GB1596295A

GB1596295A - Optical apparatus and method

Info

Publication number: GB1596295A
Application number: GB1200477A
Authority: GB
Original assignee: SIRA INSTITUTE
Current assignee: SIRA INSTITUTE
Priority date: 1978-04-28
Filing date: 1978-04-28
Publication date: 1981-08-26

Description

(54) OPTICAL APPARATUS AND METHOD (71) We, SIRA INSTITUTE LIMITED, a British Company of South Hill, Chislehurst, Kent BR7 5EH. 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:- The present invention relates to optical apparatus and method.

In particular, the present invention relates to optical apparatus and methods for determining the position of radiation.

It is frequently desired to determine the position of radiation such as a beam of radiation or a focused area of radiation. The position may be indicative of a number of factors which are being determined and specific instances will be given throughout the Specification.

According to a first aspect the present invention provides apparatus for determining the position of a beam of radiation, comprising a radiation modifying means mounted to receive the beam of radiation, and means for detecting a characteristic of the radiation passed thereto from the modifying means, said modifying means being adapted to modify said characteristic of the radiation as detected by the detector means, as, in use, the beam of radiation incident on the modifying means is moved with respect to the modifying means, the value of said characteristic of the beam of radiation detected by the detector means continuously varying as the beam is moved across the modifying means in a predetermined direction, whereby the value of the characteristic of the radiation detected by the detector means is related to the position of the beam of radiation at the modifying means.

Throughout the statements of invention and the claims the term "beam of radiation" should be taken to include a parallel, converging or diverging beam of radiation having a generally defined cross-sectional area.

As examples the characteristic may be the wavelength of the radiation or may be the angle of polarisation if the beam of radiation is polarised, but we prefer to utilise as the characteristic of the radiation, its intensity. In this case the means for detecting the characteristic of the radiation may be a simple radiation detector.

The modifying means may comprise a layer of a material through which the beam may pass with some attenuation of intensity, the thickness of the plate varying in the predetermined direction; or a photographic plate which has been exposed in such a way that its opacity varies in the predetermined direction; or a layer comprising successive light and dark bands, the relative widths of the bands varying in the predetermined direction; or a plate carrying a metallic film, the thickness of the metallic film varying in the predetermined direction; or a photographic or photo-etched area mask carrying a fine pitch array of interlocking triangular-shaped opaque and transparent areas, the mean transmission characteristic varying in the predetermined direction; or a rotating cylindrical light chopper with profiled vanes, the transmission characteristic of which varies in the predetermined direction.

In a preferred arrangement, an initial beam is split into two beams and means are provided for directing each of said two beams to respective determining apparatus each as aforesaid, the two determining apparatus being arranged mutually orthogonally with respect to the initial beam so that the position of the initial beam in any direction transverse its axis is determined.

According to a further aspect of the invention there may be provided apparatus for determining the position of a beam of radiation comprising, transverse the path of the beam, a first central radiation detector for detecting the beam, which central radiation detector is surrounded by an annular radiation detector for detecting scattered radiation, and beam splitter means for passing the beam to apparatus as aforesaid.

In a preferred arrangement the modifying means comprises a diffuser which may be generally planar, the incident beam, in use, forming a defined area on the diffuser and the detector means is situated in a plane containing the normal to the diffuser at the defined area at an angle other than zero to said normal. The characteristic detected in this case is the intensity of radiation received by the detector which varies as the defined area is moved relative to the diffuser.

In this case there may be provided two said detector means, their said planes in which they are situated being orthogonal.

There may be provided two detectors in the or each said plane, one on each side of said normal.

The diffuser may be transmitting diffuser whereby the detector means is situated on the opposite side of the diffuser from the incident beam path or a reflective diffuser whereby the detector means is situated on the same side of the diffuser as the incident beam path. It is preferred that the diffuser approximates to a Lambertian diffuser which may be of opal glass.

The invention also provides radiation position detector means comprising a diffuser for receiving the radiation and on which the incident radiation forms a defined area and detector means for detecting said radiation transmitted or reflected from the diffuser, said detector means being situated in a plane containing the normal to the diffuser at said defined area and at an angle other than zero to said normal, whereby the output signal of the detector means is related to movement of the radiation across the diffuser.

