JP2009513362A - Laser marking method for surface - Google Patents

Abstract

A method is provided for laser marking a surface having a reflective surface region that is relatively non-diffusive. The method includes a process of irradiating selected marking areas of a relatively non-diffusive relative surface area with laser light to produce a relatively diffusive reflective area. The radiation process includes illuminating a partially overlapping region to create a marking region.
[Selection] Figure 4

Description

  The present invention relates to a surface laser marking method.

  Laser marking on the surface is well known and the marking device typically has a laser with an output greater than 1 W, which is a spot with sufficient energy density to permanently mark the material by thermal or photochemical changes. Can be focused by lens to size. Any suitable wavelength laser that can be efficiently absorbed by the surface to be marked can be used.

A typical example is a CO ₂ laser with an infrared wavelength that quickly marks a plastic. Referring to FIG. 1, the apparatus comprises a laser 100, and a beam 110 collimated by a beam expander 102 is directed to a galvanometer scanner 104 that includes galvanic scanning mirrors 106, 108, which are diffracted by the beam. Quickly deflect 110 to a focusing lens 112, which focuses the laser beam onto the substrate surface 114 to be marked. The scanning mirrors 106, 108 oscillate at high speeds to rapidly move the laser beam across the surface in two orthogonal directions to produce accurate marks, typically at 0.1-10 m / s. To do.

  Such marking mechanisms can be divided into two categories: marking by surface removal and marking by surface deformation.

  In the first example, the material removed by the laser produces a visible mark by melting, evaporation or photochemical decomposition in the case of a high energy ultraviolet laser. The resulting surface morphology produces a readable high contrast mark.

  In the second example, laser radiation affects the composition of the material to produce high contrast marks without removing the material. This can locally melt the material to oxidize or chemically change it to form a visible mark.

The method employed will vary according to the material being marked. For example, glass has high reflectivity at long laser wavelengths (> 95% for 10.6 μm CO ₂ laser). At short wavelengths the reflectivity decreases to allow sufficient energy absorption to produce the mark. A 1.064 μm wavelength Nd: YAG laser is typically used when the reflectivity is 85-95% according to the surface finish. Shorter wavelengths are also efficient.

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

  A particular problem arises, for example in the case of aluminum. Aluminum is extremely difficult to mark with a laser because it is highly reflective at infrared and visible wavelengths. Even if more marks are obtained, they tend to be low contrast and are therefore difficult to read optically.

  A known method that can be understood from FIGS. 2 (a) and (b) is to anodize aluminum to allow marking. Anodizing is an electrochemical process whereby the surface of the aluminum workpiece 200 is coated with a deep layer 202 of aluminum oxide. This oxide layer is harder than bulk aluminum and resists collisions. This oxide layer can also be colored for cosmetic purposes.

  Aluminum oxide strongly absorbs laser radiation in the infrared. Thus, the laser beam 204 can be used to selectively remove the oxide layer 202 from the reflective aluminum 200 to produce a high contrast mark 206 (FIG. 2b) from which the oxide layer is removed. it can. This is an industry standard method for realizing laser marks on aluminum.

  However, the disadvantage of this method is that the marked area no longer has the advantages of resistance to corrosion and the robustness of the anodized layer. Also, with regard to the appearance of the mark, the mark is limited to have a bright reflective appearance with respect to a single color or colored anodized layer.

  An alternative known method is to make a layer anodized with chemical additives so that when irradiated with laser light, the layer changes color rather than being removed. Yet another method is to apply an intermediate layer to the material, for example in the form of a coating or thin film. The laser irradiated thin film adheres locally to the substrate, after which the unirradiated material can be removed to leave marks.

  However, all these methods require the generation and application of a layer or thin film of anodized material, which increases the operational complexity and time.

  The invention is set forth in the following claims. Because of the recognition that all that is required is to create a relatively diffusive reflective area on a reflective surface that is not relatively diffusive, a simple and quick processing method is provided. Furthermore, the marks or indicators generated in this way are easily readable and can be positioned so that the reading beam detector only detects diffusive reflected light, allowing a wide range of detector possibilities This enables a simple reading structure with various positions.

