WO2007025972A2

WO2007025972A2 - Optical scanning device

Info

Publication number: WO2007025972A2
Application number: PCT/EP2006/065787
Authority: WO
Inventors: Andrew Forrest
Original assignee: Imperial Innovations Ltd
Priority date: 2005-08-30
Filing date: 2006-08-29
Publication date: 2007-03-08
Also published as: WO2007025972A3; GB0517630D0; GB2429764A

Abstract

An optical scanning device includes a deflection element [2] that scans a radiation beam via a first optical system [3] over a surface [6]. The radiation reflected by the surface is converged on a detection system [8] by a second optical system. The first and second optical system together image the scanning centre of the deflection element on the detection system. The distance between the deflection element and the first optical system is preferably in the range 0.6 f1 to 0.9 f1, where f1 is the focal length of the first optical system.

Description

Optical Scanning Device

Field of the Invention The invention relates to a scanning device for scanning a surface along a scan line using a radiation beam and collecting radiation from the surface on a detection system. Such a device can be used for inspecting the surface of an object, for example for reading a predetermined pattern present on the object or detecting flaws such as cracks in the surface.

Background of the Invention

A device of this type is described in US patent 4,295,743. This known device is designed for detecting faults in surfaces. It forms a spot scanning over a line on the surface under investigation. An optical system converges radiation reflected by the surface on a detection system. The detection system comprises several detectors for detecting radiation reflected from the surface at different angles. A disadvantage of the device is the relatively complex optical system.

It is an object of the invention to provide a less complex scanning device having a high angular resolution and a high spatial resolution.

Summary of the Invention

This object is achieved by a scanning device for optically scanning a surface along a scan line, which device comprises a radiation source for supplying a radiation beam for scanning the surface, a detection system for detecting radiation from the surface, a deflection element having a scanning centre for scanning the radiation beam, a first optical system having a first focal length fi for focussing the radiation beam from the deflection element to a scanning spot on the surface, the scanning centre being arranged at a distance less than the first focal length from the first optical system, a second optical system having a second focal length f₂ for converging radiation from the surface on the detection system, and the combination of the first optical system and the second optical system imaging the scanning centre on the detection system. The scanning device combines a high spatial resolution over the scan line and a small spot on the detection system, providing a high angular resolution.

The first optical system converges the radiation beam to a focus on or close to the surface. Hence, the scanning spot on the surface formed by the radiation beam will have a relatively small diameter, achieving the high spatial resolution. The focus of a radiation beam is the smallest cross-section of a converging radiation beam. The position of the scanning centre within a focal distance from the first optical system provides an improved spatial resolution over the scan line, in particular near the scan extremes. The convergence of the radiation beam combined with the position of the scanning centre makes the layout of the optics of the scanner asymmetrical with respect to the scan line. As a consequence, the radiation beam incident on the surface is not displaced parallel to itself during scanning but the angle of incidence on the surface of the principle ray of the radiation beam changes along the scan line.

The imaging of the scanning centre on the detection system allows matching of the luminosity of the first and second optical system to that of the detection system and collection of radiation from the surface in a small spot on the detection system. The small spot makes it possible to use a relatively small detection system without making the scanning device more complex. A smaller detection system improves the angular resolution of the device, reduces capture of stray light and increases its temporal frequency response. It also avoids the problem of prior art scanning devices that collect radiation from the surface on a small detection system via the deflection element. The use of the deflection element in the radiation coming from the surface reduces the maximum angle of radiation from the surface that can be collected. The scanning device according to the invention can collect radiation from the surface being scanned on a relatively small detector without a deflection element in the optical path between the surface and the detection system. The position of the detection system is preferably within 1/3 f₂ from the position of the image of the scanning centre; more preferably within 1/10 f₂. The two optical systems must image the scanning centre on the detection system in the scanning plane. The imaging properties in the plane perpendicular to the scanning plane may be different. In a preferred embodiment the optical systems have the same imaging characteristics in both planes, and the scanning centre will be imaged on the detection system in both planes. In a preferred embodiment of the scanning device the distance between the scanning centre and the first optical system is in the range 0.6 fi to 0.9 fi

This distance of the scanning centre provides a substantially straight scan line, a substantially constant resolution over the scan line and a substantially constant position versus angle characteristic. The distance between the scanning centre and the first optical system is preferably in the range from 0.7 fi to 0.85 fi to reduce the size of both the scanning spot and the spot on the detection system.

