JP2016109593A - Refractive index distribution measurement method, refractive index distribution measurement device, and optical element manufacturing method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To measure the refractive index of a lens to be inspected with high accuracy in a nondestructive way.SOLUTION: A lens 60 to be inspected is installed on a reference base 50 having the shape of a form at edges of the lens 60 to be inspected, and the transmission wavefront in a plurality of wavelengths at edges of the lens 60 to be inspected is measured. The refractive index at edges of the lens 60 be inspected is specified on the basis of the transmission wavefront in the plurality of wavelengths at edges of the lens 60 to be inspected and the refractive index of the reference base 50.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a refractive index measuring method and a refractive index measuring apparatus for measuring a refractive index of an optical element.

  As a method for measuring the refractive index of an optical element, a method is known in which measurement is performed by a minimum deflection angle method or a V-block method after processing the optical element into a prism shape. However, since measurement by these methods requires processing of the optical element, processing takes time and the cost further increases. In the case of an optical element manufactured by a mold, the stress accumulated in the optical element is released during processing. Since the refractive index of the optical element changes due to the stress release, the refractive index cannot be measured accurately. Therefore, a method for measuring the refractive index of an optical element without processing the optical element is required.

  In the measurement method disclosed in Patent Document 1, a test lens and a glass sample whose refractive index and shape are known are immersed in a first matching liquid having a refractive index substantially equal to the refractive index of the test lens and interfered. Measure streaks. Further, the interference fringes are measured by immersing the lens to be examined and the glass sample in a second matching liquid having a refractive index slightly different from the refractive index of the first matching liquid. Then, the shape, refractive index, and refractive index distribution of the test lens are obtained from the measurement result using the first matching liquid and the measurement result using the second matching liquid. The refractive index of each matching liquid needs to be slightly different from the refractive index of the lens to be measured within a range where interference fringes are not too dense.

Japanese Patent Laid-Open No. 02-008726

  In the measurement method disclosed in Patent Document 1, a matching liquid having a refractive index substantially equal to the refractive index of the test lens is required. However, the matching liquid having a high refractive index has a low transmittance. For this reason, when the interference fringes of the optical element having a high refractive index are measured by the measurement method disclosed in Patent Document 1, only a small signal can be obtained from the detector, and the measurement accuracy is lowered.

  An object of the present invention is to provide a refractive index measuring method and a refractive index measuring apparatus capable of measuring a refractive index of a lens to be examined with high accuracy in a nondestructive manner.

  The refractive index measurement method according to one aspect of the present invention is such that a test lens is arranged on a reference table having a shape of a mold at the end of a test lens, and a transmitted wavefront at the end of the test lens is measured at a plurality of wavelengths. And a step of specifying the refractive index of the end of the test lens based on the transmitted wavefront of the end of the test lens at a plurality of wavelengths and the refractive index of the reference table.

  An optical element manufacturing method including the steps of molding an optical element and evaluating the optical performance of the molded optical element by measuring the refractive index of the optical element using the refractive index measurement method. This constitutes another aspect of the present invention.

  Further, the refractive index measuring apparatus as still another aspect of the present invention includes a reference table having a shape of an end mold of a test lens, and a transmission wavefront of the test lens arranged on the reference table having a plurality of wavelengths. It has the measurement means to measure in.

  According to the present invention, it is possible to measure the refractive index of a lens to be examined with high accuracy in a nondestructive manner.

The figure which shows schematic structure of the refractive index measuring apparatus of Example 1 in this invention. FIG. 3 is a diagram illustrating the shape and refractive index of a reference table and a test lens in Example 1. 3 is a flowchart showing a procedure for calculating a refractive index of a test lens in Example 1. FIG. 3 is a diagram illustrating interference light detected by a detector in the first embodiment. FIG. 3 is a diagram showing a reference table having a shape different from that of the reference table in the first embodiment. The figure which shows the interference light detected by the detector in the reference stand of the shape different from the reference stand in Example 1, and the reference stand. The figure which shows schematic structure of the refractive index measuring apparatus of Example 2 in this invention. 9 is a flowchart showing a procedure for calculating a refractive index of a test lens in Example 2. The figure which shows the manufacturing process of the manufacturing method of the optical element of this invention.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

  FIG. 1 shows a schematic configuration of a refractive index measuring apparatus according to a first embodiment of the present invention. The refractive index measuring apparatus of the present embodiment is configured based on a Mach-Zehnder interferometer. The measuring device includes a light source 10, an interference optical system, a reference table 50, a detector 80, and a computer 90, and measures the refractive index at the end of the lens 60 to be examined. In this embodiment, the test lens 60 is a lens having a negative refractive power, but the refractive index can be measured as long as it is a refractive optical element regardless of whether the refractive power is positive or negative.

