Note: Descriptions are shown in the official language in which they were submitted.
=
CA 02812819 2013-03-27
- 1 -
Process for testing a laser device
The invention is concerned with methods for testing a laser device that is
capable
of being utilised for machining an object and that has been set up to emit
pulsed
focused laser radiation. In particular, the methods are to enable an
examination
of the positioning accuracy of the radiation focus of the laser radiation.
For the purpose of machining objects deep within the object material it is
known
to use ultra-short-pulse laser radiation with pulse durations within the
femtosec-
ond range (where appropriate, extending into the single-digit picosecond
range),
which is able to bring about a laser-induced optical breakthrough at the focus
and, resulting from this, a photodisruption that is substantially restricted
to the
area of focus. The prerequisite for this is a transparency of the object being
machined in respect of the laser radiation, which obtains, for example, in the
case of machining operations on the human eye above a wavelength of about
300 nm. In the case of laser machining of the human eye, fs laser radiation is
employed, in particular, for the purpose of generating incisions in the cornea
or
in other tissue parts of the eye, for instance within the scope of a LASIK
treat-
ment (LASIK: laser in-situ keratomileusis) for generating a flap, in the
course of a
corneal lenticle extraction for the purpose of generating a lenticular
intracorneal
lamella, or in the course of a corneal keratoplasty for the purpose of
excising a
piece of corneal tissue to be replaced or transplanted.
In all these forms of machining, a high positioning accuracy of the laser
focus in
all three spatial coordinates in the target tissue is required, in which
connection
currently conventional accuracy requirements stipulate a few pm and, in the
best-possible case, permit positioning tolerances of only 1 pm or 2 pm.
At least the fs laser devices employed in laser surgery on the human eye often
possess a mechanical interface unit, sometimes designated as a patient
adapter,
with a contact element that is transparent to the laser radiation and that
exhibits
a contact surface which has to be brought into planar abutting contact with
the
surface of the eye or generally with the object to be machined. The interface
unit is, for example, an exchangeable module that can be coupled to focusing
optics of the laser device. The contact element with its contact surface may
serve as positional reference for the adjustment of the position of the
radiation
'
4
CA 02812819 2013-03-27
- 2 -
focus. Insofar as the eye is applied onto the contact element, a precise
machin-
ing of the eye is possible, assuming a precise referencing of the focal
position in
relation to the contact surface.
The invention aims to make available to the user of an fs laser device a
routine
test that enables a simple examination of the accuracy of the positioning of
the
focus, in particular in the direction of propagation of the laser radiation
(hereinaf-
ter called the z-direction). The test and its result are preferentially to be
capable
of being documented in straightforward manner.
For the purpose of examining the spatial location and orientation of the
contact
surface of a contact element, designed as an applanation plate, of a laser
device,
US 2006/0114469 A1 proposes moving the radiation focus along predetermined
circular paths and registering, with a photodetector, plasma sparks which
arise if
the focus at the edge of the applanation plate impinges on the latter.
In contrast, the invention provides a process for testing a laser device that
has
been set up to emit pulsed focused laser radiation, the focal position of
which is
adjustable both in and across the direction of propagation of the laser
radiation,
the laser device exhibiting a contact element that is transparent to the laser
ra-
diation, with an abutment surface for abutment of an object to be machined,
the
process comprising the following steps:
- applying onto the abutment surface a test object that is transparent to the
laser
radiation at least in a machining region,
- beaming the laser radiation into the test object bearing against the
abutment
surface and in the process moving the focal position in accordance with a
prede-
termined test pattern for the purpose of generating enduring machining struc-
tures in the test object.
The invention furnishes the user with a simply applicable method with which it
can be determined within the daily routine whether the laser device complies
with the demands made of the positioning accuracy of the radiation focus. For
this test a suitable test object (reference sample) is made available which,
after
the test has been carried out, can be preserved and archived in the long term.
