JP5596511B2 - Fizeau lens - Google Patents

Description

  The present invention relates to a Fizeau lens.

Conventionally, a Fizeau lens used in a Fizeau interferometer has a configuration in which a negative meniscus lens having a concave surface facing the object side and a positive lens group, and the most image side surface of the positive lens group is a Fizeau surface. It has been.
For example, in Patent Document 1, in order from the incident side of an incident light beam, a negative meniscus lens having a concave surface facing the incident side, a positive lens group, and a positive meniscus lens having a concave surface functioning as a reference spherical surface on the test spherical surface side The reference lens for an interferometer used in an interferometer configured by the above is described.

Patent No. 4094301

However, the conventional Fizeau lens as described above has the following problems.
In the technique described in Patent Document 1, since the concave surface of the negative meniscus lens that is the first lens faces the incident side, the incident light beam incident on the first lens is refracted by the concave surface and diverged from the concave surface. Is expanded. For this reason, at least the lens diameter of the second lens needs to be larger than the lens diameter necessary for the first lens, and one or more lenses having a larger lens diameter than the first lens are required. However, there is a problem that the maximum lens diameter of the lens to be increased becomes larger than the size of the light beam diameter of the incident light beam. For this reason, there is a problem that the lens weight of the Fizeau lens also increases.
Further, the maximum lens diameter of the lens constituting the Fizeau lens means that the diameter of the Fizeau lens is increased because the outer diameter of the Fizeau lens is regulated. Therefore, there is a problem that the diameter size and weight of the lens holder of the Fizeau lens are increased.
In addition, since the concave surface of the negative meniscus lens that is the first lens faces the incident side, there is a problem that the lens peripheral portion of the first lens protrudes to the incident side and the lens length becomes long.

  The present invention has been made in view of the above-described problems, and an object of the present invention is to provide a Fizeau lens capable of reducing the maximum lens diameter and reducing the size and weight.

In order to solve the above-described problems, in the invention described in claim 1, an incident light beam used in a Fizeau interferometer is collected toward a reference light beam forming a reference wavefront of interference and a test surface. A Fizeau lens having a Fizeau surface that is split into a measurement beam to be radiated as an outermost lens surface, the negative meniscus lens having a convex surface directed toward the object side in order from the object side on which the incident beam is incident, and a positive And a positive lens group having a refractive power and having the Fizeau surface disposed closest to the image side.
In the present specification, the negative meniscus lens means a meniscus lens having negative refractive power. The positive meniscus lens means a meniscus lens having a positive refractive power.
The positive lens group means a lens group having a positive refractive power as a whole.

  According to a second aspect of the present invention, in the Fizeau lens according to the first aspect, the lens surface of the positive lens group that faces the negative meniscus lens has a convex surface on the object side.

  According to a third aspect of the invention, in the Fizeau lens according to the first or second aspect, the F number is smaller than 1.5.

  According to the Fizeau lens of the present invention, since the negative meniscus lens arranged closest to the incident side is arranged with the convex surface facing the incident side, an increase in the beam diameter due to the negative meniscus lens can be suppressed, and the maximum lens diameter The effect that it can reduce, and can achieve size reduction and weight reduction.

It is typical sectional drawing including the optical axis which shows schematic structure of the Fizeau interferometer which concerns on embodiment of this invention. It is typical sectional drawing containing the optical axis of the Fizeau lens which concerns on embodiment of this invention. It is a typical optical path diagram explaining the effect | action of the Fizeau lens which concerns on embodiment of this invention. It is typical sectional drawing including the optical axis of the Fizeau lens which concerns on the modification of embodiment of this invention. FIG. 6 is an aberration diagram for an example (Example 1) of the Fizeau lens according to the embodiment of the present invention. It is an aberrational figure of the Example (Example 2) of the Fizeau lens which concerns on the modification of embodiment of this invention.

Embodiments of the present invention will be described below with reference to the accompanying drawings.
FIG. 1 is a schematic cross-sectional view including an optical axis showing a schematic configuration of a Fizeau interferometer according to an embodiment of the present invention. FIG. 2 is a schematic cross-sectional view including the optical axis of the Fizeau lens according to the embodiment of the present invention. In FIG. 2, symbol I indicates an image plane. A numerical example of the Fizeau lens of this embodiment will be described later as Example 1.

Below, while explaining the structure of the Fizeau interferometer of this embodiment, the structure of the Fizeau lens of this embodiment is demonstrated.
As shown in FIG. 1, the Fizeau interferometer 20 of this embodiment is a Fizeau interferometer that observes and measures the shape of the test surface 5 a of the subject 5 using interference fringes.
The schematic configuration of the Fizeau interferometer 20 includes a laser light source 6 (light source), a beam splitter 8, a collimator lens 9, a Fizeau lens 10, and an imaging unit 7 (imaging unit). Each of these members is fixed to a housing or a support member (not shown) so that the relative positional relationship with each other is maintained.
The imaging unit 7, beam splitter 8, collimator lens 9, and Fizeau lens 10 are arranged on the optical axis P ₀ of the Fizeau interferometer 20 in this order.

