Method for mounting or determining the position of a semi-finished optical product on a manufacturing mandrel during the manufacture of a spheric or aspheric lens

Abstract

In a method for mounting or for determining the position of a semi-finished optical product (3) on a manufacturing mandrel (2) during the manufacture of a spheric or aspheric lens, the position of the semi-finished product (3) on the manufacturing mandrel (2) is determined using an optical method and / or an adjustment of the semi-finished product (3) on the manufacturing mandrel (2) is carried out with the aid of an optical method, in which a transparent optical auxiliary element (5) with a polished upper surface and a lower surface which is adapted to the second surface of the semi-finished product (3) is applied with a medium (7) in between for index adaptation to the second surface of the semi-finished product (3), and is trans-illuminated with at least one optical beam (6) in such a way that the optical beam is reflected in each case partially on the upper surface of the auxiliary element (5) and on the first surface of the semi-finished product (3), and at least the beam portion which is reflected by the first surface of the semi-finished product (3) is detected using a spatially resolving optical detector.

Description

Technical application area

[0001] The present invention relates to a method for mounting or fixing or for determining the position or orientation of an optical semi-finished product on a manufacturing mandrel during the production of a spherical or aspherical lens, in which the semi-finished product has a spherical or aspherical first surface that is finished on one side and polished, and a second surface opposite it which still needs to be ground and polished to complete the lens, and is placed on the manufacturing mandrel and cemented with the first surface.

[0002] In industry, the use of high-performance, high-precision, and lightweight optomechanical imaging systems is becoming increasingly important. The use of aspherical surfaces is playing a growing role, as it allows for a reduction in the number of lenses and a significant minimization of dimensions and weight.

[0003] When manufacturing an aspheric surface, there is a risk of decentering, manifesting as both a vertex displacement and tilting (vertex offset and axial tilt), and these can occur completely independently of each other. Therefore, aspheric semi-finished products must be subjected to specific centering rules during the grinding and polishing process and positioned accordingly on the production spindles (mandrels) to prevent rejects during production. A subsequent centering process, as is common in the production of spheres, is then no longer possible.

[0004] In addition to surface form defects, industrial manufacturing requires a fast and easy-to-use characterization and modification of internal and external centering errors for the perfect assembly of lens multiples. These requirements result in increased demands on measuring instruments and on the in-situ qualification of samples under production conditions. In the production of aspheric lenses, centering errors > 0.5' sometimes lead to high rejection rates. A precise measurement of the internal centering error of aspheric lenses is not yet available. Interferometric measurements are also unsuitable for centering measurement. Point sensor-based sensors are unsuitable because they have too many degrees of freedom, leading to numerous errors. Measurement with point sensors is only possible with well-centered systems.

[0005] The most precise possible mounting of semi-finished aspheres onto manufacturing mandrels is a challenge that must be overcome to produce precise aspheres with centering errors of less than 30 arcseconds and a lateral offset L of less than 5 µm. Tactile instruments can be used for this purpose, but this requires significant investment, accuracy is limited, and measurement times for high spatial resolution are very long. Furthermore, tactile instruments can scratch optical components. The centering dimension itself cannot be measured from both sides in a single setup. The asphere must be fixed in a special holder and exchanged or rotated. This leaves behind specific clamping errors that can exceed 1 µm, resulting in tilt angle errors of more than one arcminute for small radii < 10 mm.

[0006] The centering process for a spherical lens is typically performed using a clamping and centering machine. Clamping is achieved via precisely manufactured bells on accurately aligned spindles. The lens centers itself through a self-centering process if the tangential angles at the lens edge are sufficiently large. If the centering process is very successful, the optical axis generated by the two centers of curvature is parallel to the cylinder axis and coincides with the cylinder axis after centering. If this is not the case, for example, with a meniscus lens with a large radius or a homocentric lens, the mandrel centering method is generally used. The lens is then adjusted and bonded to a mandrel using an optical measuring instrument such as an autocollimator.

[0007] Generally, the reference for aligning a spherical-aspherical lens is based on a mechanical surface such as the edge cylinder. In most cases (especially with a short edge cylinder), the spherical surface (Base A) and the edge cylinder (Base B) are used, as shown in Figure 1 The aspherical axis then has an inclination and a lateral displacement L from the reference axis at the vertex of the asphere. This is defined in ISO 10110-6 and is also referred to as "external centering error" because it relates to external dimensions and / or external surface references.

