GB2170314A

GB2170314A - Measurement of depth or relief

Info

Publication number: GB2170314A
Application number: GB08426661A
Authority: GB
Inventors: Peter Martin Zollman; Julian Graham Edwards; Keith Gilbert Doyle
Original assignee: Zed Instruments Ltd
Current assignee: Zed Instruments Ltd
Priority date: 1984-10-22
Filing date: 1984-10-22
Publication date: 1986-07-30
Also published as: GB8426661D0

Abstract

Apparatus for scanning or mapping a surface 45 of varying depth or contour comprises an optical 'read' head which includes optical beam-splitting and polarising elements 20, 22, 27 as well as a thin, flexible mirror 30 which is vibrated at a high frequency by an acoustic output, e.g. that of a loudspeaker. A beam of light, e.g. from a laser 12, is focussed via the mirror so as to produce a spot image on the surface at one position thereof. A photodetector unit 55 senses the light reflected from the target. The intensity of the light reflected varies as the light spot moves into and out of focus as a function of the vibration of the mirror and the variation in the depth of the surface. The time difference between successive maxima of the photodetector output signal is then determined. In one application, the surface is moved helically relatively to the 'reading' head and the output of the photodetector unit is used to modulate the output of a 'writing' head, such as a powerful laser used to engrave a polymeric material, e.g. a flexographic printing roller. <IMAGE>

Description

SPECIFICATION Measurement of depth or relief This invention concerns apparatus for, and a method of measuring depth or relief. More particularly, the invention relates to measurement of depth or relief by means of non-contact scanning apparatus and method as well as to utilisation of the measurement results to produce a copy or reproduction of the surface the depth or relief of which has been scanned.

Our U.S. Patents Nos. 4,232,345 and 4,323,928 describe a method of and apparatus for producing a seamless or continuous copy of a two-dimensional, unjointed original.

Although the invention described in these patents has been very successful, it is not capable of producing good-quality copies of, or faithfully reproducing, artwork which is three-dimensional, i.e. has a dimension of depth, especially varying depth. This is because the 'read' scan of the original artwork scans for optical density. Yet in many fields there is a need for such copying or reproduction, e.g. in producing textured surfaces by techniques such as flexographic printing.

Non-contact electro-optical scanning apparatus is also known which employs the principle of optical triangulation to find the exact position or 'micro-displacement' of a target surface, see e.g. J. Phys. E.: Sci. Instrum., Vol.

17, 1984, pages 864-866.

However, this known apparatus is capable of providing a depth or contour measurement over a very limited range only and the measurement is performed at a different point for every depth.

The present invention seeks to provide a method of, and apparatus for non-contact electro-optical measurement of the depth or contour of a surface and for the mapping or scanning of such a surface. In one important application, the results of the scan are used to reproduce or copy the scanned surface on a copy surface.

According therefore to one aspect of the present invention there is provided non-contact scanning apparatus for scanning a surface of varying depth or contour, comprising lightdirecting means for directing light at said surface to produce an image thereon, focussing means for focussing the image at a reference focal plane disposed at a datum position of said surface, sensing means for sensing light reflected from said surface and producing an output representative of the amount of light reflected, means operative in each scan for dynamically and repeatedly varying the focal length of the said focussing means about the reference focal plane, and detecting means for detecting successive maxima in said output.

Preferably, a virtual source of light is provided and said means for varying the focal length is an actuator for reciprocating the said virtual source about its reference position; the virtual source may be a mirror the reflective surface of which is flexibly and smoothly deformable according to a desired curvature. In a preferred embodiment, the mirror is a thin membrane-like member deformed by a driving device, e.g. acoustically coupled to an electroacoustic transducer such as a loudspeaker to vibrate at the frequency (preferably the natural frequency of the mirror) of an A.C. power supply to the transducer.

Advantageously, the sensing means include photoelectric sensors the output of which is analysed to determine the separation in time of successive peaks; this is then converted into values expressed as depth. The depth values are then expediently utilised to modulate the output of a 'writing' or engraving tool, e.g. a laser, to reproduce the original surface on a copy surface such as a polymeric flexographic printing roller.

In another aspect of this invention, there is provided copying apparatus comprising an optical reading device for optically scanning a master surface of varying depth or contour, depth measuring means for measuring the depth of successive scan positions relative to a reference depth value, and for providing an output representative of the measured depth values, a writing device for producing on a copy carrier a surface of varying depth or contour, and control means for modulating the output or operation of the writing device in accordance with the output of said measuring means.