In Fizeau interferometer 20, subject 5, for example, holder with movable stage by (not shown) such as, the position and inclination adjustment with respect to the optical axis P ₀ in a state facing the Fizeau surface 4b of the Fizeau lens 10 to be described later Is held to be able to.
In FIG. 1, as an example, an example in which the test surface 5a is a lens having a convex surface is illustrated. However, even if the test surface 5a is a concave surface, the distance between the Fizeau lens 10 and the subject 5 is illustrated. By changing, observation and measurement by interference fringes can be performed.
The subject 5 is not limited to a lens, and may be an optical element such as a reflection mirror, for example.

The laser light source 6 generates a coherent light beam that irradiates the subject 5 and the Fizeau lens 10 as a laser beam Q ₁ that is a divergent light that travels on the optical axis P ₁ that intersects the optical axis P ₀ of the Fizeau interferometer 20. It is a light source and is arranged on the side of the beam splitter 8.
The configuration of the laser light source 6 is not particularly limited as long as the laser light that is a divergent coherent light beam can be emitted. For example, a laser diode can be employed.

Beam splitter 8 reflects the laser beam Q ₂ the optical axis P ₁ of the laser beam Q ₁ emitted from the laser light source 6 to the collimator lens 9 side at the beam splitter surface 8a, the optical axis P ₀ of the Fizeau interferometer 20 And an optical path branching element that transmits a later-described test surface reflected light beam Q ₅ and Fizeau surface reflected light beam Q ₆ incident along the optical axis P ₀ from the collimator lens 9 side to the imaging unit 7 side.

The collimator lens 9 is an optical element that condenses the laser light Q ₂ reflected by the beam splitter 8 to form a laser light Q ₃ that is a parallel light beam. The lens optical axis is the optical axis P _{0 of the} Fizeau interferometer 20. Are arranged coaxially.

The Fizeau lens 10 condenses and transmits a part of the laser beam Q ₃ (incident beam from the light source) that has been converted into a parallel beam by the collimator lens 9, and forms a subject beam 5 as a measurement beam Q ₄ converted into a spherical wave. It irradiates toward the test surface 5a of a condenser lens for guiding the collimator lens 9 side test surface reflection light beam Q ₅ reflected by the test surface 5a of the measuring beam Q ₄ is condensed.
The Fizeau lens 10 includes a Fizeau surface 4b made of a highly accurate concave spherical surface having a radius of curvature equal to the distance from the lens surface to the condensing position as the final lens surface on the test surface 5a side.
In this embodiment, the surface accuracy of the Fizeau surface 4b is λ = 632.8 (nm) and PV (peak to valley) is λ / 20 or less.

Further, the Fizeau lens 10 has a lens configuration that collects a parallel light beam and vertically enters the Fizeau surface 4b along its normal line (detailed configuration will be described later).
Therefore, in the laser light Q ₃ incident from the collimator lens 9 side, the other laser light except for the transmitted light is reflected at the light output angle of 0 degrees in the Fizeau surface 4b, a collimator as the wavefront corresponding to the shape of the Fizeau surface 4b A Fizeau reflected light beam Q ₆ (reference light beam) traveling toward the lens 9 is formed.

The test surface reflected light beam Q ₅ has an incident optical path to the test surface 5 a in a state in which the position of the test sample 5 is adjusted so that the center of curvature of the test surface 5 a coincides with the condensing position of the Fizeau lens 10. The light enters the Fizeau lens 10 backward. In this case, the test surface reflection light beam Q ₅ and Fizeau surface reflected light beam Q _6, depending on the shape error of the test surface 5a to standard reference plane Fizeau surface 4b, and interference fringes for the optical path difference is generated form Will do.

Imaging unit 7 is a camera that captures an image of interference fringes formed me by the test surface reflection light beam Q ₅ and Fizeau surface reflection light beam Q ₆ are superimposed, the imaging lens 7b and the image sensor 7a is not shown It is housed in a camera housing.
Imaging lens 7b is disposed to be focused at the focal position at the focal position of the optical axis P ₀ and coaxially and collimator lens 9, the test surface reflection light beam Q ₅ and transmitted through the beam splitter 8 is focused through the collimator lens 9 the Fizeau surface reflection light beam Q ₆ is condensed, a condensing optical system for projecting the image pickup element 7a.
The imaging element 7a is to photoelectrically convert and image an image of the light projected by the imaging lens 7b. For example, an imaging element such as a CCD or a CMOS element can be employed. The video signal photoelectrically converted by the image sensor 7a is sent to a monitor (not shown) so that it can be displayed on the display screen of the monitor.

