EP1687588A2

EP1687588A2 - Process and apparatus for measuring the three-dimensional shape of an object

Info

Publication number: EP1687588A2
Application number: EP04803125A
Authority: EP
Inventors: Bernard Tilkens; Yvon Renotte; Vincent Moreau
Original assignee: Universite de Liege
Current assignee: Universite de Liege
Priority date: 2003-11-17
Filing date: 2004-11-10
Publication date: 2006-08-09
Also published as: WO2005049840A2; WO2005049840A3

Abstract

A process for measuring a three dimensional (3D) shape of an object, comprising: 1.providing a light, 2.splitting said light into two spatially separated and orthogonally polarised signals, 3.filtering and projecting said signals on a surface of said object thus forming an interference lines pattern, and 4.viewing an image of said surface, that includes said lines pattern.

Description

Process and apparatus for measuring a 3D shape of an object.

FIELD OF THE INVENTION 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 .

BACKGROUND OF THE INVENTION Systems and methods for surface measurement are generally well known and have found application through industry and science. 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. In this known process and apparatus, 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. In this process and apparatus of US-2003/0043387, 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. These known process and apparatus require in addition that the measurement be performed in controlled environmental light. As a consequence they are not adapted to objects which are lighted with an intense light, particularly monuments in daylight, and this constitutes an additional disadvantage. In 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. In this prior art process, 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. Method and apparatus of WO-03/014663 discussed hereabove do not solve the aforesaid problems and disadvantages regarding the accuracy in the 3D measurement of objects, the stability of projected pattern, and regarding measurement in daylight. 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. In these known methods and apparatuses, 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 .

SUMlARY OF THE INVENTION

Accordingly it is 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. In such methods, 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. As an example, in the moire method, the 3D shape of the object is synthesized from a comparison of the image of the aforesaid lines pattern with a reference lines pattern. In the process according to the invention 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. In a preferred embodiment of this invention the light is a substantially monochromatic laser light. An advantage of this embodiment of the invention is that laser light is a coherent light. In this preferred embodiment of the invention 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. According to a particular feature of the 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 . To form the fringed interference lines pattern on the surface of an object, 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. In a particular embodiment of this invention, the aforesaid two spatially separated and orthogonally polarized signals are respectively a circular right polarized signal and a circular left polarized signal. In another particular embodiment of this invention 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. Subsequently in this specification said transverse electric polarized signal will be designated "TE signal" and said transverse magnetic polarized signal will be designated "TM signal". In this embodiment, 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) . In this means 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. In a preferred embodiment of this invention, 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. In a particular embodiment of this invention using a substantially monochromatic light, 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. In this particular embodiment of the invention, 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. In an advantageous embodiment of the process according to the invention, 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. For example, after 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.

More generally in this embodiment of the invention 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. Having several different fringed interference lines patterns of the same object allows the advantage of a more precise definition of the 3D shape and dimensions of the object. More specifically this allows optimum information and precision on the dimensions and the general topography of the object surface as well as on details of this topography. 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 said first interference lines pattern; and - in a second step, (vi) substituting said planar dielectric substrate having a definite optical thickness in step (ii) by another planar dielectric substrate having another definite optical thickness, thus forming a modified polarization sensitive device, (vii) directing both TE signal and TM signal of step (i) onto the modified polarization sensitive device of step (vi) , in order to have said TE signal reflected on said grating and said TM signal reflected on said planar outside face of said another planar dielectric substrate ; (viii) filtering both reflected TE and TM signals from step (vii) through the polarizing system of step (iii) ; (ix) directing the signal from said filtering of step (viii) onto said surface of said object thus forming a second interference lines pattern on said object; and (x) viewing a second image of said object that includes said second interference lines pattern. In steps (ii) 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. In step (i), 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) . In another advantageous embodiment of the process according to the invention, 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. In this advantageous embodiment of the invention, 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. In the advantageous embodiment just described 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. In this advantageous embodiment 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. In this interesting application of the process according to the invention, 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. In the apparatus according to the invention, 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. In a preferred embodiment of the apparatus according to the invention 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) . In another preferred embodiment, 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. This apparatus is specially adapted to measure the 3D shape of objects or monuments which are outside and illuminated with daylight. For this application, 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. In a further preferred embodiment of the apparatus according to the invention, 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. In this embodiment of the invention, 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. Information about this feature of the invention is provided above, in relation with the process according to the invention. In the preferred embodiment which has just been described, the dielectric substrate may generally be a glass sheet. In an advantageous form of execution of this preferred embodiment the planar dielectric substrate has a thickness which is adjustable. For this end 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. In another example, the planar dielectric substrate may comprise two (or more) juxtaposed and index matched sheets of different optical thicknesses. With any one of these forms of execution, 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. The reason and advantages of this particularity of the invention have been explained above in relation with the process according to the invention. In a still further embodiment of the apparatus according to the invention, 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. In some modification of the embodiments described above of the apparatus according to the invention, the Bragg grating of the polarization splitting units may be replaced by a particular holographic grating, generally called Polarization Sensitive Hologram (PSH) . 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 Liege - 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 OMNIDEX™ .

