Optics: measuring and testing – By polarized light examination – With light attenuation
Reexamination Certificate
1999-03-10
2001-03-27
Kim, Robert (Department: 2877)
Optics: measuring and testing
By polarized light examination
With light attenuation
C356S512000
Reexamination Certificate
active
06208416
ABSTRACT:
This invention relates to measuring the shape of objects.
The use of projected fringes for measurement of surface profile is a well-developed technique as described for example by V. Srinivasan, H. C. Liu and M. Halioua, “Automated phase-measuring profilometry of 3-D diffuse objects,” Appl. Opt. 23 3105-3108 (1984). Parallel fringes are projected onto the object surface, either by a conventional imaging system or by coherent light interference patterns. If the projection and observation directions are different, the phase distribution of the measured fringe pattern includes information on the surface height profile of the object. The sensitivity can be improved by viewing through a line grating: the technique is then referred to as projection moiré.
Automated analysis of the fringe patterns is normally carried out either by the Fourier transform method (M Takeda, H Ina and S Kobayashi, “Fourier-transform method of fringe-pattern analysis for computer-based topography and interferometry”, J Opt. Soc. Am 72 156-160 (1982)), or by phase-stepping of the fringe patterns (J. H. Bruning, D. R. Herriott, J. E. Gallagher, D. P. Rosenfeld, A. D. White and D. J. Brangaccio, “Digital wavefront measuring interferometer for testing optical surfaces and lenses”, Appl. Opt. 13 2693-2703 (1974)). Both produce wrapped phase maps (i.e., phase values lying in the range −&pgr; to +&pgr;); the 2&pgr; phase jumps must then be removed by the process known as phase unwrapping to recover the surface shape. Phase unwrapping is normally carried out by comparing the phase at neighbouring pixels and adding or subtracting multiples of 2&pgr; to bring the relative phase between the two pixels into the range −&pgr; to +&pgr;. This causes problems when the technique is applied to real engineering objects since such objects often contain sudden jumps in profile. It can then become impossible to unwrap correctly across such real discontinuities and large (multiples of 2&pgr;) phase errors can propagate across the image. The problem can be reduced in principle by measuring several phase maps at different sensitivities. Only one unwrapped map will then be consistent with all the wrapped maps. White light interferometry can also measure absolute surface heights, but is difficult to apply when the height variations exceed the range of piezo-electric translators (PZT), typically a fraction of a mm.
The present invention provides methods and apparatus which can rapidly and unambiguously measure the profiles of discontinuous objects.
The invention comprises a method for measuring the shape of objects which might have surface discontinuities using projected fringes, in which the pitch of the fringes is varied over time. Each time the fringe pitch is changed a sequence of phase-stepped images of the fringe pattern is recorded, from which a complex intensity map is calculated. The sequence of such maps forms a three-dimensional complex intensity distribution. The phase of this distribution is then unwrapped along the time axis or else the Fourier transform along the time axis is calculated to yield data representative of the shape of the surface.
Intensity maps may be recorded as pixel images.
The pitch of the fringes may be varied by piezo-electric device means.
The fringes may be produced by Michelson interferometer means, and the phase and spatial frequency of the fringes may then be varied by plane mirrors forming part of the interferometer means.
The fringes may, however, be generated by interference between laser light traversing dissimilar paths of which at least one is variable for changing phase and spatial frequency.
The fringes may also be generated from incoherent light by a spatial light modulator (SLM) which can be operated to change the phase and spatial frequency of the fringes.
The invention also comprises apparatus for measuring shape of objects which might have surface discontinuities using projected fringes comprising fringe generating means projecting fringes on the object to be measured, phase stepping means to vary the phase of the fringes and fringe pitch varying means adapted to vary the pitch of the generated fringes over time, recording means adapted to record the sequence of phase-stepped images as the fringe pitch is varied over time and computing means programmed to calculate a three-dimensional complex intensity distribution either to unwrap the phase of this distribution along the time axis or to calculate its Fourier transform along the time axis to yield data representative of the shape of the surface.
The change in the fringe pitch between any two successive phase maps must be sufficiently small that the resulting unwrapped phase change lies in the range −&pgr; to +&pgr; for all points within the measurement region. This is equivalent to the requirement that, provided the position of the central fringe remains unchanged during the changing of the fringe pitch, no more than one fringe is added to the projected fringe pattern between the acquisition of two successive phase maps. The requirement may however be relaxed in certain circumstances.
The fringe generating means may comprise Michelson interferometer means in which the fringe phase and pitch varying means comprise plane mirror means. The interferometer means may comprise a beamsplitter cube with orthogonal adjacent faces and adjustable mirrors in juxtaposition to said adjacent faces of which one mirror may be adjustable towards and away from its associated face (for phase changing) and the other adjustable as to tilt (for spatial frequency changing). Alternatively the tilt and translation operations may be combined into a single mirror mount. At least one mirror may be adjustable by means of a piezo-electric device.
The fringe generating means may comprise laser illuminated optical fibre means providing two dissimilar light paths. The phase varying means may comprise optical fibre length varying means, which may comprise a piezo-electric cylinder around which the fibre is wrapped and which is electrically actuated to vary its diameter. A second piezo-electric device can relatively shift adjacent light output ends of the optical fibres to change the fringe spatial frequency.
The fringe generating means may comprise an incoherent, white light source and a spatial light modulator driven by an external signal source so as to produce synthetic fringes such that the transmitted light intensity has an approximately sinusoidal profile. The arrangement may comprise a condenser lens system and a projection lens projecting the fringes onto the object to be measured.
The transmittance of the spatial light modulator may be wavelength-dependent thereby allowing several fringe patterns of differing phase or spatial frequency to be projected simultaneously as patterns of differing colour within a single projected image.
The recording means may comprise a pixel image recorder such as a video system in which the object is imaged by a lens on to the light-sensitive image detector. The sensitivity of the image detector may be wavelength-dependent thereby allowing several fringe patterns of differing phase or spatial frequency to be recorded as a single coloured image.
The computing means may comprise a processor arranged to operate in real time by carrying out simple arithmetic operations on an entire pixel image fame simultaneously. Parallel processors or a fast serial processor could be used. It is of course not always required to operate in real time. The processor may have sufficient random access memory to store the entire three-dimensional complex intensity distribution so that the one-dimensional Fourier transform can be calculated along the time axis.
REFERENCES:
patent: 5321497 (1994-06-01), Ai et al.
Database Inspec.—Institute of Electrical Engineers, Stevenage, GB, Inspec No. 5556642, Feb. 1997—Saldner H.O. et al: “Profilometry using temporal phase unwrapping and a spatial light modulator-based fringe projector”, XP002033884, see abstract, & Optical Engineering, Feb. 1997, SPIE, USA, vol. 36, mo. 2, ISSN 0
Huntley Jonathan Mark
Saldner Henrik
Kim Robert
Lee Andrew H.
Loughborough University Innovations Limited
Renner , Otto, Boisselle & Sklar, LLP
LandOfFree
Method and apparatus for measuring shape of objects does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method and apparatus for measuring shape of objects, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method and apparatus for measuring shape of objects will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2546466