Telecentric interferometer, method for determining characteristics of an incident light field, and interferometer assembly - Patents.com

Abstract

A telecentric interferometer, a method for determining characteristics of an incident light field, and an interferometer assembly are disclosed. The telecentric interferometer includes a front optical element having an exit pupil, a first optical system having a first interferometer arm, a second optical system having a second interferometer arm, a detector, and a shift unit positioned between the front optical element and the detector. The first optical system and the second optical system have identically positioned object-side principal planes with the same object-side focal length, the same Gaussian image distance, and the same object-side optical axis. The exit pupil of the front optical element is separated from the object-side principal plane by the object-side focal length. The shift unit shifts the image-side principal plane of the first optical system and / or the image-side principal plane of the second optical system so that the image-side principal plane of the first optical system is different from the image-side principal plane of the second optical system and the optical path length of the first interferometer arm is equal to the optical path length of the second interferometer arm.

Description

[Technical Field]

[0001] The present disclosure relates to a telecentric interferometer, a method for determining a characteristic of an incident light field using a telecentric interferometer, and an interferometer assembly comprising a telecentric interferometer and an illumination device. [Background technology]

[0002] Optical interferometers can be used to reconstruct the phase and / or amplitude of an optical field. For example, optical interferometers allow the reconstruction of three-dimensional images. In particular, the spatial position (e.g., location and / or structure) or local refractive index of an object or individual points of an object can be determined from the intensity of the optical field that has interacted with the object, measured by a detector. In addition, there is interest in measuring pure phase objects, i.e., objects that cannot be seen by conventional microscopy (sometimes referred to as quantitative phase contrast microscopy).

[0003] Conventional optical systems can have limitations in the lateral alignment and / or resolution of the interferometer and / or the axial resolution (i.e., depth of field), making image reconstruction difficult. Additionally, conventional optical systems can suffer from chromatic and / or monochromatic aberrations (errors in imaging) that require expensive and complex correction.

[0004] U.S. Patent Application Publication No. 2017 / 242398 A1 discloses a birefringent lens in an in-line interferometer for use in microscopy. The design is optimized for microscopy of incoherent light sources. U.S. Patent Application Publication No. 2017 / 0329280 A1 discloses an apparatus including a beam expander for generating an interferogram generated by an incoherent point source. This uses a highly refractive lens within the interferometer, which implies complex requirements due to inherent aberrations and the need for achromatic and chromatic balancing of aberration correction between the two interferometer arms.

[0005] U.S. Pat. No. 9,417,610 B1 discloses an interferometer that uses a concave mirror to generate a significantly out-of-focus interferogram. Therefore, spot overlap is not important in this publication. The purpose of this device is to image a phase aberration source via a mirror into the upstream optics of the interferometer so that the resulting phase aberration can be compensated for in the measured image. This applies to mutually incoherent light sources. The placement of the imaging element in the interferometer (its design) is determined by the physical location of a phase perturbation layer in the middle of the optical path from the object to the front optics. The interferometer disclosed in this publication is not telecentric.

[0006] In an interferometer, typically, two central beams (i.e., along the axis of symmetry) are superimposed on the detector. However, objects that deviate from the symmetry (e.g., differ from the central beam) can experience degradation in quality. In conventional configurations, the quality of the interference usually varies across the field of view. This is particularly important for coherent light sources, since in these sources all field components can interfere with each other. The quality of the interference can be measured, among other things, by the quality of the superposition (for images using one emitting point source).

[0007] In conventional interferometers, interference cannot usually be observed with mutually incoherent light sources (e.g., spots) if the overlap of the spots (e.g., light fields or parts of light fields that are images of points such as light sources) is inaccurate. This is because coherence is only given to light within one and the same light source. However, spot interference can be achieved by defocusing, i.e., blurring the spots. This blurring uses the property that the overlap of inaccurately overlapped, i.e., laterally shifted, Gaussian spots still produces a Gaussian spot at the center position. This method benefits from the automatic suppression of non-aligned parts of the light field due to non-interference. However, this method may not be applicable to coherent light (e.g., quantitative phase difference), because inaccurate or approximate overlaps result in new (but erroneous) interference with other spots. Furthermore, real spots in optical systems are not Gaussian spots but arise from Fresnel diffraction at the exit pupil of the optical system (e.g., "Born Principles of Optics", M. Born and E. Wolf, Cambridge University Press, 8 th (See [link missing from image], [link missing from image], and [link missing from image].) Therefore, non-central superposition of these composite spots can result in non-rotationally symmetric superposition and significant interferogram evaluation problems, and therefore artifacts. Traditional methods can also suffer from the interferometer's resolution, which is greatest near the focus. Therefore, in interferometry, it is generally advantageous to superpose the images of different spots near the focus, which implies "as accurately as possible." Therefore, pinpoint superposition devices are required, especially for applications using (partially) coherent light.

[0008] Quantitative phase microscopy (i.e., self-interference with partially coherent light) is of great technological importance. Therefore, interferometers suitable for both coherent light and light from mutually incoherent sources are needed. Furthermore, the combined evaluation of light with different degrees of coherence may enable new sampling possibilities.

[0009] Conventional interferometer configurations are usually based on the principle of generating only approximate overlap of spots, i.e., the above-mentioned accuracy cannot be achieved with non-axial spots. This can already be recognized by using different refractive powers in the optical systems of the two interferometer arms without appropriate compensation by lenses outside the interferometer. In this case, telecentricity cannot be achieved for both arms of the interferometer. In this case, telecentricity can only be achieved if the corresponding compensation is achieved by appropriately optimized lenses outside the interferometer. If these optimized lenses are not provided outside the interferometer, the entire optical system cannot be telecentric in the sense of the present application. In particular, conventional optical systems may not have the compensation necessary to be telecentric in the sense of the present application.

[0010] It may also be beneficial for the interferometer to be suitable for a wide spectral range, such as wavelength multiplexing to achieve good visibility of the interference fringes, synthetic wavelength applications, or simply to use all available light for the measurement.

[0011] Furthermore, conventional optical systems may not be telecentric, particularly not telecentric in image space (so-called image space telecentricity). Therefore, there is a need to provide an improved interferometer that is image space telecentric for both arms and uses no lenses / mirrors or only lenses / mirrors with very low refractive power in the interferometer arms of the device. Summary of the Invention

[0012] According to some aspects of the present disclosure, a telecentric interferometer includes a front optical element having an exit pupil, a first interferometer arm that is part of a first optical system and is located on the image side of the front optical element, a second interferometer arm that is part of a second optical system and is located on the image side of the front optical element, a detector that is located on the image side of both the first optical system and the second optical system, and a shift unit that is located between the front optical element and the detector. The first optical system and the second optical system have the same object-side focal length and the same Gaussian image distance. The first optical system and the second optical system have identically positioned object-side principal planes with the same object-side optical axis. The exit pupil of the front optical element is separated from the object-side principal plane by the object-side focal length. The shift unit shifts the image-side principal plane of the first optical system and / or the image-side principal plane of the second optical system so that the image-side principal plane of the first optical system is different from the image-side principal plane of the second optical system and the optical path length of the first interferometer arm is equal to the optical path length of the second interferometer arm.

[0013] According to some aspects of the present disclosure, a method for determining a characteristic of an incident light field using a telecentric interferometer includes propagating the incident light field through an exit pupil of a front optical element; splitting the incident light field into a first portion propagating along a first optical system and a second portion propagating along a second optical system; shifting an image-side principal plane of the first optical system relative to an image-side principal plane of the second optical system using a shift unit such that the image-side principal plane of the first optical system is different from the image-side principal plane of the second optical system and the optical path length of the first interferometer arm is equal to the optical path length of the second interferometer arm; combining the first portion of the incident light field and the second portion of the incident light field into an exit light field; and measuring an interference pattern of the exit light field using a detector.

[0014] Those skilled in the art will recognize additional features and advantages upon reading the following detailed description, and upon viewing the accompanying drawings. [Brief explanation of the drawings]

[0015] The present disclosure is illustrated by way of example, and not by way of limitation, in the figures of the accompanying drawings in which like reference numerals refer to similar or identical elements and in which elements are not necessarily drawn to scale.

[0016] Elements of the drawings are not necessarily to scale relative to each other. Same reference numbers indicate corresponding like parts. Features of the various illustrated embodiments may be combined unless they are mutually exclusive. Examples are shown in the drawings and detailed in the following description. [Figure 1] 1 illustrates a schematic diagram of a telecentric interferometer according to an aspect of the present disclosure. [Figure 2] FIG. 1 is a diagram illustrating the principle of a telecentric optical system. [Figure 3A] 1A and 1B illustrate schematic diagrams of aspects of a telecentric interferometer according to examples of the present disclosure. [Figure 3B] 1A and 1B illustrate schematic diagrams of aspects of a telecentric interferometer according to examples of the present disclosure. [Figure 4] 1 illustrates a schematic diagram of an interferometer according to an aspect of the present disclosure. [Figure 5A] 1 illustrates a schematic diagram of an interferometer according to an aspect of the present disclosure. [Figure 5B] 1 illustrates a schematic diagram of an interferometer according to an aspect of the present disclosure. [Figure 6A] 1A and 1B illustrate schematic diagrams of aspects of a telecentric interferometer according to examples of the present disclosure. [Figure 6B] 1A and 1B illustrate schematic diagrams of aspects of a telecentric interferometer according to examples of the present disclosure. DETAILED DESCRIPTION OF THE INVENTION

[0017] The following detailed description refers to the accompanying drawings, which form a part of this disclosure, and in which, by way of illustration, specific examples of interferometers and / or methods are shown. Further examples will be apparent.

[0018] Throughout this description, whenever possible or useful, and whenever not otherwise specified, the notation "Field Guide to Geometrical Optics" by John E. Greivenkamp (SPIE, 2004) will be used (hereafter abbreviated as "Greivenkamp").

[0019] Hereinafter, the positional relationship refers to a position in the optical path of the interferometer. For example, if a first component is positioned "downstream" ("upstream") of a second component, the first component is located after (before) the second component in the optical path. If a first component is positioned "between" a second component and a third component, the first component may be located downstream of the second component and upstream of the third component, or vice versa. In some examples, the "downstream" ("upstream") position refers to the image side (object side). "Lateral" refers to a direction perpendicular to the optical axis.

[0020] The optical axis can be considered to be defined locally by the optical element and / or optical component. Every optical element with refractive power has an optical axis given by the symmetry of the optical element. In so-called "aligned" optical systems with several optical elements, the optical axes of the optical elements are aligned, i.e., coincide. Mirrors do not have refractive power, and the optical system can have different optical axes before and after the mirror. An optical system is aligned if the ray before the mirror that is on the optical axis is also the ray on the optical axis after the mirror. Other optical elements that do not have refractive power can be treated similarly. In so-called tunnel diagrams, all optical axes of an aligned optical system are depicted with one axis. The ray extending along the optical axis is called the central ray. When an optical system is aligned, the central ray passes through the entire optical system on the optical axis. The optical axis can also be called the axis of symmetry or the z-axis.

[0021] Throughout this specification, an optical component (sometimes referred to as an "optical element") may comprise or be at least one of a mirror, a concave mirror, a convex mirror, a beamsplitter (e.g., a beamsplitter cube), a lens, or a dielectric plate. Other optical components are also possible.

[0022] Hereinafter, the geometric optical path length of an optical path refers to the geometric length of the optical path. In other words, the geometric optical path length is the Euclidean distance between the start and end points of the optical path, integrated along a ray. The geometric optical path length between two optical components in the first or second interferometer arm does not include the length between the principal planes of the two optical components. That is, only the length outside the area between the principal planes is counted. For this purpose, the optical path length along the central ray or the z-axis (axis of symmetry) can be used, and the principal planes are determined by Gaussian sweep for all individual optical components in the (first) optical system comprising the first interferometer arm and / or by Gaussian sweep for all individual optical components in the (second) optical system comprising the second interferometer arm, respectively. In contrast, the term "total geometric optical path length" refers to the case where the area between the principal planes is not subtracted. Herein and below, the geometric optical path length and total geometric optical path length are always measured between the object and the detector, unless another reference point is explicitly specified. The optical path length is the total geometric optical path length multiplied by the refractive index of the objects located in the optical path. For different refractive indices along the optical path, the optical path length is given by the line integral over the refractive indices.

[0023] Unless otherwise specified below, the object-side characteristics (e.g., object-side focal length) of an optical component (or optical system) may be the same as the image-side characteristics (e.g., image-side focal length) of the optical component (or optical system).

[0024] When an optical component is referred to by only one principal plane without further distinction, the first and second principal planes are assumed to be coincident. A skilled optical engineer can generalize to finite principal plane divisions.

