GB1592511A

GB1592511A - Surface inspection apparatus

Info

Publication number: GB1592511A
Application number: GB2083577A
Authority: GB
Original assignee: Ferranti PLC
Current assignee: Ferranti International PLC
Priority date: 1977-05-18
Filing date: 1977-05-18
Publication date: 1981-07-08
Also published as: DE2820910A1; JPS53143357A

Description

(54) IMPROVEMENTS RELATING TO SURFACE INSPECTION APPARATUS (71) We, FERRANTI LIMITED a Company registered under the Laws of Great Britain of Hollinwood in the County of Lancaster, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to. be particularly described in and by the following statement: This invention relates to surface inspection apparatus and in particular to optical inspection apparatus for providing an indication of the physical characteristics of the surface being inspected.

It is known to examine the physical characteristics of a surface by directing a beam of light onto the surface at an oblique angle and measuring the intensity of light reflected at angles surrounding the specular angle. The specular angle is that at which light would be reflected if the surface were to be replaced by a perfect mirror. By observing the angular distribution of the light at various locations on the surface an indication of the characteristics of the surface can be obtained.

Examination at each location usually takes the form of moving a collimating receiver across the scattered beam to measure the intensity of light received at different angles in turn, which requires moving between the different inspection locations and by the time taken to make the total number of measurements required.

Inspection may be speeded up by apparatus in which the incident beam is scanned rapidly across the surface (which may itself be moving) and detecting light reflected by means of a receiver housed in an enclosure to which light is admitted by a slit of predetermined narrow width in the direction of scan. Light reflected at the specular angle will only enter the enclosure by way of the slit when incident upon a particular part of the surface but reflected light scattered by a surface portion adjacent to said part will result in a received signal the intensity of the scattered light depending on the distance of the portion from said particular part, that is, the angle at which the light is scattered to enter the slit, and the amount increases as the light gets closer to the particular part at which specular reflection occurs.As the beam scans the surface the signal produced by the receiver will increase and decrease, the variation in amplitude relating to the angular distribution characteristic of the reflective qualities of the surface, but instead of being formed by detecting the intensity of light, reflected from a single surface point, by a receiver positioned at a range of angles, the variation results from the detection at different angles for different adjacent parts of the surface by a fixed receiver.

The enclosure may have a plurality of apertures in order to obtain distributions for different areas of the surface which are all scanned in turn by the beam.

Such apparatus, however, while it may give a signal enabling a full analysis of surface characteristics to be made requires either that the scanning apparatus is carried close to the surface locations at which measurements are being made and so retaining problems caused by the manoeuvering of the apparatus close to the surface, or is carried away from the surface requiring large aperture optics to direct a beam onto the surface and collect light reflected therefrom.

It is an object of the present invention to provide a surface inspection apparatus of simple construction which mitigates some or all of the above disadvantages.

According to the present invention surface inspection apparatus includes a laser arranged to produce a continuous beam of light, optical coupling means operable to direct the light from the laser to an inspection head arranged to be mounted adjacent a portion to be inspected of the surface, the inspection head containing optical means arranged to direct the beam onto the surface at an oblique angle thereto to illuminate said portion and a plurality of photodetection elements located in the plane of the light incident on, and specularly reflected from, the surface each to receive light reflected from the portion of the surface at a different one of a range of angles centred on the specular angle, and circuit means operable to interrogate each photodetection element in turn to produce a signal whose amplitude variation with time corresponds to the variation of intensity of reflected light with angle of the range. The surface inspection apparatus may include means operable to move the inspection head relative to the surface so as to change the portion of surface illuminated.

The photodetection elements may comprise an integrated array of addressable photodiodes responsive to interrogation signals applied to each diode in turn to provide serially on a single path output signals representing the intensity of illumination incident on each photodiode as it is interrogated.

The inspection head may include a focussing arrangement to focus the beam onto the surface and it may be arranged to ensure that reflected light always engages the photodetection elements. This may be achieved by arranging the focussing arrangement to provide a beam focussed to a circular spot on the portion of the surface to be inspected and in which light reflected at the specular angle is caused to diverge after reflection in a plane extending transversely to the plane of the light incident on, and specularly reflected from, the surface.

The optical coupling means may comprise an extensible tube fixed at one end to the laser and at the other end carrying the inspection head. The tube may comprise a plurality of sections each capable of being coupled to like sections to change the overall length of the tube and each section may have focussing means therein arranged so that the overall focal length of the focussing means is independent of the length of the extensible tube.

