Sectioning a transparent material using optical radiation

Abstract

A method for creating cuts in a transparent material using optical radiation, wherein the optical radiation is focused at a focus in the material and the focus is displaced along a trajectory, wherein a periodic, crossing Lissajous figure is used as trajectory as viewed perpendicular to a main direction of incidence at the radiation.

Claims

1. A method for sectioning a crystalline lens or a lens capsule of the eye using optical radiation, comprising: focusing the optical radiation along a main direction of incidence and at a focus located within the material; and chopping a volume of the crystalline lens or the lens capsule of the eye by cuts having a grid structure by shifting the focus along a path having the form of a crossing Lissajous figure perpendicular to the main direction of incidence and by repeating the Lissajous figure either at several height levels that are stacked along the direction of incidence or while adjusting the focus along the direction of incidence.

2. The method according to claim 1, further comprising additionally shifting the focus back-and-forth along the main direction of incidence of the radiation.

3. The method according to claim 1, further comprising creating the Lissajous figure by superimposing a first harmonic oscillation with a second harmonic oscillation, wherein both harmonic oscillations have frequencies which amount to different integer multiples of a base frequency, and the frequency of the first oscillation is at least twice the base frequency.

4. The method according to claim 3, wherein the frequency of the first oscillation is at least three times the base frequency.

5. The method according to claim 1, further comprising using the Lissajous figure to defines a cut area such that the Lissajous figure comprises sections of the path extending beyond said cut area and the optical radiation is switched off or modified on said sections of the path extending beyond said cut area such that it does not create cuts in the transparent material on said sections of the path extending beyond said cut area.

6. The method according to claim 1, wherein the cuts have a grid structure and further comprising repeating the Lissajous figure in several height levels that are stacked along the direction of incidence.

7. The method according to claim 6, further comprising providing, between at least two height levels, an intermediate plane lying parallel to the height levels and forming a contiguous cut surface in the intermediate plane.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention is explained in further detail below by way of example with reference to the attached drawings which also disclose features essential to the invention. There are shown in:

(2) FIG. 1 is a schematic representation of a treatment apparatus for ophthalmology procedures, in particular for correcting defective vision,

(3) FIG. 2 is a schematic representation with regard to the structure of the treatment apparatus of FIG. 1,

(4) FIG. 3 depicts a basic principle for introducing pulsed laser radiation into the eye with the treatment apparatus of FIG. 1,

(5) FIG. 4 is a schematic representation to illustrate a grid of cuts which is to be created with the treatment apparatus of FIG. 1,

(6) FIG. 5 is a schematic representation to illustrate the creation of the schematically represented cut of FIG. 4,

(7) FIG. 6 is a representation similar to FIG. 5, but with a delimitation of cuts to a machining zone and

(8) FIGS. 7-38 are schematic representations of paths which can be used in the creation of cuts according to FIG. 5 or 6 with the treatment apparatus of FIG. 1.

DETAILED DESCRIPTION

(9) FIG. 1 shows a treatment apparatus 1 for eye surgery. For example an eye-surgery process which is similar to that described in EP 1 159 986 A2 or U.S. Pat. No. 5,549,632 can be carried out with it. The treatment apparatus 1 creates a material cutting in transparent material using treatment laser radiation 2. This material cutting can be e.g. a creation of cuts, in particular the treatment apparatus for correcting defective vision can bring about a change on an eye 3 of a patient 4. The defective vision can include hyperopia, myopia, presbyopia, astigmatism, mixed astigmatism (astigmatism in which there is hyperopia in one direction and myopia in a direction at right angles thereto), aspheric errors and higher-order aberrations. The material cutting can, however, also be used in the field of ophthalmology on other tissues of the eye, e.g. for sectioning the crystalline lens in cataract surgery. Where reference is made to eye surgery below, this is to be understood in each case only by way of example and not as limiting.

(10) In the embodiment example described, the components of the apparatus 1 are controlled by an integrated control unit, which, however, can of course also be formed as a stand-alone unit.

(11) FIG. 2 shows the treatment apparatus 1 schematically. In this variant it has at least three pieces of devices or modules. Laser device L emits the laser beam 2 onto the material, e.g. the eye 3, and adjusts the position of the focus in the material in three spatial directions x, y, z. The adjustment along the main direction of incidence of the optical radiation (z-axis) is called z-axis adjustment, the x- and y-axis adjustment is for example, carried out perpendicular to the z-axis by scanners.