The above described apparatus may be used to detect flaws in a reflective surface whereby a beam of radiation is reflected from the surface and the position of the beam is detected, and in a preferred arrangement said reflective surface is provided by tin plate. The beam of radiation is preferably scanned across the reflective or tin plate surface.

The invention also provides a method for determining the position of a beam of radiation comprising directing the beam of radiation at a modifying means the modifying means being arranged to modify a characteristic of the radiation as detected by a detector means so that as the beam of radiation is moved across the modifying means the value of the characteristic of the radiation detected by the detector means varies continuously and is related to the position of the beam of radiation at the modifying means.

In a preferred arrangement there is provided a method for determining the position of a beam of radiation comprising splitting the beam into two beams and determining the position of each of the two beams in accordance with the above method, in mutually orthogonal directions.

Furthermore the invention provides a method for determining the position of a beam of radiation comprising providing a first detector for detecting scattered radiation and detecting the position of the central portion of the beam by either of the aforesaid methods.

Said methods may be used to detect flaws in a reflective surface by reflecting a beam of radiation from said surface (which may be provided by tin plate) and detecting the position of said beam by said methods.

Preferred embodiments of the invention will now be described by way of example only and with reference to the accompanying drawings in which: Fig. 1 is a diagrammatic lay-out of apparatus for inspecting a flat reflective sample incorporating the apparatus of the invention, Figure 2 illustrates diagrammatically apparatus according to one aspect of the invention, Figure 3 illustrates a preferred arrangement of position sensitive apparatus of the type included in Figure 2, Figures 4 and 5 illustrate output signals of the apparatus of Figure 3, Figure 6 illustrates diagrammatically apparatus according to a further aspect of the invention of a generally similar type to that illustrated in Figure 2, Figure 7 illustrates diagrammatically part of apparatus according to a further aspect of the invention, Figure 8 illustrates the output signal from the apparatus of Figure 7, Figure 9 illustrates a further apparatus according to the invention, Figure 10 illustrates a further apparatus according to the invention, and, Figure 11 illustrates an alternative arrangement of the apparatus of Figure 10.

Throughout the specification we shall refer to radiation. It will be understood that this covers all wavelengths of radiation and in particular optical wavelengths (light), ultraviolet, and infrared radiation.

Furthermore, the apparatus described is shown as incorporating lenses but other equivalent devices such as focusing mirrors and other focusing apparatus may be utilised in their place as appropriate.

Furthermore, where the apparatus is arranged so that radiation is transmitted through a material, it will be understood that in certain circumstances the apparatus may be modified so that the radiation is reflected from the material. In particular, as will be clear later, the attenuator of Figure 2 is a transmission attenuator but a reflection attenuator may equally be used.

The broader aspects of the present invention relate to apparatus for determining the position of radiation, in particular a beam of radiation. However, in order to understand these broad aspects of the invention, it is considered preferable to provide an example of their use and this is provided by the arrangement of Figure 1.

The apparatus of Figure 1 is intended to examine a generally flat plate of a semireflective or reflective material for damage.

The material may comprise, for example, a metal such as tin plate and the apparatus may be adapted for examining the tin plate on a production line for faults such as bruising, scratches or dents. The apparatus of Figure 1 is an experimental arrangement but may be readily adapted for use in a production environment.

In Figure 1 there is provided a radiation source in the form of an optical wavelength laser 11, radiation from which is passed through a beam shaper 12 of known optical components for shaping the beam of radiation from the laser to produce a desired cross section, in this case, a circular cross section. A plane mirror 13 is provided for reflecting the beam from the beam shaper 12 onto the surface of a reflective sample of, for example, tin plate 14 which is mounted on an X-Y positioning table 16.

As mentioned above the described arrangement is an experimental arrangement and the X-Y positioning table allows the beam to be scanned across the tin plate 14.

In a production environment, however, the tin plate 14 would normally be in strip form and moving in a first direction parallel to its length and a conventional scanning arrangement may be provided for scanning the beam from the plane mirror 13 transversely to the direction of movement of the tin plate 14 so that the entire surface of the tin plate 14 is scanned.

The beam reflected from the tin plate 14 is passed to a plane mirror 17 and thence to a position determining apparatus 18. A power supply 19 is provided for the apparatus 18 and the output of the apparatus 18 is passed to an oscilloscope, pen recorder or other recording means 21.