Alternatively the optical coupling means may comprise at least one flexible optical fibre fixed at one end to the laser and carrying at the other end the inspection head.

In this specification light is intended to include electromagnetic radiation at either side of the visible spectrum that is, infra red and ultra violet.

Embodiments of the invention will now be described by way of example with reference to the drawing accompanying the provisional specification, in which: Figure 1(a) is a sectional elevation through the optical parts of surface inspection apparatus according to the present invention Figure l(b) is a distribution curve showing the distribution of intensity of reflected light with deviation of the angle of reflection from the specular angle, Figure l(c) illustrates the output signal of the inspection apparatus, Figure 2(a) and 2(b) are sectional elevations through alternative arrangements of optical parts shown in Figure 1, Figure 2(c) is a cross-section through the optical part shown in Figure 2(b), and Figures 3 and 4 are sectional elevations through different arrangements of surface inspection apparatus to illustrate different optical coupling means.

Referring to Figure 1 apparatus 10 for inspecting the characteristics of a surface 11 comprises a light source 12 in the form of a He-Ne laser which produces light continuously in the form of a narrow substantially parallelsided beam 13. Attached to the laser so as to receive the beam 13 is optical coupling means in the form of a tube 14 carrying at the end remote from the laser an inspection head 15 forming an axially closed extension of the tube 14. The inspection head has an aperture 16 located at the side thereof and arranged in operation to overlie a portion of the surface 11 to be inspected. The inspection head contains a mirror 17 mounted adjacent the closed end and inclined to the beam 13 so as to direct it at the aperture 16 to strike the surface 11 at an oblique angle.A plurality of photo-detection elements are also contained in the form of an integrated array 18 of photodiodes which array is located to receive the beam after reflection by the surface 11. The diodes of the array are closely spaced and the array extends in the plane of the beam incident on and specularly reflected from the surface. The centre of the array is located so as to receive light reflected at the specular angle and the rest of the array is able to receive light scattered by the surface, that is, reflected at angles in a range of angles including, and centred on, the specular angle.

The diodes of the array are all connected to a single output lead 19 and are interrogated in turn, that is, the array is scanned (by circuitry not shown) to feed a signal related to the intensity of the illumination striking the addressed diode to the output lead 19.

Referring to Figure l(b) the function shows (for a surface other than a perfect reflector in which all light is reflected at the specular angle) the distribution of reflected light of intensity (I) against angle (z) from the specular angle.

The width and height of the function are both dependent on the nature of the surface but the general shape is applicable to most surfaces.

Figure 1 (c) shows the output signal which may be expected for a single scan of the array in terms of the output voltage against time. It will be seen that the output signal bears a strong resemblence to the distribution function and in terms of height and widths may be interpreted to provide the reflection characteristics of the surface of the location illuminated.

It will be appreciated that the optical components of the inspection head may be aranged - so as to cause the beam to strike the surface at an angle suited to the characteristics of the surface and the access to the surface available for the inspection head. In Figure 2 an arrangement is shown in which the tube of the optical N coupling means extends orthogonally to the surface. The beam 13 is deflected by a wedgeshaped prism 20 to strike the surface 11 at an oblique angle by way of an opening 21 in the end wall of the inspection head. The array 18 of photodiodes is located so as to receive light reflected from the surface by way of the opening 21.

Figures 2(b) and 2(c) show sectional side and cross-sectional elevations respectively of an alternative arrangement in which the plane of light incident on, and reflected by, the surface is orthogonal to the longitudinal axis of the tube. The light is redirected by a pair of inclined mirrors 22, 23 through an aperture 16 and the array 18 is located to receive light reflected by way of the aperture.

Figure 2(b) also shows some additional features which may be applied to the arrangements shown in Figure 1(a) and 2(a). The inspection head 15 may be provided with feet 24 and the head may be rotatable relative to the tube (if the beam 13 is transmitted along the axis of the tube) so that the interior of a closed body can be inspected.

The He-Ne laser employed has a typical beam divergence of about 1 milliradian from an initial diameter of about 0.8 mm. and for other than the shortest path lengths between the laser and surface it is desirable to employ a lens (25, Figure 2(b)) to accept the beam and focus it into the surface.

As the array 18 consists of a line of physically small photodiodes any tilting of the surface may result in the plane of the incident and reflected light not intersecting the array. Such an effect may be obviated causing the reflected beam to diverge in a plane transversely to the plane of the incident and reflected beams. The divergence may be caused by forming the lens 25 from two cylindrical elements. The elements are arranged to focus the beam to a circular or elliptical spot at the surface but to diverge after reflection in a plane transversely to that containing the incident and reflected beams so as always to engage the array.

