EP3894233A1

EP3894233A1 - Method of imperceptible marking

Info

Publication number: EP3894233A1
Application number: EP19798287.9A
Authority: EP
Inventors: Martin Kucera; Vladislav LANG
Original assignee: University of West Bohemia
Current assignee: University of West Bohemia
Priority date: 2018-11-12
Filing date: 2019-11-05
Publication date: 2021-10-20
Anticipated expiration: 2039-11-05
Also published as: WO2020099177A1; CH716802B1; GB201818362D0; GB2578889A; EP3894233B1; DE112019003468T5; CZ202123A3

Abstract

The invention is directed to a method of imperceptibly marking an object and a method of detecting such marking. The object suitably has a first and a second surface parallel to the first surface wherein the second surface is imperceptibly marked, which marking is detected by illuminating the first surface with electromagnetic radiation. The reflected radiation reveals the marking made on the second surface. Such markings may be used for security purposes, such as to prevent unauthorised copying or to verify the authenticity of a product.

Description

METHOD OF IMPERCEPTIBLE MARKING

The invention is directed to a method of imperceptibly marking an object and a method of detecting such marking. The marking may be used for security purposes, such as to prevent unauthorised copying or to verify the authenticity of a product.

Background of the Invention

Invisible or imperceptible marking systems which are ordinarily not visible to the naked eye, but are revealed under certain specific circumstances, are extremely useful in a variety of contexts, including product authentication, identification and anti-counterfeiting and theft prevention and recovery. One common method of invisible marking is the use of an ink which fluoresces upon illumination with UV light.

It is also sometimes desirable to reduce the size of the marking that needs to be invisible to a size that cannot be seen with the naked eye. Detection of such markings is only possible with the use of a microscope. This method is used e.g. for marking of electronic components.

Another known method is the irreversible modification of the product using a laser. The marking becomes visible when irradiated with electromagnetic radiation at a wavelength outside the visible range of 400 to 700 nm. This method is primarily applied to plastics.

According to J. Phys. D: Appl. Phys. 42 (2009) 042004, laser markings are also detectable using the difference in condensation between the laser- marked areas and unmarked areas.

Infrared Physics & Technology 43 (2002) 171-174 teaches detection of invisible laser markings overlaid with paint layers using an Infrared-reflection technique in conjunction with a focal plane camera. EP0714353 mentions the invisible marking of glass and plastic using a method of carbon dioxide laser to create stress that is invisible to the naked eye, but which is capable of being rendered visible under polarized light.

Chinese magic mirrors are known for capturing an embossed pattern on the back of the mirror which is subsequently revealed on a screen placed in front of the mirror when a light is shone on the polished front face of the mirror. This is described in, for example, Eur. J. Phys., 27, (2006) 109-1 18.

However, these mirrors require casting of pattern on the back side of the substrate, which is visible on the back surface. Furthermore, casting has limited variability in terms of the pattern produced.

All of the methods mentioned above have some disadvantages. Some methods are time-consuming, affect the properties and appearance of the substrate, while some methods require sophisticated detection using special lights such as polarised light, UV light, etc.

The invention addresses these and other problems of the prior art.

Summary of the invention

According to a first aspect, the invention provides a method of imperceptibly marking the first surface of an opaque object comprising the steps of:

i. providing an opaque object comprising a first surface and a

substantially coplanar second surface;

ii. distressing the second surface;

characterised in that imperceptible marks are produced on the first surface, which are detectable by illuminating the first surface with electromagnetic radiation and measuring the reflected radiation.

According to a second aspect, the invention provides a distressed material obtainable by a method of the invention. According to a third aspect, the invention provides a label comprising the distressed material of the invention.

Brief description of the Figures

Figure 1 is a photograph

Figure 2 is a photograph

Figure 3 is a photograph

Figure 4 is a photograph

Figure 5 is a photograph

Figure 6 is a photograph

Figure 7 is a photograph

Figure 8 is a photograph

Figure 9 is a photograph

Figure 10 is a photograph

Figure 1 1 is a photograph

Detailed Description of the Preferred Embodiments

The present invention provides a method of altering the reflective properties the first surface of a material comprising distressing the second surface. Using the inventive method, the appearance of the first surface is minimally affected, such that ordinarily the difference in reflective properties cannot be detected except under certain specific conditions.

According to a preferred embodiment, the present invention provides a method of marking a first surface of a material by exposure of the opposite (second) surface to a laser, which is configured to trace a pattern on the second surface. The laser treatment forms markings on the first surface that are invisible to the naked eye. The laser marking may be detected when electromagnetic radiation, preferably light, preferably in the visible region of the spectrum, such as in the range of 360 to 780 nm, is shone on to the first surface of the substrate at an angle such that the reflected light forms an image on a suitable viewing surface. The pattern marked by the laser on the material is visible in the reflection.

