How to measure optical lens surfaces

Abstract

In a method for measuring an optical lens surface, a measuring beam (26, 27, 28) is generated using a light source (21) having a certain size, collimated via an optical arrangement (23) and detected by an optical detector (25) after reflection at the lens surface to be measured. Between the light source (21) and the optical detector (25), an aperture (24) is used to limit the measuring beam (26, 27, 28), which acts as a field stop causing vignetting. The field stop is blurred and imaged as a first light spot (26a) on the optical detector (25) by the measurement (beam 26, 27, 28) after reflection at a first lens surface. At least one second light spot (27a) obtained by reflection of the measuring beam (26, 27, 28) at a second lens surface of the optical lens arrangement (10) is also detected simultaneously by the optical detector (25). From the intensity distribution of both light spots (26a, 27a), the angle of each lens surface relative to the optical axis of the measurement beam (26, 27, 28) can be determined simultaneously for both lens surfaces, thereby providing a method for measuring lens surfaces of an optical lens system that allows for rapid measurement of the lens surfaces without the need for complex optical setups.

Description

[Technical field]

[0001] The invention relates to a method for measuring the surface of an optical lens, in particular for performing a centering measurement of an aspheric lens, in which the reflection of the lens surface is measured by means of an optical measuring device, in which an aperture is used that limits the optical measuring radiation, which is blurred and imaged as a light spot on an optical detector of the measuring device.

[0002] During the manufacture of aspheric lenses, manufacturing errors or tolerances may cause deviations from the desired aspheric shape, which must be identified for quality assurance reasons. This can be done by a non-contact centering measurement, which can detect the apex offset and axial tilt of the aspheric lens. In a centering measurement, the position of the center of curvature (apex) of the spherical part of the aspheric lens surface is first determined, followed by a measurement in the aspheric part, i.e. in the peripheral area outside the apex. [Background technology]

[0003] Patent document 1 describes a method for determining the position of the axis of symmetry of an aspheric lens surface relative to a reference axis. In this method, the position of the center of curvature of the spherical part of the lens surface is first determined, and then the wobbling movement of the aspheric part is measured by rotation of the lens about the rotation axis in order to determine the position of the axis of symmetry of the aspheric lens surface relative to the rotation axis. This measurement is performed by means of an autocollimator through reflection on the lens surface to be measured.

[0004] From DE 10 200 03 133 A1 a method and a measuring device are known for contactless measurement of angles or angle changes at an object, which can in particular also be used for measuring aspheric lens or mirror surfaces. In this method, an optical measuring beam is generated by means of a (finite size) light source (extended light source), which illuminates through a diaphragm acting as a field diaphragm and is detected by means of an optical detector after reflection at the surface to be measured. Here, the illuminated diaphragm is blurredly imaged as a light point on the optical detector via an optical system. By determining the position of the diaphragm image relative to a reference position, the angle between the measuring surface and the optical axis at the intersection of the measuring surface and the optical axis of the measuring beam can be determined.

[0005] From US Pat. No. 5,399,543 a method is known for measuring an optical lens arrangement based on the back reflection of different lens surfaces, in which relatively complex imaging optics are used to generate different image planes at the estimated positions of the centres of curvature of the lens surfaces.

[0006] US Pat. No. 5,399,633 shows a method for measuring optical lenses with an autocollimation telescope, in which the individual lens surfaces are measured one after the other by focusing on the respective corresponding lens surface. From US Pat. No. 5,399,633 a further method for measuring optical lens surfaces is known, in which a separate imaging optics is used for each lens surface, where each lens surface is measured respectively only from the side towards which the lens is directed. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] European Patent No. 1918687 [Patent Document 2] European Patent No. 1636542 [Patent Document 3] European Patent No. 3037800 [Patent Document 4] International Publication No. 2014 / 114444 [Patent Document 5] U.S. Patent No. 7,286,212 Summary of the Invention [Problem to be solved by the invention]

[0008] The object of the invention is to provide a method for measuring optical lens surfaces of an optical lens arrangement, which allows for a rapid measurement of at least two lens surfaces of the optical lens arrangement without the use of complex imaging optics. [Means for solving the problem]

[0009] The above problem is solved by a method according to patent claim 1. Advantageous configurations of the method are the subject matter of the dependent claims or will become apparent from the following description and the exemplary embodiments.

