Integrated optical device for contactless measurement of altitudes and thicknesses

An optical device for contactless measurement of height and/or thickness. The optical device having an axial chromatic aberration in order to encode the height and/or thickness information of an object positioned in the measurement field. The optical system is anchored in a confocal architecture. A detection system decodes the information through a detection system allowing the wavelength(s) focused on the surface(s) of the object to be discriminated. A plurality of points can be measured simultaneously or successively.

The present invention relates to a device for contactless optical measurement that is single-point, for which a single point of the object is measured, or multipoint, dedicated to the measurement of height(s) and/or thickness(es). This measurement device is based on the principle of chromatic confocal microscopy and thus has the properties related to the confocality of an optical system associated with a controlled axial chromatic aberration.

The confocal optical system requires having a single-point illumination system. The image of the source point is focused on the object to be measured, and the light reflected or backscattered by the object is in turn imaged onto a confocal diaphragm (spatial filter) positioned upstream of the photodetector. The source point, the object and the diaphragm are confocal. With respect to full-field microscopy, a confocal system allows the axial and lateral selectivity to be increased. The light coming from a plane located outside of the focal plane, as well as the light coming from a point outside of the optical axis, is filtered by the confocal diaphragm, thus improving the signal-to-noise ratio and the lateral resolution of the optical system.

The control of the axial chromatic aberration provides a confocal optical system with the chromatic dispersion of the spectrum of the source of light along its optical axis. This controlled optical aberration allows the encoding of the height or of the thickness according to the wavelength. The decoding of this information is carried out via a system capable of analysing the wavelengths of the spectrum backscattered by the object.

The combination of these two principles into a shared architecture allows the observation and measurement of a set of points of the space included in the measurement volume of the measurement apparatus without being disturbed by its neighbours. The measurement volume is defined by the illuminated lateral field (defined by the set of illuminated points), spread out along the optical axis and limited to within the axial chromatic aberration of the optical system. This type of measurement device allows very good performance to be obtained, in particular in terms of axial and lateral resolution.

In order to carry out their functions, these measurement apparatuses consist:Of a polychromatic source,Of an optical system having a controlled axial chromatic aberration,Of a spectral analysis system such as a spectrometer,Of means for processing the signal, calculation and data transmission.

The invention described by the reference patent [1], relates to a method and a measurement apparatus based on the principle of chromatic confocal microscopy, according to the prior art. The device mentioned, uses a polychromatic source, a holographic or diffractive lens such as a Fresnel lens in order to generate an axial chromatic aberration, and a spectral analysis device in order to determine the highest-energy wavelength corresponding to the wavelength perfectly focused on the object. This embodiment allows the position of an object with respect to a reference surface positioned upstream of the object to be measured. Said measurement apparatus is single-point. This patent is a reference for chromatic confocal measurement systems, and the apparatuses of measurements of the prior art all have at least one optical fibre associated with a coupler such as a beam splitter cube, or a fibre coupler, whether for the chromatic confocal measurement devices provided with a lateral field or for the particular case, much more widespread in the industry of three-dimensional measurement, of the single-point measurement apparatus, for which a single point of the object is measured. The referenced patents [2], [3] and [4] will now be examined.

The invention described by reference patent [2], relates to a chromatic confocal measurement apparatus provided with a lateral field according to the prior art.

FIG. 1illustrates an embodiment of a chromatic confocal measurement apparatus provided with a lateral field according to the prior art [Cf. Reference 2], such that the lateral field represents a line. Other embodiments for this measurement apparatus exist in such a way as to obtain, for example, a field having a rectangular shape, but they are not shown here.FIG. 1illustrates a chromatic confocal measurement device according to the prior art, consisting of an optoelectronic box100, of a measurement head200, connected by an optical cable300.

The optoelectronic box100consists of a polychromatic source10, a spectral analysis device20, computer and electronic means30for processing the signal, calculation, data transmission, control and configuring the measurement apparatus, and a power supply block40. This chromatic confocal measurement apparatus provided with a lateral field allows an object400having a surface to be measured positioned in its measurement volume500to be measured.

The measurement head200consists of a set of optical lenses50provided with a controlled axial chromatic aberration. This set of optical lenses50can be separated into two subsets: a collimator51and a chromatic lens52. The collimator51is thus totally devoid of an axial chromatic aberration, and its role is to collimate the beam coming from the ends73of the optical cable300and to manage the magnification of the set of optical lenses50and thus manage the size of the spot on the object400. The chromatic lens52is a set of lenses allowing the desired chromatic aberration to be obtained. This chromatic aberration can also be generated by using diffractive lens such as a Fresnel lens [Cf. Reference 1 and 4]. The measurement head200also contains a beam splitter parallelepiped60, the role of which is to direct the beams coming from the ends73of the optical cable300towards the set of optical lenses50up to the object400and to direct the beams backscattered by the object400and coming from the set of optical lenses50towards the ends74of the optical cable300. This combination/separation assembly is shown inFIG. 1, by a beam splitter parallelepiped60but can also be the coupling zone of a set of fibre couplers.

In the case in which, the combination/separation assembly is a beam splitter parallelepiped60, the optical cable300consists of N fibres70(N being an even positive integer) divided into N/2 illumination fibres71, the role of which is to guide the light coming from the polychromatic source10towards the measurement head200, and N/2 return fibres72allowing the light coming from the measurement head200to be guided towards the spectral analysis device20. The ends73and74, respectively, of the fibres71and72near the measurement head200are optically conjugated with the surface of the object400. The ends75and76, respectively, of the fibres71and72near the optoelectronic box100are positioned, respectively, facing the polychromatic source10and at the input of the spectral analysis device20.