(12) The operation of the laser device L is fully automatic, controlled by integrated or separate control device C. In response to a corresponding start signal the laser device L starts to deflect the laser beam 2 and thereby creates cuts which are constructed in a manner yet to be described.

(13) The control device C operates according to control data which either have been produced by it or have been supplied to it. In the latter case, which is shown in FIG. 2, the control data necessary for operation are supplied from a planning device P to the control device C beforehand as a control data set via control lines not identified in more detail. The determination or transmission of the control data takes place prior to operation of the laser device L. Naturally, the communication can also be wireless. As an alternative to direct communication, it is also possible to arrange the planning unit P physically separated from the laser device L, and to provide a corresponding data transmission channel.

(14) In ophthalmology, the defective vision of the eye 3 is preferably measured with one or more pieces of measuring device M before the treatment apparatus 1 is used. The measured values are then supplied to the control device or the planning device P and form the basis for the production of the control data. In particular, the position and/or extent of an area to be treated, in particular to be sectioned, can be measured.

(15) The control device or the planning device P produces the control data set from the measurement data which have been determined, e.g. for the eye to be treated. They are supplied to the planning device P via an interface S and, in the embodiment represented, come from measuring device M which has previously taken measurements of the eye of the patient 4. Naturally, the measuring device M can transfer the corresponding measurement data to the planning device P or directly to the control device C in any desired manner.

(16) Preferably, the control data set is transmitted to the control device and, further preferably, operation of the laser device L is blocked until there [exits] exists a valid control data set at the laser device L.A valid control data set can be a control data set which in principle is suitable for use with the laser device L of the treatment apparatus 1. Additionally, however, the validity can also be linked to further tests being passed, for example whether details additionally stored in the control data set concerning the treatment apparatus 1, e.g. an apparatus serial number, or concerning the patient, e.g. a patient identification number, correspond to other details that for example have been read out or input separately at the treatment apparatus as soon as the patient is in the correct position for the operation of the laser device L.

(17) The transmission of the measurement data and/or of the control data can be by means of memory chips (e.g. by USB or memory stick), magnetic storage (e.g. disks), or other data storage devices, by radio (e.g. WLAN, UMTS, Bluetooth) or wired connection (e.g. USB, Firewire, RS232, CAN-Bus, Ethernet etc.). A direct radio or wired connection has the advantage that the use of incorrect measurement data is ruled out with the greatest possible certainty. This applies in particular when the patient is transferred from the measuring device

(18) M or pieces of measuring device to the laser device L by means of a support device (not represented in the figure) which interacts with the measuring device M and the laser device L respectively such that the respective devices recognize whether the patient 4 is in the respective position for measurement or introduction of the laser radiation 2. The transmission of the measurement and defective-vision data to the treatment apparatus 1 can also take place simultaneously with bringing the patient 4 from the measuring device M to the laser device L.

(19) According to one example embodiment, it is ensured by suitable means that the control device or the planning device P always produces the control data set belonging to the patient 4 and an erroneous use of an incorrect control data set for a patient 4 is as good as ruled out.

(20) In the embodiment described, the laser radiation 2 is emitted as a pulsed laser beam focussed into the material, e.g. the eye 3. The pulse duration produced by the laser device L in this case is e.g. in the femto-second range, and the laser radiation 2 acts by means of non-linear optical effects in the material, e.g. the crystalline lens or cornea. The laser beam has e.g. 50 to 800 fs short laser pulses (for example 100-400 fs) with a pulse repetition frequency of between 10 kHz and 10 MHz. The type of material-cutting effect which the treatment apparatus 1 uses with the laser radiation, however, is of no further relevance for the following description, in particular there is no need to use pulsed laser radiation. The only important thing is that a focus of machining radiation 2 in the material is shifted along a path.

(21) The treatment apparatus 1 forms a cut in the material, the shape of which cut depends on the pattern with which the laser-pulse foci are/become arranged in the tissue. The pattern in turn depends on the path along which the focus is shifted. The path predetermines target points for the focus position at which one or more laser pulse(s) is (are) emitted and ultimately defines the shape and position of the cut.

