Laser apparatus for repetitively marking a moving sheet

The energy of a laser beam (32) is used to form images at intervals along a lengthwise moving web (10). The images are formed along a plurality of axes spaced transversely of the web so that after slitting and cutting an image appears on each sheet. The laser beam (32) is directed across the web and is repetitively and successively switched by rotating optical mirrors in assemblies (71a-71g, 72a-72g) located at intervals across the web to scan image-bearing masks (FIG. 2) rotating with the mirrors. The beam impinges on respective mirrors of successive assemblies. The mirror assemblies are conical mirror segments (78) or plane mirrors cooperating with a fixed conical mirror segment (FIG. 5) to produce a focussed line (82) that is fixed in space and scans the rotating mask. The scanning line is re-imaged (90) on the web (10).

This invention relates to a method and apparatus for marking moving sheet 
material by the action of light thereon. The invention has particular 
application to the marking of a longitudinally moving web of material such 
as paper. A particular concern of the invention is with using laser energy 
to mark the material. It will be understood that in this context "light", 
whether from a laser or other source, includes radiation both within and 
without the visible spectrum. "Marking" of a sheet is used herein to mean 
effecting a change in the sheet by impinging light on the sheet. Such a 
change may, for example, be thermally induced or photo-chemically induced, 
and may or may not be visible. The practice of the invention finds 
particular application where the marking to be applied to the sheet is in 
accord with some prescribed pattern or design. The invention will be 
particularly described in relation to marking paper with a visible image. 
It is long-established in paper making to water-mark the web on the 
Foudrinier wire. The web is so marked at sufficient intervals both across 
and along the web such that when it is finally sliced and cut into 
individual sheets, each sheet will bear a water-mark associated with the 
paper manufacturer. In practice water-marking in the conventional fashion 
is only economical for relatively expensive grades of paper that are 
produced at relatively slow speed. Conventional water-marking is not 
normally applied to papers in which the water-marking may be obscured by 
subsequently applied coatings. It is not applied with the paper that is to 
be used in the manufacture of carbonless-copy paper, except for 
letter-head or relatively low tonnage premium qualities, primarily on the 
economic ground abovementioned. Nonetheless, the manufacturer of such 
papers may wish to identify the paper with himself, since cut sheet as 
packed and sold will normally bear the label of some other company. Thus 
the need arises for some other means of marking which is identifiable but 
is not too obtrusive for the use for which the paper is intended. 
Many materials such as metal, plastics and paper can be marked by use of 
laser light which may remove a coating layer or act on a surface layer of 
a base material. Such action may be a removal or "burning" of material or 
other thermally induced change, such as a colour change, depending on the 
material in question. An example of the use of lasers for this purpose is 
described in the journal "Laser Focus", July 1975, pages 28 to 32 under 
the heading "Fast laser pulses can etch a pattern on a moving part on a 
production line". Apparatus for this general purpose is available from 
various manufacturers. In the commercially available apparatus a mask 
containing the pattern to be marked on the product is imaged onto the 
product. The mask is in the form of a stencil through which light is 
transmissible. The whole mask is illuminated with a laser pulse to produce 
a pulsed image on the product. By using short pulses of sufficient energy, 
the apparatus can clearly mark a rapidly moving surface. 
Such apparatus can be used for the marking of paper. In principle it can be 
used on a paper making machine or in other machines for processing 
manufactured web, such as coating machines that apply the microcapsules 
for carbonless copy paper. Such a web may be typically up to several 
meters wide and is normally subsequently longitudinally slit into reels 
which may then be used to provide sheets either in pre-cut form or to 
provide continuous stationery. Thus each longitudinal section to be slit 
from the web requires to be marked at intervals along its length. Whilst 
in principle this can be done in the pulse imaging system described with 
the aid of beam splitters, beam splitting divides the pulse energy 
requiring an increase in the power of laser source for a required image 
intensity at the web. The pulse power required is relatively high since 
the whole mask area as imaged on the web is illuminated at one time and 
the effective dwell time on the moving web is necessarily short in order 
to prevent blurring of the image. Typically a web may be moving at speeds 
up to 20 m/s. 
There have been numerous proposals to use laser energy to perforate 
cigarette tipping paper. Here the object is to form small perforations not 
to make any image on the paper. Such proposals have used optical switching 
means, possibly combined with focussing arrangements, to direct a laser 
beam to successive locations across and along the web. These proposals 
involve the use of staggered arrays of optical elements, often arranged in 
a rotary fashion. Examples are found in U.K. patent specifications GB Nos. 
1603752, 2022492, 2027628 and 2032323; and in European specifications EP 
No. 0021165 and 0047604. Optical switching arrangements are also disclosed 
in GB No. 2074341 and EP No. 0042173. An alternative to optical switching 
is disclosed in GB No. 2118882 in which a web of cigarette tipping paper 
passes over a drum containing spaced circumferential rows of apertures at 
which individual lasers located in the drum are directed. 