[0008] If the spherical-aspherical lens is shifted and tilted so that the aspherical axis coincides with the reference axis (reference axis = aspherical axis) (cf. Figure 2The centering error is reduced to a single aspect: the lateral displacement of the center of curvature C1 relative to the aspherical axis, or the equivalent tilt angle σ, in the same way as for a spherical lens. This is also referred to as the "internal centering error." The internal centering error of an aspheric lens is thus an inherent displacement of the center of curvature of the spherical surface relative to the aspherical axis of the second surface. This internal centering error is an inherent error of the lens itself. State of the art

[0009] The schematic sequence of the manufacturing process of double aspheres is shown in Figure 3 This is shown. First, a glass blank 1 is applied to a manufacturing mandrel 2 ( Fig. 3a ). The glass blank 1 is ground and polished on the first aspherical surface ( Fig. 3b The polishing process in the polishing machine is not shown here. The next step involves initial edge finishing ( Fig. 3c). The semi-finished product 3 with the finished ground and polished first aspherical surface is cemented onto the mandrel 2 ( Fig. 3d ) and then ground and polished on the second aspherical surface ( Fig. 3e The polishing process is not shown. Finally, the final edge finishing takes place ( Fig. 3f ).

[0010] In the production of double aspherics, the grinding of the first aspheric surface is part of the process. Fig. 3b This is relatively straightforward, as it is measured on mandrel 2, allowing for quality control. The crucial step takes place during the re-clamping or re-cementing process ( Fig. 3dThe second aspheric surface is ground and polished, and the edge is subsequently machined. Precise alignment on mandrel 2 before grinding the second aspheric surface is essential. This is currently done using measuring probes. Once mounted, the second aspheric surface is ground and polished, and the edge is then finished. When aspheric surface 4 is removed from mandrel 2 for final measurements, it cannot be remounted. This means that the aspheric surface cannot be corrected and, if the specifications are not met, is rejected. Automatic re-correction after grinding the second aspheric surface is also not possible. Typically, due to inadequacies in the re-cementing process from one lens surface to the other, or the lack of intermediate results from a centering error measurement, this manufacturing technique results in centering errors of approximately 3-10 arcminutes in the final product.

[0011] With the current state of the art, the production of precision aspheres using conventional grinding and polishing techniques with an undefined cutting edge involves mounting a cut or milled glass block or a pre-pressed blank (so-called blank) onto a mandrel and initially processing it in the grinding machine. In the preliminary process, the sphere, or in the case of aspheres, the best-fit sphere (BFS), is first ground using a diamond-tipped cup tool or a diamond pellet-coated forming tool. After grinding the first surface, the rim cylinder is then typically edge-centered in the same machine using another machining spindle.

[0012] The mandrel with the test specimen is removed from the machine, the lens is de-blocked, and then re-blocked onto the mandrel on the first, ground surface to grind the second side. Depending on the requirements, the lens edge can be centered during the re-blocking process (preferably with thermal wax) using, for example, a mechanical or optical dial gauge. The machining is then carried out as on the first side, and the lens edge, which has an allowance (approx. 0.2 to 0.02 mm), is recentered again. This ensures that the aspheric surface aligns precisely with the cylinder edge.

[0013] The subsequent polishing process is carried out in the CNC polishing machine. Because this side of the asphere is perfectly aligned with the spindle, experience has shown that the polishing process yields an optimal aspherical surface with a perfectly centered asphere axis across all areas of the asphere—center, zone, and edge. Since the CNC control precisely follows the path of the asphere on this centered surface, eliminating any decentering or eccentricity, the exact same footprint is expected across the entire surface, ensuring a flawless polishing process. Therefore, there are no polishing errors due to differing footprints or varying dwell times, resulting in a perfectly rotationally symmetrical surface, even at the vertex of the lens.

[0014] In the next step, the lens is repositioned so that the aspherical surface is facing downwards. It may be coated with a protective lacquer to prevent damage. The main task now is to optimally align the already finished and, if applicable, lacquered lower surface with the mandrel axis so that the lower aspherical axis is parallel and coincident with the mandrel axis and thus with the machining axis. In the simplest case, this adjustment is achieved by pressing the heated lens onto the ring edge using the warm wax putty applied to the heated mandrel (approx. 60°C), and mechanically centering the rim cylinder. However, significant deviations are to be expected here due to the influences listed below: Mandrel runout: 1 to 20 microns (from high quality to standard) Mandrel concentricity: 2 to 50 microns (from high quality to standard) Paint variation (first-order analysis): 10 to 20 microns Cylinder roundness: 1–5 microns Cylinder roughness: 1–10 microns Rq Temperature differences during blocking (20°–65°C) Dial gauge is usually not accurate enough

[0015] When blocking the semi-finished product onto the mandrel, various influences become apparent which significantly affect the centering process and centering accuracy, and can thus lead to an internal centering error in the production of the asphere.