The principle of a chromatic confocal measurement apparatus provided with a lateral field is the following: The polychromatic source10emits radiation guided by a set of N/2 illumination fibres71, up to a measurement head200. The N/2 ends73of the illumination fibres71are spatially organised in such a way as to define a lateral measurement field (a line, a disc, or a rectangle for example). The light rays emitted by each of the N/2 illumination fibres71are propagated into the measurement head200provided with lateral fields and with an axial chromatic aberration. The wavelengths belonging to the spectrum of the polychromatic source10are thus dispersed along the optical axis. A continuum of wavelengths is generated for each point of the field, thus generating a measurement volume500in the space of the object400. For each point of the field one wavelength is perfectly focused on the object400, with the condition that the surface area to be measured of the object400is less than the measurement volume500. Each wavelength perfectly focused for each point of the lateral field is backscattered by the objet400. The backscattered light is propagated in the opposite direction in the measurement head and then through a combination/separation assembly60and is focused on the N/2 ends74of the N/2 return fibres72. The N/2 ends74are conjugated with the N/2 ends73, and consequently the ends73and74have exactly the same spatial organisation (line, disc, rectangle or others). The N/2 return fibres72guide each wavelength, perfectly focused on the object400for each point of the lateral field, towards the input of a spectral analysis device20. The spectral analysis device20—a grating spectrometer, for example—is provided with a photodetector21that allows the N/2 spectra corresponding to the N/2 measurement points forming the lateral field to be visualised. Each spectrum shows a peak of intensity corresponding to the wavelength perfectly focused on the object400for each point of the field. The computer and electronic means30allow each of the spectra to be processed, the position of the N/2 peaks to be determined, the N/2 equivalent heights to be calculated and its data to be simultaneously transmitted through a data transmission cable32to a computer600. The measurement of N/2 heights distributed in the lateral field is thus carried out simultaneously.

In another embodiment described by reference patent [2], it is also possible to position a scanning mirror in the measurement head200between the collimator51and the chromatic lens52in order to successively measure a set of points.

The invention described by reference patent [3], relates to a line chromatic confocal measurement apparatus according to the prior art.

Other embodiments exist for this type of chromatic confocal measurement device provided with a lateral field [Cf. Reference 3]. The embodiment described in patent [3] is similar to the previous one, it specifies that it is also possible to introduce a matrix of micro-mirrors or a matrix of liquid crystals that can be sequentially switched via electronic means for periodic modulation. This matrix positioned either between the illumination cable71and the measurement head200or between the polychromatic source10and the illumination cable71allows the points belonging to the lateral field to be illuminated sequentially. The contribution of this patent with respect to the previous one [Cf. Reference 2] is in the possibility of successively illuminating the neighbouring points of the same line, which is not possible in the context of reference patent [2].

The patent application [Cf. Reference 4] relates to a single-point chromatic confocal measurement device allowing movements of the surface of an object400placed in the measurement volume500to be measured. The device described by said patent application uses a polychromatic source10and more particularly a white LED, a diffractive lens52that introduces a chromatic aberration in the direction of its optical axis, a lens called “objective”, equivalent to the collimator51described in reference [2], which is positioned closer to the object400than the diffractive lens52. The role of the “objective” lens51, by positioning it between the object400and the diffractive lens52, is to preserve the precision of the measurement apparatus regardless of the axial position of the surface of the object400in the measurement volume500of the apparatus. The author of said patent application, compares the response of the spectral analysis device20according to the position of the lens called “objective”51. It appears that this response remains constant when the objective lens51is positioned between the diffractive lens52and the object400, giving the system the property of having a constant precision over its entire range of measurement. On the contrary, when the “objective” lens51is positioned upstream of the diffractive lens52, the response of the spectral analysis device20, varies according to the axial position of the object400inside the measurement volume500, which is a property known to a Person Skilled in the Art. However, the global response of the measurement system also depends on the response of the spectral analysis device20, and depends, de facto, on its intrinsic configuration. Thus, for example, the response of a grating spectrometer is much different than that of a prism spectrometer since the formula of the prisms giving the angular dispersion according to the wavelength is much different than the formula of the gratings. A Person Skilled in the Art, must thus mention this effect, which causes the variation in the global response of the apparatus, and a fortiori in the precision of the apparatus in its range of measurement, according to the architecture of the spectrometer20used, regardless of the position of the lens called “objective”51. Besides the claim related to the use and the positioning of the lens called “objective”51, this patent application, reference [4], describes a system using a fibre optic coupler. The fibre optic coupler replaces the illumination fibre71associated with an element60for combination/separation of beams and with a return fibre72. This fibre coupler thus allows, the luminous flux to be guided, between an optoelectronic box100consisting of a white LED source10, a spectral analysis device20, and a control unit equivalent to the computer and electronic means30for processing the signal, calculation, data transmission, control and configuring the measurement apparatus described in reference [2], and a measurement head200consisting in particular of the lenses called lens51and diffractive lens52.

This fibre architecture, systematically used by the companies that design and/or manufacture these chromatic confocal measurement apparatuses, is also described in said patent application as providing flexibility in order to facilitate the movement of the measurement head200with respect to the controller100. Moreover, it is also mentioned that the use of a fibre coupler allows the need to use spatial filters to be eliminated, the latter being embodied by the end of the illumination fibre of the fibre coupler. A Person Skilled in the Art is aware of the advantages provided by the use of the fibres70, whether in terms of flexibility but also in terms of ease of production, yet the chromatic confocal measurement device according to the present invention also allows the constraints related to the use of these optical fibres to be totally or partly eliminated while preserving the flexibility of the architecture according to the prior art.

The two embodiments of a chromatic confocal measurement device provided with a lateral field according to the prior art [Cf. References 2 and 3], describe systems comprising at least one optical cable300of N optical fibres70connecting an optoelectronic box100to an optical head200, or, if the spectral analysis device20and the polychromatic source10are not grouped together in the same optoelectronic box100, of two optical cables300of N/2 optical fibres71and72connecting, respectively, the polychromatic source10to the measurement head200and the measurement head200to the spectral analysis device20. The N/2 ends of the fibres71and72near the measurement head200form, respectively, a plurality of source points and a plurality of spatial filters that are optically conjugated. Likewise, the embodiments of a single-point chromatic confocal measurement device according to the prior art [Cf. Reference 1 and 4], also describe systems comprising at least one fibre70or a fibre coupler connecting an optoelectronic box100to an optical head200. A Person Skilled in the Art considers the single-point chromatic confocal measurement device to be a specific case of the chromatic confocal measurement apparatus provided with a lateral field for which, a single point of the object400is measured (height or thickness) and for which, the number of fibres70used is reduced to N=2 or to a fibre coupler.

