JP6700699B2 - Refractive index distribution measuring method, refractive index distribution measuring apparatus, and optical element manufacturing method - Google Patents

Description

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

  In the lens manufacturing method using a mold, three physical quantities of shape, refractive index, and refractive index distribution deviate from design values. In particular, the refractive index distribution generated inside the lens adversely affects the optical performance. Therefore, in manufacturing a molded lens, a technique for nondestructively measuring the refractive index distribution of the lens after molding is required under the condition that the shape and the refractive index are different from the designed values.

  In the measurement method disclosed in Patent Document 1, the first and second reference elements having a refractive index substantially the same as the refractive index of the test object and having a surface shape opposite to the surface shape of the test object are used for the test object. And the matching oil is filled in the gap between the first and second reference elements and the object to be measured to form a measuring cell. Then, the interference fringes of the measurement cell are measured, the phase distribution data is calculated from the interference fringes, and the difference from the preset reference value is obtained to obtain the refractive index distribution of the test object.

JP, 2014-196966, A

  In the measurement method disclosed in Patent Document 1, the reference element having a refractive index substantially the same as the refractive index of the test object and the matching oil are used to reduce the influence of refraction due to the shape of the test object. The refractive index distribution of is calculated. However, in the lens manufacturing method by molding, not only the shape of the test object but also the refractive index of the test object deviates from the design value for each molding condition. Difference occurs.

  Due to this difference in refractive index, a shape error of the test object (deviation from the design value of the shape) appears as an error in the refractive index distribution. Furthermore, the difference in the refractive index between the test object and the reference element itself leads to an error in calculating the refractive index distribution. Therefore, in order to obtain the refractive index distribution with high accuracy, it is necessary to perform calculation to correct the influence of the shape error of the test object and the refractive index error of the test object (deviation of the refractive index from the design value).

  An exemplary object of the present invention is to provide a refractive index distribution measuring method and a refractive index distribution measuring device capable of nondestructively and highly accurately measuring the refractive index distribution of a lens.

A refractive index distribution measuring method according to one aspect of the present invention uses a first reference lens whose shape and refractive index are known, a second reference lens whose shape and refractive index are known, and a medium, A measuring step of measuring a transmitted wavefront of the unit to be inspected constituted by sandwiching, a shape and a refractive index of the first reference lens, a shape and a refractive index of the second reference lens, and a transmitted wavefront of the unit to be tested. And a correction step for calculating a refractive index distribution of the lens under test by correcting a refractive index distribution error derived from an uncertain constant component of the transmitted wavefront of the unit under test. And

  Incidentally, a method of manufacturing a lens, including a step of molding the lens, and a step of evaluating the optical performance of the molded lens by measuring the refractive index distribution of the lens using the above-mentioned refractive index distribution measuring method, It constitutes another aspect of the present invention.

Further, a refractive index distribution measuring device according to still another aspect of the present invention uses a medium, a first reference lens having a known shape and refractive index, a second reference lens having a known shape and refractive index, and a medium. Measuring means for measuring a transmitted wavefront of a unit to be inspected constituted by sandwiching the lens to be inspected, shape and refractive index of the first reference lens, shape and refractive index of the second reference lens, and Using the transmitted wavefront of the unit, the refractive index distribution error derived from the uncertain constant component of the transmitted wavefront of the unit to be tested is corrected, and a calculating means for calculating the refractive index distribution of the lens to be tested, It is characterized by having.

  According to the present invention, the refractive index distribution of a lens can be measured nondestructively and with high accuracy.

The figure which shows schematic structure of the refractive index distribution measuring apparatus of Example 1 in this invention. 6 is a flowchart showing a procedure of calculating the refractive index distribution of the lens to be inspected in Example 1. FIG. 4 is a diagram showing a dispersion curve of the refractive index of the lens under test and the first and second reference lenses in the first embodiment. FIG. 3 is a diagram showing a coordinate system defined on a lens to be inspected in Example 1 and an optical path of a light beam in a measuring device. FIG. 4 is a diagram showing a plurality of arrangements in which the unit to be inspected has different inclinations with respect to the light to be inspected in Example 1. 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.

