GB2307550A - Illumination of a test sample by scanning a line of coherent radiation - Google Patents
Illumination of a test sample by scanning a line of coherent radiation Download PDFInfo
- Publication number
- GB2307550A GB2307550A GB9523973A GB9523973A GB2307550A GB 2307550 A GB2307550 A GB 2307550A GB 9523973 A GB9523973 A GB 9523973A GB 9523973 A GB9523973 A GB 9523973A GB 2307550 A GB2307550 A GB 2307550A
- Authority
- GB
- United Kingdom
- Prior art keywords
- line
- sample
- coherent radiation
- laser
- radiation
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Withdrawn
Links
Classifications
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
- G01B11/00—Measuring arrangements characterised by the use of optical techniques
- G01B11/24—Measuring arrangements characterised by the use of optical techniques for measuring contours or curvatures
- G01B11/25—Measuring arrangements characterised by the use of optical techniques for measuring contours or curvatures by projecting a pattern, e.g. one or more lines, moiré fringes on the object
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
- G01B11/00—Measuring arrangements characterised by the use of optical techniques
- G01B11/16—Measuring arrangements characterised by the use of optical techniques for measuring the deformation in a solid, e.g. optical strain gauge
- G01B11/161—Measuring arrangements characterised by the use of optical techniques for measuring the deformation in a solid, e.g. optical strain gauge by interferometric means
- G01B11/162—Measuring arrangements characterised by the use of optical techniques for measuring the deformation in a solid, e.g. optical strain gauge by interferometric means by speckle- or shearing interferometry
Abstract
A sample (2) is illuminated for inspection by scanning a line (4) of coherent radiation across its surface in a direction which may be perpendicular to the line, using a line generator (5) and moveable mirror (7). The line has substantially uniform intensity along its length. In shearography the scan is synchronised with the framescan of a video camera (8) to capture an image. The illumination technique is also suitable for holography, electronic speckle pattern interferometry, speckle interferometry and particle image velocimetry.
Description
METHOD AND APPARATUS FOR INSPECTING PB TESTING A SAMPLEnx OPTICAL METROLOGY This invention relates to a method and apparatus for inspecting or testing a sample, such as an aircraft skin panel, by optical metrology and is particularly, but not exclusively, concerned with such a method and apparatus applicable for optical non destructive testing by shearography for aerospace components.
Coherent optical techniques such as holography, electronic speckle pattern interferometry (ESPI), speckle interferometry, particle image velocimetry (PIV) and shearography are currently being utilised for applications such as non destructive testing (NDT), vibration analysis, object contouring, stress and strain measurement, fatigue testing, deformation analysis and fluid flow diagnosis. All of these techniques have associated drawbacks with performance being to some extent a trade off against specific disadvantages inherent in the individual techniques.
For example shearography has high sensitivity and tolerance to environmental noise but is of limited application because of difficulties in inspecting large areas due to inefficiencies in the laser power available and optical beam expansion and delivery systems. Additional problems are encountered with a relatively low signal to noise ratio.
There is thus a need for an improved method and apparatus of inspecting or testing a sample by optical metrology which at least reduces a number of the foregoing problems associated with current coherent optical techniques as set forth above.
According to a first aspect of the present invention there is provided a method of inspecting or testing a sample by optical metrology, in which the sample is illuminated by scanning a line of coherent radiation across a surface of the sample.
This technique is applicable to many types of coherent optical processors such as holography, electronic speckle pattern interferometry (ESPI), speckle interferometry, particle image velocimetry (PIV), and shearography.
Preferably the radiation is provided by a laser source and the line of coherent radiation has substantially uniform intensity.
Conveniently the method utilises shearography in which the sample is observed by a video camera and in which the line of coherent radiation is scanned across the sample in synchronism with the frame scan of the camera.
Advantageously the method is operated with zero shear in the direction of scan of the line of coherent radiation.
According to a second aspect of the present invention there is provided apparatus for inspecting or testing a sample by optical metrology including a source of coherent radiation, means for forming a line of coherent radiation from said source and means for scanning the line of coherent radiation across a surface of the sample.
Preferably the source of coherent radiation is a laser operable to provide a line of coherent radiation with substantially uniform intensity.
Conveniently the apparatus includes a video camera operable to view the sample and provide an output signal, a processor operable to receive the output signal and extract therefrom the frame rate of the camera, and wherein the line scanning means incorporates a moveable mirror for reflecting a line of laser radiation, with the processor being further operable to synchronise the video camera scan frame rate with the rate at which the line of coherent laser radiation is scanned across the sample by the moveable mirror.
