JP2004226502A - Interferometer reference lens - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an interferometer reference lens whose constitution is made more compact without reducing a converging angle while securing a satisfactory aberration performance. <P>SOLUTION: The reference lens is constituted by arranging a 1st positive lens whose convex faces an incident side, a 2nd positive meniscus lens whose convex faces the incident side, a 3rd positive meniscus lens whose convex faces the incident side and a 4th meniscus lens whose convex faces the incident side in this order from the incident side of the incident luminous flux, and also, the lens is constituted so as to satisfy conditional expressions (1) and (2), then, the reduction of the reference lens in size and weight is attained, and also, the convex spherical surface of a comparatively large depth angle can be measured while suppressing increase in spherical aberration. <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a reference lens for an interferometer used in an optical interferometer, and for example, to a reference lens for an interferometer used in a Fizeau interferometer applicable to, for example, surface accuracy measurement of an optical component.
[0002]
[Prior art]
Generally, an interferometer is often used to measure the surface accuracy of an optical component such as an optical lens or a mirror. In particular, a Fizeau interferometer having a reference lens manufactured with high precision is widely used as an interferometer having a simple structure and a relatively wide field of view and capable of measuring with high precision.
[0003]
In this Fizeau interferometer, measurement of a spherical surface (hereinafter, referred to as a spherical surface to be measured) such as a lens surface is performed as follows. First, a parallel incident light beam is condensed by a reference lens, converted into a spherical wave, and radiated to a spherical surface to be measured. At this time, the direction of the normal line (the traveling direction) of the spherical wave is made to coincide with the direction of the normal line of the test spherical surface, whereby the vertical reflected light beam (hereinafter, referred to as test light) from the test spherical surface. obtain. On the other hand, a part of the incident light beam is reflected on an exit surface (hereinafter, referred to as a reference spherical surface) of the reference lens which is closest to the test spherical surface to obtain a reflected light beam (reference light). Then, by causing the test light and the reference light to interfere with each other to generate interference fringes and observing the interference fringes, the shape of the test spherical surface can be measured.
[0004]
For a reference lens used in a Fizeau interferometer, the numerical aperture NA on the spherical surface side to be measured is sufficiently large (NA ≧ 0.806), and the expected angle (the maximum outer diameter of the spherical surface to be measured from the center of curvature of the spherical surface to be measured) is determined. Even if it is a convex spherical surface to be detected having a large expected angle θ1, a spherical wave luminous flux converging at a convergence angle θ2 larger than the expected angle θ1 is formed, and the surface accuracy is improved over the entire spherical surface to be detected. There is one that can be measured with high accuracy (for example, see Patent Document 1). In addition, the present applicant previously disclosed a very compact interferometer reference lens having a small lens outer diameter of at most 130.0 mm while securing a sufficiently large numerical aperture NA (for example, NA ≒ 0.833). (See Patent Document 2). Note that the relationship between the numerical aperture NA and the convergence angle θ2 is
NA = sin (θ2 / 2)
It is expressed as
[0005]
[Patent Document 1]
JP 2000-346613 A [Patent Document 2]
Japanese Patent Application No. 2002-029195 [0006]
[Problems to be solved by the invention]
However, the reference lens described in Patent Literature 1 or Patent Literature 2 is suitably used for measuring a spherical surface to be inspected having a larger radius of curvature and a larger outer diameter, and the number of constituent lenses is large. Both size and weight are relatively large. Therefore, when a spherical surface to be inspected whose curvature radius and outer diameter are not so large is set as an object to be measured, it lacks compactness and is difficult to handle. Further, the above-mentioned reference lens is expensive due to its configuration. Therefore, a more compact and inexpensive reference lens has been desired.
[0007]
The present invention has been made in view of such a problem, and an object of the present invention is to provide a reference lens for an interferometer having a more compact configuration without reducing the convergence angle while ensuring good aberration performance. It is in.
