WO2009047350A1

WO2009047350A1 - A system and method for monitoring a laser drilling process

Info

Publication number: WO2009047350A1
Application number: PCT/EP2008/063677
Authority: WO
Inventors: Tony Flaherty; Bernard Fabien
Original assignee: National University Of Ireland, Galway
Priority date: 2007-10-11
Filing date: 2008-10-10
Publication date: 2009-04-16

Abstract

A system for monitoring a laser drilling process comprising an illumination source (15) and a processor (1). The illumination source (15) provides illumination light (2) to a drilling zone and the processor (1) captures the light (2) from the illumination source (15) reflected from the drilling zone during the drilling process and detects when the drilling process is complete based on the captured illumination light.

Description

Title

A system and method for monitoring a laser drilling process

Field of the Invention

The present invention relates to laser drilling of materials. More particularly, the invention relates to a system which enables a laser drilling process to be viewed in real time and provides for the automatic detection of the breakthrough point of the drilling process.

Background to the Invention

Laser drilling is a fast, efficient method for producing high precision, small diameter holes in a material. Typically a pulsed laser with pulse repetition frequencies in the range 50 Hz -10OkHz is used. Due to variations in material thickness or consistency, the number of pulses required to drill through a sample of material can vary significantly. This can lead to both the problems of under-drilling and over-drilling. In under-drilling, the hole is found to be too small, or the drilling is found not to be fully complete. In over drilling, the hole is too big. This may additionally lead to damage being done to a backing material, especially in a multi-layered system or in a small-bore tube.

To alleviate the above mentioned problems, breakthrough sensors are often used. These sensors determine when the breakthrough in the drilling process is reached i.e. that point at which the drilled hole is first opened. A number of types of breakthrough sensors are available. These include magnetic, acoustic and optical sensors.

Optical rear-side sensors, which detect light passing through the material, are the simplest type of breakthrough sensor. However, these sensors suffer severe practical limitations, in that it is impractical to apply them to parts with geometries more complex than simple sheet material. Front-side breakthrough sensors are therefore preferable.

Existing front-side breakthrough sensors generally sense, using photodiodes or cameras, some portion of the reflected laser light. The sensors often also monitor the intensity or temporal profile of the optical radiation emitted from the plume of hot ionised matter emitted from the surface of the sample which is generated during the laser drilling process, known as plasma. The drawback of this sensor is that optical emissions generated during drilling can make it difficult to image the process in detail, as the laser-drilling process generates a high-brightness plasma in the vicinity of the laser- material interaction, which can saturate an optical detector. This makes it difficult for the operator to align and optimize the drilling process. Capturing a high-quality image is therefore difficult.

The response of the sample to the laser during and immediately after irradiation can give valuable information to the process developer in determining thermal and mechanical stability of the sample and in finding optimal laser parameters for drilling or machining of the part.

There is a therefore a need in the art for a system that combines the functionality of viewing the drilling process in detail, as well as obtaining quantitative information on breakthrough, to provide an optimal hole drilling process. This system should be able to provide for the end point detection of both blind holes and through holes in a sample.

Summary of the Invention

The present invention provides a system for monitoring a laser drilling process comprising an illumination source for providing illumination light to a drilling zone; and a processor for capturing the light from the illumination source reflected from the drilling zone during the drilling process and for detecting when the drilling process is complete based on the captured illumination light.

By capturing only the illumination light, rather than the laser light and/or the plasma radiation, it enables the drilling process to be clearly visualized in real time. Furthermore, by detecting when the drilling process is complete based on the captured illumination light only, the technique of the present invention enables the endpoint to be accurately detected. Preferably, the detection comprises monitoring for a change in the intensity of the captured illumination light.

The monitoring for a change in the intensity of the captured illumination light provides for an accurate endpoint detection.

The system may further comprise a visual display for outputting the intensity profile of the captured illumination light.

