GB1568212A - Interferometric methods for measuring surface displacements or differences in surface shape - Google Patents

Description

(54) IMPROVEMENTS IN OR RELATING TO INTERFEROMETRIC METHODS FOR MEASURING SURFACE DISPLACEMENTS OR DIFFERENCES IN SURFACE SHAPE (71) 1, THE SECRETARY OF STATE FOR DEFENCE, LONDON, do hereby declare the invention, for which I pray that a patent may be granted to me, and the method by which it is to be performed, Ito be particularly described in and by the following statement:- This invention relates to interferometric methods for measuring surface displacements or differences or changes in surface shape.

The use of double-exposure speckle interferometry for the measurement of surface displacement or distortion is known. A general description of the method is given, for example, by Butters and Leendertz in "Measurement and Control", Vol 4, December 1971, pp349-354. Broadly the method consists in recording the speckle paturrn derived from a surface by illuminating the surface with coherent light and superimposing light received from the surface and light from a reference beam from the light source, and comparing this recorded pattern with a second such pattern derived from the same surface after its displacement, distortion or other spatial modification, or derived from a second surface with which the first surface is to be compared.The two patterns can be recorded on photographic films for subsequent comparison, but, as described in the above paper, it is also possible to receive the patterns on the screen of a TVtype camera (eg, a vidicon tube) whose output is fed to a data store for subsequent electronic comparison, eg, by subtraction.

Two problems can arise with the method as applied hitherto. Firstly, surfaces whose roughness is small compared with the wavelength of the light act as specular reflectors.

In consequence such surfaces will not scatter light over large enough angles to be observable from any but the specular direction. Moreover the light which does scatter in the specular direction causes highlights to be produced in the image plane of the surface. These consequences prevent the derivation of a satisfactory recordable speckle pattern from such surfaces.

A second problem arises if the surface roughness is comparable with the wavelength of the light, as with some machined surfaces.

This causes a decorrelation between the speckle patterns of different surfaces. Such decorrelation seriously reduces the visibility of any constant-profile discrepancy fringes, i.e.

fringes which represent zones where the subtraction of the two patterns yields zero, and hence where there is a displacement, distortion or difference of approximately nA/2 wavelengths (influenced to some extent by the geometry of the illumination and viewing directions) as between these zones on one surface and on the other surface.

One feature of the present invention alleviates the first of these problems (the consequences of specular reflection) and, in combination with a further feature, permits alleviation of the second problem (decorrelation). In particular this combination of features allows speckle interferometry to be applied to the measurement of machined surfaces.

According to one feature of the present invention there is provided a method of measuring surface displacements, or differences or changes in surface shapes, by speckle interferometry wherein a first object surface is illuminated by an object beam of coherent radiation, the radiation specularly reflected from said surface is focussed to produce an image of said surface on a radiation-sensitive detector, and a reference beam of radiation from the same coherent source is superimposed upon the imaged surface at the detector, modifying thereby the speckle pattern formed by the object beam at the detector, and wherein said modified speckle pattern is compared with a second such pattern derived from the same or a second object surface, characterised in that the object beam is caused to be diffused before illuminating the first or each object surface, so that the detector recieves radiation from all of the relevant surface and the formation of highlights is suppressed.

The diffusing (or scattering) of the object beam can be effected by directing it through, or reflecting it from, a surface of suitable roughness in relation to the wavelength of the radiation, in a known manner.

According to a further feature the present invention may also comprise the use of infrared coherent radiation, instead of visible light as hitherto in speckle interferometry, in order to alleviate the aforementioned problem (decorrelation) which arises with some machined surfaces. The wavelength of obtainable infra-red coherent radiation, eg, the 10 6 micron radiation from a CO2 laser, is large compared with the surface roughness of such surfaces and the decorrelation is reduced to an extent which allows fringes to be visible.

To enable the nature of the present invention to be more readily understood, attention is directed, by way of example, to the drawing accompanying the Provisional Specification which is a schematic optical diagram of a speckle interferometer embodying the present invention.

