled by way of a conveying means. The substrate is preferably controlled so that radiation emitted by the radiation source is incident on the substrate in a direction which is substantially perpendicular to its surface.

The substrate may be controlled so as to move substantially continuously at a substantially continuous speed. In such embodiments, the speed at which the output point returns to the start position is preferably substantially larger than that of the substrate. Advantageously, this limits the distance that the substrate moves while the output point returns to the first position. For such embodiments, the distance through which the output point moves is preferably smaller than the width of the beam emitted by the radiation source by at least the distance through which the substrate moves whilst the radiation source returns from the second position to the first position. This ensures that, for such an embodiment wherein the substrate moves substantially continuously, the method can achieve substantially no gap between adjacent pixels.

Therefore, in order to ensure that there is substantially no gap between adjacent pixels the output point must return to the first position within a required fly back time. The speed requirement for fly back time may be relaxed if the laser power of the radiation source is increased. For example, for an embodiment wherein the fly back time is 1 % of the total scan time, by increasing the power of the radiation source by around 10% the required fly back time may increase by a factor of 10.

An arrangement wherein the one or more arrays of sources comprise two dimensional arrays increases the time allowed for the output point to return to from its stop position to its start position whilst ensuring that there is substantially no gap between adjacent pixels, as will be explained further below.

For an embodiment comprising a linear array of radiation sources, and assuming that the substrate moves continuously, each source must track and irradiate a single pixel and then return to its start position in the time that it takes the substrate to move a distance equal to the width of a single pixel. Therefore, for such one dimensional arrays the speed of return from the stop position to the start position for the tracking system needs to be fast in relation to the speed of the tracking from the start to stop positions.

However, for a two dimensional array with n rows of radiation sources, n rows of pixels can be irradiated simultaneously. Therefore, in order to ensure no gaps between pixels, and assuming again that the substrate moves continuously, each source must track and irradiate a single pixel and then return to its start position in the time that it takes the substrate to move a distance equal to the width of n pixels. Therefore the time available for the tracking system to return from the stop to the start position is greater than or equal to the time required to scan one dot or pixel dimension. In this case therefore there is no requirement for¹ the tracking device to have the return speed to be much greater than the scan speed. Such a configuration therefore allows a larger range of optical scanning technologies to be used.

Alternatively, the substrate may be controlled so as to move incrementally. Said incremental movement may be made up by repeating the following steps: moving the substrate by a distance substantially equal to the width of the projection of the beam emitted by the radiation source onto the substrate during a first time period; and keeping the substrate at rest for a second time period. In such embodiments, the radiation source can return to the start position during the second time period, whilst the substrate is at rest. Furthermore, provided that the second period is sufficiently long, in such an embodiment, the speed at which the output point returns to the start position need not be significantly larger than the speed of the substrate. The method may further comprise the step of intentionally introducing a mismatch between the motion of the output point and the motion of the substrate in order to create coloured fringe patterns as required or desired or provide fine control of line width in the direction perpendicular to the motion of substrate.

In order that the invention can be more clearly understood embodiments thereof are now described further below, by way of example, with reference to the accompanying drawings, of which:

Figure 1 is a schematic representation of an apparatus according to the present invention;

Figure 2 shows a specific embodiment of an apparatus according to the present invention;

Figures 3-la to 3-le show schematically how the beam steering unit may track the beam of radiation from the radiation source across the substrate by translation of the output point;

Figures 3-2a-3-2e show schematically how the beam steering unit may track the beam of radiation from the radiation source across the substrate by rotation of the output point; and

Figures 4a-4d show further specific embodiments of apparatus according to the present invention.

Referring to figure 1, a schematic representation of an apparatus 10 according to the present invention is shown. The apparatus 10 is suitable for irradiating a substrate 5 that moves relative thereto and it comprises: a radiation source 1 ; a beam steering unit 2; a monitoring means 3; and a processing means 4. The radiation source 1 is operable to output a beam of radiation 11 and the beam steering unit 2 is operable to receive this radiation 11 and transmit it to an output point 21. The output point 21 is operable to move between at least a start position and a stop position. The monitoring means 3 is operable to monitor the position and speed of the substrate 5 which it is desired to irradiate and to transmit this information to the processing means 4. In turn, the processing means 4 is operable to receive this information from the monitoring means 3 and transmit instructions to: the radiation source 1 so as to control the emission of radiation therefrom; and the beam steering unit 2 so as to control the movement of the output point 21.

