Optics: measuring and testing – Material strain analysis – By light interference detector
Reexamination Certificate
1999-12-06
2001-09-04
Kim, Robert H. (Department: 2877)
Optics: measuring and testing
Material strain analysis
By light interference detector
C356S450000
Reexamination Certificate
active
06285447
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a method for increasing the signal-to-noise ratio in non-destructive testing of a sample by a phase-stepped optical inspection system which, particularly, but not exclusively, is suitable for use with optical shearography employing a shearing interferometer.
A particular type of phase-stepped optical inspection system for non-destructive testing or evaluation of a sample is an optical shearography system using a shearing interferometer.
Such contemporary shearography systems measure the surface form of an object in a static position and after it has been subjected to a stressing force. Usually, the stress is administered by a pressure reduction chamber or by thermal loading. The two images that characterize the unstressed and stressed states are subsequently subtracted to yield an image that contours the stress-induced distortion. If the stressing force has been applied effectively, then sub-surface defects in the test object, such as disbonding between the skin and core materials of a composite panel, may be visualized in this image.
Unfortunately, there are several factors that limit the effectiveness of shearography for non-destructive testing (NDT) namely:
1. Shearography is based on speckle interferometry, which is inherently noisy.
2. If the stressing force is applied gradually over an extended period, then the initial and final images may become de-correlated, due to environmental instability, and no result is produced.
3. The dynamic range of shearography is restricted, because of the limited spatial resolution of video based image acquisition.
4. Transient features, such as air currents, can mask the presence of defects.
A conventional technique known as phase stepping helps reduce image noise by eliminating stationary intensity patterns and enhancing image contrast. However, even if phase stepping is employed, the results are often too noisy and not sufficiently repeatable to be dependable for non-destructive testing or evaluation of safety critical structures.
There is thus a need for an improved method whereby the noise of shearography images is reduced and the repeatability of results is improved to an extent where shearography becomes viable for production non-destructive testing.
SUMMARY OF THE INVENTION
According to one aspect of the present invention, there is provided a method for increasing the signal-to-noise ratio in non-destructive testing of a sample by a phase-stepped optical inspection system in which,
correlated speckle images successively stepped in phase are generated and captured of a sample to be tested or evaluated, by illuminating the sample, while in a static unstressed state, with coherent radiation,
the sample is illuminated with coherent radiation, stressed incrementally at predetermined stress increments, and correlated speckle images successively stepped in phase are generated of the incrementally stressed sample at the predetermined stress increments and captured,
the phase-stepped speckle images of the unstressed sample and the phase-stepped speckle images of the incrementally stressed sample are used to calculate the phase before and after stressing and differenced to extract the incremental phase change,
the magnitude and sign of each incremental phase change is inspected, and if the phase difference between successive measurements increases by more than &pgr;, 2 &pgr; subtracted from the measurement values, or if the phase difference between successive measurements decreases by more than &pgr;, 2 &pgr; is added to the measurement value and
the phase at each point in each speckle image is calculated with improved accuracy by summing the nearest neighbor phase differences in the image that are weighted by the square of their respective modulations, where the modulation is a measure of intensity variation with phase variation and the result of this calculation is normalized by dividing by the sum of the modulations.
Preferably, the predetermined stress intervals employed are dependent on the material and structure of the sample.
Conveniently, the phase differences are summed using a modulation filter in the form of a 3×3 convolution mask.
Advantageously, the optical inspection system utilized is an optical shearography system incorporating a shearing interferometer, and the modulation is a measure of intensity variation with phase variation in the shearing interferometer.
REFERENCES:
patent: 6043870 (2000-03-01), Chen
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patent: 0875744A2 (1998-04-01), None
patent: WO 90/07751 (1990-12-01), None
Sjoedahl M. et al.: “Three Dimensional Deformation Field Measurements with simultaneous TV Holography and Electronic Speckle Photography”, Applied Optics, US, Optical Society of America, Washington, vol. 36, No. 16, Jun. 1, 1997 (1997-06-01), pp. 3645-3648, XP000657622, ISSN: 0003-6935, p. 2, left-hand col, last line; right-hand col., In. 1, p. 3, left-hand col.
Liu-Sheng Wang et al.; “Additive-Subtractive Speckle Interferometry: Extractio of phase Data in Noisy Environments”, Optical Engineering, US, Soc. Of Photo-Optical Instrumentation Engineers. Bellingham, vol. 35, No. 3, Mar. 1, 1996 (1996-03-01), pp 794-801, XP000597470, ISSN: 0091-3286.
Nakadate S et al.: “Fringe Scanning Speckle-Pattern Interferometry”, Applied Optics, Jul. 15, 1985, USA, vol. 24, No. 14, pp 2172-2180, XP000885048, ISSN: 0003-6935.
Parker Steve Carl Jamieson
Salter Phillip Langley
BAE Systems PLC
Kenyon & Kenyon
Kim Robert H.
Lee Andrew H.
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