Optics: measuring and testing – By light interference
Reexamination Certificate
2001-02-26
2004-06-01
Glick, Edward J. (Department: 2882)
Optics: measuring and testing
By light interference
C356S035500
Reexamination Certificate
active
06744517
ABSTRACT:
The present invention relates to a measurement method arid measurement apparatus employing an interferometer arranged to form patterns of interference fringes.
Interferometers are well-known, and the testing and measurement of: optical components, from simple spectacle lenses to astronomical telescopes requires an interferometer system of one sort or another. Interferometers are also now routinely applied in engineering for the measurement of mechanical and thermal behaviour of materials and components.
Conventionally, for the most accurate measurement these interferometer systems are constructed from high-quality optical elements and include fine controls for precise alignment. The need for high quality, precise components makes interferometer systems expensive and places practical restrictions on the aperture of the instrument. Typically, the controls are adjusted to reduce the number of interference fringes formed in the observer's field of view before the test or measurement is performed to a minimum, and ideally zero. Then, an object to be tested is inserted in one arm of the interferometer, or the interferometer is perturbed (altered) in some other way. If the interferometer was initially set up to produce a fringe-free field, then all interference fringes appearing in the test interferogram are due to the perturbation.
In conventional interferometer measurement applications, a few fringes in the initial (i.e. reference) interferogram may be tolerated, if the test/measurement perturbation results in an interference pattern having a large number of fringes. The underlying imperfections in the unperturbed interferometer may be ignored.
If, however, the test/measurement perturbation itself only introduces a small number of fringes, then the underlying imperfections cannot simply be ignored.
Techniques are known for removing the effects of aberrations in the reference interferogram so as to display an image from a test component which is free from spurious fringes generated by an imperfect optical system. The method for applying the correction is, however, both elaborate and slow. From one or more interferograms of the reference and test object he phase distributions are calculated. The method typically necessitates the conversion of at least three test interference fringe patterns (interferograms) and at least three reference interferograms into digital images to facilitate processing. The three or more reference and test interferograms are phase stepped (shifted) from each other by pre-determined amounts. These phase shifted patterns are generated sequentially by the appropriate phase shifting of fringes, for example by a piezoelectric transducer-(PZT)-driven mirror or wavelength modulation.
Once the phase distributions (phase maps) have been calculated an unwrapping procedure is then applied to the phase maps. As the test phase maps also contain the reference information, subtraction of the reference map from the test map results in the presentation of the test information only. As a consequence or the delay, the subtraction is usually performed off-line and post-operatively. In addition, the approach may fail because the phase calculation and unwrapping procedures will not tolerate interferograms with excessive numbers of closely spaced fringes or fringes which are contorted.
It is also known to derive an accurate phase map of the optical path perturbation resulting from a test component by deliberately introducing carrier fringes (a spatial carrier) into the test interferogram by, for example, tilting a mirror in the interferometer, and performing a Fourier transform analysis method. Rather than requiring a least three reference interferograms, with the Fourier transform method only one fringe pattern having a spatial carrier is enough for the analysis. However, it requires more computation for Fourier transformation and filtering and cannot be conducted in real-time. Therefore it has not been easy to accelerate fringe analysis for quick applications such as feedback control of optical instruments and real-time monitoring of dynamic phenomena.
The paper “Video-rate fringe analyzer based on phase-shifting electronic moiré patterns”, Kato et al, Applied Optics, Nov. 10, 1997, Vol. 36, No. 32, p8403—describes a fringe analyzer that delivers the phase distribution at a video-rate from a fringe pattern containing a spatial carrier. It is based on parallel generations of three phase-shifted moiré patterns through electronic multiplication with computer-generated reference gratings and low-pass filtering. The phase distribution is derived by the subsequent parallel processing of these patterns on the basis of a three-step phase-shifting algorithm.
Image processing involving digital subtraction of images is known in digital speckle pattern interferometry (described, for example, in “Speckle Metrology”, Ed. R. S. Sirohi, Marcel Deker, Inc. New York, 1993, p125) and in document analysis (described, for example, in “A new method for displaying indented and other markings on documents”, C. Forno, Science and Justice 1995, 35 (1) 45-51), and in “More technique by means of digital image processing”, K. J. Gasvick, Applied Optics 1983, 22 (23) 3543-8.
Moiré fringe generation is a known process whereby the intensity distributions of two dissimilar grid patterns are combined, for example by superimposition, as described in Chapter 6, “Handbook of Experimental Mechanics”, Society for Experimental Mechanics Inc, Prentice Hall, Englewood Cliffs, N.J. 07632, USA 1987, ISBN:0-3-377706-5. By superimposing the dissimilar grids, a moiré fringe pattern is generated which represents the local differences between the spatial frequencies of the grids.
According to a first aspect of the present invention, there is provided a measurement method comprising the steps of:
arranging an interferometer to form a first interference fringe pattern comprising at least ten interference fringes;
recording an image of said first interference fringe pattern;
perturbing an optical path in the interferometer to form a second interference fringe pattern comprising at least ten interference fringes; and
combining an image of said second interference fringe pattern with the recorded image of the first interference fringe pattern to produce a further image comprising a moiré fringe pattern arising from a difference or differences between the first and second interference fringe patterns.
Thus, it is no longer necessary to align the interferometer with great precision to produce a substantially fringe free reference (i.e. first) interference fringe pattern before the test or measurement is performed (i.e. before the interferometer is perturbed/altered).
The moiré fringe pattern produced by combining the first and second interference fringe patterns is determined by the perturbation itself, and not by the underlying Imperfections and misalignments of the unperturbed interferometer.
In this new approach, all the errors of a poor quality, misaligned system are accepted and then eliminated by the combination process, producing a moiré fringe pattern. The method enables very large aperture optical systems for traditional and engineering interferometers to be constructed from inexpensive and basic components.
A conventional high quality optical measurement interferometer will typically comprise optical components having surfaces manufactured to tolerances of better than &lgr;/10 or even &lgr;/100 where &lgr; is the wavelength of light input to the interferometer.
With the inventive method, imperfections in optical components as large as 100&lgr; or greater may be tolerated.
The interferometer used in the present invention may be an optical interferometer, or alternatively may be an interferometer arranged to form an interference pattern from incident electromagnetic radiation having different wavelength.
In a basic form, the method may be implemented by recording the first image on, for example, a photographic film. The subsequent interference fringe pattern, produced by perturbing the interferometer system, may then be
Forno Colin
Whelan Maurice
European Community Represented By Commision of the European Comm
Glick Edward J.
Ho Allen C.
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