  First Embodiment FIG. 1 shows a schematic configuration of a refractive index distribution measuring device according to a first embodiment of the present invention. The refractive index distribution measuring device of the present embodiment is constructed based on a Mach-Zehnder interferometer. The measuring device has a light source 10, an interference optical system, a unit to be inspected 200, a compensating plate 130, a detector 80, and a computer 90, and measures the refractive index distribution of the lens to be inspected 60. In this embodiment, the lens to be inspected is a lens having a positive refractive power, but the refractive index distribution can be measured by the refractive optical element regardless of whether the refractive power is positive or negative.

  The light source 10 is a light source (for example, a supercontinuum light source) that can emit light of a plurality of wavelengths. Light of a plurality of wavelengths passes through the monochromator 20 and becomes quasi-monochromatic light. The light passing through the monochromator 20 becomes a divergent wave through the pinhole 30 and becomes a 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 splits the light that has passed through the collimator lens 40 into reference light that does not pass through the test lens and test light that passes through the test lens. Is guided to the detector 80.

  The unit 200 to be inspected includes a lens 60 to be inspected, a first reference lens 120, a second reference lens 125, and a medium 71. The first reference lens 120, the medium 71, the lens 60 to be inspected, the medium 71, and the second reference lens. They are arranged in the order of 125.

  The first reference lens 120 has a refractive index equal to that of the lens 60 under test at a specific wavelength. Since the lens 60 to be inspected has a refractive index distribution, the refractive index of the lens 60 to be inspected refers to a specific part of the refractive index of the lens 60 to be inspected. The specific part may be any part.

The second reference lens 125 is made of the same material as the first reference lens 120. The refractive index of the medium 71 does not have to be equal to the refractive index of the lens 60 to be measured (for example, when the refractive index n _{d of the} lens 60 to be measured is about 1.9, the refractive index n _d of the medium 71 is about 1). About 7). The closer the refractive index of the medium is to the refractive index of the lens 60 to be tested, the more the influence of refraction on the surface of the lens 60 to be tested can be reduced, but the medium can be air.

  The first reference lens 120 has a flat surface and a surface having a shape substantially equal to the surface shape of the first surface of the lens 60 to be tested, and the second reference lens 125 has a surface shape of the second surface of the lens 60 to be tested. It has a surface and a flat surface having substantially the same shape. That is, the surfaces of the first reference lens 120 and the second reference lens 125 opposite to the surface facing the lens 60 under test are flat surfaces.

  In this embodiment, the surfaces having substantially the same shape are, for example, surfaces having the same shape within a manufacturing error range of the surface shape of the lens under test. The surface shapes and refractive indexes of the first reference lens 120 and the second reference lens 125 are known. The first reference lens 120 and the second reference lens 125 are lenses whose refractive index distributions can be ignored, and are manufactured by grinding/polishing, for example. The unit to be inspected 200 is arranged on a rotary stage 140 having a rotation axis in a direction perpendicular to the incident light to be inspected.

  The test light reflected by the beam splitter 100 is reflected by the mirror 106 and transmitted through the test unit 200. On the other hand, the reference light transmitted through the beam splitter 100 is transmitted through the compensation plate 130 and is reflected by the mirror 105. The compensating plate 130 is a glass block for reducing the optical path length difference between the test light and the reference light. If the coherence length of the light transmitted through the monochromator 20 is long, the compensating plate 130 is not necessary. The reference light and the test light are combined by the beam splitter 101 to form interference light.

  The refractive index of the medium 71 can be calculated by measuring the temperature of the atmosphere near the medium 71 using a thermometer (not shown) and converting the measured temperature into a refractive index. However, in this embodiment, the calculation of the refractive index of the medium 71 is not essential.

  The mirror 105 is driven in the arrow direction in FIG. 1 by a drive mechanism (not shown). The drive direction is not limited to the arrow direction in FIG. 1, and may be any direction as long as the optical path length difference between the reference light and the test light is changed by driving the mirror 105. The drive mechanism of the mirror 105 is composed of, for example, a piezo stage or the like. The driving amount of the mirror 105 is measured by a length measuring device (for example, a laser displacement meter or an encoder) (not shown), and is controlled by the computer 90. The optical path length difference between the reference light and the test light is adjusted by the drive mechanism of the mirror 105.