Because enduring machining structures are generated in the test object within
the scope of the test, the test and its result can also readily be
reconstructed at a
CA 02812819 2015-03-26
- 3 -
later time. The test object may, for example, be disc-shaped or plate-shaped.
Suitable by
way of material of the test object that is transparent to the laser radiation
is, for
example, PMMA (polymethyl methacrylate), though other materials, in particular
other
transparent, non-absorbing plastic materials, are by no means ruled out.
Certain exemplary embodiments can provide a process for testing a laser device
adapted
to emit pulsed focused laser radiation having a focal position which is
adjustable both in
and across a direction of propagation of the laser radiation, the laser device
including a
contact element that is transparent to the laser radiation and has an abutment
surface for
abutment of an object to be machined, the process comprising the following
steps:
applying onto the abutment surface a test object that is transparent to the
laser radiation
at least in a machining region, the test object comprising: an interlayer that
can yield a
plurality of discoloration zones in response to the laser radiation; and one
or more
reference markings marking a specified piercing region indicating where a
particular
discoloration zone corresponding to a particular depth appears only if the
laser device is
calibrated; beaming the laser radiation into the test object bearing against
the abutment
surface and moving the focal position in accordance with a predetermined test
pattern to
generate permanent machining structures in the test object, the test pattern
comprising
instructions to move the focal position to a sequence of z-depths including
the particular
z-depth, the focal position generating the machining structures comprising the
discoloration zones ascending in a stair-like manner in the direction of
propagation of the
laser radiation towards an outer surface of the test object facing the
abutment surface;
and determining a z-calibration of the laser-device by: if the particular
discoloration zone
corresponding to the particular depth appears at the specified piercing
region,
determining that the laser-device is calibrated; and if otherwise, determining
a correction
for correcting the z-calibration from a gradient of the stair pattern.
CA 02812819 2015-03-26
- 3a -
In one configuration of the process, the machining structures include one or
more
discolouration zones which contrast optically with surrounding material
regions of the test
object. Whenever a discolouration is mentioned here, this is not to be
understood as a
reference to the creation or changing of a genuine colour. Since the radiant
energy
beamed in may result in a local photodisruption of the material of the test
object, the
discolouration may consist, for example, in a mere local darkening
(blackening) or in the
creation of a milky/dull patch. The discolouration may accordingly consist in
a local
change in brightness or in a change in the grey level of the material of the
test object by
virtue of an arbitrary modification of the material which has been triggered
by the laser
radiation. In any case, in this configuration the interaction of the laser
radiation with the
material of the test object brings about the creation of a zone that can be
detected with
the naked eye or/and with a camera-based image-recognition system and that
stands out
optically in relation to the surrounding material regions of the test object
and accordingly
constitutes an optical contrast with these surrounding material regions.
The machining structures preferably include at least one first discolouration
structure
optically contrasting with surrounding material regions of the test object,
which ascends
in the test object along a direction of structural extent extending across, in
particular
in rectilinearly oblique manner, the direction of propagation of the laser
radiation
(z-direction) as far as an outer surface of the test object facing towards the
abutment
surface. The first discolouration structure may, for example, be represented
by a stripe
pattern consisting of a plurality of successive discolouration stripes along
the direction of
structural extent. Alternatively, the first discolouration structure may be
represented by a
flat discolouration surface ascending obliquely in relation to the direction
of propagation
of the laser radiation as far as the outer surface of the test object.
=
CA 02812819 2013-03-27
- 4 -
In the case of the configuration of the first discolouration structure as a
stripe
pattern, the discolouration stripes with their stripe plane are preferentially
ori-
ented orthogonally to the direction of propagation of the radiation. Stripes
fol-
lowing one another pairwise may in this case have a mutual separation in the
direction of propagation of the radiation of at most 10 pm, better at most 8
pm,
still better at most 6 pm and, for example, 5 pm. In the case of observation
of a
projection of the stripe pattern onto a plane that is orthogonal to the
direction of
propagation of the radiation, stripes following one another pairwise may have
mutual separation. It is, of course, not excluded that, in the case of such a
pro-
jection observation, stripes following one another pairwise are substantially
free
from any separation from one another.