The embodiments, which use an optical cable300of N fibres70, are very ingenious, since they allow the measurement head to be moved far from the computer and electronic means30for processing the signal, calculation, data transmission, control and configuring the measurement apparatus and from the power supply block40. This allows the operator to configure the measurement apparatus and carry out measurements while being remote from the production line, for example. Moreover, this embodiment simplifies the integration of the measurement apparatus because the filtering holes are embodied by the end of each of the N fibres70contained in the optical cable300, thus avoiding the difficulty of introducing N filtering holes into the device. However, of portion, their length, their more or less great capacity to resist mechanical stresses of friction, shearing, twisting or elongation, their loss of photometric transmission according to the radius of curvature imposed, the optical cables300are subject to wear when the measurement apparatus is subjected to numerous repetitive cycles comprising rapid accelerations and/or to rotary movements, which is exactly the case when these measurement apparatuses are used on production units for size control. Finally, the maintenance of the chromatic confocal measurement apparatus provided with a lateral field, if one or more fibres70belonging to the optical cable300break, quickly becomes complicated and requires the return of the measurement apparatus to the site of the manufacturer in order to change the entire optical cable (since changing a single fibre70is not possible for reasons of integration via gluing into grooves most often in the shape of a V). In order to prevent these breakdowns that are impossible to repair on site, the user may be forced to store one or more replacement apparatuses, in order to be able to carry out a standard replacement in case of a breakdown. This constraint has an impact that is often too great on the use and maintenance costs.

The optical cables300used in chromatic confocal measurement apparatuses provided with a field as implemented in the prior art, are thus a constraint for integrators and users that the invention described here allows to be totally or partly overcome.

The purpose of the invention is to eliminate the technical difficulties mentioned above. Thus, the invention, by eliminating all or a portion of the optical cables300, promotes the robustness and compactness of the measurement apparatus. The purpose of the invention is to eliminate all or a portion of the optical cables300that are present in all the current chromatic confocal measurement apparatuses, whether single-point or provided with a lateral field.

The invention thus allows the constraints of integrating the measurement apparatus into an industrial environment, for example on a production line, to be overcome.

The invention also allows, the measurement device to be integrated into a motorised movement system, such as a Coordinate-Measuring Machine (CMM) that is most often in the form of a measurement gantry provided with translation axes and/or rotation axes, or also, of a robot arm with a plurality of axes of rotation. The absence of optical cables facilitates the integration and the use of the measurement apparatus according to the invention in this type of system.

Three embodiments allow all or a portion of the optical cables300to be eliminated in order to partly or totally overcome the constraints resulting from this architecture called “modular” because it consists of two totally separate element, an optoelectronic box100and an optical head200, connected by an optical cable300. These three embodiments are named as follows and will be described below:

2. Optical head200with a device for integrated spectral analysis20,

3. Optical head200with a polychromatic source10and a device for integrated spectral analysis20.

For these three embodiments, there are different versions that will be described below. These various embodiments will be illustrated and described as belonging to the family of the chromatic confocal apparatuses for measuring height and/or thickness provided with a lateral field, but the invention also relates to the single-point chromatic confocal apparatuses for measuring height and/or thickness that represents a specific case of the multipoint configuration (provided with a lateral field).

FIG. 2illustrates a first preferred embodiment of a chromatic confocal apparatus for measuring height and/or thickness provided with a lateral field according to the invention, for which a polychromatic source10is integrated into the measurement head200, here called Optical head200with an integrated polychromatic source10. Moreover, the measurement head200, also contains electronic means31allowing the intensity and the emission frequency of the polychromatic source10to be controlled, optionally a power supply41, a set of lenses50, and a light guide subassembly80. The function of the light guide subassembly80consisting, according toFIG. 2and for example, of N/2 illumination fibres71and of a beam splitter parallelepiped60is to guide the light from the polychromatic source10, injected into the N/2 ends75of the N/2 illumination fibres71, to the set of lenses50and to structure the light according to an object lateral field11consisting of a plurality of source points embodied by the N/2 ends73of the N/2 illumination fibres71. The set of lenses50, which can consist of an achromatic collimator51and a chromatic lens52allows both the N/2 spots of light to be focused on the object400according to an image lateral field12, but also the desired axial chromatic aberration that corresponds to the range of measurement of the chromatic confocal apparatus to be generated. The collimator51is thus totally devoid of an axial chromatic aberration, and its role is to collimate the beam coming from the light guide subassembly80, and to manage the magnification of the set of optical lenses50and thus to manage the spot size on the surface of the object400positioned in the measurement volume500of the apparatus. The chromatic lens52, which can be a set of lenses or a diffractive lens such as a Fresnel lens [Cf. Reference 1 and 4], allows the desired chromatic aberration to be obtained.

This first preferred embodiment only partly overcomes the constraints related to the chromatic confocal measurement apparatus provided with a lateral field according to the prior art, since an optical cable300of N/2 return fibres72(N being a positive even integer) is necessary in order to guide the backscattered light coming from the measurement head200to the optoelectronic box100. The N/2 ends74of the N/2 return fibres72inside the measurement head200, are positioned in the conjugated-image plane of the N/2 ends73of the N/2 illumination fibres71and are organised according to a conjugated image lateral field13strictly identical to the object lateral field11since the set of lenses50like any optical assembly used for a forward and return trajectory has unity magnification. The N/2 ends74act as filtering holes forming a plurality of spatial filters, giving this apparatus the property of being confocal.

In this first embodiment, the optoelectronic box100contains, a spectral analysis device20, the last element of which is a photodetector21, computer and electronic means30for processing the signal, calculation, data transmission, control and configuring the measurement apparatus, and a power supply block40. The N/2 ends76of the N/2 return fibres72are connected at a spectral analysis device20input and are organised in such a way as to obtain a remote image lateral field14provided with a spatial sampling compatible with the spectral analysis of each of the points from the image lateral field12. For example,FIG. 2shows a remote image lateral field14that follows a line, but there are other organisations for this remote image lateral field14, for example such as a non-regular grid (inter-fibre space in the horizontal direction different than that in the vertical direction), allowing the imaging of a plurality of spectra on the same line.

