Method of laser marking a body of material having a thermal conductivity approximately equal to that of glass

A method of providing a body of material (14), having a thermal conductivity approximately equal to that of glass, with a sub-surface mark. A beam of laser radiation (12) to which the material (14) is substantially opaque is directed to surface of the body, so as to cause beam energy to be aborbed at the surface of the material in an amount sufficient to produce localised stresses within the body (14) at a location spaced from the surface without any detectable change at the surface, the localised stresses thus produced being normally invisible to the naked eye but capable of being rendered visible under polarised light.

BACKGROUND OF THE INVENTION 
The present invention relates to a method of providing a body of material 
with a sub-surface mark that is invisible to the naked eye but which is 
capable of being rendered visible under polarized light. 
Many products are packaged in containers of glass or plastics and there has 
been a desire for many years to provide a method of marking containers of 
this type so that once a mark has been applied, it cannot be removed. 
Clearly such a method of marking would have a wide range of applications, 
not least in combating parallel trading. 
In the past, in order to produce an indelible mark, manufacturers have 
relied, almost exclusively, on surface marking. However, the problem with 
this type of mark is that it may be either destroyed by removing that part 
of the surface on which the mark is applied, or imitated by the 
application of an identical mark on a substitute container. 
In order to overcome these problems, the Applicant developed a method and 
apparatus for providing a body of material with a sub-surface mark which 
are described in International Patent Publication No. WO 92/03297. The 
method described comprises the steps of directing, at a surface of the 
body, a high energy density beam to which the material is transparent and 
bringing the beam to a focus at a location spaced from the surface and 
within the body so as to cause localised ionization of the material and 
the creation of a mark in the form of an area of increased opacity to 
electromagnetic radiation substantially without any detectable change at 
the surface. This provided the advantage that the resulting mark was both 
difficult to imitate and near impossible to remove. 
In order to provide a method of marking having further advantages, it can 
be desirable that the resulting mark is invisible to the naked eye. In 
this way, a potential counterfeiter will not only have difficulty in 
removing or imitating the mark, but will also run into problems in 
locating the mark in the first place. 
U.S. Pat. No. 3,657,085 describes a method of proving a sub-surface mark 
using an electron beam but also mentions the possibility of using a laser 
beam as an alternative. The object of the U.S. patent is to provide a 
method of marking an article, such as a spectacle lens, with an 
identification mark which is normally invisible but which can be rendered 
visible when required. To this end, the electron, or laser beam, is 
directed onto a mask placed over the spectacle lens so that that part of 
the beam passing through the cut-out portions of the mask, impinges upon 
the material of the spectacle lens. The beam is scattered by collisions 
with the molecules of the material that makes up the lens with the result 
that the kinetic energy of the beam is absorbed as heat producing 
permanent stress patterns within the lens. These stress patterns are 
invisible to the naked eye but may be rendered visible by double 
refraction in polarized light. 
When referring to the possible use of a laser beam, U.S. Pat. No. 3,657,085 
does so in conjunction with the marking of mass coloured materials, i.e. 
materials having a chromophore throughout their bulk and not simply ones 
provided with a coloured surface layer. It is this chromophore that 
absorbs the laser radiation and, in doing so, generates sufficient 
localised heating to produce permanent stress patterns within the 
material. Since the resulting mark is spaced from the surface of the 
material, the material must be at least partially transparent to the laser 
radiation used in order to allow the laser radiation to penetrate the 
material to the required depth. 
SUMMARY OF THE INVENTION 
In contrast, according to a first aspect of the present invention, there is 
provided a method of providing a body of material with a sub-surface mark 
comprising the steps of directing at a surface of the body a beam of laser 
radiation to which the material is substantially opaque, the beam energy 
absorbed at the surface of the material being sufficient to produce 
localised stresses within the body at a location spaced from said surface 
without any detectable change at said surface, the localised stresses thus 
produced being normally invisible to the naked eye but capable of being 
rendered visible under polarized light. 
Advantageously the mark created by the localised stresses may be 
representative of one or more numerals, letters or symbols or a 
combination thereof. 