[0008]
[Means for Solving the Problems]
The reference lens for an interferometer according to the present invention collects the reference light obtained by reflecting a part of the incident light beam on the reference spherical surface and the light beam that has passed through the reference spherical surface toward a focal point that coincides with the center of curvature of the reference spherical surface. Interferometer configured to measure the shape of the test sphere by interfering with the test light obtained by reflecting the light on the test sphere having a center of curvature coinciding with the focal point after the light is illuminated. Reference lens for an interferometer, which is, in order from the incident side of the incident light beam, a positive first lens having a convex surface facing the incident side, and a positive second lens having a meniscus shape having a convex surface facing the incident side And a positive third lens having a meniscus shape with a convex surface facing the incident side, and a fourth lens having a meniscus shape with a convex surface facing the incident side are arranged and configured. (1) And it is obtained so as to satisfy (2).
2.2 ≦ f3 / f ≦ 2.8 (1)
0.75 ≦ R5 / f ≦ 0.83 (2)
Here, f indicates the focal length of the entire reference lens system for the interferometer, f3 indicates the focal length of the third lens, and R5 indicates the radius of curvature of the third lens on the incident side surface. The “positive lens” means a lens having a positive refractive power.
[0009]
In the reference lens for an interferometer according to the present invention, the first lens functions to focus the incident light beam and reduce the light beam diameter. Subsequent second and third lenses exhibit a function of condensing the incident light beam transmitted through the first lens stepwise so as to gradually increase the convergence angle. Further, the fourth lens functions to focus the incident light flux emitted from the third lens toward the focal point. In addition, a proper positive power range of the third lens is defined by the conditional expression (1). Thus, the maximum radius of curvature and the outer diameter of the measurable spherical surface to be measured are defined, and the radius of curvature of a favorable range in the subsequent fourth lens is defined. Further, an appropriate radius of curvature on the exit-side surface of the third lens is defined by the conditional expression (2), thereby enabling aberration correction.
[0010]
It is preferable that the reference lens for an interferometer according to the present invention is further configured to satisfy the following conditional expression (3).
3.6 ≦ f2 / f ≦ 5.4 (3)
Here, f2 indicates the focal length of the second lens.
[0011]
It is preferable that the reference lens for an interferometer according to the present invention is further configured to satisfy the following conditional expression (4).
0.8 ≦ NA (4)
Here, NA indicates the numerical aperture on the test spherical surface side of the entire interferometer reference lens system.
[0012]
In the reference lens for an interferometer according to the present invention, it is preferable that the refractive indexes of the first to fourth lenses are all 1.75 or more.
[0013]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
[0014]
First, a Fizeau interferometer 1 to which an interferometer reference lens (hereinafter simply referred to as a reference lens) according to an embodiment of the present invention will be described with reference to FIG. FIG. 1 shows a configuration example of a Fizeau interferometer 1 according to the present embodiment. The Fizeau interferometer 1 includes a laser light source 2, a laser beam diverging lens 3, a half mirror 4, a collimator lens 5, a reference lens 6 (6A to 6G), an imaging lens 8, and an image sensor 9. It is provided with. The reference lens 6 (6A to 6G) is selected from any one of seven different cross-sectional configuration examples described later, but is collectively referred to as “reference lens 6” when no distinction is necessary. I do. On the optical axis Z1, an imaging element 9, an imaging lens 8, a half mirror 4, a collimator lens 5, and a reference lens 6 are arranged in this order. On the other hand, the laser beam diverging lens 3 and the laser light source 2 are sequentially arranged in the axial direction passing through the half mirror 4 and orthogonal to the optical axis Z1. The laser beam diverging lens 3 has a function of expanding the beam diameter of the laser beam emitted from the laser light source 2. The half mirror 4 has an inclination of 45 ° with respect to the optical axis Z1, and reflects a part of the irradiated light beam and transmits the remaining light beam. The collimator lens 5 has a function of converting an incident light beam into a parallel light beam. The reference lens 6 is a lens group in which an anti-reflection film is applied to all surfaces except for a reference spherical surface F, which will be described later. The spherical surface 7 to be measured, which is an object to be measured, is disposed such that the center of curvature coincides with the focal point P. Note that, in the example of FIG. 1, the description is made on the assumption that the test spherical surface 7 is a convex spherical surface. Here, the surface of the reference lens 6 that faces the test spherical surface 7 (the surface closest to the test spherical surface 7) is referred to as a reference spherical surface F. The center of curvature of the reference spherical surface F also coincides with the focal point P. The imaging lens 8 condenses the light reflected on the reference spherical surface F and the light reflected on the test spherical surface 7 and projects interference fringes on the image sensor 9. The imaging element 9 has a function of capturing the projected interference fringes as image data.