Specification GB No. 2161752A discloses the application of laser energy to 
a web at selected points. GB No. 2133352 describes how a laser beam can be 
used to mark a moving product by use of electronically-controlled beam 
deflection to produce indicia on a dot matrix basis. 
It is also known to laser engrave materials, including paper. Such a system 
is described in GB No. 2126955 in which the engraving image and the object 
to be engraved are essentially raster scanned by a focussed laser beam. 
This is not suitable for multiple marking of a moving web. 
There will be described more fully below apparatus in which a continuous 
wave CW laser is utilised to provide the marking of the web at several 
locations transversely of the longitudinally moving web and repetitively 
along the web. The input beam is sequentially switched to the several 
output locations by optical elements and may be arranged to provide almost 
continuous use of the laser beam energy. Effectively the optical switching 
elements serve to sequentially establish a plurality of optical paths 
terminating at the output locations but having a common input for 
receiving the laser beam. It is of particular interest to mark the web 
with an image which is derived from an image-bearing mask. The apparatus 
to be described scans the mask which enables a lower power beam to be used 
than would be the case for imaging the whole mask. 
To effect scanning, the incoming laser beam is focussed to a predetermined 
configuration which is scanned across the mask so as to modulate the 
intensity distribution. The scanning of each mask is performed in the 
apparatus described below by focussing the beam to an elongate 
configuration that is fixed in space and moving the mask through the 
focus. Such a focus is required for each output location across the web. 
The foci intercepted by the respective masks may lie in a plane that is 
essentially at the web surface. This is a contacting system in which the 
web is conveniently guided over the surface of a rotating drum. The masks 
are located at the surface and the other optical elements are contained 
within the drum. It is preferred, however, to use a non-contacting 
arrangement in which the mask-scanning foci are formed at intermediate 
points in the respective optical paths and are re-imaged and thus focussed 
onto the web by output optics in each path. The re-imaged line foci at the 
web are modulated in accord with the mask image as the mask is scanned. 
Since the modulated line at the web is also spacially fixed, the web 
itself provides the necessary scanning movement to reproduce the image on 
it. 
Two techniques will be described for performing beam-switching and 
focussing. The first uses conical mirror segments to perform both beam 
switching and focussing. This is done in practice by mounting the segments 
in a continuously rotating structure. The second technique employs conical 
mirror segments to perform the focussing and plane mirror assemblies to 
perform the beam switching. In this case the conical mirror segments are 
fixedly mounted and the plane mirror assemblies are mounted to a rotary 
structure. Conical mirrors have the property of producing a line focus as 
discussed above. 
In implementing the first technique the beam-switching and focussing to 
lines may be conveniently performed by sets of conical mirror segments 
arranged in a rotary structure whose axis of rotation extends transversely 
of the web. Each set of conical mirror segments is located in an annulus, 
the sets being transversely spaced across the web. The segments of each 
set are angularly spaced and the sets are angularly offset so that as seen 
by an incoming laser beam they are interleaved so that the beam impinges 
on only one conical mirror segment at a time and preferably on mirror 
segments from different sets in succession as the sets rotate. With the 
aid of other optical elements the beam can be brought to its line foci at 
the positions required for imaging on the paper. 
By orientation of the conical mirror segments to have their cone axes 
coincident with the axis of rotation, each line focus produced by the 
incoming beam impinging on a moving mirror is fixed in space and the 
successive line foci produced by any one set of conical mirrors lie at the 
same fixed spacial point. In this case the line foci are not only fixed in 
a direction transverse to the direction of web movement but are fixed in 
the longitudinal direction also. The scanning of the relevant mask by such 
a fixed line is effected by moving the mask relative to the line beam. 
Conveniently the masks can be carried by, or otherwise rotate 
synchronously with, the structure carrying the mirror segments. One 
arrangement is to have a respective mask for each mirror segment that is 
positioned to traverse the focussed line as the mirror traverses the laser 
beam. 
The second technique has one fixed conical mirror in the optical path to 
each output location across the web. These fixed mirrors are conveniently 
stationed in line with the respective longitudinal sections of the web to 
be marked. Adjacent each conical mirror there is a respective set of plane 
mirror assemblies arranged in an angularly offset manner for rotation to 
repetitively switch the beam to the conical mirror. The sets of plane 
mirror switching assemblies are themselves angularly offset one from 
another, that is they are progressively angularly staggered across the web 
so that as seen by the incoming beam the mirrors are interleaved to 
provide a repeated sequence of markings across the web. 