  The light source 10 is a light source (for example, white LED) that can emit light having a plurality of wavelengths. Light having a plurality of wavelengths passes through the spectroscope 20 and becomes quasi-monochromatic light. The light that passes through the spectroscope 20 becomes a divergent wave through the pinhole 30 and becomes parallel light through the collimator lens 40.

  The interference optical system includes beam splitters 100 and 101 and mirrors 105 and 106. The interference optical system divides the light that has passed through the collimator lens 40 into test light that passes through the test lens 60 and reference light that does not pass through, and causes the test light and reference light to interfere with each other to detect the interference light. The light is guided to the vessel 80.

  The test lens 60 is arranged on the reference table 50 with a test solution (liquid) 70 interposed. FIG. 2A is a perspective view of the test lens 60 installed on the reference table 50. The reference table 50 has the shape of the test lens 60 so that the end of the test lens 60 fits the reference table 50. In FIG. 1, the reference table 50 and the test lens 60 are shown in a cross-sectional view parallel to the vertical direction and the optical axis direction. Note that the surface of the end portion of the test lens 60 does not need to be a mirror surface, and may be a sanded surface.

The reference table 50 has a refractive index dispersion different from that of the test lens 60 and has a refractive index equal to the refractive index of the test lens 60 at a specific wavelength. FIG. 2B is a diagram illustrating the refractive index dispersion of the reference table 50 and the test lens 60. FIG. 2 (b) indicates that the refractive index of the refractive index and the test lens 60 of the reference platform 50 are equal at a particular wavelength lambda _0. In this embodiment, the shape and refractive index of the reference table 50 are set to known values.

  The refractive index of the test solution 70 does not need to match the refractive index of the test lens 60 (for example, the refractive index of the test lens 60 is about 1.9, and the refractive index of the test solution 70 is about 1.7. do it).

  The test light reflected by the beam splitter 100 is reflected by the mirror 106 and passes through the reference table 50, the test solution 70, and the end of the test lens 60. On the other hand, the reference light that has passed through the beam splitter 100 passes through the reference base 50 and is reflected by the mirror 105. Since both the test light and the reference light pass through the reference base 50, the optical path length difference between the test light and the reference light is reduced. That is, the reference base 50 through which the reference light is transmitted functions as a compensation plate. Of course, a compensation plate may be inserted in the optical path of the reference light instead of the reference table 50. The test light and the reference light are combined by the beam splitter 101 to form interference light.

  The interference light formed by the beam splitter 101 is detected by a detector 80 (for example, CCD or CMOS) through the imaging lens 45. The interference signal detected by the detector 80 is sent to the computer 90. The detector 80 is arranged at a conjugate position with respect to the position of the test lens 60 and the imaging lens 45. That is, the image of the test lens 60 and the interference fringes are formed on the detector 80.

  The computer 90 has calculation means for calculating the refractive index of the lens 60 to be measured based on the detection result of the detector 80 and control means for controlling the wavelength transmitted through the spectroscope 20, and is composed of a CPU and the like.

FIG. 3 is a flowchart showing calculation means for calculating the refractive index of the end portion of the test lens 60 in this embodiment. First, the test lens 60 is placed on the reference table 50 with the reagent solution 70 interposed therebetween (S10). While controlling the wavelength with the spectroscope 20, the transmitted wavefront (interference light) at the end of the lens 60 to be measured at a plurality of wavelengths is measured (S20). A specific wavelength λ ₀ is determined from transmitted wavefronts at a plurality of wavelengths (S30).

FIG. 4 shows the interference light detected by the detector 80. If there is a difference between the refractive index at the end of the test lens 60 and the refractive index of the reference base 50, interference fringes appear at the end of the test lens 60 as shown in FIG. When the wavelength is swept by the spectroscope 20, there is a wavelength at which the interference fringes of the test lens 60 are minimized. The wavelength is a specific wavelength λ ₀ , and at λ ₀ , the end of the test lens 60 and the refractive index of the reference base 50 coincide. From the above, based on the refractive index of the reference table 50 at the specific wavelength λ _0, the refractive index of the end of the lens 60 to be tested is specified (S40).

