JP2017513043A - A method for setting tolerances on optical surfaces using local pupil regions - Google Patents

Abstract

A method for setting a tolerance on an optical surface of a lens element of an optical system based on an interferogram of the optical surface is disclosed. This method defines, for an optical surface, a local pupil region having a corresponding field point in the image plane, a tolerance T for the characteristics of the optical surface in the local pupil region, and the performance of the optical system for the field point. The step of determining based on the metric, the step of applying a polynomial to the surface shape of the interferogram over the local pupil region to obtain the coefficient C relating the characteristics of the optical surface to the performance metric, and the field points , Comparing the tolerance T with the coefficient C to see if C <T. In order to set tolerances for the entire optical surface, this process may be repeated for different local pupil region positions with respect to the interferogram to set tolerances for the entire optical surface.

Description

  The present disclosure relates to tolerance setting for an optical surface, and more particularly, to a method for setting tolerance for an optical surface of an optical system using a local pupil region.

  The disclosures of all publications or patent documents cited herein are incorporated by reference in their entirety.

  All optical systems are composed of one or more optical elements in the form of refractive lenses, mirrors, beam splitters and the like. All methods of manufacturing optical elements produce some kind of surface shape error that degrades optical performance compared to the ideal performance that can be obtained if the optical element is perfectly fabricated. Thus, in general, optical systems require that the optical surfaces of the optical elements of the system be made to be within selected tolerances. Such tolerances are a number of values measured over the effective diameter of the optical element, such as optical magnification, surface non-uniformity, root mean square (RMS) deviation from the ideal surface, surface gradient, and power spectral density. Applicable to surface parameters.

  Modern optical design software allows for modeling the optical performance of an optical system based on surface errors for each optical surface in the system. A surface error is localized and can have a relatively low frequency, such as a localized gradient error. Such a surface error can adversely affect the imaging performance in a small part of the imaging region of the optical system. However, when the tolerance is set for the surface parameters as described above, if the analysis is performed over the entire effective diameter, such a localized surface error may not be sufficiently emphasized. Furthermore, the predetermined tolerance is preferably related to a performance metric of the optical system in which the optical element is placed. Two examples of performance metrics are field curvature (field flatness) and distortion.

One aspect of the present disclosure is a method of setting a tolerance for an optical surface of a field lens element of an optical system having a pupil and an image surface, the optical surface having an effective diameter and a total area AS. This method
a) a step of measuring an interferogram of an optical surface, wherein the interferogram measures the surface shape of the optical surface over the entire effective diameter of the optical surface;
b) defining a local pupil region having an area AR and a position in the optical surface, the local pupil region having a corresponding field point in the image plane;
c) determining a tolerance T for at least one characteristic of the optical surface in the local pupil region based on at least one performance metric of the optical system for the field point;
d) applying a polynomial to the surface shape of the interferogram over the local pupil region, the polynomial including at least one coefficient C relating at least one characteristic of the optical surface to at least one performance metric. A process that is
e) comparing the tolerance T with at least one coefficient C to see if C <T for the field point;
It has.

Another aspect of the present disclosure is a method of setting a tolerance on an optical surface of a lens element of an optical system having a pupil and an image surface based on an interferogram of the optical surface. This method
a) defining, for an optical surface, a local pupil region having corresponding field points in the image plane;
b) determining a tolerance T for at least one characteristic of the optical surface in the local pupil region based on at least one performance metric of the optical system for the field point;
c) applying a polynomial to the surface shape of the interferogram over the local pupil region, the polynomial comprising at least one coefficient C relating at least one characteristic of the optical surface to at least one performance metric. A process that is
d) comparing the tolerance T with at least one coefficient C to determine if C <T for the field point;
It has.

Another aspect of the present disclosure is a method of setting a tolerance on a field surface of an optical element of an optical system based on an interferogram representing a surface shape of the field surface. This method
a) defining, for the field plane, a local pupil region having a corresponding field point FP in the image plane of the optical system;
b) determining a tolerance T for at least one characteristic of the field plane in the local pupil region based on at least one performance metric of the optical system for the field point;
c) fitting a polynomial to the surface shape of the interferogram over the local pupil region to obtain at least one polynomial coefficient C relating at least one characteristic of the field surface to at least one performance metric;
d) comparing the tolerance T with at least one coefficient C to determine if C <T for the field point;
It has.