In a preferred embodiment the combination of the first optical system and the second optical system forms a telescopic system. The telescopic system converges an incoming collimated radiation beam to a scanning spot in the focal plane between the first and second optical system. The surface to be scanned is preferably arranged in this plane. The term 'telescopic system' means that the first and second optical systems are separated by a distance f 1 + f2 with a tolerance of 10% of the distance. A further reduction of the size of the spot on the detection system can be achieved when a distance between the scanning spot and the second optical system is in the range from 1.05 f₂ to 1.25 f₂.

In a specific embodiment of the scanning device according to the invention the detection system comprises a plurality of detectors for detecting radiation emanating from the surface at different angles. Combined with the small spot on the detection system, it provides for a high angular resolution.

The first and/or second optical system may include lenses. However, the first optical system and/or the second optical system is preferably a concave mirror, preferably a concave parabolic mirror. Scan mirrors have the advantages of having less weight than scan lenses, providing a more compact device, not suffering from chromatic aberration and being cheaper to manufacture. The first and second focal length may have different values, providing freedom of design. For example, a long first focal length gives a small scanning spot and, hence, a good spatial resolution and a short second focal length reduces the size of the scanning device without affecting the spatial resolution. In a special embodiment of the device the first focal length is substantially equal to the second focal length. This allows both elements to have the same design. Moreover, it also allows the first optical system and the second optical system to be integrated in a single optical element, thereby reducing the cost of the device. The design of the optical scanning system generally causes the scan length on the second optical element to be larger than that in the first optical element. Hence, when using the single optical element, the radiation emanating from the surface preferably crosses a centre of the single optical element during scanning, providing the largest length available in the element for the beam coming from the surface.

The angular range over which radiation from the surface is captured can be increased by arranging a first cylindrical optical element having a cylinder axis parallel to the scan line between the surface and the second optical system. The element converts angular incidence into displacement in the plane vertical to the scanning plane. A further increase can be achieved by arranging a second cylindrical optical element between the surface and the first cylindrical optical element.

Further features and advantages of the invention will become apparent from the following description of preferred embodiments of the invention, given by way of example only, which is made with reference to the accompanying drawings.

Brief Description of the Drawings

Fig. Ia shows a first embodiment of the scanning device according to the invention drawn in a plane perpendicular to the scanning plane,

Fig. Ib shows the first embodiment of the scanning device according to the invention drawn in the scanning plane, Fig. 2 shows an embodiment of the detection system of the scanning device,

Fig. 3a shows a schematic layout of the optical system of the scanning device in a plane perpendicular to the scanning plane for explanation of the focussing of the collimated scanning beam,

Fig. 3b shows a schematic layout of the optical system in the scanning plane for explanation of the geometry of the scanning principal ray, and

Fig. 4 shows a second embodiment of the scanning device according to the invention.