(22) A possible mode of operation of the laser beam 2 is indicated schematically in FIG. 3. It is focussed into the material, e.g. the cornea 5 or lens of the eye, by means application of a lens system of the laser device L not identified in more detail. As a result there forms in the material a focus 6 in which the energy density of the laser radiation is so high that, in combination with the pulse length, a non-linear effect occurs. For example, each pulse of the pulsed laser radiation 2 can create at the respective site of the focus 6 an optical breakthrough in the material, e.g. in the cornea 5 or lens, which is indicated schematically in FIG. 3 by way of example by a plasma bubble. As a result, material, e.g. tissue, is cut owing to this laser pulse. When a plasma bubble forms, the tissue layer cutting comprises a larger zone than the spot covered by the focus 6 of the laser radiation 2, although the conditions for creating the breakthrough are achieved only in the focus. In order for an optical breakthrough to be created by every laser pulse, the energy density, i.e. the fluence, of the laser radiation must be above a certain threshold value which is dependent on pulse length. This relationship is known to a person skilled in the art from, for example, DE 695 00 997 T2.

(23) Alternatively, a material-cutting effect can also be produced by the pulsed laser radiation by emitting several laser radiation pulses in one area, wherein the spots 6 overlap for several laser radiation pulses. Several laser radiation pulses then interact to achieve a tissue-cutting effect. For example the treatment apparatus 1 can use the principle which is described in WO 2004/032810 A2.

(24) The treatment apparatus 1 creates in the cornea 5 or in the lens of the eye 1 a sectioning of the tissue by forming cuts as a grid (naturally, this is not limited to ophthalmology). By this grid represented schematically in FIG. 4, transparent material 1a is to be sectioned, with the result that it can be removed.

(25) In the transparent material 1a, a grid of cuts 10, 11, 12 is created by shifting the focus 6 of the laser radiation 2, which propagates along a direction of propagation 3a, inside the transparent material 1a along a path 6a. The path 6a is chosen such that it follows the cuts 10, 11 and 12 and travels along these.

(26) When controlling the laser device L, the control device C makes sure that the cuts 10, 11 and 12 are constructed by the trajectory 6a only contrary to the main direction of propagation 3a of the laser beam 2. Otherwise, the focus 6 would be disrupted by material cuts that are already present (for example cuts) in the incident light cone 4a. This problem is in principle posed in the case of material cutting by focussed optical radiation and is particularly great at crossover points 5a of the cuts 10, 11 and 12 in the use described by way of example.

(27) The cut 12 represented by a dashed line illustrates that the path 6a has to be arranged in different height levels in order to avoid the mentioned laser focus disruption in the incident light cone 4a. The path 6a is therefore formed such that it works through all cuts 10, 11 and 12 in the lowest height level first, i.e. in the height level that is the furthest removed from the laser device L in relation to the main direction of incidence 3a. If, with the path 6a, the cut lines of all cuts 10, 11 and 12 were worked through with it in this height level, the same is carried out for the next height level which lies closer to the laser device L in relation to the main direction of incidence 3a.

(28) In order to avoid a time-consuming deceleration and re-positioning or acceleration of the focus deflection, the grid of cuts represented schematically in FIG. 4 is generated by movement of the biaxial deflection of the laser device L on a path 23 in the shape of a Lissajous figure, which is represented in FIG. 5. The Lissajous figure only has to be provided in respect of the x-/y-deflection, thus as seen along the main direction of incidence 3a. Only from this view does a closed, thus periodic, path 6a must occur.

(29) There are thus two variants: Firstly, the path 6a can lie in one height plane. Then the z-position of the focus 6 remains constant as long as the Lissajous figure is travelled, and the path 6a is also closed in three-dimensional space. The control data or the control by the control device thus effect in each height level a closed path 23 on which the focus is guided. This is called variant 1 below.

(30) Secondly, the z-adjustment can adjust the position of the focus, while the Lissajous figure is being travelled. This is called variant 2 below. Unless differences in these two variants are explicitly discussed, the statements made here apply to both variants.

(31) As a Lissajous figure forms by a superimposition of harmonic oscillations, x-/y-deflection device with harmonic sine or cosine movements can be moved continuously. A high working speed is the result.

(32) In sections 23 of the path 6a which run outside of the zone in which the intended cuts are to be created (the cut 10 is also drawn in by way of example in FIG. 5), the laser beam is blanked, i.e. switched off or deactivated in respect of its treatment effect. The laser radiation is active and cuts tissue only in sections 24 of the path 6a which run inside the zone in which cuts are to lie. FIG. 5 shows, dotted, the sections 23 of the path 6a, in which the laser beam is blanked. The sections 24 of the path 6a in which the radiation brings about a material cutting are drawn in with continuous lines.

