JP2016109594A - Refractive index distribution measurement method, refractive index distribution measurement method, and optical element manufacturing method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To measure the refractive index distribution of a lens to be inspected at low cost with high accuracy.SOLUTION: A reference lens 50 whose shape and refractive index are known is sandwiched using two lenses 60 to be inspected and a liquid 70 to configure a unit 100 to be inspected, and the transmission wavefront of the unit 100 to be inspected is measured. A wavefront aberration that is a difference between the transmission wavefront of the unit 100 to be inspected and the transmission wavefront of a unit to be inspected that is configured with two reference lenses to be inspected that have specific refractive index distribution is calculated. The average value of refractive index distribution of the two lenses 60 to be inspected is calculated using the wavefront aberration.SELECTED DRAWING: Figure 1

Description

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

  A lens manufacturing method using a mold generates a refractive index distribution inside the lens. The refractive index distribution inside the lens adversely affects the optical performance. Therefore, a technique for measuring the refractive index distribution in a non-destructive manner after molding is necessary for manufacturing a molded lens.

  In the measurement method disclosed in Patent Document 1, the specimen 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 specimen to interfere. Measure streaks. Further, the interference fringes are measured by immersing the test object and the glass sample in a second matching liquid having a refractive index slightly different from the refractive index of the first matching liquid. And the shape and refractive index distribution of a test object are calculated | required from the measurement result by a 1st matching liquid, and the measurement result by a 2nd matching liquid. The refractive index of each matching liquid needs to be slightly different from the refractive index of the test object as long as the interference fringes are not too dense.

  In the measurement method disclosed in Patent Document 2, the first transmitted wavefront is measured by placing the test object in a first medium having a first refractive index lower than the refractive index of the test object. Further, the second transmitted wavefront is measured by placing the test object in a second medium having a second refractive index lower than the refractive index of the test object and different from the first refractive index. Then, using the measurement results of the first and second transmitted wavefronts and the respective transmitted wavefronts when the reference specimen having a specific refractive index distribution is arranged in the first and second media, The refractive index distribution of the test object is calculated by removing the shape component of the test object.

Japanese Patent Laid-Open No. 02-008726 JP 2010-151578 A

  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. When the measurement accuracy of the transmitted wavefront decreases, the calculation accuracy of the refractive index distribution also decreases.

  With the measurement method disclosed in Patent Document 2, the refractive index distribution of an optical element having a high refractive index can be measured. However, since the refractive index of the test object is different from the refractive index of the medium, the wavefront transmitted through the test object has a large aberration. It is difficult to measure the transmission wavefront of large aberration with high accuracy. Further, when measuring a test object having negative power such as a concave lens, an illumination lens having a larger diameter than that of the test object and a wavefront sensor having a large screen are required. Large-diameter illumination lenses and large-screen wavefront sensors are very expensive. In addition, a manufacturing error or an arrangement error of the illumination lens appears as a refractive index distribution calculation error of the test object.

  An object of the present invention is to provide a refractive index distribution measuring method and a refractive index distribution measuring apparatus capable of measuring a refractive index distribution of a test lens with high accuracy at low cost.

  The refractive index distribution measuring method as one aspect of the present invention measures a transmitted wavefront of a test unit configured by sandwiching a reference lens having a known shape and refractive index using two test lenses and a liquid. Difference between the measurement step to be performed, the transmitted wavefront of the test unit, and the transmitted wavefront of the reference test unit configured by sandwiching the reference lens using two reference test lenses having a specific refractive index distribution And calculating a wavefront aberration and calculating an average value of refractive index distributions of the two test lenses using the wavefront aberration.

  An optical element manufacturing method comprising: molding an optical element; and evaluating the optical performance of the molded optical element by measuring the refractive index distribution of the optical element using the refractive index distribution measuring method. The method also constitutes another aspect of the present invention.

  In addition, a refractive index distribution measuring apparatus according to still another aspect of the present invention includes a test unit configured by sandwiching a reference lens having a known shape and refractive index using two test lenses and a liquid. The wavefront that is the difference between the measurement means for measuring the transmitted wavefront, the transmitted wavefront of the test unit, and the transmitted wavefront when two reference test lenses having a specific refractive index distribution form the test unit And calculating means for calculating an average value of refractive index distributions of two test lenses using wavefront aberration.

  According to the present invention, the refractive index distribution of the lens to be measured can be measured with high accuracy at low cost.