[0025] A telecentric interferometer is provided. Telecentricity in object space or image space requires that the chief ray be parallel to an axis in that space. As defined by Greivenkamp, ​​the chief ray starts at the edge of the object (i.e., a point at the boundary of the field of view), passes through the center of a pupil (e.g., an entrance pupil), and leaves the center of a further pupil (e.g., an exit pupil), defining the respective image height and (optional) other pupil position. As a result of image-space telecentricity, the apparent system magnification remains constant even if the detection plane is displaced from its focal position (i.e., defocused). Although the image is blurred, the center of each spot remains at the same lateral position, and the size or magnification of each individual spot remains constant even if it is blurred. Image-space telecentricity is independent of the position of the object or detector. Therefore, an image of the same size is produced regardless of the distance between the lens and the object or detector. In this disclosure, for a general object in the field of view, the ray from the edge of the object that passes through the center of the pupil may be referred to as the chief ray.

[0026] In the interferometer described in the present invention, telecentricity (image side) may particularly mean that the optical system of a first interferometer arm (e.g., a Gaussian sweep of a component of the first interferometer arm) may have the same focal length as the optical system of a second interferometer arm (e.g., a Gaussian sweep of a component of the second interferometer arm). The two interferometer arms may also be referred to as the "interference part" of the interferometer. The interferometer provided herein may be easy to implement and may enable superposition of interference images with image-side telecentricity that is perfect in the paraxial Gaussian approximation. Furthermore, the positions of optical elements may be selectable independently of the object or object-related properties such as aberrations so that strict image-side telecentricity conditions are met.

[0027] In an image-side telecentric system, different chief rays intersect the exit pupil of the front optical element at the center of the pupil and become rays parallel to the optical axis on the image side of the optical system or detector. This applies to both the first and second interferometer arms of the interferometer. Therefore, a virtual point source at the center of the exit pupil can generate a plane wave on the detector (in a paraxial Gaussian approximation). That is, all of these points can have the same phase difference relative to the chief ray as it passes through the two interferometer arms. Therefore, a virtual point source at the center of the exit pupil can generate a plane wave on the detector. That is, all object points can have the same phase difference relative to the corresponding chief ray as it passes through the two interferometer arms. The central ray can be the chief ray of an on-axis object. The different chief rays can have the same phase difference, equal to the phase difference of the central ray.

[0028] Telecentric interferometers, at least according to some aspects of the present disclosure, may be suitable (e.g., without realignment) for only a single wavelength (e.g., one color) or for multiple wavelengths (e.g., the entire visible spectrum). For this purpose, using an achromatic optic at the focus may not be sufficient, but the optical path difference between the arms of the interferometer needs to be chromatically corrected depending on the configuration (this is another type of achromatism), and / or even the fringe spacing can be kept chromatically constant with appropriate correction.

[0029] Throughout this disclosure, the term "image space telecentric system" may refer to, or even be used synonymously with, an image space telecentric lens or an image space telecentric lens system that is part of an interferometer and provides telecentricity. According to some examples, a telecentric interferometer includes a first interferometer arm and a second interferometer arm. The first interferometer arm may be part of a first optical system, and the second interferometer arm may be part of a second optical system. Each of the first optical system and the second optical system includes optical components. In some examples, the first optical system and the second optical system may share at least one optical component but differ in at least another optical component. The shared optical component may not be part of the first interferometer arm or the second interferometer arm. That is, the first optical system and the second optical system may differ within at least the first interferometer arm and the second interferometer arm.

[0030] The first and second interferometer arms may be provided by a beam splitter that splits the incident light field into two light fields. Hereinafter, the terms "light field" and "light beam" may be used interchangeably (i.e., "light field" is "light beam" and vice versa) unless explicitly stated otherwise. The light field (or light beam) includes multiple light rays. The first of the two light fields may propagate along the first interferometer arm, and the second of the two light fields may propagate along the second interferometer arm. The light field (or spot) may have a chief ray that can be defined as the ray passing through the center of the exit pupil of the front optics. After propagating along the separate interferometer arms, the two light fields may be recombined using a beam combiner. The initial beam splitter may be used as the beam combiner (e.g., a Michelson-type configuration), or the beam combiner may be a separate optical component (e.g., a Mach-Zehnder-type configuration).

[0031] For example, the beam splitter and / or beam combiner may comprise or be a beam splitter cube, a prism, a semi-transparent mirror, or a pellicle, or a dielectric component. Depending on the application, chromatic effects may be compensated or corrected to the extent necessary. Those skilled in the art will understand that any optical component suitable for splitting and / or combining a light field may be used as a beam splitter and / or beam combiner.

[0032] The beam splitter that defines the two interferometer arms can be part of the entrance optics of the interferometer. The beam combiner can be part of the exit optics of the interferometer. If the beam splitter is the same as the beam combiner, the beam splitter can be part of the entrance and exit optics.

[0033] The first optical system and the second optical system may have the same object-side focal length and the same Gaussian image distance. Therefore, the term "object-side focal length" ("Gaussian image distance") refers to the first object-side focal length (first Gaussian image distance) of the first optical system and the second object-side focal length (second Gaussian image distance) of the second optical system, respectively. The Gaussian image distance of an optical system is the distance between the principal plane and the conjugate plane (sometimes called the "object plane" on the object side and the "image plane" on the image side). It may also be possible for the first optical system and the second optical system to have the same object-side focal length and the same Gaussian image distance.

[0034] The first and second optical systems may have identically positioned object-side principal planes and identical object-side optical axes. The "object-side optical axis" may be the optical axis originating from the front optical element. The identical focal lengths (also referred to as "optical focal lengths" or "front focal lengths") of the first and second optical systems and the identical object-side principal planes may enable the overlap of image-side telecentric beams. If the distances between the exit pupil and the principal planes are not identical, the chief ray of any object point passing through the center of the exit pupil at an angle greater than 0° will strike the principal planes of the first and second optical systems at different lateral distances from the axis. This means that the chief rays of the two interferometer arms may not have the same ray axis and therefore will not be positioned above each other at the ray centers. In such a scenario, which may be the case with a conventional interferometer, no or only a small interference signal may be measured, which may adversely affect the resolution of the interferometer.

[0035] In at least some examples, the image-side principal plane of the first optical system and the image-side principal plane of the second optical system are parallel. The image-side principal plane of the first optical system and the image-side principal plane of the second optical system may be different (e.g., may be at different positions along the optical path of the central ray). The image-side conjugate plane of the first optical system may be different from the image-side conjugate plane of the second optical system. The image-side focal length of the first optical system (sometimes referred to as the "back focal length" of the first optical system) and the image-side focal length of the second optical system (sometimes referred to as the "back focal length" of the second optical system) may be equal. For example, the image-side conjugate plane of the first optical system is positioned at the position of the image of the object emerging from the optical path via the first interferometer arm. Similarly, the image-side conjugate plane of the second optical system is positioned at the position of the image of the object emerging from the optical path via the second interferometer arm.

[0036] The telecentric interferometer may include a detector located on the image side of both the first optical system and the second optical system. For example, the detector may be or include a CMOS sensor having multiple pixels. For a given object position, the detector may be configured to be near a conjugate plane of the first optical system and a conjugate plane of the second optical system, which may be different from the conjugate plane of the first optical system. For example, the detector may be located between the image-side conjugate plane of the first optical system and the image-side conjugate plane of the second optical system. In such a scenario, neither the image from the first interferometer arm nor the image from the second interferometer arm is focused on the detector, but both images are slightly out of focus.

[0037] In some examples, the image-side optical axis of the first optical system is the same as the image-side optical axis of the second optical system. Additionally or alternatively, the optical systems may be aligned. In this case, the interferometer may be a perfectly centered optical system. Alternatively, the image-side optical axis of the first optical system may be shifted laterally relative to the image-side optical axis of the second optical system. Such a lateral shift may also be referred to as "lateral shear." The lateral shear may be selected so that adjacent spots still at least partially overlap. This may be useful for coherent light sources, where even different spots may interfere with each other. In the following, a spot may be a light field or a portion of a light field that is an image of a point-like light source. The lateral shear may be achieved, for example, by slightly rotating a physical optical component located in one of the pupils of the optical system. In some embodiments of the present disclosure, this shear may be set to zero or a finite value by adjustment.

[0038] In at least some examples, a telecentric interferometer includes a front optical element (sometimes referred to as a "front optical element"). The first optical system and the second optical system may be located on the image side of the front optical element. When the interferometer is in use, an object to be inspected by the interferometer may be located on the object side of the front optical element. The front optical element includes an exit pupil. The exit pupil may be mapped to infinity by the first optical system and / or the second optical system. It may be possible for both the first optical system and the second optical system to satisfy the image-side telecentric condition, but the image-side principal planes of the two optical systems may be different.

[0039] The exit pupil of the front optical element can be separated from the object-side principal plane of the first optical system and the second optical system by the object-side focal length. This position of the exit pupil can, for example, provide a telecentric system (e.g., an image-space telecentric system) in both arms of the interferometer. An image-space telecentric interferometer provides telecentricity on at least the image side of the optical system.

[0040] The telecentric interferometer may include a shift unit. The shift unit is located between the front optical element and the detector. For example, at least one component of the shift unit may be located in the first interferometer arm and / or the second interferometer arm. In some examples, both the first optical system and the second optical system may constitute a part of the shift unit, e.g., both the first interferometer arm and the second interferometer arm may constitute a part of the shift unit. Alternatively, the shift unit may be located solely in the first interferometer arm or the second interferometer arm.

[0041] The shift unit shifts the image-side principal plane of the first optical system and / or the image-side principal plane of the second optical system. In some examples, the shift unit shifts the image-side principal plane of the first interferometer arm and / or the image-side principal plane of the second interferometer arm. The shift unit may be adapted such that the image-side principal plane of the first optical system is different from the image-side principal plane of the second optical system and the optical path length of the first interferometer arm is equal to the optical path length of the second interferometer arm. The optical paths of the first interferometer arm and the second interferometer arm may each be determined relative to a central ray along a symmetry axis of the system. In particular, the optical paths may be determined between corresponding starting points of the rays on the object point and corresponding ending points of the rays on the detector. Because the detector cannot be in the same image plane for both arms (because the arms are different), the mapping from the object to the detector is not aberration-free, and rays other than the central ray may have a phase difference. This may cause an interference pattern to appear.

[0042] The object-side principal plane of the first optical system is identical (e.g., identically positioned) to the object-side principal plane of the second optical system. However, the image-side principal plane of the first optical system and the image-side principal plane of the second optical system may be different. Adjusting the identity of the optical paths may be achieved, for example, by shifting a mirror in one of the arms of the interferometer. In this case, the mirror may be part of the shifting unit.

[0043] In some examples, the first interferometer arm and / or the second interferometer arm may have a finite focal length as long as each optical system within the arm is constructed with optical elements having a finite focal length. In other words, a portion of the first optical system and / or the second optical system located within the first interferometer arm and / or the second interferometer arm, respectively, has a finite focal length (using Gaussian sweep). In contrast, if a portion of the first optical system within the first interferometer arm and / or a portion of the second optical system within the second interferometer arm include only a dielectric plate, the first interferometer arm and / or the second interferometer arm, respectively, have an infinite focal length. As long as the interferometer arms include one or more optical elements with absolute optical power greater than zero, the magnitude of the absolute focal length of each optical element divided by the beam diameter (or the diameter of the exit pupil of the front optical element) may be greater than 10, or greater than 20, or greater than 30, or greater than 50, or even greater than 100. The first interferometer arm and / or the second interferometer arm may be capable of including only elements with negative refractive power, i.e., concave lenses or convex mirrors, and plane mirrors, respectively.

[0044] The positions of the pupil, principal plane, and optical elements may be given by the configuration of the interferometer, but the position of the image plane may depend on the position of the object. The position of the detector (which may not be counted as an optical element) is appropriately selected taking the object into consideration. Focusing in the usual sense is not required. The object may be an inhomogeneity, a disturbance, or a group of scattering centers (hereinafter also referred to as "disturbance"). A disturbance is an object that disrupts or scatters the linear propagation of light. In optical theory, all such objects can be considered as sources of spherical waves (or equivalent electromagnetic modes) and can be imaged by an optical system. The optical field of the disturbance may also be called a "spot." The imaging distance refers to these disturbances and the corresponding image.

[0045] The shift by the shift unit may correspond to a defocus of the first interferometer arm (and / or the first optical system) relative to the second interferometer arm (and / or the second optical system). Here, the focal length and the distance of each conjugate plane from the respective principal plane may be identical for the first interferometer arm and the second interferometer arm. "Defocus" in this context refers to a situation in which the detection plane and the image plane (conjugate plane) do not coincide. In contrast, in an optical system without defocus, the detection plane and the image plane usually coincide. Since the shift (or defocus) is performed separately for the first interferometer arm and the second interferometer arm, the term "defocus of the first interferometer arm relative to the second interferometer arm" (or vice versa) is used. When describing an optical system using Gauss's equation, this may mean that the specified distance to the principal plane is the same for the first interferometer arm and the second interferometer arm. This may apply to the object and image as well as the pupil of the optical system for each of the first interferometer arm and the second interferometer arm. These two design criteria (regarding image and pupil position) can be met simultaneously.

