Method and device for assessing an optical element

Abstract

The invention relates to a method (10) for assessing the quality of an optical element (100) of a lens comprising a plurality of optical elements, each optical element (100) comprising at least one surface to be assessed, the method (10) comprising the following steps: - optically measuring (12) a surface to be assessed in order to obtain a measured surface, the measured surface comprising topographic data of the surface to be assessed; - from the measured surface, determining (14) a calculated surface representing the measured surface; - determining a centre of the calculated surface; - determining an optical axis from the calculated surface; - generating (16) at least one cross-section of the calculated surface passing through the centre of the surface and including the optical axis; and - obtaining (18), from the at least one cross-section, topographic deviations of the calculated surface with respect to a theoretical surface. The invention also relates to an assessment system implementing such a method.

Description

[0001] Method and device for evaluating an optical element

[0002] Technical field

[0003] The present invention relates to a method for evaluating the quality of an optical element of a lens comprising a plurality of optical elements. The invention also relates to an evaluation system implementing such a method.

[0004] The field of the invention is, in a non-limiting manner, that of optical control and measurement systems, in particular for the manufacture of optical elements or components, implemented in particular in imaging objectives.

[0005] State of the art

[0006] Imaging lenses typically consist of one or more lenses and possibly other components, such as plates or filters, for shaping optical beams. These optical elements, or lenses, may be assembled as a stack along an axis in a holder such as a barrel.

[0007] When manufacturing optical elements, such as lenses or objectives with multiple lenses, it may be necessary to control or measure shapes of the interfaces, or surfaces, of the optical elements.

[0008] Optical elements, such as optical assemblies or imaging lenses, generally consist of one or a plurality of lenses and possibly other components for shaping optical beams. These components, or lenses, may be assembled as a stack in a holder such as a barrel.

[0009] The optical performance of such an optical element, for example an imaging lens, depends essentially on the manufacturing precision of the optical components (such as lenses) which compose it, and on the precision with which they are positioned in the assembly, both with respect to their optical axes and their respective distances, if applicable.

[0010] It is therefore necessary to check the components and their assemblies. In particular, in an assembly it may be necessary to check the shape of the interfaces of the optical components or lenses, in order to determine, for example, non-compliant or poorly positioned, deformed or inclined elements.

[0011] A device and method for controlling shapes of optical elements such as lenses are described in US 9,658,129 B2. The shapes of surfaces are measured using an interferometric technique, in particular to determine the vertex or apex. However, these measurements do not provide an indication of shape but only of control points.

[0012] Statement of the invention

[0013] It is an object of the invention to propose a method and a system for evaluating the quality of an optical element intended to be implemented in an imaging objective.

[0014] Another aim of the present invention is to propose a method and an evaluation system whose measurements can be directly used to set up a sorting based on the imaging quality of the optical elements.

[0015] It is also an aim of the present invention to propose a method and an evaluation system allowing a characterization of an objective in which the evaluated optical element must be implemented, taking into account the topographical deviations measured on one or more surfaces of the optical element, or even taking into account said measured topographical deviations to compensate them at least partially.

[0016] At least one of these aims is achieved with a method for evaluating the quality of an optical element of a lens comprising a plurality of optical elements, each optical element comprising at least one surface to be evaluated, the method comprising the following steps: optically measuring a surface to be evaluated to obtain a measured surface, the measured surface comprising topographical data of the surface to be evaluated,

[0017] - from the measured surface, determination of a calculated surface representing the measured surface,

[0018] - determination of a center of the calculated surface,

[0019] - determination of an optical axis from the calculated surface,

[0020] - generation of at least one cross-section of the calculated surface passing through the center of the surface and including the optical axis, and obtaining, from the at least one cross-section, topographic deviations of the calculated surface from a theoretical surface.

[0021] The method according to the present invention makes it possible to evaluate or control the quality of an optical element intended to be part of a lens, by measuring each of the surfaces of this optical element. For each surface, topographic data, representing the shape of the surface, are collected and used to determine a calculated surface. The calculated surface can for example result from a least squares method between the measured surface and a theoretical surface.