(33) FIG. 5 furthermore shows the spacing 8 between the height levels in which in each case the same Lissajous figure is travelled. The desired sectioning of the transparent material 1a achieved only when the path 6 follows Lissajous figures that are congruent in all individual height levels. In order to guarantee this feature, the Lissajous figure of the path 6a is a closed Lissajous figure in each height level.

(34) The x-/y-deflection movement of the focus 6 follows the equations

(35) $x=\underset{i}{\overset{}{.Math.}}{A}_x^i\cos \left(2f_x^{i}t+{}_x^i\right),y=\underset{i}{\overset{}{.Math.}}{A}_y^i\cos \left(2f_y^{i}t+{}_y^i\right)$
if the x- and y-scanners are controlled with control signals Sx and Sy according to the equations

(36) $\mathrm{Sx}=\underset{i}{\overset{}{.Math.}}a\left(f_x^i\right)A_x^i\cos \left(2f_x^{i}t+{}_x^i+\left(f_x^i\right)\right),\mathrm{Sy}=\underset{i}{\overset{}{.Math.}}a\left(f_y^i\right)A_y^i\cos \left(2f_y^{i}t+{}_y^i+\left(f_y^i\right)\right)$

(37) For the sake of simplicity, a possible scaling factor which describes the proportionality between the amplitude of the deflection movement and the control signal is here assumed to be 1. The term i is to be understood as an index and not as a power.

(38) In the equations a(f) describes a frequency-dependent amplitude attenuation and (f) describes a frequency-dependent phase retardation. Both of these frequency-dependent functions describe the response behaviour of the x-/y-deflection to the control signals. At very slow frequencies a=1 and =0 holds, and the scanners follow the control signals exactly. If the control frequencies rise, a(f) is reduced. The amplitude of the deflection movement thus becomes smaller than the controlling amplitude, and the phase retardation grows. The deflection movement follows the control signals only with some retardation. The frequency response from a and can be determined once for the scanner elements used and can be held available in the generation of the control signals.

(39) In order to create the Lissajous figure in respect of the x-/y-deflection, integer multiples of a base frequency f.sub.0 are chosen for all frequencies of the summands for x- and y-deflection,

(40) $x=\underset{i}{\overset{}{.Math.}}{A}_x^i\cos \left(2N_x^i{f}_{0}t+{}_x^i\right),y=\underset{i}{\overset{}{.Math.}}{A}_y^i\cos \left(2N_y^i{f}_{0}t+{}_y^i\right),\mathrm{with}{N}_{\mathrm{xy}}^i\left[1,2,3,\mathrm{.Math.}\right],$
whereby the path 6a is repeated with the base frequency f.sub.0.

(41) To create the cuts 10, 11 and 12, according to variant 1 the path 6a can comprise, in each height level, one full pass (or, as mentioned in the general part of the description, several passes) through the Lissajous figure.

(42) To create the cuts, according to variant 2 it is, however, likewise possible to carry out the shift along the main direction of incidence 3a during the Lissajous figure. The control device or the control data produced by the planning device effecting a z-adjustment of the focus 6 either in a short path section, which then represents a transition between two height levels of variant 1, or effecting a continuous z-adjustment of the focus 6. In relation to the above-mentioned equations, in which the scanning path is repeated with a base frequency f.sub.0, the z-coordinate of the focus 6 moves, in the case of continuous z-adjustment of the focus 6, in the time period 1/f.sub.0, by the desired distance 8 which path sections lying one above the other are to have.

(43) FIG. 6 shows, by way of example, a sectioning of a crystalline lens 30, which is carried out for cataract surgery. The treatment apparatus 1 creates cuts 31, by the path 6a following a Lissajous figure. In sections 32 of the path which lie inside the lens 30, the laser radiation 2 is active. In sections 33 which lie outside of the crystalline lens 30, the laser radiation 2 is blanked. Naturally, this principle can also be applied to other materials and illustrates that the combination of activating the laser radiation inside a desired volume and blanking outside of the desired volume makes possible a rapid construction of crossing cuts.