BRIEF DESCRIPTION OF THE DRAWINGS

Details and features of the present invention will become apparent to those skilled in the art from the following detailed description of the attached drawings, wherein 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. In these figures same reference numerals denote same components.

DETAILED DESCRIPTION OF SOME EMBODIMENTS 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) . In this apparatus, 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. To measure the 3D shape of an object, 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. When the surface of this object is rigorously flat, the fringes of the projected lines pattern (13) are rectilinear. These fringes are deformed when the surface of the object is not flat. When a voltage is applied to the liquid crystal cell 8, this cell introduces a relative difference between the optical paths of the TM and the TE signal components of the light from the source 1. The optical length of this modification depends on the amplitude of the voltage. So by successively applying different voltages to the liquid crystal cell 8, a series of three or more shifted interference lines patterns 13,14 and 15 are successively projected on the object. When the polarization splitting unit 3 is in first position, on figure 1, 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 Λ/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. When 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'. In a particular embodiment of the apparatus of figure 1 and 2 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.

Claims

1. A process for measuring a three-dimensional (3D) shape of an object, comprising a. providing a light ; b. forming a interference lines pattern of said light on a surface of said object ; and c. viewing an image of said surface, that includes said line pattern ; 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.

2. Process according to claim 1, characterized in that said light is a substantially monochromatic light.

3. Process according to claim 2, characterized in that said substantially monochromatic light is a laser light.

4. Process according to any one of claims 1 to 3, characterized in that said two orthogonally polarized signals are respectively a transverse electric polarized signal (TE) and a transverse magnetic polarized signal (TM) .

5. Process according to any one of claims 1 to 3, characterized in that said two orthogonally polarized signals are respectively a circular right polarized signal and a circular left polarized signal.

6. Process according to any one of claims 2 to 5, characterized in performing said viewing of said lines pattern image through an interferential filter which is transparent to said substantially monochromatic light but substantially opaque to electromagnetic radiations outside said substantially monochromatic light.

7. Process according to claim 6, characterized in that said interferential filter is substantially opaque to radiations in the UV-visible-NIR spectrum, which are outside the radiation wavelength range of said monochromatic light.

8. Process according to any one of claims 2 to 7, characterized in that the object is lighted with an outside light having a range of wavelengths which includes the wavelength range of said monochromatic light and in that, inside said wavelength range of the monochromatic light, said monochromatic light has a global power which is greater than the global power of said outside light.

9. Process according to any one of claims 1 to 8, characterized in that it 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 filter 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.

10. Process according to any one of claims 2 to 9, characterized in that said splitting said light into a transverse electric polarized signal (TE) and a transverse magnetic polarized signal (TM) comprises directing said light onto a polarization sensitive Bragg grating on a planar dielectric substrate, in order to have said TE signal reflected on said grating and said TM signal reflected on the planar interface of the dielectric substrate opposite to said grating.