According to yet another aspect of the invention, there is provided a method of scanning a surface of varying depth or contour, comprising directing light at said surface to produce an image thereon, focussing the image at a reference focal plane disposed at a datum position of said surface, sensing light reflected from said surface and producing an output representative of the amount of light reflected, in each scan dynamically and repeatedly varying the focal length of the said focussing means about the reference focal plane, and detecting successive maxima in said output.

In a further aspect of the invention, there is provided a method of copying or reproducing a master surface of varying depth or contour, comprising optically scanning said surface, measuring the depth of successive scan positions relative to a reference depth value and providing an output representative of the measured depth values, producing on a copy carrier a surface of varying depth or contour by means of a 'writing' device, and modulating the output or operation of the writing device in accordance with the output of the measured depth values.

The invention will now be described, merely by way of example, with reference to and as shown in the accompanying diagrammatic drawings, wherein: Figure 1 is a schematic cross-section of a preferred embodiment of apparatus according to the invention, Figure 2 is a waveform diagram, and Figure 3 is a block diagram of apparatus for copying a surface having depth or relief and including apparatus shown in Fig. 1.

Referring first to Fig. 1, there is shown a non-contact electro-optical scanning apparatus for scanning a surface having depth or relief and generally designated 10. The apparatus includes a casing 11 accommodating, as a preferred light source, a laser 12, e.g. a 2 mW polarised helium-neon laser, the E-vector of the light being directed into the plane of the Figure. However, it should be appreciated that any collimated light beam from a multidirectional source could be used. The beam of light from the laser 12 passes along a first axis 13 through a focussing lens 14 and a diffuser 15 which forms a bright spot acting as a virtual source of light. From the diffuser 15 light passes through a pinhole 17 and a collimating lens 18 to a first beam-splitter 20.

A part of the beam goes through the beamsplitter 20 along axis 13 and is absorbed by the internal wall of the casing.

The first beam-splitter 20 deflects the rest of the beam by 90t along a second axis 21 and sends it, without any change in polarisation, to a second beam-splitter 22 which is a polarising device. The device 22 is so chosen that all the light with the particular polarisation, i.e. with the E-vector perpendicular to the plane of the diagram is reflected whereas light with the E-vector in the plane of the diagram would pass straight through. The reflected beam is now turned by a further 90- along a third axis 25 which is parallel with the first axis 13. After passing through a quarter-wave plate 27 secured to a mount 28, the emergent beam is circularly polarised and impinges on a mirror 30. The plate 27 is at a slight angle to the optical axis 25.

The mirror 30 is a flexible and vibratable body, e.g. a thin glass disc polished and coated with aluminium. The mirror 30 is mounted on a fused silica ring 32 secured on a tube 33 which surrounds a loudspeaker 35.

The loudspeaker 35 is thus acoustically coupled to the mirror 30 to vibrate the latter. The loudspeaker is connected to a high-frequency A.C. power supply of e.g. 2.5 kHz, the drive frequency being preferably so chosen as to coincide with the natural (resonant) frequency of vibration of the mirror. The loudspeaker 35 is mounted on a base 36 connected to the casing 11 and its position is settable by adjusting screws 37.

Light reflected by the mirror 30 passes back along axis 25 through the quarter-wave plate 27 where its polarisation is changed once again so that the E-vector is now in the plane of the 4xagram. This beam then passes straight through, undeflected or unreflected, the beam-splitter 22 to a lens 40 mounted in screw-in cap 41, there being a set of such caps 41 carrying different lenses 40.

The target, i.e. the surface the depth variations of which is to be mapped, is adjacent the lens 40 and is designated 45.

The optical system described so far will produce a small spot image, typically about 0.1 mm in diameter, focussed at a reference or datum plane which represents the mean depth of the target surface. The brightness of the actual spot will then depend on the deviation of its position from the actual focal plane.

However, since the mirror is vibrated at high frequency (in our example the target area is scanned 2500 times per second) the actual surface will move into and out of focus twice in each cycle, i.e. twice for each successive peak of the curvature of the mirror surface.

This results in a very important advantage of the present invention, namely that it enables very rapid measurements e.g. here 2500 independent measurements per second. This will be referred to again below: at this stage it should merely be noted that the amount or intensity of light reflected from the target 45 will vary as a function of the variation in the focussing position of the optical system.

