WO1990005907A2 - Inspection apparatus - Google Patents

Abstract

An optical apparatus including focusing means (41), an optical axis, an object point (31) and an image point on said optical axis, said object point being movable along the optical axis with respect to the focusing means, means being provided to change the effective optical distance between either the object point and the focusing means or between the image point and the focusing means to maintain the object point at a fixed point even if the image point moves.

Description

INSPECTION APPARATUS

The present invention relates to an optical apparatus preferably an inspection apparatus. The inspectio apparatus which will be described as a preferre embodiment of the invention is an apparatus fo inspecting a surface of, for example, a panel of a article such as a motor car but is not restricte thereto.

There is a considerable requirement for the inspectio of surfaces, and in particular, the surfaces o articles. Many articles require inspecting, fo example domestic goods and motor cars in particular There are a number of difficulties to be overcome i properly inspecting the surface of such articles.

One of the problems is that, particularly in the cas of motor cars, they are irregularly shaped and on production line a variety of different shaped moto cars may succeed each other down the production line The inspection apparatus should be able to cope wit such changes.

It is necessary usually to move the optical syste close to the surface to be inspected, and any movemen of the optical system away from that surface cause problems of focusing of the beam of radiation.

Through the specification we will refer to "light" "optical" and like expressions which refer to radiatio of visible wavelengths. It will be understood that th apparatus may be suitably modified to deal wit radiation of other wavelengths such as ultra violet o infra red and such apparatus is included in the scop 2 -

of the invention.

According to a first aspect, the present invention provides an optical apparatus including focusing means, an optical axis, an object point and an image point on said optical axis, said object point being movable along the optical axis with respect to the focusing means, means being provided to change the effective optical distance between either the object point and the focusing means or between the image point and the focusing means to maintain the object point at a fixed point even if the image point moves. Preferably the focusing means remains stationary, and the means for changing the effective optical distance comprises a movable non-focusing element. Preferably means may be provided to move said movable non-focusing element in such a way that continuous changes in the continuous movement of the object point is compensated so that the image point remains stationery at all times. Preferably said means may comprise a movable mirro means, the mirror means being in the form of tw mirrors at an angle to one another and being rotatable together about an axis to one side of the optical axis

According to a second aspect of the invention, there i provided an optical apparatus (which need not be a inspection apparatus), including optical means fo varying the focusing effect of a focusing component said optical apparatus including a main optical axis the focusing component being on said main optical axi and having an optical axis, the focusing componen being tilted so that the two optical axes are tilte with respect to each other and means being provided t pass a beam of radiation through said focusin component at a variable transverse position wit - 3 -

respect to the main optical axis to thereby vary th focusing effect of said focusing component.

The means to move the beam may comprise a tiltabl mirror.

According to a third aspect, the invention may compris an inspection apparatus, including means to pass a bea of radiation across a surface to be inspected bea shaping means being provided whereby, different point during said movement, the cross section of the beam o radiation is changed by beam shaping means so as t detect different types of fault. Thus, if the beam o radiation is scanned across the surface in two opposit directions, whilst scanning in one diretion, the cros section of the beam at the surface may be of a firs shape, and when scanning in the opposite direction, th cross section may be of a different shape. Typically, one of these shapes may be circular, and the other slo shaped .

According to a fourth aspect, the invention ma comprise an optical apparatus in which a composite bea comprising a plurality of beams of separate, distinct wavelengths is provided, whereby some optica components in the apparatus sensitive to wavelength ma affect the different beams in a different manner, an other optical components not sensitive to wavelengt may affect all of the beams in a similar manner.

The invention may also provide according to a fift aspect an inspection apparatus in which means i provided to pass a beam of radiation to a surface to b inspected, means is provided to collect the beam fro the surface, and means is provided to determine th 4 -

sharpness of the cross-section of the beam collected from the surface under inspection. Thus, said collected beam may be passed over an opaque edge, and then to a radiation detector, and means being provided to determine the shape of the beam cross section from the output of the signal detector. Thus a fuzzy beam cross section will be detected as providing a different change of signal level from a sharp beam.