An embodiment of the present invention will be described by way of example with reference to the drawings.
In overview, the marking system can be understood with reference to FIG. The laser produces a beam that can be scanned in the direction normally designated by A and B by the scanner 300 with respect to a workpiece 302 having a reflective surface or surface area that is not normally diffusive, such as a specular or transmissive surface. To do. The laser illuminates the selected marking area (s) 304, particularly traversing the marking area (s) to roughen these areas, so that they usually have diffusive reflective or scattering properties. Is done. As a result, a readable surface indicator such as a barcode can be generated on the surface 302 by appropriately patterning the marking area 304.

  The method described with reference to FIG. 3 is particularly advantageous when the indicator comprises a bar code that can subsequently be read using the system outlined in FIGS. 4 (a) and (b). It is valid. The system includes a read beam generator 400 such as a low power laser that can scan in direction C along a barcode 402. Referring to FIG. 4A, the laser 400 directs the reading beam 404a toward the workpiece 406. When the read beam 404a is incident on a non-diffusible reflective area 408a, such as a specular area as shown in FIG. 4, the beam is specularly reflected as shown at 404b. Referring to FIG. 4b, the read beam is directed to a roughened region 408b as shown at 404c and the beam is scattered or diffusely reflected as shown at 404d.

  If a bar code negative in FIG. 3 is generated, ie a roughened surface area corresponding to the white (scattering) area of a normal bar code, then the detector 410 will scatter rather than specularly reflected light 404b, for example Bar code 402 is read by a detector that is positioned to collect light scattered from the white area corresponding to the roughened area. It can be seen that it is reconstructed in exactly the same way as if it were a barcode. In particular, the roughened area scatters more light back to the detector than the non-diffusible raw area, so that the laser marked area shows white and the unmarked reflective surface shows black. Show.

  As described above, in the following more detailed description, embodiments of the present invention use a high power pulsed laser to generate a barcode that can be read by a conventional barcode reader. Techniques and procedures for changing roughness are provided.

Referring back to FIG. 1, the surface roughening method uses an infrared Q-switched solid state laser such as the commercial Starlase AO2 Nd: YAG laser supplied by Crawley Powerlase Ltd, UK. The pulse laser 100 provides energy of about 10 ^{−3 to} 10 ⁻² J with a pulse rate of 10 ^{−7 to} 10 ⁻⁸ s with a repetition rate of kHz. The output beam is collimated, expanded by a beam expander 102, and directed to a galvanometer scanner 104. The laser beam is deflected to a focusing lens 112 by a scanner 104 that includes two adjustable mirrors 106 and 108. A target or workpiece 114 composed of uncoated aluminum in this embodiment is pre-determined from the lens 112 to provide a constant spot size across the incidence plane, in the embodiment of the present invention, on the order of 200 μm or less. Located in the plane of incidence held at a vertical distance. Mirrors 106 and 108 are configured to control the position of the laser spot focused in two orthogonal directions within a predetermined incident plane.

  The scanner operates to move the laser spot around the target incidence plane at a speed on the order of 0.1 to 10 m / s to provide radiation at a selected surface area of the target 114. The scanning mirror can be any suitable type of control device (including any hardware) for physically moving the mirror that is interfaced with computer-based software pre-programmed in the desired pattern for radiation, such as a barcode pattern. (Not shown). For example, a scanner of the type available from Scanlab GmbH (Munich, Germany) can be used.

  The output of the laser is matched to the target 114 for proper energy absorption. The selected vertical distance from the lens to the plane of incidence and the selected laser parameters (such as pulse energy, pulse width, radiation and repetition rate) are ultimately the power incident on the target 114 as described in detail below. Control the density. In addition, the sequence of pulses is overlapped as described in detail below.

  The most important optical parameters are power density, pulse duration, pulse energy, repetition rate, and series of pulse train overlap techniques to achieve surface roughening, i.e., relatively diffuse reflective areas rather than material removal Understanding the effect of each of these parameters allows for the identification of an appropriate mode of operation. For optimal operation, the power density must be sufficient to achieve localized phase changes and is a function of pulse duration, focal spot size, and pulse energy. A reasonably high energy density at the workpiece allows efficient coupling to this reflective material, with each pulse removing a few cubic microns of material to create small pits in the substrate. If thousands of pulses per second are available, the material can be quickly machined out to produce a surface with controlled roughness.

  The pulse duration is important, short pulses allow for rapid material removal, allow controlled or regular roughening, and prevent bulk substrate damage.