The plane mirror assemblies preferably use pairs of mirrors, specifically 
at right angles. A first mirror of the pair is aligned to come into a 
position facing the associated conical mirror as rotation progresses. The 
second is radially displaced from the first to intercept the beam so as to 
reflect it to the first. A particular adaptation of this arrangement is to 
have the second mirrors of the pairs alternately positioned at greater and 
lesser radii around the set, i.e. on inner and outer circles. With such an 
arrangement the incoming beam is laterally displaced between the inner and 
outer circle radii by an input beam chopper that is rotated synchronously 
with the rotation of the switching mirrors. 
The fixed conical mirrors produce line foci as already described and the 
image-bearing masks are carried with the rotating plane mirrors so as to 
be scanned by the line foci in the manner already discussed. Preferably 
the modulated line foci are re-imaged onto the web. 
According to one aspect of the present invention there is provided a method 
of marking sheet material moving along a predetermined path comprising the 
steps of: 
repetitively establishing by optical path-switching means an optical 
connection between an input portion of an optical path and an output 
portion of the optical path, which output portion terminates at the moving 
sheet at a location that is spacially fixed in a direction transverse to 
the direction of sheet movement, directing a beam of light along the input 
portion of the optical path at least at times when said optical connection 
is established whereby the beam of light impinges on the moving sheet at 
said location, said beam of light causing a change in the sheet to mark 
same at spacial intervals in the direction of movement; characterised in 
that the beam of light is focussed to a predetermined configuration at 
said location, and characterised by 
modulating the predetermined beam configuration by means of an 
image-bearing mask to mark the sheet in accord with the image. 
The configuration abovementioned may be an elongate configuration, such as 
a straight line, extending transversely to the direction of movement of 
the sheet. 
In the embodiments to be described a scanning movement is effected between 
the mask and the predetermined beam configuration. The beam may be 
focussed to its predetermined configuration at a point preceding the 
aforesaid location and the configuration re-imaged at the location. The 
scanning movement between mask and beam is effected at this preceding 
point. 
There will be described respective optical arrangements in which the beam 
focussing is performed by the optical path switching means and by means 
separate from the switching means. 
More particularly, the method of the invention may be used to mark a sheet 
along at least one further axis in the direction of sheet movement and to 
this end preferably comprises repetitively establishing by said optical 
switching means an optical connection between said input portion and a 
further output portion of a further optical path, which further output 
portion terminates at a further location that is spacially fixed in a 
direction transverse to the direction of sheet movement and is 
transversely offset from the first-mentioned location, 
the optical connections established in the first-mentioned optical path 
being interleaved in time with those established in the further optical 
path, 
the direction of the beam of light along said input portion occurring at 
least when the optical connections to the first-mentioned and further 
output portions are established, whereby the beam of light also impinges 
on the moving sheet at said further location, 
and focussing said beam of light at said further location to a 
predetermined configuration and modulating this latter configuration by 
means of an image-bearing mask to also mark the sheet at spacial intervals 
in the direction of movement along an axis transversely offset from the 
first-mentioned markings. 
In another aspect the invention provides an optical apparatus for use in 
producing an image on a moving sheet of material, comprising a beam entry 
point for receiving a laser beam in a predetermined alignment, at least 
one mask-receiving means for receiving a mask bearing an image, optical 
means including first and second optical elements carried by a structure 
repetitively movable in a predetermined path to intercept the beam at 
different times in each such movement to switch the beam to the or 
different ones of the mask-receiving means, and said optical means being 
adapted to focus the received laser beam to a line at the mask-receiving 
means to which the beam is switched, the mask-receiving means and optical 
means being so arranged to provide a relative motion between the line and 
the mask-receiving means at which it is focussed for scanning a mask 
received therein. 
The structure mentioned in the preceding paragraph may be rotatably 
mounted, whereby each of said elements describes an arcuate path, and may 
have drive means coupled to it to continuously rotate same. 
In one embodiment of the optical apparatus described below, the first and 
second elements are adapted to focus the beam intercepted thereby to a 
line fixed in space as the intercepting elements move and in which the 
mask-receiving means is or are mounted to said structure to move in 
synchronism therewith so as to effect scanning with respect to the 
relevant spacially fixed line. These first and second elements may 
comprise conical mirror segments mounted to have their cone axes 
coincident with the axis of rotation of the structure. They may be mounted 
to intercept the received laser beam at different points along the beam to 
provide respective line focusses that are spacially separated. 
In one preferred apparatus, the first element is one of a first set of like 
elements mounted to intercept the received beam at the same first point 
along its path and at different times during the movement of said 
structure, and the second element is one of a second set of like elements 
mounted to intercept the received beam at a second point along its path 
and at different times during the movement of the structure that are 
interleaved with the beam interceptions of the first set of elements. 
As mentioned above, the mask receiving means may be mounted to the 
structure and in this case the elements of the first set may provide 
respective line segments at the same first spacial point upon their 
respective interceptions of the beam and the elements of the second set 
provide respective focussed lines at the same second spacial point upon 
their respective interceptions of the beam. More particularly where 
conical mirror segments are employed, then preferably the elements of the 
first set have conical surfaces that are segments of a first cone and the 
elements of the second set have conical surfaces that are segments of a 
second cone. 