An apparatus for inspecting the panels of a motor car will now be described as an example of apparatus according to the invention and with reference to the accompanying drawings in which

Figure 1 is a diagrammatic side view of an inspectio apparatus of the invention in the form of inspectio apparatus,

Figure 2 is a diagrammatic perspective view of th apparatus of ^"Figure 1,

Figure 3 is a diagram showing the optical components o the apparatus of Figure 1,

Figures 4A and 4B diagrammatic are side and plan view respectively of a first part (first dynamic focusin means) of the optical apparatus of Figure 3,

Figure 5 is a view similar to Figure 4A showin diagrammatically the operation of the optical apparatu of Figures 4A and 4B,

Figure 6 is a diagrammatic side view of the principl of operation of another part (the second dynami focusing means) of the optical apparatus, - 5 -

Figure 7 is a more detailed diagrammatic side view of the second dynamic focusing means of the optical apparatus of Figure 3,

Figures 8A, B and C which are vertically aligned with one another show, respectively, a grating with two types of light beams passing through the grating, the output signals corresponding to the two light beams, and the threshold signals corresponding to the light beams.

Figure 9 is a diagrammatic view of part of the apparatus which shows a further dynamic focusing effect .

The apparatus of the present invention is intended amongst other things to detect defects such as dents, so called distinetiveness of image (DOI) and diffuse reflectancy defects and surface colour changes.

Figure 1 illustrates an inspection apparatus 10 according to the invention. The inspection apparatus is arranged to inspect the front, rear and upper painted panels of a motor car 11 on a production line. The side surfaces may be inspected by two generally less mobile inspection means (not shown but utilising the same principles as the apparatus to be described) on each side of the car as the car moves past. The inspection apparatus 10 comprises a frame 12 mounted so as to be movable up and down and forwardly and rearwardly with respect to the axis of the motor car 11, and to tilt about a transverse axis H. These degrees of motion are shown by the arrows 13 and the movement may be controlled by hydraulic rams or by a - 6 -

robot arm (not shown). When inspecting the rear of the car, the apparatus may carry out a more complex movement. Instead of the frame 12 sloping rearwardly as shown in Figure 1, the frame may slope forwardly.

The frame 12 mounts a first upper housing 14 housing the majority of the optical components, and a lower member 16. As is clear from Figure 2 which shows a perspective front view, the lower member 16 comprises an extended grating 17 which extends generally across the width of the car, and slightly downwardly on each side of the car. The lines 20 of the grating 17 are shown in Figure 2 grossly exaggerated in width fo clarity. Behind the grating 17 (behind, in the sens that it is behind the grating 17 from the point of vie of an incident light beam) and in contact therewith is a generally coextending retro reflective screen 18 an in front of the grating 17 is a generally coextendin filter wedge 19 (18 and 19 not being shown in Figure for clarity). The filter wedge 19 is of a pink colou and its optical density increases from its upper edg 19A to the lower edge 19B. The increase in density ma be provided by having a thicker lower edge 19B, th material of the wedge 19 having the same optica density per cubic mm, or alternatively the "wedge" 1 may be of the, same thickness throughout but produced i such a way that its optical density increases from it top edge 19A to its lower edge 19B.

From Figures 1 and 2 it will be seen that the optica components within the upper housing 14 provide a outgoing beam 22 of radiation (details of which will b given hereafter) which is scanned (see Figure 2) alon a scan line 21 across the surface of the upper panel of the car 11. The beam is specularly reflected t pass through the filter wedge 19, through the grating 17, (the beam being scanned perpendicularly to the grating lines 20) to the retro reflective screen 18, is then retro reflected from that screen back through the grating 17, wedge 19 and is rereflected from the car panel to the optical apparatus within the upper housing 14. It is-well known that retro reflective material reflects an incident beam back along the incident path with a small degree of scatter.

