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

Definitions

• the present invention relates to a process and apparatus for measuring the three-dimensional shape (or 3D shape) of an object, specifically adapted to structured light projection and interferometric measuring methods .
• Structured light projection methods are suitable to measure the three-dimensional-shape (or 3D shape) of objects.
• a structured light projection method which is commonly used for 3D shape measurements is known in the art as fringes projection or moire method.
• This known moire method usually comprises projecting a periodic light pattern, that can be the result of the interference of two coherent beams, forming a deformed line pattern on said object, and synthesizing the 3D shape of the object from said deformed lines pattern and a reference lines pattern.
• An application of the structured light projection technique is to measure the 3D shape of objects, mechanical pieces and machine pieces in factories and laboratories.
• US-2003/0043387 discloses a process and apparatus using a moire method for measuring the 3D shape of an object.
• a beam of light from a white light source is condensed and projected through a projection grid to the object under measurement, forming a grid image of lines pattern on the object, this grid lines pattern being deformed corresponding to the 3D shape of the object.
• This deformed lines pattern image is focused on an image pickup image surface of a CCD camera after being passed through a reference grid.
• the use of a pair of physical or material grids requires intricate mechanical means which constitutes a disadvantage and may further be a cause of a lack of precision in the measurement.
• O-03/014663 there is provided a method of calculating the three dimensional surface coordinates of a point on the surface of an object, which comprises illuminating the object with a set of fringes (which may be interference fringes) , and capturing a plurality of images of the surface with a camera with different fringes phase settings.
• techniques which are suggested to produce the fringes include the Lloyd's mirror technique, the Fresnel bi- prism, the Michelson interferometer and the projection of fringes from a mask.
• Both US-3 897 136 and US-2002/0003628-A1 relate to methods and apparatuses for measuring various parameters of test objects by forming moire patterns through the superposition of grating structures.
• a coherent light wavefront is split by a polarizing beam splitter into two spatially separated and orthogonally polarized wavefronts.
• One of both wavefronts serves as a reference wavefront.
• the other wavefront (called the object wavefront) is transmitted to the test object and either reflected from or transmitted through the object.
• the object wavefront is then superimposed with the reference wavefront to form an interference pattern that can be analysed by moire methods .
• an object of this invention to overcome these prior art problems. It is particularly an object of this invention to provide an improved process and apparatus for measuring a three-dimensional (3D) shape of an object.
• a more specifically object of this invention is to provide a new and improved process and apparatus for measuring a 3D shape of an object using an interferometric fringes projection method.
• a further object of this invention is to provide a interferometric fringes projection process and apparatus having an improved accuracy and insensitiveness to vibrations.
• a still further object of this invention is to provide an improved interferometric fringes projection process and apparatus, adapted to the measurement of the 3D shape of objects illuminated with uncontrolled light such as statues, sculptural statues, buildings, memorials, etc. in daylight.
• An additional object of this invention is to provide a interferometric fringes projection apparatus comprising only a limited number of mechanical means, if any. Accordingly in accordance with one aspect of the present invention there is provided a process for measuring a three-dimensional (3D) shape of an object, comprising providing a light ; forming an interference lines pattern of said light on a surface of said object ; and viewing an image of said surface, that includes said lines pattern ; this process being characterized in that said forming an interference lines pattern includes splitting said light into two spatially separated and orthogonally polarized signals and directing both signals onto said surface.
• the process according to the invention belongs to the fringes projection methods. Fringes projection methods are well known in the art and commonly used for measuring the 3D shape of objects.
• a light is projected from a source onto the object subjected to measurement, and is so controlled to form a light lines pattern on a surface of the object.
• An adequate analysis or treatment of an image of this lines pattern provides information on the 3D shape of the object.
• the 3D shape of the object is synthesized from a comparison of the image of the aforesaid lines pattern with a reference lines pattern.
• the term "light” has a general definition and designates any electromagnetic wave in the visible and invisible spectrum. Accordingly in the process according to the invention the light may be either an electromagnetic wave of the visible spectrum or an electromagnetic wave of the invisible spectrum.