The figure which shows schematic structure of the refractive index distribution measuring apparatus of Example 1 in this invention. 5 is a flowchart showing a procedure for calculating a refractive index distribution of a test lens in the first embodiment. FIG. 3 is a diagram illustrating a coordinate system defined on a unit to be tested in Example 1 and an optical path of a light beam in the measurement apparatus. The figure which shows schematic structure of the refractive index distribution measuring apparatus of Example 2 in this invention. 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 distribution measuring apparatus according to a first embodiment of the present invention. The measuring apparatus of the present embodiment includes a light source 10, a pinhole 30, a test unit 100, stages 40 and 41, a Shack-Hartmann sensor 80, and a computer 90. The average value of the refractive index distributions of the two test lenses 60 is obtained. measure. The two test lenses 60 are concave meniscus lenses manufactured under the same conditions, and the two test lenses 60 exhibit substantially equal optical characteristics.

  The light source 10 is a light source (for example, a HeNe laser) that can emit quasi-monochromatic light. The light from the light source 10 becomes a divergent wave through the pinhole 30 and enters the unit 100 to be tested installed on the stage 40. The light transmitted through the unit 100 to be tested becomes convergent light and is detected by the Shack-Hartmann sensor 80.

  The test unit 100 includes two test lenses 60, a reference lens 50 having a known shape and refractive index, and a test solution 70, and the two test lenses 60 sandwich the reference lens 50 via the test solution 70. It has a structure. The test unit 100 is a lens having positive power as a whole. The test unit 100 is configured so that the surface of the test lens 60 having the larger curvature faces the reference lens 50, thereby reducing the aberration of the transmitted wavefront of the test unit 100 and transmitting the wavefront. Is preferably measured with high accuracy.

  The reference lens 50 has a surface shape substantially equal to the shape of the surface 60b having a large curvature among the surfaces 60a and 60b of the lens 60 to be examined. The substantially equal surface shape is, for example, a surface shape that coincides with the surface 60b of the lens 60 to be tested within a range of manufacturing error of the surface shape of the lens 60 to be tested. The reference lens 50 is a homogeneous lens with a negligible refractive index distribution, and is manufactured by grinding or polishing, for example. The material of the reference lens 50 is, for example, the same material as the lens 60 to be examined.

  In this embodiment, it is not necessary to make the refractive index of the test lens 60 and the refractive index of the test solution 70 coincide with each other, and the test solution 70 may be a liquid having a refractive index lower than that of the test lens 60. For example, when the refractive index of the test lens 60 is about 1.9, the refractive index of the test solution may be about 1.7.

  The stage 40 and the stage 41 can be driven in an arrow direction (optical axis direction) in FIG. 1, and a relative distance between the pinhole 30, the unit 100 to be tested, and the Shack-Hartmann sensor 80 can be adjusted.

  The computer 90 calculates calculating means for calculating the refractive index distribution of the two test lenses 60 based on the detection result of the Shack-Hartmann sensor 80 and the positions of the stages 40 and 41, and control for controlling the positions of the stages 40 and 41. It has means and consists of a CPU or the like.

  FIG. 2 is a flowchart showing a calculation procedure for calculating the average value of the refractive index distribution of the test lens 60 in this embodiment. In this embodiment, first, the test unit 100 is configured by sandwiching the reference lens 50 using the two test lenses 60 and the test solution 70 (S10). In this embodiment, the test unit 100 is installed on the stage 40, and the stages 40 and 41 are driven so that the relative positions between the pinhole 30 and the test unit 100 and between the test unit 100 and the Shack-Hartmann sensor 80 are relative. The distance is adjusted (S20).

Then, the transmitted wavefront W _m (x, y) of the unit 100 to be tested is measured (S30). The transmitted wavefront W _m (x, y) of the unit 100 to be tested is expressed as Equation 1.

L _A (x, y), L ₁ (x, y), δ ₁ (x, y), L _B (x, y), δ ₂ (x, y), L ₂ (x, y), L _C (X, y) is the geometric distance between the components along the ray shown in FIG. The light rays in FIG. 3B indicate light rays that pass through the point (x, y) inside the unit 100 to be tested shown in FIG. FIG. 3B is drawn ignoring the deflection of the light beam due to refraction at each surface. L ₁ (x, y) and L ₂ (x, y) are the thicknesses of the two test lenses 60, and L _B (x, y) is the thickness of the reference lens 50. δ ₁ (x, y) and δ ₂ (x, y) are gaps that appear when the surface shapes of the two test lenses 60 and the reference lens 50 are slightly different, and the gap is filled with the test solution 70. Yes.

n ₁ ^Lens (x, y) and n ₂ ^Lens (x, y) are the refractive indexes of the two test lenses 60 averaged in the direction of light passing through the point (x, y). n ^Oil is the refractive index of the test solution 70, and n ^Ref is the refractive index of the reference lens 50.

