WO2007049064A1

WO2007049064A1 - A method of laser marking a surface

Info

Publication number: WO2007049064A1
Application number: PCT/GB2006/004038
Authority: WO
Inventors: Paul Harrison; Matt Henry; Jozef Wendland
Original assignee: Powerlase Limited
Priority date: 2005-10-28
Filing date: 2006-10-27
Publication date: 2007-05-03
Also published as: EP1963044A1; JP2009513362A; GB0522087D0

Abstract

A method of laser marking a surface having a relatively non- diffusely reflective surface region is provided. The method comprises irradiating with laser light a selected marking region of the relatively non-diffusely relative surface region in order to create a relatively diffusely reflective region. The radiation process comprises irradiating partially overlapping region to create the marking region.

Description

A METHOD OF LASER MARKING A SURFACE

The invention relates to a method of laser marking a surface.

Laser marking of surfaces is well known, the marking apparatus typically comprising a laser of >1W output power, which can be focussed by means of a lens to a spot size with sufficient energy density to indelibly mark a material through a thermal or photochemical change. Any appropriate wavelength laser can be used that will be efficiently absorbed by the surface to be marked.

A typical example is that of a CO₂ laser with a wavelength in the infrared which would rapidly mark plastic. Referring to Fig. 1 the apparatus includes a laser 100, the beam collimated with a beam expander 102 and directed into a galvanometric scanner 104 including galvanometric scanning mirrors 106, 108 which rapidly deflect the beam 110 into a focussing lens 112, which focuses the laser beam on the substrate surface 114 to be marked. The scanning mirrors 106, 108 dither at high speed to rapidly move the laser beam across the surface in the two orthogonal directions to create precise marks at high speed, typically 0.1 - 10 m/s.

Mechanisms for such marking can be divided into two categories: marking by surface removal and marking by surface modification.

In the first instance removing material with a laser creates a visible mark: either by melting, vaporisation or photochemical decomposition, in the case of high- energy ultraviolet lasers. The resulting surface morphology creates a readable high contrast mark.

In the second instance the laser radiation affects the material composition to create a high contrast mark without material removal. It may locally melt the material, cause it to oxidise or chemically alter to form a visible mark. The approaches adopted vary dependent on the material being marked. For example, steel is highly reflective at longer laser wavelengths (> 95% for CO₂ lasers at 10.6μm). At shorter wavelengths, reflectivity reduces to allow sufficient energy absorption to create a mark. Typically a Nd: YAG laser at 1.064μm wavelength would be employed where reflectivity is 85-95% depending on surface finish. Shorter wavelengths would also be effective.

Some materials are difficult to mark because of their optical properties or because the resulting marks have poor contrast. Examples would include uncoated aluminium (reflective), glass (transparent and brittle) and ceramics (reflective and poor quality marks).

Particular problems arise for example in the case of aluminium. Aluminium is extremely difficult to mark with a laser as is it highly reflective at infrared and visible wavelengths. In addition if a mark is achieved it tends to be of low contrast and so difficult to read optically.

A known strategy which can be understood from Figs. 2a and 2b, is to anodise the aluminium to allow marking. Anodising is an electro-chemical process by which the surface of an aluminium workpiece 200 is coated with a deep layer 202 of aluminium oxide. This oxide layer is harder than bulk aluminium and corrosion resistant. This oxide layer can also be coloured for cosmetic purposes.

Aluminium oxide is strongly absorbing of laser radiation in the infrared. So it is possible to selectively remove the oxide layer 202 from the reflective aluminium 200 beneath using a laser beam 204 to create a high contrast mark 206 (Fig 2b) where the oxide layer has been removed. This is an industrially standard way of achieving a laser mark on aluminium. However a disadvantage with this approach is that the region that is marked no longer benefits from the corrosion resistance and hardness of the anodised layer. Also in terms of mark appearance, the mark is limited to having a bright reflective appearance against the plain or coloured anodised layer.

An alternative known strategy is to make the anodised layer with chemical additives such that the layer changes colour, rather than being removed, when irradiated with laser light. A further alternative strategy is to apply an intermediate layer to the material for example in the form of a coating or a film. The laser irradiated film adheres locally to the substrate after which the non- irradiated material can be removed to leave a mark.

However all of these approaches require the creation and application of a layer of anodised material or film increasing the complexity of the operation and making it more time consuming.