Such an arrangement allows the motion of the output point 21 to substantially match that of the substrate 5 whilst radiation is emitted therefrom so that the projection of the radiation beam onto the substrate remains substantially stationary relative to the substrate. For example, output point 21 may move parallel to the substrate 5 and at substantially the same speed as the substrate 5. Alternatively, the output point 21 may rotate so that the angle with which the radiation is incident upon the substrate is altered continuously from a first angle to a second angle. Advantageously, this allows the spatial intensity distribution of the radiation incident upon the substrate 5 to be substantially the same as the spatial intensity distribution emitted by the radiation source 1.

Furthermore, the above arrangement allows for the output point 21 to return to its start position whilst it is not emitting radiation. This can allow a plurality of regions or pixels of the substrate to be irradiated sequentially as it moves relative to the apparatus 10 whilst ensuring that the spatial intensity distribution of the radiation incident upon the substrate 5 remains substantially the same as the spatial intensity distribution of the beam emitted by the radiation source 1.

Referring to figure 2, an apparatus 100 according to the present invention is shown. The apparatus 100 comprises: radiation sources 101a, 101b, 108; beam steering units 106; a monitoring means 1 18; and a processing means 116. In the embodiment depicted the apparatus comprises: an R radiation source 101a; an NIR or visible radiation source 101b; and a UV radiation source 108.

A reactive substrate 11 1 is moved relative to the apparatus 100 by a conveying means (not shown), The speed and position of the substrate 111 is determined by the monitoring means 1 18, which may comprise any suitable sensor such as a position encoder. The monitoring means is further operable to send a signal to the processing means 1 16 conveying this information.

In turn, the processing means 1 16 is operable to control the emission of radiation from the radiation sources 101a, 101b, 108 and their power levels via suitable drive electronics 115, 119. The processing means 116 is also operable to control the beam steering units 106, and in particular the position of the output point (not shown), via suitable drive electronics 112, 113.

Each of the radiation sources 101a, 101b comprises a module of NIR or visible laser diodes, which may or may not comprise photo-diode monitors. The output of each of the radiation sources 101a, 101b is coupled via fibre optics 110, which may comprise either a circular or a square core waveguide, to an optical switch multiplexer 102. The optical multiplexer 102 is operable to route the output from a spare source to the output that corresponds to a failed primary source and this redirection is driven by suitable electronics 114 under the control of the processing means 116. The output from the optical multiplexer 102 is coupled to optical fibres 1 10 (either square or circular cores) and the opposite ends of these optical fibres 110 are arranged into a linear or two-dimensional array 103 as is desired or required. The output from this array 103 of optical fibres is conditioned with a beam transformation module 104 that converts the beam shape and intensity distribution to the required format and limits beam divergence with a combination of refractive and/or diffractive micro-optical elements. The output from the beam conditioner is directed to an adaptive pixel mask 105 where the wave front of each source is modified such that the appropriate intensity distribution for each pixel is formed at the substrate. The pixel mask 105 is driven by electronics 117 and controlled by the main controller 116. The output from the pixel mask 105 is directed to a beam steering unit 106 which operates under the control of the processing means 1 16 via drive electronics 112. The beam steering units 106 will be described in greater detail below.

Similarly, the UV radiation source 108 comprises a module of emitters and monitor detectors. Although not shown, optical beam switches or multiplexers, beam shaping and transformation elements and an adaptive pixel mask may be included if desired or required. The output from the UV radiation source 108, monitor and modulator module is directed to a beam steering unit 106, which operates under the control of the processing means 116 via drive electronics 113.

Each of the beam steering units 106 is operable to receive the radiation emitted by the radiation source with which it is associated and transmit it to an output point, wherein said output point is operable to move between at least a start position and a stop position. In operation, the speed and position of the substrate 111 are determined by the monitoring means 118. The processing means 116 uses this information to control the actuation of the radiation sources 101, 108 and the position of the output point of each beam steering unit 106. Once the radiation source has been actuated by the processing means 116, the output point of the corresponding beam steering unit 106 moves from its start position at a scan speed which is substantially the same as that of the substrate 111. The dot formed by the beam of each element in the array 103 is tracked for a time that allows the beam to perform this scan and return to its first position in the time it takes the substrate 111 to move one dot width.