In the apparatus there may be first and second fixed devices for re-imaging 
the focussed lines at said first and second points respectively to third 
and fourth spacially fixed points respectively. 
In another embodiment of the apparatus of the invention, the optical means 
includes at least one fixed optical element for focussing the beam to a 
line, and the afore-mentioned first and second optical elements comprise 
plane mirrors. In this case the first and second elements may be mounted 
for beam interception at different points along the beam and a respective 
fixed optical element is then associated with the first and second 
elements to receive the beam intercepted thereby and focus same to a line. 
The or each fixed optical element may comprise a conical mirror segment. 
More particularly in this embodiment, preferably the first element is one 
of a first set of like elements mounted to perform beam interception at a 
first point and at different times during the movement of said structure, 
and said second element is one of a second set of like elements mounted to 
perform beam interception at a second point displaced from the first point 
in the direction of beam impingement thereon and at different times during 
the movement of said structure that are interleaved with the beam 
interception of the first set of elements. A single respective fixed 
optical focussing element is associated with each of said first and second 
sets of elements, and each optical element of the first and second sets 
comprises a pair of plane mirrors. These plane mirrors of each pair may be 
set at right angles to one another, to reverse the direction of the beam 
impinging thereon. 
In a preferred arrangement for the just-discussed embodiment the optical 
elements of the first set are offset from those of the second set as seen 
normal to the direction of beam impingement and further comprises a 
further optical path switching means for receiving an input beam directed 
to said entry point and for switching said beam to first and second paths 
intercepted by said first and second sets of elements respectively, said 
further optical path switching means being operable in synchronism with 
the movement of the structure. 
It is also an aspect of the present invention to provide apparatus for 
repetitively marking a moving sheet of material with an image comprising: 
optical apparatus of the invention as set forth above 
means for generating and providing a laser beam at said beam entry point of 
the optical apparatus; and 
means for moving a sheet along a predetermined path to receive images of 
the or each mask scanned by a line segment. In this marking apparatus, the 
means for generating and providing the laser beam preferably includes a 
continuous wave laser, means for monitoring the alignment of the laser 
beam at the entry point to provide a signal indicating error in alignment 
and means in the laser beam path to said beam entry point for adjusting 
the alignment of the laser beam in accord with said error signal. It may 
further comprise means connected in a feedback path with the laser to 
monitor and regulate the laser output power.

In the figures, like reference numerals refer to like parts. 
Referring to FIG. 1 a web of paper 10 is shown as moving lengthwise from 
right to left in the direction of arrow B along a path guided by 
appropriate rollers (not shown). For the purposes of this description, it 
is assumed that the web will eventually be slit into a number of 
longitudinal sections, such as 10a-10d and that each section will be cut 
into individual sheets. The purpose of the apparatus is to mark each of 
the eventual sheets with a word or logo indicative of the manufacturer. 
Mounted along the path of the web is an optical system 20 shown more fully 
in FIGS. 2 and 3 to provide a series of marks along each longitudinal 
section 10a-10d of the web at intervals which will allow the mark to 
appear on each cut sheet. The optical system 20 receives a laser beam at 
an input location or beam entry point 22. The beam is generated by a 
continuous wave laser 30, such as a 1 KW or greater carbon dioxide gas 
laser. The beam 32 is shown in dot-dash line and is transmitted through an 
enclosed optical path 34 to the beam entry point 22. The exact path will 
depend on local circumstances but includes a beam modification unit (BMU) 
40 to control the beam at the entry point 22. The beam is guided in its 
path with the aid of plane mirrors such as 38a,b,c. 
The BMU 40 is effectively a telescope whose function is to provide a beam 
of required size and divergence at the entry point 22 from the beam 
emergent from the laser 30. The emergent beam will have a beam size and 
divergence particular to a given laser and thus the optical design of the 
BMU is dependent on a knowledge of the beam characteristics of the laser 
employed. The design of the BMU optics is a matter within the competence 
of those skilled in laser systems and need not be further discussed here. 
The beam characteristic at point 22 may be monitored at that point or as is 
illustrated, it may be monitored in terms of the beam emergent from BMU 40 
by a partial mirror 42 for tapping off a small proportion of the beam to a 
detector 44. Such detectors are known in the laser art and will not be 
described further. 