[0016] DE 2113142 A1 describes a method for determining the position or orientation of a lens in a lens holder, in which the position or orientation can be determined using an optical method and thus an adjustment of the lens can also be carried out.

[0017] EP 1997585 A1 deals with the attachment of a shell as a processing device to a spectacle lens. The correct attachment position of the shell to the lens is determined using an optical illumination and imaging system.

[0018] The object of the present invention is to avoid or at least reduce the internal centering error during the manufacturing process of spherical-aspherical lenses or double-sided aspherical lenses. Description of the invention

[0019] The problem is solved by the method according to claim 1. Advantageous embodiments of the method are the subject of the dependent claims or can be found in the following description or the exemplary embodiments.

[0020] The proposed method serves to mount or fix an optical semi-finished product onto a manufacturing mandrel during the production of a spherical or aspherical lens. It can also be used to determine the position or orientation of the optical semi-finished product on a manufacturing mandrel during the production of a spherical or aspherical lens. In the proposed method, the optical semi-finished product already has a spherical or aspherical first surface that has been ground and polished on one side, and a second surface opposite this, which still needs to be ground and polished to complete the lens. In the method, the semi-finished product is placed onto the manufacturing mandrel with its first surface facing down, preferably adjusted in the process, and then cemented or blocked. The method thus involves re-cementing the semi-finished product on the mandrel as described above. 3D figureThe process is characterized by the fact that the adjustment, position, or orientation of the semi-finished product is checked or determined using a special optical method, and / or the adjustment (in particular the centering process mentioned above) is carried out using this special optical method. In this optical method, a transparent optical auxiliary element with a polished upper surface and a lower surface adapted to the second surface of the semi-finished product is applied to the second surface of the semi-finished product via an index-adapting medium, in particular an immersion oil, as an intermediate layer. The optical auxiliary element is illuminated with an optical beam (or multiple optical beams) such that the optical beam is partially reflected at both the upper surface of the auxiliary element and the first surface of the semi-finished product.At least the portion of the beam reflected from the first surface of the semi-finished product, preferably both reflected beam portions, is / are detected with at least one spatially resolving optical detector.

[0021] This procedure makes it possible to obtain a reflection of the measuring beam or optical beam from the first surface of the semi-finished product, even though the second surface is not yet polished. This allows the semi-finished product to be aligned on the production mandrel before cementing, based on the captured reflections. The production mandrel is preferably rotated during the measurement. The use of the reflection from the already finished spherical or aspherical first surface is only made possible by the attached auxiliary element and the medium used for index adjustment. Alternatively, the measurement can be performed not for alignment, but simply to determine the position or orientation of the semi-finished product, particularly its lower surface, in order to then correctly machine the second surface based on this information.

[0022] For a flat second surface of the semi-finished product, an auxiliary element is preferably used, which is designed as a plane-parallel plate or as a concave or convex lens with a flat lower surface. For a convex or concave second surface of the semi-finished product, an auxiliary element is preferably used that has a concave or convex lower surface and a flat, concave, or convex upper surface.

[0023] In the proposed method, the auxiliary element is preferably designed to completely or almost completely cover the second surface of the semi-finished product. Preferably, the measurements with the optical beam or with two optical beams to detect the reflected beam components are carried out both in the center and at the edge of the semi-finished product. Brief description of the drawings

[0024] The proposed method is explained in more detail below using exemplary embodiments in conjunction with the drawings. These show: Fig. 1 an exemplary representation of the external centering error of a spherical-aspherical lens with an inclined aspherical axis and lateral offset L at the vertex of the aspherical surface (vertex); Fig. 2 an exemplary representation of the internal centering error of a spherical-aspherical lens with the aspherical axis as the reference axis and the offset of the spherical center of curvature; Fig. 3 the schematic sequence of the manufacturing process for the production of a double aspherical lens; Fig. 4 a representation of the determination of the orientation of an optical semi-finished product on a manufacturing mandrel according to an exemplary embodiment of the proposed method with a plane-parallel plate as an optical auxiliary element; Fig. 5 two exemplary representations of the use of differently designed auxiliary elements depending on the shape of the semi-finished product in the proposed method; and Fig.6 Two exemplary representations of the measurement of the semi-finished product at two different positions during the design of the . Figure 5 . Ways to implement the invention

[0025] The proposed method is used in the manufacturing process of a spherical or aspherical lens, as exemplified in the introductory description in conjunction with Figure 3 was described.