Advantageously the moveable mirror is drivable in a scanning movement via a sinusoidal wave form or via any other shaped wave form operable to counteract angular variation in the reflectance of the sample and, matched to the inertial response of the mirror.
Preferably the line forming means is a line generator located between the laser and the moveable mirror.
For a better understanding of the present invention, and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings in which:
Figure 1 is a schematic diagram of a conventional optical illumination system not according to the present invention.
Figure 2 is a graphical representation of intensity distribution along the dotted line A-A in Figure 1 of intensity in lumens versus distance in centimetres.
Figure 3 is a diagrammatic representation of a method of inspecting or testing a sample by optical metrology according to one embodiment of the present invention, and
Figure 4 is a graphical representation of the intensity of illumination in lumens distribution along the dotted line
B-B in Figure 3 against distance in centimetres.
Figure 5 is a diagrammatic representation of a method and apparatus according to a further embodiment of the present invention, and
Figure 6 is a diagrammatic illustration, in part to an enlarged scale, of scanned illumination applied to a sample according to a method of the present invention utilising shearography.
A method and apparatus according to the present invention for inspecting or testing a sample by optical metrology is suitable for many current coherent optical techniques such as holography, electronic speckle pattern interferometry, speckle interferometry, particle image velocimetry and shearography.
These techniques are in turn suitable for employment in non destructive testing, vibration analysis, object contouring, stress and strain measurement, fatigue testing, deformation analysis and fluid flow diagnosis. For the purposes of simplicity the method and apparatus will be described in terms of shearography for non destructive testing of aerospace components such as aircraft skin panels.
A shearography system works by generating two laterally displaced images of a test sample. In practice, this is achieved using a shearing element, of which there are many variants and imaging optics. When the sample is illuminated using coherent radiation such as usually visible radiation from a laser, these twin images are modulated by a speckle pattern due to the high coherence of the light. These two images interfere to form a macroscopic speckle pattern, which may be recorded electronically using a charge coupled device (CCD) and a framestore.
Interferometric images or fringe patterns may be generated by subtracting two speckle patterns of the sheared twin image, where the second speckle pattern is recorded after the test specimen has been subject to a stressing force, such as thermal, pressure or vibration. If an appropriate stressing force is applied defects in the structure of the sample are revealed by highly characteristic 'figure-of-eight' fringes.
In practice, the resulting fringe patterns are noisy due to spurious intensity variations and, consequently, the sensitivity of the technique is reduced. It has been proposed that the noise may be suppressed if the phase difference between the sheared images is extracted from the interferograms. The phase difference is calculated by subtracting the interferometric phase of the surface before and after a stressing force has been applied. The interferometric phase may be calculated by capturing a sequence of four fringe patterns by a camera and framestore, that have been successively incremented in phase by s/2.
These four images may then be processed mathematically in a computer to yield the phase image.
However the application of shearography to routine aerospace non-destructive testing limited by problems such as insufficient laser power simultaneously to inspect large areas, inefficient beam expansion and delivery systems and non-uniform illumination.
Figure 1 of the accompanying drawings is a diagrammatic representation of a typical apparatus for providing laser illumination from a laser source 1 to a sheet like sample 2 such as a metal or composite skin panel for an aircraft. The laser beam from the source 1 is passed via diverging optics 3 such as lens, fibre or spatial filter, to form a spot like illuminated area on the sample 2. Figure 2 shows the intensity of illumination distribution along the dotted line
A-A in lumens with distance in centimetres from which it can be seen that the intensity profile is a Gaussian distribution profile.A currently used laser source 1 for coherent metrology is a TEMPO laser which when used with beam expansion systems employing refractive optics such as the diverging optics 3 in Figure 1 produce highly non uniform illumination with the Gaussian profile as shown in Figure 2.
With such a conventional system the average pixel intensity value is typically only 65 when the brightest pixels are reaching a saturation value of 255 if the beam is used to illuminate a charge coupled device. As the dynamic range of current charge coupled device sensors is limited problems tend to occur when recording images of samples illuminated for shearography purposes by laser beams with a Gaussian profile.
For example the signal to noise ratio of shearography images is a strong function of the intensity at each pixel in the image because average speckle modulation is approximately directly proportional to speckle intensity. Hence the signal becomes very noisy at the edges of the image when Gaussian beam illumination is used.