[0015]
In the Fizeau interferometer 1 having such a configuration, the surface accuracy of the spherical surface 7 to be measured is measured as follows. First, a laser beam having a desired wavelength is emitted from the laser light source 2. The diameter of the laser beam is expanded by the laser beam diverging lens 3. A part of the expanded laser beam is reflected by the half mirror 4 in a right angle direction, that is, a direction along the optical axis Z1. The reflected laser beam enters the collimator lens 5 while further expanding the beam diameter. The laser beam passes through the collimator lens 5 and is converted into a parallel laser beam, and then enters the reference lens 6 as it is. The light beam incident on the reference lens 6 is incident on the reference spherical surface F, and most of the incident light beam is transmitted without being refracted. However, the reference spherical surface F is left with a slight reflectance by, for example, applying no anti-reflection film or applying an anti-reflection film. For this reason, a part of the incident light beam is reflected here vertically without passing through the reference spherical surface F. This reflected light beam becomes the reference light. On the other hand, the light beam transmitted through the reference spherical surface F is focused toward the focal point P. In the example of FIG. 1, since the test spherical surface 7 exists between the reference spherical surface F and the focal point P, a vertical reflected light beam from the test spherical surface 7, that is, the test light is generated. However, when the test spherical surface 7 is concave, the light beam emitted from the reference spherical surface F is once focused on the focal point P, and then irradiates the test spherical surface 7 while expanding again to generate test light. The reference light and the test light travel along the path of the incident light in reverse. The light sequentially follows the reference lens 6 and the collimator lens 5, travels straight through the half mirror 4, and reaches the imaging lens 8. The test light and the reference light are condensed by the imaging lens 8 and form interference fringes on the image sensor 9. The interference fringes represent the surface shape of the spherical surface 7 to be measured, and the surface accuracy of the spherical surface 7 to be measured can be measured by observing or analyzing the interference fringes.
[0016]
Next, the reference lens 6 according to the present embodiment will be described in more detail with reference to FIGS. 2 to 8 show configuration examples of the cross sections of the reference lenses 6A to 6G, and correspond to Numerical Examples 1 to 7 (FIGS. 9 to 15) described later.
[0017]
First, the configuration of the reference lens 6 will be described.
[0018]
2 to 8, the side indicated by reference sign Zinc is the incident side (light source side), while the side indicated by reference sign Zemit is the exit side (test spherical surface side). The symbol Ri indicates the radius of curvature on the i-th (i = 1 to 8) optical surface (Si) that sequentially increases toward the emission side, with the most incidence side surface (incident light beam incidence surface) as the first. The symbol Di indicates a surface interval between the i-th optical surface (Si) and the (i + 1) -th optical surface (Si + 1) on the optical axis Z1. Here, D8 indicates the distance between the surface S8 and the focal point P, and in this case, coincides with the radius of curvature R8. The symbol Lj indicates a j-th (j = 1 to 4) lens sequentially increasing toward the emission side, with the lens on the most incidence side being the first lens. Further, the symbol θ2 indicates the convergence angle θ2 of the incident light beam focused toward the focal point P.
[0019]
The reference lens 6 has, in order from the incident side of the incident light beam, a positive first lens L1 having a convex surface facing the incident side and a positive first lens L having a meniscus shape having a convex surface facing the incident side along the optical axis Z1. The two lenses L2, the positive third lens L3 having a meniscus shape with the convex surface facing the incident side, and the fourth lens L4 having the meniscus shape with the convex surface facing the incident side are provided. The concave surface on the exit side of the fourth lens L4, that is, the incident light beam exit surface S8 is the reference spherical surface F.