The invention is set out in the claims. Because of the recognition that all that is required is to create relatively diffusely reflecting regions on a relatively non- diffusive reflective surface, a simple and speedy processing approach is provided. Furthermore, a mark or indicia created in this manner is easily readable as a reading beam detector can be positioned so as only to detect diffusely reflected light allowing a simple reading configuration with a wide range of possible positions for the detector. Embodiments of the invention will now be described, by way of example, with reference to the drawings, of which:

Fig. 1 is a schematic diagram showing a known laser marking system; Fig. 2a is a schematic side view showing marking an anodised aluminium substrate; Fig. 2b is a schematic side view showing a marked anodised aluminium substrate; Fig. 3 is a schematic view showing a marking scheme according to an embodiment of the present invention;

Fig. 4a and 4b are schematic views showing a reading scheme according to an embodiment of the present invention; Fig. 5 shows a pattern of laser pulses, each leaving a pit or crater on the surface creating a roughened region.

In overview, a marking system can be understood with reference to Fig. 3. A laser generates a beam scannable in directions designated generally A and B by a scanner 300 relative to a workpiece 302 having a non-diffusely reflecting surface or surface region such as a generally specularly reflective or transmissive surface. The laser irradiates a selected marking region or regions 304 and, in particular, traverses the marking region or regions so as to roughen those regions such that they are rendered generally diffusely reflective or scattering. As a result, a readable surface indicia such as a barcode can be created on the surface 302 by patterning the marking regions 304 appropriately.

The approach described with reference to Fig. 3 is particularly advantageous when the indicia comprises a bar code which can subsequently be read using the system shown in overview in Figs. 4a and 4b. The system includes a reading beam generator 400 such as a low power laser scannable in direction C, along the barcode 402. Referring to Fig. 4a, the laser 400 directs a reading beam 404a towards the workpiece 406. When the reading beam 404a is incident on a non-diffusely reflecting region 408a, such as a specularly reflective region as shown in Figs 4, then the beam is specularly reflected as shown at 404b. Referring to Fig. 4b, when the reading beam is directed as shown at 404c to a roughened region 408b then the beam is scattered or diffusely reflected as shown at 404d.

Where, in Fig. 3, a negative image of a bar code is created, the roughened regions corresponding to the white (scattering) regions of a conventional bar code it will be seen, therefore, that if a detector 410 is positioned so as to collect scattered light 404d rather than, for example, specularly reflected light 404b, the barcode 402 is reconstructed in exactly the same manner as if it were a conventional barcode read by a detector positioned to collect scattered light from white regions corresponding to the roughened region. In particular, because the roughened area scatters more light back to the detector than the non-diffusive untreated areas, the laser marked region reads white whilst the unmarked reflective surface reads black.

As discussed, in more detail below, embodiments of the invention provide a technique and procedure for altering the surface roughness of target materials, using a high-power pulsed laser, to create barcodes which are readable in conventional barcode readers.

Referring once again to Figure 1, the surface roughening approach uses an infrared Q-switched solid-state laser such as the commercially available Starlase AO2 Nd: YAG laser supplied by Powerlase Ltd, Crawley, UK. This pulsed laser 100 provides energy of the order of 10^"3 to 10^"2 J in 10^"7 to 10^"8 s duration pulses with kHz repetition rates. The output beam is collimated and expanded by a beam expander 102 and directed into a galvanometric scanner 104. The laser beam is diverted by the scanner 104, comprising two adjustable mirrors 106 and 108, onto a focusing lens 112. A target or workpiece 114, comprising, in this embodiment of uncoated aluminium, is positioned in an incident plane held a predetermined perpendicular distance from the lens 112 so as to provide a constant spot size, in the present embodiment of the order of < 200 μm, across the incident plane. The mirrors 106 and 108 are configured to control the positioning of the focused laser spot in two orthogonal directions within the given incident plane.

The scanner operates to move the laser spot around the target incident plane, at rates of the order of 0.1 to 10 m/s, to provide irradiance at selected surface areas of the target 114. The scanning mirrors are controlled by a controller (not shown) of any appropriate type comprising for example, hardware for physically moving the mirrors interfaced with computer-based software pre-programmed with a desired pattern for irradiance such as a barcode pattern. For example, a scanner of the type available from Scanlab GmbH (Munich, Germany) may be used.

The output of the laser is matched to the target 114 to provide suitable energy absorption. The chosen perpendicular distance from lens to incident plane and the selected laser parameters (such as pulse energy, pulse width, irradiance and repetition rate) ultimately control the power density incident on the target 114, as discussed in more detail below. Furthermore, the sequence of pulses are overlapped as will be described in greater detail below.