The output of the beam steering unit 106 connected to the NIR radiation source 101a is directed into a beam combining unit 107 where it is combined with the output of the beam steering unit 106 connected to the source of UV radiation 108. The output of the beam combining unit 107 passes through a focusing element 109 and is directed towards the substrate 1 11.

The output of the beam steering unit 106 connected to the NIR or visible radiation source 101b passes through a second focusing element 109 and is directed towards the substrate 11 1.

The focusing elements 109 may comprise any suitable optical elements, for example, a lens, or a combination of lenses. The focusing elements 109 may include dynamic focus control. The focusing elements 109 are arranged such that the pixels formed by one element are overlapped by those formed by the second.

The above described embodiment is suitable for use with a reversible medium which requires the following steps to effect colour change: 1. Each pixel is activated with NIR and simultaneously each pixel is illuminated with UV as required to form blue; and

2, individual pixels are illuminated with NIR or visible to form red and/or yellow as required/desired.

Although one type of medium for use with the apparatus and method of the present invention has been described above, as will be appreciated by one of ordinary skill in the art, other types of media may be utilised.

Figures 3- la to 3-le show, schematically, how the beam steering unit of the present invention may track the beam of radiation from the radiation source across the substrate by translation of the output point 21. Each of these sub-figures shows a radiation source 1, a beam steering unit 2 which comprises an output point 21 and a substrate 5. The substrate 5 moves relative to the radiation source 1 at speed v.

As shown in figure 3-la, at the start of the irradiation of each pixel the output point 21 is disposed in a start position. The sequence is initiated by the processing means (not shown) actuating the radiation source 1 and causing the output point 21 to move towards a stop position at a first speed, v_ls which is substantially equal to that of the substrate 5. As can be seen in figure 3- lb, since the relative speed of the output point 21 and the substrate 5 is substantially zero, initially the region A of the substrate which is irradiated by the radiation does not change. The output point 21 continues to move at first speed, vi, until it reaches the stop position (figure 3-lc). Preferably, the distance between the start and stop positions is smaller than the width of the radiation beam at the substrate. Once the output point 21 reaches the stop position, the radiation source 1 is stopped and the output point 21 moves back towards the start position at a second speed, v_2j which is preferably significantly larger that that of the substrate 5. Once the output point 21 reaches the start position, the sequence may start again so that the next region of the substrate B may be irradiated. Preferably, the distance between the start and stop positions and the second speed, v₂, are chosen so that there is substantially no gap between one pixel A and the next B.

Figures 3 -2a to 3-2e show, schematically, an alternative method, by which the beam steering unit of the present invention may track the beam of radiation from the radiation source across the substrate by rotation, of the output point 21. Each of these sub-figures shows a radiation source 1, a beam steering unit 2 and a substrate 5. The substrate 5 is moves relative to the radiation source 1 at speed v.

As shown in figure 3 -2a at the start of irradiation the output from the beam steering unit 2 is disposed in the start position and incident on a first region of the substrate A. The sequence is initiated by the processing means (not shown) actuating the radiation source 1 and causing the output point to rotate with an angular velocity v_la so that the projection of the radiation on to the substrate moves at a first speed vi which is substantially equal to the velocity of the substrate 5. As can be seen in figure 3-2b, since the relative speed of the projection of the radiation on to the substrate and the substrate 5 is substantially zero, initially, the region A of the substrate which is irradiated does not change. The output point 21 continues to rotate at angular speed via until it reaches the stop position (figure 3-2c). Preferably, the distance through which the projection of the beam on the substrate moves as the output point 21 moves between the start and stop position is smaller than the width of the beam at the substrate. Once the output point 21 reaches the stop position the radiation source is stopped and the beam steering unit moves the beam back to the stall position at a second angular speed v_2a (figure 3 -2d) so that the projection of the radiation on to the substrate moves at a second speed v₂. Preferably, v₂ is significantly larger than that of the speed of the substrate 5. Once the beam reaches the start position, the sequence may start again so that the next region B of the substrate may be irradiated. Preferably, the distance between the start and stop positions and the second angular speed, v_2aj are chosen so that there is substantially no gap between one pixel A and the next B.

Figures 4a to 4d show further specific embodiments of apparatus according to the present invention. Each of these shares several common elements with the embodiment shown in figure 2 and discussed above. Below only the differences between each of the further embodiments and that shown in figure 2 will be emphasised.