[0022] One or more cross-sections of the calculated surface are then generated. These cross-sections of the measured surface pass through the center of the calculated surface and include the optical axis of the surface. The center is determined from the topographic measurements. These cross-section views passing through the center of the surface are particularly useful for operators. Indeed, it provides them with a visualizable two-dimensional representation, allowing them to better assess the extent and / or nature of topographic deviations, simultaneously across the entire plane of the section.

[0023] Finally, topographic deviations, or errors, of the calculated surface compared to a theoretical surface are obtained from at least one cross-section.

[0024] The lens can be an imaging lens, especially for smartphones.

[0025] According to one embodiment, the topographic deviations can be calculated for corresponding points of the cross-section of the calculated surface and the theoretical surface, the points being corresponding along a local axis of interest intercepting said calculated and theoretical surfaces at these points.

[0026] In one example, the local axis of interest may correspond to one of the directions of a local optical beam in the objective in which the optical element, for example a lens, is assembled.

[0027] According to other examples, the local axis of interest may correspond to

[0028] - an axis of symmetry of one of the surfaces of the optical element,

[0029] - the axis of an assembly barrel, the normal axis of a support face of a lens, a common mean axis theoretically defining these axes, or a weighted average of these different axes.

[0030] Advantageously, the step of obtaining topography deviations may comprise a step of obtaining slope and / or curvature deviations, relative to the theoretical surface.

[0031] It is indeed important to know the slope differences because they can lead to differences in the refraction angles of a light beam incident on the surface concerned.

[0032] The refraction angle deviation is reflected along the entire length of the beam's optical path in the lens, containing the surface of interest, up to the sensor. This deviation is therefore more decisive for the quality of the lens than a deviation in the Z position at which the beam intercepts the surface.

[0033] A deviation in the local curvature also has an amplified effect on the focal length even if the topographic error along the Z axis is minimal.

[0034] Thus, the method makes it possible to achieve sensitivity to the real optical effect by classical topography measurements.

[0035] Advantageously, the center of the calculated surface can be materialized on the measured surface, on a theoretical surface and / or on a difference between the measured surface and the theoretical surface.

[0036] The center of the surface is indeed an important piece of data for an operator implementing the method according to the invention. An incorrect surface center can lead to the visualization of potentially non-existent defects. It is then appropriate to ensure the display and / or marking of the surface calculated by this data.

[0037] Advantageously, the method according to the invention may further comprise a step of displaying the coordinates of the direction vector of the optical axis and / or the center of the surface calculated by a display module.

[0038] The method according to the invention may further comprise a step of defining at least one conformity indicator of the measured surface based on the topography deviations. Advantageously, the conformity indicator may be supplemented by data on the difference between the evaluated surface and the theoretical surface such as:

[0039] - the position between points, and / or

[0040] - the physical and / or optical thickness along an axis of interest.

[0041] The at least one compliance indicator may be compared to at least one threshold to decide the compliance of the assessed surface, and / or the compliance of the entire objective.

[0042] At least one threshold of a compliance indicator can be obtained by comparing the optical effects (refraction, etc.) of the theoretical surface and the calculated surface.

[0043] At least one set of compliance indicator thresholds can be obtained by taking into account effects of variation in refraction angles, and / or effects of variations in the concentrating power of the optical beam.

[0044] According to one embodiment, the step of defining the at least one conformity indicator can be carried out by comparing a slope and / or a curvature of the calculated surface and the theoretical surface, for corresponding points of the cross-section of the calculated surface and the theoretical surface.

[0045] The step of defining the at least one conformity indicator can be carried out by calculating a threshold value of a refraction angle value of a light beam by the evaluated surface.

[0046] Slope and / or curvature deviations can be used, in particular, as metrics for implementing quality control during the manufacture of an optical element for a lens.