Next, details of the lens configuration of the Fizeau lens 10 of the present embodiment will be described with reference to FIG.
Fizeau lens 10 toward the image side is laser light Q ₃ is a parallel beam image surface I is formed from the object side to be incident, a first lens L1 (negative meniscus lens), a second lens L2 (positive lens) The third lens L3 (positive lens) and the fourth lens L4 (positive lens) are arranged in this order and are accommodated in a lens holder (not shown).

  The first lens L1 is composed of a single lens of a negative meniscus lens in which the first surface 1a facing the object side is a convex spherical surface and the second surface 1b on the image side is a concave spherical surface.

The second lens L2 is composed of a single convex convex lens having a third surface 2a made of a convex spherical surface and a fourth surface 2b made of a flat surface from the object side.
In the second lens L2 of the present embodiment, the circumference of the incident light beam having a light beam diameter equal to the lens effective diameter of the Fizeau lens 10 is slightly smaller than the lens effective diameter of the Fizeau lens 10 on the third surface 2a. The air incident on the top is spaced apart. Further, the radius of curvature of the fourth surface 2b is configured to be larger than the radius of curvature of the third surface 2a.

  In the following, in order to distinguish from the effective lens diameter of the Fizeau lens 10, the diameter of the effective area on the lens surface of the single lens constituting the Fizeau lens 10 is referred to as a lens diameter. The lens outer diameter refers to the diameter of a lens outer shape in which a holding portion or the like is provided outside the lens diameter.

  The third lens L3 is composed of a single lens of a positive meniscus lens in which the fifth surface 3a facing the object side is a convex spherical surface and the sixth surface 3b on the image side is a concave spherical surface.

  The fourth lens L4 is composed of a single lens of a positive meniscus lens in which the seventh surface 4a facing the object side is a convex spherical surface and the Fizeau surface 4b is provided on the image side.

  Thus, in the present embodiment, the second lens L2, the third lens L3, and the fourth lens L4 are all positive lenses (lenses having positive refractive power). For this reason, the lens group composed of these lenses is a positive lens group having positive refractive power and having the Fizeau surface disposed closest to the image side.

Curvature radius of first lens L1, second lens L2, third lens L3, and fourth lens L4 (hereinafter collectively referred to as “first lens L1 to fourth lens L4”) The surface spacing, the refractive index of the glass material, and the like are the necessary F-numbers under the condition that when the incident light beam is a parallel light beam, the incident light beam on the Fizeau surface 4b is perpendicularly incident along the normal line of the Fizeau surface 4b. It can be appropriately set according to the refractive power for obtaining (FNo) and the required aberration.
The present invention is particularly suitable for a bright Fizeau lens, that is, a Fizeau lens with a small Fno. For this reason, Fno is suitable for a Fizeau lens smaller than 1.5.
This is because if Fno is 1.5 or more, the aberration correction effect by the negative meniscus lens is enhanced.

  As in the present embodiment, in order to have three positive lens groups, it is preferable that the refractive index of the glass material used for the first lens L1 to the fourth lens L4 is large. For example, when Fno is 0.6, it is preferable to use a glass material having a refractive index with respect to d-line greater than 1.75.

Next, the operation of the Fizeau interferometer 20 will be described focusing on the operation of the Fizeau lens 10.
3A and 3B are schematic optical path diagrams for explaining the operation of the Fizeau lens according to the embodiment of the present invention. FIG. 3A shows the case of this embodiment, and FIG. 3B shows the case of a comparative example.

In order to observe or measure the test surface 5a with the Fizeau interferometer 20, the arrangement position of the subject 5 is adjusted to a position where the center of curvature of the test surface 5a substantially coincides with the condensing position of the Fizeau lens 10. . Then, light the laser light source 6 to emit laser light Q _1.
The laser beam Q ₁ is reflected by the beam splitter surface 8 a of the beam splitter 8, travels on the optical axis P ₀ as the laser beam Q _2, is converted into a parallel beam by the collimator lens 9, and becomes the laser beam Q ₃ , and is applied to the Fizeau lens 10. Incident.
When the laser beam Q ₃ are incident on the Fizeau lens 10, it is condensed by following the optical path shown in FIG. 2, for normal incidence along the normal to the Fizeau surface 4b. Laser light Q ₃ incident on the Fizeau surface 4b is partially transmitted to the image plane I side, it is emitted as the measuring light beam Q _4, and other parts, is reflected by the Fizeau surface 4b, Fizeau surface reflection light beam Q ₆ The incident optical path is reversed.

