TESTING DEVICE AND METHOD FOR MEASURING THE HOMOGENEITY OF AN OPTICAL ELEMENT

Abstract

A testing device for measuring the homogeneity of an optical element in a beam path of the testing device and related method. The testing device includes an interferometer, which comprises a monochromatic light source, an adjustable objective, a reference surface associated with a surface of the optical element to be tested or an interferometry surface, and an analysis unit for the interference of the wave fronts of the light reflected by the reference surface and the associated surface of the optical element to be tested or of the interferometry surface. The testing device and method facilitate highly precise measurement of the homogeneity of an entire optical element—not merely individual surfaces. The method is suitable for the highly precise measurement of plastic lenses or other injection molded components for refractive laser eye surgery for example.

Claims

1.-42. (canceled)

43. A method for measuring homogeneity of an optical element having at least one non-planar surface according to principles of an interferometer, the method comprising: generating interference of wavefronts of reflected light from a reference face that is not part of the optical element to be tested and an associated surface of the optical element to be tested; arranging the surface of the optical element to be tested, which is associated with the reference face, in a beam path of the interferometer in such a way that light used for measurement must pass the optical element to be tested in order to be reflected at the surface associated with the reference face.

44. The method as claimed in claim 43, further comprising compensating a monochromatic aberration by a specified geometry of the optical element to be tested.

45. A method for measuring homogeneity of a an optical element having at least one non-planar surface according to principles of an interferometer, the method comprising: generating interference of wavefronts of the reflected light from a reference face and an interferometry surface; arranging the optical element to be tested in a beam path of the interferometer in such a way that light used for measurement passes through the optical element to be tested, both before and after it has been reflected at the interferometry surface; and compensating a monochromatic aberration occurring as a result of a specified geometry of the optical element.

46. The method as claimed in claim 44, further comprising, for the purposes of compensating the monochromatic aberration, arranging an optical compensation element in the beam path at a smallest possible distance from the optical element to be tested.

47. The method as claimed in claim 45, further comprising, for the purposes of compensating the monochromatic aberration, arranging an optical compensation element in the beam path at a smallest possible distance from the optical element to be tested.

48. The method as claimed in claim 43, further comprising: first, measuring an ideal optical reference element and recording data of which as a reference measurement, next, measuring the optical element to be tested, the data of which are recorded as measurement of the optical element to be tested; and last, subtracting the data of the reference measurement from the data of the measurement of the optical element to be tested.

49. The method as claimed in claim 45, further comprising: first, measuring an ideal optical reference element and recording data of which as a reference measurement, next, measuring the optical element to be tested, the data of which are recorded as measurement of the optical element to be tested; and last, subtracting the data of the reference measurement from the data of the measurement of the optical element to be tested.

50. The method as claimed in claim 43, further comprising positioning the optical element to be tested with a defined deviation and non-concentrically in relation to a test apparatus which implements the principle of the interferometer.

51. The method as claimed in claim 45, further comprising positioning the optical element to be tested with a defined deviation and non-concentrically in relation to a test apparatus which implements the principle of the interferometer.

52. The method as claimed in claim 43, further comprising subtracting low-frequency homogeneity defects to render high-frequency homogeneity defects identifiable.

53. The method as claimed in claim 45, further comprising subtracting low-frequency homogeneity defects to render high-frequency homogeneity defects identifiable.

54. The method as claimed in claim 43, further comprising separating the components of defects of the homogeneity of the optical element caused by the two surfaces and the volume of the optical element by virtue of two further measurements being implemented according to the principles of interferometry, including in a first additional measurement, assigning a first new reference face to a first surface which represents an original light-entry surface of the optical element to be tested, in order to represent the surface defects of this first surface, in a further additional measurement, rotating the optical element to be tested through 180° and, once again, assigning a reference face to a second surface of the optical element to be tested, in order to represent the surface defects of this second surface, combining the first additional measurement and the further additional measurement by calculation with the original measurement in order to constitute the homogeneity of the volume of the optical element to be tested.