[0047] Indeed, it is possible to calculate a set of values ​​of the metrics (for example, maximum refraction angle, maximum curvature, average refraction angle deviation, average curvature deviation) per surface area, or for the entire surface with possibly a weighting per area. This definition step then allows the establishment of a quality control for a manufacturing process of a lens for an objective. According to one embodiment, the step of defining the at least one conformity indicator can be carried out by calculating a deviation between an optical axis and / or an optical center and an assembly axis of the lens and / or an assembly center, for at least one surface of the optical element.

[0048] Indeed, the deviation between the optical axis of an optical element and an assembly axis of the lens can alter the conformity of the entire lens. Indeed, this deviation can cause an asymmetry of the optical transfer function, and generally a degradation of the latter at least over a part of the field of an image produced with the lens.

[0049] In a complementary manner, it is also possible to use such a deviation occurring on several lenses of the lens stack, by rotating each of said lenses by a certain angle specific to each lens, to realign these lenses at least partially according to a new optical center different from the assembly center. The observed deviation can thus be partly compensated, or a compromise which improves the performance of the assembled lens can be found, even if the optical centers are not rigorously realigned on the same new common center. In this case, it is appropriate to consider an indicator of conformity of deviation between the optical center and the assembly center for each surface of each lens, not around the zero value (along two perpendicular axes), but around two non-zero values.

[0050] We can also consider an indicator of compliance of deviation between the optical axes and assembly axes, possibly around non-zero values.

[0051] Advantageously, the step of defining the at least one conformity indicator can be carried out by calculating a threshold value of at least one quality indicator of the lens in which the optical element, and therefore the evaluated surface, is intended to be implemented. This involves evaluating the performance that can be achieved with this lens containing the optical element concerned, measured using the following indicators: characteristics of the transfer function, chromatic aberration (chromatism), geometric aberration, so-called 3rd order aberrations such as coma, astigmatism, focal plane curvature, etc.

[0052] The method according to the invention may further comprise the evaluation of a quality metric of an image or of the transfer function (Modulation Transfer Function, MTF) obtainable with said evaluated surface.

[0053] A threshold of a compliance indicator can be obtained by comparing the optical effects of the theoretical surface and the calculated surface.

[0054] It is thus possible to determine the loss of performance of the objective containing the surface being evaluated directly as a function of the topographical deviations, and in particular as a function of the slope and / or curvature deviations of this surface. This determination is carried out by simulating the performance of the objective, in particular the image quality that can be obtained with it, with the measured surface present in the objective.

[0055] Based on this quality metric, automated sorting can be performed during the manufacturing of optical elements.

[0056] Minimum and maximum thresholds can be considered around unaffected deviations for centering deviations, which amounts to considering minimum and maximum values ​​that are not symmetrical or not equal in absolute value. Different minimum and maximum thresholds (not equal in absolute value) can also be considered for other indicators of slope deviation and curvature deviation, or even optical or physical distance, to achieve at least partial deviation compensations during lens assembly.

[0057] Optical distance means the product of the physical distance multiplied by the group index or the refractive index at a measurement or use wavelength, which corresponds to the propagation delay of an optical wave packet or the phase delay of the wave front propagating in said lens.

[0058] Advantageously, the step of defining the at least one conformity indicator can be carried out by calculating a threshold value of a combination of a plurality of quality indicators of the objective. The measurement step of the method according to the invention can comprise an interferometric measurement.

[0059] According to one example, the measuring step may comprise a measurement by holographic optical profilometry.

[0060] The interferometric measurement can be carried out using a known interferometric or profilometric measuring device.

[0061] According to another aspect of the invention, there is provided a system for evaluating the quality of optical elements of a lens comprising a plurality of optical elements, each optical element comprising at least one surface to be evaluated, the system comprising: an optical measuring device, a digital processing module, a display module, the system being configured to implement the steps of the method according to the present invention.