  The measurement method of the present embodiment measures the refractive index of the test lens 60 without using a test solution (matching solution) having a refractive index substantially equal to the refractive index of the test lens 60. Therefore, the refractive index of an optical element having a high refractive index (up to 1.8 or more) from which an effective matching liquid cannot be obtained can be measured nondestructively and with high accuracy.

  In this example, the wavelength was scanned with a combination of a light source that emits light of a plurality of wavelengths and a spectroscope. A white LED is used as a light source that emits light of a plurality of wavelengths, but a supercontinuum light source, a short pulse laser, and a halogen lamp can be substituted. In addition, a wavelength swept light source or a multiline laser that emits a plurality of wavelengths discretely may be used. A light source that emits light of a plurality of wavelengths is not limited to a single light source, and a plurality of light sources may be combined.

  In this embodiment, a Mach-Zehnder interferometer is used as an interference optical system for causing the test light and the reference light to interfere with each other. However, a Fizeau interferometer or a Twiman Green interferometer can be used instead. A shearing interferometer such as a Talbot interferometer or a Shack-Hartmann sensor may be used instead of the interference optical system that causes the test light and the reference light to interfere with each other.

  The reference table 50 of the present embodiment is composed of one component as shown in FIG. 1 and FIG. 2A, but may be composed of two or more components as shown in FIG.

  In addition, the reference table 50 in the present embodiment has a shape in contact with the outer peripheral portion of the lens 60 to be tested, as shown in FIG. Instead, the range where the reference table 50 and the test lens 60 are in contact may be only one cross section shown in FIG. 1 (surface including the straight line AB in FIG. 4). That is, the reference table 50 may have a shape as shown in FIG. In the present invention, the reference table having the shape of the mold of the end portion of the test lens means a reference table having a shape that fits the end portion of the test lens on the plane including the straight line AB in FIG.

FIG. 6B shows the state of interference light detected when the reference table 50 has a shape as shown in FIG. If attention is paid only to the interference fringes on the straight line AB, the specific wavelength λ ₀ can be determined similarly to the above method.

In this embodiment, a specific wavelength λ ₀ at which the refractive index of the end of the test lens 60 is equal to the refractive index of the reference base 50 is determined, and the refractive index of the end of the test lens 60 at the specific wavelength λ ₀ is determined. Was calculated. Instead of determining the specific wavelength λ ₀ , the refractive index of the end of the lens 60 to be tested can be calculated from the value of the transmitted wavefront and the shape of the reference table 50. For example, at the wavelength λ ₁ , when the optical path length difference between the points A and B in FIG. 4 is 5λ ₁ (for five interference fringes), the refractive index N ^Lens (λ ₁ ) at the end of the lens 60 to be measured and the reference The difference from the refractive index N ^Block (λ ₁ ) of the table 50 is calculated by Equation 1.

L is the geometric distance of the light beam that passes through the end of the lens shown in FIG. 1 (point A in FIG. 4), and can be calculated from the shape of the reference table. Since the refractive index N ^Block (λ ₁ ) of the reference table 50 is a known amount, the refractive index N ^Lens (λ ₁ ) at the end of the lens 60 to be measured can be calculated. However, the usable condition of this calculation method is that the interference fringes can be resolved in the detector 80.

  FIG. 7 shows a schematic configuration of the refractive index measuring apparatus according to the second embodiment of the present invention. The refractive index measurement apparatus of the present embodiment measures a transmitted wavefront using a wavefront sensor. The measuring device has a light source 11, a reference table 50, a Shack-Hartmann sensor 81, and a computer 90, and measures the refractive index of the end of the lens 60 to be examined. In this embodiment, the test lens 60 is a lens having a positive refractive power. In this embodiment, a method for measuring the refractive index of the end of the test lens 60 when the difference between the shape of the end of the test lens 60 and the shape of the reference table 50 is large will be described. The same configurations as those in the first embodiment will be described with the same reference numerals.