It will be understood that if the optical arrangement '2 produces a circular beam and the tin plate 14 is perfectly flat then the scatter pattern incident on the apparatus 18 will be a main circular specular lobe, characterised by a well defined boundary of small angular subtense, outside which the scatter intensity rapidly diminishes, and background scatter, generally of much lower intensity than the specular lobe, which extends to large angles about the specular direction. If, however, there is a fault in the tin plate 14 then as the beam traverses the fault the scatter pattern will vary. If there is a small bruise on the tin plate 14 then the main specular lobe will no longer be circular, the centre of the lobe will move and there may be some scattered light surrounding the lobe.If there are scratches on the tin plate 14 then the shape of the lobe may remain circular but there will be much more scattered light. If there is a fold in the tin plate 14 then the lobe will become eliptical and move. With many types of fault in the tin plate 14 as the beam scans across the fault the centre of the lobe will be displaced from its normal position.

The apparatus 18 is intended to detect these changes in the position of the image of the lobe and thereby indicate faults in the tin plate 14.

As described, the apparatus of Figure 1 is rather sensitive to tilt of the tin plate. This can be overcome to a certain extent by, for example, re-arranging the apparatus to reflect the radiation from mirror 13 onto the tin plate sample, and providing in the path of the reflective light a retroreflector. A retroreflector is a reflecting device in which most (and in a perfect case, all) of the incident radiation is reflected back along the incident path. Under those circumstances, of course, the light would then be reflected back along the path towards the mirror 13 and can be separated by means of a suitably arranged beam splitter.

A first arrangement of beam positioning determining apparatus 18 will be described but before doing so the principle of this apparatus will be better illustrated by reference to Figure 2. In Figure 2, the beam axis is illustrated by reference 26 and a simple radiation detector is provided by photocells 27. Interposed transverse the beam axis 26 is a means 28 for varying the characteristic of the beam. In the present instance this is provided by an attenuator in which the degree of attenuation varies progressively from a minimum adjacent edge 29 to a maximum adjacent edge 30.

Thus if the beam of radiation passes through the attenuator 28, a greater amount of the radiation is passed if the beam passes adjacent the edge 29 and progressively less is passed as the beam transverses the attenuator 28 towards the edge 30. If the radiation is light of optical wavelength then this may be most easily arranged by rendering the edge 30 more opaque than the edge 29 the opacity varying, preferably linearly, from the edge 30 to 29. It will thus be understood that for a constant input beam intensity the output of the detector 27 will vary depending on the position of the beam across the attenuator 28 between the edges 29 and 30. If the beam traverses the attenuator 28 adjacent the edge 29 then the maximum signal is recorded by detector 27 and if the beam traverses the attenuator 28 adjacent the edge 30 then a minimum signal is recorded by detector 27.This therefore provides a simple means of determining the position of the beam between the edges 29 and 30.

We have described in the above paragraph this aspect of the invention in its simplest form. However, it will be appreciated that the attenuator may be replaced by other means for varying other characteristics of the beam. Thus, for example, the means 28 may be arranged to rotate the plane of polarisation of transmitted light to an extent which varies continuously from the edge 29 to the edge 30. Thus, for example, it may be provided that the plane of polarisation of the light will not be varied adjacent the edge 29 whereas it is rotated through, for example, 90" adjacent the edge 30. The detector 27 should then, of course, be arranged to be sensitive to the plane of polarisation of the light. Other characteristics may similarly be varied by the means 28.

However, if the means 28 is simply to vary the intensity of the beam passing therethrough (as in the case of the attentuator) then a number of arrangements may be used. The means 28 may be made of a material which attentuates the beam passing therethrough and the thickness of the material may vary from a minimum at edge 29 to a maximum at edge 30.

Alternatively, the attenuator may be of a constant thickness and may be, for example, a photographic plate which has been exposed in such a way that the opacity at edge 30 is greater than the opacity at edge 29. Other arrangements may include successive light and dark bands parallel to the the edges 29 and 30, the dark bands being broader adjacent the edge 30 than they are at edge 29 or a metallic film with a suitable thickness gradient, of a variety of photographic or photo-etched area masks in which the mean transmission characteristic results from a fine-pitch array of interlocking triangular-shaped opaque and transparent areas. The attenuator may comprise a rotating cylindrical light chopper with suitably profiled vanes to yield the desired linear characteristic from the edge 30 the edge 29 on a time averaged basis.