• a light in the UV-visible-Near IR electromagnetic spectrum (of from approximately 300 nm to approximately 1,500 nm) is however preferred.
• the light is a substantially monochromatic laser light.
• An advantage of this embodiment of the invention is that laser light is a coherent light.
• the laser light beam may be provided by any adequate laser source. Gas laser sources are convenient. NdYAG diode pumped solid state (DPSS) laser sources are preferred in the sense of this invention.
• the aforesaid light is spatially separated into two orthogonally polarized signals, which are then projected in a common polarization state in order to form the fringed interference lines pattern.
• Polarization of light is well known in the art, and means to polarize light are thus also well known.
• the invention is thus a new application of a known physical property of light to an interferometric fringes projection 3D measuring process, in order to improve this fringes projection 3D measuring process and avoid the disadvantages of the fringes projection 3D measuring processes of the prior art .
• the aforesaid two spatially separated and orthogonally polarized signals are first passed through an adequate polarizing filter and subsequently projected onto the object under measurement.
• the filter consists in a polarizing system which is adapted to project both aforesaid orthogonally polarized signals in a common polarization state which is subsequently directed onto the object to form thereon the fringed interference line pattern.
• the aforesaid two spatially separated and orthogonally polarized signals are respectively a circular right polarized signal and a circular left polarized signal.
• the aforesaid two spatially separated and orthogonally polarized signals are respectively a transverse electric polarized signal and a transverse magnetic polarized signal. This embodiment of the invention is preferred.
• a preferred means to polarize the light into a TE signal and a TM signal comprises directing said light onto a polarization sensitive Bragg grating interleave between an incident medium and a planar dielectric substrate, in order to have said TE signal reflected on said grating and said TM signal reflected on the outer face of said planar dielectric substrate. (This outer face of the planar dielectric substrate is the face which is opposite to the planar interface between said grating and said incident medium) .
• the light is generally directed onto the polarization Bragg grating so as to make a Brewster angle with the planar interface between the Bragg grating and the incident dielectric medium.
• Brewster angle is well known in the art and corresponds to the incident angle of a light beam for which the reflection of the TM signal component of the light is theoretically zero. This angle depends on the refractive index of incident and transmission space.
• the incident and the planar dielectric substrates are generally made of glass.
• the incident medium is a right angle glass prism and the Brewster angle is 45°.
• a step of the process according to the invention comprises viewing an image of the fringed interference lines pattern formed on the surface of the object subjected to the process.
• This step is generally made by means of a camera or a similar device.
• this step of viewing the fringed interference lines pattern image is performed through an interferential pass-band filter which is transparent to said substantially monochromatic light but substantially opaque to electromagnetic radiations outside said substantially monochromatic light.
• the substantially monochromatic light is preferably situated in the visible spectrum, more preferably inside the UV-visible-NIR spectrum (NIR means Near Infra-Red) and the interferential filter is substantially opaque to electromagnetic radiations of the visible spectrum, more preferably of the UV-visible-NIR spectrum with the exception of the or those of said substantially monochromatic light.
• the monochromatic light used in this particular embodiment is preferably a laser light.
• the particular embodiment just described is well adapted to the 3D measurement of objects which are outside, such as buildings, monuments, etc., normally lighted by daylight. Therefore when using a monochromatic light with a range of wavelengths in the visible spectrum (or the UV- visible-NIR spectrum) , it is advantageous that this monochromatic light has a global power which is greater than the global power of the outside light (for example daylight) 'in the same spectral window.
• this process comprises in a first step, (a) splitting said light into a first spatial state of two spatially separated and orthogonally polarized signals ; (b) filtering both orthogonally polarized signals of step (a) through a polarizing system adapted to project said signals in a common polarization state, (c) directing the signal from said filtering of step (b) onto said surface of said object thus forming a first interference lines pattern on said object; and (d) viewing a first image of said object that includes said first interference lines pattern; and in at least a second step, (e) splitting said light into a second spatial state of two spatially separated and orthogonally polarized signals; (f) repeating steps (b) and (c) of said first step, thus forming a second interference lines pattern on said object; and (g) viewing a second image of said object that includes said second interference lines pattern.