Advantageously the beam of laser radiation may be concentrated so as to 
form an illuminated spot at a location on the surface of the body, the 
spot being movable relative to the body to be marked, thereby enabling the 
mark created by the localised stresses to be of a predetermined shape. 
Preferably the spot may be moved relative to the body to be marked in such 
a way as to produce an elongate region of localised stresses that when 
rendered visible under polarised light gives the appearance of a line. 
Alternatively, the spot may be moved relative to the body to be marked in 
such a way as to produce a series of spaced apart regions of localised 
stresses that when rendered visible under polarised light gives the 
appearance of a series of dots. In particular, the series of spaced apart 
regions of localised stresses may be formed by moving the spot at a 
constant speed relative to the body to be marked and periodically varying 
the power density of the beam. Alternatively, the series of spaced apart 
regions of localised stresses may be formed by maintaining the power 
density of the beam substantially constant and varying the time the spot 
is used to illuminate successive locations on the surface. To this end the 
spot may be moved relative to the body to be marked at a speed that varies 
periodically between zero and 3000 mm/s whilst still maintaining an 
average speed in the range from 2 to 3 m/s. Preferably the beam energy 
absorbed at successive locations on the surface may vary smoothly from one 
location to the next. Preferably the laser radiation may have a power 
density at the spot of up to 10 kW/cm.sup.2. 
Advantageously the beam of laser radiation may be caused to illuminate a 
mask placed in front of the body to be marked, the mask having one or more 
apertures, thereby enabling the mark created by the localised stresses to 
be of a predetermined shape. 
Advantageously the beam of laser radiation may be generated by a CO.sub.2 
laser. 
Advantageously the body of material may be transparent to electromagnetic 
radiation at wavelengths within the visible region. Alternatively, the 
body of material may be opaque to electromagnetic radiation at wavelengths 
within the visible region such that the localised stresses may only be 
seen by optical instruments operating at an appropriate wavelength within 
the electromagnetic spectrum. 
According to a second aspect of the present invention there is provided a 
body of material comprising a region of localised stresses at a location 
spaced from a surface of the body and without any detectable change at 
said surface, the localised stresses extending from one edge of a 
lens-shaped mark of substantially convex cross-section. 
Advantageously the body of material may be transparent to electromagnetic 
radiation at wavelengths within the visible region. In particular, the 
body of material may be of glass or plastics. Alternatively, the body of 
material may be opaque to electromagnetic radiation at wavelengths within 
the visible region such that the localised stresses may only be seen by 
optical instruments operating at an appropriate wavelength within the 
electromagnetic spectrum. 
Advantageously the mark created by the localised stresses may be 
representative of one or more numerals, letters or symbols or a 
combination thereof. 
Advantageously the body of material may be a container.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION 
An apparatus capable of performing the method of marking of the present 
invention is shown in FIG. 1. As can be seen, this apparatus comprises a 
source 10 which produces a beam of laser radiation 12 which is directed so 
as to impinge upon a body of material 14 that, in the present example, is 
in the form of a bottle. Since the eventual sub-surface mark is intended 
to be normally invisible to the naked eye but capable of being rendered 
visible to the eye under polarized light, the bottle 14 is chosen to be of 
a material such as glass or plastics that is transparent to 
electromagnetic radiation within the visible region of the electromagnetic 
spectrum. Furthermore, the source 10 is selected in such a way that the 
material of the bottle 14 is substantially opaque to the beam of laser 
radiation 12 produced by the source. 
In the particular embodiment illustrated in FIG. 1, the source 10 comprises 
an RF excited simulated continuous-wave carbon dioxide (CO.sub.2) laser 
that emits a beam of laser radiation 12 having a wavelength of 10.6 .mu.m 
and which is consequently invisible to the naked eye. Having been emitted 
from the CO.sub.2 laser, the beam of laser radiation 12 is incident upon a 
first reflecting surface 16 that directs the beam 12 through a beam 
expander 18 and a beam combiner 20 to a second reflecting surface 22. A 
second source of laser radiation, in the form of a low power He-Ne 
(Helium-Neon) laser 24, is disposed adjacent to the CO.sub.2 laser 10 and 
emits a secondary beam of visible laser radiation 26 with a wavelength of 
632.9 nm. The secondary beam 26 impinges upon the beam combiner 20 where 
it is reflected towards the second reflecting surface 22 coincident with 
the beam of laser radiation 12 from the CO.sub.2 laser 10. Thus the 
necessary properties of the beam combiner 20 are that it should transmit 
electromagnetic radiation with a wavelength of 10.6 .mu.m whilst 
reflecting electromagnetic radiation with a wavelength of 632.9 nm. In 
this way the He-Ne laser beam 26 provides the combined CO.sub.2 /He-Ne 
beam 12,26 with a visible component that facilitates optical alignment. 