Light reflected from the target surface 45 will then pass back along the axis 25 through the lens 40, straight through the polarising beam splitter 22 and through the quarter-wave plate 27 (change of polarisation again) to the mirror 30. From the mirror 30 light is reflected back through the quarter-wave plate 27 so that its polarisation is now restored to the original one. In this state of polarisation the light is reflected by the beam splitter 22 along axis 25 to the beam splitter 20. Here about half of it is reflected back along axis 13 towards the laser 12 and the remainder passes through the beam splitter 20 along axis 21, through a collecting lens 50 and a second pinhole 52. The size of this pinhole 52 is preferably matched to the size of the spot on the target 45 to improve contrast. Light emerging from the pinhole 52 falls on a photoelectric detector 55 mounted on and electrically connected to a printed circuit board 56 carrying circuitry which is schematically illustrated in Fig. 3. The detector 55 produces an electrical waveform which is an analogue of the intensity of light impinging on it and a typical waveform is illustrated in Fig. 2.

As may be seen in Fig. 2, in which the abscissa is time and the two ordinates are the electrical output (voltage) of the photodetector and the curvature of the mirror 30, respectively, the voltage 60 exhibits two maxima or peaks 61 symmetrically disposed about an instant in time when the mirror curvature 65 is at a maximum.

In the time interval between two successive maxima of mirror curvature 65 two such vol tage peaks 61 occur. The peaks 61 are sharp and well-defined: between successive peaks the voltage waveform 60 is essentially flat.

This voltage waveform 60 thus corresponds to the explanation given above, namely that for each cycle of vibration of the mirror 30 the spot image on the target surface 45 will move into and out of focus twice. Sharp focus will occur at the instant where the distance of the target surface 45 from the lens 40 coincides with the distance to the focus from the latter. Thus the amount of light reflected by the surface 45 and detected by the detector 55 will produce correspondingly two relatively closely spaced peaks 61. The separation t between respective pairs of peaks 61 is thus representative of the depth of a given point of the target surface 45, given that the frequency of vibration is constant.

An important (but not exclusive or exhaustive) application of the invention described so far is the reproduction or copying of the relief or contour of the target surface 45, hereafter referred to as the 'master surface 45'. The copy surface may be a printing broiler, e.g. a flexographic printing roller. A preferred embodiment of such a copying system is shown in Fig. 3. The master surface 45 is mounted on a rotary 'master' cylinder 70, rotated in use, by means not shown, in the sense of arrow 71. The apparatus 10 described with reference to Fig. 1 is represented by a block 72 and lens 73, hereafter referred to as optical reader head 72. Actuating means, not shown, cause relative linear movement in the direction of arrow 74 between the cylinder 70 and reader head 72, whereby the master is helically scanned.The output of the photodetector 55 is amplified by an amplifier 75 which may (or may not) form part of the circuitry on the printed circuit board 56 of Fig.

1. The amplified output is substantially of the waveform 60 shown in Fig. 2.

Circuit block 77 finds the peaks 61 in the waveform 60 and circuit block 78 measures the time separation t between the peaks 61.

Circuit block 80 then converts the values of t into values of depth and produces an output the waveform 82 of which is schematically illustrated. This waveform 82 is an analogue of the relief or contour of the master surface 45. The output of waveform 82 is then passed to a laser control circuit 85 which controls the operation of a 'writing' or power laser 86, e.g. a CO2 laser. The light output of laser 86 is focussed by a lens 87 at a copy surface 88 which is mounted on a rotary 'copy' cylinder 89 rotated in use in a sense indicated by arrow 90, by means not shown.

Non-illustrated means produce relative linear movement between the 'writing head' or laser 86 and the copy cylinder 89, whereby the copy surface 88 is helically scanned and engraved.

It will be appreciated that the preferred embodiment of the invention described above provided for a highly accurate apparatus and method of depth measurement as well as contour copying, at very high speed.

It should also be appreciated that the results of the 'read' scan could be stored and then retrieved at a later time and/or at a different physical location to perform the 'write' scan.

The apparatus is compact and by making good use of polarisation phenomena, the system is economical with light. Furthermore the system is insensitive to mechanical misalignments such as e.g. that of the acoustic driver for the vibrating mirror, because of the arrangement of the optical path: thus, the 'in' and 'out' paths between the mirror and the target are always the same as each other even if the mirror is twisted away from its nominal position.

By providing a set of lenses 40 the focal length may be varied to suit the target, i.e. to set the focal plane to the mean depth of the target surface.

If the power of the lenses 18, 50 and the diameters of the pinholes 17, 52 are chosen appropriately, then the size of the image spot on the photodetector always corresponds, apart from a scaling factor, to that on the target surface, whatever the focal length of the actual lens 40 employed.