Finally, the optoelectronic box is connected to the power grid via a power supply cable42, to a computer600with a data transmission cable32, and to the measurement head200with an electric cable43. However, a Person Skilled in the Art does not exclude a configuration for which the power supply block40and/or the power supply block41is a rechargeable battery and for which, no or respectively, only one power supply cable42or one power supply cable43is necessary during the phases of battery recharge. Likewise, in the near future, the possibility of transmitting the data in wireless mode can also be imagined, which would also allow the data transmission cables32to be eliminated.

Thus, this first preferred embodiment of a chromatic confocal measurement apparatus provided with a lateral field according to the invention allows the cable of N/2 illumination fibres71connecting the optoelectronic box100and the measurement head200to be advantageously replaced by a power supply cable43. The substitution of the cable of N/2 illumination fibres71by a power supply cable43represents a development of interest for the integrators of a size-control system, who have moreover widely experienced the reliability, the robustness and the durability of electric cables and/or cables of data transmissions in industrial inspection systems installed on a production line.

The difference between this first preferred embodiment of a chromatic confocal measurement apparatus according to the invention and the measurement apparatuses according to the prior art lies in the integration of the polychromatic source10into the inside of the measurement head200and in the use of a suitable light guide subassembly80. The integration of the polychromatic source(s)10into the inside of the measurement head200, also involves the integration of the electronic means31allowing the intensity and the emission frequency of the polychromatic source10to be controlled and optionally also the integration of its power supply41. Below, theFIGS. 3A, 3B, 3C and 3Dthat represent the various embodiments of a light guide80associated with the polychromatic source(s)10are described.

FIG. 3Aillustrates, a first preferred embodiment of the polychromatic source10subassembly associated with a light guide80, in which the light guide80contains N/2 illumination fibres71and a beam splitter parallelepiped60. These N/2 illumination fibres71are organised differently at their two ends75and73near the polychromatic source10and the device60for combination/separation of beams, respectively. The N/2 ends75near the polychromatic source10, are grouped together and positioned facing the polychromatic source(s)10in order to collect as much luminous flux as possible. In order to do this, and for example, the N/2 ends75of the illumination fibres71can be grouped together into a circular cable. Obviously, other configurations are possible, for example, the N/2 ends75can be separated and grouped together into a plurality of different cables and positioned facing a plurality of polychromatic sources10. Finally, for example, one or more lenses can also be positioned between the polychromatic source(s)10and the cable or cables containing the N/2 ends75of the illumination fibres71, in order to improve the flux collection. The N/2 ends73of the illumination fibres71near the assembly60for combination/separation of the light are organised in such a way as to obtain the desired field and spatial sampling. This field can be a line, a rectangle, a disc, or any other geometric shape allowing the object400to be measured in an optimal manner. Thus, for example, in the case of a distribution of the N/2 ends73of illumination fibres71in a line with a constant inter-fibre space, as shown inFIG. 3A, a Person Skilled in the Art would favour the use of grooves in the shape of a V, allowing a very precise alignment of the N/2 ends73of the illumination fibres71, as well as excellent regularity of the distance between the fibres.

FIG. 3Billustrates, a second preferred embodiment of the polychromatic source10subassembly associated with a light guide80, in which the light guide80is a portion of a set of N/2 fibre couplers comprising, for each of them, N/2 illumination fibres71, N/2 return fibres72and N/2 object fibres77. The ends75of the N/2 illumination fibres71are near the polychromatic source10. The N/2 ends75are grouped together and positioned facing the polychromatic source(s)10in order to collect as much luminous flux as possible, as described above, in the first preferred embodiment of the polychromatic source10subassembly associated with a light guide80. The ends73of the N/2 object fibres77are near the set of lenses50not shown inFIG. 3B, and are organised in such a way as to obtain the desired field and spatial sampling, as described above, in the first preferred embodiment of the polychromatic source10subassembly associated with a light guide80. For example, a transverse cross-section of the ends73of the object fibres77in a linear organisation is shown inFIG. 3B. Finally, the N/2 return fibres72, which belong to the N/2 fibre couplers, do not belong to the light guide80subassembly. The assembly60for combination/separation of beams is in this case, the set of the N/2 zones of couplings of the N/2 fibre couplers. For reasons of ease of representation,FIG. 3Bschematically illustrates the N/2 zones of couplings via a single block, yet is it obvious that this assembly60for combination/separation of beams consists of N/2 distinct zones of couplings.

This second preferred embodiment of the polychromatic source10subassembly associated with a light guide80, does not provide a notable benefit with respect to the first embodiment illustrated byFIG. 3A. The fibre coupler allows, however, the elimination of the complicated phase of integration that consists of aligning the ends74of the return fibres72with the ends73of the illumination fibres71, inevitable for the first preferred embodiment of the polychromatic source10subassembly associated with a light guide80. On the other hand, the N/2 fibre couplers, used in this second preferred embodiment of the polychromatic source10associated with a light guide80, forms a source of backscattered parasite light that is much more significant than that of the coupler of the beam splitter parallelepiped60type, thus limiting the turndown ratio of the photodetector21.

FIG. 3Cillustrates, a third preferred embodiment of the polychromatic source10subassembly associated with a light guide80, in which the light guide80consists of a lens for shaping the beam53, of a homogeniser54, of a mask55having holes. The lens for shaping the beam53can consist of one or more lenses and has the function of shaping the beam coming from the polychromatic source(s)10and injecting this beam at the input of the homogeniser54, the role of which is to make the luminous flux uniform over the entire lateral field11in order for each measurement point to carry the same energy. This homogeniser54can be, for example, a waveguide having a square or circular cross-section, consisting of two lenses having different refractive indices no and n1, as described in the transverse cross-section56, in order to guide the light in the material having the index no (the material having the index n1can optionally be air). The mask of holes55, a transverse cross-section11of which is shown, is generally positioned immediately after the homogeniser54, and can ideally be glued to the latter. This mask55of holes allows the definition of the matrix of measurement points that will be imaged on the object400and thus the definition of the objet later field11. The masks of holes55are created via evaporation/deposition of a metal, then the organisation of the holes is carried out via photolithography, the zones of the mask55covered with a metal layer have an optical transmittance of almost zero and only the zones devoid of a deposit transmit the light coming from the homogeniser54. The mask of holes55allows the light to be structured according to an organisation that can have various shapes, such as a line, a disc, a rectangle or any other geometric shape allowing the object400to be measured in an optimal manner. The mask of holes55allows excellent repeatability of the patterns to be obtained, both in terms of shape and in terms of positioning of the patterns or holes in the matrix of patterns. The patterns or holes are for example circular, but can also be square or of any other geometric shape. This third preferred embodiment of the polychromatic source10subassembly associated with a light guide80, thus uses a light guide80that has the advantage of eliminating the need to use illumination optical fibres71, but a Person Skilled in the Art is aware of the loss of flux, and thus of the loss of photometric efficiency caused by this configuration.