Figure 3 shows in diagrammatic form the optical components within the upper housing 14. The optical apparatus includes a first laser 23A providing a beam at a first wavelength (633 nm), the beam being passed to a first beam combining mirror 24. Also passed to mirrorS (24B, 24C) are two further beams from two further lasers (23B, 23C), via a single mode polarisation preserving optical fibre 26 , the two further laser beams being of 458 nm and 514 nm wavelength respectively.

The combined beam is reflected onto an optical axis 27 by a mirror 28. The combined beam passes to a first dynamic focusing means 25 to maintain the beam in focus as it scans across the car. As is also illustrated in Figures 4A and 4B, the dynamic focusing means 25 includes a rotatable tilt mirror 29, comprising a mirror arranged at an angle to the optical axis 27 so as to deflect the incoming beam. The mirror 29 being arranged so as to be tiltable about an axis 31 which is in the plane of the mirror surface, the optical axis 27 also passing through the axis 31. Thus, by varying the tilt of the mirror 29, the beam path reflected from the mirror 29 may be reflected by an angle between A and B (see Figure 4A). The beam reflected from mirror 29 is passed to a tilted lens 32 and thence along a beam path to a pair of 90° roof mirrors 33. When the tilt mirror 29 is at a position such that the angle of reflection of the beam is A then the reflected beam passing between the tilted lens 32 and the mirrors 33 also passes through a pair of lenses 36,37.

The beam passing back through the tilted lens 32 is reflected from the mirror 29 along an axis 3u at an angle to axis 27 (one lies behind the other in the view of Figure 3 but see Figure 4B) and passes through lens 20, and past mirror 28. The beam continues to a scanning mirror 38 which oscillates about an axis J which is in the plane of the mirror 33 and also passes through the optical axis 30. The outgoing beam 22 is thereby passed down to the surface of the car as shown in Figures 1 and 2 and is scanned thereacross by the mirror 38. The beam is reflected from the surface of the motor car to the grating 17 as already described and back by the retroref lector 18. The return beam 42 is received by the oscillating mirror scanner 38 where it is descanned.

Unlike the outgoing beam 22 which is a narrow-focuse (combined) laser beam, the returning beam 42 include image forming information as will be clear later and i of greater width than outgoing beam 22. Thus, th oscillating scanner mirror 38 has relatively larg transverse dimensions. The mirror 38 reflects th incoming beam 39 back to a mirror 39, the mirror 3 having a central hole 34 through which the optical axi 30 passes, the majority of the beam striking the mirro 39 which reflects it to a lens 41 which focuses th - 9 -

incoming beam 42 to a point behind the surface of a mirror 45A of mirror pair 45A,45B.

The mirror pair 45A, 45B form a second dynamic focusing means 43, which is illustrated in more detail in Figure 7. Effectively, however, the second dynamic focusing means 43 causes the incoming beam to focus at a fixed image point P (shown in Figure 7) and lens 46 focuses the image point P onto a mirror 47. The mirror 47 is a mirror with a central hole. The lenses 41 and 46, together with the second dynamic focusing means 43 combine to image onto (or in fact just behind) the plane of the mirror 47 the surface of the panel of the motor car being inspected. This means that light which originates from the panel of the motor car (scattered light only) is focused down to a narrow cone and passes through the central hole to an optical fibre 48. The remaining component of the incoming light beam, which is in fact specularly reflected light from the panel, because it does not have a point of origin on the surface of the panel of the motor car, will not be tightly focused at that point and will therefore effectively form a circular shaped cross section at the mirror 47, thereby effectively apparently forming a halo around the central cone of scattered light. Thus the specular component of the incoming beam 42 will be reflected by the mirror 47 to a lens 49, beam splitters 51,52 being provided to split the beam into its blue (488 nm) component and green (514 nm) component. The blue and green components are collected by respective lenses 53,54 and passed to respective optical fibres 56,57. The remaining red part of the beam (633 nm) passes through the second beam splitter 52 and is collected by lens 58 and passed to an optical fibres 59 (the centre of the beam) and 60 (the outer ring of the beam) .