Detailed Description of the Invention

Fig. 1 shows diagrammatically a first embodiment of the scanning device according to the invention. Fig. Ia and Ib show a view of the device in a plane perpendicular to the scanning plane and in the scanning plane, respectively. Only the principal ray of each radiation beam is shown. The arrows on the principal rays indicate the direction of propagation of the radiation. The scanning device comprises the following elements. The radiation beam for scanning is generated by a radiation source 1, e.g. a laser. A deflection element 2 scans the radiation beam over the surface. The deflection element may be any element that changes the direction of a radiation beam, e.g. a galvanic mirror, as drawn in the Figure, or a polygon mirror. The scanning beam is reflected by an optical element 3, drawn as an off-axis section of a circularly symmetric parabolic mirror. The mirror has a relatively small height in a direction perpendicular to the scanning plane and a relatively large height in the scanning plane. A first folding mirror 4 and a second folding mirror 5 reduce the building height of the scanning device. The folding mirrors are elongate and preferably flat. The path may be folded in other ways. After reflection on the folding mirrors the radiation beam impinges on the surface 6 under investigation. Radiation reflected from the surface returns through the optical path and is condensed by an optical element 7 onto a detection system 8. The optical element 7 reduces the size and deviation in the scan direction of the spot on the detection system during a scan. The radiation beam 10 emitted by the radiation source 1 is a collimated beam. The deflection element 2 changes the radiation beam 10 to a scanning beam 11, which scans approximately the height of the mirror 3, as shown Fig. Ib, where the scanned beam is drawn at the end of the scan. The converging beam 12 reflected from the mirror 3 is folded twice by the folding mirrors 4 and 5. Since the parabolic mirror 3 should be used at as near normal incidence as possible, the scanning beam 11 must be as close as possible to the first folding mirror 4. After the second folding mirror 5 the converging beam 12 comes to a focus as scanning spot 13 on the surface 6. During scanning, the scanning spot traces a substantially straight line, the scan line 14. Radiation reflected from the surface in the scanning spot forms a diverging beam 15, which is transformed to a substantially collimated beam 16 by reflection on mirror 3. Since the strip traced by the diverging beam 15 on the mirror 3 during scanning is longer than the strip traced by the scanning beam 11 on the mirror 3, the folding mirrors and the mirror 3 are arranged such that the strip traced by the diverging beam 15 crosses the centre of the mirror 3, thereby providing more space for the strip. The lens 7 converges the collimated beam 16 onto the detection system 8. The size of the scanning device can be further reduced by arranging a folding mirror in the collimated beam 16. Fig. 2 shows a specific embodiment of the detection system 8 in a plane perpendicular to the scanning plane. The detection system is a quadrant detector having four detectors 17a, 17b, 17c and 17d arranged in a square. The principal ray of the reflected beam intercepts the centre of the quadrant. The double arrow 18 indicates the direction corresponding to the direction of scanning. When the surface 6 is a perfect mirror, the optical elements can be lined out such that the converging beam 16 impinges on the centre of the quadrant detector. In that case each of the four detectors will receive an equal amount of light, on condition that the radiation beam has a circular symmetric intensity distribution in a cross section of the beam. When the surface has a slope in the scanning plane, the spot on the detection system will move in the direction of the double arrow 18. When the surface has a slope in the plane perpendicular to the scanning plane, the spot will move in a direction perpendicular to the double arrow 18. An electronic circuit, not shown in the drawing, has as input the four electric outputs of the detectors 17a to 17d. The total intensity of the converging beam 16 is represented by an output signal being the sum of the detector signals 17a+17b+17c+17d, the slope in the scanning plane by the combination of detector signals (17a + 17b) - (17c + 17d) and the slope in the plane perpendicular to the scanning plane by (17a + 17d) - (17b + 17c). The slope signals may be normalised by the sum of the detector signals. The analogue output signals of the electronic circuit can be converted to digital signals and used in a video processor, where colours can be used to indicate slope or height of the surface in a display image. The output signal of a photodiode near the end of the scan line can be used as a synchronisation signal for building the display image. The detection system may also be a 4x4 array of detectors or larger instead of the 2x2 array shown in Figure 2.

In another embodiment of the detection system a central, square detector has four detectors around it, one adjacent each of its sides. A measure of the total intensity and the slopes in two perpendicular directions can be obtained from suitable combinations of the output signals of the detectors. An example of such an embodiment is a CCD detector. When the slope in only one direction is required, the detectors for the slope in the perpendicular direction can be omitted from the detection system. In cases where the distribution of the radiation reflected from the surface is circularly symmetric, the detection system may be a linear array of two or more detectors; a detector at the end of the array may be positioned to intercept the principal ray of the reflected beam.

Design parameters of the optical system of the scanning device shown in Figure Ia and b will now be elucidated with reference to Fig. 3. Figure 3a shows a schematic layout of the optical system in a plane perpendicular to the scanning plane. The first optical system for converging the scanning beam 11 to the scanning spot 13 is represented by a lens 23. The second optical system for converging the diverging beam 15 on the detection system 8 is represented by a lens 24. In the scanning device shown in Fig. Ia and b the first and second optical element are combined in a single optical element, i.e. the mirror 3. Although the first and second optical system have the same optical properties in the embodiment of Fig.1 and the drawing of Fig. 3, this is not necessary. The first optical system, lens 23 in the Figure, has a focal length Ii₁; the second optical system, lens 24 in the Figure, has a focal length f₂. The position of the surface 6 is indicated by the dashed line 25, the distance between the dashed line and lens 23 being d₃ and between the dashed line and lens 24 d₄. The measurement of a distance is taken from the principal point of the lens at the side of the dashed line. The principal point of a mirror is the vertex, i.e. the point of intersection of the optical axis and the mirror surface.