To achieve surface roughening, that is a relatively diffusely reflecting region rather than material removal, the most important optical parameters are power density, pulse duration, pulse energy, repetition rate and a technique of overlapping of a train of pulses; an understanding of the effects of each of these parameters allows identification of an appropriate working regime. For optimum operation, power density must be sufficient to achieve a localised phase change and is a function of pulse duration, focal spot size and pulse energy. Appropriately high energy densities at the work piece allow effective coupling to this reflective material; each pulse removing a few cubic microns of material to create a tiny pit in the substrate. With thousands of pulses available per second, it is possible to rapidly machine away material to create a surface with a controlled roughness.

Pulse duration is important; short pulses allow rapid removal of material allowing controlled or regular roughening and avoiding bulk substrate damage. Sufficient pulse energy and power density is required to create a useful volume of material removal per pulse. In particular, sufficient absorption is required to initiate a phase change in the target. Once the surface begins to melt or vaporise, the absorption co-efficient rises exponentially; this can make laser processing energy efficient even with metals which are normally considered highly reflective.

A sufficient repetition rate is required at these parameters to allow fast enough roughening to make for a viable industrial process. Control of pulse overlap allows control of the roughness. This process can produce regular or irregular surface morphology depending on parameters employed to achieve a sufficient roughening effect.

All parameters are selected to provide roughening (i.e. relatively diffusely reflective or scattering region on a relatively non-diffusively reflective surface). The region must provide sufficient contrast that the detector distinguishes them, for example, average surface roughness (Ra) greater than 5 μm.

A single laser pulse causes the formation of a small crater or pit in the surface of the target as material is removed. Each pit roughens the surface very slightly but using a combination of overlapping pits the entire surface is roughened to the extent that it becomes distinguishable from the relatively non-diffusively reflective regions.

The material removal rate increases with laser pulse repetition rate. However, this is only partially explained by the increased average laser power delivered to the target. Each laser pulse can be considered as raising the temperature of the target and vaporising material. It follows that higher removal rates will be achieved if less laser energy is used heating the target up to the vaporisation point. The higher the substrate initial temperature, the more laser pulse energy is used to actually remove material rather than just heat it up. Higher material removal rate can be achieved by increasing the substrate temperature. In an embodiment, this is accomplished by overlapping laser pulses so that each point on the surface is subjected to several consecutive laser pulses. Due to the special profile of the laser beam and the thermal diffusivity of the substrate, each laser beam acts both to input heat into the substrate and to remove material. The effect of subjecting the surface to several consecutive laser pulses, each of which inputs heat into the substrate, is to significantly raise the average surface temperature. However, if the temperature is raised too high a melt pool is created and material removal is very violent and fast but the quality of the mark is poor and thermal damage to the target may result. The optimum surface temperature depends on the material properties, the pulse overlap, pulse repetitions frequency and irradiance.

The partial heating regime described above provides an efficient and cost effective method of removing surface material. It will be appreciated however that it is possible to slow this process down to allow cooling of the substrate, for example if the temperature is approaching a point at which a melt pool would be created. Additionally, it is possible to use a laser with negligible heat transfer to remove surface material, such as an ultra fast laser or Excimer.

Although material removal rate determines how quickly the target can be marked, the surface roughness of the mark determines how efficient it is as a diffuse reflector. Embodiments of the present invention consider the trade-off between material removal rate and surface diffuseness.

Correlating the movement of the focal spot and the laser repetition rate, allows for a controlled overlapping of a sequence of pulses. A suitable overlap, combined with the appropriate power density and target material, creates well- defined regions of sufficiently increased surface roughness. In one embodiment, surface roughening is achieved on an aluminium target using the laser parameters given in the following paragraph and traversing the focal spot across the target as follows (refer to Fig 5 for clarity): In the first stage, a straight line (in the A direction) of overlapping pits is created by moving the focal spot 50% the spot diameter between pulses; when the desired length is achieved, one step equal to 50% the spot diameter is made in the orthogonal direction (B direction); steps of 50% the spot diameter are then used in the opposite direction to the first stage (the negative A direction) to return the pulse to the original A co-ordinate; a step of 50% the spot diameter in the same orthogonal direction (positive B direction) is used to move the spot along; and the sequence repeated until a roughened area with the desired dimensions has been formed. This may be repeated for a small number of passes. Alternatively the pits may not be overlapped but sufficiently close to cause appropriate roughening.