  Sufficient pulse energy and power density are required to perform an effective amount of material removal per pulse. In particular, sufficient absorption is required to initiate the target phase change. As the surface begins to melt or evaporate, the absorption coefficient increases exponentially, which can make laser processing energy efficient, even for metals that are usually considered highly reflective.

  Sufficient repetition rates are required with these parameters to allow sufficiently high speed roughening for viable industrial processes. Control of pulse overlap allows control of roughening. This process can produce a regular or irregular surface morphology depending on the parameters used to achieve a sufficient roughening effect.

  All parameters are selected for roughening (i.e., relatively diffusive reflective or scattering regions on a reflective surface that is not relatively diffusive). The regions must provide sufficient contrast, for example with an average surface roughness (Ra) greater than 5 μm, so that the detector can distinguish them.

  A single laser pulse results in the formation of small craters or pits on the surface of the target as material is removed. Each pit has a very slightly rough surface, but using a combination of overlapping pits, the entire surface is roughened to such an extent that it can be distinguished from a relatively non-diffusive reflective area.

  The material removal rate increases with the repetition rate of the laser pulses. However, this is explained in part by the increased average laser power transmitted to the target. Each laser pulse can be thought of as raising the temperature of the target and the evaporation material. As a result, a high removal rate is achieved if less laser energy is used to heat the target to the vaporization point. The higher the initial temperature of the substrate, the more laser pulse energy is required to actually remove the material rather than simply heating it.

  High material removal rates can be achieved by increasing the temperature of the substrate. In one embodiment, this is accomplished by overlapping the laser pulses so that each point on the surface receives several consecutive laser pulses. Due to the special profile of the laser beam and the thermal diffusion of the substrate, each laser beam both applies heat to the substrate and removes material. The effect of exposing the surface to several successive laser pulses, each applying heat to the substrate, significantly increases the average surface temperature. However, if raised too high, a molten pool is created and material removal becomes very violent and fast, but the quality of the mark can be reduced and thermal damage to the target can occur. The optimum surface temperature depends on the material properties, pulse overlap, pulse repetition frequency, and radiation.

  The partial heating mode described above provides an efficient and cost effective way to remove surface material. However, it will be appreciated that if the temperature approaches the point at which, for example, a molten pool is created, the speed of this process can be reduced to allow the substrate to cool. Furthermore, it is possible to use a laser such as an ultra high speed laser or excimer with negligible heat transfer to remove surface material.

  The material removal rate determines the rate at which the target can be marked, while the roughness of the mark surface determines its efficiency as a diffusive reflector. Embodiments of the present invention allow for a compromise between material removal rate and surface diffusivity.

  The correlation between focal spot movement and laser repetition rate allows controlled overlap of the sequence of pulses. Proper power density and proper overlap combined with the target material produce well defined regions of sufficiently increased surface roughness. In one embodiment, surface roughening is achieved with an aluminum target using the laser parameters given in the following paragraphs and moving the focus across the target as described below (for clarity). For this purpose, see FIG. In the first stage, the overlapping pit (A direction) straight lines are generated by moving 50% of the focal spot diameter between pulses, and when the desired length is achieved, the spot diameter of 50% One step equal to is performed in the orthogonal direction (B direction) and the 50% spot diameter step is then used in the opposite direction (negative A direction) to return the pulse to the origin A of the coordinates. , A 50% spot diameter step in the same orthogonal direction (positive B direction) was used to move the spot and the sequence was repeated until a roughened area with the desired size was formed . This can be repeated with a few passes. Instead, the pits are not overlapped, but can be close enough to cause proper roughening.

  In a variant not shown, a line of pulses (each pulse with a fixed pulse and a pulse overlap) overlaps an adjacent pulse line by a certain amount. Based on the laser setting used, the scan line overlap can be anywhere between 0 and 99%. In one embodiment, the scan line overlap is 20-50%.

  Those skilled in the art will recognize that other techniques for overlapping pulses can be used to increase the average temperature of the target during laser marking.

In particular, according to one embodiment, surface roughening can be achieved on an aluminum target at ambient temperature by a laser with a wavelength of 1064 nm, where aluminum is more than 80% reflective but has a short pulse duration and small focus. The selected spot size tied to allows sufficient energy transfer to the aluminum. The optimal state of aluminum is observed with a pulse energy of 10 mJ and a pulse duration of 90 ns. However, proper surface roughening is observed in the pulse energy range (4-25 mJ) and duration (30-160 ns). The optimal focal spot size is 160 μm and is obtained with radiation exceeding 10 ⁷ W / cm ² (most preferably 5.5 × 10 ⁸ W / cm ² ) at a scanning speed of 1.5 m / s.