Once combined, the two coincident beams 12,26 are reflected at the second 
reflecting surface 22 to a third reflecting surface 28, and from the third 
reflecting surface 28 are further reflected towards a fourth reflecting 
surface 30. From the fourth reflecting surface 30 the combined beam 12,26 
is reflected yet again toward a head unit 32 from whence the combined beam 
12,26 is finally directed towards the bottle 14. In order to facilitate 
marking at different heights from the base of the bottle 14, the third and 
fourth reflecting surfaces 28 and 30 are integrally mounted, together with 
the head unit 32, so as to be adjustable in a vertical plane under the 
action of a stepping motor (not shown). 
Within the head unit 32 the combined CO.sub.2 /He-Ne beam 12,26 is 
sequentially incident upon two movable mirrors 36 and 38. The first of the 
two mirrors 36 is disposed so as to be inclined to the combined beam 12,26 
that is incident upon it as a result of reflection from the fourth 
reflecting surface 30 and is movable in such a way as to cause the beam 
reflected therefrom to move in a vertical plane. The second of the two 
mirrors 38 is similarly inclined, this time to the beam 12,26 that is 
incident upon it as a result of reflection from the first mirror 36, and 
is movable in such a way as to cause the reflected beam 12,26 to move in a 
horizontal plane. Consequently, it will be apparent to those skilled in 
the art that the beam 12,26 emerging from the head unit 32 may be moved in 
any desired direction by the simultaneous movement of the first and second 
mirrors 36 and 38. In order to facilitate this movement the two movable 
mirrors 36 and 38 are mounted on respective first and second galvanometers 
40 and 42. Whilst it is recognised that any suitable means may be provided 
to control the movement of the two mirrors 36 and 38, the approach adopted 
combines a speed of response with an ease of control that represents a 
significant advantage over alternative control means. 
Emerging from the head unit 32, the combined beam 12,26 is concentrated by 
passing through a lens assembly 44 which may include one or more lens 
elements. A first lens element 46 brings the beam 12,26 to a focus at a 
chosen location on the surface of the bottle 14. As is well known, the 
maximum power density of the beam 12,26 is inversely proportional to the 
square of the radius of the beam 12,26 at its focus which in turn is 
inversely porportional to the radius of the beam 12,26 that is incident 
upon the focusing lens 46. Thus for a beam 12,26 of electromagnetic 
radiation having a wavelength .lambda. and a radius R incident upon a lens 
of focal length f, the power density at the focus E, is to a first 
approximation, given by the expression: 
##EQU1## 
where P is the power produced by the laser. From this expression the value 
and purpose of the beam expander 18 is readily apparent since increasing 
the radius of the beam R serves to increase the power density E at the 
focus. In addition, the lens element 46 is typically a short focal length 
lens having a focal length in the range between 70 mm and 80 mm so that 
power densities in excess of 6 kW/cm.sup.2 may be readily achieved at the 
focus of the beam 12,26. 
A second lens element 48 may be placed in series with the focusing lens 
element 46 in order to compensate for any curvature of the surface of the 
bottle 14. It will be recognised that such a correcting lens will not be 
required if the body to be marked 14 presents a substantially planar 
surface to the incident beam and the need for such an element may be 
negated altogether if the first element 46 is of variable focal length and 
comprises, for example, a flat field lens. However, it is to be noted that 
the use of one or more optical elements is a particularly simple and 
elegant way of ensuring that the beam 12,26 is focused on the surface of 
the body 14 irrespective of any curvature thereof. 