Figure 4a shows a configuration 200 which also comprises three radiation sources (an NiR radiation source 10 la, an NIR or visible source 101b and a UV radiation source 108), however, the three optical systems 401, 402, 403 are adjacent. Preferably, the NIR or visible source 101b is operable to emit visible radiation in the range 400-680nm. More preferably, the NIR or visible source 101b is operable to emit visible radiation in the range 520-680nm. Each of the optical systems 401, 402, 403 may comprise some or all of the subassemblies described previously. The three optical systems 401, 402, 403 are arranged so that the dots or pixels produced by each subsystem coincide.

Media which are suitable for use with the various embodiments of the present invention described herein include colour forming substrates that may incorporate various forms of a diacetylene material. Diacetylenes (DA) that require heating to form an active state are particularly applicable. There are two types of DAs: 'heat activated⁵ (HA) and 'reversible'. The HA version is transformed from an inactive form to an active form when it is heated above a critical temperature and allowed to cool and re-crystalize in the active form. The 'reversible' form of the DA is essentially only active, to UV radiation, within a particular temperature range or above a critical temperature. When the substrate is allowed to cool it returns to the inactive state. In either case the active phase of the material is sensitive to UV radiation and on exposure polymerizes to form a colour, particularly, but not exclusively, the colour blue. In addition, the activation temperature and colour transformation temperatures can be different and are preferred to be different so that activation and colour formation can be differentiated.

Suitable media characteristics for use with configuration 200 are described below.

Example substrate 1 (Heat Activated)

This substrate requires the following steps:

1. Activate the substrate by heating individual pixels with NIR to the activation temperature as required;

2. Wait for a cooling/ re- crystallisation time;

3. Expose individual activated pixels to UV radiation (preferably in the wavelength range 245nm - 350nm) to form the first colour (blue) as required; 4. Expose un-activated individual pixels to intense/ higher fiuence UV radiation (preferably in the wavelength range 245nm ~ 350nm) to form the third colour (yellow) as required; and

5. Illuminate individual first colour (blue) pixels with NIR or visible radiation (400-680nm) to form the second colour (red). Alternatively, the following sequence of steps may be used:

1. Expose un-activated individual pixels to intense UV radiation (preferably in the wavelength range 245nm - 350nm) to form the third colour yellow as required;

2. Activate individual pixels by heating with NIR to the activation temperature as required;

3. Wait for a cooling/ re-crystallisation time;

4. Expose individual activated pixels to UV (preferably in the wavelength range 245nrn - 350nm) to form the first colour (blue) as required; and

5. Illuminate individual first colour (blue) pixels with NIR or visible radiation (400-680nm) to form the second colour (red) as required.

Example substrate 2 (Heat Activated)

This substrate requires the following steps:

2. Wait for a cooling/ re-crystallisation timer

3. Expose individual activated pixels to UV (preferably in the wavelength range 245nm - 350nm)radiation to form first colour (blue) as required;

4. Illuminate individual first colour (blue) pixels with NIR or visible radiation (400-680nm) to form second colour (red) or third colour (yellow) as required.

Example substrate 3 (Heat Activated - high temperature) The activation temperature of this material is above the second colour (red) and third colour (yellow) transition temperatures. The sequence of steps required is:

1. Activate each pixel with NIR to a temperature which is above the third colour (yellow) thermal transition;

2. Illuminate with UV (preferably in the wavelength range 245 nm - 350nm) to form the first colour (blue), either:

a. flood illuminate if the background is not sensitive to UV; or b. illuminate only the required pixels if background stability is compromised by exposure to UV radiation and

3. Heat the required first colour (blue) pixels with NiR or visible radiation (400nm-680nm) to produce the second colour (red) and/or the third colour (yellow) as required.

Figure 4b shows a configuration 300 that differs from apparatus 100 in that it comprises a broad UV source 220 which is used to flood illuminate the substrate rather than the source 108. A first NIR system 301 is used to activate each pixel as required and the broad UV source 220 (preferably in the wavelength range 245nm - 350nm) flood illuminates the substrate to form the first colour (blue) as required. A shield 221 is provided to establish an area free from the UV radiation. Once an area of the substrate moves past the shield, a second NIR or visible subsystem 302 (preferably in the wavelength range 400nm - 680nm) illuminates individual pixels with NIR or visible radiation to form the second colour (red) or third colour (yellow) as required. Suitable media characteristics for use with configuration 300 are described below.