Using a small laser spot with high pulse power can achieve the required power density (> 10 ⁷ W / cm ² ), but it takes a long time to roughen a given area. It will be appreciated that appropriate operating parameters can be determined for other materials by routine experimentation.

  The preferred pulse repetition rate is 20 kHz, but surface roughening is observed in the range of 3-50 kHz. Pulse overlap is important to produce the most efficient rough surface.

In embodiments of the present invention, laser roughening can be achieved with a laser spot of any spatial profile, but is preferably a Gaussian, “Super Gaussian” or homogenized profile (ie, “Top”). Hat profile). In embodiments of the present invention, “Super Gauss” Starlase AO2 (M ² = 22) was used to give better control over crater size and uniformity. However, localized hot spots and non-uniform profiles that contain large variations in intensity across the spot diameter are in fact desirable as well, as this can produce more localized roughening. It will be apparent to those skilled in the art that In addition, diffractive optical elements (DOE) or beam masking techniques can be used to generate a radiation pattern controlled by a laser having an arbitrary spatial profile.

  A focusing lens having a long focal length is advantageous in that it provides tolerance for accurate positioning of the target with respect to the plane of incidence.

  The parameters specified in the previous embodiment are optimized using aluminum as the target material for marking. The roughened layer is about 50 μm deep, and no dependence on the grain size is seen. The surface finish before marking is preferably selected for optimum performance. Prior to laser roughening, the material should be substantially non-diffusible to light, including the 635 nm red laser diode light normally used to read barcodes. Also, any resulting oxide layer needs to exhibit the same non-diffusive properties to allow diffusive reflections from regions that are intentionally laser-roughened to be distinguishable. . The material must have a reasonably smooth finish before marking to prevent undesired reflection diffusion etc., and the finish should be given by laser polishing as known by those skilled in the art Can do.

  The described laser marking type embodiment is particularly suitable for materials that are usually highly reflective and difficult to laser mark, such as metal. Furthermore, the embodiment provides a one-step rapid manufacturing process and achieves a speed of a few seconds per mark. Furthermore, the need to coat the target surface is first eliminated, and various problems and disadvantages associated with such coating are overcome. Furthermore, the bulk properties of the marked component are not affected. Roughening the surface of the uncoated aluminum produces a mark that is relatively low contrast but readable by the eye.

  The small size of the laser spot used for roughening allows high resolution barcodes, which means that more information per unit area is available. These barcodes are given directly to the target, can be driven by software, and are difficult to counterfeit or damage. Embodiments of the present invention have been found to be particularly efficient when using aluminum as a target.

The techniques described herein can be extended to any laser capable of achieving the specifications described herein, and any target material (eg, copper or silver) that is provided with the correct proper energy coupling from the laser to the target. It will be recognized that it can. Different wavelength lasers can be used to achieve the same roughening effect. Candidates include 10.6 μm wavelength Q-switched CO ₂ lasers, 532 nm, 355 nm and 266 nm frequency converted solid state lasers, and ultraviolet excimer lasers.

  Any target material with substantial optical properties of any wavelength, including transparent materials such as glass, is suitable for this process, in which case the barcode “black” instead of having a specular region. The portion is provided by a non-roughened transmissive area through which the reading beam passes and the light is collected only from the area roughened by the detector.

  Carefully controlled channels of roughened and smooth areas can be generated according to embodiments of the present invention, such as a machine readable indicator such as a barcode as described below. It can be used to represent

  A conventional bar code reader of the type shown in FIGS. 4a and 4b is more particularly a laser diode which usually emits a read beam in the red part of the spectrum, but any suitable laser. And a low power laser that can be implemented, a scanning device such as a mirror (not shown) that vibrates rapidly to scan the laser beam in one dimension along the bar, and a target area directed A set of collection optics that act as an objective lens, a light detection system such as a regular photodiode that responds to the light collected by the collection optics, controls the scanning device, interprets the photodiode signal, and barcodes And a controller that uses hardware and software to provide an output representative of. A typical bar code consists of a series of black lines of varying thickness printed on a white background. When a low power laser scans across the barcode 402, the black and white lines reflect the incident light back in the direction of the photodiode in different fractions, allowing detection and subsequent barcode reconstruction. To.