The provision of a visual display enables a user to view the drilling process in detail in real time, and therefore make use of the visual information being provided in order to optimise the drilling process. It can also be used to aid in trouble-shooting of the drilling process.

Preferably, the illumination source has a wavelength range different to the wavelength range of the laser being used for the drilling process and the light generated at the drilling zone by the drilling process.

A filter may be adapted to only pass reflected light to the processor which corresponds to the wavelength range of the illumination source.

By the illumination source having a wavelength different to the wavelength range of the laser being used for the drilling process, and by the use of a filter adapted to filter out the optical emissions generated from the surface of the sample which have been found to make it difficult to view the drilling process, and only pass light corresponding to the wavelength range of the illumination source, a clear view of the drilling process is provided.

Preferably, an indicator is generated when the change in the intensity of the captured illumination light corresponds to a threshold value which has been determined for the type of material being drilled to indicate that the drilling process is complete. The threshold value will also depend on whether a blind hole or a through hole is to be drilled in the sample. The use of an indicator provides for the automatic notification that the drilling process is complete.

The indicator may be a control signal to the laser used in the drilling process to stop or block its optical emission.

By the indicator being a control signal to stop the laser drilling process, it ensures that the drilling process is stopped as soon as the endpoint has been reached, or at a specified time thereafter, in order to prevent overdrilling of the sample.

The processor may comprise a detector for capturing the reflected light and a signal processing unit for detecting when the drilling process is complete.

The system may further comprise a drilling laser.

The laser beam may be moveable over a given range.

By the laser beam being moveable over a given range, it enables a number of holes to be drilled in a target sample without requiring the sample to be moved.

The movement of the laser beam may be provided by moveable optics.

Preferably, the moveable optics comprise a galvanometer scanner.

The illumination source may be synchronisable to power off during a rapid change in the position of the laser beam.

By enabling the illumination source to be synchronisable to power off during a rapid alteration in the position of the laser beam, for example when moving from one machined feature to another, it prevents any motion blur which could result when the galvanometer scanner is moved to direct the drilling laser to a new position. The powering off of the illumination source may be controlled by a controller of the laser.

In one embodiment, the illumination source may comprise one or more high brightness light emitting diodes (LEDs).

The light emitting diodes may be fast switching light emitting diodes.

In another embodiment, the illumination source may comprise a laser.

By using a laser as the illumination source, a bright, intense and monochromatic source is provided, which enables high speed imaging to be achieved.

The system may further comprise a beam deflector for enabling the passage of the laser beam from the laser illumination source to the drilling zone, and directing the light from the laser illumination source reflected from the drilling zone to the processor.

The beam deflector may comprise: a polarizing beam splitter for polarising the laser beam; and a quarter wavelength waveplate for transforming the polarisation of the laser beam from the laser illumination source between linear and circular.

This enables the returning reflected beam from the laser illumination source to be directed by the beam-splitter towards the processor for capturing and detecting the reflected light.

By using a polarising beam splitter, power loss is reduced when compared to a simple beam splitter.

Alternatively, the beam deflector may comprise a beam splitter.

The laser illumination source may be arranged so that it is aligned and coincident with the drilling laser at the drilling zone. This focusing of the illumination source at the drilling zone increases the illumination intensity, further enabling high speed imaging to be provided.

In another embodiment, the laser illumination source may comprise a separate harmonic beam of the drilling laser.

This has the advantage of not requiring a separate illumination source to the light source already provided by the drilling laser.

The invention also provides a method for monitoring a laser drilling process by a drilling laser comprising: providing illumination light to a drilling zone; capturing the illumination light reflected from the drilling zone during the drilling process; and detecting when the drilling process is complete based on the captured illumination light.

Preferably, the step of detecting comprises monitoring for a change in the intensity of the captured illumination light.

The method may further comprise the step of outputting the intensity profile of captured illumination light to a visual display.