[0046] The shift may change the axial position of a first light field (e.g., a first portion of the incident light field) propagating through a first optical system relative to a second light field (e.g., a second portion of the incident light field) propagating through a second optical system. The lateral position of the first light field relative to the second light field is typically not changed by the shift. However, if the first and second light fields experience a relative lateral shift (so-called "lateral shear") by the shift unit (or by another optical component of the interferometer), such lateral shift may be compensated for by, for example, a slight tilt of the interferometer due to at least one of the mirrors of the first interferometer arm and / or the second interferometer arm and / or a physical component located in one of the pupils.

[0047] The first light field may be superimposed with the second light field after propagating through the first and second interferometer arms, respectively. The first spot of the first light field and the second spot of the second light field may interfere with a common exit spot of the exit light field. The first spot of the first light field and the second spot of the second light field may arise from the same entrance spot of the incident light field. Thus, the incident light field may be superimposed with its defocused self, thereby generating an interference pattern that may enable the characteristics (e.g., phase or amplitude) of the incident light field to be determined. Due to the defocusing design, the two interfering light fields may have the same beam opening angle and / or the same divergence angle with the same curvature. For a light field or spot, the divergence angle can be measured at the beam waist, i.e., the position with the smallest beam radius, and represents the angle of incidence and divergence of the light field at this point (beam divergence angle).

[0048] The effects of defocusing in interferometers, particularly reference-beam-free interferometers, are described in detail in U.S. Patent Application Publication No. 2022 / 034645 A1, which also describes extracting characteristics of an incident light field using an interferometer with a defocusing unit. The disclosure of that U.S. Patent Application Publication regarding the evaluation and overall concept of the interferometer is incorporated herein by reference. The present disclosure uses a similar defocus (shift) concept as U.S. Patent Application Publication No. 2022 / 034645 A1, but in combination with a novel telecentric design.

[0049] In addition to imaging errors, in some aspects of the present disclosure, it may be beneficial to compensate for chromatic changes in optical path length. Therefore, using high-power lenses in the interferometer arms is disadvantageous because of the unavoidable errors associated with them. When high-power lenses are present in the interferometer arms, image components outside the center of the optical system may not interfere as desired. Conventional interferometers with high-power lenses can only be used with incoherent light sources.

[0050] By using a chromatically compensated interferometer, it may also be possible to use the interferometer on a coherent light field (obtaining a quantitative phase contrast). Interferometer errors have a particularly strong effect on coherent light, since the electric field is quadratically included in the correlation term measured from the two interferometer arms. In this case, evaluation can be performed according to the method of U.S. Pat. No. 10,823,547 (B2).

[0051] According to some examples, the shift unit includes at least one weak lens located in one of the first interferometer arm or the second interferometer arm. For example, the shift unit includes a first weak lens (e.g., a convex weak lens) located in the first interferometer arm and / or a second weak lens (e.g., a concave weak lens) located in the second interferometer arm. The first weak lens may be different from the second weak lens. The shift unit may further include a strong lens located on the image side of the first weak lens and / or the second weak lens. Each of the lenses may be, for example, a standard lens or a mirror lens. The strong lens may be able to compensate for different refractive effects of the first weak lens and the second weak lens, resulting in both optical systems telecentrically imaging the exit pupil of the front optical element.

[0052] The weak lenses (e.g., the first weak lens and / or the second weak lens) may have a finite focal length. The absolute focal length (i.e., its absolute value) of the weak lenses may be greater than zero and less than infinity. The strong lenses may also have a finite focal length. The absolute value of the focal length of the strong lenses may be greater than zero and less than infinity. The weak lenses (e.g., the first weak lens and / or the second weak lens) may have a focal length (absolute value) of at least 100 mm, or 200 mm, or 300 mm, or 400 mm, or 500 mm, or 1000 mm, or even 1500 mm. Separately or in combination, the absolute value of the focal length of the weak lenses (e.g., the first weak lens and / or the second weak lens) divided by the beam diameter may be at least 10, or 20, or 30, or 50, or 100, or 150. In general, the weak and / or strong lenses (eg, in the form of a standard lens or mirror) can be only convex, only concave, or one arm can be concave and the other convex.

[0053] In some examples, a weak lens (e.g., a first weak lens and / or a second weak lens) may be separated from the strong lens by the focal length of the strong lens. Both the first weak lens as part of the first optical system and the second weak lens as part of the second optical system may be separated from the strong lens by the focal length of the strong lens. The weak lens may not affect the focal length of the dual system (i.e., the combination of the weak lens and the strong lens). However, the weak lens may affect the axial position of the image-side principal plane of the dual system. Thus, the weak lens may be able to shift the image-side principal plane in different ways relative to the first optical system and the second optical system.

[0054] The strong lens may be located outside the first and second interferometer arms, e.g., downstream of the first and second interferometer arms. For example, the strong lens may be part of the first and second optical systems. That is, the strong lens may be part of both optical systems rather than part of either of the interferometer arms. Thus, the strong lens may be a common strong lens for the first and second optical systems. The strong lens may be part of the output optics. The output optics may be located between one interferometer arm and the detector on the other.

[0055] The first weak lens and / or the second weak lens may be spaced from the principal plane of the strong lens by the object-side focal length of the strong lens. The first weak lens and / or the second weak lens may have a larger absolute object-side focal length than the strong lens. For example, the object-side absolute focal length of the first weak lens and / or the second weak lens may be at least an order of magnitude larger than the object-side focal length of the strong lens. In other cases, the absolute focal length of the weak lens may be two, three, five, or even ten times larger than the absolute focal length of the strong lens. The refractive power of the weak lens may be correspondingly weaker than the strong lens.

[0056] In some examples, the shift unit includes a first weak lens in the first interferometer arm and a second weak lens in the second interferometer arm. The focal lengths (e.g., object-side focal lengths) of the first weak lens and the second weak lens may be equal in magnitude but opposite in sign. For example, the first weak lens may be a converging lens and the second weak lens may be a diverging lens, or vice versa. Separately or in combination, the first weak lens may be a concave lens or a convex mirror, or the second weak lens may be a concave lens or a convex mirror. The object-side focal length of the strong lens may be smaller in magnitude than the focal lengths of the first weak lens and the second weak lens.

[0057] The first weak lens and / or the second weak lens and / or the strong lens may comprise multiple optical elements that provide the focal length characteristics described above. Using a combination of lenses as the first weak lens and / or the second weak lens and / or the strong lens instead of only a single lens may allow the optical characteristics of the lens system to be adapted. For example, using a combination of lenses may suppress geometric and / or chromatic aberrations.

[0058] A weak lens has a weak refractive power and therefore has negligible monochromatic aberration effects. Monochromatic optical aberrations disrupt image-side telecentricity and may be of primary concern in the described optical system if they are different in the first and second interferometer arms.

[0059] In at least some instances, telecentric interferometers are configured in a Michelson configuration. In such configurations, the beam splitter that defines the two interferometer arms can also be used as a beam combiner. Each interferometer arm allows two passes of the optical field traveling through it.

[0060] In some examples, the interferometer is configured in a Michelson configuration, and the shift unit includes a first weak lens and a second weak lens. The shift unit further includes a strong lens. The first weak lens may be a concave mirror or a plane mirror, and the second weak lens may be a convex mirror, or vice versa. Therefore, the focal lengths of the first weak lens and the second weak lens may have opposite signs. The absolute values ​​of the focal lengths of the first weak lens and the second weak lens may be identical in magnitude.

[0061] In some examples, the shift unit of the telecentric interferometer comprises a dielectric plate. The dielectric plate may be located in the first interferometer arm or the second interferometer arm, or both, and the dielectric plates may be different. The shift unit may comprise an additional dielectric plate located in the other interferometer arm. For example, a dielectric plate may be located in the first interferometer arm and an additional dielectric plate may be located in the second interferometer arm, or vice versa. The dielectric plate may not have converging or diverging properties and / or may have a focal length close to infinity (when considered as an optical system). In other words, the dielectric plate may not be a lens. The entrance surface of the dielectric plate may extend substantially parallel to the exit surface of the dielectric plate.

[0062] The refractive index and / or thickness of the dielectric plate can be selected so that the optical path length of the first interferometer arm is equal to the optical path length of the second interferometer arm, and the geometric optical path length of the first interferometer arm is different from the geometric optical path length of the second interferometer arm. The optical path length can be measured from the object to the detector. The detector may not be located in one of the image planes (conjugate planes) of the first optical system and the second optical system. In fact, the geometric optical path lengths from the object to the conjugate planes of the first interferometer arm and the second interferometer arm are the same. The thickness of the dielectric plate is its extension between its entrance surface and its exit surface. The refractive index of the dielectric plate can be changed, for example, by changing the material of the dielectric plate.

[0063] The dielectric plate may be made of a single dielectric material or may comprise several dielectric plates and / or layers, at least two of which are made of different dielectric materials. Several dielectric plates and / or layers may be stacked together and / or there may be a space between at least two of the dielectric plates and / or layers.

[0064] For example, a dielectric plate can be positioned in the first interferometer arm. The geometric optical path length of the first interferometer arm is selected to be different from the geometric optical path length of the second interferometer arm without any dielectric plate. For example, in a Michelson-type configuration, the mirror in the first interferometer arm can be positioned slightly offset (e.g., in the axial or z-direction) compared to the mirror in the second interferometer arm. Without the dielectric plate, the optical path lengths of the two interferometer arms would also be different in such a configuration. The dielectric plate (e.g., the material and / or thickness of the dielectric plate) can be selected to compensate for this difference in optical path length between the two interferometer arms that would occur if no dielectric plate were inserted in the first interferometer arm. The dielectric material (or multiple dielectric materials, in the case of several dielectric plates and / or layers) can be selected so that this compensation can be achieved for several different wavelengths. However, the geometric optical path length remains unchanged by the dielectric plate, except for the principal plane division, which should be taken into account when determining the geometric optical path. The effects of both the principal plane division and the optical path length change thus produce the desired shift effect. That is, the first interferometer arm and the second interferometer arm have the same optical path length but different geometrical optical path lengths.

[0065] The geometric optical path length of the first interferometer arm differs from the geometric optical path length of the second interferometer arm by the optical path length compensated for by the dielectric plate. This also applies, mutatis mutandis, if a dielectric plate is positioned in the second interferometer arm or if both interferometer arms comprise a dielectric plate. In the latter case, the dielectric plates of the two interferometer arms have different refractive indices, dispersions, and / or thicknesses.

[0066] According to some examples, alignment of an interferometer may include an alignment step (e.g., a final alignment step) that includes adjusting an axial mirror position in at least one of the interferometer arms so that there is no longer an optical path length difference between the first interferometer arm and the second interferometer arm. This alignment step may be used in combination with other examples described herein (e.g., for a shift unit including a lens). For example, in the case of a shift unit including a dielectric plate, alignment may be performed for multiple wavelengths despite dispersion in the dielectric material. An alignment step that adjusts the optical path length by adjusting the position of a mirror may not be available in a so-called common-path configuration.

[0067] According to at least one example, the telecentric interferometer further includes an input optical element. The input optical element is located between one front optical system and the other first and second interferometer arms. The input optical element can be part of both the first and second optical systems. The input optical element is adapted to adjust the object-side focal length of the first optical system and the object-side focal length of the second optical system. The input optical element can be used to match the first and second optical systems to the exit pupil of the front optical system. In some examples, the input optical element includes at least one of a relay lens, a tube lens, or an achromatic tube lens. The input optical element can include a beam splitter that defines the two interferometer arms.

[0068] An interferometer (e.g., two interferometer arms including a beam splitter and / or a beam combiner) may include several optical components. At least one of the optical components (e.g., a mirror) may be located in a different spatial position in the first optical system than the optical components in the second optical system. This allows for the optical path length to be adjusted by spatially shifting one of the optical components. For example, by moving a mirror, it is possible to zero the optical path length difference between the two interferometer arms. In this example, the interferometer avoids a common-path configuration in which the two optical paths of the beam splitter are realized by polarization filters. It is envisioned that a birefringent material, such as a liquid crystal-based device, could be used to achieve this optical path adjustment. This would allow for the optical path length difference between the two polarizations to be set, but compensation by the device's dielectric layers would be subject to dispersion. Compensating for this dispersion would be very complex and expensive. Therefore, this device is only suitable for a narrow wavelength range. Therefore, the use of a common-path interferometer may be ruled out.

[0069] In some examples, the telecentric interferometer may include an output optical element positioned between the first and second interferometer arms on the one hand and the detector on the other hand. The output optical element may be part of both the first and second optical systems. For example, the output optical element may include a beam combiner that combines the light fields propagated through the interferometer arms. In some examples, the output optical element may include a strong lens that is part of the shift unit. The output optical element may be configured to adjust the magnification, image focal length, and / or the position of the principal planes of the first and second optical systems to enable sufficient focus on the detector.