  In the example, the tolerance setting is performed for local pupil regions at different positions with respect to the interferogram, and the positions of the different local pupil regions have different corresponding field points in the image plane. In this specification, the movement of the local pupil region relative to the interferogram is referred to as “scanning” of the local pupil region.

  Also, in the example, the optical system includes a number of field surfaces, and the tolerance setting method is applied to one or more field surfaces, such as all field surfaces.

  Additional features and advantages are set forth in the following detailed description, and from which, the description will be readily apparent, in part, to those skilled in the art or in the specification and claims, rather than only in the accompanying drawings. It will be understood by implementing the described embodiments. Both the general description so far described and the following detailed description are merely exemplary and are intended to provide an overview or framework for understanding the nature and characteristics of the claims. Should be understood to be.

  The accompanying drawings are included for further understanding and are incorporated in and constitute a part of this specification. The drawings illustrate one or more embodiments and, together with the detailed description, serve to explain the principles and operations of the various embodiments. Accordingly, the present disclosure will be more fully understood from the following detailed description, which should be read in conjunction with the accompanying drawings.

1 is a schematic diagram of a general purpose optical system including at least one field lens group having at least one field lens element and a pupil group optionally having a pupil and optionally one or more pupil lens elements; FIG. 1B is an optical diagram of an example of the general purpose optical system of FIG. 1A disclosed in US Pat. No. 2,696,758, entitled “Wide-angle photographic objective lens”, where the pupil of the optical system is a local pupil region Is defined on the surface of the element in the optical system. It is the elements on larger scale of the lens surface of a field lens element, and has shown the light beam which demarcates a local pupil on the lens surface. FIG. 4 is a front view of an exemplary rotationally symmetric field lens element illustrating the position and size of a local pupil region of the lens surface. The interferogram for an example of a field lens surface is illustrated and the illustrated local pupil region is moved (scanned) relative to the interferogram. FIG. 4 is a schematic diagram illustrating movement (scanning) of an exemplary local pupil region, and in a certain advance distance example, the step proceeds on the interferogram stepwise from a solid circle to a dashed circle and further to a dotted circle. Is shown in FIG. 6 is a schematic diagram illustrating the movement (scanning) of an exemplary local pupil region, step by step on the interferogram from a solid circle to a dashed circle and then to a dotted circle in another example of different advance distances. Shown to go forward. It is an optical diagram of an example optical system, and the surface of the front field lens element has an axial upper surface error that can affect the focusing position of the axial light beam. For the same optical system (square) having a field lens element including a non-ideal surface with a nominal optical system (triangle), an image plane position IP (μm) and a focus position fp (wavelength) over the field (image plane) It is a figure depicting the relationship of λ). FIG. 7 is a diagram showing distortion aberration as a vector, showing the influence of the central artifact on the field lens element similar to the surface error shown in FIG. 6, and the starting point of each vector V is the ideal position of each light beam. is there.

  Reference will now be made in detail to various embodiments of the disclosure, examples of which are illustrated in the accompanying drawings. Wherever possible, the same or similar reference numbers and symbols will be used throughout the drawings to refer to the same or like parts. The drawings are not necessarily to scale, and one of ordinary skill in the art will know where the drawings are simplified to illustrate the main aspects of the present disclosure.

  The following claims are incorporated into and constitute a part of this detailed description.

  In some of the drawings, Cartesian coordinates are shown for reference, but are not intended to limit direction or orientation.

  FIG. 1A is a schematic diagram of a general-purpose optical system 10. The optical system 10 includes an object plane OP, an optical axis A1, and an image plane IP that can actually be an image curved surface instead of a plane. The optical system 10 includes at least one lens element L, has at least one field lens group GF having at least one field lens element LF, and has at least one lens pupil P (or aperture stop APS) and 1 And a pupil group GP which may have one or more pupil lens elements LP. Generally, the pupil lens element LP is placed relatively close to the pupil P or conjugated to the pupil P, while the field lens element LF is placed relatively far from the pupil P or P conjugated. The tolerance setting method of the present disclosure is mainly applied to the field lens element LF. In the context of the tolerance setting method shown here, how the pupil lens element LP and the field lens element LF can be distinguished will be described in greater detail below. The optical system 10 is not limited to a specific type of optical system. Thus, in various non-limiting examples, the optical system may be an infinite focus system, a focus system (sometimes referred to as a “non-infinite focus system”), an asymmetric system, and the like.