  The interference light formed by the beam splitter 101 is detected by a detector 80 (for example, CCD or CMOS) via 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 position conjugate with the position of the subject lens 60 with respect to 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 calculates the refractive index distribution of the lens under test based on the detection result of the detector 80, the wavelength of the light transmitted through the monochromator 20, the drive amount of the mirror 105, and the rotation amount of the rotary stage 140. It has a control means for controlling, and is composed of a CPU and the like.

  FIG. 2 is a flowchart showing a calculation procedure for calculating the refractive index distribution of the lens 60 to be inspected. In the present embodiment, first, the test lens 60 is sandwiched by using the first reference lens 120, the second reference lens 125, and the medium 71 to configure the test unit 200 (S10).

As shown in FIG. 3, the first reference lens 120 and the second reference lens 125 have a refractive index dispersion different from that of the lens 60 to be inspected, and a refraction equal to the refractive index of the lens 60 to be inspected at a specific wavelength λ ₀ . Have a rate. In general, the absolute value of the refractive index of a molded lens changes greatly depending on the molding conditions. If the first and second reference lenses have the same refractive index dispersion as the lens under test 60, the two curves in FIG. 3 have almost the same slope, and therefore the intersection of the two curves (specific wavelength λ ₀ ) May not exist. Therefore, in this embodiment, it is desirable that the first reference lens 120 and the second reference lens 125 be made of a material different from the material of the lens 60 to be tested.

Next, the test unit 200 is placed on the test optical path (S20). While controlling the wavelength with the monochromator 20, the transmitted wavefronts of the unit under test 200 at a plurality of wavelengths are measured (S30). Then, the specific wavelength λ ₀ is determined from the transmitted wavefront of the unit under test 200 at a plurality of wavelengths (S40).

The specific wavelength λ ₀ is a wavelength at which the refractive index at a specific position inside the lens 60 to be inspected matches the refractive index of the first and second reference lenses. At wavelengths having the same refractive index, the change in the wavefront becomes small before and after the unit under test 200, so that the specific wavelength λ ₀ can be determined from the size of the transmitted wavefront. In particular, since the transmitted wavefront becomes large in a portion where the second surface of the lens 60 to be inspected has a steep inclination with respect to the first surface, it is easy to determine the specific wavelength λ ₀ by using that portion. For example, the transmitted wavefront near the cut end (edge) of the lens to be inspected is suitable for determining the specific wavelength λ ₀ .

The specific wavelength λ ₀ can also be determined from the interference fringes rather than the transmitted wavefront. When the specific wavelength λ ₀ is determined from the interference fringes, the interference fringes at a plurality of wavelengths are measured while controlling the wavelength with the monochromator 20, and the wavelength (=specific wavelength λ ₀ ) at which the interference fringes are the smallest is determined. Good.

The transmitted wavefront is measured by the fringe scan method using the driving of the mirror 105. The transmitted wavefront W(λ ₀ , x, y) of the unit under test at a specific wavelength λ ₀ is expressed by Equation 1.

L _A (x, y), L _B (x, y), L (x, y), L _C (x, y), and L _D (x, y) are the light rays shown in FIG. It is the geometric distance between each component along. The light ray of FIG. 4B indicates a light ray passing through a point (x, y) inside the lens 60 to be inspected shown in FIG. FIG. 4B is drawn by ignoring the deflection of the light beam due to the refraction on each surface.

L(x,y) is the thickness of the test lens 60, L _A (x,y) is the thickness of the first reference lens 120, and L _D (x,y) is the thickness of the second reference lens 125. L _A (x, y) and L _D (x, y) are assumed to be measured by a stylus type surface shape measuring method or the like, and are defined here as known quantities. L _B (x, y) and L _C (x, y) are the second surface of the first reference lens 120 and the first surface of the lens 60 to be tested, and the second surface of the lens 60 to be tested and the second reference lens 125. The first surface of 1 is a gap generated when the surface shapes thereof are slightly different, and the gap is filled with the medium 71.

n ^sample (λ ₀ , x, y) is the refractive index of the lens 60 to be measured at a specific wavelength λ ₀ . n ₀ (λ ₀ ) is a known refractive index of the first reference lens 120 and the second reference lens 125 at a specific wavelength λ ₀ . In the present embodiment, it is assumed that the first reference lens 120 and the second reference lens 125 have the same refractive index and are uniformly distributed in the lens. n ^medium (λ ₀ ) is the refractive index of the medium 71 at a specific wavelength λ ₀ .