Example substrate 4 (Reversible)

This substrate requires the following steps:

1. Activate the substrate by heating individual pixels with NIR to the activation temperature as required and irradiate with UV (preferably in the wavelength range 245nm - 350rrm) to form the first colour (blue);

2. Illuminate individual first colour (blue) pixels with NIR or visible radiation (400-680nm) to form the second colour (red) or third colour (yellow) as required.

Figure 4c shows another configuration 400 that uses a single NIR source 101 and associated optical system and a broad flashed UV source 220. In this embodiment there is a single NIR subsystem and a pulsed broad UV source, Furthermore, in this embodiment during the time that is required for the output point (not shown) of the beam steering unit 106 to track a single dot width on the substrate 11 1, the substrate 11 1 is exposed to a sequence of NIR, UV and NIR radiation. In particular, the NIR source 101 may be actuated while the output point moves from its first position, to approximately half way between its first position and its second position so as to activate the substrate 111 ; next the UV lamp is flashed for initial polymerisation and initial colour formation; and finally while the output point moves from approximately half way between its first position and its second position to its second position the NIR source 101 is activated again to effect the further colour changes. Alternatively, for approximately the first half of the radiation time the substrate may be exposed to NIR and UV simultaneously and for the remainder of the radiation time the substrate may be exposed to NIR radiation only.

Suitable media characteristics for use with configuration 400 are described below.

Example substrate 5 (Reversible)

This substrate requires the following steps:

1. Activate the substrate by heating individual pixels with NIR to the activation temperature as required

2. Irradiate with UV (preferably in the wavelength range 245nm - 350nm) to form the first colour (blue);

3. Illuminate individual first colour (blue) pixels with NIR or visible radiation (400-680nm) to form the second colour (red) or third colour (yellow) as required.

Alternatively, the following sequence of steps may be used:

1. Activate the substrate by heating individual pixels with NIR to the activation temperature as required and irradiate with UV (preferably in the wavelength range 245nm - 350nm) to form the first colour (blue);

2. Illuminate individual first colour (blue) pixels with NIR to form the second colour (red) or third colour (yellow) as required.

The embodiment 500 shown in Figure 4d is a variant that uses two monolithic source arrays: an NIR or visible source array 101 and a UV sources array 108 with a beam displacement switch and beam transformation and tracking options as described in the sections above. It should be noted that this variation can also be applied to the configurations shown in figures 4a, 4b and 4c above. For embodiments that require simultaneous emission of NIR and UV radiation or NIR, visible and UV radiation a particularly attractive combination will be to employ a source, such as certain laser systems, that emits NIR and UV radiation or NIR, visible and UV radiation simultaneously as with second and fourth harmonic generation of visible and UV radiation from the fundamental NIR radiation.

In each of the embodiments discussed above, single or multiple mechanical tilting mirrors may be used to keep the radiation beams synchronised to the substrate. Or, alternatively, single or multiple wavelength dependent scanning technologies, such as electro-optic or acousto-optic, could be employed the appropriate technology depends on product speed.

It is of course to be understood that the invention is not to be restricted to the details of the above embodiments which have been described by way of example only.

In particular, as will be appreciated by those skilled in the art, in each of the embodiments discussed above, the order of certain elements in each optical system may be altered without departing from the intended scope of the present application.

Claims

An apparatus which is suitable for irradiating a substrate which moves relative thereto, the apparatus comprising: a radiation source which is operable to emit a beam radiation; a beam steering unit which is operable to receive the radiation emitted by the radiation source and transmit it to a moveable output point; a monitoring means which is operable to monitor the motion of a substrate which it is desired to irradiate; and a processing means which is operable to receive information from the monitoring means and in response thereto transmit instructions to: the radiation source so as to control the emission of radiation therefrom; and the beam steering unit so as to control the movement of the output point

An apparatus as claimed in claim 1 wherein the output point is operable to move so that the projection of the beam of radiation emitted by the output point on to the substrate is substantially stationary with respect to the substrate.

An apparatus as claimed in claim 2 wherein the movement of the output point comprises a translation and wherein the output point is operable to move parallel to the substrate and at substantially the same speed as the substrate.