(44) The following features can additionally be realized:

(45) The laser radiation can also be blanked on particular sections of the path inside an area in which cuts lie if the course of the path on the Lissajous figure in those areas does not correspond to a desired cutting pattern.

(46) The height levels need not be planes in the mathematical sense. In particular in the case of a curvature of the image field and/or when curved material is machined, the height levels can be curved 2D manifolds.

(47) In the control of scanners, the amplitude attenuation and phase retardation of the x-/y-deflection device can be determined and taken into account by providing corresponding counterbalancing offsets of the amplitude and phase of the deflection control at high frequencies.

(48) The frequencies of the deflection on the Lissajous figures can take into account the maximum spacing of spots which successive laser pulses are to strike.

(49) Naturally, the cutting pattern created with the Lissajous figure can be supplemented by further cut elements, for example a cylinder jacket as outer delimitation of the treated volume area. In order additionally to also divide the parts of the transparent material 1a created by the cuts 10, 11 and 12 perpendicular to the direction of propagation 3a, cuts which extend substantially perpendicular to the main direction of incidence 3a can also be created between individual height levels.

(50) The cuts created using the Lissajous figure can be used to section eye tissue, for example the crystalline lens or the cornea. It is also possible to effect by the crossed cuts a targeted weakening of a material. In the field of eye surgery, this can be e.g. an intrastromal weakening of the cornea in order to influence the balance between intraocular pressure and cornea strength such that a desired change in the shape of the front surface of the cornea is achieved.

(51) FIGS. 7 to 38 show examples of patterns which the Lissajous figure can have when viewed along the main axis of incidence 3a. Further patterns are also possible.

(52) The Lissajous figures of FIGS. 7 to 18 are based on x-/y-deflection with single multiples of the base frequency, the Lissajous figures of FIGS. 19 to 38 are based on mixed multiples of the base frequency. The individual parameters are as follows:

(53) TABLE-US-00001 Nx Ny Ax Ay FIG. 7 1 2 2 1 90 FIG. 8 3 4 1.1 0.95 30 FIG. 9 5 6 1.02 0.96 18 FIG. 10 7 8 1.01 0.97 12.86 FIG. 11 2 3 1.5 1.05 45 FIG. 12 4 5 1.25 1 22.5 FIG. 13 6 7 1.15 1.03 15 FIG. 14 8 9 1.09 1.01 11.25 FIG. 15 1 3 2 1 90 FIG. 16 3 5 1.6 1.01 30 FIG. 17 5 7 1.33 1.02 18 FIG. 18 7 9 1.2 1.03 12.86

(54) TABLE-US-00002 N.sub.x.sup.i N.sub.y.sup.i Ax.sup.i Ay.sup.i .sub.x.sup.i .sub.y.sup.i FIG. 19 1; 2 1; 2 1; 0.63 1; 0.63 0, 90 90; 0 FIG. 20 1; 2 1; 2 0.65; 1 0.65; 1 0; 90 90; 0 FIG. 21 1; 2 1; 2 1.5; 0.75 1.5; 0.75 0; 270 0, 90 FIG. 22 1; 2 1; 2 1; 1 1; 1 0, 90 90; 0 FIG. 23 1; 3 1; 3 1; 0.45 1; 0.45 0; 90 90; 0 FIG. 24 1; 3 1; 3 1; 0.75 1; 0.75 0; 90 90; 0 FIG. 25 1; 3 1; 3 0.64; 1 0.64; 1 0; 90 90; 0 FIG. 26 1; 3 1; 3 0.65; 0.65 0.65; 0.65 0; 90 90; 0 FIG. 27 1; 4 1; 4 1; 0.35 1; 0.35 0; 90 90; 0 FIG. 28 2; 3 2; 3 1; 0.45 1; 0.45 0; 90 90; 0 FIG. 29 1; 4 1; 4 1; 0.7 1; 0.7 0; 90 90; 0 FIG. 30 1; 4 1; 4 0.7; 1 0.7; 1 0; 90 90; 0 FIG. 31 2; 3 2; 3 0.5; 1 0.5; 1 0; 90 90; 0 FIG. 32 1; 4 1; 4 1; 1 1; 1 0; 90 90; 0 FIG. 33 2; 3 2; 3 1.1; 0.85 1.1; 0.85 0; 90 90; 0 FIG. 34 1; 5 1; 5 1.3; 0.55 1.3; 0.55 0; 90 90; 0 FIG. 35 1; 5 1; 5 1; 0.6 1; 0.6 0; 90 90; 0 FIG. 36 1; 5 1; 5 0.8; 1.3 0.8; 1.3 0; 90 90; 0 FIG. 37 1; 5 1; 5 0.7; 0.7 0.7; 0.7 0; 90 90; 0 FIG. 38 1; 5 1; 5 1; 1 1; 1 0; 90 90; 0