L is the sum of L _A (x, y), L _B (x, y), L (x, y), L _C (x, y), and L _D (x, y). That is, there is a relational expression of Expression 2. The right side of Expression 2 is a known amount and is substantially equal to the design value of the thickness of the lens 60 to be tested.

C is an arbitrary constant. The interferometer cannot measure the wavefront, that is, the constant component (DC component) of the equiphase surface. Therefore, the wavefront measured by the interferometer contains an uncertain amount C of a constant component. The term of n ₀ (λ ₀ )L in Expression 1 is a constant term and is a part of the constant term C, but is expressed separately from the constant term C for later calculation. Expression 1 can be transformed into Expression 3 using Expression 2.

Both sides of Equation _3, if divided by the _{L- (L A (x, y} ) + L D (x, y)), the refractive index distribution is expressed by Equation 4 is calculated. The second term on the right side and the third term on the right side in Equation 4 correspond to the calculation error of the refractive index distribution.

  The third term of the equation (4) is a component derived from the uncertain constant term C that cannot be measured by the interferometer, and therefore a large refractive index distribution error may occur depending on the value of C. Therefore, the following correction calculation is performed.

At a specific position (x ₀ , y ₀ ) inside the lens 60 to be inspected, the refractive index of the lens 60 to be inspected is equal to the refractive indices n ₀ (λ ₀ ) of the first and second reference lenses (n ^sample (n ^sample ( λ ₀ , x ₀ , y ₀ )=n ₀ (λ ₀ )). At this time, Expression 3 is expressed as Expression 5 at the position (x ₀ , y ₀ ). Equation 3 after subtracting by Equation _{5, L- (L A (x} , y) + L D (x, y)) if divided by, as in Equation 6, the error of the refractive index distribution derived from the constant term C A refractive index distribution in which no component is mixed can be obtained.

The constant term C may be set as C=0 if the difference between the refractive index of the lens under test 60 and the refractive indices of the first and second reference lenses is small and the refractive index distribution of the lens under test 60 is close to zero. no problem. However, in general, the refractive index of the lens 60 to be tested changes depending on the mold, and a refractive index distribution also occurs. Therefore, if C=0, the accuracy deteriorates. In this embodiment, the transmitted wavefront is measured at a specific wavelength λ _{0 at} which the refractive index of the lens under test 60 is equal to the refractive indexes of the first and second reference lenses, and the constant term C is calculated by the equations 5 and 6. Since the refractive index distribution error derived from is eliminated, the accuracy is high.

  The surface shape of the first surface of the test lens 60 and the surface shape of the second surface of the first reference lens 120, and the surface shape of the second surface of the test lens 60 and the surface shape of the first surface of the second reference lens 125. However, as long as they match within a manufacturing error of the surface shape of the lens to be inspected, the second term on the right side of Expression 6 can be ignored. That is, the approximation of Expression 7 holds.

From the above, the refractive index distribution GI(λ ₀ , x, y) of the subject lens 60 at the specific wavelength is calculated as in Expression 8. Right-hand side of Equation 8 in the denominator _{L- (L A (x, y} ) + L D (x, y)) , since approximately equal to the design value of the thickness of the lens _{60, L- (L A (x} , y) + _L D (x, y) may be substituted at the design value of the thickness of the subject lens 60 instead of).

  The measuring method of the present embodiment is based on the refractive index distribution caused by the shape error of the lens 60 to be inspected (deviation from the design value of the shape) and the refractive index error of the lens 60 to be inspected (deviation of the refractive index from the design value). The calculation error is corrected using the shape and refractive index of the reference lens. Therefore, the refractive index distribution of the optical element can be measured nondestructively and with high accuracy.

The measurement method of the present embodiment measures the refractive index distribution of the lens 60 to be measured without using a medium (matching liquid) having a refractive index substantially equal to the refractive index of the lens 60 to be measured. Accordingly, the refractive index distribution of an optical element of an effective high refractive index matching liquid is not obtained (n _d to 1.8 or higher) can also be calculated with high accuracy.