To the end of each of fibre 48,56,57,59 60 there i attached a respective light sensitive detector 63 to 6 for detecting light passing along the respective fibre Outputs from each of the detectors (b3) to (67) ar passed to a central computer processor (72) to proces the infromation and produce outputs.

The mode of operation of the apparatus will now b described.

On a production line the motor car moves in th direction of the arrow 15 and the frame 12 is moved by for example, a motion system eg a robot so as to b swept over the upper, front and rear surfaces of th motor car 11. Because of its configuration it is no necessary for the apparatus 10 to closely follow th surface of the motor car 11 as with our earlie arrangement shown in our earlier PCT patent applicatio number PCT/GB86/00399, but the lower housing 16 of th apparatus 10 may be spaced some reasonable distance typically 200-500 mm from the surface of the panel o the car.

It will be understood that the beam which is scanne across the surface of the motor car 11 along scan li 21 comprises three colours, (633 nm, 488 nm , and 51 nm) .

The red (633 nm) and green (514 nm) laser bea components will pass through the grating 17 without a variable modulation, because of the colour, (yellow) the grating. Tne blue (488 nm) beam will be affected by the gratin because the yellow grating acts as if it were a blac and white grating. Thus, as the laser beam scans ove the car body, the returned blue laser beam will b modulated by the effect of the grating.

This modulated (blue) signal, the specular component o which is detected by light sensitive detector 64, ca be used to detect dents and distinetiveness of imag (DOI). The dents are detected by comparing th modulated signal received by the detector 64 with tha expected for that part of the body.

Thus the outgoing beam 22 is reflected from the surfac of the panel of the car. If the surface of the car wa exactly flat, then as the outgoing beam 22 is scanne across the flat surface it would strike the lines 20 o the grating 17 at regular intervals and the retur signal would be modulated in a regular fashion However, because the surface of the car body is not flat (eg: includes dents or is naturally curved) th outgoing beam is reflected at an angle which varie depending upon the curvature of the car surface an this affects the modulation of the beam by the gratin 17. Thus, if the outgoing beam scans across an area o car panel surface which is convex then the ra reflected from the body surface to the grating 17 will move across the grating 17 more rapidly than if the ca panel surface was flat, and the modulation of th signal will be at a greater frequency than otherwise Similarly, if the outgoing beam scans across a concav part of the car panel's surface, then the reflecte beam will move across the grating 17 more slowly tha if the car panel surface was flat and so the modulate signal be at a lower frequency. A perfect panel whic - 12 -

includes natural curves will produce a modulated signal where the frequency varies with time in a known way.

Thus to detect dents (or waviness of panels) the frequency of the modulated signal is compared with that expected for a perfect panel.The size of dents which can be detected depends upon the spatial frequency of the grating 17. A fine grating will allow smaller dents to be detected. The pitch of the grating may be in the range lm^'m - 20mm, typically 12mm. This pitch is small enough to detect dents and waviness. The blue (488 nm) beam may also be utilised to provide a measure of the di s t i netiveness of image. The term "distinctiveness of image" (DOI) is a well known term in the art and effectively measures the "shinyness of the surface". The manner in which this is measured is shown in Figures 8A-8C.

The three figures, Figures 8A-8C are aligne vertically. Thus, considering the left hand sides o the figures, if we assume that the paint surface of th vehicle is very shiny, then it will produce a ver sharp distinct image of the blue beam and this shar beam cross section 70 (which is typically o approximately 0.25 mm diameter at the car surface an 2-3mm where it strikes the grating 17) is shown i Figure 8A. The output signal of detector 64 is show in Figure 8B for a beam of beam cross section 70 passe from a transparent part 68 to yellow line 69 of th grating 17.