[0070] Light rays have a linear phase in their propagation direction, e.g., the z-direction. Near the beam waist, an additional phase effect, the so-called Gouy phase, exists. The change in Gouy phase near the focal point extends over a Rayleigh length in the z-direction. The Rayleigh length is defined for the light field so that the area of ​​the light field doubles (image blur) at a distance of one Rayleigh length from the beam waist. In the image-side telecentric interferometer disclosed herein, interference can be measured so that both the fields across the first and second interferometer arms can be measured near their respective focal points. To this end, interference can be measured for a detector position between the two image planes (principal planes) of the first and second interferometer arms, respectively. For a detector position that is exactly centered, the two Gouy phases of the two fields can be exactly opposite, i.e., contribute twice to the interference. However, the linear phase may not contribute to the interference except for the global phase (provided the propagation direction is perpendicular to the detection plane). This may be the case in an image-telecentric configuration.

[0071] According to some aspects of the present disclosure, the separation of the major planes of the two interferometer arms divided by the Rayleigh distance may be less than 2, or less than 5, or less than 10, or less than 20, or less than 30, or less than 50, or less than 100. The Rayleigh length or Rayleigh distance refers to the light field leaving the output optics and for this purpose may be determined for a point source and may also be determined for the light field incident on a detector.

[0072] An image-side telecentric interferometer according to the present disclosure may have, for each object point (spot), overlapping chief rays that are parallel to the optical axis in the image space of the two interferometer arms. Therefore, the chief rays may strike the detector at the same point for the two interferometer arms. The quality of compliance with this requirement can be measured by the (lateral) alignment error and / or the Gouy error.

[0073] The (lateral) alignment error may be the lateral distance of the chief rays of the first and second interferometer arms at the intersection of the detection plane (assumed to be symmetric between two conjugate planes and determined for a typical object distance for the application) divided by the minimum diffraction-induced spot size of the spot image on one of the principal planes (i.e., in focus). The lateral alignment error is typically determined at the edge of the field of view. In some examples, an interferometer according to the present disclosure may have an alignment error of less than 0.2, or less than 0.5, or less than 1, or less than 2, or less than 5, or less than 10, or less than 15, or less than 20, or less than 30, or less than 50.

[0074] The Gouy error can be determined as follows: The chief rays of the first and second interferometer arms relative to the spot can enclose angles greater than zero with the optical system axis (=axis of symmetry). The larger angle of the two interferometer arms is used to determine the optical path length of the oblique optical path between the image (conjugate) planes of the first and second interferometer arms according to geometric rules. This extension shifts the Gouy phase between the principal planes and can be avoided by good telecentricity. "Good" in this case may mean that the chief ray hits the detector at an angle of exactly 90°. The optical path length divided by the Rayleigh length is referred to as the "Gouy error." The Gouy error is determined at the edge of the field of view. In some examples, an interferometer according to the present disclosure can have a Gouy error of less than 0.1, or less than 5e-2, or less than 1e-2, or less than 5e-3, or less than 1e-3, or even less than 1e-4.

[0075] According to some examples, at least one of the first optical system or the second optical system includes a correction unit. For example, both the first optical system and the second optical system may include a correction unit. The correction unit is adapted to correct a chromatic change in the optical path length in the first optical system and / or the second optical system due to the shift unit. This chromatic change may be caused, in particular, by a shift in the optical path length.

[0076] In contrast, achromatic lenses are corrected to provide the sharpest possible image for a wide range of wavelengths. This is particularly beneficial when the focal length is wavelength-independent. A sharp image implies the lowest possible wavefront error. This relates to the difference in optical path length relative to a reference wavefront. For example, the optical path length of the chief ray itself does not remain constant in a typical achromatic lens. This is particularly true when both the first and second optical systems contain different achromatic lenses. In this case, the difference in optical path length between the central ray of the first optical system and the central ray of the second optical system may be wavelength-dependent due to unavoidable dispersion in the dielectric medium of the lenses. Therefore, in addition to their achromatic properties, achromatic lenses can benefit from a correction unit for the optical path of the central ray. Therefore, even by adjusting the optical path length by moving one optical element of the interferometer arm (e.g., by changing the position of a mirror), it may be difficult to zero out the optical path length difference for all wavelengths. As a result, different wavelengths have different relative phase positions with respect to, for example, the central ray (which may correspond to the center of the telecentric interference spot). The visibility of the interference is significantly reduced until it disappears. If this is the configuration, a compensation unit is recommended.

[0077] Aberration compensation in lenses (sometimes referred to as chromatic compensation) and the correction unit introduced herein typically relate to different aspects. For example, an aberration-corrected lens (e.g., an achromatic doublet) may cause a wavelength-dependent optical path length difference between two interferometer arms, which can be corrected by the correction unit. However, the correction unit may be adapted to correct such aberration effects in the difference between the optical path lengths of the first and second interferometer arms (e.g., measured for the central ray of the first and second interferometer arms). Such aberration effects may be caused by the shift unit and / or by further optical components of the first and / or second optical systems.

[0078] The focal length of the correction unit may approach infinity. In other words, the correction unit may have no divergence or convergence properties. The correction unit may be translationally invariant in a direction perpendicular to the optical axis. In other words, the correction unit may have no divergence or convergence properties at all. The correction unit may also laterally correct the light field propagating through the first optical system and / or the second optical system such that the light field traveling along the optical axis before propagating through the first optical system and / or the second optical system also travels along the optical axis after propagating through the first optical system and / or the second optical system.

[0079] In some examples, the correction unit may include or consist of at least one dielectric plate (e.g., a planar dielectric plate). The at least one dielectric plate may be located in one of the first interferometer arm or the second interferometer arm, or the first interferometer arm and the second interferometer arm may each include at least one dielectric plate, and the optical flats of the dielectric plates in the interferometer arms differ between the first interferometer arm and the second interferometer arm (e.g., with respect to at least one of their material, their refractive index, their dispersion, and their thickness). Using at least one dielectric plate as the correction unit may make it possible to correct for chromatic path length effects.

[0080] In some examples, the correction unit may include or consist of at least one optical flat slightly tilted relative to the optical axis. The at least one optical flat may be located in one of the first interferometer arm or the second interferometer arm, or each of the first interferometer arm and the second interferometer arm may include at least one optical flat. Alternatively, or in combination, the correction unit may include or consist of a first prism and a second prism oriented opposite to each other so that only a small gap exists between the tilted surfaces. The second surfaces of the first prism and the second prism in the beam path are parallel to each other. The entire optical system may act like a dielectric plate whose thickness can be changed by moving the prisms laterally toward each other.

[0081] When an interferometer includes a strong lens and a weak lens as described above, it may be possible to realize a telecentric configuration in which only the weak lens is positioned in one of the two interferometer arms (or each weak lens is positioned in either one of the two interferometer arms). For example, if the weak lenses are concave or convex mirrors, respectively, chromatic effects that change the optical path difference between the first interferometer arm and the second interferometer arm are not present in the interferometer. In this case, chromatic correction can be omitted, and the interferometer may not include a chromatic correction unit.

[0082] In some examples, the interferometer includes a color correction unit. The correction unit can be adapted to keep the optical path length difference between the first interferometer arm and the second interferometer arm constant across a color range. For example, the color correction unit can be part of the correction unit or can be separate from the correction unit. The interferometer can be color corrected to measure light having a spatial coherence length that is a multiple (1, 2, 5, 10, 25, 50, 100, 500, or 1000) of the wavelength of the light.

[0083] The inventors of this application have found that combining a shift unit and a correction unit simplifies the design of a telecentric interferometer compared to other optical systems. The spectral width of the required color correction is determined according to the requirements of the applicable wavelength band. A dielectric plate is suitable for a simple configuration (LED light or superluminescent LED light as an illumination device for the interferometer or photoluminescence measurement), while for a wider range, a design with a curved mirror and an achromatic color per se is possible. In this case, the color correction unit can be omitted. The most advanced requirements can be achieved with color spot correction (color balance shift unit). All approaches result in an optical system with image-side telecentricity for both interferometer arms and equal optical paths for the central ray.

[0084] The measured interference pattern of the interferometer may include interference patterns, interference rings, and / or interference fringes. These structures have different phase positions at different wavelengths, but the central ray (parallel to the symmetry axis or z-axis) has the same phase position (zero interference) according to the described and implemented optical path length chromatic correction (correction unit). Different color phase positions other than the principal ray are referred to as "spot chromaticity." Based on the constant phase position of the central ray in the image, one can count "2π" phase paths of interference, and this number determines the preferred temporal coherence length of the light. For example, if the maximum phase difference in a phase-interleaved phase image is 4π, then the measurement light may require temporal coherence of at least two wavelengths. The arrangement of the principal planes (and therefore conjugate planes) may result in a phase shift of rays other than the central ray. The position of the interference fringes may be wave-dependent away from zero-order interference (referred to as "spot chromaticity" in this disclosure). It is possible to shift the principal planes (and therefore the conjugate planes) depending on the wavelength (for example, by using a weakly chromatic lens) so that the central ray has a constant zero interference, but the interference fringes shift with wavelength by the variable principal planes in the direction opposite to the spot chromaticity so that the entire interference pattern remains stationary. This allows not only the chromaticity in the optical path length but also the spot chromaticity to be corrected, so that, for example, white light with a coherence length of one wavelength can be measured without a filter. A shift unit with this property is called a color balance shift unit. It may be desirable to correct for weakly chromatic lenses with a color correction unit.

[0085] In some examples, the correction unit comprises at least two dielectric elements having different refractive indices. Separately or in combination, the shift unit comprises a dielectric plate that is also part of the correction unit.

[0086] In some examples, the shift unit may include at least one optical component having optical power (e.g., refractive power). The optical component having refractive power may be a curved mirror and / or a lens (e.g., a weak lens and / or a strong lens). In the following, the term "lens" refers to both a refractive lens and a curved mirror. The Gaussian sweep of all optical elements in the first interferometer arm and / or the Gaussian sweep of all optical elements in the second interferometer arm, such as the correction unit (typically having zero refractive power) and the lens (having refractive power) in the first and second interferometer arms, may have a finite focal length (i.e., a refractive power different from zero) in this case.

[0087] In an alternative example of an interferometer, the first interferometer arm and / or the second interferometer arm may not include a beam expander, such as a ferroelectric lens, a ferroconcave mirror, a ferroconvex mirror, or a combination of these optical components. The beam expander is an afocal system (Keplerian or Galilean telescope). The strong optical component is an optical component with an F-number (absolute value) of less than 50, or less than 30, or less than 20, or less than 10, or less than 5, or less than 2. For example, some of the shift units in the first interferometer arm and / or the second interferometer arm may not include such a beam expander. In particular, the shift unit may differ from such one or more beam expanders. For example, if the shift unit includes a weak lens and a strong lens, the weak lens may be positioned in one of the interferometer arms and may differ from a true afocal system (here, excluding afocal systems such as a mirror or a dielectric plate), and the ferroelectric lens may be positioned outside the interferometer arms. An afocal system in this context is a telescope such as a Keplerian or Galilean telescope. The weak lens can be in the interference section of the optical system (i.e., the first and / or second interferometer arms), while the strong lens can be outside the interference section. For example, unless precautions are taken, a ferroelectric lens used as a beam expander (afocal system) can introduce associated monochromatic and chromatic aberrations that may require correction (if the lens is in the interferometer section). Furthermore, chromatic correction (related to optical path length) may need to be performed. Such corrections are typically complex and expensive due to the large number of optical components involved. For these and other reasons, configurations with a beam expander in the interference section have several disadvantages compared to the compact configurations according to aspects of the present invention. For the same reason, the interferometer arms generally may not include a strong optical lens or a compound lens system. However, the interferometer may include such lenses outside the interferometer arms. In this context, a lens is considered strong if its focal length is less than 8, 6, 4, 2, 1.5, 1.0, or 0.5 or 0.2 times the largest linear dimension of the optical system.The dimension of an optical system for this purpose is the total geometric length of the beam path in the interferometer. In this context, the first interferometer arm and the second interferometer arm begin in the beam path at a first point where the beams in the first interferometer arm and the second interferometer arm experience different positions or different optical transformations (e.g., by lenses). The interferometer arms end at a position where the beam paths of the two beams in the first interferometer arm and the second interferometer arm overlap due to interference.

[0088] In some examples, the telecentric interferometer includes a phase shift unit disposed in or downstream of at least one of the first interferometer arm or the second interferometer arm. For example, the phase shift unit may be disposed in the first interferometer arm and / or the second interferometer arm. Separately or in combination, the phase shift unit may be disposed between a beam combiner that combines the two interferometer arms and a detector. The phase shift unit may include at least one of a piezoelectric crystal-based movable mirror (e.g., in the first interferometer arm and / or the second interferometer arm), a circular polarizer, a linear polarizer, a quarter-wave plate (e.g., an achromatic quarter-wave plate), or a polarization-sensitive detector. Using the phase shift unit, images with different relative phase shifts can be obtained, from which interference terms at both phase positions (e.g., real and virtual positions) can be determined. Therefore, the phase shift unit can adjust the zero optical path difference between the two interferometer arms by an angle value smaller than 180° (or smaller than 360°) or smaller than half a wavelength (or a whole wavelength). The goal can be to determine a complex interference term (also called a "complex interferogram") from the different images. The complex interference term IF(x,y) can be given by: IF(x,y) = Conjugate (E2(x,y)) * E1(x,y) (from the other image). IF(x,y) is a complex quantity determined by evaluation from the intensity image. E1(x,y) and E2(x,y) denote the electric fields arising from the first interferometer arm and the second interferometer arm, respectively, in complex notation for the point (x,y) on the detector. IF(x,y) is the interference quantity determined via the evaluation unit. IF(x,y) contains the phase information of the optical field measured by self-interference. Therefore, a device according to the present disclosure may comprise a phase shifting unit and an evaluation unit for determining the complex interferogram IF.