Next, the transmitted wavefront W _sim (x, y) of the reference test unit configured by sandwiching the reference lens 50 using two reference test lenses having a specific refractive index distribution is calculated (S40). . This step assumes a test lens (reference test lens) having the same shape as the test lens 60 and having a specific refractive index distribution, and the reference test lens is arranged at the position of the test lens 60 in step S30. In this state, the transmitted wavefront is calculated. In this embodiment, for the sake of simplicity, the refractive index of the reference lens is assumed to be uniform. As the value of the shape and refractive index of the reference test lens, for example, the design value of the test lens 60 is used. W _sim (x, y) is expressed by Equation 2.

n ₀ ^Lens is the refractive index of the reference test lens, and L (x, y) is the thickness of the reference test lens. In this embodiment, it is assumed that a surface having a large curvature (a surface corresponding to the surface 60b of the test lens 60) among the surfaces of the reference test lens has a surface shape similar to the surface shape of the reference lens 50. Therefore, the gap filled with the reagent solution 70 is omitted.

Then, the wavefront aberration W (x, y) that is the difference between the transmitted wavefront W _m (x, y) measured in step S30 and the transmitted wavefront W _sim (x, y) calculated in step S40 is calculated (S50). . The wavefront aberration W (x, y) is expressed as Equation 4 using the relational expression of Equation 3.

  Finally, the average value of the refractive index distributions of the two test lenses is calculated using the wavefront aberration W (x, y) as shown in Expression 6 using the approximate expression of Expression 5 (S60).

  In the present embodiment, lenses manufactured under the same conditions are used as the two test lenses 60. The lens production under the same conditions refers to, for example, lens production using a mold. In the lens production by the mold, a large number of lenses are produced under the same conditions. Since the two lenses taken out from them have substantially the same physical properties, the average value of the refractive index distributions of the two test lenses is substantially equal to the refractive index distribution of each of the test lenses alone.

  Shape control and shape measurement of a surface with a large curvature like the surface 60b are difficult. Therefore, it is difficult to grasp the actual shape of the surface 60b. If there is a discrepancy (shape error) between the assumed shape and the actual shape, a difference occurs in refraction at the interface, and the difference appears as a difference in transmitted wavefront. It is necessary to determine whether the difference in the transmitted wavefront is derived from a shape error or a refractive index distribution. In the present embodiment, the reference lens 50 and the test solution 70 are sandwiched by a surface 60b having a large curvature among the surfaces of the two test lenses 60. As a result, the influence of refraction on the surface 60b becomes almost zero, and only the influence of refraction on the surface 60a having the smaller curvature needs to be considered. Since a surface with a small curvature like the surface 60a is simple in both shape control and shape measurement, the wavefront component derived from the shape error and the wavefront component derived from the refractive index distribution are easily separated.

  In this embodiment, a test unit 100 (convex lens) having a positive power is configured by using two test lenses 60 having a convex surface, a reference lens 50, and a test solution 70. Since refraction at the interface occurs only on the surface 60a having a small curvature, the entire power of the unit 100 to be tested is small. As the power is reduced, the transmitted wavefront aberration is also reduced, which facilitates measurement of the transmitted wavefront. In addition, since the test unit 100 has a positive power, in order to measure the transmitted wavefront of the test unit 100, a light source, a pinhole, and a small screen size wavefront sensor may be used. If the number of optical elements in the system is small, it is difficult for system errors caused by manufacturing errors and arrangement errors of the respective optical elements to be mixed. Further, since a large-diameter illumination lens and a large screen wavefront sensor are not required, the cost can be reduced.

  Since the test unit 100 has a symmetrical structure in which the reference lens 50 and the test solution 70 are sandwiched between the two test lenses 60, the test unit 100 is easy to hold. In addition, due to the contrasting structure, the end portion (edge) of the test lens 60 through which normal light does not pass is transmitted through the light, so that the measurable area is wide.

In this embodiment, approximation of Formula 5 is used when calculating the average value of the refractive index distribution of the two test lenses 60. If the difference (shape error) δ ₁ (x, y), δ ₂ (x, y) between the surface shapes of the two test lenses 60 and the reference lens 50 is large, the approximation of Equation 5 does not hold and the refraction is performed. The calculation accuracy of the rate distribution decreases. Therefore, when the shape error is large, the shape error can be removed by a method using the following two types of test solutions.