In this drawing a CO2 laser 1 produces a coherent infra-red radiation output of 10'6 microns wavelength and, as usual in interferometry, has a coherence length at least twice the object depth of the surface to be examined. The output of laser 1 is directed at a wedge-shaped beam-splitter 2 whence most of the output continues as the object beam 14 and a smaller fraction is reflected as the reference beam 15.

Object beam 14 is reflected by plane mirror 3 into a beam-expanding lens 4 which diverges the beam over the area of a reflecting scatter-plate 5. Radiation is scattered from plate 5 on to the object surface 6 which is a machined surface roughness is comparable with the wavelength of visible light but small compared with 10 6 microns. The object surface 6 specularly reflects and scatters this radiation towards the object lens aperture 7 and object lens 8. Lens 8 forms an image of the object surface on the infra-red detector 13.

Detector 13 is suitably an infra-red sensitive vidicon-type tube.

The reference beam 15 is directed by a plane mirror 9 towards a focussing lens 10 which focusses this beam on to a pinhole 11.

The radiation which passes through the pin- hole is superimposed on the object beam radiation at the wedge-type beam combiner 12.

Apart from the use of infra-red radiation instead of visible light, and the introduction of the scatter-plate 5, the above-described arrangement will-be familiar to those working in the field of the speckle interferometry.

They will also be familiar with the optical components (lenses, etc) suitable for use in such an arrangement when operated with infra-red radiation.

The scatter-plate (or diffuser) 5 is a familiar optical component in other applications, comprising a roughened surface coated with a reflecting material such as aluminium, the roughness of the surface being comparable with the wavelength in use both in depth and laterally. In the present instance it is conveniently made by roughening a glass surface with carborundum before coating with aluminium. In the drawing the scatter-plate is shown as a flat reflector, but it can alternatively be concave or convex, or some complex shape, or a mulitiplicity of separate plates.Instead of a reflection scatter-plate, a transmitting scatterer, can be used, eg, of suitable infrared transmitting material The illuminated area of the scatter plate is made sufficiently large to cause all points of interest on the object surface 6 to receive radiation and reflect some of it into the object lens aperture 7.

The consideration affecting the optical design of the surface-imaging and beamcombining parts of the arrangement are substantially the same as those for speckle interferometry using visible light. For example the radiation from the object surface is superimposed on the reference beam in such a way that spatial frequency content of the radiation at the detector 13 is determined only by the object lens aperture 7. (As when using visible light, the aperture 7 is arranged to match the speckle size to the detector resolution in a known manner; the larger the aperture, the smaller the speckle size.) Thus the pinhole 11 is used to obtain a good quality reference beam whose phase and amplitude will vary: only slowly across the detector in comparison with those of the object beam.For the same reason the pinhole 11 is desirably so located that its virtual image lies in the plane of the object lens 8 and in the centre of aperture c7;ibed arrangement 7.

The above-described arrangement has enabled surface profile measurements to be made to an estimated accuracy of i 5 microns on surfaces having a roughness not exceeding 0-8 microns CLA (centre line average).

In a modified arrangement for combining the object and reference beams, the wedgetype beam combiner 12 is replaced by a plane mirror inclined to deflect the object beam to a detector located in line with the reference beam from pinhole 11. The mirror has a hole therein to allow the reference beam to pass therethrough to the detector.

At infra-red wavelengths, eg, 10 6 microns, there is an advantage in using speckle interferometry rather than infra-red holography for such measurements. This is because the spatial resolution of suitable available elec tronic infra-red detectors is much larger than such wavelengths, ie, separation of the order of this wavelength cannot be resolved. With holography the detector resolution must be comparable with the wavelength; by contrast, with speckle interferometry the speckle size can be matched to the detector resolution by adjusting the object-lens aperture, as previously mentioned.

Although described with particular reference to the use of a vidicon tube or other photo-electric device as detector, which allows the successive speckle patterns to be stored for subsequent processing, other forms of infra-red image detectors can be used.