[0010] In the proposed method, an optical measurement beam is generated using a light source with a magnitude (finite size), collimated through an optical device, directed onto a first lens surface of the object of measurement of an optical lens device, and detected with at least one spatially resolved optical detector after reflection at the first lens surface of the object of measurement. In the method, an aperture is used between the light source and the optical detector to limit the measurement beam, which acts as a field stop causing vignetting, and is blurredly imaged by the measurement beam as a light spot (hereinafter referred to as the first light spot since it reflects at the first lens surface of the object of measurement) on the optical detector and detected with the optical detector.

[0011] Here, the light source with the dimension is configured to provide a uniform surface brightness, for example by using one or more LEDs preceded by a diffuser, or preceded by a condenser, or preceded by a combination of a diffuser and a condenser. The spatially resolving optical detector can be formed, for example, by a CCD or CMOS area sensor. Of course, other spatially resolving optical detectors may also be used. It is also possible to arrange several spatially resolving detectors side by side.

[0012] The aperture limiting the optical measurement beam is preferably formed by a diaphragm with a fixed or adjustable diaphragm opening. This aperture can also be formed by a suitable lens mount of the lens of the optical device. The aperture is arranged at a suitable position in the beam path of the measurement beam, preferably between the optical device for collimation, preferably an autocollimation optics, and an optical lens device with a measurement lens surface or in said optical device, in order to obtain a vignetting effect that generates a blurred light spot on the detector. The optical lens device can refer to a single lens, for example an aspheric lens with a one-sided or two-sided aspheric surface, i.e. one aspheric surface and one spherical surface, or an aspheric lens with two aspheric lens surfaces, or an arrangement of several lenses spaced apart from each other.

[0013] Here, in the proposed method, at the same time as the detection of the first light point, at least one second light point, resulting from the reflection of the measurement beam at a further lens surface (hereinafter referred to as the second lens surface) of the optical lens arrangement, is also detected by means of an optical detector. This second light point also results from an aperture limiting the measurement beam acting as a field stop causing vignetting, which aperture is imaged (after reflection at the second lens surface) on the optical detector. The intensity distributions of both light points detected simultaneously by means of the optical detector are then evaluated in order to determine the angle between the first lens surface and the optical axis of the measurement beam at the intersection of the first lens surface with the optical axis of the measurement beam, and the angle between the second lens surface and the optical axis of the measurement beam at the intersection of the second lens surface with the optical axis of the measurement beam, respectively. This angle can be determined, for example, in the same way as described in the above-mentioned patent document 2. Moreover, the respective angles at the measurement positions (intersections of the optical axis of the measurement beam and the lens surface) can be determined by displacing the respective light points with respect to a known reference position (zero position) on the optical detector. The vignetting effect of the aperture limiting the measurement beam makes it possible to determine the position of the light spot very accurately via the intensity distribution produced by the light spot, which has a V-shape along a line passing through the center of the light spot.

[0014] It has been found that in the proposed method, when a measuring device with a vignetting field stop is used, the light spot generated by a further second lens surface on which the measuring beam strikes after passing through the first lens surface can also be detected together with the first light spot on the detector and evaluated accordingly. The second lens surface can refer, for example, to the rear lens surface of the lens or to the inner lens surface (in the case of cemented lenses). Both light spots on the detector must be sufficiently distinguishable or separable from one another in order to determine the angle. If this is not the case during the measurement, the lens arrangement or the lens is displaced perpendicularly to the optical axis of the measuring beam until both light spots are sufficiently distinguishable or separable. Here, the distance of the displacement is preferably known and taken into account during the evaluation. Thus, the proposed method allows at least two lens surfaces to be measured simultaneously in terms of their angle with respect to the optical axis of the measuring beam at the respective measuring positions without the use of additional or complex optical devices. Therefore, when measuring an aspheric surface, the exact position of the aspheric surface, in particular the tilt and lateral offset, can be determined by measuring the aspheric surface portion in a single rotational measurement of at least 180 degrees (or correspondingly multiple measurements at different positions on one half of the aspheric surface).