In a preferred embodiment, the machining structures generated in the test
object
include a second discolouration structure optically contrasting with
surrounding
material regions of the test object, which forms one or more reference
markings
for the piercing region of the first discolouration structure through the
outer
surface of the test object. Depending on the location of the piercing region
of
the first discolouration structure relative to the reference markings, a
statement
is possible about the quality of the z-calibration of the laser device.
Advanta-
geously, the reference markings mark a specified piercing region of the flrst
discolouration structure through the outer surface of the test object.
Depending
on whether the first discolouration structure pierces the outer surface of the
test
object inside or outside the specified piercing region, the test can be
described as
passed or not passed. Such an evaluation is particularly easy both for the
user
and for an automated, camera-based evaluating system. For the purpose of
marking the specified piercing region, the reference markings may, for
example,
form a pair of marking lines running alongside one another in parallel with a
separation.
The test pattern may provide for the generation of a third discolouration
struc-
ture optically contrasting with surrounding material regions of the test
object,
which runs along the outer boundary of a predetermined available positioning
field for the focal position that is orthogonal to the direction of
propagation of the
radiation. This available positioning field represents the maximal scan region
in
which the radiation focus can nominally be adjusted in a transverse plane
relative
to the direction of propagation of the radiation (hereinafter called the x,y
plane).
-
. = CA 02812819 2013-03-27
- 5 -
The boundaries of this scan region may, for example, have been defined by con-
structional or other structural features of the scanner used for x,y
deflection of
the laser radiation or/and by control-engineering presets. Frequently the maxi-
mal scan region in the x,y plane is defined by a circle. By the third
discolouration
structure being generated directly on the outer boundary of the nominally
avail-
able maximal x,y scan region, it can easily be discerned whether the maximal
scan region can actually be traversed by the scanner. For if interruptions
arise in
the third discolouration structure, this is an indication that in the
interruption
region the nominally available maximal scan region cannot actually be fully
util-
ised.
In an alternative configuration, the machining structures may include one or
more cut surfaces by which the test object is separated into at least two
partial
objects detached from one another. By subsequent gauging of at least a frac-
tional number of the partial objects, equally a statement is possible about
the z-
positioning accuracy of the radiation focus and hence about the z-calibration
of
the laser device. This gauging may, in turn, be carried out by the user
himself,
or it may be carried out in automated manner by means of a suitable gauging
system.
According to a further development, the cut surfaces may separate out from the
outer surface of the test object bearing against the abutment surface a, for
ex-
ample, plate-shaped or disc-shaped partial object, the partial object that is
sepa-
rated out possessing uniform thickness or possessing several step-like regions
of
differing thickness which are offset from one another.
The test object is preferentially manufactured in its machining region from a
material that is transparent in the visible wavelength region and at the wave-
length of the laser radiation.
The test object may be formed homogeneously at least in its machining region.
Alternatively, it is conceivable that the test object is of multilayer
construction at
least in its machining region and the interactive reaction to the laser
radiation
beamed in is different in different material layers of the test object.
CA 02812819 2013-03-27
- 6 -
In order to hold the test object on the contact element during the implementa-
tion of the test, suction force may be employed. For this purpose the contact
element or a retaining body carrying said contact element may have been con-
structed with one or more suction chambers which are open towards the test
object and which are capable of being evacuated by an evacuation pump.
It has already been mentioned that the test object or its parts can be
enduringly
preserved after the generation of the machining structures. The archiving of
the
machined test object (or of its parts) is expediently undertaken with assigned
date or/and time data which advantageously provide an indication of the time
of
the implementation of the test. Accordingly, at a later time it is possible to
re-
construct at any time which archived test object is associated with the test
last
carried out prior to an eye treatment and whether this test was successful or
not.