[0089] According to some embodiments, the phase shift (by the phase shift unit) may be different from the principal plane shift (by the shift unit). The phase shift may be performed to determine a complex interference term from the detected intensity. The principal plane shift may be performed to obtain sufficiently different electric fields E1 and E2 in different interferometer arms.

[0090] A further aspect of the present disclosure is directed to a method for determining a characteristic of an incident light field using a telecentric interferometer according to examples described herein. The characteristic of the incident light field may be or may include at least one of the phase of the incident light field or the amplitude of the incident light field. The incident light field may originate from an object. For example, the input light field may be a coherent field, a partially incoherent field, or a fully incoherent field. The incident light field may include several light rays. The incident light field may be monochromatic or polychromatic. A central ray of the incident light field may define an optical axis of the interferometer.

[0091] According to some examples, the method includes propagating an incident light field through an exit pupil of a front optical element and splitting the incident light field into a first portion (e.g., a first light field that may be denoted as E1) and a second portion (e.g., a second light field that may be denoted as E2), where the first portion propagates along the first optical system (e.g., along a first interferometer arm) and the second portion propagates along the second optical system (e.g., along a second interferometer arm). A central ray of the first (second) portion may define an optical axis of the first (second) interferometer arm.

[0092] The method may further include shifting the image principal plane of the first optical system relative to the image principal plane of the second optical system using a shift unit such that the image principal plane of the first optical system is different from the image principal plane of the second optical system and the optical path length of the first interferometer arm is equal to the optical path length of the second interferometer arm. The optical path length is measured between the object and the detector. The first portion measured on the detector is thereby defocused with respect to the second portion measured on the detector.

[0093] In some examples, the first and second portions are combined into an exit light field. An interference pattern (e.g., a complex interference pattern) of the exit light field can be measured with a detector. The interference pattern can result from interference between a first portion of the incident light field and a second portion of the incident light field that is shifted relative to the first portion. Thus, the incident light field can interfere with its shifted (e.g., defocused) self. Thus, the telecentric interferometer can be a reference-beam-free interferometer.

[0094] Due to the shift of the principal planes, the first and second light fields may have different propagation lengths when they interfere on the detector, but the optical path length of the central ray of the two light fields is the same. Therefore, the central ray may exhibit an optical path difference of zero. Since the image is typically not aberration-free (or out of focus), the other rays of the light fields have optical path differences that express themselves when the interference changes. Therefore, on the detector, an interferogram is measured in intensity. From the measurement of the different interferograms at different phase positions (e.g., via a phase shift unit), a complex interferogram IF can be determined.

[0095] The first and / or second light field can be constructed from the complex interferogram. This can correspond to the physical and / or mathematical propagation (so-called "traceback") of the first and / or second light field back to the exit pupil, from where it follows the optical path of the other interferometer arm (e.g., the second interferometer arm in the case of tracing back the first light field, or the first interferometer arm in the case of tracing back the second light field) to the detector. At the detector, this field corresponds to the field from the other interferometer arm. Thus, the first and second light fields can be related. Since the optical imaging ratio of the first and second light fields can be the same up to their respective principal planes, the light fields can be identical (albeit at different positions) on this plane. Therefore, to bring the fields to the same position, for example, the first light field can be moved from the second light field to the principal plane of the second interferometer arm, and vice versa for the first light field. This can be done so that the central rays have the same optical path length at the superposition position. Similarly, this procedure can be performed for conjugate planes, where the principal planes are replaced by conjugate planes. In this case, the two light fields differ only in the portion of propagation from the principal plane of the first interferometer arm to the principal plane of the second interferometer arm. Therefore, it may not be necessary to retrace the light path back to the exit pupil and then again to the principal plane or conjugate plane. This greatly simplifies the analysis and evaluation.

[0096] However, this can only be true for the entire field of view if the optical system is telecentric on the image side. This is because only then is the overlap exact and the propagation direction is at a 90° angle to the detection plane. The relationship between the first and second light fields can therefore be expressed by the light propagation over the distance of the principal plane division. Mathematically, this is a unitary transformation (also called a mapping). The unitary transformation can be represented by a propagation matrix U ("mapping U"). The propagation can be mathematically expressed, for example, as the Fresnel integral or the propagator of the Helmholtz equation, or the Fresnel diffraction integral, the Huygens-Fresnel principle, or the Debye integral (see, for example, "Born Principles of Optics", M. Born and E. Wolf, Cambridge University Press, 8 th printing 2013, Chapter 8.2).

[0097] Therefore, the interferometer may be suitable for measuring the properties of the light field (complex interferogram IF), the interpretation of which may be based on knowledge of the principal plane division (and / or knowledge of the mapping [unitary transformation] of the first light field to the second light field).

[0098] In at least some examples, the interferometer, e.g., the length of the interferometer arm, may be calibrated before, during, and / or after the object is measured with the interferometer. Calibration may include selecting a detector reference portion (e.g., a section of the detector) that detects a known reference interferogram. Calibration may include measuring an interferogram at the detector reference portion. In some examples, the reference interferogram may additionally or alternatively be measured with the object. The known reference interferogram may be compared to the measured interferogram. The measured interferogram may be calculated by subtracting a global phase (i.e., e) from the reference interferogram. iφ) The measured interferogram can be calibrated to a fixed phase of a known reference interferogram. The calibrated interferogram can be invariant in global phase even when changes in the optical path length of the interferometer are small.

[0099] The calibration may be performed using a calibration light source, for example a laser source, which may be blocked during measurement of the image of the object, however in such a case an additional detector may be required.

[0100] To evaluate the complex interferogram IF, it may be advantageous to distinguish whether the measurements were made with spatially coherent or incoherent light.

[0101] For spatially coherent light, the procedure is detailed in US Patent Application No. 10,823,547 (B2). The mapping U introduced in this disclosure, which maps a first light field to a second light field, corresponds to the propagator mapping U in US Patent No. 10,823,357 (B2). To apply this method, the intensities of the first and second light fields (i.e., |E1| 2 and |E2| 2 ) may need to be known. Measuring this quantity can be done, for example, by blocking one of the interferometer arms and measuring only the intensity arising from the other interferometer arm. Alternatively or additionally, a separate second camera for intensity imaging can be integrated into the setup, in addition to the interferogram, for example via a beam splitter. As a result of the calculation, the individual complex fields E1(x,y) and / or E2(x,y) are determined from the complex interferogram IF(x,y). This can be the quantity determined in quantitative phase contrast microscopy. Therefore, this device can be used as a quantitative phase microscope.

[0102] A different situation is when the complex interferogram IF is recorded with incoherent light. This procedure is shown in International Patent Application Publication US 2022 / 034645 (A1). This publication refers to the situation where, for a given object position, self-referencing a light beam (e.g., a spot) with itself results in a complex point spread function. The point spread function includes part of the diffraction of light at the exit pupil of the front optical element (see, for example, "Born Principles of Optics", M. Born and E. Wolf, Cambridge University Press, 8 th (See U.S. Patent Publication 2013, Chapter 8.8). Due to image-side telecentricity and perfect overlap, the shape of the point spread function does not change with the lateral displacement of the object point in the field of view. Image information can be reconstructed by refolding the point spread function, as described, for example, in U.S. Patent Application Publication No. 2022 / 034645 A1. Refolding is the mathematical inverse operation of folding. The point spread function is different for different z positions of the object point. Therefore, different z positions can be distinguished in the image. This method therefore enables 3D reconstruction of the object. The folding and refolding process can be called propagation. The corresponding function is also called the propagation operator.

[0103] According to at least some aspects of the present disclosure, the measured interference spot can be brought into focus or defocused via a mathematical propagation operator. Thus, an interferogram for the new focus position can be electronically calculated. Given such an arbitrary focus position, the set of Gaussian interference spots IF(x,y) can be, for example, point-wise squared. The new interferogram IF'(x,y) = IF(x,y) 2A new image is formed corresponding to the image. In this image, each spot can be resharpened by squaring, i.e., as it propagates and focuses, it becomes a sharper spot. Furthermore, at half the lateral distance, ghost spots can be generated between every pair of different spots. These ghost spots resist resharpening, resulting in a sharper image but containing artifacts. The squaring procedure is usually performed at an arbitrarily selected electronic distance position. Comparing the results at different orthogonal distances, it is found that the phase of each individual squared single spot (referred to as a single spot) differs from the phase of the superposition of different spots, i.e., the intermediate ghost spot. The different behavior of the single spot and the ghost spot makes it possible to determine the image parts resulting from artifacts over a number of different frames. The resharpened image can thus be freed from artifacts.

[0104] According to at least some aspects, a method for determining characteristics of an incident light field may include generating at least two intermediate interference patterns from a measured interference pattern, e.g., at mutually different focal points and / or different electronic focal positions. For example, the intermediate interference patterns may be generated electronically, e.g., by convolving the measured interference pattern with a propagation kernel for a selected propagation distance. The at least two intermediate interference patterns may then be processed using algebraic techniques. For example, the algebraic techniques may include at least one of subtracting the processed images from each other (e.g., using a linear pixel-wise complex weighting function), pixel-wise squaring, and / or multiplying with a chirp function. Before or after processing, a filter function may be applied to the at least two intermediate interference patterns, thereby creating a resultant image. The filter function may be, for example, a linear combination of the intermediate images.

[0105] These method steps (i.e., generating an intermediate interference pattern, processing the intermediate interference pattern, and applying a filter function) can be repeated iteratively, for example, at least twice. The resulting image created in the previous iteration cycle can be used as the measured interference pattern in the next iteration cycle. In each cycle, the algebraic techniques of the previous cycle can be used, or new pixel-by-pixel manipulation rules can be applied. This results in a sharper image, i.e., fine features can be better detected. The resulting image may be less noisy, since it is averaged over several images. The resulting image may have (electronically) modified values ​​of the z-component.

[0106] Compared to the original data, the resulting image can be focused on a different plane than the detector plane, which can correspond to electronic focusing and refocusing, which can compensate for aberrations of the optical system used (e.g., the object, the front optical element, the entrance optical element, and / or the first optical system and / or the second optical system) and can result in a sharper reproduction of at least part of the image.

[0107] According to at least some embodiments, the method may include reshaping the measured interference pattern by electronically deconvolving the measured interference pattern with a reshaping function. The reshaping may be performed before generating the at least two intermediate interference patterns. The reshaping function may be selected to avoid steps at the intermediate interference patterns. For example, to facilitate subsequent processing, it is possible to remove the diffraction effects of the exit pupil of the front optical element in a separate step. For this purpose, a diffraction pattern is determined for a specific focus position (or z-position). The result may be referred to as a reshaping function. The correction may be performed, for example, by refolding, which corresponds to the inverse of convolution. This method makes it possible to remove the effects of diffraction for a set of object points in the interferogram. Such a corrected interferogram consists of a superposition of Gaussian-like complex interference spots.

[0108] This method can benefit from the incoherence of resolvable object points relative to one another. Therefore, the measured interference pattern (interferogram) can be a superposition of complex point spread functions. This method can therefore measure the interference of isolated object points.

[0109] Incoherent illumination may be referred to as "ideally incoherent" when different points in the object field, which can be resolved by a front optic, receive mutually incoherent illumination light. According to some aspects of the present disclosure, some or all of the rays of a light field originating from an object may be coherent with one another. In this context, the term "coherent light field" may refer to a situation in which light rays interfere, provided that their optical path difference is within the coherence length. The term incoherent light is used to refer to a light field in which light rays are coherent only if they originate from the same point source. Furthermore, in the coherent case, the structure of the light field is assumed to be characterized by point-like disturbances, which result in a radially scattered light field. When the term "imaging" is used for this situation, it means that the point-like disturbances are imaged. It may not be possible to image a background field, which may be present in some cases, such as a field from bright-field illumination in a microscope, using an interferometer according to aspects of the present disclosure. Point-like disturbances can result in both light being emitted in other directions or light being lost (like a shadow). Perturbations in the light field from coherent illumination can be elastic scatterers; that is, if the optical path difference is smaller than the coherence length, the exiting light fields have a complex phase relationship, and light fields from different scatterers can interfere. The interference relationship is typically more complex for coherent light than for incoherent light. An object can emit a coherent scattered light field only if it is coherently illuminated. Coherence may require that all disturbances that become interfering in the interferometer be within the coherence volume of the illumination. In other words, within spatial and temporal coherence, disturbances are usually illuminated coherently. Illumination can be performed so that the illumination light enters the interferometer and reaches the detector (bright-field illumination) or so that the illumination light does not enter the interferometer (dark-field illumination).