According to the present invention a sample may be inspected or tested by optical metrology involving illuminating the sample 2 by scanning a line 4 of coherent radiation across a surface of the sample 2 as shown in Figure 3 of the accompanying drawings. The radiation is provided by a laser source 1 and the line 4 of coherent radiation has substantially uniform intensity. A line of uniform intensity may be generated from a line generator 5 which may conveniently be a Lasiris line scanner commercially available.
The beam from the line generator 5 is passed to a reflective surface 6 and from thence to scanning means conveniently in the form of a moveable mirror 7 whereby the line 4 can be moved up and down the sample 2 such as in the direction of the arrow C. Figure 4 shows the intensity distribution along the dotted line BB in the line of radiation 4, which in practice has a finite width approximating to a strip, along the length of the dotted line BB and also uniform perpendicular thereto in the direction of the arrow C. It can be seen from Figure 4 that the intensity distribution along the line 4 is substantially constant. When utilised for shearography purposes such line scanning illumination instead of the divergent wave front shown in Figure 1 produces high quality fringe patterns.
Figure 5 of the accompanying drawings shows apparatus according to one embodiment of the present invention as applied to shearography. In respect of this apparatus components already described with respect to Figures 1 and 3 of the accompanying drawings will be given like reference numerals and will not be further described in detail. The apparatus includes a video camera 8 operable to view the sample 2 and provide an output signal 9. Conveniently the camera 8 is a charge coupled device (CCD) imaging camera which may be used for shearography and for electronic speckle pattern interferometry. The apparatus includes a processor 10 which receives the signal 9 and extracts the frame rate of the camera 8.The processor 10 further operates to synchronise the video camera scan frame rate with the rate at which the line of coherent laser radiation 4 is scanned in the direction
C across the sample 2 by the moveable mirror 7. Preferably the line 4 is synchronised to the frame capture at 25Hz. Thus in effect the processor 10 yields frame synchronising pulses which can be filtered to drive the moveable mirror 7 directly so that the scanning mirror 7 is synchronised to the video signal to yield a stable image on the monitor. This is necessary otherwise banding will be observed in the image when the frequency drifts away from the frame frequency.
Alternatively, as illustrated in Figure 5, the filtered synchronising pulses can trigger a signal generator 11 which is used to drive the moveable mirror 7. Different types of wave form for mirror scanning can be used of which the simplest is sinusoidal, but any other shaped wave forms can be used to counteract angular variations in the reflectance of the sample provided that they are matched with the inertial response of the mirror 7.
In shearography two adjacent object regions, separated by the shearing distance, are brought to focus at the same point in the image plane. This is illustrated in Figure 6. If the two contributing regions are displaced in a direction C perpendicular to the projected laser strip 4, then due to the
Gaussian distribution across the width of the strip, as shown in the enlarged oval D, the intensities from these regions will be mismatched. This will result in a reduction of the fringe contrast. Hence the line width is important in determining the fringe contrast obtained with shearography, and there must be zero shear in the direction C of motion of the laser strip 4.For example, the speckle diameter d.p.Ckl. is given by equation 1: dapockle = 2-44k F ... (1)
where X is the wavelength of the light and F is the F.No of the imaging lens. Typically for a frequency doubled Nd:YAG laser (532nm) and an F. No of 5.6 this leads to a speckle diameter of 7ym. The diameter of the area ds on the sample 2 contributing to this speckle is given by:
ds - 2.44via ... (2)
where v is object-to-lens distance and a is the diameter of the imaging lens aperture. Typical viewing distance is 1.5m and the aperture diameter is 3mm.
Hence, the effective spot diameter 12 on the sample 2 contributing to each individual speckle is 0.6mm. With a laser line of width 1 to 2mms the fringe contrast falls off for the first few fringe orders, due to the intensity mismatch. However, for thicker laser lines more fringe orders are visible and fringe contrast is improved.
In Figure 6 reference numeral 13 represents idealised shearing optics and reference numeral 14 represents speckle on a video camera 8. Additionally in the oval D on beam profile 15 is shown at 16 the speckle capture zone. On beam profile 17 is shown at 18 the mismatch in beam profiles.
The use of scanned illumination according to the present invention means that in all, more than 90%, of the laser energy is delivered to the inspection area. Most panels are rectangular and are better suited to illumination by a rectangular beam. Also low reflectivity surfaces may be inspected due to the increased laser power at the sample, and larger panels areas may be inspected simultaneously, due to the increased laser power at the sample. High accuracy beam alignment is not necessary because spatial filtering is unnecessary and launch optics are not used. The beam is highly uniform, and because of this, the signal-to-noise ratio is optimised over the whole field-of-view.