The method may further comprise prior to the step of capturing the illumination light of performing the step of filtering the reflected light from the drilling zone so as to pass only the light which corresponds to the wavelength range of the illumination source. Preferably, the method further comprises the step of generating an indicator when the change in the intensity of the captured illumination light corresponds to a threshold value which has been determined for the type of material being drilled to indicate that the drilling process is complete. The threshold value will also depend on whether a blind hole or a through hole is to be drilled in the sample.

The indicator may be a control signal to the laser being used in the drilling process to stop or block its optical emission.

The method may further comprise the step of synchronising the illumination source to power off during rapid change in the position of the drilling laser beam.

Preferably, the method further comprises the initial step of: measuring the value of the intensity of the illumination light at the process endpoint for the same type of material as that on which the drilling process is to be performed; and setting the threshold value to this value.

The illumination source may comprise a laser, and the method may further comprise the steps of: providing a beam deflector for enabling the passage of the laser beam from the laser illumination source to the drilling zone; and directing the light from the laser illumination source reflected from the drilling zone for capture.

The beam deflector may further carry out the steps of: polarising the laser beam from the laser illumination source; and transforming the polarization of the laser beam from the laser illumination source from linear to circular on the path of the laser beam from the laser illumination source to the drilling zone, and from circular to linear on the return path of the reflected laser beam from the drilling zone.

The method may further comprise the step of arranging the laser illumination source so that it is aligned and coincident with the drilling laser at the drilling zone. In another embodiment of the invention, the illumination source comprises a separate harmonic beam of the drilling laser, and the method further comprises the step of providing a filter adjacent the drilling laser to separate the harmonic beams of the drilling laser for use as the illumination source and the laser beam for drilling.

The invention also provides a laser drilling system comprising: a laser and the monitoring system for the laser drilling process.

These embodiments will now be described with reference to and/or as illustrated in the accompanying drawings.

Brief Description of the Drawings

Figure 1 shows a block diagram of the system of the present invention for monitoring a laser drilling process;

Figure 2 shows a detailed view of the optical paths present at the focus point of a target material on which the drilling process of the present invention is being performed;

Figure 3 describes an embodiment of the invention where the illumination source comprises a laser; and

Figure 4 describes an embodiment of the invention where the illumination source comprises a separate harmonic beam of the drilling laser.

Detailed Description of the Drawings

The present invention will now be described with reference to the accompanying figures 1 to 4.

A laser 6 is provided for drilling or cutting a hole in a piece of a target material or sample 12. The drilling laser 6 may be a continuous beam laser or a pulsed laser. In the described embodiment, the laser is a pulsed laser with pulse repetition frequencies in the range 50 Hz-IOOkHz. In the embodiment shown, the point of focus of the laser beam 14 emitted from the laser 6 can be altered over a given range without moving the laser beam 14 using moveable optics. In the described embodiment, the optics comprise a galvanometer scanner 7. The galvanometer scanner 7 consists of a pair of mirrors 8 and 9, mounted orthogonally, on two low inertia oscillating motors, or galvanometers. The motors are computer- controlled, and allow the deflection of the laser beam over a large two-dimensional, or three-dimensional space. The galvanometer scanner 7 directs the laser beam 14 onto the target sample 12 using the pair of orthogonal mirrors 8 and 9. The laser beam exits the galvanometer scanner through a flat-field focusing lens 10, which focuses the laser beam 14 to a point on the focal plane, where the target 12 is placed. Using the galvanometer scanner, the laser beam 14 can therefore be scanned rapidly from one position to another on the target sample 12, in order to be aimed at particular focus points 13 on the sample 12, depending on the position on the sample 12 where holes are desired to be drilled. These holes may be either through holes or blind holes.