It will be appreciated that when mirrors (17, 22,23) are employed these may be curved with either spherical or cylindrical surfaces to effect any focussing of the beam required.

In order to investigate fully the surface 11, the inspection head is moved over the surface after each scan of the array so that the surface is illuminated at a different location.

At each location the array may be scanned before further movement to provide a series of pulses in an envelope similar to that of Figure 1(c) but displaying the parameters associated with the surface at the point illuminated. This procedure is repeated for as many locations as it is desired to inspect. However, the time taken to scan an integrated array of photodiode is so short that a surface location may be examined while the inspection head is moving relative to the surface.

In order to inspect surfaces requiring the inspection head to be located at distances say 1 metre or more from the laser it is necessary to compensate for divergence of the laser beam.

Whereas simple focussing optics are required for short distances between the laser and inspection head for longer distances large diameter lenses would be required and in this situation a telescopic lens may be employed at the laser source to focus the beam onto the lens in the reading head.

An alternative arrangement is shown in Figure 3 in which the optical coupling means comprises an extensible tube formed of sections 26 each of which may be coupled to other like sections to form the tube and each of which contained focussing elements 27 to maintain the beam parallel. In this way the length of the tube may be changed at will without the need to refocus the beam onto the inspection head.

The tube sections may be removable, as shown, or, where focussing elements are omitted, may be formed of segments which slide within one another telescopically.

Yet another arrangement is shown in Figure 4 in which the optical coupling means comprises a flexible light guide 28 formed from one or more optical fibres and including the necessary electrical connections for the inspection head 5. The inspection head may thus be inserted into a closed object to inspect an inner surface and may be located and moved, where appropriate, by means of magnetic coupling (29) through the wall of the object.

It will be appreciated that optical inspection apparatus according to the present invention car be made physically small, as the source of light is a laser remote from the inspection head the limiting factor in size of the inspection head being the size of the photodiode array. Arrays are available commercially in integrated semiconductor form in which several hundred photodiodes are contained in an array length of 1 0mum., making possible to resolve reflected light distribution to better than 1% degree with the array 10 mm. from the surface. This of course enables such optical inspection apparatus to be built of compact size. An example of such an array is the C series solid state line scanners of Reticon Corp., Mountain View, California.

WHAT WE CLAIM IS: 1. Surface inspection apparatus including a laser arranged to produce a continuous beam of light, optical coupling means operable to direct the beam of light from the laser to an inspection head arranged to be mounted adjacent a portion to be inspected of the surface, the inspection head containing optical means arranged to direct the beam onto the surface at an oblique angle thereto to illuminate said portion and a plurality of photodetection elements located in the plane of the light incident on, and specularly reflected from, the surface each to receive light reflected from the portion of the surface at a different one of a range of angles centred on the specular angle, and circuit means operable to interrogate each photodetection element in turn to produce a signal whose amplitude variation with time corresponds to the variation of intensity of