[0015] The proposed method will now be explained in more detail on the basis of example embodiments with reference to the drawings. [Brief description of the drawings]

[0016] [Figure 1] FIG. 2 is a schematic diagram of an exemplary optical setup that can be used in carrying out the proposed method. [Diagram 2] FIG. 13 is an example of the use of the optical configuration described above when measuring an aspheric surface by the proposed method. [Diagram 3] FIG. 11 is a diagram of a further example of implementing the proposed method using such an arrangement. [Figure 4] FIG. 13 is an example of measuring an aspheric surface by the proposed method. [Diagram 5] FIG. 13 is a further example of measuring an aspheric surface by the proposed method. [Figure 6a] FIG. 13 is an example of measuring a double aspheric surface by the proposed method. [Figure 6b] FIG. 13 is an example of measuring a double aspheric surface by the proposed method. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0017] In the proposed method, the blurred exit aperture is imaged on the detector as an image of the limiting aperture or aperture stop of the optically effective system used in the method by at least two at least partially reflecting lens surfaces, which can be configured as spherical or aspherical. In order to ensure this kind of effect of the aperture stop as a field stop, the aperture stop is positioned at a suitable position in the beam path of the measurement beam. Here, the emitted measurement beam bundle or object beam bundle is not stationary, as is the case, for example, in conventional autocollimators with a line of sight. Rather, a light source with a magnitude (object illumination surface) serves as an object, which preferably has at least the same dimensions or the same size as the detection surface of the detector, so that each object illumination point corresponds to a corresponding conjugate image point position, which corresponds to a tilt α of the corresponding main beam through the center of the aperture stop. This angle of the main beam is determined and evaluated upon detection. It is particularly preferred that the object illumination surface is selected to be larger than the detection surface. The in principle blurred imaging of the exit pupil of the entire system on the detector allows various blurred exit apertures on the detector to be evaluated in the longitudinal direction from different lens surfaces (usually with different radii). Here, it is only important that the beams at the lens surfaces have different (separable) surface inclination angles. Thus, more than two surfaces can also be detected. To optimize the measurement, it is advantageous to adjust the beam divergence by suitable auxiliary lenses or, usually, by suitable adjustment of optical devices such as lenses, so that the light spots on the detector are sufficiently separated from one another. Here, the light spots are therefore adjusted via optical elements, possibly even via adjustment of the aperture size, in such a way that both light spots do not overlap and can be separated from one another.

[0018] Firstly, FIG. 1 shows a schematic diagram of an exemplary configuration of a measurement system that can be used in the proposed method. The figure shows a (finite size) light source (diffuse light source) 21, also called surface emitter in this patent application, which in this example is formed by illuminating a diffuse light disk 21b from a radiation source 21a, for example an LED. A measurement beam bundle 28 emerging from this light source 21 is irradiated into the optical system via a beam splitter 22, deflected by 90 degrees and illuminates through an aperture stop 24. At the lens 10 to be measured, the beam bundle is reflected and, after passing through the beam splitter 22 again via the objective lens 23, is imaged on a photosensitive detector 25. Here, reflections occur not only on the lens surfaces of the lens 10 facing the measurement system, but also on those that do not face the measurement system. If the lens surfaces at the intersection with the optical axis 29 of this measurement system are not exactly at an angle of 90 degrees to this axis, a light point that is deviated from the intersection of the detector 25 with the optical axis 29 is imaged on the detector 25 for each of the lens surfaces. Here, as described in more detail in, for example, US Pat. No. 5,399,633, the light spot is caused by the vignetting effect of the aperture stop 24 and allows accurate measurement of the angular deviation from the 90 degree angle.

[0019] Furthermore, Fig. 2 shows a schematic diagram of the measurement of, in this case, a spherical lens 10, with the measuring device of Fig. 1, also referred to as sensor 20 in this and the following examples. In front of this sensor 20, in these examples, an auxiliary lens 50 for beam shaping is placed. For the measurement, the lens 10 to be measured is preferably placed on a lens receiver 30, for example a ring edge or a three-point support, on a rotating stage 40, which can be rotated around a rotation axis 41 (rotation C). Here, the lens receiver 30 also allows for a precise referencing of the sensor 20 to the lens to be measured, for example by placing a polished sphere of known precision on the lens receiver 30 and calibrating the device with this sphere. Thus, with the sensor 20 or a similar suitable instrument, a reference axis can be established and the lens support can be referenced as a reference surface of the lens to be measured.