In any event it may prove desirable to have means of monitoring the 
alignment of the beam at the entry point 22 as well as its intensity. Beam 
intensity is of importance in order to obtain an intensity at the web 
surface that is sufficient to produce a visually acceptable mark without 
undue damage to the material. To this end a separate monitoring 
arrangement 46 for tapping off a small proportion of the beam and 
detecting its alignment and intensity may be located at or adjacent the 
point 22. The alignment measurement may be used to generate a control 
signal for one of the earlier beam deflecting mirrors, such as 38b. The 
mirror may be movably mounted to be motor driven along orthogonal axes by 
means diagrammatically indicated at 48 and responsive to the alignment 
control signal applied over line 50. The intensity measure signal is 
applied over line 52 as a feedback signal to laser 30. A conventional 
modern laser is electronically controlled to establish its output power 
and thus is readily amenable to being connected into a power control 
feedback loop. There is advantage in measuring intensity as near to the 
final point of application of the beam as possible so as to not only 
compensate for fluctuations in the operation of laser 30 but also in the 
beam path. 
Having described the manner in which a laser beam is established, the laser 
processing optical system 20 for marking the web will now be described 
with reference to FIGS. 2 and 3. The system will be described primarily 
with reference to the optical elements employed for marking of one 
longitudinal section 10a of the web. Similar arrangements for marking the 
other sections are spaced transversely across the web and are angularly 
interleaved in a manner that will be discussed subsequently. 
Although reference has been made to an input location or beam entry point 
22, there is no precisely defined point as such. The apparatus thus far 
described provides a laser beam 32 of predetermined size and alignment to 
enter the optical system shown in FIGS. 2 and 3. As discussed beam 
alignment and intensity can be monitored at a point adjacent the input 
location 22 which may be regarded as a notional plane intersecting the 
beam entry into the optical system as indicated in FIG. 3. At this point 
the beam will be of circular cross-section having a diameter of say 8 mm. 
The optical system 20 includes a rotary construction, conveniently 
supported within a cylindrical drum 60 which is mounted for rotation about 
its axis 62. A motor drive (not shown) is provided to rotate the drum at a 
uniform speed or at a speed synchronous with web speed. The drum 60 is 
mounted above and spaced from the path of the web 10 with the drum axis 
normal to the direction of longitudinal movement of the web. In each 
marking of the web the optical system focuses a line of laser light on the 
surface of the web. To ensure accurate positioning of the web at the 
impingement of the line on the web, the web is preferably guided as shown 
in FIG. 4, where the web passes over a free-running roller 64 whose 
surface is normal to the optical axis at the point of impingement. For 
ease of illustration and understanding in FIGS. 2 and 3, the web is 
assumed to be moving in a plane beneath the drum 60. 
In its hollow interior the drum carries a plurality of sets of mirror 
segments the sets being axially spaced across the drum. For clarity of 
illustration only one set 70 is fully shown comprising, for example, seven 
segments 71a-77a, arranged in an annulus, equi-angularly spaced. Each 
segment is formed from a copper block having one surface 78 shaped and 
polished in the form of a segment of a cone whose axis coincides with the 
rotation axis 62 of the drum 60. The segments are mounted at the interior 
surface of the drum and may be located by an internal annular wall 61. 
Thus as mounted within the drum the conical mirror faces 78 form segments 
of a frusto-cone coaxial with the drum axis with the cone apex pointing 
away from the direction of the incoming laser beam 32. The laser beam axis 
is parallel to axis 62 and its radial offset from the axis the same as 
that of the segments so that each conical mirror segment intersects and 
traverses the beam as the drum is rotated. FIGS. 2 and 3 show the beam at 
the centre point of its impingement on one of the mirror segments 71a. 
It is a characteristic of the conical mirror, rotated in this orientation, 
that as long as the laser beam impinges on it, it will focus the beam into 
a spacially fixed line segment. This line segment lies on the cone axis in 
the absence of any other optics. As the drum rotates the segments 71a-77a 
perform both beam switching and focussing the beam to a line. The 
line-focussed beam will be referred to as a line beam. 
The optical system 20 further includes a plane mirror 80 fixedly supported 
within the drum 60, and positioned and oriented to produce the line focus 
at the wall of the drum. The line beam produced by segments 71a-77a exits 
the drum through respective apertures 81a-87a in its wall that are aligned 
with the mirror segments. It will be appreciated best from FIG. 2 that as 
each segment 71a-77a intercepts the incoming laser beam 32 it produces for 
the duration of the interception a spacially fixed line beam at a point 82 
that lies in the axial direction and thus scans at least part of the 
associated aperture 81a-87a moving transversely past it. The optical axis 
through point 82 is radial to the drum axis so that the line beam lies in 
the drum wall and the extended optical axis intersects the web normal to 
its plane and to the direction of movement. Exterior to the drum the 
optical system 20 includes an imaging lens 90 that is mounted on the 
extended optical axis to image the line beam onto the web surface. Thus as 
the line beam scans one of apertures 81a-87a, its image, also fixed in 
space, scans a portion of the moving web surface. If the line beam is 
modulated at the aperture the modulation is reproduced in the line beam 
acting on the web surface so as to mark it. By placing a means for 
modulating the line beam, that is for selectively masking the beam, in the 
aperture, the image carried by the modulating means is reproduced on the 
paper surface. 