[0020]
In FIG. 2, the first lens L1 has a biconvex shape. However, the first lens L1 may have a meniscus shape as shown in FIGS. 3 to 8, or may have a plano-convex shape. Further, the fourth lens L4 is not limited to the positive refractive power, but may have a negative refractive power.
[0021]
The reference lens 6 is configured to satisfy the following conditional expressions (1) and (2) in order to ensure good aberration performance and compactness. Here, f indicates the focal length of the entire system of the reference lens 6A, f3 indicates the focal length of the third lens L3, and R5 indicates the radius of curvature of the incident surface S5 of the third lens L3.
2.2 ≦ f3 / f ≦ 2.8 (1)
0.75 ≦ R5 / f ≦ 0.83 (2)
[0022]
It is preferable that the reference lens 6 be configured so as to further satisfy the following conditional expression (3) in order to secure more excellent aberration performance and ease of manufacturing. Here, f2 indicates the focal length of the second lens L2.
3.6 ≦ f2 / f ≦ 5.4 (3)
[0023]
It is desirable that the reference lens 6 be further configured to satisfy the following conditional expression (4) in order to measure the spherical surface 7 to be examined having a larger prospective angle θ1. Here, NA is the numerical aperture on the test spherical surface side of the entire reference lens 6A.
0.8 ≦ NA (4)
[0024]
Further, the reference lens 6 is configured so that all of the first to fourth lenses L1 to L4 have a refractive index Nj (j = 1 to 4) of 1.75 or more, in order to make it easier to manufacture. It is desirable.
[0025]
Next, the optical function and effect of the reference lens 6 will be described.
[0026]
The light beam incident on the reference lens 6 sequentially passes through the first to fourth lenses L1 to L4, and is converged toward the focal point P.
[0027]
Here, the first lens L1 mainly exhibits a light beam focusing function. The light beam focusing function is a function of focusing the light beam diameter of the incident light beam. Assuming that the incident light beam incident surface S1 is concave toward the incident side, the radius of curvature R8 of the reference spherical surface F (surface S8) of the reference lens 6 increases, but the size of the entire reference lens 6 increases. . Further, since the power shared by the subsequent lens increases, it becomes difficult to correct aberrations with a small number of lenses. On the other hand, in the present embodiment, since the first lens L1 is a positive lens whose convex surface (incident light beam incident surface S1) faces the incident side, the incident light beam is focused by transmitting through the first lens L1. And the luminous flux diameter is reduced. As a result, the reference lens 6 can be configured with a small number of lenses, and its maximum diameter can be reduced.
[0028]
Since the second and third lenses L2 and L3 each have a positive meniscus shape with the convex surface facing the incident side, it is possible to avoid a sharp bending of the light beam when passing through each surface. . That is, it is possible to reduce the occurrence of spherical aberration in each lens while converging the numerical aperture NA of the incident light beam so as to gradually increase. However, the beam diameter gradually decreases as the convergence angle θ2 (numerical aperture NA) increases.
[0029]
The fourth lens L4 has a surface S8 (reference spherical surface F) that is concave toward the exit side so that the convex spherical surface 7 to be measured can be measured, and transmits a light beam emitted from the third lens L3. It functions to form a spherical wave having the focal point P as the center of curvature and having the same radius of curvature as the surface S8. The spherical wave emitted from the surface S8 advances toward the focal point P while gradually reducing the radius of curvature, and is perpendicularly incident on the test spherical surface 7 (the normal direction of the test spherical surface 7 matches the normal direction of the wavefront). Incident). Therefore, when the test spherical surface 7 is a convex spherical surface, the radius of curvature R8 of the surface S8 defines the maximum radius of curvature that can be measured. In the case of a concave spherical surface, there is naturally no limitation on the radius of curvature R8. Further, since the surface S7 on the incident side has a convex shape toward the incident side, it is possible to efficiently correct spherical aberration.