  In this embodiment, as shown in FIG. 1, the refractive index distribution of the lens 60 to be measured is measured in a configuration in which the light to be measured is vertically incident on the lens 60 to be measured, but as shown in FIG. On the other hand, the refractive index distribution of the lens 60 to be inspected can be measured even if the light to be inspected obliquely enters. By measuring the refractive index distribution of the test lens 60 with the oblique incidence arrangement, the refractive index distribution of the test lens 60 in the optical axis direction can be calculated. The optical axis direction refers to the direction from the first surface apex of the lens 60 to be inspected to the second surface apex.

  First, the transmitted wavefront of the unit to be measured 200 is measured in a plurality of arrangements in which the unit to be measured 200 has different inclinations with respect to the light to be measured. The tilt angle of the unit to be inspected 200 with respect to the light to be inspected is adjusted by controlling the rotation amount of the rotary stage 140 by the computer 90.

  Then, in a plurality of arrangements, the refractive index distribution of the subject lens 60 is calculated. The refractive index distribution calculated in this example is a refractive index distribution projection value obtained by projecting the three-dimensional refractive index distribution of the lens 60 under test in the transmission direction of the test light. The projection value of the refractive index distribution of the lens 60 to be measured with the oblique incidence arrangement has the refractive index distribution information of the lens 60 to be measured in the optical axis direction according to the tilt angle. If the refractive index distribution projection value is calculated in a plurality of arrangements, the refractive index distribution information in the optical axis direction of the subject lens 60 can be extracted.

  5A and 5B show how a light beam travels when the unit 200 to be inspected is tilted with respect to the light to be inspected. As the tilt angle of the unit under test 200 increases, the refraction angle on the surface of the unit under test 200 increases. The larger the refraction angle, the larger the shift amount of the test light that is transmitted with respect to the incident test light. Further, due to the influence of the refraction, there is a problem that the incident angle of the test light on the test lens 60 is smaller than the tilt angle of the test unit 200. If the angle of incidence is small, information about the refractive index distribution of the lens 60 under test in the optical axis direction cannot be obtained.

  5C and 5D show a method for solving the above problem. The influence of refraction is reduced by inserting prisms 150, 151, 155, and 156 having an apex angle that is the same as the tilt angle of the unit under test 200 in front of and behind the unit under test 200.

  The prisms 150, 151, 155, 156 are made of the same material as the first and second reference lenses. A medium 71 is applied to the gaps between the prisms 150, 151, 155, 156 and the unit under test 200 to reduce the influence of refraction in the gaps.

  As shown in FIGS. 5C and 5D, by using the prism having the same apex angle as the tilt angle of the unit under test 200, the projection value of the refractive index distribution at the same incident angle as the tilt angle of the unit under test 200 can be obtained. can get.

  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, the wavelength is scanned by a combination of a light source that emits light of a plurality of wavelengths and a monochromator. A supercontinuum light source is used as a light source that emits light of a plurality of wavelengths, but an optical frequency comb, a super luminescent diode (SLD), a short pulse laser, and a halogen lamp can be used instead.

  Instead of a combination of a light source that emits light of a plurality of wavelengths and a monochromator, a wavelength swept light source may be used, or a multiline laser that discretely emits a plurality of wavelengths may be used. The 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. This embodiment is sufficient if it is a light source that emits light of two or more types of wavelengths.

  In this embodiment, a Mach-Zehnder interferometer is used for the interference optical system. Instead, a Twyman Green interferometer or a Fizeau interferometer can be substituted.

  In this embodiment, the difference between the surface shape of the second surface of the first reference lens 120 and the surface shape of the first surface of the lens 60 to be inspected, and the surface shape of the second surface of the lens 60 to be inspected and the second reference lens. A method of measuring the refractive index distribution in the case where the difference from the surface shape of the first surface is larger than that in the first embodiment will be described. In this embodiment, this difference in surface shape (shape component) is removed by using transmitted wavefronts at two different wavelengths.

  FIG. 6 is a diagram showing a schematic configuration of a refractive index distribution measuring apparatus according to the second embodiment of the present invention. In this embodiment, the transmitted wavefront is measured using the Shack-Hartmann sensor 81 without using the interference optical system used in the first embodiment. The light source 11 is a multi-line gas laser (for example, an argon laser or a krypton laser), and the lens 60 to be inspected is a lens having a negative refractive power. The same components as those in the first embodiment will be described with the same reference numerals.