• This embodiment of the invention may comprise more than the aforesaid first and second steps.
• the aforesaid second step it may comprise a third step which includes (h) splitting said light into a third spatial state of two spatially separated and orthogonally polarized signals; (k) repeating steps (b) and (c) of said first step, thus forming a third interference lines pattern on said object; and (1) viewing a third image of said object that includes said third interference lines pattern.
• the aforesaid second step may be followed by n successive steps (n being a whole number at least equal to 1), each step m (among these n steps) including (e m ) splitting said light into a m eme spatial state of two spatially separated and orthogonally polarized signals; (f m ) repeating steps (b) and (c) of said first step, thus forming a m eme interference lines pattern on said object; and (g m ) viewing a m eme image of said object that includes said m eme interference lines pattern.
• the advantageous embodiment of the invention just described allows the formation of two (or more) different fringed interference lines patterns on the object, characterized by different fringe spacing.
• a preferred method to perform the advantageous embodiment just described hereabove comprises (i) polarizing the light into two superimposed orthogonally polarized signals consisting respectively in a transverse electric polarized signal (TE) and a transverse magnetic polarized signal (TM) ; and, - in a first step, (ii) directing both TE signal and TM signal of step (i) onto a polarization sensitive Bragg grating on a planar dielectric substrate having a definite optical thickness, in order to have said TE signal reflected on said grating and said TM signal reflected on the outside planar face of said planar dielectric substrate ; (iii) filtering both reflected TE and TM signals from step (ii) through a polarizing system adapted to project said signals in a common polarization state, (iv) directing the signal from said filtering of step (iii) onto said surface of said object thus forming a first interference lines pattern on said object; and (v) viewing a first image of said object that includes
• step (i) and (vii) of this preferred method the so-called planar outside face of the planar dielectric substrate is a face of this substrate which is opposite to the planar interface between the Bragg grating and the planar dielectric substrate.
• any convenient means may be used to polarize the light into superimposed TE and TM signal components.
• a means which is preferred in step (i) comprises a polarizing filter with polarization axis forming an angle of 45 ° in respect to TE and TM directions.
• the series of steps (vi) to (x) is similar to the series of steps (ii) to (v) , with the exception of the polarisation sensitive devices used respectively in step (ii) and step (vi) .
• Both polarisation sensitive devices used respectively in step (ii) and in step (vi) comprise a Bragg grating on a planar dielectric substrate, but the optical thickness of the planar dielectric substrate in step (ii) is different from the one in step (vi) .
• this process comprises providing a source of light, and in a first step, (j) splitting said light into a set of two spatially separated and orthogonally polarized signals ; (jj) filtering both orthogonally polarized signals of step (j) through a polarizing filter adapted to project said signals in a common polarization state; and (j j j ) directing the signal from said filtering of step (jj) onto said surface of said object thus forming a first interference lines pattern on said object; and (jv) viewing a first image of said object that includes said first interference lines pattern; and in at least a second step, (vj) before step (jj) of filtering both orthogonally polarized signals of step (j), introducing a relative difference between the respective optical paths of said two orthogonally polarized signals, so as to form in step (jjj) an interference lines pattern which is shifted with respect of the interference lines pattern of the aforesaid first step.
• steps (j), (jj), (jjj) and (jv) are similar to steps (a) , (b) , (c) and (d) of an advantageous embodiment described hereabove and to steps (i) , (iii) , (iv) and (v) of a preferred method of embodiment described hereabove.
• Step (vj) comprises increasing the path of one of both orthogonally polarized signals in comparison with the path of the other polarized signal.
• the fringed interference lines pattern obtained in stage (jjj) following stage (vj) in said second step is shifted with respect of the one obtained in said first step before stage (vj ) .
• Stage (vj) may be repeated two or more times (generally from 3 to 10 times) using different optical path differences to obtained a series of shifted images of the fringe interference lines pattern.