  The light source 11 is a wavelength swept light source (for example, a semiconductor laser). The wavelength of the light source 11 is controlled by the computer 90. The light emitted from the light source 11 passes through the pinhole 30 and becomes a divergent wave, and becomes parallel light by the collimator lens 40. The light that has passed through the collimator lens 40 passes through the lens 60 to be tested placed on the reference table 50 via the test solution, and is detected by the Shack-Hartmann sensor 81.

  Since the reference table 50 in this embodiment is made of the same material as the lens 60 to be tested, it has substantially the same refractive index dispersion as the lens 60 to be tested. Further, the reference base 50 in this embodiment is composed of two parts 50a and 50b as shown in FIG. The component 50 a has a planar shape, and the component 50 b has a shape close to the second surface of the test lens 60. The shape close to the second surface of the test lens 60 means, for example, the shape of the design value of the second surface of the test lens 60.

  In the present embodiment, the second surface of the lens 60 to be examined has a shape error from the design value, and the difference from the shape of the reference table 50 (component 50b) is large. When the shape error is large, the influence of the shape error (shape component) appears on the transmitted wavefront, so that the refractive index measurement accuracy decreases. In this embodiment, two types of test solutions having different refractive indexes are used in order to remove the influence of the shape error. FIG. 8 is a flowchart showing the calculation means for calculating the refractive index of the end of the lens 60 to be tested in this embodiment.

First, the test lens 60 is placed on the reference table 50 with the first reagent solution 70 (for example, refractive index 1.7) interposed therebetween (S110). While controlling the wavelength of the wavelength swept light source, the first transmitted wavefront W ₁ (λ) at the end of the test lens 60 at a plurality of wavelengths is measured (S120). The first transmitted wavefront W ₁ (λ) is expressed by Equation 2.

Here, N ^Lens (λ) is the refractive index of the end of the test lens 60, N ^Block (λ) is the refractive index of the reference base 50, and N ₁ ^Oil (λ) is the refractive index of the first test solution 70. . y is the coordinate shown in FIG. 7, δ (y) is the geometric distance of the light beam transmitted through the test solution shown in FIG. 7, and L (y) −δ (y) is the object shown in FIG. This is the geometric distance of the light beam that passes through the end of the analyzing lens 60. When the test lens 60 has a shape as designed, δ (y) = 0. That is, δ (y) is an amount derived from a shape error (shape component). Formula 2 is transformed into Formula 4 using the approximate formula of Formula 3.

Next, the test lens 60 is placed on the reference table 50 with the second reagent (for example, refractive index˜1.75) interposed therebetween (S130). Then, the second transmitted wavefront W ₂ (λ) at the end of the test lens 60 at a plurality of wavelengths is measured (S140). The second transmitted wavefront W ₂ (λ) is expressed by Equation 5 in the same manner as the _first transmitted wavefront W ₁ (λ). N ₂ ^Oil (λ) is the refractive index of the second reagent.

Finally, when the shape component δ (y) is eliminated from the first transmitted wavefront W ₁ (λ) and the second transmitted wavefront W ₂ (λ), the refractive index N ^Lens (λ ) Is calculated as in Equation 6 (S150).

  Using the apparatus and method described in the first and second embodiments, the result of the measured refractive index can be fed back to a method for manufacturing an optical element such as a molded lens.

  FIG. 9 shows an example of a manufacturing process of an optical element using molding.

  The optical element is manufactured through an optical element design process, a mold design process, and an optical element mold process using the designed mold. The molded optical element is evaluated for its shape accuracy, and when the accuracy is insufficient, the mold is corrected and the molding is performed again. If the shape accuracy is good, the optical performance of the optical element is evaluated. By incorporating the refractive index calculation flow described with reference to FIGS. 3 and 8 into this optical performance evaluation step, it is possible to mass-produce optical elements molded using a high refractive index glass material as a base material. If the optical performance is low, the optical element whose optical surface is corrected is redesigned.

  The embodiments described above are merely representative examples, and various modifications and changes can be made to the embodiments when the present invention is implemented.