  The unit to be tested 200 is arranged in the medium 70 filled in the container 50. As shown in FIG. 6, the unit to be inspected 200 according to the present embodiment includes a first reference lens 120, an inspected lens 60, a second reference lens 125, a medium 70, and auxiliary glasses 127, 128, 129. It is a rectangular parallelepiped (or a cylinder). The first surface and the second surface of the lens 60 to be inspected are aspherical surfaces. The second surface of the first reference lens 120 is a spherical surface close to the aspherical shape of the first surface of the lens 60 to be tested, and the first surface of the second reference lens 125 is a non-spherical surface of the second surface of the lens 60 to be tested. It is a spherical surface close to a spherical shape.

In this embodiment, the refractive index n ^sample (λ, a, b) near the cut end (edge) of the lens 60 to be inspected is known. The refractive index of the lens 60 to be inspected can be measured by, for example, the minimum deviation method by processing the vicinity of the edge of the lens 60 to be inspected into a prism shape. The first reference lens 120, the second reference lens 125, and the auxiliary glasses 127, 128, 129 are all made of the same material, and all the surface shapes and the refractive index n ₀ (λ) are known amounts.

  The light emitted from the light source 11 is dispersed by the monochromator 20, passes through the pinhole 30, becomes divergent light, and becomes parallel light by the collimator lens 40. The parallel light passes through the unit to be tested 200 arranged in the medium 70 in the container 50 and is detected by the Shack-Hartmann sensor 81. The signal detected by Shack-Hartmann 81 is sent to the computer 90. A thermometer (not shown) is arranged in the container 50. The refractive index of the medium 70 is calculated by the computer 90 using the temperature of the medium 70 measured by a thermometer and the temperature coefficient of the refractive index of the medium 70.

  A method of calculating the refractive index distribution of the lens 60 to be tested in this example will be described below. In this embodiment, first, the unit to be inspected 200 is configured by sandwiching the lens to be inspected 60 between the first reference lens 120, the second reference lens 125, the medium 70, and the auxiliary glasses 127, 128, 129, and the unit to be inspected 200. Are placed on the optical path in the container 50.

Then, the first transmitted wavefront W(λ ₁ , x, y) of the unit under test 200 at the first wavelength λ ₁ (for example, λ ₁ =457 nm) and the second wavelength λ ₂ (for example, λ ₂ = The second transmitted wavefront W(λ ₂ , x, y) of the unit under test 200 at 514 nm) is measured. The transmitted wavefront W(λ, x, y) at the wavelength λ is expressed by Equation 9. Furthermore, Expression 9 can be transformed into Expression 11 by using the approximation of Expression 2 and Expression 10. Expression 11 is expressed as Expression 12 at the coordinates (a, b). Formula 13 is a formula in which Formula 12 is subtracted from Formula 11 to eliminate the constant term C. The meanings of the symbols in the formulas are the same as in the first embodiment.

The shape and refractive index of the first reference lens 120, the shape and refractive index of the second reference lens 125, the transmitted wavefront of the unit under test 200 at the first wavelength, and the transmitted wavefront of the unit under test 200 at the second wavelength. By using the difference (shape component) L _B (x,y)−L _B (0,0)+L _C (x,y)− between the surface shapes of the first and second reference lenses and the lens 60 to be tested. L _C (0,0) can be removed.

The refractive index distribution GI(λ ₁ , x, y) of the lens 60 to be measured at the first wavelength is calculated as in Expression 15 using the approximation in Expression 14. The refractive index distribution GI(λ ₂ , x, y) of the lens 60 to be tested at the second wavelength is expressed by Expression 16 using Expressions 14 and 15.

  In this embodiment, when the surface shape difference between the first and second reference lenses and the test lens is large, the surface shape difference (shape component) is removed by using the transmitted wavefronts at two different wavelengths, and the refractive index. The distribution was calculated. In order to remove the shape component, two types of media having different refractive indexes may be used instead of the two types of wavelengths. If the unit 200 to be measured is configured by using two types of media having different refractive indices and the transmitted wavefront of the unit 200 to be measured is measured, the shape component can be removed to calculate the refractive index distribution. The method is as follows.

The test lens 60 is sandwiched between the first reference lens 120, the second reference lens 125, the _first ^medium having the first refractive index n ₁ ^medium (λ), and the auxiliary glasses 127, 128, 129, It constitutes a first unit to be tested. Then, the first transmitted wavefront of the first unit under test is measured. The first transmitted wavefront W ₁ (λ,x,y) of the first unit to be tested is expressed by Expression 17.