The lenses 23 and 24 form a telescopic system such that (fi + f₂) = (d₃ + d₄). A deviation of up to 10% of fi + f₂ may arise in the value of d₃ + d₄ when minimising the diameter of the scanning spot 13 and the diameter of the collimated beam 16 where it impinges on cylindrical lens 7. Since the scanning beam 21 incident on the lens 23 is a collimated beam, the lens 23 forms the scanning spot in the focal plane of the lens. The values of fi and d₃ should preferably be substantially equal to have the smallest cross-section of the radiation beam on the surface under investigation when using a collimated beam 21. Since the lenses 23 and 24 are rotationally symmetric in this schematic layout, the collimated scanning beam will in the scanning plane also converge to a scanning spot on the surface. Therefore, Figure 3a also represents the lenses 23, 24 and the radiation beams in the scanning plane, when drawn in the situation where the scanning beam is halfway the scan range.

Fig. 3b shows a schematic layout of the optical system in the scanning plane. Whereas in Fig. 3a the two lines of the beam 21 indicate the extent of the beam, in Fig. 3b the drawn lines represent the principle ray of the radiation beam from the deflection element 26 to the detection system 28. Figure 3b shows two positions of the radiation beam. One line shows the case where the scanning spot is at one end 29 of the scan line, the other line the case where the scanning spot is at the other end 30 of the scan line.

The distance d₂ between the scanning centre 27 and the lens 23 is equal to 0.79 fi for optimum values of scan range, size of the scanning spot and the spot on the detection system and scan distortion. The scanning centre is the point at which the principle ray of the scanning beam when half-way the scan range intersects the plane that has the smallest area within which or the shortest line on which the principle rays at different scan positions cross the plane. Scan distortion includes the two errors of the spot not moving in an exactly straight line across the surface and not moving at a constant velocity along the scan line. From the right-hand side of the lens 23 the scanning centre appears to be located at a distance di from the lens 23; the figure shows this as the virtual image 31 of the scanning centre. The relation between di, d₂ and fi (all values taken as positive) is given by the lens formula:

When d₂ has the above value of 0.79 fi, the distance di has a value of 4.7 fi. The distance d₄ is optimised away from the exact requirement for a telescopic system to achieve a smaller spot on the detection system, resulting in a value of 1.1 f₂. The distance d₃ is optimised to reduce the size of the scanning spot by optimizing aberrations, resulting in a value of 0.99 fi. The scanning centre 27 is imaged by lenses 23 and 24 on the detection system at a distance d₅ from lens 24. The values of the distances are governed by the lens formula:

1 _ 1 J_

/₂ d_x + d₃ + d₄ d₅

When d₂ has the above value of 0.79 fi and fi = f_2j the distance d₅ has a value of 1.2 f₂. Figure 2b shows clearly that the radiation beam traces a longer line on the lens 24 than on the lens 23. The above calculation for the schematic layout of the optical system can be transferred directly to the embodiment of the scanning device shown in Fig. Ia and b.

The actually built embodiment of the scanner shown in Figs. Ia and Ib uses a 15 mW Helium-Neon laser as radiation source. The diameter of the scanning beam is generally within the range 5 to 20 mm and in this embodiment of the scanning device it is 15 mm. The deflection element is an octagonal scanning prism rotating at 3600 rpm. The parabolic mirror 3 has a focal length f of 2 metre and a diameter of 500 mm in the scan direction. The scan length on the surface is 400 mm. The maximum size of the scanning spot over the length of the scan is 0.1 mm, giving a spatial resolution of 1 in 4000. The optical element 7 is a lens having a focal length of 50 mm, which converges the 40 mm diameter beam to a spot of less than 1 mm diameter on the detection system. The detector system includes four detectors in a row. The detectors are photomultipliers to obtain sufficient bandwidth and a sufficiently large photosensitive area, 32 MHz and 20 mm diameter in the present case. The diameter of the spot on the detection system is about 40 mm. The optical system changes the position of the converging beam on the detection system by 38.4 mm for a one degree slope on the surface 6. The total capture range is -2 to +2 degrees, corresponding to a slope range of -1 to +1 degrees because of the doubling effect of the reflection on the surface. The detection system can be arranged to intercept radiation only within a specific 4 degrees interval of the angular spectrum, e.g. within the range 0 to 4 degrees or 3 to 7 degrees. Such a detection system is particularly useful when the light reflected from the surface is circularly symmetric or there is only a limited angular range of interest. The angular resolution is one degree.

Figure 4 shows another embodiment of the scanning device according to the invention. Similar elements have the same reference numerals as in Figure Ia and Ib. Only the principal ray of each radiation beam is shown. The device has a first optical system in the form of a parabolic mirror 41 for converging the scanning beam 11 to the scanning spot 16 on the surface 6 under investigation. A first folding mirror 42 changes the direction of the scanning beam. A second folding mirror 43 changes the direction of the diverging beam coming from the surface. A second optical system in the form of a parabolic mirror 44 converges the diverging beam 15 onto the detection system 8. The design of the optical system of this embodiment follows the principles set out in relation to Figure 2a and 2b.