[0062] Description of figures and embodiments

[0063] Other advantages and characteristics will appear on examining the detailed description of non-limiting examples, and the attached drawings in which:

[0064] - [Fig.l] Figure 1 is a schematic representation of a non-limiting exemplary embodiment of a measuring device that can be implemented within the framework of the present invention;

[0065] - [Fig.2] Figure 2 is a schematic representation of a non-limiting exemplary embodiment of an evaluation method according to the present invention;

[0066] - [Fig.3] Figure 3 an example of a surface measured with the method according to the present invention; and

[0067] - [Fig.4] Figure 4 shows examples of measurements obtained with the method according to the present invention.

[0068] It is understood that the embodiments which will be described below are in no way limiting. In particular, it is possible to imagine variants of the invention comprising only a selection of characteristics described below isolated from the other characteristics described, if this selection of characteristics is sufficient to confer a technical advantage or to differentiate the invention compared to the state of the prior art. This selection includes at least one preferably functional characteristic without structural details, or with only part of the structural details if this part alone is sufficient to confer a technical advantage or to differentiate the invention compared to the state of the prior art.

[0069] In particular, all the variants and embodiments described can be combined with each other if there is no technical obstacle to this combination.

[0070] A system for evaluating the quality of an optical element of a lens according to the present invention comprises an optical measuring device for performing measurements of the surfaces of the optical element as well as a digital processing module for obtaining and processing topographical information of the measured surfaces.

[0071] Figure 1 is a schematic representation of an exemplary measuring device that may be implemented in a system according to the present invention.

[0072] The measuring device 1000 comprises a full-field low-coherence interferometric device of the Michelson or Linnik type. The interferometer is formed by a splitter element 104, for example in the form of a cube or a splitter plate, with a measuring arm which directs a measuring beam 106 towards a surface to be measured of an optical element 100, and a reference arm with a mirror 105 to form a reference beam 116.

[0073] The interferometer is illuminated by a light source 112 via a lighting splitter element 103, for example in the form of a cube or a splitter plate. Depending on the detection modes implemented, the source 112 may be low coherence, with a coherence length of the order of a few microns or a few tens of microns for example, or more coherent, with a coherence length of a few hundred microns or more. It may in particular comprise, for example, a superluminescent diode (SLD), a laser diode, a thermal light source (halogen lamp, etc.) or a supercontinuum source. It may also comprise a filtering device, for example with a grating and a slit, or interference filters, to adjust the coherence length as required.It can be arranged to emit in visible or near infrared wavelengths, around one or more wavelengths.

[0074] Of course, the separating elements 103, 104 can be non-polarizing, or polarizing and associated with quarter-wave plates to make lossless couplers.

[0075] The measurement beam 106 and reference beam 116, reflected respectively in the two arms of the interferometer, are directed via the illumination splitter plate 103 towards a camera 101 with a sensor or detector 102 comprising a detection matrix, for example of the CMOS or CCD type.

[0076] When the difference in optical paths between the measurement and reference beams is less than the coherence length of the source 112, interference is obtained on the detector 102.

[0077] The device 1000 according to the embodiment shown in Figure 1 also comprises a focusing lens or objective 107, and a tube lens 109, arranged to define a conjugate object plane of an image plane formed on the detector 102. The reference arm further comprises an objective 110 which also defines, with the tube lens 109, a reference object plane conjugate to the image plane of the detector 102.

[0078] The device 1000 is a full-field imaging device, which makes it possible to image surfaces of the optical element to be evaluated 100 on the detector 102.

[0079] Typically, the device 1000 includes optical components for focusing the illumination beam into the back focal plane of the focusing lens 107 and the reference arm lens 110. The illumination beams are not shown in the figure for clarity.

[0080] The device 1000 also comprises a displacement means 108 whose function is to move the object plane conjugate with the image plane formed by the detector 102, so as, for example, to image the surfaces or interfaces of the object on the detector 102. This displacement means 108 may comprise a system for moving the focusing objective 107 or lenses of this objective, for example with a linear or helical translation device. Alternatively or in addition, this displacement means 108 may comprise a translation device or stage for moving the device 1000 relative to the optical element to be measured 100, or vice versa.