In a modification not shown, lines of pulses (each pulse having a fixed pulse to pulse overlap) overlay adjacent lines of pulses by a certain amount. Depending on the laser settings used, scan-line overlap can be anything between 0 and 99%. In an embodiment, the scan-line overlap is 20 to 50 %.

The skilled person will be aware that other techniques for overlapping pulses can be used to raise the average temperature of the target during laser-marking.

In particular, according to one embodiment, surface roughening is achievable on an aluminium target at ambient temperature with a laser of wavelength 1064 nm, at which aluminium is > 80% reflective but the short pulse duration and small focused spot size selected allow for sufficient energy transfer to the aluminium. Optimal conditions for aluminium are found with a pulse energy of 10 mJ and a pulse duration of 90 ns. However, adequate surface roughening is found over a range of pulses energies (4 to 25 mJ) and durations (30 to 160 ns). The optimal focused spot size is 160 μm with irradiances greater than 10⁷ W/cm² (most preferably 5.5 x 10⁸ W/cm²) at a scanning rate of 1.5 m/s. Using a smaller laser spot but higher pulse power could achieve the necessary power density (> 10⁷ W/cm²) but it would take longer to roughen a given area. It will be seen that appropriate operational parameters can be determined for other materials by routine experimentation.

The preferred pulse repetition rate is 20 kHz, however surface roughening has been observed in the range 3 to 50 kHz. The overlapping of pulses is important to create the most effective roughened surface.

In embodiments of this invention, laser roughening can be achieved with a laser spot of any spatial profile but preferably Gaussian, "Super-Gaussian" or homogenised (i.e. a "top-hat" profile). In the present embodiment the "Super-Gaussian" Starlase AO2 (M = 22) was used to give better control over the size and homogeneity of the craters. However, it will be apparent to the skilled person that a non-uniform profile containing localised hot spots and large fluctuations in intensity across the spot diameter may, in fact, be equally desirable as this is likely to produce a more localised roughening. Furthermore, a Diffractive Optic Element (DOE) or beam masking technique may be employed to produce a controlled irradiance pattern with a laser with any spatial profile.

A focusing lens with a long focal length is preferential to provide tolerance on the precise positioning of the target relative to the incident plane.

The parameters specified in the above embodiment have been optimised using aluminium as the target material for marking. The roughening layer is of the order of 50 μm deep and no dependence on grain size is found. The surface finish prior to marking is preferably selected for optimum performance. Before laser-roughening, the material needs to be substantially non-diffusive to light, including the 635 nm red laser-diode light conventionally used to read the barcodes. Also, any resulting oxide layers would need to exhibit the same non- diffusive nature to allow diffuse reflections off intentionally laser-roughened regions to be distinguishable. The material must have a reasonably smooth finish prior to marking to avoid unwanted reflections diffuse or otherwise and the finish may be imparted by laser polishing as known by the skilled reader.

Advantageously, embodiments of the described type of laser marking are particularly suitable for materials normally considered highly reflective and difficult to laser mark, such as metals. Furthermore, embodiments provide a one-stage rapid manufacturing process; and achieve a rate of a few seconds per mark. Moreover, the need to coat a target surface first is eliminated, overcoming various problems and disadvantages associated with such coating. Furthermore, the bulk properties of the marked component are unaffected. Roughening the surface of the uncoated aluminium surface creates a mark with relatively low contrast but that is readable by eye.

The small size of the laser spot used for roughening allows for high resolution barcodes; meaning more information per unit area. These barcodes are applied directly to a target, can be driven by software and are difficult to counterfeit or damage. Embodiments of the invention have been found to be particularly effective with aluminium as the target.

It will be appreciated that the technique described herein can be extended to any laser capable of achieving the specifications set out herein and any target material (e.g. copper or silver) given correct appropriate energy coupling from laser to target. Alternative wavelength lasers could be employed to achieve the same roughening effect. Candidates would include Q-switched CO₂ lasers at 10.6 μm wavelength, frequency converted solid state lasers at 532 nm, 355 nm and 266 nm wavelength and Excimer lasers in the ultraviolet.

Any target material with substantial optical properties in at any wavelength would be suitable for this process including transparent materials such as glass in which case instead of having specularly reflecting regions, the "black" portion of the barcode would be provided by un-roughened transmissible regions through which the reading beam simply passes such that light is only collected from the roughened region by a detector.

It has been demonstrated that carefully controlled channels of roughened and smooth regions can be created according to embodiments of this invention which can, for example, be used to represent a machine readable indicia such as a barcode as described below.