The first reference lens 120, the second reference lens 125, the _second ^medium having the second refractive index n ₂ ^medium (λ) different from the _first refractive index n ₁ ^medium (λ), and the auxiliary glasses 127 and 128. , 129 to sandwich the lens 60 to be inspected to form a second unit to be inspected. Then, the second transmitted wavefront of the second unit under test is measured. The second transmitted wavefront W ₂ (λ, x, y) of the second unit to be tested is expressed by Expression 18.

  Finally, using the shape and refractive index of the first reference lens 120, the shape and refractive index of the second reference lens 125, the first transmitted wavefront, and the second transmitted wavefront, the shape of the lens 60 to be inspected. The refractive index distribution is calculated by removing the components. The constant term C can be erased by the same method as in Expressions 12 and 13. The refractive index distribution GI(λ, x, y) of the lens 60 to be inspected is represented by Expression 19.

  Instead of using two kinds of wavelengths and two kinds of media, a transmitted wavefront of a reference unit to be measured, which is constructed by sandwiching a reference sample lens having a specific refractive index distribution using the first and second reference lenses, is used. Thus, the influence of the difference in surface shape between the first and second reference lenses and the lens 60 to be inspected can be reduced.

In this embodiment, a uniform refractive index distribution of the refractive index n ^sample (λ,a,b) is assumed as the refractive index distribution of the reference test lens. The design value of the test lens 60 is used as the shape of the reference test lens. The transmitted wavefront W _sim (λ, x, y) of the reference measurement unit configured by sandwiching the reference measurement lens by using the first reference lens and the second reference lens is represented by Expression 20. .. However, C'is an arbitrary constant.

δL _B (x, y) is the difference between the first surface shape of the lens under test 60 and the first surface shape of the reference lens under test, and δL _C (x, y) is the second surface shape of the lens under test 60 and the reference This is the difference from the second surface shape of the lens under test. Using the transmitted wavefront W _sim (λ, x, y) of this reference unit under test, the influence of the difference in surface shape between the first and second reference lenses and the lens under test 60 (the second term on the right side of Expression 11). Can be reduced.

The difference between the transmitted wavefront W(λ,x,y) (Equation 11) of the unit to be measured 200 and the transmitted wavefront W _sim (λ,x,y) (Equation 20) of the reference unit to be inspected is as shown in Equation 21. Represented by.

Since δL _B (x, y) and δL _C (x, y) are values that are as small as the manufacturing error of the lens under test 60, the approximation of Expression 22 is established. If the constant term CC′ is deleted in the same manner as in Expressions 21 and 13 from Expressions 21 and the approximation of Expression 22 is used, the refractive index distribution GI(λ, x, y) of the lens 60 under test is It is expressed as 23.

  In this embodiment, the Shack-Hartmann sensor 81 is used to measure the transmitted wavefront of the unit under test 200. A shearing interferometer such as a Talbot interferometer may be used instead of the Shack-Hartmann sensor.

  In this embodiment, the wavelength is controlled using the monochromator 20, but a wavelength selection filter may be used instead of the monochromator.

  In this embodiment, a light source (multi-line gas laser) that emits a plurality of wavelengths is used. However, when a method using two types of media or a method using a reference test lens is used, a light source with a single wavelength (for example, , HeNe laser).

  It is also possible to feed back the result of the refractive index distribution measured using the apparatus and method described in the first and second embodiments to the method for manufacturing an optical element such as a molded lens.

  FIG. 7 shows an example of a manufacturing process of an optical element using molding.

  The optical element is manufactured through an optical element designing step, a die designing step, and an optical element molding step using the designed die. The shape accuracy of the molded optical element is evaluated, and if the accuracy is insufficient, the mold is corrected and molding is performed again. If the shape accuracy is good, the optical performance of the optical element is evaluated. By incorporating the refractive index distribution measuring method of the present invention in this optical performance evaluation step, it becomes possible to mass-produce optical elements molded using a high refractive index glass material as a base material. If the optical performance is low, redesign the optical element with the corrected optical surface.