An apparatus as claimed in claim 2 wherein the movement of the output point comprises a rotation of the output point so that the angle with which the radiation is incident upon the substrate is altered continuously from a first angle to a second angle.

An apparatus as claimed in claim 4 wherein the range of angles of incidence is less than or equal to 10 degrees.

6. An apparatus as claimed in claim 4 or claim 5 wherein for the entire range of angles the radiation emitted by the radiation means is incident on the substrate in a direction which is substantially perpendicular to its surface.

7. An apparatus as claimed in any one of claims 4 to 6 wherein the rotation is achieved by any suitable means, for example, by rotation of a mirror at the output point.

8. An apparatus as claimed in any preceding claim wherein the output point is operable to move so that the distance that the projection of the beam of radiation onto the substrate moves is substantially the same as, or slightly smaller than, the width of the beam of radiation.

9. An apparatus as claimed in any preceding claim wherein monitoring the motion of a substrate involves monitoring the position and/or speed of a region of the substrate upon which the radiation is incident.

10. An apparatus as claimed in any preceding claim wherein the spatial intensity distribution emitted by the radiation source is substantially uniform over the beam profile.

1 1. An apparatus as claimed in any one of claims 1 to 9 wherein a non-uniform spatial intensity distribution is emitted by the radiation source so as to control the pattern of colour within a single dot or pixel.

12. An apparatus as claimed in any preceding claim wherein the spatial intensity distribution emitted by the radiation source may be time dependent so that different distributions are used for subsequent irradiations.

13. An apparatus as claimed in any preceding claim wherein the radiation emitted by the radiation source is of a suitable wavelength and intensity to effect a colour change to the area of the substrate upon which it is incident.

14. An apparatus as claimed in any preceding claim wherein the radiation source comprises one or more arrays of multiple sources.

15. An apparatus as claimed in claim 14 wherein the one or more arrays of multiple sources comprises a linear array or a two dimensional array.

16. An apparatus as claimed in claim 15 wherein said two dimensional arrays are regular two dimensional arrays.

17. An apparatus as claimed in claim 15 wherein adjacent rows or columns of the two dimensional arrays are staggered.

18. An apparatus as claimed in claim 17 wherein for each such row or column in the array there is at least one other row or column that is aligned therewith.

19. An apparatus as claimed in any one of claims 14 to 18 wherein the beam steering unit comprises a plurality of output points, one for each source.

20. An apparatus as claimed in claim 19 wherein the processing means is able to move each of the output points independently.

21. An apparatus as claimed in any preceding claim wherein the radiation source is operable to emit more than one wavelength of radiation.

22. An apparatus as claimed in claim 21 wherein the radiation source comprises a first array of multiple laser sources which are operable to emit infra-red (I ) or near infra-red (NIR) radiation and a second array of multiple laser sources which are operable to emit ultra-violet (UV) radiation.

23. An apparatus as claimed in claim 21 wherein the radiation source comprises: a first array of multiple laser sources which are operable to emit infra-red (IR) or near infra-red (NIK) radiation; a second array of multiple laser sources which are operable to emit ultra-violet (UV) radiation; and a third array of multiple laser sources which are operable to emit infra-red (IR), near infra-red (NIR) radiation or visible radiation.

24. An apparatus as claimed in claim 21 wherein the radiation source comprises: a first array of multiple laser sources which are operable to emit infra-red (IR) or near infra-red (NIR) radiation; a second sources which comprises a lamp operable to emit ultra-violet (UV) radiation; and a third array of multiple laser sources which are operable to emit infra-red (IR), near infra-red (NIR) radiation or visible radiation.

25. An apparatus as claimed in claim 21 wherein the radiation source comprises: a first array of multiple LED sources which are operable to emit infra-red (IR) or near infra-red (NIR) radiation; a second array of multiple LED sources which are operable to emit ultra-violet (UV) radiation; and a third array of multiple LED sources which are operable to emit infra-red (IR), near infra-red (NIR) radiation or visible radiation.

26. An apparatus as claimed in any preceding claim wherein the apparatus further comprises a conveying means which is operable to move a substrate relative to the radiation source.

27. An apparatus as claimed in claim 26 wherein the conveying means arranges the substrate so that radiation emitted by the radiation means is incident on the substrate in a direction which is substantially perpendicular to its surface.