A configuration similar to that described inFIG. 3C, would involve using a mask55consisting of a plurality of parallel solid lines, rather than a plurality of lines of holes, which would allow a gain in photometric efficiency, but this leads to a loss of confocality in one direction. This loss of confocality manifests itself as a loss of lateral resolution caused by the fact that each measurement point also receives the optical information of its close neighbours, this phenomenon also called cross-talk can highly deteriorate the measurement in terms of lateral resolution but also in terms of axial precision, especially for the measurement of objects that is photometrically non-uniform and/or comprises local discontinuities. Finally, it is also possible to not use a homogeniser54, which would lead to a different photometric response according to the point observed on the object400. This has the effect of highly limiting the turndown ratio, since, in this configuration, the points located in the middle of the field emit a more intense signal than the points located at the edge of the field.

FIG. 3Dillustrates, a fourth preferred embodiment of the polychromatic source10subassembly associated with a light guide80, but which differs from the previous ones in the following: the polychromatic sources10are micro-LEDs positioned upstream of the assembly60for combination/separation of beams, and the propagation occurs under free field conditions between the matrix of micro-LEDs10and the assembly60for combination/separation of beams. This fourth preferred embodiment considerably simplifies the overall architecture, since each micro-LED forms a single-point source point that is directly imaged by the set of lenses50on the object400. This matrix of micro-LEDs has a dual function, it both forms the polychromatic sources10and also defines the object lateral field11. This set of micro-LEDs, the luminous intensity per unit of surface area of which is indeed compatible with the chromatic confocal measurement apparatus according to the invention, is a rather recent technology from the microelectronics industry that thus enjoys production methods that allow emitting-surface sizes of up to 20 μm in diameter and a very precise and extremely repeatable structured organisation to be achieved. Moreover, it is also possible to individually address each of the micro-LEDs belonging to this matrix, thus providing the possibility of illuminating each of the LEDs successively or illuminating groups of LEDs successively, which also corresponds to one of the limitations of the prior art solved in a complex manner with matrix of micromirrors, by the reference patent application [3]. Thus, in the embodiment described byFIG. 3D, it is possible to define very precisely the measurement field that can represent a line, a rectangle, a disc, or any other geometric shape allowing the object400to be measured in an optimal manner. This fourth preferred embodiment allows the compactness of this subassembly to be increased, excellent control of the measurement field to be obtained (its shape, the periodicity of its patterns or holes), the need to use illumination optical fibres71to be eliminated, optimal photometric efficiency to be obtained due to the absence of a means for guiding or for transporting energy80between the polychromatic source(s)10and the assembly60for combination/separation of the light.

FIG. 4Aillustrates, schematically, a second preferred embodiment of a chromatic confocal measurement apparatus provided with a lateral field according to the invention called Optical head200with an integrated spectral analysis device20, for which the spectral analysis device20, the last element of which is a photodetector21, is integrated into the measurement head200. For this second preferred embodiment of a chromatic confocal measurement apparatus provided with a lateral field according to the invention, the polychromatic source(s)10is (are) thus integrated into an independent source box700that also contains the electronic means31allowing the intensity and the emission frequency of the polychromatic source10to be controlled, optionally a power supply41.

In addition to the spectral analysis device, the measurement head200contains a set of lenses50, an assembly60for combination/separation and computer and electronic means30for processing the signal, calculation, data transmission, control and configuring the measurement apparatus. The set of lenses50, which can consist, for example, of an achromatic collimator51and a chromatic lens52, allows the N/2 spots of light coming from the N/2 ends73of the illumination fibres71to be focused on the object400, and also allows the desired axial chromatic aberration that furthermore corresponds to the measurement range of the chromatic confocal apparatus to be generated. The collimator51is thus totally devoid of an axial chromatic aberration, and its role is to collimate the beam coming from the N/2 ends73of the illumination fibres71, and to manage the magnification of the set of optical lenses50and thus manage the size of the spot on the surface of the object400positioned in the measurement volume500of the apparatus. The chromatic lens52is a set of lenses allowing the desired chromatic aberration to be obtained. This chromatic aberration can also be generated by using a diffractive lens such as a Fresnel lens [Cf. Reference 1 and 4]. The measurement head200also contains an assembly60for combination/separation, the role of which is to direct the beams coming from the object lateral field11defined by the N/2 ends73of the illumination fibres71, towards the set of optical lenses50up to the object400according to an image lateral field12, and to direct the beams backscattered by the object400and coming from the set of optical lenses50towards the spectral analysis device20that is integrated into the measurement head200. The path of the beams backscattered from the object400to the input of the spectral analysis device20runs through a fibre link, consisting of N/2 return fibres72, the N/2 ends74of which near the beam splitter parallelepiped60are conjugated with the N/2 ends73of the illumination fibres71and define the conjugated image lateral field13that must be strictly identical to the object lateral field11in order to not lose information during this optical coupling. The N/2 ends74act as filtering holes, forming a plurality of spatial filters, giving this apparatus the property of being confocal. At the input of the spectral analysis device20, the N/2 ends76of the N/2 return fibres71are organised in such a way as to obtain a remote image lateral field14provided with spatial sampling compatible with the spectral analysis of each of the points from the image lateral field12. For example,FIG. 4Ashows a remote image lateral field14that follows a line, but there are other organisations for this remote image lateral field14, for example such as a non-regular grid (inter-fibre space in the horizontal direction different than that in the vertical direction), allowing the imaging of a plurality of spectra on the same line. Finally,FIG. 4Ashows an optical cable300consisting of N/2 illumination optical fibres71, the N/2 ends73of which act as source points, in this case, the cable300allows the luminous flux to be transported and structured according to a lateral field11. A new embodiment of the measurement apparatus corresponding to an optical head200with an integrated spectral analysis device20is described below and illustrated inFIG. 4B, for which the optical cable300is only used to transport the luminous flux.