[0081] The device 1000 may also optionally comprise a displacement means 111 for varying the length of the reference arm, for example in the form of a translation stage 111 moving the reference mirror 105. The objective 110 of the reference arm may also be adjustable to maintain the reference mirror 105 in an object plane conjugate to the image plane formed by the detector 102. This displacement means 111 makes it possible to adjust the relative difference in optical paths between the measuring arm and the reference arm of the interferometer, so as to adjust the area of ​​appearance of the interference fringes.

[0082] The system according to the invention further comprises a digital processing module, configured to produce, from the measured interference signals, topographic information of the surfaces measured according to a field of view. This processing module comprises at least one computer, a central or computing unit, a microprocessor and / or suitable software means.

[0083] Of course, the measuring device may alternatively comprise a low coherence point-mode interferometric device. In this case, a plurality of interferometric signals are acquired by scanning the entire field of view at a plurality of measurement points on the surface, in order to obtain topographic information over the entire surface.

[0084] With the interferometric device 1000, when a surface or interface 120 of the optical element 100 appears in the coherence zone, an interference structure is obtained on the sensor resulting from the interference between the measurement and reference beams for the field of view. Topographical information of the interfaces can be deduced from these interference structures.

[0085] A system according to the invention, for example implementing an interferometric device according to the embodiment shown in Figure 1, or another topographical measurement device, can be used to implement the steps of a method according to the invention which will be described subsequently.

[0086] Figure 2 is a schematic representation of a non-limiting exemplary embodiment of a measuring method according to the invention.

[0087] The method, shown in Figure 2, comprises a step 12 of measuring a surface to be measured, belonging to an optical element to be evaluated.

[0088] According to a preferred embodiment, the measurement is carried out by interferometric techniques, for example using a device such as shown in Figure 1.

[0089] In the measurement step 12, interferometric signals corresponding to the surface to be measured 120 are detected. The coherence zone corresponding to the reference beam has been positioned at this surface 120 previously, as detailed above with reference to FIG. 1.

[0090] The measured interferometric signals consist of interferograms from which it is possible to obtain optical distances, in the Z direction, which are counted relative to a position reference of the interferometer, for a position (X, Y) in the field of view. This makes it possible to know the relative position of each measured surface.

[0091] When measurements are performed with full-field interferometers, as illustrated in Figure 1, full-field interference structures resulting from interference between a measurement beam and a reference beam for the entire field of view are obtained directly on the optical sensor.

[0092] From the interference signals, topographic data of the measured surface can be obtained. Topographic data represents the optical or geometric shape of the surface. All interference signals for the surface to be measured are then digitally processed to derive topographic data.

[0093] The optical shape is called "apparent" because it is affected by any surfaces or interfaces crossed. It is deduced from interferometric measurements. To obtain real geometric shapes of the surfaces of the optical element, the optical shapes of these must be corrected for propagation effects. Different known methods can be used to determine optical and / or geometric shapes.

[0094] According to one embodiment, profilometric methods can be used for the digital processing of the measured signals. Profilometry is based on the processing of interferogram sequences.

[0095] In this type of method, the measurement and reference beams are preferably adjusted so as to be incident on the detector with propagation directions that are essentially parallel or coincident, or slightly inclined, so as to produce flat or weakly modulated hue interferograms.

[0096] A first example of such a profilometric method implements algorithms based on phase-stepping interferometry (PSI), in which a plurality of interferograms are acquired for a plurality of optical path difference or phase values ​​between the measurement and reference beams. Then, the phase and possibly the amplitude at any point in the field of view of the interference signal thus formed are determined by applying a known algorithm. The optical shape of the surface can then be determined from the phase.

[0097] A second example of a profilometric method implements algorithms based on vertical scan interferometry (VSI). For this, for the surface considered, a plurality of interferograms is acquired for a plurality of optical delays between the measuring and reference arms of the interferometer, in a range of optical delays preferably extending beyond the coherence length of the light source. By detecting at each point of the detector the optical delay for which the difference in optical paths between the beams is zero, the optical shape of the surface is directly obtained, from which the geometric shape can be deduced.