The output of the detector 27 may be passed to a suitable signal processing apparatus to provide an indication of the position of the input beam across the means 28.

As will be well understood the arrangement of Figure 2 will only determine the position of the beam in one transverse direction. However, by splitting the beam into two component parts and arranging a similar attenuator orthogonal to the attenuator 28 across the second part of the beam, the position both in the X and Y direction transverse the axis of the beam may be similarly determined. Such an arrangement forms the basis of the apparatus shown in Figure 3.

The apparatus of Figure 3 comprises a first beam splitter 32 and a photo-detector 33, 34, the photo-detector 33 having a circular detector surface and the photodetector 34 having an annular detector surface surrounding the photodetector 33. A second beam splitter 36 is provided and two sets of attenuators 37, 38 and respective photodetectors 39, 40 are also provided.

The arrangement is such that radiation from the diffusing screen 14 passes to the beam splitter 32. Part of the light passes through the beam splitter onto the two photodetectors 33, 34. The optical system is arranged such that the specular lobe is incident on the central photodetector 33 and scattered light is detected by the photodetector 34. The other part of the light from the central lobe incident on the beam splitter 32 passes to the beam splitter 36 where it is again divided, one half passing through the attenuator 37 to the detector 39 and the other half passing through the attenuator 38 to the photodetector 40. The two attenuators 37, 38 are arranged orthogonally with respect to one another.

The attenuators 37, 38 may be of any of the forms described with reference to Figure 2.

It will be understood that the photodetector 34 measures the amount of light scattered over a wide angle. The specular lobe is split into two channels by the beam splitter, one of which is passed direct to the photodetector 33 which thereby measures the intensity of the specular lobe. The photodetector 33 is arranged so that its light collecting surface is such a size that the specular lobe will not normally depart from that surface.

However, the movement of that specular lobe caused by faults in the tin plate will be indicated bv the photodetectors 39, 40.

As before, the outputs of the various photodetectors may be fed to a signal processing apparatus which may produce a suitable output to indicate the position of the specular lobe as follows. If we consider the output signals of the detectors 33, 34, 39 and 40 respectively to be indicated by the letters T S X Y respectively then S provides the wide angle scatter measurement, T provides the absorption measurement, and X and Y are sensitive to movement of the specular lobe. They can be normalised with respect to T to yield X'=X/T and Y'=Y/T so that X' and Y' are true position sensitive outputs. As indicated in Figure 1, the output from the signal analysing apparatus may be passed to the oscilloscope 21 from which movement of the specular lobe from its central position can readily be indicated or may be fed to other recording or processing apparatus.

In practice, detectors 33, 39 and 40 from which positional information is derived may show inherent positional sensitivity due to non-uniformity of the photosensitive area. This non-uniformity, superimposed on the linear attenuator characteristic, would result in a non-linear, possibly ambiguous response to movement of the specular lobe. It is therefore preferable to include additional diffusing or optical scrambling means between the attenuator and the detector in order to ensure that the detector photocathode is uniformly illuminated with light. Suitable scrambling means include, for example, a simple diffusing screen, an integrating sphere or an incoherent optical fibre bundle.

The advantage of the arrangement of Figure 3 is that it is possible to measure and therefore take into account tilt of the strip of tin plate. In a production environment this otherwise causes a problem. Also by a suitable filtering of the output signals X, Y one may determine sudden changes in X and Y which are caused by local mechanical damage to be distinguished from the slowly varying effects of strip tilt.

Figure 4 shows the output signal X for two successive scans over the same damaged area of strip; in the case of the trace (a) the attenuator was not mounted in front of the photodetector whereas in trace (b) the attenuator was included. It will be noted that the introduction of the attenuator between the photodetector and the tin plate sample provides an indication of the movement.

Figure 5 shows similar signals to those in Figure 4 with an area-mask attenuator formed from two chrome on glass linear gratings held in close contact with a small wedge angle between the two sets of parallel lines (Moire fringe effect). As before trace (a) was obtained with no attenuator and trace (b) with the attenuator in position.

Note the separate effects of slowly varying sheet tilt (this particular tin plate sample was severely bent) and localised mechanical damage (wrinkles).

