JP2010060493A - Method of evaluating shape of aspheric lens - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method of evaluating the shape of an aspheric lens for accurately evaluating the aspheric shape in a short time without complicating an interferometer and without increasing cost. <P>SOLUTION: A system S for evaluating the shape of the aspheric lens irradiates a reference plane 5a with the optical component of a part of parallel measuring light, and irradiates an aspheric plane 7a to be inspected of the aspheric lens 7 with the remaining optical component via a null lens 6. An interference fringe is formed by making return measuring light that is reflected on the aspheric plane 7a to be inspected and returned via the null lens 6 interfere with return reference light that is reflected on the reference plane 5a and returned. The shape of the aspheric plane 7a to be inspected is evaluated based on the interference fringe. The null lens 6 is formed of a single lens, the first lens plane 6a has an aspheric shape substantially similar to a standard aspheric shape of the aspheric plane 7a to be inspected, and the second lens plane 6b is a flat plane perpendicular to the lens optical axis 6c of the null lens. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a method for evaluating the shape of an aspherical lens that determines whether or not the aspherical shape of an aspherical lens is acceptable using an interferometer having a reflective null lens.

  Imaging lens systems used in imaging optical systems such as digital cameras are highly demanded for miniaturization and compactness along with improvement in optical performance. In recent years, aspheric lenses have been frequently used in imaging lens systems in order to satisfy such demands. Yes. A mold press lens made of a glass material is known as an aspheric lens. When the shape error of the aspherical surface of the aspherical lens is large, it is impossible to obtain desired aberration characteristics as the entire imaging lens system. Therefore, it is necessary to accurately measure the aspherical shape of the aspherical lens after manufacture, and to remove the one having a large shape error.

  Conventionally, for measuring the aspherical shape of an aspherical lens, a contact-type measuring device that measures the shape of the aspherical surface by bringing a contactor into contact with the aspherical lens (for example, a form-talissurf manufactured by Taylor Hobson, Inc. Matsushita Electric Industrial Co., Ltd. UA3P) is used. The shape measurement using such a surface shape measuring apparatus has a problem that it takes time to measure the shape of the entire aspheric surface. Therefore, it is conceivable to measure the shape of the aspherical surface using an interferometer such as a Fizeau interferometer that is frequently used for measuring the shape of the spherical lens.

  A measuring method for measuring the shape of an aspheric surface using an interferometer is disclosed in Patent Documents 1 and 2. In the method of measuring an aspherical shape disclosed in Patent Document 1, a part of light from a light source is reflected by a reference surface, and another part of light is reflected by a test aspherical surface via an aspherical wave generating means. The measurement light reflected by the test aspheric surface is returned to the aspherical wave generation means, and the interference light is formed by causing the reference light reflected by the reference surface to interfere with the measurement light from the aspherical wave generation means. The present invention relates to an aspheric shape measuring method for measuring the shape of a test aspheric surface by observing, and is characterized in that the aspheric wave generating means and the test aspheric surface are brought into contact with each other. A null lens is used as the aspherical wave generating means, and as shown in FIG. 6 of the document 1, a combined lens composed of a biconvex lens, a cemented lens of a biconcave lens, and a biconvex lens as a null lens system. Is used. By bringing the null lens into contact with the test aspheric surface, it is possible to accurately determine the positions of the null lens and the test aspheric surface in the optical axis direction.

The aspherical shape measurement method disclosed in Patent Document 2 is a method that employs an electronic moire method, and is based on interference between the test light reflected by the aspherical surface of the workpiece to be measured and returned again through the null lens and the reference light. The interference fringe image is detected by the two-dimensional image detection device, the error of the null lens is evaluated in advance, and a pattern corresponding to the error is converted into reference image data and written in the reference image memory in advance. The obtained interference fringe image is subjected to image calculation based on the reference image data, and the fringe pattern displayed on the television monitor represents the shape error of the aspherical surface of the pure workpiece to be measured. This avoids a decrease in accuracy in determining whether the workpiece to be measured is good or bad due to null lens design and production errors. As the null lens, a combined lens composed of a plurality of lenses is used as described in FIG.
JP 09-329427 A JP 10-132531 A

  In the method of measuring the aspherical shape of an aspherical lens using an interferometer that uses a null lens, the null lens is designed and produced at a wavelength order or less in order to generate an aspherical wave that matches the aspherical surface of the aspherical lens by the null lens. It is necessary to do in. Therefore, the design and production of a null lens is not easy and the cost is high. In addition, if the null lens is not perfect, the inspection operator will judge the quality of the aspheric surface to be examined by looking at the fringe pattern of the interference fringes distorted due to the error of the null lens, and accurate shape evaluation will not be possible. It will be possible.