The process may furthermore include an enabling of the laser device for a
laser
treatment of the human eye if a success of the test of the laser device is
deter-
mined, or a disabling of the laser device for a laser treatment of the human
eye if
a failure of the test of the laser device is determined. This enabling or
disabling
can be brought about by a program-controlled control unit of the laser device,
whereby a disabling of the laser device can be cancelled, for example, only by
a
following successful implementation of a renewed test. The determination of
whether or not the test was carried out successfully can be undertaken by the
control unit, for example on the basis of a user input via an input device of
the
laser device. For this purpose the control unit may, for example, prompt the
user, by means of an appropriate prompt on a monitor, to enter his/her own
assessment of the result of the test, for instance via a keyboard, a pointing
de-
vice or some other form of input device. Depending on which evaluation the
user enters, the control unit determines whether or not the test was
successful
and thereupon brings about an enabling or disabling of the laser device. In
the
course of the archiving of the test object, the evaluation entered by the user
may
also be stored.
In an alternative configuration, the evaluation of the test result can be
under-
taken automatically by a suitable gauging and evaluating system.
=
CA 02812819 2013-03-27
- 7 -
The invention will be elucidated further in the following on the basis of the
ap-
pended drawings. Represented are:
Figure 1: a greatly schematised block diagram of a laser machining device in
the
course of implementation of a calibration test according to an embodiment,
Figure 2: schematically, a test object that is used for the calibration test,
with
machining structures introduced therein, according to an embodiment,
Figure 3: details of the machining structures introduced into the test object
shown in Figure 2,
Figures 4a to 4c: schematically, different test results,
Figure 5: an example of reference markings that can be introduced into a test
object within the scope of a calibration test as part of the machining
structures,
Figure 6: a test object with a machining structure introduced therein
according to
a further embodiment,
Figures 7a and 7b: exemplary test results for a test object provided with the
reference markings shown in Figure 5,
Figure 8: a test object according to a further embodiment,
Figure 9: schematically, an example of a partitioning of a test object into
partial
objects within the scope of a calibration test and
Figure 10: a further example of a partitioned test object.
Reference will firstly be made to Figure 1. The laser machining device shown
therein ¨ denoted generally by 10 ¨ serves for placing incisions in the human
eye
by means of laser technology. It will be understood that this is only an exem-
plary application; in principle, the laser machining device 10 may also serve
for
other machining purposes.
-
=
. CA 02812819 2013-03-27
- 8 -
The laser machining device 10 includes a laser-source 12 which emits a pulsed
laser beam 14 with pulse durations within the femtosecond range, for instance
within the three-digit femtosecond range. Focusing optics 16 focus the laser
beam 14 onto a focal point 18, the position of the beam focus 18 in the
direction
of beam propagation (z-direction) and also in a plane perpendicular thereto
(x,y
plane) being adjustable by means of scan components 20 which here, for the
purpose of better clarity of layout, are represented as a unified functional
block.
The laser-source 12 and the scan components 20 are capable of being controlled
by a program-controlled control unit 22, to which a monitor 24 and also an
input
device 26 (e.g. keyboard, pointing device, etc.) are attached.
For the purpose of positionally accurate coupling of the laser radiation into
the
eye to be machined, the laser machining device 10 possesses an interface unit
(patient adapter) 28 which is detachably coupled to a casing of the focusing
optics 16 and which exhibits a contact element 30 transparent to the laser
radia-
tion and also a holder 32 for the contact element 30. In the exemplary case
shown, the contact element 30 is constructed as a plane-parallel applanation
plate; however, it may be fashioned otherwise on its side facing towards the
eye
or/and on its side facing away from the eye; for example, it may be concave or
convex. The side of the contact element 30 facing towards the eye forms an
abutment surface 34 which can serve as positional reference for the eye to be
treated. With a view to treating an eye, the latter is brought into abutting
con-
tact with the abutment surface 34, in which connection a suction ring, not
repre-
sented in any detail, may previously be mounted onto the eye in a manner
known as such, said suction ring being, in turn, capable of being firmly
coupled
with the interface unit 28, for example by suction force.