As seen in FIGS. 3 and 10 the modulating means 84 is a stencil cut with a 
trade name, logo or whatever image is to be reproduced on the paper and 
that image is scanned by the line beam. The stencil mask is a thin sheet 
of reflective copper located in the aperture at the outer drum surface. 
Thus the point 82 of the line beam focus is at the drum surface. At the 
beam scans the image opening in the stencil the modulated line reproduces 
the image on the web as is best seen in FIG. 3. If the peripheral speed of 
the drum is the same as that of the web, the mask image will be reproduced 
exactly on the paper; if not, the reproduced image aspect ratio will not 
be 1:1. The maximum width--in the direction of the line beam--of the 
stencil opening is less than the length of the line segment. In 
reproducing an image from a stencil, it has to be appreciated that the 
imaging lens 90 produces a reversed image. 
The illustrated mirror segment set has seven apertures and thus seven 
identical masks are to be located in the respective apertures if identical 
marks are to be employed throughout. 
It will be appreciated that the arrangement thus far described will produce 
a series of spaced marks, seven per rotation of the drum, along a 
longitudinal axis of the web. Thus this series of marks may be aligned 
along one of the longitudinal sections, such as 10a, into which the web is 
to be later slit. The beam switching technique is extended to further 
laterally spaced optical systems whereby a plurality of longitudinal 
series of marks may be made spaced transversely across the web, so as to 
mark the four longitudinal sections 10a-10d shown in FIG. 1. 
To this end the drum 60 is provided with three further sets of seven mirror 
segments 71b-77b, 71c-77c and 71d-77d. The segments of each set are 
equiangularly spaced about the axis 62 but the sets are angularly offset 
from one another. The sets are axially displaced from one another along 
axis 62 so that the drum 60 contains four optical beam-switching, 
focussing and imaging systems 20 as already described in FIG. 3. That is, 
in addition to the set of mirror segments there is an associated set of 
aligned drum apertures, each containing a stencil mask, and a respective 
plane mirror 80 and imaging lens 90 for projecting a line beam onto the 
web surface. The four transversely-spaced plane mirrors 80 and imaging 
lenses 90 are supported on gantries (not shown) extending across the web 
and provide four transversely fixed outputs for marking the web. 
The construction and operation of the complete laser processing optics may 
be better understood by referring to FIG. 2. This shows in phantom the 
next set of segments 71b-77b spaced further along the axis but angularly 
offset from the segments 71a-77a. The next two sets are likewise offset 
such as is illustrated by way of example for segments 74a, 74c, 74d 
mutually offset with and between segments 74a and 75a. Thus as seen along 
the axis the segments of the sets are interleaved and the beam is 
successively switched by a respective segment from each set. 
This arrangement successively produces a mark in each of the four web 
sections 10a-10d in a sequence that is repeated seven times in each 
rotation of the drum in the illustrated case. 
Both the number of mirror segments in a set and the number of sets can be 
varied. There is advantage in having the annulus traced by the laser beam 
32 relative to the drum filled with mirror segments in making maximum use 
of the CW laser power. At the far side of the drum from the beam entry 
absorbent material can be placed to absorb any laser light that passes the 
optical system. In trying to utilise the greatest number of segments 
regard has to be had to the necessity that the beam should impinge on each 
mirror segments long enough to scan the stencil mask image. Thus the 
angular extent of the segment cannot be reduced too far. Additionally 
account needs to be taken of the physical width of the beam relative to 
the mirror segment. In a prototype apparatus the beam had a diameter of 8 
mm. The segments were about 35 mm wide in the direction of rotation so 
that the beam was fully intercepted by the mirror over a distance of about 
19 mm. The full intensity of the line-focussed beam is thus only realised 
over this lesser distance. 
In the practical implementation of the above described apparatus, it is 
recommended that the following provisions also be made. 
All stationary optics in the system are cooled to prevent possible 
distortion caused by overheating. A temperature controlled coolant is used 
and maintained above the `dew point` to prevent condensation on the mirror 
surfaces. The imaging lens assemblies are supplied with `dry` and `oil 
free` compressed air to prevent debris from collecting on the lens 
surfaces, and to stop `beam blocking` caused by vapour and debris from the 
web transport apparatus and related processes absorbing a significant 
portion of the laser beam. In order to assist efficient use of the laser 
energy vapour and debris from such processes must be effectively removed 
from the optical system, particularly at the beam-switching mirrors, using 
a combination of compressed air jets, extraction devices and shielding 
nozzles. 