A first and a second cylindrical optical element, in this embodiment a plane- convex cylinder lens 45 and a convex-concave cylinder lens 46 are arranged between the scanning spot 16 and the second folding mirror 43. The cylinder axis of the two lenses is in the direction of the scan line. The cylindrical optical elements converge the rays coming from the surface in the scanning plane, thereby increasing the range of angles that can be captured. To achieve a maximum angular capture range the focal length of the cylindrical lenses is chosen as short as compatible with positioning the lenses in the scanning device. The surface preferably includes the focal axis of the two cylinder lenses. The optical element 7 in front of the detection system 8 includes two cylindrical lenses having a total focal length of approximately 12 mm. The embodiment of Figure 5 can capture rays in an angular range of -20 to +20 degrees in a plane perpendicular to the scanning plane. The detection system may be arranged to intercept radiation only within the range 0 to 40 degrees in that plane. The range can be separated into at least 16 channels measuring radiation from different angles in the 40 degree range.

The scanning device according to the invention is a general purpose scanner for retrieving total reflectivity information from the surface but also angle- resolved reflectivity information. The information can be obtained for different wavelengths and polarisations by changing the radiation source of the scanning device or adding radiation sources for simultaneous measurements or placing filters or polarisers over the detection system. The polarisation response can be measured at two perpendicular polarisations at the same time. Similarly, different detectors can measure different parts of a wavelength spectrum concurrently. The scanning device is very suitable for inspecting the surface of objects, in particular for quality checks of products. When using a polariser and an analyser, the scanning device can detect very slight dirt and blemishes on surfaces, such as those of optical elements. The scanning device also allows the detection of for example cracks in the glazing of tiles, scuff and finger marks on surfaces, texture differences of surfaces, surface flatness, etc. Although the scanning device operates in reflection on the surface, it can likewise operate in transmission.

The above embodiments are to be understood as illustrative examples of the invention. Further embodiments of the invention are envisaged. It is to be understood that any feature described in relation to any one embodiment may be used alone, or in combination with other features described, and may also be used in combination with one or more features of any other of the embodiments, or any combination of any other of the embodiments. Furthermore, equivalents and modifications not described above may also be employed without departing from the scope of the invention, which is defined in the accompanying claims.

Claims

1. A scanning device for optically scanning a surface along a scan line, which device comprises: a radiation source for supplying a radiation beam for scanning the surface; a detection system for detecting radiation from the surface; a deflection element having a scanning centre for scanning the radiation beam; a first optical system having a first focal length fi for focussing the radiation beam from the deflection element to a scanning spot on the surface; the scanning centre being arranged at a distance less than the first focal length from the first optical system; a second optical system having a second focal length f₂ for converging radiation from the surface on the detection system; and the combination of the first optical system and the second optical system imaging the scanning centre on the detection system.

2. A scanning device according to claim 1, wherein the distance between the scanning centre and the first optical system is in the range 0.6 fi to

0.9 fi.

3. A scanning device according to any one of claim 1 or 2, wherein the distance between the scanning centre and the first optical system is in the range from 0.7 fi to 0.85 fi.

4. A scanning device according to claim 1, 2 or 3, wherein the combination of the first optical system and the second optical system forms a telescopic system.

5. A scanning device according to any one of claim 1 to 4, wherein a distance between the scanning spot and the second optical system is in the range from 1.05 f₂ to 1.25 f₂.

6. A scanning device according to any one of claim 1 to 5, wherein the detection system comprises a plurality of detectors for detecting radiation from the surface at different angles.

7. A scanning device according to any one of claim 1 to 6, wherein the first optical system and/or the second optical system is a concave mirror.

8. A scanning device according to any one of claim 1 to 7, wherein the first focal length is substantially equal to the second focal length.

9. A scanning device according to claim 8, wherein the first optical system and the second optical system are integrated in a single optical element.

10. A scanning device according to claim 9, wherein during scanning the radiation from the surface crosses a centre of the single optical element.

11. A scanning device according to any one of claim 1 to 8, wherein a cylindrical optical element having an axis parallel to the scan line is arranged between the surface and the second optical system.

12. A scanning device according to claim 11, wherein a second cylindrical optical element is arranged between the surface and the cylindrical optical element.