In Figure 8A, a beam cross section 71 is shown for beam reflected from a less shiny surface. It will b seen that the central area 71A of the cross section i surrounded by a "halo" outer area 71B. As a result 13 -

the detected signal shown on the right hand side of Figure 8B changes from a peak value to the lower value by means of a less steep slope compared with the signal to the left of Figure 8A. By providing threshold values at A, B, C and D it is possible to provide a measure of the slope and hence the "halo" effect. In this way, therefore, it is possible to measure the degree to which the beam cross section is sharp and hence the "distinctiveness of image". More thresholds may be used.

The green (514 nm) laser beam will also not be affected by the grating, as it scans across the grating, but will be attenuated by the pink perspex wedge 19. The pink perspex wedge is used to measure the vertical position at which the green beam strikes the retro reflective screen 18. Referring to Figure 1 the wedge 19 is increasingly opaque to the green laser beam as one moves from the upper 19A to the lower 19B edge, Clearly, if the beam strikes the pink wedge towards its lower edge, then the beam will be attenuated to a greater extent than if it strikes the wedge near its upper edge. Thus, the signal output of light sensitive detector 65 which effectively measures the amplitude of the specularly reflected component of the green laser beam will vary depending upon the position in which the green laser beam strikes the pink wedge 19 and this is in turn, as is clear from Figures 1 and 2, a measure of the angle at which the incoming beam is reflected from the surface of the vertical body 11. The normal position in which the beam strikes the pink wedge 19 will be known (from, for example, a computer memory) and changes, particularly rapid changes, from that known position and hence known value of the output signal of detector 65 will be a further measure of defects such as dents in the body surface.

As has already been described, the red (633 nm) lase beam will not be affected by the grating nor indeed b the pink wedge 19 and so, the specular component of th returning red beam which is detected by the detector 66,67 is also to be used to provide information abou discrete defects such as dirt inclusion in the pain and scattering type defects -s-uch as scratches o sanding marks.

The arrangement of the optics is such as to focus ont the front end of the optical fibres 59 and 60 an imag of the retroreflector 18. Thus, the central fibre 5 will pick up the specular light and the surroundi optical fibre 60 will pick up light which has bee scattered.

As already described, the diffuse light in the thr colours from the car body surface is collect separately by the fibre optic 48. In order to obtai relevant colour information it is necessary to separa the three colours by optical means, such as bea splitters like the beam splitters 51,52. The thr beams may then be separately detected and the colour the body surface determined from those three separa beams. If, however, it is only desired to look at t combined beam, then the fibre optic 48 is arranged pass the combined beam to the light sensitive detect 63 and the light sensitive detector 63 may be used provide information regarding gross surface defects highly curved areas where the specular beam may deflected so much that it does not come into conta with a retro reflective screen. - 15

As already discussed, the apparatus also provides two dynamic focusing systems. As the upper housing 14 is at some distance from the position where the beam strikes the surface of the motor car 11, as the depth of focus of the beam at the surface of the car panel is only about 62 mm it is necessary to provide a first focusing means 25 to focus the outgoing beam onto the surface and a second focusing means 43 to focus the returning beam onto parts of the optical apparatus.

The first dynamic focusing means 25 for focusing the outgoing beam comprises, as already described, the rotatable tilt mirror 29, the tilted lens 32, and roof mirrors 33.

It will be understood that by tilting the mirror 31 the beam (which is a laser beam of very small transverse dimensions) is swept from one edge (the lower edge in figure 4A) of the lens 32 to the opposite (upper) edge. As is clear, the beam passes from the lens 32 to the roof mirrors 33 and back to the lens 32. Thus, if the beam is adjacent the lower edge of the lens 32 (and ignoring for the moment the lenses 36,37) then the distance from the lens 32 to the roof mirrors 33 and back to the lens 32 is much less than if the beam is adjacent the upper edge of the lens 32. The effect of this is shown in Figure 5 in which the roof mirrors have been omitted and the optical system opened out showing the beam passing from the lens 32 back to the lens 32. In fact, we show two beams Bl and B2 which although narrow (their widths are much exaggerated in the Figure for clarity) are not collimated and show the focusing effect of the lens 32. The input field or object plane which is curved is indicated as O and the image plane is indicated as I (which is also curved but - 16 -

is predominantly at an angle to the optical axis).