28. An apparatus as claimed in claim 26 or claim 27 wherein the conveying means moves the substrate substantially continuously.

29. An apparatus as claimed in claim 26 or claim 27 wherein the conveying means may move the substrate incrementally.

30. An apparatus as claimed in any one of claims 26 to 27 wherein the conveying means moves the substrate at a substantially continuous speed.

31. An apparatus as claimed in any preceding claim wherein the processing means is operable to ensure that whilst the radiation source is emitting radiation the output point moves with a direction and speed substantially matching that of the region of the substrate upon which the radiation is incident.

32. An apparatus as claimed in any preceding claim wherein the output point has a start position and the processing means is operable to ensure that when the radiation source stops emitting radiation the output point returns to the start position.

33. An apparatus as claimed in claim 32 wherein the time taken for the output point to return from a stop position, at which the radiation source stops emitting radiation, to the start position is substantially shorter than the time taken to move the output point from the start position to the stop position.

34. An apparatus as claimed in any preceding claim wherein the shape of the beam emitted by the radiation source is square or rectangular.

35. An apparatus as claimed in claim 34 wherein the square or rectangular beam shapes are achieved by use of any or all of the following: square or rectangular apertures; diffraction beam shaping; a short length of square or rectangular section optical fibre connected to the end of an optical fibre and mounted into beam optics; square or rectangular waveguides including monolithic blocks with refractive index changes written into the block by direct femto-second lasers, proton exchange_s metal in diffusion; epitaxial grown waveguides; tapered waveguides; or liquid waveguides.

36. An apparatus as claimed in any preceding claim when dependent either directly or indirectly upon claim 14 further comprising micro-optical elements to correct for positional errors in the array of multiple sources.

37. An apparatus as claimed in any preceding claim when dependent either directly or indirectly upon claim 14 further comprising apparatus to adjust the timing of the operation of the radiation sources so as to correct for positional errors in the array of multiple sources.

38. An apparatus as claimed in any preceding claim further comprising micro-optics which are used to minimise divergence of the beam and hence beam size.

39. An apparatus as claimed in any preceding claim wherein the apparatus further comprises a pixel mask which is operable to transform the shape and spatial

' intensity distribution of the beam as desired or required.

40. An apparatus as claimed in claim 39 wherein the pixel mask allows for the spatial intensity distribution produced to change on a dot to dot basis in real time.

41. An apparatus as claimed in claim 39 or claim 40 wherein the pixel mask comprises a tilting mirror MEMS device to generate the required pattern.

42. An apparatus as claimed in any one of claims 39 to 40 wherein the pixel mask comprises a patterned transparent electrode structure deposited onto an electro

ns - optic crystal to create mico-pixels wherein the phase mask can be altered by altering the voltage that is applied to each micro-pixel.

43. An apparatus as claimed in any one of claims 39 to 40 wherein the pixel mask comprises any or all or the following: optically addressed spatial light modulators (OASLM), magneto-optics, liquid crystals.

44. An apparatus as claimed in any preceding claim wherein the beam steering unit comprises any or all of the following: electro-optic devices; acousto-optic devices; micro mechanical devices; or mechanical tilting mirrors.

45. An apparatus as claimed in any preceding claim wherein the beam steering unit is wavelength dependent.

46. An apparatus as claimed in any one of claims 1 to 44 wherein the beam steering unit is independent of the wavelength of the radiation it transmits.

47. An apparatus as claimed in any preceding claim wherein the beam steering unit comprises any or all of the following:

(a) Electro-optic technology employing Pockels effect, Kerr effect, or total internal reflection (TIR) in materials such as:- Lithium Niobate, SBN60 (Sr o.e Bao.4 Nb₂O_s), SBN-61, SBN-75, SBN50, KTN (KTa0₃ & KNb0₃), Silicon Carbide, KDP, KD*P (larger r63 coeff.), ADP, Potassium Niobate-3, BBO beta barium borate Zinc Sulphide, Zinc Selenide, BSO, Zinc Telluride, CsH2AS04, Barium Titanate, Barium Calcium Titanate, Barium Strontium Titanate, Lithium Tantalate, Strontium Titanate, Silver Arsenic Sulfide, Potassium Tantalum Niobium Oxide, Gallium Arsenide, Copper Chloride, Quartz Lithium lodate, Polymers, Chalcogenides, BSKNN (Ba_2-x Sr_x Ki_-yNa_y Nb₅0₁₅), PBN60 (Pbi.x Ba_x Nb₂0₅), KN,SNN; (b) Acousto-optic devices employing Tellurium Dioxide, Quartz, fused quartz, Lead Molybdate (PbM0₄), extra-dense flint glass, SPS, KRS-5 carbon disulphide, nitro-benzene, nitro-toluene, Lithium Niobate including Lithium