(55) The following example is given for determining the path:

(56) The path 6a shall section a circular zone with a diameter of e.g. 5 mm over a height extension of e.g. 1 mm according to FIG. 12 into e.g. 44 sections. A maximum distance of e.g. ds=3 m of neighbouring laser pulse positions and dT=3 m of two paths lying one above the other in the main direction of incidence is to be observed. The laser device L emits laser pulses e.g. with a frequency f.sub.L=500 kHz. The speed v of the movement of the scanner position should not exceed v=ds*f=1.5 m/s, but should, as far as possible, lie at this maximum to achieve a short treatment time.

(57) The following applies to the speed along the path:
v={square root over ({dot over (x)}.sup.2+{dot over (y)}.sup.2)},
wherein {dot over (x)}, {dot over (y)} are the time derivatives of the x- and y-coordinates of the focus position.
With
x=A.sub.x cos(2f.sub.0N.sub.xt),y=A.sub.y cos(2f.sub.0N.sub.yt+)(by way of example x=0)
the time derivatives are:
{dot over (x)}=2f.sub.0N.sub.xA.sub.x sin(2f.sub.0N.sub.xt),{dot over (y)}=2f.sub.0N.sub.yA.sub.y sin(2f.sub.0N.sub.yt+).

(58) The speed v becomes maximum at time t, if the sine term of both {dot over (x)} and {dot over (y)} assume the maximum value 1 or 1 and is then:
{circumflex over (v)}=2f.sub.0{square root over ((N.sub.xA.sub.x).sup.2+(N.sub.yA.sub.y).sup.2)}.

(59) The cosine terms of x and y are then precisely 0 and the laser focus is then located exactly on an optical axis defining the main direction of incidence. At the position v does not quite achieve the maximum value {circumflex over (v)} but is upwardly limited by it and can be estimated without great errors by {circumflex over (v)} (e.g. for the path according to FIG. 16).

(60) The dimensions of the path is set via the amplitudes A.sub.x and A.sub.y. The minimum dimension is to cover the 5-mm diameter area. To create cuts which cross perpendicularly on the optical axis <x,y>=<0,0>, there must be at the axis position {dot over (x)}={dot over (y)}, thus N.sub.xA.sub.x=N.sub.yA.sub.y.

(61) To generate the depicted FIG. 12, an N.sub.x of 4 and an N.sub.y of 5 are necessary. A.sub.y must correspond to the desired diameter of e.g. 5 mm, and A.sub.x then is A.sub.x=N.sub.yA.sub.y/N.sub.x=6.25 mm. With these values, 222.14 mm*f.sub.0 is calculated for {circumflex over (v)}.

(62) Thus if v=1.5 m/s is to be achieved, the periodic path must be repeated with f.sub.0=6.75 Hz. For the scanners, the cosine frequencies follow f.sub.x=4*6.75 Hz=27 Hz and f.sub.y=33.75 Hz, which is set e.g. in the case of mirrors driven by galvanometer. For a height of 1 mm and with a distance between paths lying one above the other of dT=3 m, 333 passes through the path are stacked above the other. At f.sub.0=6.75 Hz, this takes barely 50 s.

(63) The base frequency is found analogously for the remaining Lissajous figures or other geometric constraints, assuming the given or desired boundary conditions. E.g. for FIG. 8 with otherwise identical parameters results in a base frequency f.sub.0=9.5 Hz introducing the 333 cutting levels could be introduced in 35 s.

(64) The size of the parts which are created in the material by the sectioning depends, as FIGS. 7 to 38 show, on the Lissajous figure used. If the side length of a part is denoted q and the radius of the zone in which the sectioning is to take place (circle in FIGS. 7 to 38) is denoted r, the following applies to

(65) FIGS. 7-10: q=2*r/N.sub.y

(66) FIGS. 11-14: q=2*r/N.sub.x

(67) FIGS. 15-18: q=2*r/N.sub.x