In an alternative embodiment, distressing of the second surface may be accomplished by mechanical means. Suitable mechanical means include scoring, sand blasting, shot blasting, scratching or embossing. The first surface of the material is minimally affected, such that ordinarily the difference in reflective properties cannot be detected except under certain specific conditions.

Although the methods of the invention can be applied to a range of materials, they are most suited to laminar or substantially laminar materials, namely those having first and second surfaces that are substantially parallel. The first and/or second surfaces of such laminar materials are preferably substantially planar, although they may also be be textured, e.g. having a degree of rugosity, curved and/or inclined.

The methods of the invention are useful for processes such as applying labels with bar codes, date codes, QR codes, serial numbers, part numbers or adding copyright / trademarks and logos to a number of different surfaces in many industries. Another use is the addition of a date value to materials such as metals and plastics; examples are a best before date or maintenance due date. Additionally the marking methods of the invention is useful in the creation of banknotes, ID and smartcards offering high degrees of fraud prevention.

Although laser marking is known and used for some of these purposes, it suffers from the disadvantage that it is visible in normal use. This is undesirable in certain circumstances, as it may detract from the aesthetic appearance of the article being marked. Moreover, in certain applications such as fraud prevention, it may be desirable to not disclose the existence of a security feature, which can remain concealed until its present needs to be revealed by an authenticating authority. It has been found that the process according to the present invention is simpler and easier to perform compared to the prior art mentioned above. Additionally, an adaptation of the applied marking ensures precise placement of the mark which may also be very fine and detailed. Furthermore, in contrast to prior art methods, it is very easy to adapt the inventive method to mark a different pattern on consecutive pieces of material, as, for example, marking a number of components to enable individual identification of each one.

Suitable laser marking systems are continuous wave (cw) or pulsed CO2 lasers and yttrium aluminium garnet (YAG), e.g., Nd:YAG lasers where the marking is accomplished by the heat of the applied laser beam. The wavelengths of the pulses produced by these systems are within the visible or infrared spectrum. Such lasers are conventionally employed for engraving, soldering and welding wherein, the case of marking, the surface layer of the material is melted, ablated or vaporized to produce discernible indicia or pattern.

CO2 lasers may also be used. These have been principally employed for marking plastic surfaces, such as 1C packages.

It is preferred that the laser employed in the process of the invention is a fibre laser. A fibre laser is a laser whose lasing medium is an optical fibre that is doped with a suitable dopant such as ytterbium, neodymium, erbium or thulium.

A preferable laser marking system for implementing the invention is a pulsed fiber laser, such as a pulsed fibre laser average output power of from 5 to 100 W, preferably from 10 to 50 W, more preferably from 15 to 40 W, such as around 20 W. A suitable laser marking system used for the purpose of demonstrating the effects of the invention is pulsed fiber laser SPI G3 HS of average output power 20 W available from SPI Lasers UK Ltd, of

Southampton, United Kingdom. The power density required to stimulate thermal interactions at the surface of the material will, of course, depend upon the material of the body and the speed at which the beam is scanned. Materials such as Perspex® may be marked using a beam having a power density of as little as approximately 50 W/cm², while to mark metals it is necessary for the beam have a power density approximately 1 MW/cm². Bodies made of glass fall between these two extremes and may be marked using a beam having a power density of in excess of 300 W/cm² and a scanning speed of 3 m/sec.

The laser is preferably focussed on the second surface of the material as a spot. In order to form the pattern on the second surface, it is necessary to move the spot over the second surface. The laser spot may be guided by any suitable means. Alternatively, the material may be moved to produce pattern by the laser beam. Generally, the spot will be guided by a laser head, being the assembly from which the laser beam exits toward the work piece. A laser head generally does not contain the laser source, but the focusing optics, a protection glass, and also additional features e.g. to direct a gas flow to the work zone. The light may enter the laser head via a high power optical fiber cable. Preferably, the laser head includes a galvanometer scanner. This is a highly dynamic electro-optical component that uses a rotatable low-inertia mirror to position a laser beam with high precision and repeatability.