[0030]
Conditional expression (1) defines an appropriate positive power range of the third lens L3. When the value goes below the lower limit of conditional expression (1), the power of the third lens L3 becomes too strong, and the thickness of the peripheral portion of the third lens L3 becomes thin, which makes it difficult to manufacture. Alternatively, the radius of curvature R8 of the surface S8 (reference spherical surface F) becomes small, so that a convex spherical surface having a relatively large radius of curvature cannot be measured. Further, spherical aberration cannot be sufficiently corrected. On the other hand, when the value exceeds the upper limit of the conditional expression (1), the power of the third lens L3 becomes weak, so that the radius of curvature R7 on the incident surface S7 of the subsequent fourth lens L4 becomes too large. Therefore, high-order spherical aberration occurs with high sensitivity to the dimensional error of the surface S7, and extremely high dimensional accuracy is required in manufacturing.
[0031]
Conditional expression (2) defines an appropriate radius of curvature R5 on the exit-side surface S5 of the third lens L3. When the value goes below the lower limit of conditional expression (2), the light beam from the second lens L2 is sharply bent, and spherical aberration is likely to occur. Further, since the angle for viewing the surface S5 becomes large, uniform polishing becomes difficult. On the other hand, when the value exceeds the upper limit of the conditional expression (2), the light beam from the second lens L2 cannot be sufficiently condensed. Therefore, the curvature radius R7 of the subsequent fourth lens L4 must be reduced accordingly. Therefore, spherical aberration is likely to occur in the fourth lens L4.
[0032]
Conditional expression (3) defines an appropriate positive power range of the second lens L2. If the lower limit of conditional expression (3) is exceeded, the power of the second lens L2 will be too strong. For this reason, the sensitivity to the spherical aberration caused by the change in the distance D4 between the surface S4 of the second lens L2 and the surface S5 of the third lens L3 is changed to the spherical aberration caused by the change in the center thickness D7 of the subsequent fourth lens L4. Therefore, it becomes difficult to perform correction by adjusting the surface interval or the like. On the other hand, when the value exceeds the upper limit of the conditional expression (3), the power of the second lens L2 becomes insufficient, so that it is necessary to extremely increase the power distribution that the subsequent third lens L3 bears. For this reason, the angle at which the surface S5 of the third lens L3 is viewed becomes too large, and uniform polishing of the surface S5 becomes difficult. Further, if the first lens L1 bears the power instead of the third lens L3, the convergence angle becomes too large immediately after the light enters the reference lens 6, that is, at the time when the light passes through the first lens L1, and the luminous flux The diameter becomes smaller. As a result, the radius of curvature R8 of the surface S8 (reference spherical surface F) is reduced, and the maximum measurable radius of curvature of the convex spherical surface (test spherical surface 7) is reduced.
[0033]
In addition, since the first to fourth lenses L1 to L4 all have a refractive index Nj (j = 1 to 4) of 1.75 or more, appropriate power distribution in the first to fourth lenses L1 to L4 is performed. You. When the refractive index Nj of any of the first to fourth lenses L1 to L4 is less than 1.75, the radius of curvature R5 of the third lens L3 or the radius of curvature R7 of the fourth lens L4 becomes too small, and the surface S5 or the surface It becomes difficult to perform uniform polishing in S7.
[0034]
As described above, according to the present embodiment, by satisfying each of the conditional expressions having the above-described configuration, it is suitable for a Fizeau interferometer and has a standard excellent in balance between optical performance and practical use. You can get a lens. Specifically, various aberrations such as spherical aberration are suppressed without increasing the diameter of the incident light beam, and a relatively large convergence angle θ2 can be ensured even with a small number of lenses of four. This makes it possible to measure a convex spherical surface (the spherical surface 7 to be measured) having a relatively large prospective angle θ1 while being lightweight and compact as a whole.
[0035]
【Example】
Next, several examples of the reference lens according to the above embodiment will be described. Hereinafter, first to seventh numerical examples (Examples 1 to 7) will be described collectively.