In the Fizeau lens 10, a positive lens group having a positive refractive power including a first lens L1, a second lens L2, a third lens L3, and a fourth lens L4 having a negative refractive power is disposed in order from the object side. As a whole, the light collecting optical system has a positive refractive power. Therefore, the positive lens group has a function of condensing the laser beam Q _3, the first lens L1 has a function of correcting aberration occurring in the positive lens group.

Hereinafter, along the optical path of the laser beam Q _3, illustrating the details of the action of Fizeau lens 10.
The most object side of the Fizeau lens 10, since the first lens L1 is arranged, the laser light Q ₃ are being refracted by the first surface 1a is convex, and reaches the second surface 1b being condensed .
For example, as shown in FIG. 3A, if the effective radius of the Fizeau lens 10 is h ₀ and the peripheral ray passing through the circumference of the effective lens diameter is expressed as Q _m , the peripheral ray Q _m is expressed as follows. A point q ₁ having a light ray height from the optical axis P ₀ on the first surface 1a is incident on the point q ₁ having the height h ₀ and is refracted by the first surface 1a. As a result, a point q on the second surface 1b is lower than h _{0. 2} position is reached.
Since the first lens L1 is a negative meniscus lens, peripheral rays Q _m is the second surface 1b, is emitted in a direction ray height is increased as it travels along the optical axis P _0, the second lens L2 side Head for.
The second lens L2 is marginal rays Q _m is to be incident on the slightly smaller radius h ₁ on the circumference than the lens effective radius h ₀ Fizeau lens 10 are arranged at the air gap . In the present embodiment, since the second surface 1b of the first lens L1 is a concave surface and the third surface 2a of the second lens L2 facing the second lens L2 is a convex surface, the lens is compared with the case where the convex surfaces face each other. In the peripheral portion, the interval between the second surface 1b and the third surface 2a is easily reduced.
Thus, marginal ray Q _m is ray height is within a period not exceeding h _0, is incident on the light point q ₃ of the height h ₁ on the third surface 2a. That is, the lens diameter of the second lens L2 may be 2 · h ₁ .

The second lens L2, the third lens L3, and the fourth lens L4, since both a positive lens, peripheral rays Q _m are each time it is refracted at the lens surfaces, ray height along the optical axis P ₀ The process proceeds in the direction in which the height decreases or the increase in the light beam height is suppressed. Therefore, the beam diameter of the laser beam Q ₃ passing through the second lens L2~ fourth lens L4 is less than or lens effective diameter of the Fizeau lens 10, or as large as the ray height h ₀ on the third surface 2a be Compared to the case where the light is incident beyond, the diameter becomes smaller.

Here, the operation of the present embodiment will be described in contrast to the comparative example shown in FIG.
The first lens 23 and the second lens 24 shown in FIG. 3B show an example of a lens configuration on the object side of a conventional Fizeau lens.
The first lens 23 is a single lens of a negative meniscus lens in which the first surface 23a facing the object side is a concave spherical surface and the second surface 23b on the image side is a convex spherical surface.
The second lens 24 is a positive lens having a third surface 24a made of a convex spherical surface on the object side.

In this case, light marginal ray _{Q m} of height _{h 0} is a result of light ray height from the optical axis _{P 0} the first surface 23a is incident at point _{q 1} of _{h 0,} is refracted by the first surface 23a, The second surface 23b reaches the position of a point q ₂ ′ where the ray height is higher than h ₀ .
Since the first lens 23 is a negative meniscus lens, peripheral rays Q _m is the second surface 23b, is emitted in a direction ray height is increased as it travels along the optical axis P _0, the second lens 24 side Head for.
For this reason, the peripheral ray Q _m increases the ray height, and enters the point q ₃ ′ of the ray height h ₂ (where h ₂ > h ₀ ) on the third surface 2a.

Therefore, the lens diameter of the third surface 24 a of the second lens 24 needs to be larger than the diameter of the light beam transmitted through the second surface 23 b of the first lens 23. For this reason, the lens diameter of the second lens 24 needs to be surely larger than the diameter of the light beam 2 · h ₀ incident on the first lens 23. As a result, in the positive lens group, at least the lens diameter of the second lens 24 needs to be larger than the lens diameter necessary for the first lens 23, and the lens constituting the Fizeau lens compared to the Fizeau lens 10. The maximum lens diameter becomes large.
As the maximum lens diameter increases, the outer diameter of the lens including the region of the holding portion also increases. Accordingly, as the diameter increases, the mass of the lens and the mass of the lens holder increase. For this reason, the Fizeau lens is heavier than the Fizeau lens 10.