As shown in Figure 1, the light 2 from an illumination source 15 is reflected from the focus points 13, and passes back through the galvanometer scanner 7, towards a beam splitter 19, which directs the light towards a processor 1. The processor 1 is provided so as to capture reflected light 2 from the drilling zone present during the drilling process and detect when the drilling process is complete based on the reflected light 2. It will be appreciated that the reflected light 2 from the drilling zone is therefore captured by the processor 1 for the entire time period during which the laser beam is being focused onto the target sample 12. The nature of the reflected light which the processor 1 captures will be discussed in more detail below.

The processor 1 comprises a signal processing unit 11 and a detector 3. The detector 3 captures the reflected light 2 and outputs the profile of the captured light as an image to a visual display 4. In the described embodiment of the invention, the detector takes the form of a camera, which is a CCD or CMOS camera.

The detector 3 also passes the captured reflected light it receives to the signal processing unit 11 to monitor for a change in the intensity level of this light 2. Upon detection of a change in the intensity of the reflected light, representing the breakthrough of the drilling laser, the processor 1 can signal that the drilling should be stopped by the use of an indicator. The value of the change in intensity level which represents a breakthrough for the target sample 12 must be determined during process set up, and is explained later. This value will depend on whether a through hole or a blind hole is desired to be drilled, and also the material of the target sample. This indicator may be for example a visual or an aural indicator. Alternatively, a control "off signal from a controller (not shown), which takes a signal from the processor 1 , may be automatically sent to the drilling laser 6, or to an external device, to stop the drilling, either as soon as breakthrough has occurred, or at a specified time thereafter.

The illumination source 15 provides illumination light to the drilling zone. As shown in the figures, the illumination source is typically orientated so that its light 2 is directed to the drilling zone. The illumination source 15 is adapted to have a wavelength range different to the wavelength range of the drilling laser light 14, and to the wavelength range of the plasma radiation 17 generated from the plasma 18 during the drilling process (see Figure 2). In the described embodiment, the optical illumination source 15 comprises a high brightness illumination source, such as for example, an array of light emitting diodes (LEDs) or a laser.

An optical filter 5 positioned in front of the detector 3 of the processor 1 is selected to only pass that light 2 to the detector 3 which corresponds to the wavelength range of the illumination source 15. The filter 5 therefore ensures that plasma radiation 17 and drilling laser radiation 16 reflected from the drilling zone are rejected, so that clear visualization of the laser drilling process is made possible by means of the detector 3.

In use, the laser 6 is switched on, and the laser beam 14 is directed by means of the galvoscanner 7 to the target part 12 of the sample. Light radiating from the drilling zone is then received at the filter 5. This light includes plasma radiation 17 and laser radiation 16, as well as reflected radiation 2 from the illumination source 15, as shown in Figure 2. The filter 5 blocks the light in the wavelength range of the plasma 17 and the reflected light in the wavelength range of the drilling laser 16, with only the light corresponding to the wavelength of the illumination source 15 being permitted to pass through to the detector 3. The reflected illumination light 2 which passes through the filter is then captured by the detector 3 of the processor 1.

The intensity profile of the reflected illumination light 2 captured by the detector 3 is output to the visual display 4. The visual display 4 allows direct visualization of the drilling process, enabling a user to monitor the drilling process in real time, and also to detect the breakthrough when it occurs. This gives the benefit of being able to view the process in detail, and therefore make use of the visual information being provided in order to optimise the drilling process, such as for example to correctly align the laser beam or to adapt the laser power to the machining process. It can also be used to aid in trouble-shooting of the drilling process.

It will be appreciated that once breakthrough occurs, the light from the illumination source 15 will be channeled through the bored hole, so that the intensity of reflected light 2 will be suddenly reduced. In particular, once breakthrough occurs, the intensity profile of the reflected light across the machined area (hole) will show a sudden change, as the area around the hole reflects the illuminated light, and the centre of the hole will not reflect the light anymore (where the hole to be drilled is a through hole). This fact enables the automatic detection of when the breakthrough occurs.