  Conventionally, a null lens as an aspherical wave generating element used for measuring an aspherical shape by an interferometer is a combined lens composed of a plurality of lenses. In the case of a combination lens, there is a problem that the cost increases because it is necessary to manufacture each of the constituent lenses of the null lens with high accuracy. Further, even if each constituent lens is manufactured with high accuracy, if each constituent lens cannot be assembled with high accuracy, the quality of the test aspherical shape cannot be evaluated with high accuracy. Therefore, it is necessary to accurately position each constituent lens in the optical axis direction and to assemble them in a state where there is no eccentricity (axial deviation, tilt).

  In the measurement method disclosed in Patent Document 1, since the null lens and the test aspheric surface are brought into contact with each other, the positional accuracy of the null lens and the test aspheric surface in the optical axis direction can be improved. However, since a null lens is composed of multiple lenses, it is necessary to manufacture each component lens with high accuracy, and there is no deviation in the distance between the axes of each component lens, and no eccentricity (axial deviation, tilt). It is difficult to assemble with. Therefore, distortion appears in the fringe pattern of the interference fringes due to the manufacturing error and assembly error of the constituent lenses, and there is a possibility that the quality of the test aspheric surface cannot be accurately determined.

  In the measurement method disclosed in Patent Document 2, the distortion of the fringe pattern of the interference fringes caused by the error of the null lens is removed by image processing. For this purpose, it is necessary to evaluate the error of the null lens in advance, convert the pattern according to the error into reference image data and store it in the reference image memory, and image calculation for removing the error is required. is necessary. Therefore, there is a problem that the device configuration becomes complicated and the cost becomes high.

  An object of the present invention is to propose a shape evaluation method for an aspheric lens that can accurately evaluate an aspheric shape in a short time without complicating the interferometer and increasing the cost.

  In order to solve the above-described problems, the present invention irradiates a part of measurement light onto a reference surface, and applies another part of the measurement light to an aspherical lens to be measured via an aspherical wave generating element. The measurement light return measurement light that irradiates the test aspheric surface, reflects off the test aspheric surface, and returns via the aspheric wave generating element, and returns the measurement light return reference light reflected on the reference surface. In the shape evaluation method of the aspheric lens that evaluates the shape of the test aspheric surface based on the interference fringe, the aspheric wave generating element is a null lens made of a single lens, One first lens surface of the null lens is an aspheric surface having a shape substantially similar to the design aspheric surface of the test aspheric surface, and the other second lens surface of the null lens is perpendicular to the lens optical axis of the null lens. It is a flat surface. It is desirable to use parallel light as the measurement light.

  In the aspherical lens shape evaluation method of the present invention, a reflective null lens system comprising a single lens is used as the aspherical wave generating element. Therefore, unlike a null lens composed of a plurality of lenses, a single null lens need only be manufactured with high precision. It can be manufactured at low cost, and the influence on the fringe pattern of interference fringes due to manufacturing errors of the constituent lenses can be reduced.

  Further, since there are no assembly errors such as misalignment of the distance between the axes and eccentricity that occur when assembling a plurality of constituent lenses, the influence on the fringe pattern of the interference fringes caused by the assembly error of the null lens can be reduced.

  In addition, since one lens surface of a null lens consisting of a single lens is aspherical and the other lens surface is a plane perpendicular to the lens optical axis, the null lens can be mounted on the interferometer with high accuracy using the plane lens surface as a reference. As a result, it is possible to suppress the assembling error of the null lens.

  Therefore, according to the aspherical lens evaluation method of the present invention, the interference fringes are displayed as images, the parallel state of the displayed interference fringes is visually confirmed, and the suitability of the aspherical lens to be tested is accurately determined. It is possible to judge well.

  Here, in the method for evaluating the shape of the aspheric lens according to the present invention, the interference fringes are analyzed, and the PV value of the wavefront aberration of the return measurement light (the maximum value and the minimum value of the shape error in the test aspheric surface). And the calculated PV value is compared with a predetermined selection reference value to determine the suitability of the aspherical lens to be tested.