[0110] In at least some examples, the object is appropriately illuminated. In particular, the illumination may allow coherent and / or incoherent light rays to be emitted by the object. Due to the uniformity of the light path, the coherence requirement is also low in the coherent case. In this case, illuminators usable for the interferometer are lasers, superluminescent LEDs, or LEDs. In the incoherent case, incoherent illumination, such as critical illumination or Köhler illumination, is required. The illumination may be, for example, transmitted, reflected, or EPI illumination. It is also possible to measure photoluminescence, and the illumination is spectrally blocked. Incoherent illumination is referred to as "ideally incoherent" when different points in the object field, which can be resolved by a front optic, receive mutually incoherent illumination light. This situation can also be described by the expression "incoherent within resolution."

[0111] A further aspect of the present disclosure relates to an interferometer assembly. The interferometer assembly may include a telecentric interferometer (e.g., an image-side telecentric interferometer) according to examples described herein. The interferometer assembly may further include an illumination device. The illumination device may be adapted to illuminate the object with illumination light such that different object points of the object coherently reflect or transmit portions of the illumination light. Alternatively, the illumination device may be adapted to illuminate the object with illumination light such that different object points of the object incoherently scatter or transmit the illumination light, particularly within the resolution. This means that points located at a resolution distance of the front optics are incoherently illuminated and therefore scatter incoherent light from each other.

[0112] The illumination can be such that unscattered or specularly reflected illumination light reaches the detector (so-called "bright field"). Alternatively, unscattered or specularly reflected illumination light may not reach the detector (so-called "dark field"). The illumination can be performed so that different parts of the object are illuminated at different times (so-called "structured illumination") and / or with different wavelength ranges. The latter can be measured with wavelength resolution using different detectors via beam splitters or color filters.

[0113] A telecentric interferometer according to an embodiment of the present disclosure will be described in detail with reference to the schematic diagram of FIG. 1 . The telecentric interferometer includes a first optical system 10 and a second optical system 20. The first optical system 10 includes a first interferometer arm 11 having a shift unit 12 and a correction unit 13. The second optical system 20 includes a second interferometer arm 12 having a shift unit 22 and a correction unit 23. Other variations of the telecentric interferometer may include the first interferometer arm 11 having the shift unit 12 but not the correction unit 13, and / or the second interferometer arm 12 having the correction unit 23 but not the shift unit 22. Combinations of these configurations are possible (e.g., shift units 12, 22 in both interferometer arms 11, 21 and correction units 13, 23 in only one of the interferometer arms 11, 12). A portion of the shift unit 12 of the first interferometer arm 11 may be located outside the first interferometer arm 11 within the first optical system 10. The same may be true for the shift unit 22 of the second interferometer arm 12. The first optical system 10 may further include an input optical element 31, a beam splitter 32, a beam combiner 33, and an output optical element 34. The input optical element 31, the beam splitter 32, the beam combiner 33, and the output optical element 34 may also be part of the second optical system 20.

[0114] The telecentric interferometer includes a detector 35 and an exit pupil 42 of a front optical element (not shown in FIG. 1 ). The exit pupil 42 is located upstream (i.e., on the object side) of the first optical system 10 and the second optical system 20. The detector 35 is located downstream (i.e., on the image side) of the first optical system 10 and the second optical system 20. The detector 35 may include a plurality of pixels. The detector may be or include a CMOS sensor (e.g., a CMOS sensor array).

[0115] The first optical system 10 has an object-side focal length 141, an object-side principal plane 142, an object-side optical axis 143, a Gaussian image distance 151, an image-side principal plane 152, an image-side optical axis 153, and an image-side conjugate plane 154. The second optical system 20 has an object-side focal length 241, an object-side principal plane 242, an object-side optical axis 243, a Gaussian image distance 251, an image-side principal plane 252, an image-side optical axis 253, and an image-side conjugate plane 254. In both the first optical system 10 and the second optical system 20, the respective optical characteristics (e.g., the object-side focal lengths 141, 142, the object-side principal planes 142, 242, the object-side optical axes 143, 243, etc.) reflect the optical characteristics of the entire optical system.

[0116] First, the object sides of the first optical system 10 and the second optical system 20 will be described. The object-side principal plane 142 of the first optical system 10 is identical to (i.e., has the same position as) the object-side principal plane 242 of the second optical system 20. The object-side focal length 141 of the first optical system 10 is identical to the object-side focal length 241 of the second optical system 20. In this example, the input optical elements 31 of the first optical system 10 and the second optical system 20 are located at the object-side principal plane 142 of the first optical system 10 and the object-side principal plane 242 of the second optical system 20. Other designs are also possible according to the present disclosure.

[0117] The exit pupil 42 is positioned on the object side of the first optical system 10 and the second optical system 20. In other words, an object to be imaged (not shown in FIG. 1 ) may be positioned upstream of the exit pupil 42, and the exit pupil 42 may be positioned between one of the first optical system 10 and the second optical system 20 and the object on the other side. The distance of the exit pupil 42 to the object-side principal planes 142, 242 of the first optical system 10 and the second optical system 20 corresponds to the object-side focal lengths 141, 142 of the first optical system 10 and the second optical system 20. By positioning the exit pupil 42 in this manner, a telecentric interferometer (particularly, an image-side telecentric interferometer) can be realized. As a result, the image of the exit pupil 42 is shifted to infinity in image space, corresponding to a telecentric configuration.

[0118] Reference is now made to the first optical system 10 and the second optical system 20. The incident optical element 31 may be or may comprise at least one of a lens, a (curved) mirror, a photo lens, a tube lens, a microscope objective, a telescope, or another beam shaping element. The incident optical element 31 is located upstream of a beam splitter 32. In some examples, the beam splitter 32 may be part of the incident optical element 31. The beam splitter 32 may split the incident light field into a first light field and a second light field (not shown in FIG. 1 ). The first light field propagates through the first interferometer arm 11, and the second light field propagates through the second interferometer arm 21.

[0119] The first interferometer arm 11 and the second interferometer arm 12 have the same optical path length but different geometric optical path lengths. This is achieved by the shift unit 12 of the first optical system 10 and / or the shift unit 22 of the second optical system 20. For simplicity, the principle of the shift unit will be described for the shift unit 12 of the first optical system 10. The following description applies mutatis mutandis to the shift unit 22 of the second optical system 20. The shift unit 12 of the first optical system 10 can change (e.g., shorten or lengthen) the geometric optical path length of the first interferometer arm 11 compared to the first optical system 10 without the shift unit 12. This shifts the image-side principal plane 152 of the first optical system 10. The geometric optical path length of the first interferometer arm 11 is further adjusted to compensate for any shift or difference introduced in the optical path length of the first interferometer arm 11 compared to the second interferometer arm 21. For example, if the optical path length of the first interferometer arm 11 is extended by a length d by the shifting unit 12 of the first optical system 10, the geometrical optical path length is reduced by the length d compared to the geometrical optical path length of the first interferometer arm 11 without shifting the shifting unit 12 (and / or d is adjusted for any refractive index n in the optical path). As an example, the geometrical optical path or the optical path can be adjusted by moving at least one mirror of one of the first interferometer arm 11 and the second interferometer arm 21 of the dual path interferometer. It may be preferable that the two interferometer arms 11, 21 have separate optical paths and mirrors, i.e., they cannot have a common optical path. The shift unit 12 of the first optical system 10 and the shift unit 22 of the second optical system 20 are adapted to shift the image-side principal plane of the first optical system 10 relative to the image-side principal plane 252 of the second interferometer arm 21 so that the image-side principal plane 152 of the first optical system 10 is different from the image-side principal plane 252 of the second optical system 20 and the optical path length of the first interferometer arm 11 is equal to the optical path length of the second interferometer arm 21.

[0120] The first optical system 10 and / or the second optical system 20 may further comprise respective correction units 13, 23. The correction units 13, 23 are adapted to correct chromatic changes in the optical path length of the respective optical systems 10, 20 caused by the shift units of the respective optical systems 10, 20. The correction units 13, 23 may be, for example, dielectric plates. The focal lengths of the correction units 13, 23 may approach infinity. Unlike the depiction in FIG. 1 , the correction unit 13 of the first optical system 10 (e.g., at least a part of the correction unit 13) may be part of the shift unit 12 of the first optical system 10. The same also applies, mutatis mutandis, to the correction unit 23 and the shift unit 22 of the second optical system 20.

[0121] After passing through the first interferometer arm 11 and the second interferometer arm 21, respectively, the first and second optical fields are combined in a beam combiner 33, and the combined optical field propagates through output optics 34. For example, the output optics may comprise lenses to compensate for different focal lengths of the optical components in the first interferometer arm 11 and the second interferometer arm 21, and then form image planes of the first interferometer arm 11 and the second interferometer arm 21, respectively, near a detector 25.

[0122] Next, let us refer to the image sides of the first optical system 10 and the second optical system 20. The image-side principal plane 152 of the first optical system 10 has a different position from the image-side principal plane 252 of the second optical system 20. This is due to the shift units 12 and 22. The detector 35 is located between the image-side conjugate plane 154 of the first optical system 10 and the image-side conjugate plane 254 of the second optical system 20. The image-side conjugate planes 154 and 254 are separated from the image-side principal planes 152 and 254 of the respective optical systems 10 and 20 by the Gaussian image distances 151 and 251 of the respective optical systems 10 and 20. The detector 35 is out of focus with respect to both the image resulting from the first optical system 10 and the image resulting from the second optical system 20, but the deviation from focus is small enough to allow for the analysis of the interference pattern between the first and second light fields.

[0123] FIG. 2 illustrates the principle of a telecentric optical system for a simple optical system with only a lens 311, which represents the first optical system 10 and the second optical system 20 in this example. For simplicity, the lens 311 is depicted as a thin lens, i.e., the object-side principal plane coincides with the image-side principal plane in the drawing. The object-side focal length of the lens 311 is identical to the image-side focal length of the lens 311, i.e., the same refractive index (focal length 311f of the lens 311). The object-side optical axis is also identical to the image-side optical axis (optical axis 311a). The same optical medium is assumed in the object space and the image space.

[0124] To illustrate the effect of a telecentric system, a small arrow is shown on the left side of FIG. 2 as an exemplary object. A light field 61 having a first ray 61a, a second ray 62b, and a chief ray 61c originates from an object point. A central ray 61d extends along the optical axis 311a (for clarity, the central ray 61d is depicted slightly off-axis). In the simplified diagram of FIG. 2, the image-side optical axis coincides with the object-side optical axis. The light field 61 passes through the exit pupil 42 of the front optical element 41. For simplicity, we assume that the two principal planes of the optical system coincide. The first ray 61a and the second ray 62b may correspond to the outer rays (so-called marginal rays) of the light field 61 that can pass through the exit pupil 42. The chief ray 61c intersects the optical axis 311a at the exit pupil 42. For illustrative purposes only, an arrowhead is used as the object point from which the chief ray 61c originates. For simplicity, the front optical element 41 is positioned so that the distance from the first principal plane of the front optical element 41 to the object (arrow) is the focal length 411f of the front optical element 41. The lens 311 is spaced from the exit pupil 42 by the focal length 311f of the lens 311. The lens 311 can be a Gaussian sweep of a much more complex optical system. To simplify the drawing, a vanishing principal plane division is assumed.

[0125] The light field 61 passes through the lens 311. On the image side of the lens 311, the image position 313 is a focal length 311f away from the lens 311. The chief ray 61c intersects the optical axis 311a at the position of the exit pupil 42 and is parallel to the optical axis 311a on the image side.

[0126] This corresponds to an image-side telecentric system. Telecentricity in object space or image space requires that the chief ray 61c is parallel to an axis in object space or image space, respectively. As a result, the apparent optical system magnification remains constant even if the object or image plane is displaced from its nominal position. The image will be blurred, but the size or magnification will be correct.

[0127] The principles of the shift units 21, 22 of the telecentric interferometer according to an embodiment of the present disclosure will be described in detail with reference to the schematic diagrams of FIGS. 3A and 3B. FIG. 3A schematically shows a simplified scheme of the shift units 12, 22. The shift units 12, 22 include weak lenses 51, 52 and a strong lens 53. The strong lens 53 is present in both the first optical system 10 and the second optical system 20, but is located outside the first interferometer arm 11 and the second interferometer arm 21 (not shown in FIG. 3A). In some examples, the shift unit 12 of the first optical system 10 may include a first weak lens 51 (e.g., positioned in the first interferometer arm 11), and the shift unit 22 of the second optical system 20 may include a second weak lens 52 (e.g., positioned in the second interferometer arm 21). 3A shows the first weak lens 51 and the second weak lens 52 as one component for simplicity of illustration, however the first weak lens 51 and the second weak lens 52 are different lenses and are positioned in different interferometer arms 11, 12.

[0128] The principal plane 511 of the first weak lens 51 and the principal plane 521 of the second weak lens 52 may coincide. The principal plane 531 of the strong lens 53 is separated from the principal planes 511, 521 of the first weak lens 51 and the second weak lens 52 by the object-side focal length 53f of the strong lens 53. The principal planes 511, 521, 531 of the first weak lens 51, the second weak lens 52, and the strong lens 53, respectively, are considered in the thin lens approximation (without principal plane division).