9-4-

Niobate doped with Fe and Fe ions, Lithium tantalite, Lithium fluoride, Rutile (Ti0₂), sapphire, alpha iodic acid (IIIO );

(d) Nematic liquid crystals, Smectic liquid crystals, Blue phase liquid crystals, Cholesteric liquid crystals;

(e) Magneto-optics steering via refractive index control;

, An apparatus as claimed in any preceding claim wherein the beam steering unit employs a translation device that comprises any one or all of the following: piezo-electric devices, magnetostriction devices, voice coils or linear motor translators.

49. An apparatus as claimed in any preceding claim wherein the radiation source is suitable for effecting a colour change in the substrate.

50. An apparatus as claimed in claim 48 wherein the substrate comprises a diacetylene material.

51. An apparatus as claimed in claim 50 wherein the diacetylene material is combined with an IR or NIR absorbing material or is provided over a layer of IR or NIR absorbing material.

52. A method of irradiating a substrate comprising the steps of;

(i) providing an apparatus as claimed in any one of claims 1 to 51 ;

(ii) monitoring the motion of the substrate with the monitoring means;

(iii) operating the radiation source to emit a beam of radiation; and

53. A method as claimed in claim 52 further comprising the steps of:

(v) storing the start position of the output point; and

54. A method as claimed in claim 53 further comprising the step of repeating sub- steps (iii) to (vi) a plurality of times.

55. A method as claimed in any one of claims 52 to 54 wherein the distance tlirough which the projection of the beam of radiation emitted by the output point on to the substrate moves is substantially the same as, or slightly smaller than, the width of the beam emitted by the radiation source.

56. A method as claimed in any one of claims 52 to 55 wherein the time taken for the output point to return from a stop position, at which the radiation source stops emitting radiation, to the start position at step (vi) is substantially shorter than the time taken to move the output point from the start position to the stop position at step (iv).

57. A method as claimed in any one of claims 52 to 56 wherein the method further comprises the step of controlling the movement of the substrate relative to the apparatus.

58. A method as claimed in claim 57 wherein the substrate is controlled by way of a conveying means.

59. A method as claimed in claim 58 wherein the substrate is controlled so that radiation emitted by the radiation source is incident on the substrate in a direction which is substantially perpendicular to its surface.

60. A method as claimed in claim 58 or 59 wherein the substrate is controlled so as to move substantially continuously at a substantially continuous speed.

61. A method as claimed in claim 60 wherein the distance through which the projection of the beam of radiation onto the substrate moves is smaller than the width of the beam by at least the distance through which the substrate moves whilst the radiation source returns to the start position.

62. A method as claimed in claim 58 or 59 wherein the substrate is controlled so as to move incrementally wherein the movement is made up by repeating the following steps: moving the substrate by a distance substantially equal to the width of the beam emitted by the radiation source in a first time period; and keeping the substrate at rest for a second time period.

63. A method as claimed in any one of claims 52 to 62 further comprising the step of intentionally introducing a mismatch between motion of the output point and the motion of the substrate in order to create coloured fringe patterns as required or desired.

64. A method as claimed in claim 63 wherein the intentional mismatch between the motion of the output point and the substrate is used to adjust the width of lines created in a direction perpendicular to the direction of substrate motion.

65. A method as claimed in any one of claims 51 to 63 wherein the substrate comprises a layer of diacetylene material on a base layer.

66. A method as claimed in any one of claims 51 to 63 wherein the substrate incorporates diacetylene material.

67. A method as claimed in claim 65 or claim 66 wherein the diacetylene material or diacetylene material layer is combined with an I or NIR absorbing material or is provided over a layer of IR or NIR absorbing material.

68. A method as claimed in claim 66 or claim 67 wherein the diacetylene material or diacetylene material layer in combined with an IR or NIR absorbing material or is provided over a layer of IR or NIR absorbing material.