  FIG. 1B is an optical diagram of an example of the general purpose optical system of FIG. 1A disclosed in the specification of US Pat. No. 2,696,758 (hereinafter referred to as the '758 patent) entitled “Wide Angle Photographic Objective Lens”. is there. The optical system 10 includes a lens element L, that is, field lens elements LF1 and LF2 in the field lens group GF, and pupil lens elements LP1 to LP4 in the pupil group GP. The optical system 10 in FIG. 1B includes an object plane OP (not shown) because the optical system 10 is relatively far to the left. The optical system 10 has twelve lens surfaces S and is denoted S1 to S12.

  FIG. 1B also shows three light beams RB, denoted RB1, RB2, and RB3. Each of the light beams RB1 to RB3 travels from the object plane OP to the image plane IP through the field lens element LF, the pupil P, and the pupil lens element LP. Each of the light beams RB1 to RB3 is focused at each field point FP1, FP2, FP3 on the image plane IP. For ease of illustration, an ideal case where the image plane IP is a plane is shown. Each lens surface S1 to S12 as well as the pupil P has an associated effective diameter CA, that is, a diameter (see FIG. 3 described below for the first time). Accordingly, the effective diameter CA for the two surfaces S of the predetermined lens element L can be different, for example, as the effective diameters of the lens surfaces S3 and S4 of the lens element L2.

  The illustrated optical system 10 of FIG. 1B is a wide-angle photographic objective designed for use with film. Thus, at the time the objective is designed, it has imaging performance requirements defined by standards for film photography as is well known in the art. Imaging performance requirements are characterized by one or more performance metrics. Examples of performance metrics are field dependent (ie, position dependent on the image plane IP), field curvature, distortion, Strehl ratio, depth of focus, wavefront error in the image plane, and modulation transfer function (MTF). )including.

  Since the performance metric shows the characteristics of the image quality, there are advantages compared to the measurement of sidel aberration, which requires further processing, for example to understand the actual influence on the image quality.

  Each optical surface S1 to S12 has an optical tolerance based on the essential or desirable imaging performance of the optical system 10. The optical tolerance can be applied to one or more characteristics of a given optical surface S. The prior art approach for setting tolerances on optical surfaces is to perform interference measurements (“interferograms”) for each optical surface (S1 to S12 in the example of this optical system 10 in FIG. 1B), and then for each surface Emphasis is placed on evaluating the interferogram over the entire effective diameter. Interferogram is a phase measurement described in Bruning et al., “Digital Wavefront Interferometer for Inspecting Optical Surfaces and Lenses”, Applied Optics, Vol. 13, No. 11, November 1974, pages 2693-2703. It can be measured using well-known techniques, such as by interferometry. Interferometers that can measure interferograms for the tolerance setting method of the present disclosure are commercially available from numerous companies, including Zygo, Middlefield, Connecticut, and Beco Instruments, Inc., Plainview, NY Is possible.

The tolerance setting method of the present disclosure measures or acquires an interferogram for a predetermined surface S of a predetermined field lens element LF, and then, based on the size of the pupil projected on the surface. , Including examining regions within the interferogram. Continuing to refer to FIG. 1B and also to the enlarged views of FIG. 2 and FIG. 3, it is observed that each light beam RB passes through the corresponding region PR in the predetermined plane S. In FIG. 2, the illustrated region PR is shown highlighted with a thick black line, and in FIG. 3, it is shown as a smaller circular region within the effective diameter CA of the lens L. Each region PR has an area AR defined by a portion where the light beam RB intersects the surface S. Region PR has a radius _{r PR.}

  The area AR of the region PR is defined by projecting the pupil P onto the surface S through the optical system 10 along the optical path of the light beam RB. The position of the region PR on the predetermined surface S depends on the direction of the light beam RB. Hereinafter, the area AR is referred to as “local pupil area”, and the region PR is hereinafter referred to as “local pupil region”. The local pupil region PR corresponds to one field point FP in the image plane IP (see FIG. 1B). Naturally, in order to image an object over the entire field of view by the optical system 10, the light beam from the object plane OP needs to travel through the entire surface S of each lens element L to the image plane IP. However, the imaging performance at each field point FP is determined by the light rays passing through the corresponding local pupil region PR on each surface S of each lens element L.