A conventional barcode reader of the type shown in Fig. 4a and 4b consists more specifically of: a low power laser, usually a laser-diode emitting a reading beam in the red part of the spectrum although any appropriate laser and wavelength can be implemented; a scanning device, such as a mirror (not shown) which oscillates rapidly to scan the laser beam in one-dimension along the bar; a set of collection optics which point at the target area and act as an objective; a light detection system, usually a photodiode, which responds to light collected by the collection optics; and a control unit which uses hardware and software to control the scanning device, interpret the photodiode signal and provide an output representative of the barcode. Conventional barcodes consist of a series of black lines of varying thickness printed on a white background. Where the low power laser is scanned across the barcode 402, the black and white lines reflect a different fraction of the incident light back towards a photodiode allowing detection and subsequent reconstruction of the barcode

Embodiments of the invention utilise the recognition that the laser-roughened regions 304 scatter more light 408b than the relatively smooth untreated areas 408a of our target material 30 and that industrial barcode readers are capable of detecting the contrast in roughness produced by this technique.

In particular, it is found that at an angle to the target material normal, laser- roughened regions, created as described herein, 408b scatter significantly more incident light 404a than the naturally smooth and reflective untreated regions 408a. Figure 4a shows the barcode laser 400 incident on a relatively smooth region 408a of a target material. The aluminium surface is highly reflective and most of the light is reflected 404b away from the barcode reader. In this case, the photodiode 410 receives no substantial back scattered light. However, the situation in Figure 4b shows that incident light 404c from the laser 400 is substantially scattered 404d by the roughened region 408b of the target. The photodiode 410 collects some of this light 404c and reads it as being equivalent to "white" in a conventional barcode. Relatively smooth, untreated regions 408a are effectively invisible to the system; equivalent to "black" in a conventional barcode. Hence, whereas a conventional barcode exhibits a contrast in colour or reflectivity, barcodes created in accordance with embodiments of this invention exhibit contrast in surface roughness. In operation, the incident laser beam 404a is scanned in one direction across the barcode. As the laser passes over relatively rough 408b and smooth 408a regions, and the photodiode 410 detects the contrast in the roughness of the target the controller monitors, with suitable calibration, the frequency, duration and spacing of these marks allowing reading of the digital information stored within.

According to an embodiment the work piece to which the laser beam is directed is a metallic surface having a reflective coating such as paint or lacquer. The laser beam is used to remove the coating and expose the metallic substrate underneath. This achieves the effects as described for uncoated aluminium above, wherein the coated or painted region acts as a specular reflector and the laser machined metallic region acts as the diffusive reflector. This embodiment can find application in applying security and or traceability barcodes on components and vehicles after they have been painted.

Any appropriate reader, such as commercially available barcode readers are suitable for reading these barcodes. It is found that conventional barcode readers are remarkably tolerant of the incident angle and barcodes created according to embodiments of this invention can be read by angling to the surface anywhere in the range 5 to 85 degrees to the surface normal. The invention can find application on a production line for marking components (for example; the body, chassis or engine of a motor vehicles, or the like).

Whereas discussion is made of one dimensional barcodes, it is equally apparent that the invention could utilise any barcode symbology, or other machine readable indicia, including 2D symbologies and barcodes that can be read whether stationary or moving together with appropriate reader position so as to detect the contrast between the regions. Barcodes, based on these principles, could be created by alternative physical mechanisms for roughening the appropriate region of a surface to form diffusely reflecting portions such as solenoids or laser peening (e.g. as available from Telesis).

This technique can be used in all industrial sectors for creating readable barcodes on reflective materials like aluminium. For example, applications include: traceability of components, sub-assemblies and assemblies on a production line; part origin traceability post-sale; and authenticity verification for replacement and spare parts (the process being too difficult and expensive to counterfeit). Direct industrial application examples would include: marking body-in-white car bodies on production lines for traceability and automated production; marking components and subassemblies for traceability and automated production in automotive manufacturing; marking of high value automotive components such as engine parts for security; and the marking of airframe and turbine engine parts for safety, traceability and security. Equally, any industry where finished products, components or sub-components would benefit from having barcodes permanently marked onto them, before reaching the point-of-sale, could utilise from this technology.

Claims

CLAIMS:

1. A method of laser marking a surface having a relatively non-diffusely reflective surface region, the method comprising irradiating with laser light a selected marking region of the relatively non-diffusively reflective surface region, to create a relatively diffusely reflective region; comprising irradiating partially overlapping regions to create said marking region.