[0020] Fig. 3 shows a detailed view of the measurement of a spherical lens 10 with a sensor 20 according to the proposed method. In this figure, the sensor 20 is shown with a light source 21 having a size, a detector 25, a beam splitter 22, as well as an objective lens 23 and an aperture stop 24, which are shown in a schematic manner. Also visible in this figure is an auxiliary lens 50. The spherical lens 10 to be measured is placed on a lens receiver 30 on a rotating stage. The spherical lens 10 comprises an apex 11, an edge region 12, a rear apex 13, a rear edge region 14 and a cylindrical edge 15. In this example, it is shown that the centres of curvature 16 of the front lens surface and the centres of curvature 17 of the rear lens surface lie on the optical axis 18 of the lens, which does not coincide with the axis of rotation 41 of the rotating stage. In FIG. 3, two main beams 26, 27 are shown, which after reflection at both lens surfaces appear as blurred light points on the detector 25 through the aperture stop 24 due to vignetting. In this case, the main beam 26 reflected at the front lens surface with vignetting appears as a light point 26a on the detector, and the main beam 27 reflected at the rear lens surface with vignetting appears as a blurred light point 27a on the detector. The upper left part of FIG. 3 exemplarily shows the field of view or detection plane of the detector 25 with both light points 26a, 27b, which are clearly separated laterally so that both can be detected and evaluated simultaneously. In this way, the inclination of the lens surface at the intersection of the respective lens surface with the optical axis of the measurement beam can be determined. When the turntable rotates, both light points describe a circle or ellipse with a corresponding diameter, which corresponds to the real (front surface, surface 1) or virtual (rear surface, surface 2) surface inclination angle. This circle is shown in the field of view of the detector 25 in the upper left part of Fig. 3. Here, the meridional and sagittal beam paths can be evaluated separately. The actual surface tilt angle of the rear surface can be back-calculated through the measured optical effects (radius, refractive index, thickness) of the first (front) surface via an optical design program. The angular deviation can also be interpreted as the respective lateral offset of the meridional and sagittal centers of curvature with respect to the axis of rotation.

[0021] In the case of non-paraxial measurements, where the measurement beam strikes the first lens surface with an angle of incidence ε (vertical = angle with respect to the incidence normal), non-linear deviations in the refraction of the beam at the first lens surface become significant and may have to be taken into account when calculating the surface inclination angle of the second lens surface. In the case of centering measurements of the individual real surface inclinations, the calculation of the inclination angle of the rear surface is always influenced by the inclination of the lens surface in front of it and is calculated exactly by back-calculating the beam path. In the case of non-paraxial applications of the beam path, even an incidence angle of 15 degrees leads to errors that lead to deviations of 5%. For larger angles of incidence of 35 to 45 degrees, this error can be in the range of 200 to 300 percent.

[0022] Here, we idealize this application and roughly consider and construct the relationships. 1. Lens axis or aspheric axis = rotation axis 2. A reflective (second) surface directly behind the first surface 3. Reflected beam at the same position as the incident beam The change in angle ε' of the refracted beam is given by the refractive index n and the angle of incidence ε itself.

number

[0023] This results in a correction factor K=n×cosε / cosε′, which is used in determining the actual surface tilt angle of the second surface, which can be provided, for example, by a LUT (Look Up Table) as a function of the angle of incidence.