The abutment surface 34 of the contact element 30 constitutes a z-reference
which enables a highly precise positioning of the beam focus 18 within the
object
to be machined. For the purpose of examining the z-calibration of the laser ma-
chining device 10, the control unit 22 has been set up to bring about the
imple-
mentation of a calibration test in which by means of the laser beam 14 defined
test machining structures are worked into a test object 36 applied onto the
abutment surface 34, the test machining structures remaining enduringly in the
test object 36 and hence enabling a permanent documentation of the result of
the test. After implementation of the test, the machined test object 36 (or at
*
'
= = CA 02812819 2013-03-27
- 9 -
least a part of the same) is saved in an archive indicated schematically at
38.
Figure 1 shows some test objects already archived in the archive 38, which for
the purpose of better differentiation from the test object 36 to be machined,
bearing against the abutment surface 34, are denoted by 36'.
For the purpose of generating the test machining structures, the control
program
(not represented in any detail) of the control unit 22 contains suitable
instruc-
tions in order to move the beam focus 18 in a manner corresponding to a prede-
termined test scan pattern. A first configuration provides that the
interaction of
the laser radiation with the material of the test object 36 brings about an
endur-
ingly local discolouration in the test object 36, so that a discolouration
pattern
arises in the test object 36 in a manner corresponding to the test scan
pattern.
In the case of a test object 36 that is transparent within the visible
wavelength
region and to the laser radiation, the discolouration may, for example,
consist in
the material of the test object 36 becoming milky in the region of focus. In
an-
other configuration, the test scan pattern provides for the generation of
incisions
in the test object 36, in particular such incisions that result in a
partitioning of the
test object 36 into several partial objects. For the purpose of documenting
the
test, all the partial objects or only a fractional number of the partial
objects may
then be saved in the archive 38.
The test object 36 is, for example, a plate-like piece consisting of PMMA,
which
can be held on the interface unit 28 by suction force. For this purpose the
inter-
face unit 28 is equipped, in a manner not represented in any detail, with an
evacuation port to which an evacuation pump 42 is capable of being attached
via
an evacuation line 40.
Reference will now additionally be made to Figures 2 and 3. These Figures rep-
resent discolouration structures that, according to an embodiment, can be
intro-
duced into the test object 36 by means of the fs laser radiation of the laser
machining device 10. The discolouration structures include a stripe pattern as-
cending in the z-direction in stair-like manner, consisting of a plurality of
discol-
ouration stripes 44 oriented in each instance parallel to the x,y plane, and
also
two reference markings 46 running alongside one another in parallel along the
x-
y plane with a separation, which in the exemplary case shown appear as
straight
lines in top view of the x,y plane. Regardless of this, the two reference
markings
'
= . CA 02812819 2013-03-27
- 10 -
46 may have been formed as two-dimensionally extended reference stripes or
reference planes and possess a corresponding extent in the z-direction. But on
account of their linear appearance in top view of the x,y plane the two
reference
markings 46 will be designated hereinafter as reference lines. These reference
lines 46 define between themselves a specified piercing region 47 in which,
given
appropriate z-calibration of the laser machining device 10, the stripe pattern
formed by the discolouration stripes 44 should pierce the outer surface of the
test object facing towards the abutment surface 34 ¨ denoted by 48 in Figure
2.
Accordingly, the test scan pattern is designed in such a way that given
appropri-
ate z-calibration of the laser machining device 10 ¨ when observed in the
direc-
tion of ascent of the stripe pattern - the last visible discolouration stripe
44
should lie within the specified piercing region 47. This situation is
illustrated in
Figure 4a. Figures 4b and 4c illustrate, on the other hand, cases of an
incorrect
z-calibration of the laser machining device 10, in which the stair pattern of
the
discolouration stripes 44 pierces the outer surface 48 of the test object 36
in one
instance ahead of the specified piercing region 47 delimited by the reference
lines 46 (Figure 4b) and in the other instance behind it (Figure 4c). From the
gradient of the stair pattern, in the cases of Figures 4b and 4c the necessary
degree of correction for correcting the z-calibration of the laser machining
device
10 can readily be ascertained, or it may be decided that the process has to be
terminated.