The first lens 23, the second lens 24, a second surface 23b opposite to each other, the third surface 24a, since both are convex, not be an air gap along the optical axis _{P 0} is 0 or less. For this reason, even if the air space between the first lens 23 and the second lens 24 is adjusted, the optical path length q ₂ ′ q ₃ ′ between the surfaces in the lens periphery cannot be set to a certain value or less, and the large diameter Conversion cannot be easily suppressed.
Further, since the second surface 23b and the third surface 24a facing each other are both convex surfaces, the lens peripheral portion of the first lens 23 protrudes toward the object side, and the first lens 23 and the second lens 24 The compound lens length due to is larger than that of the Fizeau lens 10.

  In such a prior art Fizeau lens, the third surface 24a of the second lens 24 is a concave surface to reduce the air gap in the lens peripheral portions of the first lens 23 and the second lens 24, thereby shortening the compound lens length. However, if the third surface 24a of the second lens 24 is a concave surface, the positive refractive power of the second lens 24 cannot be increased so much, the lens diameter after the third lens increases, or the positive lens group. As a result, the number of lenses must be increased as a result of a decrease in the refractive power of the lens, which may cause a problem. In this case, the lens diameter of the Fizeau lens may increase, or the Fizeau lens may become heavy.

Thus, according to the Fizeau lens 10 of the present embodiment, either the beam diameter passing through the respective lenses can be below the lens effective diameter h ₀ Fizeau lens 10, or even as a lens larger effective diameter h _0, the Compared to the configuration of the comparative example, the size of the light beam diameter transmitted through each lens can be suppressed, so that the maximum lens diameter of the lens constituting the Fizeau lens 10 can be suppressed. As a result, a small-diameter Fizeau lens can be configured.
In addition, since the second surface 1b and the third surface 2a are concave and convex, respectively, the third surface 2a can be arranged so as to protrude to the second surface 1b side, so that the convex surfaces are arranged like the above comparative example. The length of the compound lens by the first lens L1 and the second lens L2 can be shortened as compared with the case where the lens faces each other and the case where the concave surfaces face each other. As a result, the lens length of the Fizeau lens 10 can be easily shortened.

Next, the operation of the Fizeau interferometer 20 will be described along the optical path after passing through the Fizeau lens 10.
As shown in FIG. 1, the measurement light beam Q _{4 that} has passed through the Fizeau lens 10 and is emitted from the Fizeau surface 4b is condensed toward the curvature center of the curvature center of the test surface 5a, while being focused on the test surface 5a. incident, reflected by the test surface 5a, the test surface reflection light beam Q ₅ is formed.
Test surface reflection light beam Q ₅ is incident on the Fizeau lens 10 by reversing the incident light path to the surface to be inspected 5a. Thus, interference fringes are formed by the test surface reflection light beam Q ₅ and Fizeau surface reflected light beam Q _6.
The test surface reflected light beam Q ₅ and the Fizeau surface reflected light beam Q ₆ are converted into parallel light beams when emitted from the first surface 1 a of the Fizeau lens 10 and are incident on the beam splitter 8 while being collected by the collimator lens 9. is transmitted through the beam splitter surface 8a, it proceeds along the optical axis P _0, is emitted from the beam splitter 8.
Then, the test surface reflection light beam Q ₅ and Fizeau surface reflected light beam Q _6, is projected on the imaging element 7a by the imaging lens 7b, the interference fringes are imaged, which is formed by these. Since the image signal of the interference fringes is displayed on a monitor (not shown), observation and measurement can be performed.

As described above, according to the Fizeau lens 10 of the present embodiment, the first surface 1a, which is the convex surface of the first lens L1 arranged closest to the incident side (collimator lens 9 side), is arranged with the first side facing the incident side. Therefore, since the increase in the light beam diameter by the first lens L1 can be suppressed, the maximum lens diameter of the lenses constituting the Fizeau lens 10 can be reduced and the diameter can be reduced.
In addition, by making the third surface 2a of the second lens L2 facing the first lens L1 convex, it is possible to reduce the lens length of the Fizeau lens 10 while suppressing an increase in the light beam diameter.

  Further, according to the Fizeau interferometer 20 including such a Fizeau lens 10, the diameter size of the Fizeau lens 10 with respect to the incident light beam diameter can be reduced, so that the apparatus can be reduced in size and weight. . Further, since the Fizeau lens 10 is reduced in weight, the replacement work of the Fizeau lens 10 performed according to the type of the subject 5 is facilitated, and the work efficiency is improved.

[Modification]
Next, a Fizeau lens according to a modification of the present embodiment will be described.
FIG. 4 is a schematic cross-sectional view including the optical axis of a Fizeau lens according to a modification of the embodiment of the present invention. In FIG. 4, symbol I indicates an image plane. A numerical example of this modification will be described later as Example 2.