Figure 6 illustrates apparatus of the type shown in Figure 3 in which scrambling means in the form of simple diffusing screens are placed between the attenuators 37, 38 and their respective detectors 39, 40 and between the beam splitter 32 and detector 33. The diffusers may comprise a simple ground glass screen 43. In the present instance the detector 34 is omitted.

Furthermore, for convenience a further mirror 42 is placed between the mirror 36 and the attenuator 38.

An alternative arrangement of beam position determining apparatus 18 will now be described but before doing so the principle of this further arrangement will be better illustrated by reference to Figure 7.

In this arrangement there is provided a Lambertian diffuser surface 50 on which radiation in the form of a beam of light falls and is scattered by reflection or transmission. The principle is similar in either reflection or transmission and for the moment we shall consider the reflection case only. Assume radiation reaches the diffuser surface 50 at a defined area 53, and the normal to the diffuser surface 50 at area 53 which is indicated by line 51. A radiation detector 52 is situated in a plane containing this normal to the diffuser at an angle 0 other than zero to the normal 51.

The distance between the normal 51 and the normal 54 from the detector 52 to the diffuser surface 50 is indicated in Figure 7 as x. The length of the normal 54 from the detector 52 to the surface 50 is indicated by y.

Figure 8 illustrates graphically the experimental results for the output of the detector plotted against Vy as the detector 52 is moved towards and away from the diffuser surface 50 and towards and away from the normal 51. The experimental curve is illustrated in dotted lines and the theoretical curve for a perfect Lambertian diffuser is illustrated by solid lines. It is clear from Figure 8 that for a large range of x/y (say between 0.3 and 0.8) the detector output is directly proportional to x. Thus, providing that the relevant parameters for x/y are met, the detector output signal will be indicative of the position of the normal 51 ie the position of the area 53 and hence the position of the incident beam.

Referring to the theoretical curve in Figure 8, the curve has zero curvature at x/y=0.5 and departs from linearity by less than 2% over a range of x/y from approximately 0.23 to 0.87. Used with correct geometry, therefore, a Lambertian or approximately Lambertian diffuser can closely simulate the functional effect of an attenuator described earlier while simultaneously overcoming any inherent positional sensitivity of the detector photocathode. The angle 0 should therefore be tan-' 0.5 or approximately 262 for a Lambertlan diffuser. The optimum viewing angle would be slightly less for the opal screen illustrated by the dotted line in Figure 8.For the opal screen, the mean viewing angle should be 240 and a usable range would be between tan-i 0.16 and tan- 0.74.

Figure 9 illustrates a first arrangement of apparatus of the invention incorporating a diffuser of the type shown in Figure 7. In Figure 9, the diffuser surface is shown at 50 and the radiation or light to be detected is restricted to move about the diffuser surface 50 within the measurement area 56.

As will be clear, two detectors 57, 58 are provided in a first plane including the normal 59 to the surface 50 the two detectors 57, 58 being on opposite sides of the normal 59.

The two further detectors 61, 62 are arranged in a further plane orthogonal to the plane of the detectors 57, 58. Each of the detectors 57, 58, 61 and 62 is at the same angle with respect to the normal 59.

As will also be clear, each of the detectors is aimed at the measurement area 56. A further detector 63 is optionally provided on the normal 59.

A signal processing apparatus is provided (not shown). This signal processing apparatus can process the signals from the various detectors as follows. If we consider that the signal from detector 61 is V1, from detector 62 is V2, from detector 57 is V3 and from detector 58 is V4, then(Vl-V2) yields a position sensitive output nominally independent of any absorption of light while(Vl+V2) monitors absorption independently of lobe movement over the scatter plane. Similar comments apply to (V3-V4) and (V3+V4).

It has been found that (yl-V2) remains 98% linear over a range of my from 0.23 to 0.77. Over the same range, (Vl+V2) is constant to within 1%. The effect for excursions of the incident beam parallel to CD (ie at right angles to the plane containing detectors 61, 62 and normal 59) within the 98% linear region is small. Thus crosstalk between the x and y channels is low.

The detector 63 is desirable in order to provide a reference against which to normalise Vl, V2, V3 and V4.

Fig. 10 illustrates an arrangement similar to Figure 9 except that detectors 61 and 58 are deleted. The reason for this is that with the inclusion of detector 63, these two detectors become unnecessary. The dimensions of a practical arrangement are illustrated in Figure 10. Furthermore, Figure 10 includes a further detector 64 utilised similarly to detector 34 in Figure 3 to monitor wide-angle scatter. The most advantageous position for a single detector of wide-angle scatter is in the plane of the diffuser surface 52 at an off-specular angle approximately 15--200 transverse to the rolling direction of the inspected tin plate strip.