A suitable laser head is a SCANLAB SCANcube 10 scanning head with f- theta lens 160 mm; spot size 65 micrometers; power: 18.2 W, frequency 200 kHz, pulse length 1 10 ns, scan speed 800 mm/s, hatch 0.010 mm, supplied by SCANLAB GmbH, Puchheim, Germany

The laser spot may be moved over the second surface of laminar material using a vector mode, in which the laser follows a designated path and forms the pattern. Alternatively, the laser may operate in raster mode. Using this method, an image is defined by a number of dots of a certain resolution. The image is then recreated on a material by passing a laser beam back and forth over the material along one axis to mark one line of dots with each pass in accordance with the information from the original image while stepping in very small increments along an orthogonal axis until the image is completed.

When the laser is operating in raster mode, it is generally pulsed. Suitably, the laser operates at a pulse frequency of from 1 kHz to 500 kHz, preferably from 10 kHz to 400 kHz, more preferably from 50 kHz to 250 kHz, such as about 200 kHz.

A pattern to be marked can be formed by using a mask through which the laser beam passes or by a focused laser beam which is moved or scanned to produce the desired indicia or pattern. This is desirable when a number of items are to be marked with identical patterns.

The invention is applicable to many kinds of opaque material. The term “opaque,” as used herein, means that the material is substantially

impenetrable to electromagnetic or other kinds of radiation, especially visible light.

In one embodiment, the substrate is any laminar material including but not limited to metal, glass, a plastics material or a ceramics material.

“Laminar material” as used herein means a piece of material having two substantially parallel faces. Suitable laminar materials include sheet metals (such as sheet steel and aluminium), glass, a plastic sheet (such as linear low density polyethylene (LLDPE), high density polyethylene (HDPE),

polypropylene (PP), polyvinyl chloride (PVC), polystyrene (PS),

polyoxymethylene (POM), a polyamide (PA) such as nylon, nylon 6 or nylon 6,6, polytetrafluoroethylene, a polyester (PE) such as or a polyurethane (PU).

The marking is performed on the second surface which becomes the rear face of the material. However, the marking remains invisible on the first surface which forms the front face of the material. The markings produced on the first surface are imperceptible, or substantially imperceptible. “Imperceptible” as used herein means that the markings cannot be detected by a normal visual inspection of the first surface.

In one aspect of the invention, the markings on the first surface are detected. The first surface is exposed to electromagnetic radiation, and reflected radiation is detected.

In one preferred embodiment, the electromagnetic radiation is light. “Light” in this context encompasses visible light having a wavelength in the range of 360 to 780 nm, infrared light having a wavelength of over 780 nm, and ultraviolet light having a wavelength of below 360 nm. In some embodiments, it is preferred that the light is in the visible region.

The detection may be accomplished by the formation of an image on a suitable surface, such as a screen. Surprisingly, the inventors have found that an image marked on the second surface can be formed in the reflection from the first surface, even though no detectable marks are present on the first surface. In this embodiment, it is preferred that the electromagnetic radiation is light in the visible region (i.e. 360 to 780 nm).

In an alternative embodiment, the reflected electromagnetic radiation may be measured using a detector. Suitable detector types include photoemission or photoelectric detectors, semiconductor detectors, photovoltaic detectors, thermal detectors, and polarization detectors.

Optionally, the rear (second) surface of the material is treated to ensure that the laser marking is invisible from the rear face of the material. The treatment may include mechanical grinding or polishing, or covering the surface with a paint.

In another embodiment, the front face of the laminar material is treated to provide a lustrous surface which enables clear contrast between the marked and unmarked areas when the light is reflected off that lustrous surface. This may suitably be achieved by e.g. polishing or burnishing. Remarkably, it has been found that the hidden pattern can still be revealed even when the first surface is painted.

Polishing of the first surface can be done at any stage in the process (i.e. prior or subsequent to distressing the second surface). Preferably, this is done prior to distressing the second surface. The skilled person will be well able to decide which polishing technique is best suited to a particular material.

In yet another embodiment, the light source is any source that radiates wavelengths between 360 - 780 nm. Accordingly, the light source may be natural sunlight, although trial and error will establish suitable alternatives. A strong, directional source of light is preferred, such as a beam of light.

Sunlight is suitable, and to artificially produce a light beam, a lamp and a parabolic reflector may be used.

In another preferred embodiment, a screen is placed in front of the front face of the laminar material onto which the reflected light off that front face is projected, which reveals the information that is laser marked on the rear face of the laminar material. The screen may be a white background.

After the inventive process has been applied, one or more further process steps may be applied to the laminar material.