In the apparatus described so far, the conical mirror segments have 
performed the dual function of optical path switching and beam focussing 
to a line. Reference will now be made to FIGS. 5 to 9 of the drawings 
which show three functional changes from the apparatus described above, 
but to produce the same end result. These changes are summarized as: 
separating the switching and focussing functions, using a combination of 
fixed and rotating mirrors to perform these functions, and specifically 
using fixed conical mirrors and plane mirror assemblies for switching; and 
using a further optical switching arrangement at the input to provide two 
sets of optical paths to use the beam as efficiently as possible. 
FIGS. 5 to 9 show an alternative form of the optical system 20 and are 
intended to receive the laser beam at the input point or plane 22 and 
manipulate the beam to mark the web at intervals along each of a plurality 
of longitudinal sections in the manner already described using a 
line-focussed beam to scan an image-bearing mask. 
FIG. 5 shows an axial section through part of a structure 100 extending 
across the web. The structure comprises a frame 110 fixedly supported to 
extend across the web and supports within it a rotary structure 120 
carrying switching mirrors. The axis of the rotary structure is denoted 
62. The frame carries a set of conical mirror segments 122a-122d each of 
which is mounted to a respectively upright support 124a-124d spaced across 
the web. The conical segments face outwardly and are mounted in a line 
equidistant from axis 62. The input beam is received at the left as seen 
in the figure but is optically switched in a manner to be described so as 
to impinge on each conical mirror from the right. The mirrors focus the 
beam to a line along an axis M--M which is the common axis of the conical 
mirrors and which is intersected by the image-bearing masks as described 
below. The lines modulated by the mask image are re-imaged by lenses 90 
onto the respective longitudinal sections of the web 10a, 10b and so on. 
The end result, therefore, is as described in the previous embodiment. 
The rotary structure 120 is supported within the frame 110 and includes a 
shaft 126 supported in bearings 127 in the end walls 112 of the frame. The 
shaft is rotated by a motor drive (not shown) and carries sets of beam 
switching mirrors spaced along the shaft to cooperate with respective 
conical mirrors 122a-122d Each set of beam switching mirrors is supported 
on what may be described as the spokes of a wheel structure whose hub is 
secured to the shaft 126 to rotate with it. The wheels 128a-128d are of 
identical construction and are located to rotate adjacent the respective 
conical mirrors 120a-120d to successively and repeatedly intercept the 
incoming beam and to direct it to the associated conical mirrors for 
successively imaging the web sections 10a-10d. To this end the wheels are 
successively angularly offset across the web as will become clearer from 
the following description. 
The construction of a wheel will be given with reference to FIGS. 5 and 6. 
FIG. 6 shows a wheel 128 as seen from the left in FIG. 5 and in the beam 
intercepting position of wheel 128a. 
The wheel 128 illustrated has eight equiangularly spaced spokes 130 
extending from a hub 132 secured to shaft 126 to a circular perimeter 134 
that rotates within the frame 110. Each spoke carries a pair of plane 
mirrors disposed at 90.degree. to one another to reverse the direction of 
the incoming beam. Each pair of plane mirrors comprises a radially inner 
mirror 136 arranged on a common circle that intercepts the line of the 
conical mirrors so that as the wheel rotates each inner mirror comes into 
facing position with the associated conical mirror as can be seen from 
FIG. 5. The outer mirrors 138 of the pairs are arranged alternatively on a 
first outer circle--mirrors 138a--and a second smaller radius outer 
circle--mirrors 138b. The separation of the outer mirrors into a regular 
sequence of circles could be extended in principle to three or more such 
circles. The effect of dividing the outer mirrors into two circles is to 
double the angle between successive mirrors on the same circle enabling 
more wheels to be utilised for a given angle subtended by each mirror. 
Supported at the outer extremity of each spoke of the wheel is a mask 
holder 140 so that as each mirror pair moves into beam intercepting 
position, it directs the beam onto the associated conical mirror to form 
the line beam spacially fixed as before to scan the mask moving with the 
wheel. 
FIG. 5 shows the beam 32a passing through an aperture 113a of the end wall 
112 parallel to the axis of rotation 62 to impinge on an outer mirror 138a 
to be reflected via the pair mirror 136 back in the axial direction onto 
the associated conical mirror segment 120a. This beam is focussed to a 
spacially-fixed line to scan the mask holder 140 at the outer extremity of 
the relevant spoke by virtue of the circumferential movement of the mask 
holder. As the structure 120 rotates the next wheel 128b brings a mirror 
138b into the alignment position. To impinge on this mirror the beam is 
switched by means to be described to a path 32b parallel and closer to the 
axis 62. This is diagrammatically shown by the dashed line optical path in 
FIG. 5 though it will be appreciated that at that point in the rotation 
this path has not yet been established. Further rotation successively 
brings into the beam interception position a mirror 138a of the wheel 128c 
and then a mirror 138b of the wheel 128d, and so on. For the sequence 
being described an odd number of wheels should be provided, that is five, 
seven and so forth. Taking five wheels by way of illustration the next 
beam interception will be by an outer mirror 138a of the fifth wheel. 