[0098] According to another embodiment, a digital holography method can be used for digital processing of the measured signals.

[0099] In a digital holography method, also called "off-axis interferometry", the measurement and reference beams are preferably adjusted so that they are incident on the detector with propagation directions inclined, or forming an angle between them.

[0100] By choosing a sufficiently high angle between the measurement and reference beams, the different diffraction orders are separated in the Fourier domain and can be filtered. The order corresponding to the real image can thus be obtained by filtering in the Fourier domain. It is then possible to illuminate the real image digitally with a digital reference wave corresponding to the reference wave used, to obtain the desired electromagnetic field. Finally, the optical shape of the surface and its geometric shape can be determined using the phase of the electromagnetic field.

[0101] In a step 14 of the method 10 as shown in Figure 2, a calculated surface is determined from the topographic data of the measured surface.

[0102] The determination of the calculated surface can be carried out, for example, using a least squares method, between the topographic data of the measured surface and the data of a theoretical surface.

[0103] An example of a calculated surface is shown in Figure 3, in which a spherical surface is represented, with heights indicated by grayscale.

[0104] In the same way, the measured surface (measured topographic data), the theoretical surface and / or the difference between the measured surface and the theoretical surface can also be represented.

[0105] The calculated surface allows us to find parameters of the surface shape.

[0106] For a spherical surface, these parameters include the radius, the position of the center of curvature, and the position of the apex. The apex is not the highest point on the surface, but the apex in the sense of the best fit obtained in step 14.

[0107] For an aspheric surface, these parameters include the coefficients of the asphere equation, the position of the apex and the center(s) of curvature.

[0108] The theoretical optical axis and the center of the calculated surface are also determined during step 12. Preferably, the parameters of the shape of the calculated surface are indicated digitally together with the graphical display of this surface, available to an operator implementing the method 10. In particular, the center of the calculated surface is materialized on the measured surface, on a theoretical surface and / or on a difference between the measured surface and the theoretical surface. For example, in Figure 3, the center 124 of the calculated surface is indicated on the measured surface.

[0109] The method 10 according to the invention further comprises a step 16 of generating at least one cross-section, either of the calculated surface, this cross-section passing through the center of the surface. The cross-section makes it possible in particular to obtain the radius and the diameter of the surface. The radii and / or diameters can be generated either at arbitrary positions or parametrically (for example, every 30°). The different radii and / or diameters obtained from the topographic measurement data can then be indicated to the operator and recorded.

[0110] In a step 18 of the method 10 according to the invention, the at least one cross-section generated in the previous step is used to determine topographical deviations of the calculated surface from a theoretical surface.

[0111] In fact, the cross-sections, or profiles, obtained for the calculated surface can, for example, be compared with those of a corresponding theoretical surface. Topographic errors are, in particular, height errors (z) in the (x, y, z) coordinate system of the measuring system.

[0112] As an example, Figure 4(a) shows a profile of a measured surface, Figure 4(b) shows an approximation of the profile of (a) by an arc of a circle, and Figure 4(c) shows the slope of the profile at a certain point. The slope determined and displayed is therefore representative of the measured surface. A slope deviation of the measured surface can then be calculated from the theoretical surface.

[0113] The method 10 according to the embodiment as shown in Figure 2, the step 18 of obtaining topographical deviations from the at least one cross-section comprises a step 20 of obtaining slope deviations and / or curvature deviations.

[0114] For example, it is possible to determine, at each point of a cross-section, a slope deviation in the vicinity of this point compared to the slope of the theoretical shape in the vicinity of this point.

[0115] The local slopes and curvatures can then be extracted from the cross-sections along the principal axes along which the chromatic errors, coma, and MTF characteristics of the objective comprising the evaluated optical element are decomposed. This requires preferential use of the local reference frame of cylindrical coordinates (radial, azimuthal, with parameters r and 0) whose choice of origin is determined by the step of determining the optical axis and optical center carried out previously.