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. end wall of the inspection head. The array 18 of photodiodes is located so as to receive light reflected from the surface by way of the opening 21. Figures 2(b) and 2(c) show sectional side and cross-sectional elevations respectively of an alternative arrangement in which the plane of light incident on, and reflected by, the surface is orthogonal to the longitudinal axis of the tube. The light is redirected by a pair of inclined mirrors 22, 23 through an aperture 16 and the array 18 is located to receive light reflected by way of the aperture. Figure 2(b) also shows some additional features which may be applied to the arrangements shown in Figure 1(a) and 2(a). The inspection head 15 may be provided with feet 24 and the head may be rotatable relative to the tube (if the beam 13 is transmitted along the axis of the tube) so that the interior of a closed body can be inspected. The He-Ne laser employed has a typical beam divergence of about 1 milliradian from an initial diameter of about 0.8 mm. and for other than the shortest path lengths between the laser and surface it is desirable to employ a lens (25, Figure 2(b)) to accept the beam and focus it into the surface. As the array 18 consists of a line of physically small photodiodes any tilting of the surface may result in the plane of the incident and reflected light not intersecting the array. Such an effect may be obviated causing the reflected beam to diverge in a plane transversely to the plane of the incident and reflected beams. The divergence may be caused by forming the lens 25 from two cylindrical elements. The elements are arranged to focus the beam to a circular or elliptical spot at the surface but to diverge after reflection in a plane transversely to that containing the incident and reflected beams so as always to engage the array. It will be appreciated that when mirrors (17, 22,23) are employed these may be curved with either spherical or cylindrical surfaces to effect any focussing of the beam required. In order to investigate fully the surface 11, the inspection head is moved over the surface after each scan of the array so that the surface is illuminated at a different location. At each location the array may be scanned before further movement to provide a series of pulses in an envelope similar to that of Figure 1(c) but displaying the parameters associated with the surface at the point illuminated. This procedure is repeated for as many locations as it is desired to inspect. However, the time taken to scan an integrated array of photodiode is so short that a surface location may be examined while the inspection head is moving relative to the surface. In order to inspect surfaces requiring the inspection head to be located at distances say 1 metre or more from the laser it is necessary to compensate for divergence of the laser beam. Whereas simple focussing optics are required for short distances between the laser and inspection head for longer distances large diameter lenses would be required and in this situation a telescopic lens may be employed at the laser source to focus the beam onto the lens in the reading head. An alternative arrangement is shown in Figure 3 in which the optical coupling means comprises an extensible tube formed of sections 26 each of which may be coupled to other like sections to form the tube and each of which contained focussing elements 27 to maintain the beam parallel. In this way the length of the tube may be changed at will without the need to refocus the beam onto the inspection head. The tube sections may be removable, as shown, or, where focussing elements are omitted, may be formed of segments which slide within one another telescopically. Yet another arrangement is shown in Figure 4 in which the optical coupling means comprises a flexible light guide 28 formed from one or more optical fibres and including the necessary electrical connections for the inspection head 5. The inspection head may thus be inserted into a closed object to inspect an inner surface and may be located and moved, where appropriate, by means of magnetic coupling (29) through the wall of the object. It will be appreciated that optical inspection apparatus according to the present invention car be made physically small, as the source of light is a laser remote from the inspection head the limiting factor in size of the inspection head being the size of the photodiode array. Arrays are available commercially in integrated semiconductor form in which several hundred photodiodes are contained in an array length of 1 0mum., making possible to resolve reflected light distribution to better than 1% degree with the array 10 mm. from the surface. This of course enables such optical inspection apparatus to be built of compact size. An example of such an array is the C series solid state line scanners of Reticon Corp., Mountain View, California. WHAT WE CLAIM IS:

1. Surface inspection apparatus including a laser arranged to produce a continuous beam of light, optical coupling means operable to direct the beam of light from the laser to an inspection head arranged to be mounted adjacent a portion to be inspected of the surface, the inspection head containing optical means arranged to direct the beam onto the surface at an oblique angle thereto to illuminate said portion and a plurality of photodetection elements located in the plane of the light incident on, and specularly reflected from, the surface each to receive light reflected from the portion of the surface at a different one of a range of angles centred on the specular angle, and circuit means operable to interrogate each photodetection element in turn to produce a signal whose amplitude variation with time corresponds to the variation of intensity of

reflected light with angle of the range.

2. Surface inspection apparatus as claimed in Claim 1 in which the photodetection elements comprise an integrated array of addressable photodiodes responsive to interrogation array of addressable photodiodes responsive to interrogation signals applied to each diode in turn to provide serially on a single path output signals representing the intensity of illumination incident on each photodiode as it is interrogated.

3. Surface inspection apparatus as claimed in Claim 1 or Claim 2 in which the inspection head includes a focussing arrangement arranged to focus the beam onto the surface.

4. Surface inspection apparatus as claimed in Claim 3 in which the focussing arrangement comprises optical elements arranged to focus the beam such that it forms a substantially circular spot on the portion of the surface to be inspected and light reflected at the specular angle is caused to diverge after reflection in a plane extending transversely to the plane of the light incident on, and specularly reflected from, the surface.

5. Surface inspection apparatus as claimed in any one of Claims 1 to 4 in which the optical coupling means comprises an extensible tube fixed at one end to the laser and at the other end carrying the inspection head.

6. Surface inspection apparatus as claimed in Claim 5 in which the extensible tube comprises a plurality of sections each capable of being coupled to like sections to change the overall length of the tube each section having focussing means therein arranged such that the overall focal length of the focussing means is independent of the length of the extensible tube.

7. Surface inspection apparatus as claimed in Claim 5 or Claim 6 in which the inspection head is coupled to the optical coupling means so as to be rotatable relative thereto.

8. Surface inspection apparatus as claimed in any one of Claims 2 to 4 in which the optical coupling means comprises at least one flexible optical fibre fixed at one end to the laser and carrying at the other end the inspection head.

9. Surface inspection apparatus substantially as herein described with reference to, and as shown in, any one of Figures 1 to 4 of the drawings accompanying the provisional specification.