The mirror 31 may be tilted in accordance with a predetermined program, which may be held in the memory of a computer, for a particular motor vehicle. Thus, the mirror 31 may be tilted at varying angles to maintain the surface of the car body 11 in focus throughout the scanning of the beam 22 across the surface of the body of the motor car 11.

Apart from utilising a computer memory of the body shape to change the focus, one may use the signals obtained from the optical head itself. If the relationship between the optical head 14 and the retro reflective screen 18 is fixed then the position of the specularly reflected spot on the retro reflective screen 18 can be measured by interpreting the signals from the blue and green beams. This, together with the knowledge of the scanning geometry is enough to predict the focus position.

At this point, it would be useful to refer to Figure 9. In this arrangement, a lens 35 corrsponding to lens 32 is not tilted, but its optical axis coincides with the optical axis of the apparatus. The lens 35 has effectively a curved input field or object plane O. The effect of this curved object plane O is to make curved image plane I which effectively varies th focus. As in Figure 5 we show two beams which althoug narrow (they are much wider in the figure for clarity are not collimated and shows the focusing effect o lens 35. A combination of the effect of Figure 9 together with that of Figures 4A and 4B can produce dynamic focusing means. - 17 -

In some circumstances, the arrangement of Figure 9 may be sufficient without the arrangement of Figures 4A and 4B.

We will now describe the second dynamic focusing means 43 for focusing the incoming beam onto the optical components .

Whilst it is theoreticaly possible to use, as a second dynamic focusing system, a system similar to the first dynamic focusing system, in practice the system would need to be able to cope with much larger beam widths and a greater range of change of focus. Thus the system would need to be very large and require special lenses. Figures 6 and 7 show a preferred second dynamic focus system as already described.

Referring firstly to Figure 6, which shows a simplified version of the path over which the incoming beam 42 passes from the panel 21 to the mirror 47. Panel 21 is shown in two alternative positions 21A,21B which illustrates the change of distance from the optical apparatus within the housing 14 to the panel 21 as the beam is scanned across the panel surface. The incoming beam 42 passes through mirror 41 which focuses an image of the panel 21 to an image point P, and this image point P is relayed via lens 46 onto the mirror 47. The second dynamic focusing means 43 is provided to vary the optical distance between the lens 41 and the image point P so that the image point P and the panel 21 remain at conjugate points with respect to the lens 41 even as the panel 21 moves with respect to the lens 41 between positions 21A and 21B.

Figure 7 shows the apparatus 43 for carrying this out. - 18 -

The incoming beam 42 which is to be focused is reflected twice at the two mirrors 45A,45B. There are shown two focal points FB and FA which correspond to the points at which the lens 41 would focus an image of the panel surface 21 when at respective positions 21B and 21A. It will be seen from Figure 7 that rotation of the mirror pair 45A, 45B about the axis 61 enables the incoming beam to be focused to the point P in both cases. Thus, considering the panel being positioned in panel position 21A in which case the image would be brought to focus at point FA, then if the mirror system is in position X3 then the optical path length R.FB must equal the optical path length R.S.T.U.V.P. Similarly, when the panel is in position 21A, the mirror pair is in the position XI and the optical path length T.FA must equal path length T.U.V.P.

By a suitable choice of angle between the mirror surfaces of the two mirrors 42A, 42B, the incoming beam 42, the distance between the axis of rotation 61 and the mirrors 45A, 45B the point P may be maintained stationary, as the mirrors 45A, 45B are rotated about the axis 61 to compensate for changes in the distance between the lens 41 and the panel 21.