The dynamic range requirements of the imaging sensor are less. The width of the illumination may be adjusted by varying the amplitude of the oscillating scanning mirror and/or varying the length of the laser strip 4. If the object reflectance is spatially variant then the dwell time of the laser line may be programmed to improve the illumination uniformity of the imaged specimen (spatial light modulation).
The scanned laser beam suffers less from coherent noise such as diffraction from dust, and cleanliness of the optics is less important as noise artefacts are averaged.
Claims (11)
1. A method of inspecting or testing a sample by optical
metrology, in which the sample is illuminated by scanning
a line of coherent radiation across a surface of the
sample.
2. A method according to claim 1, in which the radiation is
provided by a laser source and in which the line of
coherent radiation has substantially uniform intensity.
3. A method according to claim 2, utilising shearography, in
which the sample is observed by a video camera and in
which the line of coherent radiation is scanned across
the sample in synchronism with the frame scan of the
camera.
4. A method according to claim 3, in which there is zero
shear in the direction of scan of the line of coherent
radiation.
5. A method of inspecting or testing a sample by optical
metrology, substantially as hereinbefore described and as
illustrated in Figures 3, 4, 5 or 6 of the accompanying
drawings.
6. Apparatus for inspecting or testing a sample by optical
metrology, including a source of coherent radiation,
means for forming a line of coherent radiation from said
source and means for scanning the line of coherent
radiation across a surface of the sample.
7. Apparatus according to claim 6, wherein the source of
coherent radiation is a laser operable to provide a line
of coherent radiation with substantially uniform
intensity.
8. Apparatus according to claim 7, including a video camera
operable to view the sample and provide an output signal,
a processor operable to receive the output signal and
extract therefrom the frame rate of the camera, and
wherein the line scanning means incorporates a moveable
mirror for reflecting a line of laser radiation, with the
processor being further operable to synchronise the video
camera scan frame rate with the rate at which the line of
coherent laser radiation is scanned across the sample by
the moveable mirror.
9. Apparatus according to claim 8, wherein the moveable
mirror is drivable in a scanning movement via a
sinusoidal wave form or via any other shaped wave form
operable to counteract angular variation in the
reflectance of the sample and matched to the inertial
response of the mirror.
10. Apparatus according to claim 8 or claim 9, wherein the
line forming means is a line generator located between
the laser and the moveable mirror.
11. Apparatus for inspecting or testing a sample by optical
metrology, substantially as hereinbefore described and as
illustrated in Figures 3, 4, 5 or 6 of the accompanying
drawings.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB9523973A GB2307550A (en) | 1995-11-23 | 1995-11-23 | Illumination of a test sample by scanning a line of coherent radiation |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB9523973A GB2307550A (en) | 1995-11-23 | 1995-11-23 | Illumination of a test sample by scanning a line of coherent radiation |
Publications (2)
| Publication Number | Publication Date |
|---|---|
| GB9523973D0 GB9523973D0 (en) | 1996-01-24 |
| GB2307550A true GB2307550A (en) | 1997-05-28 |
Family
ID=10784344
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| GB9523973A Withdrawn GB2307550A (en) | 1995-11-23 | 1995-11-23 | Illumination of a test sample by scanning a line of coherent radiation |
Country Status (1)
| Country | Link |
|---|---|
| GB (1) | GB2307550A (en) |
Cited By (6)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| EP0875744A2 (en) * | 1997-04-30 | 1998-11-04 | British Aerospace Public Limited Company | Method and apparatus for inspecting a test piece by optical metrology |
| GB2340223A (en) * | 1998-08-01 | 2000-02-16 | British Aerospace | Laser shearing interferometer |
| US6246483B1 (en) | 1998-01-08 | 2001-06-12 | Bae Systems Plc | Apparatus and method for shearographic inspection and non-destructive testing of articles in a vacuum chamber |
| DE102011018710A1 (en) * | 2011-04-26 | 2012-10-31 | Baumer Innotec Ag | Optical sensor for, e.g. passenger car, has circuit unit that evaluates change of speckle or granulation patterns between signals from detector and determines information about change of object from change of patterns |