10 Light source 50 Reference table 60 Test lens 70 Test solution

Claims (19)

Placing the test lens on a reference table having the shape of the mold at the end of the test lens, and measuring a transmitted wavefront at the end of the test lens at a plurality of wavelengths;
Identifying the refractive index of the end of the test lens based on the transmitted wavefront of the end of the test lens at the plurality of wavelengths and the refractive index of the reference table;
A refractive index measuring method comprising:   2. The refractive index measuring method according to claim 1, wherein a transmitted wavefront at an end of the test lens is measured with a liquid interposed between the reference table and the test lens.   The refractive index measurement method according to claim 2, wherein a refractive index of the liquid is lower than a refractive index of the test lens.   The refractive index measurement method according to any one of claims 1 to 3, wherein the refractive index dispersion of the test lens and the refractive index dispersion of the reference table are different from each other.   From the transmitted wavefront at the end of the test lens at the plurality of wavelengths, determine a specific wavelength at which the refractive index of the end of the test lens and the refractive index of the reference table are equal, and at the specific wavelength The refractive index measurement method according to claim 4, wherein a refractive index of the reference table is specified as a refractive index of an end portion of the lens to be examined.   The difference between the refractive index of the end of the test lens and the refractive index of the reference table is calculated from the transmitted wavefront of the end of the test lens at the plurality of wavelengths and the shape of the reference table, and the reference 5. The refractive index measurement method according to claim 1, wherein a refractive index of an end portion of the test lens is calculated based on a refractive index of a table. Measuring a first transmitted wavefront at the end of the test lens at a plurality of wavelengths by interposing a first liquid between the reference table and the test lens;
A second liquid having a refractive index different from the refractive index of the first liquid is interposed between the reference stage and the test lens, so that the second end of the test lens at a plurality of wavelengths is provided. Measure the transmitted wavefront of
Removing the shape component of the test lens based on the first transmitted wavefront, the second transmitted wavefront, and the refractive index of the reference table, and calculating the refractive index of the end of the test lens. The refractive index measuring method according to claim 1, wherein the refractive index is measured. Molding the optical element;
Evaluating the optical performance of the molded optical element by measuring the refractive index of the optical element using the refractive index measurement method according to any one of claims 1 to 7. A method for manufacturing an optical element. A reference table having the shape of the mold at the end of the lens to be examined;
An apparatus for measuring a refractive index, comprising: a measuring unit that measures a transmission wavefront of the lens to be measured arranged on the reference table at a plurality of wavelengths.   The refractive index measuring apparatus according to claim 9, wherein a transmitted wavefront of an end portion of the test lens is measured with a liquid interposed between the reference table and the test lens.   The refractive index measuring apparatus according to claim 10, wherein a refractive index of the liquid is lower than a refractive index of the test lens.   The refractive index measurement apparatus according to any one of claims 9 to 11, wherein the refractive index dispersion of the test lens and the refractive index dispersion of the reference table are different from each other.   A specific wavelength at which the refractive index of the end of the test lens and the refractive index of the reference table are equal is determined from the transmitted wavefront at the end of the test lens at the plurality of wavelengths, and at the specific wavelength The refractive index measuring apparatus according to claim 12, wherein a refractive index of the reference table is specified as a refractive index of an end portion of the test lens.   The difference between the refractive index of the end of the test lens and the refractive index of the reference table is calculated from the transmitted wavefront of the end of the test lens at the plurality of wavelengths and the shape of the reference table, and the reference 13. The refractive index measuring apparatus according to claim 9, further comprising a calculating unit that calculates a refractive index of an end portion of the test lens based on a refractive index of a table. The measuring means includes a first liquid interposed between the reference table and the lens to be measured, measures a first transmitted wavefront at an end of the test lens at a plurality of wavelengths, and By interposing a second liquid having a refractive index different from the refractive index of the first liquid between the test lenses, a second transmitted wavefront at the end of the test lens at a plurality of wavelengths. Measure and
Calculation means for removing a shape component of the test lens based on the first transmitted wavefront, the second transmitted wavefront, and the refractive index of the reference table, and calculating the refractive index of the end of the test lens The refractive index measuring device according to claim 9, wherein   The refractive index measurement apparatus according to claim 9, wherein the measurement unit includes a Mach-Zehnder interferometer.   The refractive index measurement apparatus according to claim 9, wherein the measurement unit includes a shearing interferometer.   The refractive index measurement apparatus according to claim 9, wherein the measurement unit includes a Shack-Hartmann sensor.   The refractive index measuring apparatus according to claim 9, wherein the reference table is composed of two or more parts.

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