55. The method as claimed in claim 45, further comprising separating the components of defects of the homogeneity of the optical element caused by the two surfaces and the volume of the optical element by virtue of two further measurements being implemented according to the principles of interferometry, including in a first additional measurement, assigning a first new reference face to a first surface which represents an original light-entry surface of the optical element to be tested, in order to represent the surface defects of this first surface, in a further additional measurement, rotating the optical element to be tested through 180° and, once again, assigning a reference face to a second surface of the optical element to be tested, in order to represent the surface defects of this second surface, combining the first additional measurement and the further additional by calculation with the original measurement in order to constitute the homogeneity of the volume of the optical element to be tested.

56. The method as claimed in claim 55, further comprising utilizing the principles of a Fizeau interferometer

57. The method as claimed in claim 56, further comprising utilizing the principles of a Fizeau interferometer

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0060] The present invention should now be explained in more detail by example embodiments. In the drawing:

[0061] FIG. 1a depicts a first exemplary embodiment of a test apparatus according to the invention;

[0062] FIG. 2a depicts an interferogram generated by operation of the first test apparatus;

[0063] FIG. 1b depicts a second exemplary embodiment of a test apparatus according to the invention;

[0064] FIG. 2b depicts an interferogram generated by operation of the second test apparatus;

[0065] FIG. 1c depicts a third exemplary embodiment of a test apparatus according to the invention;

[0066] FIG. 1d depicts a fourth exemplary embodiment of a test apparatus according to the invention;

[0067] FIG. 3 depicts an optical element to be tested;

[0068] FIGS. 4a to 4c each depict different setups of an optical element to be tested and its compensation element;

[0069] FIGS. 5a and 5b depict the use of a test apparatus according to the invention for separating the components contributing to the homogeneity of the optical element to be tested;

[0070] FIGS. 6a to 6c depict different types of optical elements and their compensation elements.

DETAILED DESCRIPTION

[0071] FIG. 1a illustrates a first example embodiment of a test apparatus 1 according to the invention for measuring the homogeneity of an optical element 10. The test apparatus 1 contains an interferometer 2, the latter comprising a light source 3, which emits monochromatic light in the form of the laser beam which is coupled into the beam path 5 of the interferometer 2 via a beam splitter 4, an objective 6 which is adjustable and exchangeable and which contains a reference face 7 which in this case is arranged as last surface in the beam path 5 of the interferometer 2 and which is assigned to a surface of the optical element 10 to be tested, and an analysis unit 8 in the form of a CCD camera for the interference of the wavefronts of the light reflected by the reference face 7 and the associated surface of the optical element 10 to be tested. In this case, the positions of the light source 3 and the analysis unit 9 are interchangeable. Thus, there is equivalence between the interfering wavefronts transmitted back from the reference face 7 and the surface of the optical element 10 to be tested being guided through the beam splitter 4 to the analysis unit 8 or being deflected to an analysis unit 8 by the beam splitter 4 after the light source 3 has emitted the laser light through the beam splitter 4 to the optical element 10 to be tested. The interferometer may contain further elements, in particular also phase shifters for moving optical units and optical units for imaging the interfering wavefronts onto the CCD camera.

[0072] In the present case, the optical element 10 to be tested is a contact element for refractive surgery, i.e., a special planoconcave lens element made of plastic which must be produced with great precision in respect of its optical homogeneity and which is generated by application of an injection molding method. In this arrangement in the beam path 5 of the test apparatus 1, the optical element 10 comprises a surface 12 that faces the test apparatus 1, and in this case the interferometer 2 in particular, and a surface 11 that faces away from the test apparatus 1. According to the invention, the reference face 7 is assigned to the surface 11 of the optical element 10 that faces away from the test apparatus. In the specific case, this means that the reference face 7 likewise has concave curvature, in correspondence with the concave surface 11 of the lens elements 10 to be tested that faces away from the test apparatus 1. The laser beam emanating from the light source 3 of the interferometer 6 therefore passes through the surface 12 of the lens element 10 to be tested that faces the test apparatus 1, furthermore passes through the volume 13 of the lens element 10, is reflected at the lower side of the side 11 of the lens element 10 that faces away from the test apparatus 1, once again passes through the volume 13 and the surface 12 of the lens element 10 to be tested that faces the test apparatus 1 in order to interfere with the part of the laser beam reflected at the reference face 7. The returning, interfering wavefronts are steered through the beam splitter 4 to the analysis unit 9, i.e., the CCD camera, and lead to an interferogram 14 at this point.