[0129] The focal lengths of the first weak lens 51 and the second weak lens 52 may be different. For example, the focal length (e.g., object-side focal length) of the first weak lens 51 may have a different sign from the focal length (e.g., object-side focal length) of the second weak lens 52. The focal length of the first weak lens 51 may be the same magnitude as the focal length of the second weak lens 52. For example, the first weak lens 51 may be a diverging lens and the second weak lens 52 may be a converging lens, or vice versa. Both the focal length of the first weak lens 51 and the focal length of the second weak lens 52 may be greater than the object-side focal length 53f of the strong lens 53.

[0130] The Gaussian-swept combination of the first weak lens 51 and the strong lens 53 may have a first image-side principal plane 513, and the Gaussian-swept combination of the second weak lens 52 and the strong lens 53 may have a second image-side principal plane 523. The positions of the first image-side principal plane 513 and the second image-side principal plane 523 are different. In other words, the first image-side principal plane 513 and the second image-side principal plane 523 are offset from each other. Both the first image-side principal plane 513 and the second image-side principal plane 523 may be different from the principal plane 531 of the strong lens 53. The difference between the first image-side principal plane 513 and the second image-side principal plane 523 corresponds to the shift magnitude of the shifting units 12, 22. When both the first interferometer arm 11 and the second interferometer arm 21 are equipped with weak lenses (i.e., a first weak lens 51 and a second weak lens 52, respectively), the shift units 12, 22 are located in both the first optical system 10 and the second optical system 20.

[0131] FIG. 3B shows an example of shifting units 12, 22 combined with the first interferometer arm 11 and the second interferometer arm 21 of the interferometer. A light field 61 originating from an object passes through a front optical element having an exit pupil (not shown in FIG. 3B). The light field 61 then passes through the entrance optical element 31 of the interferometer. Imaging through the entrance optical element 31 generates a real or virtual image 314 (not shown in FIG. 3B). For example, the virtual image 314 may also be at infinity. The shifting units 12, 22 may be independent of the position of the intermediate image 314. The light field 61 is then split into a first light field 611 and a second light field 612 by a beam splitter 32, which also functions as a beam combiner 33. The first light field 611 propagates through the first interferometer arm 11, and the second light field 612 propagates through the second interferometer arm 21. The first interferometer arm 11 includes a first weak lens 51, and the second interferometer arm includes a second weak lens 52. The first weak lens 51 is a concave mirror lens that converges the first light field 611, and the second weak lens 52 is a convex mirror lens that diverges the second light field 612. After passing through the first interferometer arm 11 and the second interferometer arm 21, respectively, the first light field 611 and the second light field 612 are combined by beam splitters / combiners 32, 33, and the combined light field propagates through a strong lens 53 having a principal plane 531. The strong lens 53 is separated from the first weak lens 51 and the second weak lens 52 by an object-side focal length 53f of the strong lens 53.

[0132] The first weak lens 51 and the second weak lens 52 are both positioned at the image of the exit pupil 421 by the entrance optics 31. The first light field 611 is shifted (i.e., defocused) relative to the second light field 612 by the combination of the first weak lens 51, the second weak lens 52, and the strong lens 53, which together function as a shift unit. Thus, the first image-side principal plane 513 of the Gaussian-swept combination of the first weak lens 51 and the strong lens 53 is different from the second image-side principal plane 523 of the Gaussian-swept combination of the second weak lens 52 and the strong lens 53. The light beam is then imaged onto the detector 35 by an optical element (not shown). Due to the shift unit, the image-side conjugate plane 154 of the first optical system 10 comprising the first interferometer arm 11 is different from the image-side conjugate plane 254 of the second optical system 20 comprising the second interferometer arm 21. The detector 35 may be located between the conjugate planes 154 and 254. The image-side conjugate plane 154 of the first optical system 10 is separated from the first image-side principal plane 513 by a Gaussian image distance 541 of the first optical system 10. The image-side conjugate plane 254 of the second optical system 20 is separated from the second image-side principal plane 523 by a Gaussian image distance 542 of the second optical system 10. In the example shown in FIG. 3B , the Gaussian image distance 541 of the first optical system 10 and the Gaussian image distance 542 of the second optical system 20 are the same. The Gaussian image distances 541, 542 of the first optical system 10 or the second optical system 20 depend on the distance between the intermediate image 314 of the object (not shown in FIG. 3B ) and the object-side principal plane of the strong lens 53. However, the Gaussian image distance may be independent of the strength of the weak lenses 51, 52. This is a result of the specific design and means that the strong lens 53 compensates for the different focal lengths of the weak lenses 51, 52. This also means that the layout and refractive power of the strong lens 53 as described does not depend on the local position of the intermediate image 314, but does depend on the local positions of the weak lenses 51, 52.

[0133] FIG. 4 schematically illustrates an interferometer according to an embodiment of the present disclosure. The shift unit of the interferometer is implemented as in the example shown in FIG. 3B . That is, the shift unit includes a first weak lens 51, a second weak lens 52, and a strong lens 53. The interferometer shown in FIG. 4 includes a front optical element 41 that includes an exit pupil 42. In some examples, the front optical element 41 may be an external optical element, and the rest of the optical elements shown in FIG. 4 are part of the apparatus.

[0134] Downstream of the front optical element 41 is the entrance optical element 31. The entrance optical element 31 is followed by beam splitters / combiners 32, 33 (see also FIG. 3B above). The beam splitters / combiners 32, 33 define a first interferometer arm 11 and a second interferometer arm 21. The first interferometer arm 11 comprises a first weak lens 51 in the form of a concave mirror. The second interferometer arm 21 comprises a second weak lens 52 in the form of a convex mirror. Downstream of the beam splitters / combiners 32, 33 is a strong lens 53. The strong lens 53 is followed by several further optical elements (mirrors are shown in FIG. 4 by way of example only) and several optional further exit optics (not shown in FIG. 4). The strong lens 53 can be considered part of the exit optics of the two interferometer arms 11, 21. A detector 35 is located downstream of the strong lens 53. Detector 35 may be located between conjugate planes 154 and 254 .

[0135] The input optics 31, the first interferometer arm 11 (i.e., its optical elements), and the strong lens 53 are part of the first optical system 10. The input optics 31 and the strong lens 53, together with the second interferometer arm 21 (i.e., its optical elements), are part of the second optical system 20.

[0136] A light field 61 emanating from the object passes through the front optical element 41 and the exit pupil 42. The light field 61 is then imaged by the entrance optical element 31 into an intermediate image 314 (virtual image). The light field 61 passes through the beam splitters / combiners 32, 33 and is split into a first light field and a second light field. These pass through the first interferometer arm 11 and the second interferometer arm 21, respectively, and are then recombined into a common light field. The light field propagates through the strong lens 53 to the detector 35. By combining the first weak lens 51, the second weak lens 52, and the strong lens 53, the image-side conjugate plane 154 of the first optical system 10 is shifted relative to the image-side conjugate plane 254 of the second optical system 20. The detector can be positioned in the range between them.

[0137] 5A shows an interferometer according to an embodiment of the present disclosure. The interferometer includes a front optical element 41 with an exit pupil 42, an entrance optical element 31 with a lens 311, a beam splitter 32, weak lenses 51, 52 (e.g., first weak lens 51 or second weak lens 52) with principal planes 511, 521, a beam combiner 33, a strong lens 53 with a principal plane 531, and a detector 35. Reference numerals 143 and 243 denote optical axes.

[0138] The focal length 311f of the lens 311 of the input optical element 31 can be, for example, at least 150 mm and at most 250 mm, e.g., 200 mm. The lens 311 can be a converging lens or a diverging lens, depending on the (optional) other parts of the input optical element 31 (not shown in FIG. 5A ). For example, the lens 311 is a tube lens (e.g., an achromatic tube lens) or a relay lens. The input optical element 31 maps the exit pupil 42 of the front optical element 41 to the principal plane 511 of the first weak lens 51 (in the case of the first interferometer arm 11) and / or the principal plane 521 of the second weak lens 52 (in the case of the second interferometer arm 21). Thus, the principal plane 511 of the first weak lens 51 and the principal plane 521 of the second weak lens 52 lie within the image plane 421 of the exit pupil 42 via the input optical element 31.

[0139] Beam splitter 32 is positioned on the image side of lens 311 and defines first interferometer arm 11 and second interferometer arm 21. For simplicity, only a single interferometer arm is shown in Figure 5A, representing either first interferometer arm 11 or second interferometer arm 21. The following description applies mutatis mutandis to the other interferometer arm.

[0140] The interferometer arms 11, 21 are equipped with weak lenses 51, 52. If the interferometer arm is the first interferometer arm 11, the first interferometer arm is equipped with a first weak lens 51, and if the interferometer arm is the second interferometer arm 21, the second interferometer arm 21 is equipped with a second weak lens 52. Although the weak lenses 51, 52 are shown as diverging lenses, converging lenses are also possible. For example, the first interferometer arm 11 can be equipped with a diverging first weak lens 51 and the second interferometer arm 21 can be equipped with a converging second weak lens 52, or vice versa. The weak lenses 51, 52 can have a focal length of, for example, at least 50 cm and at most 200 cm, e.g., at least 80 cm and at most 120 cm, e.g., 100 cm (positive sign for converging lenses, negative sign for diverging lenses). The weak lenses 51, 52 can be (intentionally) colored lenses.

[0141] The beam combiner 33 (which may be a different optical component from the beam splitter 32 or the same optical component) combines the two interferometer arms 11, 21. Downstream of the beam combiner 33, a strong lens 53 is located within the interferometer. A principal plane 531 of the strong lens 53 (e.g., the strong lens 53 itself) is separated from the principal planes 511, 521 of the weak lenses 51, 52 by a focal length 53f of the strong lens 53. The focal length 53f of the strong lens 53 may be, for example, at least 5 cm and at most 30 cm, such as at least 10 cm and at most 20 cm, for example 15 cm. The detector 35 is positioned on the image side of the strong lens 53, between an image-side conjugate plane 154 of the first optical system having the first interferometer arm 11 and an image-side conjugate plane 254 of the second optical system having the second interferometer arm 21.

[0142] A light field 61 originating from an object (not shown in FIG. 5A) passes through the exit pupil 42. The light field 61 includes a first ray 61 a, a second ray 61 b, and a chief ray 61 c. The first ray 61 a and the second ray 61 b ​​are equally spaced from the chief ray 61 c. When the object point is moved to the optical axis, the chief ray 61 c becomes a central ray 61 d. The central ray 61 d is drawn slightly off-axis for better visibility. The first ray 61 a and the second ray 61 b ​​are equally spaced from the chief ray 61 c.

[0143] The optical field 61 propagates through the lens 311 and is split into two parts at the beam splitter 32. The chief ray 61c in the interferometer arms 11 and 21 intersects the optical axes 143 and 243 at the weak lenses 51 and 52. After passing through the first and second interferometer arms 11 and 21, which have the first and second weak lenses 51 and 52, respectively, the two parts are combined at the beam combiner 33 and propagate through the strong lens 53f. The combination of the first weak lens 51 and the strong lens 53 and / or the second weak lens 52 and the strong lens 53 results in a shift in the optical path length of the first interferometer arm 11 relative to the optical path length of the second interferometer arm 21. Although the conjugate plane of the first optical system with the first interferometer arm 11 is shifted by the first weak lens 51, the telecentricity of the interferometer is not affected. The same applies mutatis mutandis to the second interferometer arm 21 and the second weak lens 52. After passing through the strong lens 53, the first and second rays 61 a, 61 b ​​are still equally spaced from the chief ray 61 c (indicated by the dash-dotted arrows), which runs parallel to the optical axis 143, 243.

[0144] FIG. 5B illustrates an interferometer according to an embodiment of the present disclosure. The interferometer illustrated in FIG. 5B is configured similarly to the interferometer illustrated in FIG. 5A. Therefore, the following description will focus on the differences. In contrast to the interferometer illustrated in FIG. 5A, the interferometer illustrated in FIG. 5B includes an input optical element including a first lens 311 and a second lens 312. For example, the first lens 311 and the second lens 312 are converging lenses. However, it may also be possible for the first lens 311 and the second lens 312 to be diverging lenses, or for one of the first lens 311 and the second lens 312 to be a converging lens and the other to be a diverging lens. The Gaussian sweep of the focal length 311f of the first lens 311 and the focal length 312f of the second lens 312 illustrated in FIG. 5B may be similar to the focal length of the lens 311 illustrated in FIG. 5A. For example, the Gaussian sweep of the focal length 311f of the first lens 311 and the focal length 312f of the second lens 312 may be at least 10 cm and at most 30 cm. The distance between the first lens 311 and the second lens 312 may be selected to be equal to the sum of the focal length 311f of the first lens 311 and the focal length 312f of the second lens 312 (e.g., within a ±5% tolerance of the distance). The combination of the first lens 311 and the second lens 312 may be, for example, a Keplerian telescope or a Galilean telescope or an afocal system.