[0024] In the measurement of aspheric surfaces, the method is used in a similar manner, as shown in the example of the measurement of a one-sided aspheric surface in FIG. 4. An aspheric surface has not one but many centers of curvature, all of which lie on an axis passing through the apex (point of rotational symmetry). The aspheric surface has an aspheric axis. This axis can be inclined with respect to a reference axis, and the apex or the point of rotational symmetry of this surface can be offset laterally from the reference axis. Furthermore, FIG. 4 shows an example of a one-sided aspheric surface 10 (one side aspheric, the other side spherical) with a apex 11 or central area of ​​the front surface of the aspheric surface and a peripheral area 12 of the front surface of the aspheric surface. The sensor 20 is shown in this figure simply with only the detector 25 and the auxiliary lens 50. Both measurement beams reflected at the front and rear sides of the aspheric surface 10, i.e. the main beam 26 subjected to vignetting and the main beam 27 subjected to vignetting, are likewise shown exemplarily. The aspheric surface 10 has an apparent center of curvature 19 at the edge and a center of curvature 16 at the apex. The aspheric axis 18 is likewise indicated in the figure. Here too, the reflection of the measuring beam on both lens surfaces of the aspheric surface results in two light spots on the detector 25, so that both lens surfaces can be measured not only in the central area but also in the peripheral area. This is again shown by FIG. 5 using the same reference numbers on the basis of an example in which two sensors 20a and 20b are used so that measurements can be made simultaneously in the center of the aspheric surface 10 and in the peripheral area of ​​the aspheric surface 10.

[0025] In the case of double aspheric surfaces (with an aspheric surface on the rear side as well), a central measurement alone is not sufficient to determine the aspheric axis accurately. Here, measurements at different positions are necessary, as shown diagrammatically by Figures 6a and 6b with an additional measurement of the rear surface at the periphery using a third sensor 20c. It is also preferable to carry out simultaneous measurements of both the anterior and posterior surfaces at the periphery using only one sensor, for example sensor 20b, according to the proposed method.

[0026] The internal centering error of an aspheric lens can be determined, for example, by the following steps. a) Using a sensor 20 or a similar suitable instrument that can measure the center of curvature of the lens surfaces, for example without changing the auxiliary lens, displacing the instrument along the optical axis, or changing the focus, reference the haptic (or lens receiver 30) as the reference surface of the lens, using a sphere of the preferred approximate average radius of both lens surfaces. b) Measure the position of the center of curvature of the spherical portion of the surface of the aspheric lens to be measured. c) Measuring the aspheric portion outside the apex (for both lens surfaces simultaneously). d) Determine the inherent internal centering error by calculating both measurements together using the known design data (radius, thickness, and index of refraction, etc.).

[0027] The configuration of the optical device for measuring the lens surface may differ from the example of FIG. 1, for example by exchanging the detector and light source, using a different beam splitter, placing the vignetting-causing field stop in a different position, or other modifications, as long as the vignetting effect of the field stop or aperture stop that creates a blurred light spot on the detector remains guaranteed. [Explanation of symbols]

[0028] 10 Lenses 11. The apex or central area of ​​the front surface of the lens 12. Lens anterior peripheral area 13. Apex or central region of the posterior lens surface 14 Posterior lens peripheral area 15 Cylindrical edge of lens 16 Center of curvature of first surface 17 Center of curvature of second surface 18 Lens optical axis 19 Apparent center of curvature of aspheric surface at the edge 20 Sensors 20a First Sensor 20b Second Sensor 20c Third Sensor 21 Light sources with size 21a Radiation source 21b Diffusion optical disk 22 Beam splitter 23 Objective Lens 24 Aperture Diaphragm 25 Photosensitive Detector 26 Main beam vignetting after reflection at the front lens surface 26a Light spot on detector 27 Main beam undergoes vignetting after reflection at the rear lens surface 27a Light spot on detector 28 Measurement beam flux 29 Optical axis 30 Lens holder 40 Rotating table 41 Rotation axis 50 Auxiliary Lens