  The lens surface S has a total area AS (herein referred to as “lens surface area”) defined by the effective diameter CA of the surface and the curvature of the surface. The ratio of the local pupil area AR of the local pupil region PR to the total area AS over the effective diameter CA of the lens surface S is defined as η = AR / AS and is hereinafter referred to as “pupil area ratio”. The field lens element LF has a local pupil region PR having a pupil area ratio η smaller than the pupil lens LP. In one example, a field lens element LF is defined as a lens element L in which at least one of its lens surfaces S has a pupil area ratio of η <0.75, while a pupil lens LP has both of its lens surfaces as It is defined as a lens element having a pupil area ratio of η> 0.75. In another example, the field lens element LF is defined as a lens element L in which at least one of its lens surfaces S has a pupil area ratio of η <0.65, while the pupil lens LP is both of its lens surfaces. Is defined as a lens element having a pupil area ratio of η> 0.65.

  In one example, the predetermined lens element L can technically be classified between a field lens element and a pupil lens element. That is, the surface S furthest away from the pupil P can be a “field surface”, while the opposite surface closest to the pupil can be a “pupil surface”. Therefore, in one example, it is not distinguished between the lens elements L such as the field lens element LF or the pupil lens element LP, but the lens surface S is distinguished whether it is a field lens surface or a pupil lens surface.

  FIG. 4 is a diagram depicting contour lines of the illustrated interferogram 50 for the example surface S of the exemplary field lens element LF. The interferogram 50 is shown as having contour lines 51 representing the surface shape of the surface S. The interval between the contour lines is 0.3λ, and the minimum value / maximum value is −0.18λ to 0.2λ. However, (lambda) is the wavelength of the measurement light of the interferometer used in order to acquire an interferogram. Other representations of the surface shape such as a false color may be used. The local pupil region PR is shown as a black circle, and an arrow 52 attached to the local pupil region indicates movement (scanning) of the local pupil region.

As described above, the local pupil area AR of the local pupil region PR is defined by the size of the pupil P on the surface S of the lens element L for a predetermined field point FP on the image plane IP. The local pupil area AR is often substantially constant over the entire surface S of the field lens element LF. The tolerance setting for the surface S of the field lens element LF is realized by applying the surface error of the interferogram 50 in the local pupil region PR to a polynomial such as the following Zernike polynomial. One or more coefficients of the polynomial may then be associated with one or more performance metrics such as distortion, field flatness, slope, and defocus of the optical system. Next, in order to control the imaging performance of the optical system, one or more polynomial coefficients may be directly compared with a tolerance T defined by a performance metric. The local pupil region PR is moved (scanned) over the entire interferogram so as to cover the entire lens surface S, and a tolerance is set for each local pupil region.

In the example shown in FIG. 5A, the local pupil region PR is shown to travel (scan) on the interferogram 50 from a solid circle to a dashed circle and further to a dotted circle. . The movement in FIG. 5A includes stepping forward the local pupil region PR by an advance distance d equal to its radius r. In another example shown in Figure 5B, the scanning of a local pupil region PR is a local pupil area, by a distance equal to half its radius r _PR, including advancing stepwise. In general, any reasonable advance distance d that appropriately samples the interferogram 50 may be used. The practical limit on the shortness of the scanning distance d is based on the spatial sampling of the interferogram 50 and the degree of polynomial fitting.


As described above, the pupil area ratio η is smaller for the field lens element LF than for the pupil lens element LP. Since the field lens element (more precisely, the field lens surface) has a relatively small local pupil area AR compared to the area AS of the surface, the tolerance setting method of the present disclosure works best. If the pupil area ratio η becomes too large, the influence of the localized surface will hardly change over the effective diameter. For example, the tilt and refractive power measured over the interferogram 50 are typically measured as zero because these two measurement parameters cancel out to zero in the interferometer. However, if the local pupil area AR is sufficiently small, the refractive index and slope can vary over the local pupil region PR.


FIG. 6 shows another example of the optical system 10. The optical system 10 includes a biconvex field lens element LF1 having a front surface S including a surface error SE having a small concave depression shape. The optical system 10 also includes a parallel field-shaped second field lens element LF2. Eleven luminous fluxes RB1 to RB11 are shown together with associated field points FP in the image plane (more precisely, the image plane or image curved surface) IP.