First, the reference lens 50 is sandwiched between two test lenses 60 and a first reagent having a _first refractive index n ₁ ^Oil (for example, n ₁ ^Oil is about 1.7), Configure the unit to be tested. Then, the first transmitted wavefront of the first unit to be tested is measured. A first wavefront aberration W ₁ (x, y), which is the difference between the first transmitted wavefront and the transmitted wavefront when the two reference test lenses constitute the first test unit, is calculated. . The first wavefront aberration W ₁ (x, y) is expressed as Equation 8 using approximation of Equation 7.

Then, two of the lens 60, and a second reagent having a first refractive index _n ^{1 Oil} different second refractive index _n ^{2 Oil} _(e.g., from about 1.75 to ^{n 2 Oil)} Is used to sandwich the reference lens 50 to constitute a second unit to be tested. Then, the second transmitted wavefront of the second unit to be tested is measured. A second wavefront aberration W ₂ (x, y), which is a difference between the second transmitted wavefront and the transmitted wavefront when the two reference test lenses constitute the second test unit, is calculated. . The second wavefront aberration W ₂ (x, y) is expressed as Equation 9 using approximation of Equation 7.

Using the first wavefront aberration W ₁ (x, y) and the second wavefront aberration W ₂ (x, y), a shape error (shape component δ ₁ (x, y) + δ ₂ (x, y)) is calculated. The average value of the refractive index distribution of the two test lenses 60 is calculated as shown in Equation 10.

  The optical path length distribution (= refractive index distribution × L (x, y)) can be substituted for the refractive index distribution as a physical quantity indicating the optical performance of the molded lens. Therefore, the refractive index distribution measuring method (refractive index distribution measuring apparatus) of the present invention also means an optical path length distribution measuring method (optical path length distribution measuring apparatus).

  In this embodiment, a HeNe laser is used as the light source 10, but it is not necessary to be a laser, and an LED or the like can be used instead. In this embodiment, the Shack-Hartmann sensor is used as the wavefront sensor, but a shearing interferometer such as a Talbot interferometer may be used instead.

FIG. 4 is a diagram illustrating a schematic configuration of the refractive index distribution measuring apparatus according to the second embodiment of the present invention. In this embodiment, the transmitted wavefront is measured using a Talbot interferometer, which is a type of shearing interferometer, without using the Shack-Hartmann sensor used in the first embodiment. The Talbot interferometer includes a two-dimensional diffraction grating 85 and a detector 81 (for example, CCD or CMOS). In the present embodiment, a method of removing the shape errors δ ₁ (x, y) and δ ₂ (x, y) using transmitted wavefronts at two different wavelengths will be described. The same configurations as those in the first embodiment will be described with the same reference numerals.

The light having the first wavelength λ ₁ (for example, λ ₁ = 633 nm) emitted from the light source 10 passes through the half mirror 20 (for example, dichroic mirror) and reaches the pinhole 30. The light having the second wavelength λ ₂ (for example, λ ₂ = 442 nm) emitted from the light source 11 is reflected by the half mirror 20 and reaches the pinhole 30. The light having the first wavelength λ ₁ or the light having the second wavelength λ ₂ that has passed through the pinhole 30 and has become a diverging wave passes through the unit 100 to be converged light and is detected by the Talbot interferometer. .

  A method for calculating the average value of the refractive index distribution of the two test lenses 60 in the present embodiment will be described below. In this embodiment, first, the test unit 100 is configured by sandwiching the reference lens 50 using the two test lenses 60 and the test solution 70. The test unit 100 is installed on the stage 40, and the stages 40 and 41 are driven to adjust the relative distance between the pinhole 30 and the test unit 100 and between the test unit 100 and the Talbot interferometer.

Then, the transmitted wavefront of the test unit at the first wavelength and the transmitted wavefront of the test unit at the second wavelength are measured. For each of the first and second wavelengths, a wavefront aberration that is a difference between the transmitted wavefront of the test unit and the transmitted wavefront when the two reference test lenses constitute the test unit is calculated. The wavefront aberration W ₁ (λ ₁ , x, y) at the first wavelength and the wavefront aberration W ₂ (λ ₂ , x, y) at the second wavelength are expressed as Equation 11.

n ₁ ^Lens (λ ₁ , x, y) and n ₂ ^Lens (λ ₁ , x, y) are the refractive indexes of the two test lenses 60 at the first wavelength λ ₁ . n ₁ ^Lens (λ ₂ , x, y) and n ₂ ^Lens (λ ₂ , x, y) are the refractive indexes of the two test lenses 60 at the second wavelength λ ₂ . n ₀ ^Lens (λ ₁ ) and n ₀ ^Lens (λ ₂ ) are the refractive indexes of the reference lens at the first and second wavelengths, respectively. n ^Oil (λ ₁ ) and n ^Oil (λ ₂ ) are the refractive indexes of the test solution 70 at the first and second wavelengths, respectively.