Claims

1. A method for measuring an optical lens surface, in particular for performing centering measurements on aspheric lenses, comprising the steps of: an optical measurement beam (26, 27, 28) is generated using a light source (21) having a size, collimated via an optical device (23), directed to a first lens surface of the measurement object of the optical lens arrangement (10), and detected after reflection at the first lens surface of the measurement object using at least one spatially resolved optical detector (25); Between the light source (21) and the optical detector (25), an aperture (24) is used to limit the optical measurement beam (26, 27, 28), the aperture (24) acting as a field stop causing vignetting, through which the measurement beam (26, 27, 28) is blurredly imaged as a first light spot (26a) on the optical detector (25) and detected by means of the optical detector (25), from the intensity distribution of the first light spot (26a) on the optical detector (25) and the displacement of the first light spot (26a) relative to a reference position (zero position), an angle between the first lens surface of the object and the optical axis of the measurement beam at the intersection of the first lens surface of the object and the optical axis of the measurement beam is determined, At the same time as detecting the first light spot (26a), at least one second light spot (27a) is also detected by means of the optical detector (25), which is obtained by reflection of the measurement beam (26, 27, 28) on a second lens surface of a measurement object of the optical lens arrangement (10), 4. The method of claim 3, wherein an angle between the second lens surface of the object and the optical axis of the measurement beam (26, 27, 28) at an intersection of the second lens surface of the object and the optical axis of the measurement beam is determined from the intensity distribution of the second light spot (27a) on the optical detector (25) and the displacement of the second light spot (27a) relative to a reference position (zero position).

2. 2. The method of claim 1, characterized in that if the first light spot (26a) and the second light spot (27a) on the optical detector (25) are not sufficiently distinguishable to determine the angle, the optical lens arrangement (10) is displaced perpendicular to the optical axis until the light spots (26a, 27a) are sufficiently distinguishable.

3. 3. The method according to claim 1 or 2, characterized in that as the aperture (24) for limiting the measuring beam (26, 27, 28) a diaphragm is used having a fixed or adjustable diaphragm opening.

4. 4. The method according to claim 1, wherein an autocollimation optical system is used as the optical device (23).

5. The method according to any one of claims 1 to 4, characterized in that the light source (21) having said magnitude is provided by an illumination surface having at least the size of a detection surface of the optical detector (25).

6. A method for performing centering measurements on an aspheric lens as an optical lens arrangement (10), the aspheric lens comprising an aspheric first lens surface having a central spherical portion and a contiguous aspheric portion, 6. The method according to claim 1, wherein first a position of the center of curvature of the spherical portion of the aspheric first lens surface is determined, and then an angle between the first lens surface and the optical axis of the measurement beam (26, 27, 28) and an angle between a second lens surface of the aspheric lens and the optical axis of the measurement beam (26, 27, 28) are simultaneously measured at one or more positions of the aspheric portion.

7. 7. The method according to claim 1, characterized in that the optical lens arrangement (10) is placed on a rotation stage (40) for measurement and rotated about an axis of rotation (41) to determine the angle between the lens surface to be measured and the optical axis of the measurement beam (26, 27, 28) at a plurality of positions on a circular line or a part of a circular line around the axis of rotation (41).

8. 8. The method according to claim 7, characterized in that the optical lens arrangement (10) is rotated at least 180 degrees around the axis of rotation (41) for the measurement.

9. For the measurement, a number of sensors (20a, 20b, 20c) are used, for each of the sensors (20a, 20b, 20c), an optical measurement beam is generated using a light source having a size, collimated via an optical device, directed onto the lens surface of the object of measurement with an optical lens device, and detected after reflection at the lens surface of the object of measurement with at least one spatially resolved optical detector, an aperture is used between the light source and the optical detector to limit the optical measurement beam, the aperture acting as a field stop causing vignetting, through which the measurement beam is blurred and imaged as a light spot on the optical detector, 7. The method according to claim 1, characterized in that with each of the sensors (20a, 20b, 20c) an angle is determined between the lens surface of the object and the optical axis of the respective measurement beam at another location of the lens surface of the object.

10. 10. The method according to claim 9, characterized in that one of the sensors (2a, 20b, 20c) is used to determine angles at a position in a vertex region of the lens surface to be measured and one or more other sensors (2a, 20b, 20c) are used to determine angles at one or more positions in an edge region of the lens surface to be measured.

11. 11. The method according to claim 1, wherein in the case of non-normal incidence of the optical measurement beam (26, 27, 28) on a first lens surface of the measured object, the angle between the second lens surface of the measured object and the optical axis of the measurement beam (26, 27, 28) at the intersection of the second lens surface of the measured object and the optical axis of the measurement beam is determined using a correction factor K which takes into account the refraction of the measurement beam (26, 27, 28) at the first lens surface of the measured object.

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