11. Process according to claim 10, characterized in that said light is directed onto said polarization Bragg grating so as to make a Brewster angle with said grating.

12. Process according to any one of claims 1 to 8, 10 and 11, characterized in that it comprises (i) polarizing said light into two superimposed orthogonally polarized signals consisting in respectively 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 an outside planar face of said planar dielectric substrate ;

(iii) filtering both reflected TE and TM signals from step (ii) through a polarizing filter 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 said first interference lines pattern; and in a second step,

(vi) substituting said planar dielectric substrate having a definite optical thickness in step (ii) by another planar dielectric substrate having another definite optical thickness, thus forming a modified polarization sensitive device (vii) directing both TE signal and TM signal of step (i) onto the modified polarization sensitive device of step (vi) , in order to have said TE signal reflected on said grating and said TM signal reflected on said planar outside face of said another planar dielectric substrate ; (viii) filtering both reflected TE and TM signals from step (vii) through the polarizing system of step (iii) ; (ix) directing the signal from said filtering of step (viii) onto said surface of said object thus forming a second interference line pattern on said object; and (x) viewing a second image of said object that includes said second interference line pattern.

13. Process according to any one of claims 1 to 12, characterized in that it comprises providing a source of said 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 (jjj ) 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.

14. Process according to claim 13, characterized in that said introducing a relative difference between optical pathes of said two orthogonally polarized signals in step (vj) includes having said light from said source to pass through a liquid crystal cell and exciting said cell with an appropriate voltage.

15. Process according to any one of claims 1 to 14, used for measuring a 3D shape of an object by a moire method.

16. Process according to claim 15, characterized by synthesizing the 3D shape of the object from said interference lines pattern or any of said interference lines patterns and a reference lines pattern.

17. An apparatus for measuring a 3D shape of an object using an fringes projection process in accordance with any one of claims 1 to 16, comprising a source of light (1); 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; characterized in that said means for forming an interference lines pattern of said light includes a polarization splitting unit (3) adapted to split said light into two spatially separated and orthogonally polarized signals and a polarization filter (9) adapted to project said orthogonally polarized signals in a common polarization state to produce an interference pattern (13).

18. Apparatus according to claim 17, characterized in that said light is a monochromatic laser light.

19. Apparatus according to claim 17 or 18, characterized in that said polarization splitting unit (3) is adapted to spacially separate said light into a transverse electric polarized signal (TE) and a transverse magnetic polarized signal (TM) .

20. Apparatus according to claim 18 or 19, characterized in that it includes in addition, in front of said image obtaining unit (18), an interferential filter (17) which is transparent to said monochromatic laser light but substantially opaque to electromagnetic radiations outside said monochromatic laser light.

21. Apparatus according to claim 20, characterized in that said monochromatic laser light is inside the UV- visible-NIR spectrum and in that said interferential filter is substantially opaque to radiations of the UV-visible-NIR spectrum, with the exception of said monochromatic laser light.

22. Apparatus according to any one of claims 18 to 21, characterized in that said polarization splitting unit comprises a polarization sensitive Bragg grating (7) on a planar dielectric substrate having an adjustable optical thickness.

23. Apparatus according to claim 22, characterized in that said polarization splitting unit (3) comprises a polarization sensitive Bragg grating (7) index matched with a set of at least two planar dielectric substrates (4,5) of different optical thicknesses.

24. Apparatus according to claim 22 or 23, characterized in that said planar dielectric substrates (4,5) are so positioned that a light beam from the source of light (1), reaches the Bragg grating with a Brewster angle .

25. Apparatus according to claim 23 or 24, characterized in that said set of planar dielectric substrates (4,5) can be translated in respect to the incident beam.

26. Apparatus according to any one of claims 17 to 25, characterized in that a liquid crystal cell (8) is positioned between said source of light (1) and said polarizing filter (9) .

27. Apparatus according to any one of claims 19 to 21, characterized in that said polarization splitting unit (3) comprises a polarization sensitive hologram (PSH) (7), on a planar glass substrates (4,5).