• This embodiment of the invention allows more precise information on the relief of the surface of the object, by permitting the use of the so called "phase shifting methods" in the numerical treatment of images information.
• any known means may be used in step (vj), which is convenient to introduce a optical path change for at least one of both orthogonally polarized signal components.
• a preferred means comprises passing the light from the source of light through a liquid crystal cell and exciting said cell with an appropriate voltage. Using different successive voltages provides corresponding successive shifted interference lines patterns in step (jjj).
• the process according to the invention may be used in any interferometric methods or techniques adapted to measure the 3D shape or deformations of objects or similar.
• the process according to the invention finds thus applications in interferometric techniques for measuring the 3D shape or deformations of mechanical pieces and machine pieces in factories and laboratories.
• Another application of the process according to the invention is the fringes projection methods to measure the 3D shape dimensions of statues, buildings, memorials, land topographic survey and any other natural or artificial object, either in controlled or in uncontrolled surrounding light, particularly in daylight.
• the process according to the invention finds an interesting application in the moire technique, wherein the 3D shape of an object is synthesized from at least one projected lines pattern (preferably more than one lines pattern as explained hereabove in relation with advantageous and preferred embodiments of this invention) and a reference lines pattern.
• the reference lines pattern may be either an actual physical fringed lines pattern or a virtual pattern in computer software.
• the invention relates also to an apparatus for measuring a 3D shape of an object using an interferometric process in accordance with the invention as generally disclosed above, said apparatus comprising a source of light; a means for forming an interference lines pattern of said light on a surface of said object; and an image obtaining unit of said interference lines pattern; said apparatus being characterized in that said means for forming an interference lines pattern of said light includes a polarization splitting unit adapted to split said light into two spatially separated and orthogonally polarized signals; and a polarization system adapted to project said orthogonally polarized signals in a common polarization state.
• the source of light is preferably a source of a monochromatic laser light, advantageously a monochromatic laser light situated in the visible spectrum more preferably in the UV-visible-NIR spectrum.
• the polarization splitting unit is a unit adapted to split said light (preferably laser light) into a transverse electric polarized signal (TE) and a transverse magnetic polarized signal (TM) .
• the source of light of the apparatus is a monochromatic laser light and the apparatus includes in addition, in front of the aforesaid image obtaining unit, an interferential filter which is transparent to said monochromatic laser light but substantially opaque to electromagnetic radiations outside said monochromatic laser light.
• the laser light is generally inside the visible spectrum (preferably the UV- visible-NIR spectrum) and the interferential filter is substantially opaque to radiations of the visible spectrum (preferably the UV-visible-NIR spectrum) , with the exception of said laser light.
• the aforesaid polarization splitting unit comprises a polarization sensitive Bragg grating on a planar dielectric substrate. Details and information regarding such polarization sensitive Bragg grating on a planar dielectric substrate have been given above.
• the planar dielectric substrate is normally so positioned that a light beam from the source of light propagating in the incident medium reaches the planar interface between said incident medium and said grating with a Brewster angle.
• the dielectric substrate may generally be a glass sheet.
• the planar dielectric substrate has a thickness which is adjustable.
• the planar dielectric substrate may for example be a sheet having two convergent faces so as to have a thickness which regularly decreases from one end of the sheet to the opposite end thereof.
• the planar dielectric substrate may comprise two (or more) juxtaposed and index matched sheets of different optical thicknesses.
• the optical path of the TM signal component through the substrate is modified by translating the dielectric substrate or the entire polarisation splitting device in respect to the incident beam. This allows obtaining different fringed interferometric lines patterns of the object, characterized by different fringe spacing.
• a liquid crystal cell is positioned between the source of light and the polarization splitting unit or between the polarization splitting unit and the polarization filter.
• the Bragg grating of the polarization splitting units may be replaced by a particular holographic grating, generally called Polarization Sensitive Hologram (PSH) .