[0073] A corresponding interferogram 14, which is generated by operation of the first test apparatus 1 according to the invention when measuring the planoconcave lens element 10, is shown in FIG. 2a. The occurrence of a high spherical aberration is identifiable, and so the interference image in the interferogram 14 cannot be assessed by the naked eye or can only be assessed by a very experienced observer. In this case, this can usually only be evaluated reliably by an automated data analysis. In the case of very high spherical aberrations, the interference rings in one part of the interferogram attain such a high spatial frequency that they are no longer detectable (resolvable) even using a conventional CCD camera: An automated data analysis is no longer possible if, to this end, an appropriate high resolution of the interference image is no longer available, for example if the CCD camera has too few pixels. Then, very much outlay is required in order to obtain an appropriate resolution, i.e., an appropriate number of pixels.

[0074] FIG. 1b shows a second example embodiment of a test apparatus 1 according to the invention. With the exception of one detail, this second exemplary embodiment corresponds to the structure of the first, above-described exemplary embodiment of the test apparatus 1 according to the invention: It additionally comprises an optical compensation element 9, which is arrangeable (and arranged in this case) in the beam path 5 between the reference face 7 and the optical element 10 to be tested. Like the objective 6 and also the reference face 7, the optical compensation element 9 is exchangeable in such a way that it is possible to arrange in the beam path a compensation element 9 that fits to the optical element 10 to be tested in each case.

[0075] This optical compensation element 9 compensates one or more monochromatic aberrations due to the specified geometry of the surface 12 of the optical element 10 that faces the test apparatus. In the present case of this example embodiment, in which a planoconcave lens element 10 should be measured, the optical compensation element 9 is a planoconvex lens.

[0076] FIG. 2b now illustrates an interferogram 14 generated by operation of the second test apparatus 1. An interference image with regular straight fringes arises for an ideal optical element 10 (i.e., an optical element without defects or disturbances) as a result of an additional slight deviation from the concentricity between the test apparatus 1 and the optical element 10 to be tested, i.e., the planoconcave lens element in this case. In the case of deviations from an ideal optical element or reference element, i.e., if defects or disturbances occur (for example, bracing which is likewise optically effective), deviations 15 from the linearity of the interference fringes are identifiable in the interference image.

[0077] In order to render the measurement of the homogeneity even better evaluable, it is also possible in the example embodiment described here to initially carry out a reference measurement using an ideal optical element, i.e., an ideal lens element 10R in this case—with the same planoconvex lens as compensation element 9 that is subsequently used for measuring the lens element 10 to be tested. Then, the ideal lens element 10R is replaced by the lens element 10 to be tested, the latter is measured in the same way, and both measurements are subtracted from one another.

[0078] FIG. 1c shows a third example embodiment of a test apparatus 1 according to the invention, as an alternative to the second example embodiment. In this exemplary embodiment, the compensation element 9 is arranged in the beam path 5 directly downstream of the optical element 10 to be tested such that the surface 11 of the optical element 10 to be tested that faces away from the test apparatus and the surface of the compensation element 9 that faces the test apparatus are in contact over the entire area. Moreover, the surface 16 of the compensation element 9 that faces away from the test apparatus forms the interferometry surface to which the reference face 7 is assigned. Since such a surface 16 of the compensation element 9 that faces away from the test apparatus is freely selectable as a rule, it can for example be embodied in such a way that a planar reference face 7 can be used. If the optical element 10 to be tested contains a planar surface 12, the surface 16 of the compensation element will particularly advantageously for example likewise have a planar embodiment.