In the foregoing description the use of infra-red radiation enables speckle interferometry to be applied to the machined surface with the decorrelation at a level which allows fringes to be seen, and diffusing the infra-red object beam ensures that the infra-red radiation is imaged at the detector from all relevant parts of the object surface without the formation of highlights. In accordance with present invention diffusing the object beam is not limited to the use of infra-red radiation however, but is also applicable to speckle interferometry using visible light or any other detectable radiation.

WHAT I CLAIM IS: 1. A method of measuring surface displacements, or differences or changes in surface shape, by speckle interferometry wherein a first object surface is illuminated by an object beam of coherent radiation, the radiation specularly reflected from said surface is focussed to produce an image of said surface on a radiation-sensitive detector, and a reference beam of radiation from the same coherent source is superimposed upon the imaged surface at the detector, modifying thereby the speckle pattern formed by the object beam at the detector, and wherein said modified speckle pattern is compa)ed with a second such pattern derived from the same or a second object surface, characterised in that the object beam is caused to be diffused before illuminating the first or each object surface, so that the detector receives radiation from all of the relevant surface and the formation of highlights is suppressed.

2. A method as claimed in claim 1 wherein the diffusing of the object beam is effected by directing it through, or reflecting it from, a surface of suitable roughness in relation to the wavelength of the radiation.

3. A method as claimed in claim 1 or claim 2 wherein the radiation is infra-red coherent radiation.

4. A method as claimed in claim 3 wherein the surface or surfaces in question have a roughness comparable with the wave-length of visible light.

5. A method of measuring surface displacements, or differences or changes in surface shape, by speckle interferometry substantially as hereinbefore described with reference to the drawing accompanying the Provisional Specification.

6. A speckle interferometer substantially as hereinbefore described with reference to the drawing accompanying the Provisional Specification.

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

Claims (6)

**WARNING** start of CLMS field may overlap end of DESC **. tronic infra-red detectors is much larger than such wavelengths, ie, separation of the order of this wavelength cannot be resolved. With holography the detector resolution must be comparable with the wavelength; by contrast, with speckle interferometry the speckle size can be matched to the detector resolution by adjusting the object-lens aperture, as previously mentioned. Although described with particular reference to the use of a vidicon tube or other photo-electric device as detector, which allows the successive speckle patterns to be stored for subsequent processing, other forms of infra-red image detectors can be used. In the foregoing description the use of infra-red radiation enables speckle interferometry to be applied to the machined surface with the decorrelation at a level which allows fringes to be seen, and diffusing the infra-red object beam ensures that the infra-red radiation is imaged at the detector from all relevant parts of the object surface without the formation of highlights. In accordance with present invention diffusing the object beam is not limited to the use of infra-red radiation however, but is also applicable to speckle interferometry using visible light or any other detectable radiation. WHAT I CLAIM IS:

1. A method of measuring surface displacements, or differences or changes in surface shape, by speckle interferometry wherein a first object surface is illuminated by an object beam of coherent radiation, the radiation specularly reflected from said surface is focussed to produce an image of said surface on a radiation-sensitive detector, and a reference beam of radiation from the same coherent source is superimposed upon the imaged surface at the detector, modifying thereby the speckle pattern formed by the object beam at the detector, and wherein said modified speckle pattern is compa)ed with a second such pattern derived from the same or a second object surface, characterised in that the object beam is caused to be diffused before illuminating the first or each object surface, so that the detector receives radiation from all of the relevant surface and the formation of highlights is suppressed.

2. A method as claimed in claim 1 wherein the diffusing of the object beam is effected by directing it through, or reflecting it from, a surface of suitable roughness in relation to the wavelength of the radiation.

3. A method as claimed in claim 1 or claim 2 wherein the radiation is infra-red coherent radiation.

4. A method as claimed in claim 3 wherein the surface or surfaces in question have a roughness comparable with the wave-length of visible light.

5. A method of measuring surface displacements, or differences or changes in surface shape, by speckle interferometry substantially as hereinbefore described with reference to the drawing accompanying the Provisional Specification.

6. A speckle interferometer substantially as hereinbefore described with reference to the drawing accompanying the Provisional Specification.