The Fizeau lens 30 of this modification can be used in place of the Fizeau lens 10 in the Fizeau interferometer 20 of the above embodiment, and the convex surface is directed from the object side to the object side, as in the above embodiment. In this example, the negative meniscus lens and the positive lens group are arranged in this order, and the positive lens group is constituted by four positive lenses.
Fizeau lens 30, as shown in FIG. 4, towards the image side is laser light Q ₃ is a parallel beam image surface I is formed from the object side to be incident, a first lens L11 (negative meniscus lens), a The second lens L12 (positive lens), the third lens L13 (positive lens), the fourth lens L14 (positive lens), and the fifth lens (positive lens) are arranged in this order and are accommodated in a lens holder (not shown). .

  The first lens L11 is composed of a single lens of a negative meniscus lens in which the first surface 11a directed toward the object side is a convex spherical surface and the second surface 11b on the image side is a concave spherical surface.

The second lens L12 is composed of a single lens of a biconvex lens including a third surface 12a and a fourth surface 12b, both of which are convex spherical surfaces, from the object side.
In the second lens L12 of the present embodiment, the peripheral ray of the incident light beam having a light beam diameter equal to the lens effective diameter of the Fizeau lens 30 has a circumference slightly smaller than the lens effective diameter of the Fizeau lens 30 on the third surface 12a. The air is spaced apart so as to be incident on the top. Further, the radius of curvature of the fourth surface 12b is larger than the radius of curvature of the third surface 12a.

  The third lens L13 is composed of a single lens of a positive meniscus lens in which the fifth surface 13a facing the object side is a convex spherical surface and the sixth surface 13b on the image side is a concave spherical surface.

  The fourth lens L14 is composed of a single lens of a positive meniscus lens in which the seventh surface 14a directed to the object side is a convex spherical surface and the eighth surface 14b on the image side is a concave spherical surface.

The fifth lens L15 is composed of a single lens of a positive meniscus lens in which the ninth surface 15a facing the object side is a convex spherical surface and the Fizeau surface 15b is provided on the image side.
The Fizeau surface 15b is a highly accurate concave spherical surface having a radius of curvature equal to the distance from the lens surface to the condensing position as the final lens surface on the test surface 5a side of the Fizeau lens 30. The surface accuracy of the Fizeau surface 15b is the same as that of the Fizeau surface 4b.

  Thus, in this modification, all of the second lens L12, the third lens L13, the fourth lens L14, and the fifth lens L15 are positive lenses. For this reason, the lens group composed of these lenses is a positive lens group having positive refractive power and having the Fizeau surface disposed closest to the image side.

The radius of curvature of the first lens L11 to the fifth lens L15 and the refractive index of the glass material can be appropriately set according to the refractive power for obtaining the required FNo and the required aberration.
In this modification, since the positive lens group has four lenses, the refractive index of the glass material used for the first lens L11 to the fifth lens L15 can be a glass material having a lower refractive index than that of the above embodiment. It is. For example, when Fno is 0.7, a glass material having a refractive index of about 1.62 with respect to the d-line can be preferably used.

According to the Fizeau lens 30 of this modified example, as shown in FIG. 4, the second surface 11b of the first lens L11, which is a negative meniscus lens, and the object side of the second lens L12, as in the above embodiment, are shown. Since the convex third surface 12a is disposed so as to be opposed to the first lens L11 and the second lens L12 by appropriately setting the air distance between the first lens L11 and the second lens L12, It can be made smaller than the diameter of the light beam incident on the first surface 11a.
The second lens L12~ fifth lens L15, since both a positive lens, the beam diameter that passes through them, either be below the lens effective diameter h ₀ Fizeau lens 30 or the lens effective diameter h ₀ Even if it is large, since the size of the light beam diameter transmitted through each lens can be suppressed as compared with the configuration of the comparative example described in the above embodiment, a small-diameter Fizeau lens can be configured.
In addition, since the second surface 11b and the third surface 12a are concave and convex, respectively, the third surface 12a can be arranged so as to protrude toward the second surface 11b, so that the convex surfaces face each other or are concave. Compared with the case where they face each other, the length of the compound lens by the first lens L11 and the second lens L12 can be shortened. As a result, the lens length of the Fizeau lens 30 can be easily shortened.

In the above description, the lens (second lens) facing the negative meniscus lens (first lens) in the positive lens group is the peripheral ray of the incident light beam having a light beam diameter equal to the effective lens diameter of the Fizeau lens. However, the second lens will be described with an example in which the second lens is arranged with an air gap so that the second lens is incident on a circumference slightly smaller than the effective lens diameter of the Fizeau lens on the lens surface facing the first lens. did.
However, if the maximum lens diameter of the Fizeau lens is acceptable even if it is larger than the lens diameter of the first lens, the peripheral ray incident on the second lens is on the circumference having a diameter larger than the effective lens diameter of the Fizeau lens. It is good also as a structure which opened the air space | interval so that it may inject into.
Even in this case, when the air distance between the first lens and the second lens is the same, the maximum lens diameter of the Fizeau lens is larger than when the first lens is a negative meniscus lens having a concave surface on the object side. Can be reduced. Further, the lens length of the Fizeau lens can be further shortened.