In the interests of safety, the two lasers 10 and 24 and their respective 
beams 12 and 26 are enclosed within a safety chamber 52 as shown in FIG. 
2, with the combined beam 12,26 emerging from the safety chamber 52 only 
after passing through the lens assembly 44. Access to the two lasers 10 
and 24 and the various optical elements disposed in the path of the 
respective beams 12,26 is gained by means of a door panel 54 which is 
fitted with an interlock 56 which prevents the operation of the CO.sub.2 
laser 10 and the He-Ne laser 24 while the door panel 54 is open. 
A single phase electrical mains supply of 240 v is fed via the door panel 
interlock 56 to a mains distribution unit 58 that is disposed below, and 
isolated from, the safety chamber 52 in order to prevent any electrical 
effects from interfering with the operation of the lasers 10 and 24. From 
the distribution unit 58, mains electrical power is provided to the 
CO.sub.2 laser 10 and the He-Ne laser 24, as well as to a chiller unit 60 
that serves to cool the CO.sub.2 laser 10. In addition mains electrical 
power is also supplied to the stepping motor 34 and to a computer 62. 
Three AC/DC convertors and associated voltage regulators provide regulated 
DC voltage supplies of 12 v, .+-.10 v and .+-.28 v that are fed 
respectively to the He-Ne laser 24 to facilitate the pumping mechanism and 
to the head unit 32 where in particular, the .+-.28 v supply is used to 
power the first and second galvanometers 40 and 42 and the .+-.10 v supply 
is fed to the galvanometers to produce a predetermined movement of the 
first and second mirrors 36 and 38. Thus by using the computer 62 to 
modulate the .+-.10 v supply the various movements of the first and second 
galvanometer mirrors 36 and 38 may be made under the control of a computer 
programme. 
In use, the beam of laser radiation 12 emited by the CO.sub.2 laser 10 is 
caused to form an illuminated spot at a location on the surface of the 
bottle 14, the body to be marked. This spot may then be scanned across the 
surface of the bottle as a result of the movement of one or both of the 
galvanometer mirrors 36 and 38. 
It is well known that glass and some other materials that are transparent 
to electromagnetic radiation within the visible region of the 
electromagnetic spectrum are opaque to electromagnetic radiation having a 
wavelength of 10.6 .mu.m and that a CO.sub.2 laser produces laser 
radiation having just this wavelength. Despite this the Applicant has 
established that it is possible to provide a transparent body, such as 
glass, with a sub-surface mark using a CO.sub.2 laser. 
To understand the marking process it is important to remember that the 
absorbtion of a beam of laser radiation by a material is a progressive or 
statistical process and that the beam energy is always absorbed in a Beam 
Interaction Volume (BIV) of finite dimensions. Thus in this context a Beam 
Interaction Volume may be defined as that volume within which an 
arbitrarily large proportion, say 95%, of the incident beam energy is 
absorbed. For electromagnetic radiation within the visible region of the 
electromagnetic spectrum and a body of glass which is transparent at those 
wavelengths, the BIV may be very large compared to the dimensions of the 
body concerned. By contrast, for electromagnetic radiation having a 
wavelength of 10.6 .mu.m, experiments have shown the same body of glass to 
have a BIV having a depth in the direction of propagation of the beam of 
between 8.0 .mu.m and 16.0 .mu.m for a beam having a power density within 
the range from 6 to 10 kW/cm.sup.2. Thus, whilst for most practical 
purposes the beam of laser radiation 12 may be thought of as being 
absorbed "at the surface" of the body to be marked 14, the fact that a 
dimension of even 8.0 .mu.m is readily observed using electron 
microscopical techniques means that it is necessary to further define what 
is to be understood by the term opaque. Thus, for the avoidance of doubt, 
in the present context the term opaque, when used to describe the material 
to be marked, refers to a material capable of absorbing 95% of the energy 
of an incident beam of laser radiation within a distance which is less 
than that at which the sub-surface mark is spaced from the surface. 