[0026] Figure 4 Figure 1 shows an exemplary representation of determining the orientation of the optical semi-finished product 3 on a manufacturing mandrel 2 according to an embodiment of the proposed method. In this example, the semi-finished product has a flat second surface. A plane-parallel plate is used here as the optical auxiliary element 5. Figure 4The figure shows the semi-finished product 3 with a spherical first surface (in this example) on which a suitable lacquer has been applied. The manufacturing mandrel 2, onto which this semi-finished product 3 is placed and to which it is to be cemented, is only indicated in the figure. The plane-parallel plate 5, which has a polished upper surface, is then placed on the unworked, in this case flat, second surface. An immersion oil (not shown in the figure) for index matching is located between the underside of this auxiliary element 5 and the second surface of the semi-finished product 3.

[0027] For irradiation with the optical beam, this and other possible embodiments of the method preferably employ the so-called vignetting field-stop technology (VFS or V-SPOT), which allows both reflections—the reflection from the upper surface of the auxiliary element 5 and the reflection from the first surface of the semi-finished product 3—to be detected on a single detector without changing any focus. This technology makes it possible to precisely determine the local, meridional slope error in the zone or at the edge of the aspherical surface and, together with the centering deviation of the vertex, to determine the aspherical axis and thus the internal centering error.

[0028] By using this VFS technology with the corresponding, in Figure 4The measuring device EP shown in the figure produces the two reflections depicted in the upper right of the figure, for which the beam path of the correspondingly extended measuring beam 6 is indicated in the figure. The current position or orientation of the semi-finished product 3 can then be determined from the position of these reflections.

[0029] An exemplary implementation of fixing this semi-finished product 3 onto the manufacturing mandrel 2 is briefly explained in the following example. Example:

[0030] 1. The test specimen is a semi-finished product consisting of a sphere or asphere polished on one side and a flat ground back; 2. Application of an immersion oil, corresponding to the refractive index of the semi-finished product (1.4 - 1.6), to the back of the semi-finished product; 3. Placement of a high-quality, plane-parallel glass plate of appropriate size as an auxiliary element; 4. Adjustment of the EP (ELWIMAT) measuring device via angle α according to the Design of Experiments (DoE), so that a reflection from the front and back can be evaluated simultaneously on the detector; 5. Adjustment of the semi-finished product by appropriately positioning it on the mandrel (smallest centering circles); 6. Fixing of the semi-finished product; if necessary, filling of the gap with adhesive; 7. Allowing to cure or cool and checking the result at room temperature; 8. Removal of the flat plate, cleaning of the surfaces; 9. Grinding of the semi-finished product on the second side and then polishing (according to known manufacturing processes).

[0031] In cases where the second surface, which has not yet been fully machined, is curved, for example having a convex or concave shape, an auxiliary element 5 is used which has a correspondingly complementary shape on its underside.

[0032] To ensure that the measuring beams emanating from the measuring device strike the already processed first surface of the semi-finished product 3 as close as possible to the edge, the auxiliary element 5 can be designed with a correspondingly concave or convex upper surface, as shown in the Figures 5 and 6schematically indicated. These figures show, as examples, semi-finished products 3 with a concave and a convex (polished) aspherical first surface. The second surface is simply ground flat. The immersion oil 7 between the auxiliary element 5 and the semi-finished product 3 is indicated in these figures. The two auxiliary elements 5 are plano-concave and plano-convex elements, respectively, polished on both sides, which also function as auxiliary lenses. The use of such a plano-convex or plano-concave lens as an auxiliary element 5 instead of a plano-parallel plate is recommended for curved or negative (hollow) surfaces or for measurements at the very edge (with poor mandrels) in order to obtain better alignment of the positions of both reflections with good angular resolution.

[0033] Preferably, in the proposed method for adjustment or measurement, either two of the EP measuring devices are used for measurement at two different positions, both in the center and at the edge of the semi-finished product 3, or measurements are taken successively at the two different positions with one EP measuring device. This is in Figure 6 schematically represented.