  Embodiments of the present invention have a laser roughened area 304 that scatters more light 408 than a relatively smooth unprocessed area 408a of the target material 30 of the present invention, whose individual bar code reader is Utilizes the recognition that the contrast with the roughness generated by the technique can be detected.

  In particular, at an angle normal to the target material, the laser-roughened region 408b generated as described herein is much more incident than the natural, smooth and reflective unprocessed region 408a. It has been discovered to scatter light 404a. FIG. 4A shows a barcode laser 400 that is incident on a relatively smooth region 408a of the target material. The aluminum surface is highly reflective and most of the light is reflected in the direction of 404b away from the bar code reader. In this case, the photodiode 410 does not receive substantially backscattered light. However, the state of FIG. 4B shows that the incident light 404c from the laser 400 is substantially scattered by the roughened region 408b of the target as substantially indicated by 404d. The photodiode 410 collects some of this light 404c and reads it as equivalent to the “white” color of a normal barcode. The relatively smooth and unprocessed region 408a is effectively invisible to the system and is equivalent to the normal black “black”. Thus, normal barcodes exhibit contrast in color or reflectance, but barcodes generated according to embodiments of the present invention exhibit contrast with surface roughness.

  In operation, the incident laser beam 404a is scanned in one direction across the barcode. When the laser passes over a relatively rough surface 408b region and a smooth surface 408a region, and the photodiode 410 detects the contrast in the target roughness, the controller will adjust the frequency and duration of these marks with appropriate calibration. Periods and intervals are monitored and digital information stored therein can be read.

  According to one embodiment, the workpiece from which the laser beam is directed is a metal surface with a reflective coating such as a paint or lacquer. The laser beam is used to remove the coating and expose the underlying metal substrate. This achieves the effect as described for the uncoated aluminum described above, with the coated or painted area acting as a specular reflector and the laser machined metal area acting as a diffuse reflector. . This embodiment can find application in security applications and / or traceability barcodes on components and vehicles after painting.

  Any suitable reader, such as a commercially available barcode reader, is suitable for reading these barcodes. While normal bar code readers have significant tolerances to the angle of incidence, bar codes generated according to embodiments of the present invention can be any angle in the range of 5 to 85 degrees with respect to the surface normal. It has been discovered that can be read by. The invention can find application in the production line of marking components (e.g. motor vehicle body, chassis or engine).

  While a one-dimensional bar code has been described, the present invention is not limited to any bar code code or other machine readable indicator that includes a two-dimensional code, either stationary with an appropriate reader position to detect the contrast between regions. It is equally obvious that a bar code that can be read to see if it is working can be used. Barcodes are based on these principles and can be used to roughen appropriate areas of the surface to form diffusive reflective parts such as solenoids or laser peening (eg, as available from Telesis). It can be generated by the physical mechanism of

  This technique can be used in all industrial fields to produce readable barcodes on reflective materials such as aluminum. For example, applications include component tracking capabilities, sub-assemblies and assemblies in production lines, tracking capabilities of post-sale component manufacturers, replacement and / or authenticity of spare parts (the process is very difficult and expensive So it cannot be counterfeited). For direct industrial applications, the body in a production line for tracking capability and automated production forms a mark on the body of a white automobile, and the components and sub-components for tracking capability and automated production in automotive manufacturing It includes assembly markings, marking of expensive automotive parts such as engine parts for security, marking of airframe and turbine engine parts for safety, tracking capability and security, and the like. Similarly, any industry that benefits from barcodes with finished products, parts or subcomponents permanently marked on them can utilize this technology before reaching the sales floor.

1 is a schematic diagram showing a known laser marking system. The schematic side view which shows the marking of the anodized aluminum substrate, and the schematic side view which shows the marked anodized aluminum substrate. Schematic which shows the marking system by one Embodiment of this invention. 1 is a schematic diagram illustrating a reading method according to an embodiment of the present invention. FIG. 5 shows a pattern of laser pulses leaving pits or craters on the surface that produces the roughened area, respectively.