The arrangement shown in Figure 10 has been tested and found to work extremely satisfactorily by using back projection on to a diffusing opal glass screen through which the light is transmitted. However, in some industrial applications, vertical clearance above the inspected surface (the tin plate strip) may be limited. An alternative arrangement employing front projection (ie by reflection from the diffuser rather than transmission through a diffuser) may then be preferable and this is illustrated in Figure 11. The requirement for vertical clearance is reduced because of the folding action of the reflecting diffuser. In Figure 11, parts corresponding to other Figures have been similarly numbered. In addition to these parts there is provided a light proof box 71 to exclude ambient light.

In the arrangements shown in Figures 9, 10 and 11, one problem which is sometimes found is that the use of heavy diffusers particularly in the light transmission mode, reduces the amount of light available to be passed to the detectors. Under these circumstances, large light guides of, for example, tapered Perspex (RTM) may be provided to concentrate light on to the photo-multiplier detectors from the diffuser surface. Such an arrangement is particularly useful in order to maximise collection efficiency of the light reflected from a semibright surface like stainless steel or matt aluminium.

WHAT WE CLAIM IS: 1. Apparatus for determining the position of a beam of radiation, comprising a radiation modifying means mounted to receive the beam of radiation, and means for detecting a characteristic of the radiation passed thereto from the modifying means, said modifying means being adapted to modify said characteristic of the radiation as detected by the detector means, as, in use, the beam of radiation incident on the modifying means is moved with respect to the modifying means, the value of said characteristic of the beam of radiation detected by the detector means continuously varying as the beam is moved across the modifying means in a predetermined direction, whereby the value of the characteristic of the radiation detected by the detector means is related to the position of the beam of radiation at the modifying means.

2. Apparatus as claimed in claim 1 in which the means for detecting said characteristic is adapted to detect the intensity of the radiation.

3. Apparatus as claimed in claim 1 or claim 2 in which the means for detecting

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

Claims

**WARNING** start of CLMS field may overlap end of DESC **. viewing angle should be 240 and a usable range would be between tan-i 0.16 and tan- 0.74. Figure 9 illustrates a first arrangement of apparatus of the invention incorporating a diffuser of the type shown in Figure 7. In Figure 9, the diffuser surface is shown at 50 and the radiation or light to be detected is restricted to move about the diffuser surface 50 within the measurement area 56. As will be clear, two detectors 57, 58 are provided in a first plane including the normal 59 to the surface 50 the two detectors 57, 58 being on opposite sides of the normal 59. The two further detectors 61, 62 are arranged in a further plane orthogonal to the plane of the detectors 57, 58. Each of the detectors 57, 58, 61 and 62 is at the same angle with respect to the normal 59. As will also be clear, each of the detectors is aimed at the measurement area 56. A further detector 63 is optionally provided on the normal 59. A signal processing apparatus is provided (not shown). This signal processing apparatus can process the signals from the various detectors as follows. If we consider that the signal from detector 61 is V1, from detector 62 is V2, from detector 57 is V3 and from detector 58 is V4, then(Vl-V2) yields a position sensitive output nominally independent of any absorption of light while(Vl+V2) monitors absorption independently of lobe movement over the scatter plane. Similar comments apply to (V3-V4) and (V3+V4). It has been found that (yl-V2) remains 98% linear over a range of my from 0.23 to 0.77. Over the same range, (Vl+V2) is constant to within 1%. The effect for excursions of the incident beam parallel to CD (ie at right angles to the plane containing detectors 61, 62 and normal 59) within the 98% linear region is small. Thus crosstalk between the x and y channels is low. The detector 63 is desirable in order to provide a reference against which to normalise Vl, V2, V3 and V4. Fig. 10 illustrates an arrangement similar to Figure 9 except that detectors 61 and 58 are deleted. The reason for this is that with the inclusion of detector 63, these two detectors become unnecessary. The dimensions of a practical arrangement are illustrated in Figure 10. Furthermore, Figure 10 includes a further detector 64 utilised similarly to detector 34 in Figure 3 to monitor wide-angle scatter. The most advantageous position for a single detector of wide-angle scatter is in the plane of the diffuser surface 52 at an off-specular angle approximately 15--200 transverse to the rolling direction of the inspected tin plate strip. The arrangement shown in Figure 10 has been tested and found to work extremely satisfactorily by using back projection on to a diffusing opal glass screen through which the light is transmitted. However, in some industrial applications, vertical clearance above the inspected surface (the tin plate strip) may be limited. An alternative arrangement employing front projection (ie by reflection from the diffuser rather than transmission through a diffuser) may then be preferable and this is illustrated in Figure 11. The requirement for vertical clearance is reduced because of the folding action of the reflecting diffuser. In Figure 11, parts corresponding to other Figures have been similarly numbered. In addition to these parts there is provided a light proof box 71 to exclude ambient light. In the arrangements shown in Figures 9, 10 and 11, one problem which is sometimes found is that the use of heavy diffusers particularly in the light transmission mode, reduces the amount of light available to be passed to the detectors. Under these circumstances, large light guides of, for example, tapered Perspex (RTM) may be provided to concentrate light on to the photo-multiplier detectors from the diffuser surface. Such an arrangement is particularly useful in order to maximise collection efficiency of the light reflected from a semibright surface like stainless steel or matt aluminium. WHAT WE CLAIM IS:

1. Apparatus for determining the position of a beam of radiation, comprising a radiation modifying means mounted to receive the beam of radiation, and means for detecting a characteristic of the radiation passed thereto from the modifying means, said modifying means being adapted to modify said characteristic of the radiation as detected by the detector means, as, in use, the beam of radiation incident on the modifying means is moved with respect to the modifying means, the value of said characteristic of the beam of radiation detected by the detector means continuously varying as the beam is moved across the modifying means in a predetermined direction, whereby the value of the characteristic of the radiation detected by the detector means is related to the position of the beam of radiation at the modifying means.

3. Apparatus as claimed in claim 1 or claim 2 in which the means for detecting

said characteristic is adapted to detect radiation of an optical wavelength.

4. Apparatus as claimed in any of claims 1 to 3 in which the modifying means comprises means for transmitting the radiation.

5. Apparatus as claimed in any of claims 1 to 4 in which said modifying means comprises a layer of a material through which the beam may pass with some attenuation of intensity, the thickness of the plate varying in the predetermined direction.

6. Apparatus as claimed in any of claims 1 to 4 in which said modifying means comprises a photographic plate which has been exposed in such a way that its opacity varies in the predetermined direction.

7. Apparatus as claimed in any of claims 1 to 4 in which said modifying means comprises a layer comprising successive light and dark bands, the relative widths of the bands varying in the predetermined direction.

8. Apparatus as claimed in any of claims 1 to 4 in which the said modifying means comprises a plate carrying a metallic film, the thickness of the metallic film varying in the predetermined direction.

9. Apparatus as claimed in any of claims 1 to 4 in which said modifying means comprises a photographic or photo-etched area mask carrying a fine pitch array of interlocking triangular-shaped opaque and transparent areas, the mean transmission characteristic varying in the predetermined direction.

10. Apparatus as claimed in any of claims 1 to 4 in which said modifying means comprises a rotating cylindrical light chppper with profiled vanes, the transmission characteristic of which varies in the predetermined direction.

11. Apparatus for determining the position of a beam of radiation comprising means for splitting a beam of radiation into two beams, means for directing each of said two beams to respective determining apparatus, each as claimed in any of claims to 10, the two determining apparatus being arranged mutually orthogonally with respect to the initial beam so that the position of the initial beam in any direction transverse its axis is determined.

12. Apparatus for determining the position of a beam of radiation comprising, transverse the path of the beam, a first central radiation detector for detecting the beam, which central radiation detector is surrounded by an annular radiation detector for detecting scattered radiation, and beam splitter means for passing the beam to apparatus as claimed in any of claims 1 to 10.

13. Apparatus as claimed in any of claims 1 to 12 in which a radiation diffuser is situated between said modifying means and the detector means.

14. Apparatus as claimed in any of claims 1 to 4 in which the modifying means comprises a diffuser.

15. Apparatus as claimed in claim 14 in which the diffuser is generally planar, the incident beam, in use, forming a defined area on the diffuser and the detector means is situated in a plane containing the normal to the diffuser at the defined area at an angle other than zero to said normal.