[0036]
<Examples 1 to 7>
9 to 15 show lens data as Examples 1 to 7 corresponding to the configurations of the reference lenses 6A to 6G (FIGS. 2 to 8), respectively. In the column of the surface number Si in each drawing, the number of the optical surface that sequentially increases toward the emission side is shown with the most incidence side surface (that is, the incidence surface of the first lens L1) as the first. The column of the radius of curvature Ri indicates a value corresponding to the reference sign Ri shown in FIGS. 2 to 8, and the column of the surface interval Di indicates the value corresponding to the reference sign Di shown in FIGS. 2 to 8. The unit of the value of the radius of curvature Ri and the surface interval Di is millimeter (mm). Further, the column of the refractive index Nj shows the value of the refractive index of the j-th (j = 1 to 4) lens Lj from the incident side in the He-Ne laser beam (wavelength λ = 632.8 nm). In Examples 1 to 7, the refractive indexes Nj are all 1.79885 (> 1.75), which satisfies the conditional expression (4). Note that the lens data in each example is standardized such that the incident light beam diameter Φin is 100 mm.
[0037]
FIG. 16 shows numerical values corresponding to the numerical apertures NA, the total length TL (mm) of the reference lens, the focal length f of the entire reference lens system, and the above-described conditional expressions (1) to (3) for Examples 1 to 7. It is shown. As shown in FIG. 16, in Examples 1 to 7, the numerical apertures NA are all greater than 0.8, and are configured to increase sequentially from Example 1. Accordingly, the radius of curvature R8 of the surface S8 (reference spherical surface F) is configured to decrease in order from the first embodiment. In Examples 1 to 7, it is clear that all of the conditional expressions (1) to (3) are satisfied.
[0038]
17 to 23 are aberration diagrams illustrating spherical aberration of the reference lenses 6A to 6G of Examples 1 to 7. 17 to 23 are aberrations with respect to a He—Ne laser beam (wavelength λ = 632.8 nm).
[0039]
As can be seen from the lens data and the aberration diagrams described above, the spherical aberrations of the reference lenses 6A to 6G of Examples 1 to 7 are favorably corrected. In addition to this, a light and compact structure is realized while securing a somewhat large convergence angle θ2. Specifically, a four-lens configuration having a total lens length TL of 49.457 mm to 56.783 mm is provided, and a convergence angle θ2 of 107.50 ° to 151.86 ° is secured. In each of the first to seventh embodiments, the incident light beam diameter Φin is standardized as 100 mm, so that the maximum lens outer diameter φmax is about 102 mm even if a margin of 1 mm is secured on each side.
[0040]
As described above, the present invention has been described with reference to the embodiment and some examples. However, the present invention is not limited to these embodiments and examples, and can be variously modified. For example, the values of the radius of curvature R, the surface interval D, and the refractive index N of each lens component are not limited to the values shown in the above-described numerical examples, and can take other values.
[0041]
【The invention's effect】
As described above, according to the reference lens for an interferometer according to any one of claims 1 to 4, the positive first lens whose convex surface faces the incident side in order from the incident side of the incident light beam. A positive second lens having a meniscus shape having a convex surface facing the incident side, a positive third lens having a meniscus shape having a convex surface facing the incident side, and a positive third lens having a meniscus shape having a convex surface facing the incident side. Since four lenses are provided and conditional expressions (1) and (2) are satisfied, the reference lens can be reduced in size and weight, and a comparison can be made while suppressing an increase in spherical aberration. It is possible to measure a convex spherical surface having an extremely large prospect angle. That is, it is possible to easily obtain an interferometer reference lens in which the required performance and size are well balanced.
[Brief description of the drawings]
FIG. 1 is a configuration diagram schematically showing a Fizeau interferometer to which a reference lens according to an embodiment of the present invention is applied.
FIG. 2 illustrates a configuration example of a reference lens illustrated in FIG. 1, and is a cross-sectional view corresponding to a first numerical example (Example 1).
FIG. 3 is a cross-sectional view illustrating a configuration example of a reference lens illustrated in FIG. 1 and corresponding to a second numerical example (Example 2).
FIG. 4 is a cross-sectional view illustrating a configuration example of a reference lens illustrated in FIG. 1 and corresponding to a third numerical example (Example 3).
FIG. 5 is a cross-sectional view illustrating a configuration example of a reference lens illustrated in FIG. 1 and corresponding to a fourth numerical example (Example 4).
FIG. 6 is a cross-sectional view illustrating a configuration example of a reference lens illustrated in FIG. 1 and corresponding to a fifth numerical example (Example 5).