  In addition, all the components described in the above embodiments and modifications can be implemented by appropriately changing or deleting combinations within the scope of the technical idea of the present invention.

[Example 1]
The configuration parameters of the optical system of the first numerical example corresponding to the Fizeau lens 10 of the above embodiment are shown below. R ₁ , r ₂ ,... Is the radius of curvature (unit: mm) of each lens surface, and d ₁ , d ₂ ,... Are the surface intervals (unit: mm) between the lens surfaces. Each corresponds. In addition, n ₁ , n ₂ ,... Are the refractive indexes of each lens at the d-line, and ν ₁ , ν ₂ ,. These notations are common to all the following reference drawings.

Surface number Curvature radius Surface spacing Refractive index Abbe number
1 r ₁ = 81.623 d ₁ = 4.107 n ₁ = 1.77861 ν ₁ = 25.68
2 r ₂ = 56.014 d ₂ = 8.301
3 r ₃ = 98.551 d ₃ = 9.362 n ₂ = 1.77861 ν ₂ = 25.68
4 r ₄ = ∞ d ₄ = 0.9345
5 r ₅ = 37.691 d ₅ = 11.392 n ₃ = 1.77861 ν ₃ = 25.68
6 r ₆ = 76.145 d ₆ = 0.551
7 r ₇ = 20.102 d ₇ = 11.930 n ₄ = 1.77861 ν ₄ = 25.68
8 r ₈ = 18.749 d ₈ = 18.749
I ∞ (image plane)

In this embodiment, the focal length f is f = 37.3 (mm), and Fno is Fno = 0.6.
The lens length is 53.7 mm. Here, the lens length means the distance from the most incident side position of the first lens to the most exit side position of the most image side lens (fourth lens in this embodiment). Therefore, the lens length in this embodiment is the length in the direction along the optical axis P ₀ from the intersection of the first surface 1 a and the optical axis P ₀ to the intersection of the Fizeau surface 4 b and the peripheral ray of the measurement light beam Q _4. Say it.
In the following, the beam heights H ₁ , H ₂ ,... Of peripheral rays on each lens surface when the lens effective diameter of the Fizeau lens 10 is 60 mm and the beam diameter of the laser beam Q ₃ is made to coincide with the lens effective diameter are shown. . The unit of the beam height is mm.

Surface number Ray height
1 H ₁ = 30.000
2 H ₂ = 28.908
3 H ₃ = 29.717
4 H ₄ = 29.487
5 H ₅ = 28.308
6 H ₆ = 26.662
7 H ₇ = 19.738
8 H ₈ = 15.151

Thus, according to this embodiment, light height, ray height H ₃ of the third surface 2a is lower than the ray height H ₁ of the first surface 1a. Also, the second surface 1b from the eighth surface at a Fizeau surface 4b to the ray height H ₂ to H ₈ is lower than the ray height H ₁ of both the first surface.

FIG. 5 is an aberration diagram of the spherical aberration of the present example. In FIG. 5, the unit of the horizontal axis is mm.
As can be seen from FIG. 5, the spherical aberration of the Fizeau lens 10 of this example is corrected well.

[Example 2]
The configuration parameters of the optical system of the second numerical example corresponding to the Fizeau lens 30 of the modified example of the above embodiment are shown below. Reference numerals r _i and d _i (i = 1,..., 10) correspond to the reference numerals in FIG.

Surface number Curvature radius Surface spacing Refractive index Abbe number
1 r ₁ = 161.741 d ₁ = 5.000 n ₁ = 1.61655 ν ₁ = 36.26
2 r ₂ = 112.978 d ₂ = 10.000
3 r ₃ = 355.722 d ₃ = 15.000 n ₂ = 1.61655 ν ₂ = 36.26
4 r ₄ = -1189.176 d ₄ = 2.000
5 r ₅ = 91.505 d ₅ = 14.965 n ₃ = 1.61655 ν ₃ = 36.26
6 r ₆ = 225.636 d ₆ = 0.471
7 r ₇ = 69.152 d ₇ = 15.000 n ₄ = 1.61655 ν ₄ = 36.26
8 r ₈ = 164.051 d ₈ = 1.883
9 r ₉ = 38.408 d ₉ = 14.631 n ₅ = 1.61655 ν ₅ = 36.26
10 r ₁₀ = 41.313 d ₁₀ = 41.313
I ∞ (image plane)

In this embodiment, the focal length f is f = 70.9 (mm), and Fno is Fno = 0.7.
The lens length is 91.2 mm. Here, the lens length is a length defined in the same manner as in the first embodiment. Therefore, the lens length in the present embodiment is the length in the direction along the optical axis P ₀ from the intersection of the first surface 11 a and the optical axis P ₀ to the intersection of the Fizeau surface 14 b and the peripheral ray of the measurement light beam Q _4. That's it.
Below, the ray heights H ₁ , H ₂ ,... Of peripheral rays on each lens surface when the lens effective diameter of the Fizeau lens 30 is 106 mm and the beam diameter of the laser light Q ₃ is made to coincide with the lens effective diameter are shown. . The unit of the beam height is mm.