16. Apparatus as claimed in claim 15 in which said angle is between tan-1 0.16 and tan-' 0.74.

17. Apparatus as claimed in claim 16 in which the said angle is 24 .

18. Apparatus as claimed in any of claims 14 to 17 in which there are provided two said detector means in respective planes, said planes being orthogonal.

19. Apparatus as claimed in any of claims 14 to 17 in which there are provided two detectors in the or each said plane, one on each side of said normal.

20. Apparatus as claimed in claims 18 or 19 in which there is provided a further detector means mounted on said normal.

21. Apparatus as claimed in claims 18 to 20 in which there is provided a further detector means.

22. Apparatus as claimed in any of claims 14 to 21 in which the diffuser is a transmitting diffuser whereby the detector means is situated on the opposite side of the diffuser from the incident beam path.

23. Apparatus as claimed in any of claims 14 to 21 in which the diffuser is a reflective diffuser whereby the detector means is situated on the same side of the diffuser as the incident beam path.

24. Apparatus as claimed in any of claims 14 to 23 in which the diffuser approximates a Lambertian diffuser.

25. Apparatus as claimed in any of claims 15 to 24 in which the :ion diffuser is opal glass.

26. Radiation position detector means comprising a diffuser for receiving the radiation and on which the incident radiation forms a defined area and detector means for detecting said radiation transmitted or reflected from the diffuser, said detector means being situated in a plane containing the normal to the diffuser at said defined area and at an angle other than zero to said normal, whereby the output signal of the detector means is related to movement of the incident radiation across the diffuser.

27. Apparatus as claimed in any of claims 1 to 26 in which the detector means is connected to signal processing means in which the output signal of the detector means is processed to produce a recording or indication of the position of the radiation.

28. Apparatus as claimed in claim 1 substantially as hereinbefore described with reference to Figures 2 to 11 of the accompanying drawings.

29. Apparatus for detecting flaws in a reflective surface comprising means for providing a beam of radiation and reflecting said beam from said surface and means for passing said beam to apparatus of any of claims 1 to 27.

30. Apparatus for detecting flaws in tin plate comprising reflecting means for providing a beam of radiation and reflecting said beam from said surface and means for passing said beam to apparatus of any of claims 1 to 27.

31. Apparatus as claimed in claim 29 or 30, in which scanning means is provided to scan said beam of radiation across said reflective or tin plate surface.

32. Apparatus as claimed in claim 30 or 31 substantially as hereinbefore described with reference to Figure 1 of the accompanying drawings.

33. A method for determining the position of a beam of radiation comprising directing the beam of radiation at a modifying means, the modifying means being arranged to modify a characteristic of the radiation as detected by a detector means so that as the beam of radiation is moved across the modifying means the value of the characteristic of the radiation detected by the detector means varies continuously and is related to the position of the beam of radiation at the modifying means.

34. A method as claimed in claim 33 in which the characteristic detected is the intensity of the radiation.

35. A method as claimed in claim 33 or claim 34 in which the radiation is of optical wavelength.

36. A method as claimed in any of claims 33 to 35 in which the modifying means transmits the radiation.

37. A method as claimed in any of claims 33 to 36 in which, as the beam is moved across the modifying means, the characteristic of the radiation reflected or transmitted varies with the position of the beam on the modifying means.

38. A method for determining the position of a beam of radiation comprising splitting the beam into two beams and determining the position of each of the two beams in accordance with the method of any of claims 33 to 37 in mutually orthogonal directions.

39. A method for determining the position of a beam of radiation comprising providing a first detector for detecting scattered radiation and detecting the position of the central portion of the beam by a method according to any of claims 33 to 38.

40. A method as claimed in any of claims 33 to 38 in which the modifying means comprises a diffuser.

41. A method for detecting flaws in a reflective surface comprising reflecting a beam of radiation from said surface and detecting the position of said beam in accordance with the method of any of claims 33 to 40.

42. A method for detecting flaws in tin plate comprising reflecting a beam of radiation from said tin plate surface and detecting the position of said beam by the method of any of claims 33 to 41.

43. A method as claimed in claims 41 or 42 in which the beam of radiation is scanned across the reflective or tin plate surface.

44. A method for determining the position of a beam of radiation, as claimed in claim 33 and substantially as hereinbefore described.