The optical magnification is measured for each local pupil region PR to limit localized defocus on the image plane IP over the entire imaging region. Surface error SE exists in local pupil region PR6 associated with light beam RB6 and on-axis field point FP6. Since the surface error SE has a diameter that substantially matches the size of the corresponding local pupil region PR6, the surface error SE affects the focal point of the corresponding central field point FP6, and causes distortion aberration around the central field point. Will also affect. The change in focus as a function of field position is represented by a central ledge in the image plane (plane) IP. In FIG. 6, since the light beam RB and the corresponding field point indicate sampling with a relatively wide interval for measuring distortion, the change in distortion is not clear.


FIG. 7 is a diagram depicting the image plane position IP (μm) as a function of the focus position fp (wavelength λ) for the illustrated optical system 10. The curve indicated by the small triangle represents the focus position fp for the nominal design of the optical system 10, while the curve indicated by the square has a non-ideal surface with respect to the field lens element LF in the optical system 10. The in-focus position fp for the optical system is represented. The non-ideal surface S of the field lens element LF causes an undesirable shift of the in-focus position fp. The frequency of the deviation also increases.


FIG. 8 is a diagram showing distortion aberration as a vector, and shows the influence of the central artifact on the field lens element LF, similar to the surface error SE shown in FIG. The starting point of each vector V is the ideal position of each light beam RB, and the light beams are evenly spaced. An error in distortion caused by an artifact that is rotationally symmetric at the center of the field lens element LF is not visible to the field point FP on the axis, but is visible to the field points surrounding the field point on the axis. When the light beam RB leaves the central artifact, the distortion of the optical system 10 remains substantially unaffected.


In an example showing the tolerance setting method of the present disclosure, it is assumed that the imaging requirement of the optical system 10 causes any surface S of the field lens element LF to give distortion larger than 200 nm to the entire image on the image plane IP. Suppose you don't. That is, the tolerance of distortion is 200 nm. The method includes measuring an interferogram 50 for each surface S of each field lens element LF. In order to measure the local tilt for each local pupil region position for a given plane S, an appropriately sized local pupil region PR is scanned on the corresponding interferogram 50.


The amount of local tilt for each local pupil region R on each surface S can be associated with the amount of distortion at the corresponding field point FP on the image plane IP. This relationship can be established using standard optical design software and tolerance setting techniques. Therefore, the tolerance T for the local tilt amount is determined based on the distortion tolerance of 200 nm and the optical design of the optical system.


In one example, the inclination amount measured for each local pupil region PR appears in a polynomial fitting coefficient to the interferogram data. This makes it possible to directly compare the slope tolerance T calculated from the distortion performance metrics over one or more polynomial coefficients of the fitted interferogram over the local pupil region PR.


An example of polynomial fitting may be based on an extended Fringe Zernike polynomial set as defined below.


Z = {c · r ² / [1 + B]} + ΣC _{j + 1} ZP _j


Where z is a sag of a plane S parallel to the optical axis A1,


c is the vertex curvature,


B = [1 · (1 + k) · c ² · r ² ] ^1/2 ,


k is the conic constant,


r is the radial distance to the maximum radius R,


ZP _j is the j th Zernike polynomial that can be expressed in polar coordinates (r, θ), and


C _{J + 1} is a coefficient for ZP _j .


The second and third Zernike polynomials are ZP ₂ = R · cos θ and ZP ₃ = R · sin θ, which represents the slope at the wavefront, and tolerances are set for the corresponding coefficients C ₁ and C ₂ That is, it is limited to the one having the maximum value. The tolerance value is based on the slope required for a given surface S in a given local pupil region PR so as to generate a 200 nm distortion at the corresponding field point FP in the image plane IP. This is easily determined as described above by analyzing the optics using conventional lens design software such as CODE V sold by Synopsys, Pasadena, California.


Therefore, the tolerance setting method includes comparing the slope tolerance T with the slope coefficients C ₁ and C _{2 in} order to ascertain whether the slope tolerance meets the condition, ie, C ₁ <T and C ₂ <T. .


In one example, the tolerance T may not be a single value for all field points FP, and may vary depending on the position of the field points. Such a situation may occur when the imaging performance of a part of the field needs to be higher than the imaging performance of the other part of the field. For example, it is possible to have a greater amount of distortion at the end of the image plane IP than at the center of the image plane IP.