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

10 Light Source 60 Test Lens 71 Medium 80 Detector 90 Computer 120 First Reference Lens 125 Second Reference Lens 200 Test Unit

Claims (13)

A transmitted wavefront of a unit to be measured, which is configured by sandwiching the lens to be measured with a first reference lens having a known shape and a refractive index, a second reference lens having a known shape and a refractive index, and a medium, is measured Measurement step,
Using the shape and refractive index of the first reference lens, the shape and refractive index of the second reference lens, and the transmitted wavefront of the unit under test, an uncertain constant component of the transmitted wavefront of the unit under test is obtained. Compensating the refractive index distribution error resulting from, a calculation step of calculating the refractive index distribution of the lens under test,
A method of measuring a refractive index distribution, comprising:   The first reference lens and the second reference lens are formed of the same material, and the first reference lens and the second reference lens have a refractive index equal to the refractive index of the test lens at a specific wavelength. The refractive index distribution measuring method according to claim 1, wherein the method has the refractive index distribution measuring method. Measuring the transmitted wavefront of the unit under test at a plurality of wavelengths,
Determining the specific wavelength from the transmitted wavefront of the unit under test at the plurality of wavelengths,
The refractive index distribution measuring method according to claim 2, wherein the refractive index distribution of the lens to be tested is calculated using a transmitted wavefront at the specific wavelength. Measuring the transmitted wavefront of the unit to be inspected for a plurality of arrangements in which the inclination of the unit to be inspected with respect to the light to be inspected is different,
The refraction according to any one of claims 1 to 3, wherein the refractive index distribution in the optical axis direction of the lens under test is calculated by using the transmitted wavefronts of the unit under test in the plurality of arrangements. Rate distribution measurement method. Measuring a transmitted wavefront of the unit under test at a first wavelength and a transmitted wavefront of the unit under test at a second wavelength different from the first wavelength;
The shape and the refractive index of the first reference lens, the shape and the refractive index of the second reference lens, the transmitted wavefront of the unit under test at the first wavelength, and the unit of the unit under test at the second wavelength. The refractive index distribution according to any one of claims 1 to 4, wherein a shape component of the lens under test is removed by using a transmitted wavefront to calculate a refractive index distribution of the lens under test. Distribution measurement method. A transmitted wavefront of a first unit to be tested formed by sandwiching the lens to be tested by using the first reference lens, the second reference lens, and a first medium having a first refractive index. Measure
A second configuration formed by sandwiching the lens under test using the first reference lens, the second reference lens, and a second medium having a second refractive index different from the first refractive index. Measure the transmitted wavefront of the unit to be tested,
Using the shape and refractive index of the first reference lens, the shape and refractive index of the second reference lens, the transmitted wavefront of the first unit under test, and the transmitted wavefront of the second unit under test The refractive index distribution measuring method according to any one of claims 1 to 4, wherein the shape component of the test lens is removed to calculate the refractive index distribution of the test lens.   A shape and a refractive index of the first reference lens, a shape and a refractive index of the second reference lens, a transmitted wavefront of the unit to be tested, a specific reference lens using the first reference lens and the second reference lens. 5. The refractive index distribution of the test lens is calculated using the transmitted wavefront of the reference test unit formed by sandwiching the reference test lens having a refractive index distribution. The refractive index distribution measuring method according to any one of items. 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 7. A method for manufacturing an optical element, comprising: Measurement for measuring a transmitted wavefront of a unit to be tested, which is formed by sandwiching a lens to be tested with a first reference lens having a known shape and refractive index, a second reference lens having a known shape and refractive index, and a medium. Means and
Using the shape and refractive index of the first reference lens, the shape and refractive index of the second reference lens, and the transmitted wavefront of the unit under test, an uncertain constant component of the transmitted wavefront of the unit under test is obtained. Correcting the refractive index distribution error derived from, a calculating means for calculating the refractive index distribution of the lens under test,
A refractive index distribution measuring device comprising:   The refractive index distribution measuring device according to claim 9, wherein the first and second reference lenses have a flat surface on the side opposite to the surface facing the lens to be tested.   The refractive index distribution measuring device according to claim 9 or 10, wherein the measuring unit has a Mach-Zehnder interferometer.   The refractive index distribution measuring device according to claim 9 or 10, wherein the measuring unit includes a Shack-Hartmann sensor.   The refractive index distribution measuring apparatus according to claim 9 or 10, wherein the measuring unit has a shearing interferometer.

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