For example, the mutual z-separation of the discolouration stripes 44 ¨
denoted
in Figure 3 by d1 ¨ amounts to 5 pm. With such a value for the separation d1
it
can be ensured that the beam focus can be positioned and checked with an inac-
curacy of at most 5 pm in the z-direction.
The stripe width ¨ denoted in Figure 3 by d2¨ amounts, for example, to about
250 pm. The mutual separation of the discolouration stripes 44 in x,y
projection
- denoted in Figure 3 by d3 - may, for example, correspond to the stripe width
d2, in the present case accordingly may likewise have a value of about 250 pm.
The mutual separation of the reference lines 46 ¨ denoted in Figure 3 by d4 ¨
amounts, in the exemplary case shown, to four times the stripe width d2. In
the
case of a stripe width of 250 pm, dimension c14 therefore amounts to 1000 pm.
Via the gradient (for example, 5 pm/500 pm), a miscalibration which possibly
obtains may also be calculated.
. .
=
- CA 02812819 2013-03-27
- 11 -
For the purpose of generating the discolouration stripes 44 in the test object
36
the test scan pattern deflnes a stair pattern which consists of a plurality of
stair
steps corresponding to the discolouration stripes 44, whereby each stair step
may
be formed by several scan lines running alongside one another in the
longitudinal
direction of the steps. It will be understood that only those stair steps of
this
stair scan pattern which extend within the material of the test object 36
result in
a corresponding discolouration stripe 44 in the test object 36. As a rule, a
frac-
tional number of the stair steps of the stair scan pattern will remain outside
the
test object 36, even in the case of a slight miscalibration of the laser
machining
device 10 in the z-direction. Such stair steps of the stair scan pattern lying
out-
side the test object 36 are indicated in dashed manner in Figure 2 at 44'.
The discolouration structures that are generated in the test object 36
include, in
addition, a circular line 50 situated substantially parallel to the x,y plane,
which,
for example, may correspond to a maximally available x,y scan region of the
laser
machining device 10 or may extend within such a maximal x,y scan region with a
separation from the outer boundaries thereof. For example, the circular line
50
may be generated with a diameter such as is typical of a corneal flap
generated
by laser technology within the scope of an fs LASIK treatment. Conventional
flap
diameters lie, for example, within the range between 9 mm and 11 mm. The
circular line 50 may accordingly have, for example, a diameter of 10 mm or
11 mm. If said circular line can be discerned completely and in undistorted
man-
ner in the test object 36 as an annular discolouration, this is an indication
that at
least the scan region needed for the generation of the flap is available in
unre-
stricted manner.
Figure 5 shows a variant in which the discolouration pattern inscribed into
the
test object 36 includes two pairs of reference lines 46 at right angles to one
an-
other in x-y projection. This makes it possible to introduce into the test
object
36, in two mutually perpendicular directions in each instance, a
discolouration
pattern ascending rectilinearly in stair-like manner or otherwise.
As an alternative to a discolouration pattern ascending in stair-like manner,
Fig-
ure 6 shows a flat discolouration surface 52 which, in the manner of viewing
of
Fig. 6, which represents an x,z section, appears as a straight line. The
discolour-
ation surface 52 may, for example, be realised by a plurality of lines scans
of the
' ..