[0079] The optical element to be tested is arranged downstream of the test apparatus 1 in the beam path 5 such that light used to measure the optical element passes therethrough, as is also still the case for the compensation element 9, in order to be reflected at the surface 16 of the compensation element 9 that faces away from the test apparatus 1, i.e., at the interferometry surface. The light passes through the optical element 10 to be tested and through the compensation element 9 in such a way that there are no further interferences that are detectable by an analysis unit 8 than the interferences between the wavefronts of the light reflected at the interferometry surface 16 and at the reference face 7. These interferences provide information about the homogeneity of the optical element 10 to be tested since the light has passed through this element (forward and back) along its path to the interferometry surface. Disturbances and defects in the volume 13 or at the surfaces 11, 12 of the optical element 10 become noticeable by way of corresponding irregularities 15 in the interferogram 14, as already shown in FIG. 2b, and are easily visible on account of the use of the compensation element 9.

[0080] FIG. 1d shows a fourth example embodiment of a test apparatus 1 according to the invention, which in principle corresponds to the third example embodiment in terms of arrangement and function and the only difference is that in this case the reference face 7 is arranged downstream of the beam splitter 4. Nevertheless, completely comparable interferences between the light reflected at the surface 16 that faces away from the test apparatus 1, i.e., the interferometry surface, and the light reflected the reference face 7 become visible in the analysis unit.

[0081] FIG. 3 illustrates an optical element 10 to be tested, in this case a planoconcave lens element with two surfaces 11, 12 and the volume 13. If the light emitted by a test apparatus 1 now passes through a first surface 12 into the planoconcave lens element 10, passes through the latter and is reflected at the second surface 11, there is as a result a wave aberration W which is a function of the surface coordinates x, y perpendicular to the optical axis of the optical element to be tested, i.e., W(x, y), or else W(r, φ) if a description is implemented in polar coordinates r, φ:

W=A(n−1)+Bn+tΔn

Here moreover: [0082] A, B: are the respective deviation of the first 12 or second surface 11 from an ideal surface. A and B are likewise a function of the surface coordinates x, y (or of the polar coordinates r, φ); [0083] t: is the respective route (optical path), which extends perpendicularly or non-perpendicularly through the lens elements 10 depending on the position; [0084] n: is the refractive index; [0085] Δn: are the variations in the refractive index (likewise for the respective coordinates), which are an expression of the deviations of the homogeneity in the volume due to corresponding disturbances in the volume.

[0086] The result describes the deviation of the homogeneity of the optical element 10 to be tested, i.e., the lens element in this case which should be used as a contact element for laser eye surgery, from an ideal reference element. The influences of the deviations A, B of both surfaces 11, 12 and of the volume 13 Δn of the optical element 10 to be tested are measured in summary manner. The influence of the deviation B of the right-hand face 11, which adjoins the patient's eye during use in laser eye surgery and which is most critical during use, is however the greatest during a measurement using the method according to the invention. This applies, in particular, to the arrangement according to the invention as per FIG. 1a or 1b, in which this area 11 is used in reflection.

[0087] FIGS. 4a to 4c illustrate different setups, in each case of an optical element 10 to be tested, in this case a planoconcave lens element, and its compensation element 9. They show that it is particularly advantageous for example to arrange the compensation element 9, in this case a planoconvex compensation lens, as close as possible to the optical element 10 to be tested, as shown in FIG. 4c, because the spherical aberrations which arise at the two plane faces compensate one another almost exactly, and no defects are added at the other surfaces. For this reason, a residual error can be reduced to approximately 1/20th of the wavelength, and consequently has a negligible influence on the evaluation. By contrast, in FIG. 4a, in which work is carried out without compensation element 9, the defect of the plane face of the optical element 10 as the planoconcave lens element to be tested still exists. A significant defect likewise remains if compensation element 9 has a relatively large distance from the optical element 10 to be tested, as shown in FIG. 4b.

[0088] The geometry and arrangement of compensation element 9 and optical element 10 to be tested must be configured in such a way that there is an incidence that is as perpendicular as possible into the surface 11 of the optical element 10 that faces away from the test apparatus 1, at which surface the incident radiation should be reflected, so that the radiation takes the same path back.

[0089] In the case of lens elements 10 with a spherical configuration, the curvature of the surface at which the incident radiation should be reflected and the curvature of the associated compensation element 9 ideally have a common center.