Despite 95% of the energy of the laser radiation being absorbed within the 
BIV, the effect of the beam on the body to be marked is not confined to 
this surface region. For example, the heating effect produced by the beam 
may be felt at a location outside the BIV since glass has a significant 
coefficient of thermal conductivity. Likewise, any resulting stress 
pattern may also extend beyond the region of the glass that is directly 
affected by the laser beam, in just the same way that the stress pattern 
in a pane of glass extends beyond the tip of a crack that is propagated 
therein. Thus it will be appreciated that in principle, the physical 
consequences of irradiation can be observed at a location remote from the 
BIV. 
This situation is summarised in FIG. 3 in which there is illustrated a body 
of material having a BIV in which an arbitrary proportion of an incident 
beam energy is lost to the material. Surrounding the BIV is a Conductive 
Heating Zone (CHZ) whose boundary, like that of the BIV, must again be 
defined in terms of arbitrary limits. Beyond the Conductive Heating Zone 
lies a stressed zone in which the stresses result from thermally-induced 
changes in the physical dimensions of the material in the BIV and in all 
or part of the CHZ. The variation in magnitude of these stresses as a 
function of the radial distance from the incident beam is indicated by 
means of the curve 66 from which it can be seen that a line of peak stress 
68 may be drawn a short distance from the boundary of both the BIV and the 
CHZ. 
It has been found that using a CO.sub.2 laser having a power density of 
between 6 kW/cm.sup.2 and 10 kW/cm.sup.2 it is possible to create a mark 
within a body of glass at a depth of between 40 .mu.m and 50 .mu.m beyond 
that to which the laser radiation penetrates. This mark, which in 
cross-section has the shape of a convex lens element, typically has a 
depth (i.e. a dimension in the direction of the beam) of 10.8 .mu.m and a 
diameter of 125 .mu.m and is thought to be caused as a result of a thermal 
interaction within the glass. 
In this context it is to be be noted that the possible types of interaction 
between laser radiation and a body of material may be categorised under 
three headings dependant upon the power density of the laser radiation 
concerned. In order of increasing power density these headings are as 
follows: 
1. Photochemical interactions including photoinduction and photoactivation. 
2. Thermal interactions in which the incident radiation is absorbed as 
heat; and 
3. Ionising interactions which involve the non-thermal photodecomposition 
of the irradiated material. 
The difference between the thresholds of these three interactions is 
clearly demonstrated by comparing the typical power density of 10.sup.-3 
W/cm.sup.2 required to produce a photochemical interaction with the power 
density of 10.sup.12 W/cm.sup.2 typical of ionising interactions such as 
photoablation and photodisruption. 
The lens-shaped mark, which is invisible to the naked eye but which can be 
viewed using a compound microscope under both bright field illumination 
and when viewed between crossed polarizing filters, has been observed to 
have a sharply-defined lower edge. This observation has led to the 
speculation that the mark represents the boundary between those atoms 
within the glass that derive sufficient energy from the incident beam to 
overcome the bonds with which they are tied to their neighbours and those 
that do not. As might be expected from this model, a stressed region 
extends beyond the lower edge of the lens-shaped mark and into the body of 
the glass. This stressed region, which may have a dimension in the 
direction of the beam of up to 60 .mu.m, is also invisible to the naked 
eye but may be rendered visible under polarized light. 
It has been found that the lens-shaped mark and the associated stressed 
region may only be created using a CO.sub.2 laser beam having an energy 
density falling within a narrowly defined range. If the energy absorbed by 
the glass is too small then an insufficient thermal gradient is 
established to give rise to an observable stressed region. Conversly, if 
too high an energy is absorbed, the surface of the glass may melt or else 
the glass may crack along a line of peak stress and flake off. This 
cracking of the glass, known as "breakout", not only relieves the stress 
in what remains of the glass but also renders the mark both visible to the 
naked eye and prone to detection by surface analysis. 
In the embodiment described, the beam of laser radiation 12 is scanned 
across the surface of the bottle 14 at an average speed of 2 to 3 m/s to 
produce patterns which may be used to relate to alpha-numeric characters. 
However, rather than moving at a constant speed from one end of a straight 
line scan to the other, the beam is scanned in a series of incremental 
steps which serve to increase the definition and resolution of the 
characters thus produced. As a result, the velocity of the beam varies in 
a manner which is approximately sinusoidal between zero when the beam is 
at either end of one of its incremental steps, and so is effectively at 
rest, and approximately 3 m/s at a point midway between these two ends. 