Referring back to Figure 3, the rotable mirror 31 ma also rotate the beam to the angle A when the beam will pass through the lens 32, and then through the lenses 37,36 before being reflected at the roof mirrors 33 back through the lens system 36,37 and lens 32. Th effect of the lenses 36,37 is to spread the beam so a to provide a slot shaped cross section beam (the axi of the slot being arranged to be transverse the line o scan). In the apparatus described, as the beam 22 i scanned in a first direction (say left to right i - 19 -

Figure 2) then a beam of the circular cross section shown in Figure 8A is provided by the mirror 31 being tilted between angles A and B and when the beam is scanned in the other direction, from right to left as shown in Figure 2, then the beam is switched by tilting the mirror 31 to angle A whereby the beam passes through the lenses 36,37 to provide a slot shaped cross section beam. This slot shape has advantages for detecting the extent of a defect as will now be described.

With a small spot as shown in Figure 8A, defects such as distinctness of image (DOI) dents, orange peel and scratches can be detected as has already been described. A slot shaped laser spot can be used to detect discrete defects such as dirt inclusions and colour defects.

It also allows one to inspect the whole body of the car by means of the slit whilst only inspecting part of the body with the spot as shown in Figure 8A, which means that the scan can be slower, which produces a lower electronic band width with better signal to noise ratio, or else the body may be scanned in a shorter time.

The size of the slot shaped cross section beam is approximately 3mm long and 0.6mm wide. The size of small dirt inclusions may be measured by determining the extent to which they reduce the signal. Thus, if the slot is larger than the dirt inclusions, then not all of the signal will be attenuated and the degree of attenuation gives some measure of the extent of the dirt inclusion. Small dirt inclusions may be acceptable but larger ones may be unacceptable. - 20

The invention is not restricted to the details of the foregoing example.

We have chosen a particular set of colours: blue, green and red for the beam, yellow for the grating and pink for the wedge. Other combinations of colours, particularly for the grating and wedge may be used and the particulars are given as examples only.

The second focusing means has been provided between the lens 41 and the point P. However, it could be provided between the point P and the lens 46 or, indeed, between the lens 46 and mirror 47.

Many parts of the apparatus may be used in other types of apparatus. Thus, for example, both the first and second dynamic focusing means (referred to in the introduction as the first and second aspects of the invention) may usefully be used to focus dynamically from moving object planes onto a fixed image place (or vice versa).

Similarly the use of a beam of different cross-sections (the third aspect of the invention) at different points in the beam has widespread application in othe (usually inspection) apparatus.

Also the use of a composite beam comprising a pluralit .of beams of separate, distinct wavelengths (the fourt aspect of the invention) has many uses in othe apparatus .

The means for analysing the sharpness of a beam cross section (the fifth aspect of the invention) may also b used in other inspection apparatus.

Although the apparatus has been described in terms of inspecting a surface, the principle of the apparatus may be applied to inspecting, for example, article such transparent articles such as film whereby the radiation passes through the article. In this case, where the radiation is transmitted through the material, in addition to detecting defects on the surface of the material, bulk defects, that is defects within the bulk of the material, may also be detected.

Claims

1. An optical apparatus including focusing means (41), an optical axis, an object point (31) and an image point on said optical axis, said object point being movable along the optical axis with respect to the focusing means, means being provided to change the effective optical distance between either the object point and the focusing means or between the image point and the focusing means to maintain the object point at a fixed point even if the image point moves.

2. An optical apparatus as claimed in claim 1, characterised in that the focusing means remains stationary, and the means for changing the effective optical distance comprises a movable non-focusing element.

3. .An optical apparatus as claimed in claim 1 or 2 characterised in that means is provided to move said movable non-focusing element in such a way that continuous changes in the continuous movement of th object point is compensated so that the image point remains stationary at all times.