Therefore, a method for setting a tolerance on the optical surface S of the field lens element LF of the optical system 10 based on the interferogram of the optical surface is as follows:

a) defining, for the optical surface S, a local pupil region PR having a corresponding field point FP in the image plane IP;
b) determining a tolerance T for at least one characteristic of the optical surface in the local pupil region PR based on at least one performance metric of the optical system 10 for the field point FP;
c) applying a polynomial to the surface shape of the interferogram 50 over the local pupil region PR, the polynomial comprising at least one coefficient relating at least one characteristic of the optical surface S to at least one performance metric A process comprising C;
d) comparing the tolerance T with at least one coefficient C to determine if C <T for the field point;
It has.

Another method of setting tolerances for the optical surfaces of the field lens elements of the optical system is as follows, the optical system having a pupil P and an image plane IP, the optical surface having an effective diameter CA and It has a total area AS. This method
a) a step of measuring the interferogram 50 of the optical surface S, wherein the interferogram measures the surface shape of the optical surface over the entire effective diameter CA of the optical surface;
b) defining a local pupil region PR having an area AR and a position in the optical surface, the local pupil region having a corresponding field point FP in the image plane;
c) determining a tolerance T for at least one characteristic of the optical surface S in the local pupil region PR based on at least one performance metric of the optical system for the field point;
d) applying a polynomial to the surface shape of the interferogram 50 over the local pupil region PR, the polynomial at least one coefficient relating at least one characteristic of the optical surface S to at least one performance metric A process comprising C;
e) comparing the tolerance T with at least one coefficient C to see if C <T for the field point;
It has.

Another aspect of the present disclosure is a method of setting a tolerance on the field surface S of the optical element LF of the optical system 10 based on the interferogram 50 of the field surface. This method
a) defining, for the field plane S, a local pupil region PR having a corresponding field point FP in the image plane IP of the optical system 10;
b) determining a tolerance T for at least one characteristic of the field plane S in the local pupil region PR based on at least one performance metric of the optical system 10 for the field point FP;
c) Fit a polynomial to the surface shape of the interferogram 50 over the local pupil region PR so as to obtain at least one polynomial coefficient C relating at least one characteristic of the field surface S to at least one performance metric. Process,
d) comparing the tolerance T with at least one coefficient C to determine if C <T for the field point;
It has.

  As an example, the tolerance setting method of the present disclosure was applied to the optical system 10 of the '758 patent shown in FIG. When the optical system 10 of the '758 patent was analyzed, the distortion on the image plane IP was about 175 μm. In order to prevent generation of an undesirable amount of localized distortion by the field lens element LF, the upper limit of the local distortion is set to 50 μm for each of the first four lens surfaces S1 to S4. Design sensitivity was used to determine the tolerances set for the local pupil region R.

For the first field lens element LF1, it has been found that when the inclination is 10.87 μm over the local pupil region R of the surfaces S1 and S2, the position of the image on the image plane IP changes by 25 μm. It was. If this is applied to the Fringe Zernike polynomial, it is converted into a gradient of 17 stripes. As described above, the terms ZP ₂ and ZP ₃ of the second and third polynomials indicate the slope.

Next, for the local pupil region R of the surfaces S1 and S2 of the first field lens element LF1, the tolerance T related to the inclination is T = [C ₂ ² + C ₃ ² ] ^1/2 <17 stripes or 10.87 μm. It was calculated. The same method was used for the second field lens element LF2 and the tolerance T = [C ₂ ² + C ₃ ² ] ^1/2 <18.8 stripes or 11.7 μm. Note how the tolerance T estimated for each surface is directly compared to the numerical value calculated from the two slope polynomial coefficients.

  For the field lens elements LF1 and LF2, the size of the local pupil region R on each surface S can be determined by the peripheral ray height at the axial field position. The local pupil regions R of the surfaces S1 to S4 were found to be 30 mm, 29.26 mm, 29.2 mm and 28.8 mm, respectively.

  The tolerance setting method of the present disclosure has a number of advantages over the conventional tolerance setting method. One advantage is cost savings. By tying tolerances to the local pupil region PR (or tying tolerances to different local pupil regions), it may be possible to prevent setting too tight tolerances for one or more optical surfaces. Furthermore, in connection with using local measurements of the interferogram, not only the time but also the cost is significantly less than the production of a perfect optical system in which a large number of field points have to be examined. Furthermore, since tolerance settings for local pupil regions are applied to deviations from the ideal surface, tolerances can be easily applied to generally any type of surface shape, such as spherical, aspherical or free-form. Further, the disclosed method can be used with non-rotating symmetric optics.