Claims (31)

  In a relatively diffusive process that includes irradiating a selected marking area of a reflective surface area that is relatively non-diffusible with a laser beam and irradiating a partially overlapping area to produce the marking area. A method of forming a mark with a laser on a surface having no reflective surface area.   The method of claim 1, wherein the laser wavelength is 1064 nm.   3. A method according to claim 1 or 2, wherein the pulse energy is between 4 and 25 mJ, preferably 10 mJ.   4. A method according to any one of the preceding claims, wherein the pulse duration is 30 to 160 ns, preferably 90 ns.   The method according to claim 1, wherein the spatial profile of the laser spot is Super Gaussian. 6. A method according to any one of the preceding claims, wherein the laser is operated at a power density greater than 10 < ⁷ > W / cm < ² >, preferably 5.5 * 10 < ⁸ > W / cm < ² >.   7. A method according to any one of the preceding claims, wherein the laser is configured to focus at a spot size of 1 to 500 [mu] m.   The partially overlapping region is configured such that a relatively diffusive reflective region is generated by a first irradiation laser pulse followed by a second overlapping irradiation pulse. The method of any one of these.   9. The method of claim 8, wherein each of the first and second pulses increases an average temperature of the marking area.   10. A pair of pulses according to any one of the preceding claims, wherein the pairs of pulses overlap by 0 to 99%, preferably 40 to 60%, more preferably 50 to 55% of the diameter of the focused spot. Method.   11. A method according to any one of the preceding claims, wherein the surface is a reflective surface or a transmissive surface, particularly comprising one of aluminum, glass, silicon, copper.   12. The method according to claim 1, wherein the surface is made of a metal with a reflective coating, in particular a paint or lacquer.   13. A method according to any one of the preceding claims, comprising a method of marking a substrate surface with a machine readable indicator.   14. The method of claim 13, wherein the indicator comprises a bar code formed from the diffusive reflective area separated by non-irradiated portions of the normally non-diffusible reflective area.   The method of claim 14, wherein the diffusive reflective region corresponds to a white, reflective or scattering region of a normal barcode. In a method of controlling a laser to produce a machine readable indicator on a surface having a reflective surface area that is relatively non-diffusive,
Scanning laser light along selected areas of a reflective surface area that is not relatively diffusive to produce a relatively diffusive reflective area, and partially to produce the relatively diffusive reflective area A method including a process of irradiating an overlapping area.   In a laser apparatus that forms a mark on a surface having a reflective surface area that is relatively non-diffusible to produce a machine readable indicator, the apparatus is relatively diffusive to produce a relatively diffusive reflective area. A laser configured to illuminate a selected marking area of a non-reflective surface area and a controller for controlling laser incidence to generate the indicator, the laser comprising the relatively diffusive reflection A laser apparatus configured to irradiate a partially overlapping region to generate a region.   The apparatus of claim 17, wherein the laser wavelength is 1064 nm.   19. A device according to claim 17 or 18, wherein the pulse energy is between 4 and 25 mJ, preferably 10 mJ.   20. Apparatus according to any one of claims 17 to 19, wherein the pulse duration is 30 to 160ns, preferably 90ns.   21. Apparatus according to any one of claims 17 to 20, wherein the spatial profile of the laser spot is Super Gauss. Device according to any one of claims 17 to 21, wherein the laser is operated at a power density greater than 10 ⁷ W / cm ² , preferably 5.5 × 10 ⁸ W / cm ² .   23. Apparatus according to any one of claims 17 to 22, wherein the laser is focused with a spot size of 1 to 500 [mu] m.   Claims configured to irradiate a partially overlapping region with a first irradiating laser pulse followed by a second overlapping laser pulse to produce a relatively diffusive reflective region Item 24. The apparatus according to any one of Items 17 to 23.   25. The apparatus of claim 24, wherein in use, each of the first and second pulses increases an average temperature of a marking area.   26. The apparatus of claim 25, wherein the apparatus is configured to overlap a pair of pulses by 0-99%, preferably 40-60%, more preferably 50-55% of the diameter of the focused spot. .   27. Apparatus according to any one of claims 17 to 26, wherein the machine readable indicator comprises a bar code. In a method of generating a machine readable indicator on a surface having a reflective surface area that is relatively non-diffusive,
A method including a process for generating a relatively diffusive reflective region thereon.   A substrate having a surface with a machine readable indicator, the substrate comprising at least one non-diffusible reflective region and at least one relatively diffusive reflective region.   17. A method according to any one of the preceding claims, further comprising a process in which the laser polishes the surface area before producing the relatively diffusive reflective area.   A method or apparatus substantially as herein described with reference to the drawings.

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