Finally, from the wavefront aberration W ₁ (λ ₁ , x, y) at the first wavelength and the wavefront aberration W ₂ (λ ₂ , x, y) at the second wavelength, the shape component δ ₁ (x, y) + δ ₂ Remove (x, y) and use the approximation of Equation 12. As a result, the average value of the refractive index distribution of the two test lenses 60 at the first wavelength λ ₁ is calculated as in Expression 13. When calculating the average value of the refractive index distribution of the two test lenses 60 at the second wavelength λ ₂ , Equation 12 may be used.

  In this embodiment, when measuring the first transmitted wavefront at the first wavelength, the light of the second wavelength is cut so that the light of the second wavelength is not mixed. Similarly, when measuring the second transmitted wavefront, the light of the first wavelength is cut. To block light, the light source may be turned off or a filter may be used. When the detector 81 has a color filter mounted on each pixel, the first transmitted wavefront and the second transmitted wavefront can be measured simultaneously.

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

  FIG. 5 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. Incorporating the refractive index distribution calculation flow described with reference to FIG. 2 into this optical performance evaluation step enables mass production of 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.

  The measuring device can be applied to an application for measuring the refractive index distribution of an optical element.

DESCRIPTION OF SYMBOLS 10 Light source 50 Reference lens 60 Test lens 70 Test solution 100 Test unit

Claims (10)

A measurement step of measuring a transmitted wavefront of a test unit configured by sandwiching a reference lens having a known shape and refractive index using two test lenses and a liquid;
Wavefront aberration that is a difference between the transmitted wavefront of the test unit and the transmitted wavefront of a reference test unit configured by sandwiching the reference lens using two reference test lenses having a specific refractive index distribution A calculation step of calculating an average value of refractive index distributions of the two test lenses using the wavefront aberration;
A refractive index distribution measuring method comprising: Measuring a first transmitted wavefront of a first test unit configured by sandwiching the reference lens using the two test lenses and a first liquid having a first refractive index;
A second of a second test unit configured by sandwiching the reference lens using the two test lenses and a second liquid having a second refractive index different from the first refractive index. Measure the transmitted wavefront,
Calculating a first wavefront aberration that is a difference between the first transmitted wavefront and the transmitted wavefront of the reference test unit;
Calculating a second wavefront aberration that is a difference between the second transmitted wavefront and the transmitted wavefront of the reference test unit;
Using the first wavefront aberration and the second wavefront aberration, an average value of refractive index distributions of the two test lenses is calculated by removing shape components of the two test lenses. The refractive index distribution measuring method according to claim 1. Measuring the transmitted wavefront of the unit under test at a first wavelength;
Measuring the transmitted wavefront of the unit under test at a second wavelength different from the first wavelength;
Calculating a first wavefront aberration that is a difference between a transmitted wavefront of the test unit at the first wavelength and a transmitted wavefront of the reference test unit at the first wavelength;
Calculating a second wavefront aberration that is a difference between a transmitted wavefront of the test unit at the second wavelength and a transmitted wavefront of the reference test unit at the second wavelength;
Using the first wavefront aberration and the second wavefront aberration, an average value of refractive index distributions of the two test lenses is calculated by removing shape components of the two test lenses. The refractive index distribution measuring method according to claim 1.   The refractive index distribution measuring method according to any one of claims 1 to 3, wherein the test lens has at least one convex surface.   The refractive index distribution measuring method according to claim 4, wherein the unit to be tested has a positive power.   6. The test unit according to claim 1, wherein the test unit is configured such that a surface having a larger curvature among surfaces of the test lens faces the reference lens. Refractive index distribution measurement method. Molding the optical element;
Evaluating the optical performance of the molded optical element by measuring the refractive index distribution of the optical element using the refractive index distribution measuring method according to any one of claims 1 to 6. A manufacturing method of an optical element characterized by including. A measuring means for measuring a transmitted wavefront of a test unit configured by sandwiching a reference lens having a known shape and refractive index using two test lenses and a liquid;
Calculating a wavefront aberration which is a difference between a transmitted wavefront of the test unit and a transmitted wavefront when two reference test lenses having a specific refractive index distribution constitute the test unit; And a calculating means for calculating an average value of the refractive index distributions of the two test lenses using the refractive index distribution measuring apparatus.   The refractive index distribution measuring apparatus according to claim 8, wherein the measuring unit includes a Shack-Hartmann sensor.   The refractive index distribution measuring apparatus according to claim 8, wherein the measuring unit includes a shearing interferometer.

Images