Consequently, even though the power density of the beam is kept constant, 
different points on the surface of the bottle are exposed to different 
beam energies. It has been found that the energy density window for the 
generation of the aforementioned mark is sufficiently narrow that the 
lens-shaped mark and its associated stressed region are only observed at 
those points at which the beam is effectively at rest. The result of this 
is that under polarized light, the stressed regions created by scanning 
the laser beam across the surface of the bottle show up as a series of 
dots. Thus by controlling the movement of the galvanometer mirrors 36 and 
38, it is possible to scan the laser beam 12 across the surface of the 
bottle 14 in such a way as to "write" any desired symbol onto the bottle 
in a dot matrix format. 
In an alternative embodiment, the same dot matrix format may be achieved by 
scanning the beam across the surface of the bottle at a constant speed 
whilst periodically varying its power density between two levels either 
side of the threshold for creating the lens-shaped mark and its associated 
stress pattern. This type of varying power density might, for example, be 
achieved by superimposing a sinusoidal ripple 70 on top of a square wave 
pulse of laser radiation 72, as shown schematically in FIG. 4. Assuming 
that the threshold for creating the aforementioned mark is at a power 
level represented by the dashed line 74 one might expect to see dot-like 
regions of stress within the glass spaced apart by a distance 
corresponding to that scanned by the laser beam between successive maxima 
76 of the power density profile 78. 
In both of the foregoing embodiments it is thought that the gradual 
increase in energy absorbed by the glass at points closer to that at which 
a mark is actually created provides the glass with a limited ability to 
anneal itself. This is to be contrasted with an arrangement in which the 
laser beam is pulsed to generate a series of marks at locations spaced an 
arbitrary distance apart. The self-annealing nature of the aforementioned 
embodiments is considered to provide a marked body whose strength is not 
compromised by the marking process. 
The patterns of consecutive dots created by the methods described also 
result in a local reversal in the orientation of the stressed regions 
within the glass, and thus in the plane of polarization of any light 
caused to pass through them. This facilitates the detection of the marks 
and gives rise to a characteristic "cross-stitch" pattern, an example of 
which is shown in FIG. 5. 
In a further embodiment, rather than creating a pattern of dots, the 
described apparatus may be used to create a mark comprising one or more 
continuous lines. To this end the beam of laser radiation 12 may be 
scanned across the surface of the body to be marked at a constant 
velocity, while at the same time the power density of the beam is 
maintained at a constant level just above the threshold for creating the 
lens-shaped mark and its associated stress pattern. 
In yet another embodiment, rather than scanning the beam of laser radiation 
12 across the surface of the body to be marked 14, the beam may be used to 
illuminate a mask. By placing the mask in front of the body to be marked 
and providing the mask with one or more apertures, selected portions of 
the incident beam may be caused to impinge upon the body and so produce a 
mark of a predetermined shape. 
In order to observe the marks produced in accordance with any of the 
foregoing embodiments, the marked body may be placed between a pair of 
crossed linear polarizers and illuminated with a powerful collimated light 
beam. As a result the stressed regions are rendered visible as bright 
areas against a dark background. 
An example of an apparatus for use in viewing the marks produced in 
accordance with any of the foregoing embodiments is shown in FIG. 6 to 
comprise a housing 100 similar to that used as the base of an overhead 
projector in which there is disposed a lamp 102. The housing 100 is 
provided with an upper working surface of glass 104 and between this 
surface and lamp 102 there is provided a Fresnel lens 106 capable of 
providing basic beam collimation. Crossed linear polarizing filters 108 
are inserted between the working surface 104 and the Fresnel lens 106, 
while in order to maintain the apparatus at a safe working temperature, 
the housing 100 is provided with a fan 110, of the type used in computer 
systems, as well as a louvred opening 112 for the passage of air. A dimmer 
switch may be provided to control the intensity of the lamp 102. 
In order to observe the stressed regions within the marked body 14, the 
body is placed on top of the working surface 104 and viewed using a 
.times.10 magnifyer 114 fitted with a suitable filter 116.