  In this way, as compared with the case where the degree of bending of the fringe pattern is determined by visual observation, the determination criterion for suitability can always be kept constant, and the acceptability can be determined quickly.

  The method for evaluating the shape of an aspherical lens according to the present invention provides an aspherical error value (aspherical surface) calculated based on a measurement value obtained when the aspherical surface of the aspherical lens is measured by a surface shape measuring device. The difference between the design value and the actual shape) and the PV value of the wavefront aberration of the return measurement light obtained from the test aspheric surface is obtained. Based on the correlation, the suitability of the test aspheric surface is determined. The selection reference value for determination is preset.

  According to the measurement by the inventors of the present invention, the aspheric error value calculated based on the measured value of the aspheric surface by the surface shape measuring device, and the PV of the wavefront aberration of the return measurement light obtained using the interferometer. It was confirmed that there was a significant correlation between the values. Therefore, if the selection reference value is set based on this correlation, the pass / fail of the aspheric shape can be evaluated by the interferometer using the null lens with the same accuracy as the quality determination of the aspheric surface by the shape measurement by the surface shape measuring device. it can.

  Next, the method for evaluating the shape of an aspheric lens according to the present invention can be used to evaluate the aspheric shape of an aspheric mold lens that has been press-molded. The aspherical surface of the mold press lens is a contact type due to Tarisurf, etc. due to the lens surface shrinking due to the cooling of the mold press when releasing from the mold press mold, etc. A shape defect that cannot be detected by the shape measuring apparatus may occur. Such a shape defect faithfully appears in the fringe pattern of the interference fringes by the interferometer. For example, fine flaws appear as missing portions of interference fringes. In addition, surface information such as micro waviness, roughness, and presence / absence of grinding traces resulting from processing of the mold and mold press molding also appears in the fringe pattern of the interference fringes. Therefore, the evaluation method of the present invention is suitable for use in the evaluation of an aspheric mold press lens.

  In the shape evaluation method of an aspheric lens according to the present invention, an aspheric interference fringe is generated by an interferometer using a reflective null lens composed of a single lens as an aspheric wave generating element. One lens surface of the single lens constituting the null lens is an aspheric surface, and the other lens surface is a plane perpendicular to the lens optical axis. According to the present invention, compared to the conventional case where an aspherical surface shape evaluation is performed using a null lens composed of a plurality of constituent lenses as an aspherical wave generating element, the interferometer is not complicated and incurs high costs. It becomes possible to evaluate the aspherical shape with the same accuracy as the shape measurement by the surface shape measuring device.

  Embodiments of an aspheric evaluation system for an aspheric lens to which the method of the present invention is applied will be described below with reference to the drawings.

  FIG. 1 is a schematic configuration diagram mainly showing an optical system of an aspherical shape evaluation system according to the present embodiment. The optical system of the aspherical surface shape evaluation system S includes a light source 1 that emits coherent light with high coherence. The laser light emitted from the light source 1 is converted into divergent light L1 by the objective lens 2, and the collimator lens 3 is Then, it becomes parallel measurement light and enters the beam splitter 4 having a configuration in which an optical path separating optical element, for example, a prism is bonded.

  The parallel measurement light L2 incident on the beam splitter 4 is transmitted and incident on the reference plate 5 having the reference surface 5a at a right angle. Some of the light is reflected by the reference surface 5a and the remaining light is transmitted. The transmitted light component that has passed through the reference plate 5 is incident on the null lens 6 and is transmitted therethrough to irradiate the test aspheric surface 7a of the aspheric lens 7 to be inspected held by a jig (not shown).

  The light incident on the reference plate 5 is reflected by the reference surface 5a of the reference plate 5 to return to the beam splitter 4 via the same optical path as the return reference light L3. To the light receiving surface of the image sensor 9 such as a CCD.

  On the other hand, a part of the transmitted light component incident on the test aspheric surface 7a of the aspheric lens 7 is reflected by the test aspheric surface 7a and becomes the return measurement light L4 via the same optical path. 4 is reflected at right angles by the beam splitter 4 and enters the light receiving surface of the image sensor 9 via the imaging optical system 8 via the same optical path as the return reference light L3. An image of the test aspheric surface 7a is formed on the surface. In this way, the return measurement light L4 and the return reference light L3 enter the light receiving surface of the image sensor 9. Interference fringes are formed by the interference of these lights.