[0145] In the interferometer shown in FIG. 5B, a light field having a first light ray 61 a, a second light ray 61 b, and a chief ray 61 c propagates through the exit pupil 42 of the front optical element 41. The light field then propagates through the first lens 311 and the second lens 312. Unlike the interferometer shown in FIG. 5A (where the light field is collimated only after passing through the strong lens 53), the interferometer shown in FIG. 5B is configured such that the light field's rays 61 a, 61 b, and 61 c are collimated by the combination of the first lens 311 and the second lens 312. The collimated rays 61 a, 61 b, and 61 c then propagate through the interferometer arms 11 and 21, which include the beam splitter 32, the first weak lens 51, and / or the second weak lens 52, and the beam combiner 33. 5A, in the interferometer shown in FIG. 5B, the combination of the first weak lens 51 with the strong lens 53 and / or the combination of the second weak lens 52 with the strong lens 53 results in a shift in the geometric optical path length of the first interferometer arm 11 relative to the optical path length of the second interferometer arm 21. After passing through the weak lenses 51, 52 and / or the strong lens 53, the first light ray 61a and the second light ray 61b are still equally spaced from the chief ray 61c. After passing through the strong lens 53, the chief ray 61c travels parallel to the optical axis 143, 243. Finally, the light field is measured with the detector 35.

[0146] Further aspects of the present disclosure will be described in more detail with reference to the schematic diagrams shown in Figures 6A and 6B. Figures 6A and 6B each show a portion of an interferometer according to an example of the present disclosure. The interferometer includes an input optics 31 and a beam splitter 32 that also functions as a beam combiner 33 and defines a first interferometer arm 11 and a second interferometer arm 21. The first interferometer arm 11 includes a first mirror 351, and the second interferometer arm 21 includes a second mirror 352. The interferometer further includes a detector 35. The interferometer may include additional components not shown in Figures 6A and 6B.

[0147] In the example shown in FIG. 6A, only the first interferometer arm 11 includes a dielectric plate (first dielectric plate 361). In the example shown in FIG. 6B, both the first interferometer arm 11 and the second interferometer arm 21 include dielectric plates (first dielectric plate 361 and second dielectric plate 362). Both the first dielectric plate 361 and the second dielectric plate 362 (if present) may include only a single dielectric plate, or may include, for example, two or more dielectric plates stacked together, at least some of which may have different dielectric constants. The first dielectric plate 361 (FIG. 6A), or the first dielectric plate 361 and the second dielectric plate 362, have the function of a shift unit of the interferometer.

[0148] An incident light field 61 is split into a first light field 611 and a second light field 612 by the beam splitter / combiner 32, 33. The first light field 611 travels through the first interferometer arm 11, and the second light field 612 travels through the second interferometer arm 21. In both FIGS. 6A and 6B , the first light field 611 propagates through a first dielectric plate 361. The first dielectric plate 361 causes a change in the optical path length and geometric path length of the first light field 611. For example, if the shift is not compensated for, the first light field 611 will be delayed relative to the second light field 612, or vice versa. In other words, if the shift is not compensated for, the first light field 611 may follow the second light field 612, or the first light field may lead the second light field 612. 6A, the second light field 612 propagates through the second interferometer arm 21 without disturbance from the dielectric medium. To compensate for the shift in the optical path length of the first light field 611 in the first interferometer arm 11, the second interferometer arm 21 may have a longer geometric optical path length than the first interferometer arm 11 (or a shorter geometric optical path length if the shift causes the first light field 611 to lead the second light field 612).

[0149] 6B , the second interferometer arm 21 also comprises a dielectric plate (second dielectric plate 362). The second dielectric plate 362 changes the optical path length and geometrical path length of the second light field 612. However, the second dielectric plate 362 is different from the first dielectric plate 361. Therefore, the changes in the optical path length and geometrical path length of the first light field 611 are different from those of the second light field 612. This results in changes in the optical path length and geometrical path length of the first light field 611 relative to the second light field 612. To compensate for the relative changes in the geometrical path lengths, the geometrical path length of one of the first interferometer arm 11 and the second interferometer arm 21 is selected to be longer than the geometrical path length of the other of the two interferometer arms 11, 21.

[0150] After passing through the first interferometer arm 11 and the second interferometer arm 21, respectively, the first light field 611 and the second light field 612 are combined at the beam splitter / combiner 32, 33, and the combined exit light field is propagated to the detector 35, which measures the interference pattern of the first light field 611 and the second light field 612. The interference results from a shift in the geometrical optical path length of the first interferometer arm 11 relative to the geometrical optical path length of the second interferometer arm 21, while the optical path lengths of the first interferometer arm 11 and the second interferometer arm 21 are equal.

[0151] While specific examples have been illustrated and described herein, those skilled in the art will recognize that various alternative embodiments and / or equivalent implementations may be substituted for the specific examples shown and described without departing from the scope of the invention. This application is intended to cover any adaptations or variations of the specific examples discussed herein. Accordingly, it is intended that the present invention be limited only by the claims and their equivalents.

[0152] It should be noted that the examples of interferometers and / or methods and / or interferometer assemblies outlined herein may be used alone or in combination with other examples disclosed herein. In addition, features outlined in the context of an interferometer or interferometer assembly are also applicable to the corresponding method, and vice versa. Furthermore, all aspects of the examples of interferometers and / or methods and / or interferometer assemblies outlined herein may be combined in any manner. In particular, features of the claims may be combined with each other in any manner.

[0153] It should be noted that the description and drawings merely illustrate the principles of the proposed method and system. Those skilled in the art will be able to implement various configurations not explicitly described or shown herein, but which embody the principles of the invention and are within its spirit and scope. Furthermore, it is expressly intended that all examples and embodiments outlined herein are merely for illustrative purposes, primarily to aid the reader in understanding the principles of the proposed method and system. Furthermore, all statements herein providing principles, aspects, and embodiments of the invention, as well as specific examples thereof, are intended to encompass equivalents thereof.

Claims

1. A telecentric interferometer, the telecentric interferometer comprising: a front optical element comprising an exit pupil (42); a first interferometer arm (11) that is part of the first optical system (10) and is located on the image side of the front optical element; a second interferometer arm (22) that is part of a second optical system (20) and is located on the image side of the front optical element; a detector (35) located on the image side of both the first optical system (10) and the second optical system (20); a shift unit (12, 22) located between the front optical element and the detector (35); the first optical system (10) and the second optical system have the same object-side focal length (141, 241) and the same Gaussian image distance (151, 251); the first optical system (10) and the second optical system (20) have identically positioned object-side principal planes (142, 242) with identical object-side optical axes (143, 243); the exit pupil (41) of the front optical element is spaced apart from the object-side principal plane (142, 242) by the object-side focal length (141, 241); The shift unit (12, 22) shifts the image-side principal plane (152) of the first optical system (10) and / or the image-side principal plane (252) of the second optical system (20) to: the image-side principal plane (152) of the first optical system (10) is different from the image-side principal plane (252) of the second optical system (20); and A telecentric interferometer, in which the optical path length of the first interferometer arm (11) is shifted to be equal to the optical path length of the second interferometer arm (21).

2. the image-side principal plane (152) of the first optical system (10) and the image-side principal plane (252) of the second optical system (20) are parallel; an image-side conjugate surface (154) of the first optical system (10) is different from an image-side conjugate surface (254) of the second optical system (20); 2. The telecentric interferometer of claim 1, wherein the detector is located between the image-side conjugate surface of the first optical system and the image-side conjugate surface of the second optical system.

3. 3. The telecentric interferometer of claim 1, wherein an image-side optical axis (153) of the first optical system (10) and an image-side optical axis (253) of the second optical system (20) are identical.

4. The shift unit (12, 22) a first weak lens (51) located in the first interferometer arm (11) and / or a second weak lens (52) located in the second interferometer arm (12); a strong lens (53) located on the image side of the first weak lens (51) and / or the second weak lens (52), 4. The telecentric interferometer according to claim 1, wherein the first weak lens (51) and / or the second weak lens (52) are spaced apart from an object-side principal plane (531) of the strong lens by an object-side focal length (53f) of the strong lens (53).

5. 5. The telecentric interferometer of claim 4, wherein the strong lens (53) is part of both the first optical system (10) and the second optical system (20).

6. The shift unit (12, 22) comprises the first weak lens (51) and the second weak lens (52). the focal lengths of the first weak lens (51) and the second weak lens (52) are equal in magnitude but opposite in sign, and / or either the first weak lens (51) or the second weak lens (52) is a concave lens or a convex mirror; 6. A telecentric interferometer according to claim 4 or 5, wherein the object-side focal length (53f) of the strong lens (53) is smaller in magnitude than the focal length of the first weak lens (51) and the focal length of the second weak lens (52).

7. the interferometer is configured in a Michelson configuration; The shift unit (12, 22) comprises the first weak lens (51) and the second weak lens (52). A telecentric interferometer according to any one of claims 4 to 6, wherein the first weak lens (51) is a concave or plane mirror and the second weak lens (52) is a convex mirror.

8. The shift unit (12, 22) comprises a dielectric plate (361, 362) located in the first interferometer arm (11) or the second interferometer arm (12); The refractive index and / or thickness of the dielectric plates (361, 362) are: the optical path length of the first interferometer arm (11) is equal to the optical path length of the second interferometer arm (12); and 8. The telecentric interferometer of claim 1, wherein the geometrical optical path length of the first interferometer arm (11) is selected to be different from the geometrical optical path length of the second interferometer arm (12).

9. an incident optical element (31) located between the front optical element on one side and the first interferometer arm (11) and the second interferometer arm (12) on the other side; the entrance optical element (31) is part of both the first optical system (10) and the second optical system (20); 9. The telecentric interferometer of claim 1, wherein the input optical element (31) is adapted to adjust the object-side focal length (141) of the first optical system (10) and the object-side focal length (241) of the second optical system (20).

10. an output optical element (34) located between the first interferometer arm (11) and the second interferometer arm (12) on the one hand and the detector (35) on the other hand; The telecentric interferometer of any one of claims 1 to 9, wherein the output optical element (34) is part of both the first optical system (10) and the second optical system (20).

11. at least one of the first optical system (10) or the second optical system (20) comprises a correction unit (13, 23) adapted to correct a chromatic change in the optical path length in the first optical system (10) and / or the second optical system (20) caused by the shift unit (12, 22); Telecentric interferometer according to any one of claims 1 to 10, wherein the focal length of the correction unit (13, 23) approaches infinity.

12. the correction unit (13, 23) comprises at least two dielectric elements with different refractive indices; and / or the shifting unit (12, 22) comprises a dielectric plate (361, 362) that is also part of the correction unit (13, 23); and / or 12. The telecentric interferometer of claim 11, wherein the shift unit (12, 22) comprises at least one optical element having refractive power, and at least one of the Gaussian sweep of all optical elements in the first interferometer arm (11) or the Gaussian sweep of all optical elements in the second interferometer arm (21), including the correction unit (12, 22), has a finite focal length.

13. a phase shift unit disposed in or downstream of at least one of the first interferometer arm (11) or the second interferometer arm (21); 13. A telecentric interferometer according to any one of claims 1 to 12, wherein the phase shifting unit comprises at least one of a movable mirror, a piezoelectric crystal, a circular polarizer, a linear polarizer, a quarter wave plate, or a polarization sensitive detector.

14. A method for determining a property of an incident light field using a telecentric interferometer according to any one of claims 1 to 13, comprising the steps of: propagating the incident light field through the exit pupil of the front optical element; Splitting the incident light field into a first portion that propagates along the first optical system (20) and a second portion that propagates along the second optical system (20); Using the shift unit (12, 22), the image-side principal plane (152) of the first optical system (10) is shifted relative to the image-side principal plane (252) of the second optical system (20), the image-side principal plane (152) of the first optical system (10) is different from the image-side principal plane (252) of the second optical system (20); and Shifting the optical path length of the first interferometer arm (11) to be equal to the optical path length of the second interferometer arm (21); combining the first portion of the incident light field and the second portion of the incident light field into an exit light field; measuring an interference pattern of the exiting light field with the detector (35).

15. (i) generating at least two intermediate interference patterns from the interference patterns measured at different focal points by convolution of the measured interference patterns with a propagation kernel for a selected propagation distance; (ii) processing the at least two intermediate interference patterns using algebraic techniques; (iii) applying a filter function to said at least two intermediate interference patterns to produce a resultant image.

16. 16. The method of claim 15, comprising repeating steps (i) to (iii) iteratively, wherein the resulting image produced in a previous iteration cycle is used as the measured interference pattern in a next iteration cycle.

17. 17. The method according to any one of claims 15 to 16, wherein the method comprises, prior to generating the at least two intermediate interference patterns, reshaping the measured interference pattern by electronic deconvolution of the measured interference pattern with a reconstruction function.

18. 1. An interferometer assembly, comprising: A telecentric interferometer according to any one of claims 1 to 14; a lighting device; The illumination device illuminates an object with illumination light, and different object points on the object are coherently reflecting or transmitting a portion of the illumination light; or an interferometer assembly adapted to incoherently scatter or transmit the illumination light within the resolution;

Images