The signal processing unit 11 receives the captured reflected light 2 which has originated from the illumination source 15, and monitors its intensity to determine when a change in its intensity is registered. This is achieved by the processing unit 11 comparing the intensity of the received reflected light 2 with a previously stored threshold value, which corresponds to a value found to indicate that a breakthrough in the drilling process has been reached for the type of material being drilled. When the processing unit 11 detects a change (usually a drop) in the intensity of the received reflected light 2 being a value that matches this threshold value, it generates an indicator to signal that a breakthrough in the drilling process has occurred. As previously mentioned, the indicator may be a visual or an aural indicator. Alternatively, a control "off" signal from a controller (not shown), may take a signal from the processor 1, and be sent to the drilling laser 6 to stop or block its optical emission. It will be appreciated that the threshold value depends on the nature of the material and the geometry of the sample. Therefore, as part of the process setup, prior to monitoring for the breakthrough of a particular target material, the value of the intensity of the reflected light from the illumination source at breakthrough may be determined for the same type of material as that which is to be drilled. This can be achieved by performing a through-hole and/or a blind hole drilling on the material type, depending on whether a through hole or a blind hole is required, and measuring the value of the intensity of the illumination source at the point where the process end-point is detected, and setting this value to be the threshold value.

In one embodiment of the invention, the processor 1 can be set to detect a reduction in the intensity of the reflected illumination light 2 received in a specific area of the detector's field of view. This is achieved by the program driving the processor 1 being set to specify which part of the field of view (the region of interest- ROI) is to be acquired. The program can then measure the intensity of light on this ROI for each successive frame. It will be appreciated that the smaller the selected portion of the detector's field of view, the faster the determination of whether a breakthrough has occurred can take place, within the limits imposed by the detector technology.

In a preferred embodiment of the invention, a signal path is additionally provided between the illumination source 15 and a controller of the laser (not shown) to facilitate the synchronization of the illumination source 15 with the laser 6. A control signal may then be sent from the laser controller to power off the illumination source 15 when the drilling laser beam position is being altered. This can be carried out in particular during a rapid change in the position of the laser beam. This enables the use of the illumination source 15 in a strobe mode, and prevents any motion blur which could otherwise occur when the mirrors in the galvanometer scanner 7 are being moved to direct the laser beam to a new position, in preparation for the monitoring of the breakthrough of a new hole in the target material 12.

Figure 3 describes an embodiment of the invention where the illumination source 15 comprises a laser. The components of this embodiment are similar to those used in Figure 1 , but with the addition of a beam deflector comprising a polarising beam splitter 21 and a quarter wavelength waveplate 22.

In this embodiment, the polarising beam splitter 21 acts to allow only a linear polarisation of the light from the laser illumination source 15 to pass through. The quarter wavelength waveplate 22 positioned after the polarising beam splitter 21 then transforms the polarization from linear to circular. After the light from the laser illumination source 15 reflects from the drilling zone, the return path of the light through the quarter wavelength waveplate 22 changes the polarization back to linear, with an orientation orthogonal to that which had first passed through the polarising beam splitter 21. The polarising beam splitter 21 therefore deflects the light from the laser illumination source 15, which is reflected from the drilling zone, to the processor 1 , where it is captured and monitored for a change in intensity to indicate that the drilling process is complete.

It will be appreciated that the wavelength of the laser illumination source 15 should be chosen to be away from the peak intensity of the plasma emission spectrum 17 and the drilling laser wavelength 16, while at the same time being in the sensitivity band of the detector 3.

If desired, the beam deflector comprising the polarising beam splitter 21 and quarter waveplate 22 may be replaced by a simple beamsplitter. However, in this case, it will be appreciated that greater optical losses can be expected.