  It will be apparent to those skilled in the art that various modifications can be made to the preferred embodiments of the disclosure described herein without departing from the spirit or scope of the disclosure as set forth in the appended claims. Accordingly, the present disclosure is intended to embrace alterations and modifications as long as they fall within the scope of the appended claims and their equivalents.

  Hereinafter, preferable embodiments of the present invention will be described in terms of items.

Embodiment 1
A method for setting a tolerance for an optical surface of a field lens element of an optical system having a pupil and an image surface,
The optical surface has an effective diameter and a total area AS;
The method
a) measuring the interferogram of the optical surface, the interferogram measuring the surface shape of the optical surface over the entire effective diameter of the optical surface;
b) defining a local pupil region having an area AR and a position of the optical surface, the local pupil region having a corresponding field point in the image plane;
c) determining a tolerance T for at least one characteristic of the optical surface in the local pupil region based on at least one performance metric of the optical system for the field point;
d) applying a polynomial to the surface shape of the interferogram over the local pupil region, the polynomial relating the at least one characteristic of the optical surface to the at least one performance metric A step that includes at least one coefficient C;
e) comparing the tolerance T with the at least one coefficient C to see if C <T for the field point;
A method comprising the steps of:

Embodiment 2
Repeating the operations of d) and e) for local pupil regions at different positions with respect to the interferogram, wherein the positions of the different local pupil regions have different corresponding field points in the image plane. Embodiment 2. The method of embodiment 1 characterized by:

Embodiment 3
The step of repeating the operations d) and e) is performed by moving the local pupil region with respect to the interferogram so as to substantially cover the entire interferogram. The method according to embodiment 2.

Embodiment 4
Embodiment 2. The method of embodiment 1 wherein AR / AS <0.75.

Embodiment 5
Embodiment 6. The method of embodiment 4 wherein AR / AS <0.65.

Embodiment 6
Said at least one performance metric is selected from the group of performance metrics including field curvature, distortion, Strehl ratio, depth of focus, wavefront error in the image plane, and modulation transfer function. The method of embodiment 1 characterized.

Embodiment 7
The method of embodiment 1 wherein the tolerance T varies as a function of the position of the field point.

Embodiment 8
The method of embodiment 1, wherein the optical surface comprises a spherical surface.

Embodiment 9
The method according to embodiment 1, wherein the optical system is an afocal system.

Embodiment 10
A method for setting a tolerance on an optical surface of a lens element of an optical system having a pupil and an image surface, based on an interferogram having a surface shape of the optical surface,
The method
a) defining, for the optical surface, a local pupil region having corresponding field points in the image plane;
b) determining a tolerance T for at least one characteristic of the optical surface in the local pupil region based on at least one performance metric of the optical system for the field point;
c) applying a polynomial to the surface shape of the interferogram over the local pupil region, the polynomial relating the at least one characteristic of the optical surface to the at least one performance metric A step that includes at least one coefficient C;
d) comparing the tolerance T with the at least one coefficient C to ascertain whether C <T for the field point.

Embodiment 11
Repeating the operations of c) and d) for local pupil regions at different positions with respect to the interferogram, wherein the positions of the different local pupil regions have different corresponding field points in the image plane. Embodiment 11. The method of embodiment 10 characterized by:

Embodiment 12
12. The method of embodiment 11 wherein the tolerance T depends on the position of the field point in the image plane.

Embodiment 13
The step of repeating the operations of c) and d) is performed by moving the local pupil region with respect to the interferogram so as to substantially cover the entire interferogram. Embodiment 12. The method of embodiment 11.

Embodiment 14
The local pupil region has an area AR, and the optical surface has an area AS;
Embodiment 11. The method of embodiment 10 wherein AR / AS <0.75.

Embodiment 15
Embodiment 15. The method of embodiment 14 wherein AR / AS <0.6.

Embodiment 16
Said at least one performance metric is selected from the group of performance metrics including field curvature, distortion, Strehl ratio, depth of focus, wavefront error in the image plane, and modulation transfer function. Embodiment 12. The method of embodiment 10 characterized.