The apparatus could be modified in a number of ways. It would thus be possible to dispense with the quarter-wave plate 27 and the light splitter and polariser 22. But then the vibrating mirror would produce only one bright spot, rendering both measurement and computation more difficult and resulting in greater optical power losses.

Alternatively, the mirror could be set at e.g.

45 in the position of the beam splitter 22 to deflect the light directly towards the target surface along the axis 25. However, with this arrangement the focussed spot becomes ellip tical as the mirror curvature increases.

Claims

1. Non-contact scanning apparatus for scanning a surface of varying depth or contour, comprising light-directing means for directing light at said surface to produce an image thereon, focussing means for focussing the image at a reference focal plane disposed at a datum position of said surface, sensing means for sensing light reflected from said surface and producing an output representative of the amount of light reflected, means operative in each scan for dynamically and repeatedly varying the focal length of the said focussing means about the reference focal plane, and detecting means for detecting successive maxima in said output.

2. Apparatus according to claim 1 wherein a virtual source of light is provided and said means for varying the focal length is an actuator for reciprocating the said virtual source about its reference position.

3. Apparatus according to claim 2 wherein said virtual source is a mirror the reflective surface of which is flexibly and smoothly deformable according to a desired curvature.

4. Apparatus according to claim 3 wherein the mirror is a thin membrane-like member acoustically coupled to an electro-acoustic transducer to deform at the frequnecy (preferably the natural frequency of the mirror) of an A.C. power supply to the said transducer.

5. Apparatus according to any preceding claim wherein said light-directing means includes a plurality of polarising elements and a birefringent plate such that light from its source to the target on said surface is polarised and then from the target to the detecting means is charged back to its original state of polarisation.

6. Apparatus according to any preceding claim wherein the sensing means includes a pinhole.

7. Apparatus according to claim 6 wherein the size of the pinhole is matched to the size of the image on said surface.

8. Apparatus according to claim 6 or claim 7 wherein the sensing means include photoelectric sensors.

9. Apparatus according to claim 8 wherein the measuring means includes means for finding peaks in the output of the photoelectric sensors, means for determining the separation in time of successive peaks and means for converting the thus obtained time values to depth values.

10. Apparatus according to claim 9 wherein electric signals representative of the depth values are used as a control input to a "writing" laser.

11. Apparatus according to claim 10 wherein means are provided to produce a relative scanning movement between a "copy" surface and said "writing" laser.

12. Apparatus according to any preceding claim wherein the light-directing means includes a target lens which is one of a set of exchangeable lenses to suit the range of depth variations in said surface.

13. Copying apparatus comprising an optical reading device for optical scanning a master surface of varying depth or contour, depth measuring means for measuring the depth of successive scan positions relative to a reference depth value, and for providing an output representative of the measured depth values, a writing device for producing on a copy carrier a surface of varying depth or contour, and control means for modulating the output or operation of the writing device in accordance with the output of said measuring means.

14. Copying apparatus according to claim 13 wherein the optical reading device includes a mirror the reflective surface of which is flexi bly and smoothly deformable according to a desired curvature.

15. Copying apparatus according to claim 13 or 14, wherein the master and copy surfaces are helically scanned.

16. Copying apparatus according to any of claims 13 to 15, wherein the writing device is a laser and the copy surface is a flexographic printing roller having a laser-engravable surface.

17. A copy surface whenever produced by copying apparatus as claimed in any of claims 13 to 16.

18. Apparatus according to claim 1 or claim 13 substantially as hereinbefore described with reference to and as shown in the accompanying drawings.

19. A non-contact scanning method of scanning a surface of varying depth or contour, comprising directing light at said surface to produce an image thereon, focussing the image at a reference focal plane disposed at a datum position of said surface, sensing light reflected from said surface and producing an output representative of the amount of light reflected, in each scan dynamically and repeatedly varying the focal length of the said focussing means about the reference focal plane, and detecting successive maxima in said output.

20. A method of copying or reproducing a master surface of varying depth or contour, comprising optically scanning said surface, measuring the depth of successive scan positions relative to a reference depth value and providing an output representative of the measured depth values, producing on a copy carrier a surface of varying depth or contour by means of a 'writing' device, and modulating the output or operation of the writing device in accordance with the output of the measured depth values.

21. A method according to claim 19 or claim 20, substantially as herein described with reference to and as shown in the accompanying drawings.