  The interference fringes formed on the light receiving surface of the image sensor 9 are converted into digital image information in the image processing apparatus 10 to which the photoelectric signal of the image sensor 9 is input. The image processing apparatus 10 is mainly configured by a computer, and known interference fringe analysis software is installed in the computer. The interference fringe image is displayed on the monitor screen 11 by the interference fringe analysis software, and wavefront analysis of the interference fringes is performed.

  Here, the reflection-type null lens 6 has an aspherical shape substantially similar to the standard aspherical shape of the aspherical surface 7a to be tested of the aspherical lens 7, and an aspherical wave corresponding to the to-be-tested spherical surface 7a. This is an aspherical wave generating element. The null lens 6 is a single lens, the first lens surface 6a on the parallel measurement light incident side is an aspheric surface that is substantially similar to the standard aspheric surface of the test aspheric surface 7a, and the other second lens surface 6b is the null lens. It is a plane perpendicular to the lens optical axis 6c. The distance between the axes of the null lens 6 and the aspheric lens 7 is set so that the measurement light is perpendicularly incident on the test aspheric surface 7 a of the test aspheric lens 7.

  The measurement light that has passed through the null lens 6 is reflected by the test aspheric surface 7 a, returns to the null lens 6 again, and passes through the null lens 6. The measurement light is reflected by the test aspheric surface 7a and generates wavefront aberration. However, when the measurement light is transmitted through the null lens 6 twice, the aberration is removed and the measurement light is directed to the image sensor 9. Therefore, when the test spherical surface 7a has a standard aspherical shape, the interference fringes formed by the interference between the return measurement light and the return reference light are parallel vertical stripes, and the shape error of the test aspherical surface 7a is an interference fringe. Appears as a bend.

(Correlation between measurement by surface shape measuring device and interference fringe shape)
The present inventors investigated the correlation between the measured value by the surface shape measuring device such as Talysurf and the interference fringes using the aspherical shape evaluation system S having the above configuration. As a result, it was confirmed that there is a significant correlation between the two, and it is possible to accurately determine whether or not the aspheric shape of the aspheric lens is acceptable by observing the shape of the interference fringes. An example of the measurement results performed by the present inventors will be described below.

  First, the aspherical surface of the aspherical lens to be inspected was measured by a surface shape measuring device (Tarisurf). Table 1 shows an example of the aspheric error value (F ′ data) calculated based on the Talysurf measurement result.

  Using the above aspheric shape evaluation system S, interference fringes obtained from aspheric surfaces of the same aspheric lens were observed. FIGS. 2A to 2C show images of interference fringes obtained for “OK product”, “limit NG product”, and “NG product”. In the “OK product”, the interference fringes were straight. In the “limit NG product”, the peripheral portion bends with the central portion of the interference fringes straight. In the “NG product”, the whole bends even when the central portion of the interference fringes is straight. Therefore, when the interference fringe image is visually observed and the shape is entirely linear, it can be determined that the aspherical shape error is a acceptable product within the allowable range, and only the peripheral portion is bent. It can be determined that the aspherical shape error is in the vicinity of the limit of the allowable range, and if the shape is entirely bent, it can be determined that the aspherical shape error exceeds the allowable range. Therefore, it can be seen that the pass / fail judgment of the aspherical shape can be easily performed in a short time except in the vicinity of the limit of the allowable range.

(Correlation between measured value F ′ based on surface shape measurement and entire interference fringe PV value)
As described above, when the interference fringe image is visually observed, it is expected that a pass / fail judgment cannot be made if the aspherical shape is in the vicinity of the allowable error. Therefore, the inventors of the present invention use commercially available interference fringe analysis software, and the relationship between the entire PV value (λ) (interferometer) and F ′ (μm) (Tarisurf) of the interference fringe obtained from the aspheric surface I investigated. FIG. 3 is a graph showing the results, in which the vertical axis represents the entire surface PV (λ) and the horizontal axis represents F ′ (μm), and each measured value is plotted. In this graph, an ideal straight line L by simulation is drawn with a one-dot chain line.

  As can be seen from this graph, the square dot representing “OK judgment” gathers at the lower left of the graph, and the round dot representing “NG judgment” gathers at the upper right of the graph, and either “OK judgment” or “NG judgment” Triangular dots representing “gray determination” that cannot be clearly discriminated gather in the middle of them, and the relationship between the Talysurf measurement value F ′ and the interference fringe total height PV value is almost linear as a whole. Although the slope is slightly different from the ideal straight line L from the simulation, it can be seen that there is a high correlation between the overall PV value and F ′. The difference from the ideal straight line L is considered to be due to an error component other than the shape error component.