One advantage of the use of a laser as the illumination source 15 is that it provides a bright and monochromatic source when compared to more conventional sources. This is an important factor in high speed imaging, due to the fact that exposure time is limited by the period between two successive frames. The intensity of illumination achievable with a laser is also greater than that provided by conventional illumination sources, such as incandescent light or light emitting diodes. Furthermore, the laser illumination source 15 can be made aligned and coincident with the drilling laser 6, so as to focus illumination light only at the location where the drilling is taking place. This can be achieved using the same beam motion and focusing systems. This focused illumination, combined with the intensity of the laser source, results in the capability of high speed imaging during the machining process.

Figure 4 describes yet another alternative embodiment of the invention. In this embodiment, the illumination source 15 is provided by a separate harmonic beam of the drilling laser 6. The components of this embodiment are similar to those used in figure 3, but with the addition of a dichroic mirror 23, which is used to separate the drilling harmonic beam 14 from the illumination harmonic beam 15, and a mirror 24 to direct the illumination harmonic beam 15 into the polarizing beamsplitter 21. During operation of this embodiment, it will be appreciated that it is the light from the separate harmonic beam 15 of the drilling laser 6 reflected at the drilling zone which is captured by the processor 1 , and monitored to determine when breakthrough has been reached.

It will be appreciated that the present invention provides numerous advantages over the prior art techniques. In particular, the invention combines the functionality of visualizing the drilling process in detail, in real time, as well as obtaining quantitative information on breakthrough. This is achieved without the need to modify the system or to add optics or additional detection equipment in order to provide the analysis. It also enables process end-point to be detected for both through holes and blind holes.

The words "comprises/comprising" and the words "having/including" when used herein with reference to the present invention are used to specify the presence of stated features, integers, steps or components but does not preclude the presence or addition of one or more other features, integers, steps, components or groups thereof.

It is appreciated that certain features of the invention, which are, for clarity, described in the context of separate embodiments, may also be provided in combination in a single embodiment. Conversely, various features of the invention which are, for brevity, described in the context of a single embodiment, may also be provided separately or in any suitable sub-combination.

Claims

1. A system for monitoring a laser drilling process comprising: an illumination source for providing illumination light to a drilling zone; and a processor for capturing the light from the illumination source reflected from the drilling zone during the drilling process and for detecting when the drilling process is complete based on the captured illumination light.

2. The system as claimed in claim 1 wherein the detection comprises monitoring for a change in the intensity of the captured illumination light.

3. The system as claimed in Claim 1 or Claim 2, further comprising a visual display for outputting the intensity profile of the captured illumination light.

4. The system as claimed in any of Claims 1 to 3, wherein the illumination source has a wavelength range different to the wavelength range of the laser being used for the drilling process and the light generated at the drilling zone by the drilling process.

5. The system as claimed in Claim 4, further comprising a filter adapted to only pass reflected light to the processor which corresponds to the wavelength range of the illumination source.

6. The system as claimed in any of Claims 2 to 5, wherein an indicator is generated when the change in the intensity of the captured illumination light corresponds to a threshold value which has been determined for the type of material being drilled to indicate that the drilling process is complete.

7. The system as claimed in Claim 6, wherein the indicator is a control signal to the laser used in the drilling process to stop or block its optical emission.

8. The system as claimed in any of the preceding claims, wherein the processor comprises a detector for capturing the reflected light and a signal processing unit for detecting when the drilling process is complete.

9. The system as claimed in any of Claims 1 to 8 further comprising a drilling laser.

10. The system as claimed in Claim 9, wherein the laser beam is moveable over a given range.

11. The system as claimed in Claim 10, wherein the movement of the laser beam is provided by moveable optics.

12. The system as claimed in Claim 11, wherein the moveable optics comprise a galvanometer scanner.

13. The system as claimed in any of claims 10 to 12, wherein the illumination source is synchronisable to power off during a rapid change in the position of the laser beam.

14. The system as claimed in Claim 13, wherein the powering off is controlled by a controller of the laser.

15. The system as claimed in any of the preceding claims, wherein the illumination source comprises one or more high brightness light emitting diodes (LEDs).