| CN103185040A (en) * | 2013-04-07 | 2013-07-03 | 江苏大学 | Inlet pipe used for axial flow pump impeller inlet cross section PIV (particle image velocimetry) flow field testing |
| WO2023147939A1 (en) * | 2022-02-02 | 2023-08-10 | Hochschule Trier | System and method for shearographic measurement of large-surface objects |
Families Citing this family (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN111247401B (en) * | 2018-12-10 | 2022-04-01 | 合刃科技（深圳）有限公司 | Vibration source positioning device and method based on coherent light |
Citations (5)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| GB1407409A (en) * | 1971-10-27 | 1975-09-24 | Ferranti Ltd | Detection of blemishes in a surface |
| WO1985003776A1 (en) * | 1984-02-14 | 1985-08-29 | Diffracto Ltd. | Panel surface flaw inspection |
| US4972091A (en) * | 1989-05-16 | 1990-11-20 | Canadian Patents And Development Limited/Societe Canadienne Des Brevets Et D'exploitation Limitee | Method and apparatus for detecting the presence of flaws in a moving sheet of material |
| EP0398781A2 (en) * | 1989-05-19 | 1990-11-22 | Eastman Kodak Company | Method and apparatus for low angle, high resolution surface inspection |
| WO1993016353A1 (en) * | 1992-02-18 | 1993-08-19 | British Steel Plc | Method and apparatus for measuring the shape of a surface of an object |
-
1995
- 1995-11-23 GB GB9523973A patent/GB2307550A/en not_active Withdrawn
Patent Citations (5)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| GB1407409A (en) * | 1971-10-27 | 1975-09-24 | Ferranti Ltd | Detection of blemishes in a surface |
| WO1985003776A1 (en) * | 1984-02-14 | 1985-08-29 | Diffracto Ltd. | Panel surface flaw inspection |
| US4972091A (en) * | 1989-05-16 | 1990-11-20 | Canadian Patents And Development Limited/Societe Canadienne Des Brevets Et D'exploitation Limitee | Method and apparatus for detecting the presence of flaws in a moving sheet of material |
| EP0398781A2 (en) * | 1989-05-19 | 1990-11-22 | Eastman Kodak Company | Method and apparatus for low angle, high resolution surface inspection |
| WO1993016353A1 (en) * | 1992-02-18 | 1993-08-19 | British Steel Plc | Method and apparatus for measuring the shape of a surface of an object |
Cited By (11)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| EP0875744A2 (en) * | 1997-04-30 | 1998-11-04 | British Aerospace Public Limited Company | Method and apparatus for inspecting a test piece by optical metrology |
| EP0875744A3 (en) * | 1997-04-30 | 1999-07-21 | British Aerospace Public Limited Company | Method and apparatus for inspecting a test piece by optical metrology |
| US6031602A (en) * | 1997-04-30 | 2000-02-29 | British Aerospace Public Limited Company | Method and apparatus for inspecting or testing a sample by optical metrology |
| US6246483B1 (en) | 1998-01-08 | 2001-06-12 | Bae Systems Plc | Apparatus and method for shearographic inspection and non-destructive testing of articles in a vacuum chamber |
| GB2340223A (en) * | 1998-08-01 | 2000-02-16 | British Aerospace | Laser shearing interferometer |
| WO2000011430A1 (en) * | 1998-08-01 | 2000-03-02 | Bae Systems Plc | Apparatus and method for the non-destructive testing of articles using optical metrology |
| DE102011018710A1 (en) * | 2011-04-26 | 2012-10-31 | Baumer Innotec Ag | Optical sensor for, e.g. passenger car, has circuit unit that evaluates change of speckle or granulation patterns between signals from detector and determines information about change of object from change of patterns |
| DE102011018710B4 (en) * | 2011-04-26 | 2017-06-22 | Baumer Electric Ag | Optical sensor for motion and length measurement |
| CN103185040A (en) * | 2013-04-07 | 2013-07-03 | 江苏大学 | Inlet pipe used for axial flow pump impeller inlet cross section PIV (particle image velocimetry) flow field testing |
| CN103185040B (en) * | 2013-04-07 | 2015-09-02 | 江苏大学 | For the inlet tube of axial-flow pump impeller import cross section PIV flow-field test |
| WO2023147939A1 (en) * | 2022-02-02 | 2023-08-10 | Hochschule Trier | System and method for shearographic measurement of large-surface objects |
Also Published As
| Publication number | Publication date |
|---|---|
| GB9523973D0 (en) | 1996-01-24 |
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Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| WAP | Application withdrawn, taken to be withdrawn or refused ** after publication under section 16(1) |