This second preferred embodiment only partly overcomes the constraints related to the chromatic confocal measurement apparatus provided with a lateral field according to the prior art, since an optical cable300consisting of the N/2 illumination fibres (N being a positive even integer) is necessary in order to guide the light coming from the source box700, and more particularly from the polychromatic source(s)10, to the measurement head200. The N/2 ends75of the N/2 illumination fibres71inside the source box700are positioned facing the polychromatic source(s) in order to collect as much luminous flux as possible.

Finally, the source box700is connected to the power grid via a power supply cable42and to the measurement head via a power supply cable43and a data transmission cable33. As for the measurement head, it is connected to a computer600via a data transmission cable32. However, a Person Skilled in the Art does not exclude a configuration for which the power supply block40and/or the power supply block41is a rechargeable battery and for which, no or respectively, only one power supply cable42or one power supply cable43is necessary during the phases of battery recharge. Likewise, in the near future, the possibility of transmitting the data in wireless mode (wireless) can also be imagined, allowing the transmission of significant data streams in real time without losses, which would also allow the need for the cables of data transmissions32and33to be eliminated. Thus, this second preferred embodiment of a chromatic confocal measurement apparatus provided with a lateral field according to the invention allows the cable of N/2 return fibres72connecting the optoelectronic box100and the measurement head200to be advantageously replaced by a power supply cable43and a data transmission cable33. The substitution of the cable of N/2 return fibres72by a power supply cable43and a data transmission cable33represents a development of interest for the integrators of a size-control system, who have moreover widely experienced the reliability, the robustness and the durability of electric cables and/or cables of data transmissions in industrial inspection systems installed on a production line.

FIG. 4Billustrates, schematically, an alternative of the second preferred embodiment of a chromatic confocal measurement apparatus provided with a lateral field according to the invention called Optical head200with an integrated spectral analysis device20, which differs from the one previously described and illustrated inFIG. 4A, by the fact that, the optical cable300only contains one fibre or a plurality of fibres78(if the number of sources10is greater than one). In the case of a fibre78having a diameter that is suitable, as illustrated for example inFIG. 4B, its end79allows the flux coming from the polychromatic source10to be collected in an optimal manner, and its other end15, acts as a secondary source inside the measurement head200. In this case, the fibre78allows the source to be moved into the inside of the optical head200, and each of the first three preferred embodiments of the polychromatic source10subassembly associated with a light guide80, as illustrated inFIGS. 3A, 3B and 3C, allows the light to be structured according to an object lateral field11.

This alternative of the second preferred embodiment of a chromatic confocal measurement apparatus provided with a lateral field according to the invention called Optical head200with an integrated spectral analysis device20allows fibres78having larger diameters to be used between the source10and the optical head200, which provides greater robustness with respect to the chromatic confocal measurement apparatus provided with a lateral field according to the prior art.

For this second preferred embodiment of a chromatic confocal measurement apparatus provided with a lateral field according to the invention, for which the spectral analysis device20is integrated into the measurement head200,FIGS. 4A and 4Billustrate a fibre link between the beam splitter parallelepiped60and the spectral analysis device20, however, in the following paragraphs, other modes of propagation of the backscattered beam, in particular under free field conditions, will be described, using, respectively, theFIGS. 5A and 5Bthat represent two preferred embodiments of the measurement head200, into which, the spectral analysis device20is integrated.

FIG. 5Aillustrates a first preferred embodiment of the measurement head200, into which, the spectral analysis device20is integrated. For this first preferred embodiment, the N/2 ends73of the illumination fibres71form an object lateral field11according to an illumination line and the inter-fibre space is constant. In order to obtain this illumination line of N/2 points, a method of aligning the fibres in V-shaped grooves, for example, can be used. The image of the N/2 ends73of the illumination fibres71is formed, after a double forward and return journey through the set of lenses50and the beam splitter parallelepiped60, in an image conjugated plane upstream of the spectral analysis device20and defines the image conjugated lateral field13in which a line of N/2 filtering holes is positioned that gives the measurement apparatus the property of confocality. The image conjugated lateral field13is thus perfectly identical to the object lateral field11. The spectral analysis device20, which can be, for example, a grating spectrometer, a prism spectrometer or a spectrometer incorporating a diffractive lens, has the function of dispersing each wavelength contained in the spectrum of the polychromatic source10, and coming from each of the N/2 points of the illumination line, in a direction orthogonal to the line defined by the image conjugated lateral field13. The spectral analysis device20must be calculated in order to accept a field having a size defined by the length of the N/2 ends73of the illumination fibres71. In this first preferred embodiment of the measurement head200into which, the spectral analysis device20is integrated, the beam is propagated under free field conditions between the N/2 ends73of the illumination fibres71and the photodetector21. For example, the organisation of the spectra22on the photodetector21, resulting from the linear image conjugated lateral field13at the spectral analysis device20input, is also shown inFIG. 5A. The computer and electronic means30for processing the signal, calculation, data transmission, control and configuring the measurement apparatus allow, in particular, the signal delivered by the photodetector21to be processed and the processed data to be transmitted to a computer600via a data transmission cable32.

FIG. 5Billustrates a second preferred embodiment of the measurement head200, into which, the spectral analysis device20is integrated. For this second preferred embodiment, the N/2 ends73of the illumination fibres71form an object lateral field11according to a regular illumination grid in which the inter-fibre space is constant in both directions. For example, it is possible to obtain this regular illumination grid of N/2 points, by superimposing and gluing a plurality of V-shaped grooves. The image of the N/2 ends73of the illumination fibres71is formed, after a double forward and return journey through the set of lenses50and the beam splitter parallelepiped60, in an image conjugated plane upstream of the spectral analysis device20and defines the image conjugated lateral field13. The image conjugated lateral field13is thus perfectly identical to the object lateral field11.