  Therefore, in the image processing apparatus 10, the entire interference fringe PV value (λ) is calculated by the interference fringe analysis software, and the calculated value is compared with a preset sorting reference value. If it is smaller than that, the OK determination can be performed. That is, when the selection standard of the Talysurf measurement value F ′ is “0.155 μm”, for example, the entire surface PV value (λ) corresponding to this value is obtained from the correlation shown in FIG. It is possible to store in the image processing apparatus 10 as a selection reference value, and to perform pass / fail determination of the aspheric surface 7a of the aspheric lens 7 with high accuracy based on the selection reference value.

(Other embodiments)
In the above example, the test lens is a convex test aspheric surface 7a, and the null lens 6 is also provided with a convex aspheric surface. As shown in FIG. 4, when the test aspheric surface 7B of the test lens 7A is concave, the null lens 6A provided with the convex aspheric surface 6B is arranged behind the focal position BF of the measurement light, What is necessary is just to make it make a measurement light inject perpendicularly with respect to the to-be-tested aspherical surface 7B.

It is a schematic block diagram mainly showing the optical system of the aspherical shape evaluation system according to the embodiment of the present invention. It is an image which shows three examples of an interference fringe. It is a graph which shows the correlation with the measurement value F 'based on the whole surface PV value of an interference fringe, and surface shape measurement. It is explanatory drawing which shows the position of the null lens in case a test aspherical surface is concave.

Explanation of symbols

S aspherical shape evaluation system 1 light source 2 objective lens 3 collimator lens 4 beam splitter 5 reference plate 5a reference surface 6 null lens 6a first lens surface (aspherical surface)
6b Second lens surface (plane)
6c Lens optical axis 7 Aspheric lens 7a Aspheric surface to be tested 8 Imaging optical system 9 Imaging element 10 Image processing device 11 Monitor screen

Claims (6)

Irradiating a part of the measurement light onto the reference surface, irradiating the other part of the measurement light onto the aspherical surface of the aspherical lens to be measured through the aspherical wave generating element,
Interference is caused by interfering between the return measurement light of the measurement light reflected by the aspheric surface to be tested and returned again through the aspherical wave generating element, and the return reference light of the measurement light reflected by the reference surface and returned. Forming stripes,
In the shape evaluation method of the aspheric lens that evaluates the shape of the test aspheric surface based on the interference fringes,
The aspherical wave generating element is a null lens composed of a single lens,
One first lens surface of the null lens is an aspheric surface having a shape substantially similar to a standard aspheric surface of the test aspheric surface,
2. The shape evaluation method for an aspheric lens, wherein the other second lens surface of the null lens is a plane perpendicular to the lens optical axis of the null lens. The shape evaluation method for an aspheric lens according to claim 1,
A method for evaluating the shape of an aspherical lens, wherein parallel measurement light is used as the measurement light. In the shape evaluation method of the aspherical lens according to claim 1 or 2,
Image display of the interference fringes;
A method for evaluating the shape of an aspherical lens, wherein the shape of the interference fringe displayed on the image is visually confirmed to determine whether the aspherical shape of the aspherical lens is acceptable or not. In the shape evaluation method of the aspherical lens according to any one of claims 1 to 3,
Image analysis of the interference fringes to calculate a PV value of the wavefront aberration of the return measurement light,
A method for evaluating the shape of an aspheric lens, wherein the calculated PV value is compared with a predetermined selection reference value to determine whether the aspheric shape of the aspheric lens is acceptable or not. In the shape evaluation method of the aspherical lens according to claim 4,
An aspheric error value calculated based on a measurement value obtained when the test aspheric surface of the aspheric lens is measured by a surface shape measuring device, and a wavefront aberration of the return measurement light obtained from the test aspheric surface The correlation with the PV value of
A method for evaluating the shape of an aspheric lens, wherein the selection reference value for determining pass / fail of the test aspheric shape is preset based on the correlation.   An aspherical mold characterized by evaluating the aspherical shape of a molded aspherical molded lens using the method for evaluating the shape of an aspherical lens according to any one of claims 1 to 5. Lens shape evaluation method.