• PSH Polarization Sensitive Hologram
• This kind of polarization splitting units is well known in the art (Reconfigurable optical interconnect with holographic gratings - V. Moreau, Y. Renotte and Y. Lion - Hololab, Laboratoire de physique generale, Universite de vide - SPIE's International Technical Group Newsletter, November 2001, page 7).
• PSH which are convenient for the invention are those recorded in DuPont holographic film produced under the trademark OMNIDEXTM .
• Figure 1 is a schematic view of a particular embodiment of the apparatus of this invention in a first working position;
• Figure 2 shows the embodiment of figure 1 in a second working position.
• same reference numerals denote same components.
• the apparatus represented on figure 1 comprises a source 1 of a monochromatic laser light, a linear polarizing filter 2, a polarization splitting unit 3 (comprising elements 4, 5, 6 and 7 defined hereafter), a liquid crystal cell 8, a second linear polarizing filter 9 and a camera or image acquisition unit 18.
• the liquid crystal cell 8 is connected to a DC source (not shown) .
• the polarization splitting unit 3 comprises a Bragg grating 7 interleave and index matched between a right angle prism 6 and a set of planar glass sheets 4 and 5 with different thickness.
• the positions of the liquid crystal cell 8 and of the linear polarizing filters 2 and 9 are so selected as to be passed through substantially perpendicularly by a divergent beam of laser light from the source 1. Only the central ray of the cone of divergence of the light is represented on the figure.
• the polarization splitting unit 3 is positioned with respect to the source of light 1 in such a way that a beam of laser light 19 from source 1 makes a Brewster angle with the interface between the Bragg grating 7 and the hypotenuse of the prism 6.
• the plane of incidence 0 is defined as the plane formed by the incident light beam 19 from source 1 and the signal beam reflected by the Bragg grating 7.
• the whole polarization splitting unit 3 can be translated in the X direction perpendicular to the plane of incidence 0, in such a way that the glass sheet 4 or 5, respectively in the first position of figure 1 or in the second position of figure 2, is crossing the plane of incidence . So when the unit 3 is in its first position represented on figure 1, the light beam 19 from source 1 is split into a TE signal which is reflected on the Bragg grating and a TM signal which is reflected on the outer face of the glass sheet 4 (i.e. the face opposite to the interface between the sheet and the Bragg grating) . When the laser light 19 from source 1 makes a near 45° angle with this interface, the reflection of the TM signal on the outer face of the glass sheet is a total internal reflection.
• the linear polarizing filter 9 is oriented in such a way that its passing axis forms an angle of 45° with respect to TE and TM axis respectively, so as to transmit an equal fraction of both TE and TM signals coming from the polarization splitting unit 3.
• the linear polarizing filter is so adapted to project both signals in a common polarization state, making them producing a fringe interference lines pattern 13.
• This pattern is enlarged by passing through a projection lens 10.
• this object 16 is disposed downstream the projection lens (10) in such a way that the fringe interference lines pattern (13) is received on the surface of this object and this fringe interference lines pattern is viewed by the camera or image acquisition unit 18.
• the TE and TM signals reaching the polarizing filter 9 seem to come from two virtual sources 11 and 12 which are separated by a length equal to the thickness of the glass sheet 4 multiplied by ⁇ /2.
• the interference between coherent virtual light sources 11 and 12 results in a first set of phase shifted periodic pattern 13, 14 and 15.
• the polarization splitting unit 3 is translated to its second position, on figure 2, the TE and TM signals reaching the polarizing filter 9 seem to come from two virtual sources 11' and 12' which are separated by a length equal the thickness of the glass sheet 5 multiplied by V2.
• the interference between coherent virtual light sources 11' and 12' results in a second set of phase shifted periodic patterns 13', 14' and 15'.
• an interferential filter 17 is disposed between the camera or image acquisition unit 18 and the object.
• This interferential filter 17 is adapted to be transparent for the electromagnetic radiations of the laser light 19 from the source 1 but opaque to the UV- visible-NIR electromagnetic radiations which are outside the laser light radiations.

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• 2004
  • 2004-11-10 WO PCT/EP2004/012773 patent/WO2005049840A2/en active Application Filing
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