FIG. 7 is a cross-sectional view illustrating a configuration example of the reference lens illustrated in FIG. 1 and corresponding to a sixth numerical example (Example 6).
FIG. 8 is a cross-sectional view illustrating a configuration example of the reference lens illustrated in FIG. 1 and corresponding to a seventh numerical example (Example 7).
FIG. 9 is an explanatory diagram showing lens data as Example 1 of the reference lens shown in FIG. 2;
FIG. 10 is an explanatory diagram showing lens data as Example 2 of the reference lens shown in FIG. 3;
FIG. 11 is an explanatory diagram showing lens data as Example 3 of the reference lens shown in FIG. 4;
FIG. 12 is an explanatory diagram showing lens data as Example 4 of the reference lens shown in FIG. 5;
FIG. 13 is an explanatory diagram showing lens data as Example 5 of the reference lens shown in FIG. 6;
FIG. 14 is an explanatory diagram showing lens data as Example 6 of the reference lens shown in FIG. 7;
FIG. 15 is an explanatory diagram showing lens data as Example 7 of the reference lens shown in FIG. 8;
FIG. 16 is an explanatory diagram showing data of conditional expressions satisfied by the reference lens according to the first to seventh numerical examples.
FIG. 17 is an aberration diagram showing a spherical aberration in the reference lens of Example 1;
FIG. 18 is an aberration diagram showing a spherical aberration in the reference lens of Example 2;
FIG. 19 is an aberration diagram showing a spherical aberration in the reference lens of Example 3;
FIG. 20 is an aberration diagram showing a spherical aberration of the reference lens of Example 4;
FIG. 21 is an aberration diagram showing a spherical aberration in the reference lens of Example 5;
FIG. 22 is an aberration diagram showing a spherical aberration of the reference lens of Example 6;
FIG. 23 is an aberration diagram showing a spherical aberration of the reference lens of Example 7;
[Explanation of symbols]
R1 to R8: radius of curvature, D1 to D8: surface interval, N1 to N4: refractive index, L1 to L4: lens component, F: reference spherical surface, P: focal point, Z1: optical axis, 1: Fizeau interferometer, 2 ... Laser light source, 3 laser beam diverging lens, 4 half mirror, 5 collimator lens, 6 (6A to 6G) reference lens (reference lens) for interferometer, 7 spherical surface to be measured, 8 lens for imaging, 9 ... Image sensor.

Claims (4)

The reference light obtained by reflecting a part of the incident light beam on the reference spherical surface, and the light beam that has passed through the reference spherical surface is condensed so as to be directed to a focal point that coincides with the center of curvature of the reference spherical surface. Interferometer reference used in an interferometer configured to measure the shape of the test spherical surface by interfering with the test light obtained by being reflected by the test spherical surface having a matching center of curvature A lens,
In order from the incident side of the incident light beam,
A first positive lens having a convex surface facing the incident side;
A positive second lens having a meniscus shape with the convex surface facing the incident side,
A positive third lens having a meniscus shape with the convex surface facing the incident side,
A fourth lens having a meniscus shape with a convex surface facing the incident side, and a reference lens for an interferometer, characterized by satisfying the following conditional expressions (1) and (2): .
2.2 ≦ f3 / f ≦ 2.8 (1)
0.75 ≦ R5 / f ≦ 0.83 (2)
However,
f: focal length of the entire interferometer reference lens system f3: focal length of the third lens R5: radius of curvature on the incident side surface of the third lens. 2. The interferometer reference lens according to claim 1, wherein the reference lens is configured to satisfy the following conditional expression (3).
3.6 ≦ f2 / f ≦ 5.4 (3)
However,
f2: The focal length of the second lens. 3. The reference lens for an interferometer according to claim 1, wherein the reference lens is configured to satisfy the following conditional expression (4).
0.8 ≦ NA (4)
However,
NA: The numerical aperture on the test spherical surface side of the entire interferometer reference lens system. 4. The interferometer reference lens according to claim 1, wherein all of the first to fourth lenses have a refractive index of 1.75 or more.

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