16. The system as claimed in Claim 15, wherein the light emitting diodes are fast switching light emitting diodes.

17. The system claimed in any of Claims 1 to 14, wherein the illumination source comprises a laser.

18. The system as claimed in Claim 17, further comprising: a beam deflector for enabling the passage of the laser beam from the laser illumination source to the drilling zone, and directing the light from the laser illumination source reflected from the drilling zone to the processor.

19. The system of Claim 18, wherein the beam deflector comprises: a polarizing beam splitter for polarising the laser beam; and a quarter wavelength waveplate for transforming the polarisation of the laser beam from the laser illumination source between linear and circular.

20. The system of Claim 18, wherein the beam deflector comprises a beam splitter.

21. The system as claimed in any of Claims 18 to 20, wherein the laser illumination source is arranged so that it is aligned and coincident with the drilling laser at the drilling zone.

22. The system as claimed in any of the claims 17 to 21, wherein the laser illumination source comprises a separate harmonic beam of the drilling laser.

23. A method for monitoring a laser drilling process by a drilling laser comprising: providing illumination light to a drilling zone; capturing the illumination light reflected from the drilling zone during the drilling process; and detecting when the drilling process is complete based on the captured illumination light.

24. The method of Claim 23 wherein the step of detecting comprises monitoring for a change in the intensity of the captured illumination light.

25. The method of Claim 23 or Claim 24 further comprising the step of outputting the intensity profile of the captured illumination light to a visual display.

26. The method of any of Claims 23 to 25, wherein the illumination source has a wavelength range different to the wavelength range of the laser being used for the drilling process and the light generated at the drilling zone by the drilling process.

27. The method of any of Claims 23 to 26 further comprising prior to the step of capturing the illumination light of performing the step of filtering the reflected light from the drilling zone so as to pass only the light which corresponds to the wavelength range of the illumination source.

28. The method of any of Claims 24 to 27 further comprising the step of generating an indicator when the change in the intensity of the captured illumination light corresponds to a threshold value which has been determined for the type of material being drilled to indicate that the drilling process is complete.

29. The method of Claim 28 wherein the indicator is a control signal to the laser being used in the drilling process to stop or block its optical emission.

30. The method of any of Claims 23 to 29 further comprising the step of synchronising the illumination source to power off during a rapid change in the position of the laser beam.

31. The method of any of Claims 28 to 30, further comprising the initial step of measuring the value of the intensity of the illumination light at the process endpoint for the same type of material as that on which the drilling process is to be performed; and setting the threshold value to this value.

32. The method of any of Claims 23 to 31, wherein the illumination source comprises a laser, and further comprising the steps of: providing a beam deflector for enabling the passage of the laser beam from the laser illumination source to the drilling zone; and for directing the light from the laser illumination source reflected from the drilling zone for capture.

33. The method of Claim 32, wherein the beam deflector further carries out the further steps of: polarising the laser beam from the laser illumination source; and transforming the polarization of the laser beam from the laser illumination source from linear to circular on the path of the laser beam from the laser illumination source to the drilling zone, and from circular to linear on the return path of the reflected laser beam from the drilling zone.

34. The method of Claim 32 or Claim 33, further comprising the step of arranging the laser illumination source so that it is aligned and coincident with the drilling laser at the drilling zone.

35. The method of any of Claims 32 to 34, wherein the illumination source comprises a separate harmonic beam of the drilling laser, and the method further comprises the step of providing a filter adjacent the drilling laser to separate the harmonic beams of the drilling laser for use as the illumination source and the laser beam for drilling.

36. A laser drilling system comprising: a laser; and the system as claimed in any of Claims 1 to 8.

37. A closed surface drilled by the laser drilling system of Claim 36.

38. A tube drilled by the laser drilling system of Claim 36.

39. A system substantially as described herein, with reference to, and as illustrated in, the accompanying drawings.

TOMKINS & CO.