[0090] FIGS. 5a and 5b show the use of a test apparatus 1 according to an example embodiment of the invention for separating the components of the two surfaces and the volume of the optical element 10 that contribute to disturbances in the homogeneity of the optical element 10 to be tested. To this end, there are two further (i.e., additional) measurements according to the original principles of the Fizeau interferometer:

[0091] In a first additional measurement, shown in FIG. 5a, a first new reference face 7′ is assigned to a first surface 12 which represents the original light-entry surface of the optical element 10 to be tested, in order to represent the surface defects of this first surface 12. In this case, the light used for the measurement is incident on this first surface of the optical element and reflected there, in order to interfere with the light reflected at the reference face. Therefore, it no longer passes the volume 13 of the optical element 10 to be tested. [0092] In a further additional measurement, depicted in FIG. 5b, the optical element 10 to be tested is rotated through 180° and a reference face 7″ is once again assigned, the latter being the reference face of a second surface 11 of the optical element 10 to be tested (and, in principle, corresponds to the reference face 7 which was used when measuring the summary homogeneity of the volume 13 and the two surfaces 11, 12 in the main method), in order to represent the surface defects of this second surface 11. In this case, too, the light used for the measurement is incident on this second surface 11 of the optical element and reflected there, in order to interfere with the light reflected at the reference face. It likewise no longer passes the volume 13 of the optical element 10 to be tested. [0093] Subsequently, these two additional measurements are subtracted from the original measurement (as obtained in the main method) in order to represent the homogeneity of the volume 13 of the optical element 10 to be tested.
For a greater accuracy, the subtraction of the measurements can contain additional scalings which take account of the optical paths illustrated in FIG. 3 and/or contain the refractive index.

[0094] Thus, if the accuracy of the measurement is important instead of a fast measurement of the (summary) homogeneity and if the influences of defects or disturbances in the volume of the optical element to be tested and surface defects of the optical element to be tested are required separately, this can be easily ascertained by way of the additional method steps described here.

[0095] Here, the arrangements of FIGS. 5a and 5b for the additional measurements of the surface defects of the two surfaces 11, 12 of the optical element 10 correspond to conventional interferometry arrangements. As shown here, when measuring a planoconcave lens element 10, a plane surface is used as a reference face 7′ for measuring the planar surface 12 and a spherical reference face 7″ is used for the spherical (concave) surface 11. Hence, in the equation for W specified above, it is possible to insert A and B and the homogeneity of the volume arises after reformulating the equation. In an alternative interferometer, the reference face can also be arranged downstream of the beam splitter, as illustrated in FIG. 1d.

[0096] Finally, FIGS. 6a to 6c show different types of optical elements 10 to be tested and their compensation elements 9 in a beam path 5 of a test apparatus 1.

[0097] For measuring the homogeneity of various other conventional lens elements 10, these are arranged with similar compensation lenses 9: This is illustrated in FIGS. 6a to 6c for a biconvex lens, a planoconvex lens and a meniscus-shaped lens. As already described above, all lens elements 10 to be tested and compensation lenses 9 are arranged such that they are located as close together as possible or, in the ideal case, in contact with one another. Furthermore, the second face of the compensation lens 9 is arranged such that it is approximately concentric with the second face of the lens element 10 to be tested such that the light is not refracted and deflected there. The two lenses from FIGS. 6a and 6c can be used in the arrangement as per FIG. 1b instead of the lenses 9, 10. The two lenses from FIG. 6b can be used in the arrangement as per FIGS. 1c, 1d. An advantage here is that the light from the interferometer is reflected at the surface of the optical element (10) that faces away from the test apparatus such that this face has a dominant component in the interferogram. If the interferometry face should alternatively be located in the compensation element, then the function of the lenses 9 and 10 can also be interchanged in FIGS. 6a, 6b, 6c.

[0098] The aforementioned features of the invention, which are explained in various example embodiments, can be used not only in the combinations specified in an exemplary manner but also in other combinations or on their own, without departing from the scope of the present invention.

[0099] A description of an apparatus relating to method features is analogously applicable to the corresponding method with respect to these features, while method features correspondingly represent functional features of the apparatus described.