= CA 02812819 2013-03-27
- 12 -
beam focus running alongside one another in the plane of the discolouration
surface 52. These line scans together form a planar scan pattern, the parts of
which situated outside the test object 36 are indicated in Figure 6 in dashed
manner at 52'. In a similar manner to that in the case of the discolouration
stripes 44, the calibration accuracy of the laser machining device 10 can be
as-
sessed on the basis of the position of the piercing line at which the
discolouration
surface 52 strikes the outer surface 48 of the test object 36 (the concept of
piercing is to be understood here figuratively, since the step pattern formed
by
the discolouration stripes 44 and also the discolouration surface 52 do not,
of
course, continue outside the test object 36; for an observer looking at the
outer
surface 48, however, it looks as if the outer surface 48 is pierced by the
stripe
pattern or by the discolouration surface 52).
Figures 7a and 7b illustrate the case where, given a test object with two
mutually
perpendicular pairs of reference lines 46 (corresponding to the variant shown
in
Figure 5), a flat discolouration surface similar to the representation of
Figure 6 is
introduced into the test object 36, assigned to each pair of reference lines.
The scan lines traced by the beam focus for the purpose of realising the
disco,-
ouration surface can be discerned. These are denoted by 54 and lie
sufficiently
closely together that to the observer a two-dimensional discolouration
impression
appears. Both in Figure 7a and in Figure 7b the respective discolouration
surface
pierces the outer surface of the test object 36 within the specifled piercing
region
defined between the pair of reference lines in question, i.e. an appropriate
cali-
bration obtains.
In the embodiment shown in Figure 8 the test object 36 is of multilayer
construc-
tion and exhibits an interlayer 56 consisting of a material differing from
that in
the regions of the test object 36 situated in the z-direction above and below
the
interlayer 56. The material of the interlayer 56 displays a different
interactive
reaction with the laser radiation beamed in within the scope of the
calibration
test from that of the remaining material regions of the test object 36. For
exam-
ple, the interaction of the laser radiation with the material of the
interlayer 56
results in a discolouration that is different from that in the remaining
material
regions of the test object 36. With given z-separation of the interlayer 56
from
the outer surface 48 of the test object 36, with the aid of the mutual
separation
CA 02812819 2013-03-27
- 13 -
of the points of piercing of the discolouration surface 52 through the outer
sur-
face 48 and the interlayer 56 not only can the z-calibration be examined for a
possible z-offset but an examination for correct scaling of the z-axis of the
coor-
dinate system used by the laser machining device 10 can also be performed. For
the purpose of unambiguous determination of the point of piercing of the
discol-
ouration surface 52 through the interlayer 56, it may be advantageous to con-
struct the test object 36 with a polished, preferentially rectilinear lateral
surface,
so that both the discolouration surface 52 and the interlayer 56 are
discernible in
a lateral observation. For this purpose the test object 36 may be constructed,
for
lo example, as a half-disc or quarter-disc.
Figures 9 and 10 show, in a manner not true to scale, two embodiments in which
the machining structures introduced into the test object 36 constitute
incisions
which result in a separation of the test object 36 into partial objects 36a,
36b.
For example, a plate-like partial object (Figure 9) or a partial object
stepped in
stair-like manner (Figure 10) can be cut out of the outer surface 48 of the
test
object. By a thickness measurement of the excised partial object 36b in the z-
direction, inferences as to the accuracy of the z-calibration of the laser
machining
device 10 can subsequently be obtained. The thickness measurement may, for
example, be carried out with optical, acoustic or mechanical measuring means.
As an alternative to a gauging of partial object 36b, it is conceivable to
gauge the
z-depth of the surface depression that has arisen in the remaining partial
object
36a as a consequence of taking out partial object 36b.
After placement of the machining structures in the test object 36, the control
unit
22 of the laser machining device 10 can bring about the output of a prompt on
the monitor 24 which prompts the user to enter, via the input device 26,
whether
the calibration test was successful or not. Depending on the input of the
user,
the control unit 22 then brings about an enabling or disabling of the laser ma-
chining deice 10 for later eye operations.
In a preferred embodiment, suitable precautions may have been provided in
order to aspirate vapour or/and particles which may arise in the course of
gener-
ating the machining structures in the test object.