Surface number Ray height
1 H ₁ = 53.000
2 H ₂ = 51.868
3 H ₃ = 51.996
4 H ₄ = 52.079
5 H ₅ = 51.818
6 H ₆ = 50.627
7 H ₇ = 47.025
8 H ₈ = 45.547
9 H ₉ = 34.587
10 H ₁₀ = 29.361

Thus, according to this embodiment, light height, ray height H ₃ of the third surface 12a is lower than the ray height H ₁ of the first surface 11a. Further, the light heights H _{2 to} H ₁₀ from the second surface 1b to the Fizeau surface 15b, which is the tenth surface, take the maximum value on the fourth surface 12b, and both are higher than the light height H ₁ of the first surface. It is low.

FIG. 6 is an aberration diagram of the spherical aberration of the present example. In FIG. 6, the unit of the horizontal axis is mm.
As can be seen from FIG. 6, the spherical aberration of the Fizeau lens 30 of the present example is corrected well.

In addition, this Example can be quantitatively compared with Example 2-1 (hereinafter referred to as a conventional example) of the second embodiment of Patent Document 1 related to the prior art, so that the incident beam diameter, Fno, The refractive index of the glass material and the surface configuration from the second lens L12 to the fifth lens L15 are matched with the surface configuration from the lens 12 to the lens 15 of Patent Document 1, so that the working distances are substantially matched. For this reason, the difference of the conventional example from this example is that the convex surface of the negative meniscus lens is mainly directed to the image side.
As described in Patent Document 1, the maximum light beam diameter in the lens of the conventional example (in the term of Patent Document 1, “lens outer diameter”) is 113 mm (light height 56.5 mm). On the other hand, since the maximum light beam diameter of this embodiment is 106 mm (light beam height 53 mm), it is reduced by 15 mm (14%).
Moreover, the lens length of the conventional example is 106 mm when calculated from the numerical example of Patent Document 1 with the same definition as described above. That is, the lens length of the conventional example is the direction along the optical axis from the intersection of the lens surface of surface number 1 and the peripheral ray of the incident light beam to the intersection of the lens surface of surface number 10 and the peripheral light beam of the emitted light beam. Calculated as length.
On the other hand, since the lens length of this embodiment is 91 mm, the lens length of this embodiment is shortened by 15 mm (14%).
When comparing FIG. 9 of Patent Document 1 with FIG. 6 of this modification, the spherical aberration is smaller than 0.01 mm, but is sufficiently acceptable because the present embodiment is larger.
Thus, according to the present embodiment, it can be seen that the maximum lens diameter can be reduced and the lens length can be shortened as compared with the comparative example even when the same number of lens configurations as in the conventional example are used.

1a, 11a First surface 1b, 11b Second surface (convex surface)
2a, 12a Third surface (convex surface)
3a, 13a 5th surface (convex surface)
4a, 14a 7th surface (convex surface)
4b, 15b Fizeau surface 5 Subject 5a Test surface 6 Laser light source (light source)
7 Imaging unit 10, 30 Fizeau lens 15a 9th surface (convex surface)
15b Fizeau surface 20 Fizeau interferometer L1, L11 First lens (negative meniscus lens)
L2, L12 Second lens (positive lens group)
L3, L13 Third lens (positive lens group)
L4, L14 4th lens (positive lens group)
L15 5th lens (positive lens group)
P ₀ optical axis Q ₁ , Q ₂ , Q ₃ laser light (incident beam)
Q ₄ Measurement beam Q ₅ Test surface reflected beam Q ₆ Fizeau surface reflected beam (reference beam)
Q _m marginal ray

Claims (3)

Used as a Fizeau interferometer, the Fizeau surface that splits the incident light beam from the light source into a reference light beam that forms the reference wavefront of interference and a measurement light beam that is focused toward the test surface is the outermost lens surface. A Fizeau lens,
In order from the object side where the incident light beam enters,
A negative meniscus lens with the convex surface facing the object,
A positive lens group having positive refractive power and having the Fizeau surface disposed closest to the image side;
A Fizeau lens, characterized by being arranged. 2. The Fizeau lens according to claim 1, wherein a lens surface facing the negative meniscus lens in the positive lens group has a convex surface on the object side. The Fizeau lens according to claim 1, wherein an F number is smaller than 1.5.

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