The following Figures illustrate the method of the present invention of invisible marking on a substrate using a laser, and detection of that marking using the visible light. Examples

Example 1

A sample of stainless steel AISI304, cold rolled sheet, 70x50x1.5 mm, surface quality 2R was provided (figure 1 ). Marking of one face (the reverse face) was performed was performed using a laser marking system comprising: pulsed fiber laser SPI G3 HS of average output power 20 W; SCANLAB SCANcube 10 scanning head with f-theta lens 160 mm; spot size 65 micrometers; power: 18.2 W, frequency 200 kHz, pulse length 110 ns, scan speed 800 mm/s, hatch 0.010 mm (figure 2). The reverse face showed the engraved pattern (figure 3). The pattern on the front face could be visualised when exposed to sunlight in the reflection (figure 4). Meanwhile, the pattern remained invisible on the surface of the workpiece (figure 5).

Example 2

A sample of polyoxymethylene (POM) - black, 210x210x4.0 mm was employed (figure 6). Marking was performed by laser marking system comprising a pulsed fiber laser IPG-YLP HP 1 -100-500-500 of average output power 500 W; SCANLAB intelliSCAN 30 scanning head with f-theta lens 635 mm; spot size 65 micrometers; power: 450 W, frequency 50 kHz, pulse length 110 ns. Scan speeds of 2, 4, 6, 8, 10, 12, 14 and 16 m/s were used to mark square areas of 30x30 mm, using a hatch of 0.150 mm, of the reverse face of the sample (figure 7). When the front face was exposed to sunlight, the pattern engraved on the reverse face could clearly be seen in the reflection (Figure 8).

Example 3

A sample of stainless steel AISI304, cold rolled sheet, 90x70x1 mm, surface quality 2R was provided (figure 9). Marking was performed by laser marking system comprising: pulsed fiber laser SPI G3 HS of average output power 20 W; SCANLAB SCANcube 10 scanning head with f-theta lens 160 mm; spot size 65 micrometers; power: 18.2 W, frequency 200 kHz, pulse length 1 10 ns, scan speed 800 mm/s, hatch 0.010 mm (figure 2). A pattern was marked on the reverse face of the sample. The reverse face of the sample was painted black using LabIR® thermographic spray paint (Pilsen, Czech Republic) (Figure 10) so that the pattern visible on the reverse surface was completely obscured. The pattern marked on the reverse face was visible in the reflection of sunlight incident on the opposite (front) face (figure 1 1 ).

Claims

1. A method of imperceptibly marking a first surface of an opaque object comprising the steps of:

1. providing an opaque object comprising a first surface and a

substantially coplanar second surface; and

ii. distressing the second surface;

2. The method according to claim 1 , wherein said distressing is achieved by thermal treatment of the second surface.

3. The method according to claim 2 wherein the thermal treatment comprises exposure to a laser beam.

4. The method according to claim 1 , wherein said distressing is achieved by mechanical treatment of the second surface.

5. The method according to claim 4 wherein said mechanical treatment is selected from the group comprising scoring, etching, engraving, sand blasting, shot blasting, scratching or embossing.

6. The method according to any preceding claim wherein the material is a laminar material.

7. The method according to any preceding claim, wherein the first surface is treated to obtain lustre before distressing the second surface.

8. The method according to any of the preceding claims, wherein the material is selected from metal, glass, polymer, or a ceramic material.

9. A method according to claim 3 wherein the laser beam is supplied by a fibre laser.

10. A method according to claim 9 wherein the laser operates at a power of between 5 W and 30 W.

11. The method according to any preceding claim wherein a pattern is marked on the second surface of the material.

12. The method according to claim 1 1 wherein the laser beam is scanned over the second surface.

13. The method according to claim 12 wherein the laser beam is scanned over the second surface in a vector mode.

14 The method according to claim 12 wherein the laser beam is scanned over the second surface in a raster mode.

15. The method according to any preceding claim comprising the further step of obliterating marks on the second surface.

16. The method according to claim 15 wherein the obliteration is

accomplished by mechanical means.

17. The method according to claim 16 wherein the obliteration is

accomplished by sanding, grinding or polishing.

18. The method according to claim 15 wherein the obliteration is

accomplished by applying a coating to the second surface.

19. The method according to claim 1 1 wherein the pattern is selected from the group consisting of an image, numerical data, computer readable data, written information, or a combination thereof.

20. The method according to any preceding claim comprising the further steps of illuminating the first surface with electromagnetic radiation and detecting the reflected radiation.

21. The method according to claim 20 wherein the electromagnetic radiation is light.

22. The method according to claim 21 wherein the light is visible light in the wavelength range of 360 to 780 nm.

23. The method according to any one of claims 20 to 22 wherein the detection is accomplished by means of forming a reflected image on a screen.

24. The method according to any one of claims 20 to 23 wherein the detection is accomplished using a photodetector.

25. A distressed material obtainable by a process according to any one of claims 1 to 19.

26. A label comprising distressed material as claimed in claim 25.