This second embodiment of the measurement head200, into which, the spectral analysis device20is integrated, uses N/2 return fibres72positioned in such a way that their N/2 ends74near the beam splitter parallelepiped60are strictly organised according to the object lateral field11, that is to say, in a regular grid. The N/2 ends74positioned in a regular grid act as filtering holes, forming a plurality of spatial filters, giving this device the property of confocality.

Near the spectral analysis device20, the N/2 ends76of the return fibres72are organised according to a remote image field14in such a way as to optimise the performance of the chromatic confocal measurement apparatus provided with a lateral field according to the invention. For example,FIG. 5Bdescribes an organisation, of the N/2 ends76of the return fibres72, according to a non-regular grid (inter-fibre space in the horizontal direction different than that in the vertical direction), allowing the imaging of a plurality of spectra on the same line while guaranteeing that they do not overlap. The following paragraph will describe an embodiment of the measurement head200, into which, the spectral analysis device20is integrated, incompatible with the measurement apparatus because spectra overlap. In this configuration, a spectral analysis device20having a prism or a spectral analysis device20that uses a diffractive lens is preferred in order to also avoid any overlapping of spectral orders caused by a diffraction grating. In this second preferred embodiment of the measurement head200into which, the spectral analysis device20is integrated, the use of N/2 return fibres72is necessary in order to optimally organise the remote image lateral field14at a spectral analysis device20input. For example, the organisation of the spectra22on the photodetector21resulting from a distribution of the N/2 ends76of the return fibres72in a non-regular grid is also shown inFIG. 5B.

The computer and electronic means30for processing the signal, calculation, data transmission, control and configuring the measurement apparatus allow, in particular, the signal delivered by the photodetector21to be processed and the processed data to be transmitted to a computer600via a data transmission cable32.

In order to understand in more detail the problems related to the organisation of the ends76of the return fibres72at a spectral analysis device20input,FIG. 5Cillustrates a non-compliant embodiment of the measurement head200, into which, the spectral analysis device20is integrated. This non-compliant embodiment repeats, for the illumination, the object lateral field11in the form of a regular grid like in the second preferred embodiment of the measurement head200, illustrated byFIG. 5B, and for the return, a propagation of the beam under free field conditions, as described in the first preferred embodiment of the measurement head200, and as illustrated byFIG. 5A.

This embodiment of the measurement head200, into which, the spectral analysis device20is integrated, thus defined, is not suitable since the conjugated image lateral field13is a regular grid that cannot be imaged by a conventional spectral analysis device. Indeed, the image of an image-point matrix according to a regular grid through the spectral analysis device20leads to overlapping of spectra between neighbouring points. For example, the organisation of the spectra22on the photodetector21resulting from a distribution of the N/2 ends76of the return fibres72in a regular grid is shown inFIG. 5C, showing the problem of overlapping of spectra.

However, it is possible to design a spectral analysis device20that allows, after all, the spectra to be sufficiently separated in one direction in order to prevent any overlapping, but de facto, because of the regular grid, the points imaged in the other direction are also highly spaced apart, and require the use of a two-dimensional photodetector21, the pixels of which are rectangular with a high aspect ratio, and/or the use of a two-dimensional photodetector21having a much greater number of pixels in one direction than in the other, or the analysis of only a small number of points. The various solutions proposed, although possible to produce, do not appear wise to a Person Skilled in the Art who does not, however, eliminate them.

Thus, when the propagation occurs under free field conditions, a Person Skilled in the Art easily understands that the organisation described by the N/2 ends73of the illumination fibres71can, after all, be of any type, but that an organisation other than linear, substantially complicates the design of the spectral analysis device20. The performance of the system suffers as a result, in particular because of the large number of spectra shared by a line and thus to the small number of pixels used to image each spectrum. When an organisation described by the N/2 ends73of the illumination fibres71other than linear is preferred to describe the field in the space of the object400, a Person Skilled in the Art, prefers the use of N/2 return fibres72(Cf.FIG. 5B) positioned between the combination/separation assembly60, and the input of the spectral analysis device20, in such a way that, the ends74of the N/2 return fibres are, near the combination/separation assembly60, organised strictly in the same way as the N/2 ends73of the illumination fibres71. Near the spectral analysis device20, the N/2 ends76of the return fibres72are organised in such a way as to optimise the performance of the chromatic confocal measurement apparatus provided with a lateral field according to the invention. A linear organisation of the N/2 ends76of the return fibres72is most often preferred that involves having one spectrum per line, and thus a maximum resolution that leads to optimised precision of the chromatic confocal measurement apparatus provided with a lateral field according to the invention. A Person Skilled in the Art understands, however, that another organisation of the N/2 ends76of the return fibres72, for example, that can be a non-regular rectangular matrix (inter-fibre space in the horizontal direction different than that in the vertical direction) can allow a plurality of spectra to be placed on the same line, and thus measurement to be carried out at a rate greater than for a linear organisation. For information, the measurement rate is dictated by the capacity of the photodetector21to read and transmit images rapidly. This rate depends on the number of lines read. Since the processing of the signal is sufficiently fast in order to not be a limiting factor, the greater the number of spectra shared by a line of the photodetector21, the higher the measurement rate.

FIG. 6illustrates a third preferred embodiment of a chromatic confocal measurement apparatus provided with a lateral field according to the invention called Optical head200with integrated polychromatic source10and spectral analysis device20, for which the polychromatic source(s)10, the spectral analysis device20, computer and electronic means30for processing the signal, calculation, data transmission, control and configuring the measurement apparatus, and a power supply block40are integrated into the measurement head200. For this third preferred embodiment of a chromatic confocal measurement apparatus provided with a lateral field according to the invention, the measurement head200, also contains a light guide80, a set of lenses50. The light guide80contains, according toFIG. 6, a set of N/2 illumination fibres71and a combination/separation assembly60. As previously described, the function of the N/2 illumination fibres71is to collect, from its N/2 ends75, the light coming from the polychromatic source(s)10, guide the light towards a combination/separation assembly60, and structure the light according to an object lateral field11defined by its N/2 ends73near the combination/separation assembly60. The function of the combination/separation assembly60, which can, for example, be a beam splitter parallelepiped, is to direct the beams coming from the object lateral field11towards the set of optical lenses50up to the object400, and to direct the beams backscattered by the object400through the set of optical lenses50towards the spectral analysis device20.