Following a 45.degree. rotation of structure 120 from the position seen in 
FIG. 6, the next intercepting mirror is a mirror 138b on the next spoke of 
wheel 128a. Thus the sequence continues until after a 90.degree. rotation 
the position is again as seen in FIGS. 5 and 6. Thus the sequence repeats 
every 720/n degrees where n is the number of spoke per wheel, and 
successive wheels along the shaft are angularly offset by 
360/(n.multidot.m) degrees where m is the number of wheels. 
It will be appreciated that if all the outer mirrors were arranged on a 
common circle, which is possible, it is more difficult to pack in the 
total number of mirrors similarly to the packing shown in FIG. 2. By 
alternating the outer mirrors on inner and outer circles the angular 
packing density is halved. There is an additional benefit. As mentioned in 
connection with FIG. 3, as the beam 32 moves from the mirror segment 71a 
of one set to the segment 71b of the next set, the intensity of the line 
focus on the mask is reduced during the period that the beam is partly on 
both segments. The actual scanning of the mask should be completed in the 
period during which the beam is fully intercepted by a mirror segment. 
Reverting to FIG. 6, it will be seen that the intercepting plane mirror 
138a of wheel 124a is angularly overlapped by a mirror 138b of the next 
wheel 124b shown in dashed line. The beam switching between paths 32a and 
32b is arranged to occur at a point 142 at which the beam is fully on 
mirror 138a to be fully on mirror 138b. This overlap exists between all 
the succession of mirrors 138a and 138b to ensure the fullest use of the 
beam energy. It also has the advantage that the spoke portions do not in 
fact interface with the beam. 
The input beam at 22 in FIG. 1 is being rapidly alternated between the two 
axes 32a and 32b. To accommodate the two beam axes, the end wall aperture 
113a is supplemented by a second aperture 113b for the beam 32b or a 
single elongate slot is provided. Each of the conical mirror supports 
124a-124d has a beam transmission slot 125. This may be better seen in the 
perspective view of FIG. 7 which shows just one wheel 128, with an outer 
circle mirror 138a intercepting the beam 32a to switch it via its mirror 
pair 136 onto the associated fixed conical mirror 120 which produces the 
line beam scanning the respective mask holder 140 projecting axially from 
the relevant spoke of the wheel 129. The line is refocussed by lens 90 
onto the web 10, the line extending transversely of the web. The rotation 
of wheel 123 and movement of the web are indicated by arrows D and E 
respectively. After a sequence of beam interception by other wheels as 
described, the mirror 138b will intercept the beam which at that time will 
be switched to position 32b shown in dashed line to complete the optical 
path to conical mirror 122. 
The switching of the beam to alternate between positions 32a and 32b will 
now be described with reference to FIGS. 8 and 9. The figures illustrate a 
beam chopper 160 which displaces the incoming beam 32 between the two axes 
32a and 32b. For the arrangement of FIG. 1 the incoming beam is parallel 
to axis 62 and is to be switched between the two paths 32a and 32b also 
parallel to axis 62. FIG. 8 shows the chopper as an apertured disc 162 
having a reflective front surface as seen in the face view of the disc 
(FIG. 9) with equiangularly located aperture sectors 164. The apertured 
and non-apertured sectors 164 and 166 respectively are of equal angular 
extent. The disc is mounted at an angle (conveniently 45.degree.) to the 
beam impinging on it. Where the beam is reflected off a front surface 
sector 166 it is directed along the beam axis 32a. The disc apertures may 
either be furnished with a rear-reflecting surface, e.g. by a reflective 
disc 168 fastened at the rear or by open apertures that allow the beam to 
strike a fixed plane mirror 170 set at the same angle as disc 162. This 
produces the second switched beam 32b or 32b'. The choice of which method 
is used may depend on the displacement required between beams 32a and 32b. 
If the disc 162 with the rear reflective surface 168 is used then the disc 
thickness is obviously related to the displacement required. 
The chopper disc 162 is rotated about its axis 163 by a drive (not shown) 
that is synchronized with the rotation of structure 120. Looking again at 
FIG. 6, the angle moved by that structure in moving from the beam 
impinging on a mirror 138a of one wheel to a mirror 138b of the next wheel 
is 360/mm degrees. If there are p apertured sectors 164 in the disc 162, 
the disc will need to have moved from a reflective front sector to an 
aperture, that is 180/p degrees. Thus the rate of rotation of disc 162 
relative to that of structure 120 is nm/2p. If as shown p=n=8, then the 
ratio is simply m/2. Thus for a five or seven wheel structure the ratio is 
2.5 or 3.5. 
Also referring again to FIG. 6 the relative position or phase of disc 162 
should be set such that the incoming beam crosses the edge between sectors 
at the point 140.