[0116] The slope difference is then expressed in the reference frame of the measured surface, having the center of curvature as the origin.

[0117] Just like the center of the surface, once the slope deviations and / or curvature deviations have been determined, these are also displayed, by a display module, as a function of the radius r of the measured surface in the case of a spherical surface, or as a function of a polar angle.

[0118] These slope deviations preferably include two values ​​of slope deviations for each point, for example that of the slope along the radial axis and the slope along the azimuthal axis. For each angle 0 chosen, it is for example possible to consider the radial slope deviation as a function of r and the azimuthal slope deviation as a function of r. Similarly, for a given radius r, it is possible to consider the radial slope deviation as a function of 0 and the azimuthal slope deviation as a function of 0.

[0119] The curvature deviations preferably include two values ​​of curvature deviations at each point, the curvature deviation in the radial direction and that in the azimuthal direction, as well as the different curves that can be deduced from them as a function of r or as a function of 0.

[0120] Slope and / or curvature deviations, or errors, can be used as metrics for implementing quality control during the manufacture of an optical element, such as a lens, for an objective. Indeed, it is possible to define a set of values ​​as metrics (maximum refraction angle, maximum curvature, minimum refraction angle error, minimum curvature error) per zone, or integrated over the entire surface with possibly a weighting per zone.

[0121] In a first example, excessive or excessive curvature of a lens formed by injection molding may indicate excessive or excessive shrinkage of the material after demolding. This defect can then be corrected by adjusting the temperature, pressure and / or holding time parameters of the mold when manufacturing other lenses from the same batch, or by correcting the mold shape.

[0122] In a second example, it is possible to observe an increase or decrease in slope and / or curvature errors after a change in the cavity of a mold.

[0123] According to one embodiment, the method 10 according to the invention further comprises a step 22 of defining at least one indicator of conformity of the measured surface based on the topography deviations determined previously.

[0124] Advantageously, the step of defining the at least one conformity indicator is carried out by calculating a value of the refraction angle of a light beam by the evaluated surface. This conformity indicator is therefore directly obtained from the slope deviation or the curvature deviation described above. Indeed, the refraction angle of a light beam incident on the surface measured at a given point undergoes deviations depending on the slope and / or curvature errors at this point. This refraction angle error is reflected over the entire length of the optical path in the lens, this error being more decisive for the quality of the lens than a position error in z of the light beam on this surface.

[0125] The step of defining at least one conformity indicator is carried out to obtain at least one quality indicator of an objective conforming to at least one threshold value, this quality being able to be obtained with an objective comprising said evaluated surface, the quality indicator being chosen from:

[0126] - characteristics of the transfer function (MTF), chromatism,

[0127] - geometric aberration

[0128] - so-called 3rd order aberrations such as coma, focal plane curvature, etc.

[0129] It is thus possible to determine the loss of performance of the objective containing the surface being evaluated directly as a function of the topographical deviations, and in particular as a function of the slope and / or curvature deviations of this surface. This determination is carried out by simulating the performance of the objective, in particular the image quality that can be obtained with it, with the measured surface present in the objective.

[0130] Thus, a loss of performance of the final objective in presence can be assessed on the basis of the actual defects measured on a lens.

[0131] Based on this quality metric, automated or manual sorting can be performed during the manufacturing of optical elements.

[0132] For manual analysis and sorting by an operator, conformity criteria expressed as slope errors in a polar reference frame centered on the estimated optical axis are very advantageous.

[0133] As an example of the implementation of such automated sorting, it is possible to start from the modeling of the lens as designed, and all its performance metrics at different points of the electromagnetic field, and to perform a complete simulation calculation to obtain the reference values. Then, a second lens design can be modeled in which the theoretical surfaces of the lens(es) under test have been replaced by the surfaces as measured, keeping the nominal values ​​for all the parameters of the lens and uncharacterized lenses, and a second simulation can be performed to obtain the degradations involved on the different performance criteria (MTF, chromatism, coma, focal plane curvature) at the different points of the field.