4. An optical apparatus as claimed in claim characterised in that said means comprises a movabl mi rror .

5. An optical apparatus as claimed in claim characterised in that the movable mirror (43) comprise two mirrors (45A, 45B) at an angle to one another an being rotatable together about an axis to one side o the optical axis.

6. An optical apparatus as claimed in claim 5 characterised in that the two mirrors (45A, 45B) are at right angles to one another to form a corner cube.

7. Optical apparatus as claimed in any of claims 1 to 6 further characterised by means to pass a beam of radiation across a surface to be inspected, beam shaping means (36, 37) being provided whereby at different points during said movement, the cross section of the beam of radiation is changed so as to detect different types of fault.

8. Optical apparatus as claimed in claim 7 further characterised by means to scan a beam of radiation back and forth across a surface to be inspected, said beam shpaing means (36, 37) being operable to provide one cross section of beam whilst the beam is scanning across the surface in one direction, and a second cross section whilst the beam is scanning across the surface in the opposite direction.

9. Optical apparatus as claimed in claim 7 or 8 characterised in that one of the cross section shapes is circular, and the other slot shaped.

10. Optical apparatus as claimed in any of claims 7 to 9 characterised in that said beam shaping means (36, 37) comprises' optical components to change the shape of the beam, and means (29) to selectively pass said beam through said optical components.

11. An optical apparatus as claimed in any of claims 1 to 10 characterised in that there is provided a composite beam comprising a plurality of beams of separate, distinct wavelengths, whereby some optical components (17) in the apparatus sensitive t wavelength affect the different beams in a differen manner, and other optical components not sensitive t wavelength affect all of the beams in a similar manner.

12. An optical apparatus as claimed in any of claims to 11 characterised in that means (38) is provide t pass a beam of radiation to a surface to be inspected means (38) is provided to collect the beam from th surface, and means (36, 37) is provided to determin the sharpness of the cross-section of the bea collected from the surface under inspection.

13. An optical apparatus as claimed in claim 1 characterised in that said collected beam (70) i passed over an opaque edge (20), and then to radiation detector (64), and means (72, 70) bein provided to determine the shape of the beam cros section from the output of the signal detector.

14. An optical apparatus as claimed in any of claims to 13 characterised by optical means (25) for varyin the focusing effect of a focusing component (32), sai optical means (25) including a main optical axis (27) the focusing component (32) being on said main optica axis (27) and having an optical axis (35), the focusin component (32) being tilted so that the two optica axes (35, 27) are tilted with respect to each other an means (29) being provided to pass a beam of radiatio through said focusing component (32) at a variabl transverse position with respect to the main optica axis (27) to thereby vary the focusing effect of sai focusing component (32).

15. An optical apparatus as claimed in claim 1 characterised in that the means (29) to move the bea comprises a tiltable mirror (29).

16. An optical apparatus, including optical means (25) for varying the focusing effect of a focusing componen (32), said optical means (25) including a main optica axis (27), the focusing component (32) being on sai main optical axis (27) and having an optical axis (35), the focusing component (32) being tilted so that th two optical axes (35, 27) are tilted with respect t each other and means (29) being provided to pass a bea of radiation through said focusing component (32) at variable transverse position with respect to the mai optical axis (27) to thereby vary the focusing effect of said focusing component (32).

17. An inspection apparatus including means to pass beam of radiation across a surface to be inspected, beam shaping means (36, 37) being provided, whereb at different points during said movement, the cros section of the beam of radiation is changed so as t detect different types of fault.

18. An optical apparatus including means (23A, 23B, 23C) to provide a composite beam comprising a pluralit of beams of separate, distinct wavelengths, whereb some optical components (17) in the apparatus sensitiv to wavelength affect the different beams in a different manner, and other optical components not sensitive t wavelength affect all of the beams in a similar manner

19. An inspection apparatus in which means (38) i provided to pass a beam of radiation to a surface to b inspected, means (38) is provided to collect the bea from the surface, and means (36, 37) is provided to determine the sharpness of the cross-section of the beam collected from the surface under inspection.