[0034] The vignetting field-stop method preferably used in the proposed method, as also known from EP 1636542 B1, is a measuring principle that uses vignetting as a physical effect to measure inclination or tilt angles on surfaces by means of reflected light. The measuring beam is a quasi-parallel beam set of infinite light sources on a widely spread illuminated field near the focal point of a collimating lens. A beam splitter directs the reflected light onto a camera or a suitably spatially resolved detector. The exit pupil of this collimator system is projected into the image field behind the focal point of the collimator, in addition to a plane mirror or lens positioned in front of the system. This is called the exit port, and since the camera is located in the focal plane, there is a blurred image of the exit port on the camera.This unfocused exit port eliminates the problem of losing focus, as with a classic AC (autocollimator). This physical effect of vignetting no longer restricts the field of view. Therefore, only the camera or detector size, which preferably corresponds to the size of the illuminated field, limits the field of view. This field of view depends solely on the collimator's focal length and the size of the unfocused exit pupil (V-SPOT) on the camera. The size of the V-SPOT then depends on the distance from the exit pupil to the plane mirror, or, in the case of a curved surface, on the auxiliary lens and the radius of curvature of the DUT.

[0035] By using this method instead of a classic AC, vignetting is no longer a problem, but a simple solution for measuring surface profiles and centering deviations on non-spherical surfaces with high resolution. Reference symbol list

[0036] 1 Glass blank 2 Manufacturing mandrel 3 Semi-finished product 4 Asphere 5 Optical auxiliary element 6 Measuring beam 7 Immersion oil EP Measuring device C Centers of curvature R Radii

Claims

1. A method for mounting or fixing or for determining the position or orientation of a semi-finished optical product (3) on a manufacturing mandrel (2) during the manufacture of a spherical or aspherical lens, in which the semi-finished product (3) has a ready ground and polished spherical or aspherical first surface on one side and a second surface lying opposite the latter, which still has to be ground and polished for the completion of the lens, and is placed with the first surface on the manufacturing mandrel (2) and cemented, wherein the position or orientation of the semi-finished product (3) on the manufacturing mandrel (2) is determined with an optical process and / or an adjustment of the semi-finished product (3) on the manufacturing mandrel (2) is carried out with the aid of an optical process, in which a transparent optical auxiliary element (5) with a polished upper surface and a lower surface adapted to the second surface of the semi-finished product (3) is applied onto the second surface of the semi-finished product (3) with an interposed medium (7) for the index adaptation and is thus irradiated with at least one optical beam (6), in such a way that the optical beam (6) is partially reflected at the upper surface of the auxiliary element (5) and partially at the first surface of the semi-finished product (3) and at least the beam portion reflected by the first surface of the semi-finished product (3), preferably both reflected beam portions, are detected with at least one spatially resolving optical detector.

2. The method according to claim 1, characterised in that, in the case of a plane second surface of the semi-finished product (3), the auxiliary element (5) is designed as a plane-parallel plate or as a concave or convex lens with a plane lower surface.

3. The method according to claim 1, characterised in that, in the case of a convex or concave second surface of the semi-finished product (3), the auxiliary element (5) has a correspondingly concave or convex lower surface and a plane, concave or convex upper surface.

4. The method according to any one of claims 1 to 3, characterised in that the auxiliary element (5) is designed in such a way that it completely or almost completely covers the second surface of the semi-finished product (3), and that measurements with the optical beam (6) or with two of the optical beams (6) for detecting the reflected beam portions are carried out both in the centre and at the edge of the semi-finished product (3).

5. The method according to any one of claims 1 to 4, characterised in that, for the performance of the optical method, use is made of a measuring device (EP), in which the optical beam (6) is produced by an extended light source and is collimated via an optical arrangement before striking the semi-finished product (3), wherein an aperture limiting the optical beam (6) is placed between the light source and the optical detector, which aperture acts as a vignetting field diaphragm and through which the beam (6), after reflection at the upper surface of the auxiliary element (5) and at the first surface of the semi-finished product (3), is imaged out-of-focus as a light spot or light spots on the optical detector and is detected with the optical detector.

6. The method according to claim 5, characterised in that a diaphragm with a fixed or adjustable diaphragm opening is used as the aperture limiting the beam (6).

7. The method according to any one of claims 5 or 6, characterised in that the extended light source is provided with a light area which has at least the size of a detection area of the optical detector.

8. The method according to any one of claims 1 to 7, characterised in that the semi-finished product (3) with the manufacturing mandrel (2) is rotated about the axis of rotation of the manufacturing mandrel (2) during the measurement or the performance of the optical method.

Images