[0134] A quality metric in terms of imaging can then be assigned to a lens that cannot individually be used to form an image.

[0135] From the various performance criteria (and performance degradation brought about by imperfections measured on the lens), we can deduce an overall quality value, and one or more sorting criteria (based for example on the logical AND of the comparison of each indicator with acceptable degradation thresholds for each indicator). Each sorting criterion can correspond to a targeted quality class, for example for a premium, common, or low-cost lens model.

[0136] Of course, the invention is not limited to the examples which have just been described and numerous adjustments can be made to these examples without departing from the scope of the invention.

Claims

CLAIMS 1. Method (10) for evaluating the quality of an optical element (100) of a lens comprising a plurality of optical elements, each optical element (100) comprising at least one surface to be evaluated, the method (10) comprising the following steps: optical measurement (12) of a surface to be evaluated to obtain a measured surface, the measured surface comprising topographical data of the surface to be evaluated, - from the measured surface, determination (14) of a calculated surface representing the measured surface, - determination of a center of the calculated surface, - determination of an optical axis from the calculated surface, - generation (16) of at least one cross-section of the calculated surface passing through the center of the surface and including the optical axis, and - obtaining (18), from the at least one cross-section, topographical deviations of the calculated surface compared to a theoretical surface.

2. Method (10) according to the preceding claim, characterized in that the topographical deviations are calculated for corresponding points of the cross-section of the calculated surface and of the theoretical surface, the points being corresponding along a local axis of interest intercepting said calculated and theoretical surfaces at these points.

3. Method (10) according to claim 1 or 2, characterized in that the step (18) of obtaining topographical deviations comprises a step (20) of obtaining slope and / or curvature deviations, at at least one point of the calculated surface.

4. Method (10) according to any one of the preceding claims, characterized in that the step (14) of determining the calculated surface is carried out according to a least squares method between the measured surface and a theoretical surface.

5. Method (10) according to any one of the preceding claims, characterized in that it further comprises a step (22) of defining at least one indicator of conformity of the measured surface based on the topography deviations.

6. Method (10) according to claim 5, characterized in that the step (22) of defining the at least one conformity indicator is carried out by comparing a slope and / or a curvature of the calculated surface and the theoretical surface, for corresponding points of the cross-section of the calculated surface and the theoretical surface.

7. Method (10) according to claim 5 or 6, characterized in that the step (22) of defining the at least one conformity indicator is carried out by calculating a threshold value of a refraction angle value of a light beam by the evaluated surface.

8. Method (10) according to claim 5, characterized in that the step (22) of defining the at least one conformity indicator is carried out by calculating a deviation between an optical axis and / or an optical center and an assembly axis of the lens and / or an assembly center, for at least one surface of the optical element.

9. Method (10) according to one of claims 5 to 8, characterized in that the step (22) of defining the at least one conformity indicator is carried out by calculating a threshold value of at least one quality indicator of an objective, obtainable with an objective comprising said evaluated surface, chosen from: - characteristics of the transfer function, - chromatic aberration, - geometric aberration, - so-called 3rd order aberrations, - the curvature of the focal plane.

10. Method (10) according to the preceding claim, characterized in that it further comprises a step of assigning a quality metric of an image obtainable with an objective comprising said evaluated surface.

11. Method (10) according to any one of claims 8 to 10, characterized in that the step (22) of defining the at least one conformity indicator is carried out by calculating a threshold value of a combination of a plurality of quality indicators of the objective.

12. Method (10) according to any one of the preceding claims, further comprising a step of displaying the coordinates of the direction vector of the optical axis and / or of the center of the surface calculated by a display module.

13. System for evaluating the quality of optical elements (100) of a lens comprising a plurality of optical elements, each optical element (100) comprising at least one surface to be evaluated, the system comprising: - an optical measuring device (1000), a digital processing module, a display module, the system being configured to implement the steps of the method according to any one of the preceding claims.

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