The set of lenses50, which can consist of a collimator51and a chromatic lens52allows both the N/2 spots of light to be focused on the object400according to an image lateral field12, but also the desired axial chromatic aberration that partly defines the measurement volume500of the chromatic confocal apparatus to be generated. The beams backscattered by the object400are thus imaged through the set of lenses50in order to form a conjugated image lateral field13in the plane of the N/2 ends74of the return fibres72. Thus, the measurement apparatus consists, inter alia, of a plurality of spatial filters located in the conjugated image plane13of a set of lenses50and organised strictly identically to the source points located in an object lateral field11. These spatial filters can be embodied by the ends74of the return fibres72located between an assembly60for combination/separation of beams and a spectral analysis device20, or by a matrix of holes when the propagation between an assembly60for combination/separation of beams and a spectral analysis device20occurs under free field conditions. This plurality of spatial filters conjugated with the source points gives the measurement apparatus the property of confocality. The conjugated image field13is repeated by the N/2 ends74, then made remote and reorganised according to a remote image lateral field14, formed by the ends76of the return fibres72, at the input of the spectral analysis device20. The remote image lateral field14is provided with spatial sampling compatible with the spectral analysis of each of the points from the image lateral field12.FIG. 6shows, for example, a remote image lateral field14organised in a line of points spaced apart regularly.

FIG. 6shows a preferred embodiment of a chromatic confocal measurement apparatus provided with a lateral field according to the invention, but a Person Skilled in the Art is aware of the existence of other embodiments. All it takes to be sure of this is to combine one of the various preferred embodiments of the polychromatic source10subassembly associated with a light guide80illustrated inFIGS. 3A, 3B, 3C and 3Dwith one of the two preferred embodiments of the measurement head200, into which, the spectral analysis device20is integrated, illustrated inFIGS. 5A and 5B. These various preferred embodiments of the polychromatic source10subassembly associated with a light guide80, as well as the two preferred embodiments of the measurement head200, into which, the spectral analysis device20is integrated, are obviously compatible with this third preferred embodiment of a chromatic confocal measurement apparatus provided with a lateral field according to the invention.

This third preferred embodiment totally overcomes the constraints related to the chromatic confocal measurement apparatus provided with a lateral field according to the prior art, since the optical cables300of N/2 illumination fibres71and N/2 return fibres72(N being a positive even integer) are abandoned and advantageously replaced by a power supply cable42and a data transmission cable32that can be, for example, a CameraLink or Giga-Ethernet cable. The electric cables and/or the cables of data transmissions that have moreover been widely tested in industrial inspection systems are robust, flexible, not sensitive to the stresses of acceleration or rotation related to the movement of the measurement head, and easily replaceable in case of deterioration. De facto, this third preferred embodiment of a chromatic confocal measurement apparatus provided with a lateral field according to the invention corresponds perfectly to the industrial needs of integration. Thus, this third preferred embodiment of a chromatic confocal measurement apparatus provided with a lateral field according to the invention, by eliminating the need for any optical cables, promotes the robustness and the compactness of the measurement apparatus.

This third preferred embodiment of a chromatic confocal measurement apparatus provided with a lateral field according to the invention, also allows, the measurement device to be integrated into a motorised movement system, such as a Coordinate-Measuring Machine (CMM) that is most often in the form of a measurement gantry provided with translation axes and/or rotation axes, or also, of a robot arm with a plurality of axes of rotation. The absence of optical cables facilitates the integration and the use of the measurement apparatus according to the invention in this type of system.

The device described allows the simultaneous measurement of one or more heights and/or of one or more thicknesses of all the points of a field of the object that can potentially contain a plurality of transparent layers, either in the visible range or in the Infrared range.FIG. 7describes the signal obtained when an object400, which has a plurality of layers, is positioned in the measurement volume500of a measurement head200like that described by the third preferred embodiment of a chromatic confocal measurement apparatus provided with a lateral field according to the invention. Thus, the measurement head contains one or more polychromatic sources10, a light guide subassembly80, a set of lenses50, a spectral analysis device20, and computer and electronic means30for processing the signal, calculation, data transmission, control and configuring the measurement apparatus, and a power supply block40. The measurement head can, for example, illuminate an image lateral field12on the various surfaces of the object, and this field can then, for example, be reorganised in a line of equidistant points thus defining the remote image lateral field14, at a spectral analysis device20input. The spectral image22, collected on the photodetector21, is thus a succession of N/2 spectra parallel to each other and equidistant. Finally, the image23represents a spectrum “p” out of the N/2 spectra imaged on the photodetector21. This spectrum “p” contains 5 peaks, called “chromatic confocal peaks”, corresponding to the5interfaces of the object400. Thus, the detection of these 5 peaks and the calculation of their respective positions, allows the height of each interface to be calculated, but also the thickness of each layers of the object400if the refractive index of each of the layers is known. Consequently, if this detection and this calculation of position is applied for each of the N/2 spectra, it is possible to simultaneously calculate each height and each thickness of the object400for N/2 points of the image lateral field12.

Finally, it is possible to produce this measurement apparatus independently, in the visible or Infra Red range, in such a way that the polychromatic source10emits, respectively, radiation in the visible or Infrared spectral band, that the set of optical lenses50is calculated in order to be provided with a controlled axial chromatic aberration in the visible or Infrared spectral band, respectively, and that finally the spectral analysis device20is also designed to analyse spectra in the visible or Infrared spectral band, respectively.

Thus, the chromatic confocal measurement apparatus provided with a lateral field according to the invention allows the simultaneous measurement of one or more heights and/or of one or more thicknesses of all the points of a field of the object that can potentially contain a plurality of transparent layers, either in the visible range or in the Infrared range.

This type of device is suitable for multiple industrial uses; from wafer measurement in the semiconductor field to the measurement and control of mechanical parts on the line, or even the measurement and control of the thickness of glass or plastic films. Other fields of uses exist, the common point is the desire for a measurement system that is faster and faster, more and more compact and as flexible as possible, and it is clear here that the chromatic confocal measurement apparatus provided with a lateral field according to the invention meets these various needs.

REFERENCES

KEY TO THE FIGURES