Embodiment 17
A method for setting a tolerance on a field surface of an optical element of an optical system based on an interferogram representing a surface shape of the field surface,
The method
a) defining, for the field plane, a local pupil region having corresponding field points in the image plane of the optical system;
b) determining a tolerance T for at least one characteristic of the field plane in the local pupil region based on at least one performance metric of the optical system for the field point;
c) a polynomial in the surface shape of the interferogram over the local pupil region so as to obtain at least one polynomial coefficient C relating at least one characteristic of the field surface to the at least one performance metric And the process of applying
d) comparing the tolerance T with the at least one coefficient C to ascertain whether C <T for the field point.

Embodiment 18
The optical system includes a number of field surfaces;
18. The method according to embodiment 17, further comprising the step of repeating the operations a) to d) for each field plane.

Embodiment 19
The optical system comprises a refractive lens element;
Embodiment 18. The method of embodiment 17, wherein at least one refractive lens element comprises a field lens element.

Embodiment 20.
The local pupil region has an area AR, and the field plane has an area AS;
Embodiment 18. The method of embodiment 17, wherein AR / AS <0.75.

10 optical system 50 interferogram GF field lens group LF field lens element GP pupil group LP pupil lens element PR local pupil region

Claims (11)

A method for setting a tolerance for an optical surface of a field lens element of an optical system having a pupil and an image surface,
The optical surface has an effective diameter and a total area AS;
The method
a) measuring the interferogram of the optical surface, the interferogram measuring the surface shape of the optical surface over the entire effective diameter of the optical surface;
b) defining a local pupil region having an area AR and a position of the optical surface, the local pupil region having a corresponding field point in the image plane;
c) determining a tolerance T for at least one characteristic of the optical surface in the local pupil region based on at least one performance metric of the optical system for the field point;
d) applying a polynomial to the surface shape of the interferogram over the local pupil region, the polynomial relating the at least one characteristic of the optical surface to the at least one performance metric A step that includes at least one coefficient C;
e) comparing the tolerance T with the at least one coefficient C to see if C <T for the field point;
A method comprising the steps of:   Repeating the operations of d) and e) for local pupil regions at different positions with respect to the interferogram, wherein the positions of the different local pupil regions have different corresponding field points in the image plane. The method of claim 1 wherein:   The step of repeating the operations d) and e) is performed by moving the local pupil region with respect to the interferogram so as to substantially cover the entire interferogram. The method of claim 2.   The method of claim 1, wherein AR / AS <0.75.   5. The method of claim 4, wherein AR / AS <0.65. A method for setting a tolerance on an optical surface of a lens element of an optical system having a pupil and an image surface, based on an interferogram having a surface shape of the optical surface,
The method
a) defining, for the optical surface, a local pupil region having corresponding field points in the image plane;
b) determining a tolerance T for at least one characteristic of the optical surface in the local pupil region based on at least one performance metric of the optical system for the field point;
c) applying a polynomial to the surface shape of the interferogram over the local pupil region, the polynomial relating the at least one characteristic of the optical surface to the at least one performance metric A step that includes at least one coefficient C;
d) comparing the tolerance T with the at least one coefficient C to see if C <T for the field point;
A method comprising the steps of:   Repeating the operations of c) and d) for local pupil regions at different positions with respect to the interferogram, wherein the positions of the different local pupil regions have different corresponding field points in the image plane. The method according to claim 6.   8. The method of claim 7, wherein the tolerance T depends on the position of the field point in the image plane. A method for setting a tolerance on a field surface of an optical element of an optical system based on an interferogram representing a surface shape of the field surface,
The method
a) defining, for the field plane, a local pupil region having corresponding field points in the image plane of the optical system;
b) determining a tolerance T for at least one characteristic of the field plane in the local pupil region based on at least one performance metric of the optical system for the field point;
c) a polynomial in the surface shape of the interferogram over the local pupil region so as to obtain at least one polynomial coefficient C relating at least one characteristic of the field surface to the at least one performance metric And the process of applying
d) comparing the tolerance T with the at least one coefficient C to see if C <T for the field point;
A method comprising the steps of: The optical system includes a number of field surfaces;
10. The method according to claim 9, further comprising the step of repeating the operations a) to d) for each field plane. The optical system comprises a refractive lens element;
10. The method of claim 9, wherein at least one of the refractive lens elements comprises a field lens element.

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