2. A method as claimed in claim 1 in which the laser wavelength is

1064 nm.

3. A method as claimed in claim 1 or 2 in which the pulse energy is 4 to 25 mJ, preferably 10 mJ.

4. A method as claimed in any preceding claim in which the pulse duration is 30 to 160 ns, preferably 90 ns.

5. A method as claimed in any preceding claim in which the spatial profile of the laser spot is Super-Gaussian.

6. A method as claimed in any preceding claim in which the laser is operated with a power density greater than 10⁷ W/cm², preferably 5.5 x 10⁸ W/cm².

7. A method as claimed in any preceding claim in which the laser is focused to a spot size from 1 to 500 μm.

8. A method as claimed in any preceding claim in which the partially overlapping regions are arranged such that the relatively diffusely reflective region is created by a first irradiating laser pulse followed by a second overlapping irradiating pulse.

9. A method as claimed in claim 8 wherein each of said first and second pulses raises the average temperature of the marking region.

10. A method as claimed in any preceding claim in which a pair of pulses overlap by 0 to 99 % of the focused spot diameter, preferably 40 to 60% and more preferably 50 to 55%.

11. A method as claimed in any preceding claim in which the surface comprises one of a reflective surface or a transmissive surface, particularly, aluminium, glass, silicon and copper.

12. A method as claimed in any preceding claim in which the surface comprises a metal having a reflective coating, particularly paint or lacquer

13. A method as claimed in any preceding claim comprising a method of marking a substrate surface with a machine-readable indicia.

14. A method as claimed in claim 13 in which the indicia comprises a bar code formed of said diffusely reflective regions separated by non- irradiated portion of said generally non-diffusely reflective region.

15. A method as claimed in claim 14 in which the diffusely reflective regions correspond to the white, reflective or scattering regions of a conventional barcode.

16. A method of controlling a laser to create a machine readable indicia on a surface having a relatively non-diffusely reflective surface region comprising scanning laser light along selected regions of the relatively non-diffusively reflective surface region to create relatively diffusely reflective regions; comprising irradiating partially overlapping regions to create said relatively diffusely reflective regions.

17. A laser apparatus for marking a surface having a relatively non- diffusely reflective surface region to create a machine readable indicia, the apparatus comprising: a laser arranged to irradiate a selected marking region of the relatively non-diffusely reflective surface region to create a relatively diffusely reflective region; a controller for controlling the laser incidence to create said indicia; in which the laser is arranged to irradiate partially overlapping regions to create said relatively diffusely reflective region.

18. An apparatus as claimed in claim 17 in which the laser wavelength is 1064 nm.

19. An apparatus as claimed in either claim 17 or 18 in which the pulse energy is 4 to 25 mJ, preferably 10 mJ.

20. An apparatus as claimed in any of claims 17 to 19 in which the pulse duration is 30 to 160 ns, preferably 90 ns.

21. An apparatus as claimed in any of claims 17 to 20 in which the spatial profile of the laser spot is Super-Gaussian.

22. An apparatus as claimed in any of claims 17 to 21 in which the laser is operated with a power density greater than 10⁷ W/cm², preferably 5.5 x 10⁸ W/cm².

23. An apparatus as claimed in any of claims 17 to 22 in which the laser is focused to a spot size from 1 to 500 μm.

24. An apparatus as claimed in any of claims 17 to 23 arranged to irradiate the partially overlapping region, to create the relatively diffusely reflective region, with a first laser pulse followed by a second overlapping laser pulse.

25. An apparatus as claimed in claim 24 wherein, in use, each of said first and second pulses raises the average temperature of the marking region.

26. An apparatus as claimed in claim 25 in which the apparatus is arranged to overlap a pair of pulses 0 to 99 % of the focused spot diameter, preferably 40 to 60% and more preferably 50 to 55%.

27. An apparatus as claimed in any of claims 17 to 26 in which the machine readable indicia comprises a barcode.

28. A method of creating a machine readable indicia on a surface having a relatively non-diffusively reflective surface region comprising creating a relatively diffusely reflective region thereon.

29. A substrate having a surface with a machine readable indicia comprising at least one relatively non-diffusively reflective region and at least one relatively diffusely reflective region.

30. A method as claimed in any of claims 1 to 16 further comprising laser polishing the surface region prior to creating the relatively diffusely reflective regions.

31. A method or apparatus substantially as described herein with reference to the drawings.