Telecentric interferometer, method for determining characteristics of input light field, and interferometer assembly

Abstract

A telecentric interferometer, a method for determining characteristics of an input light field, and an interferometer assembly are disclosed. The telecentric interferometer includes a front side optic 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 displacement unit located between the front side optic and the detector. The first optical system and the second optical system have a same object-side focal length, a same Gaussian image distance, and a same object-side main plane and a same object-side optical axis. The exit pupil of the front-side optic is the object-side focal length from the object-side main plane. The shifting unit shifts an image-side main plane of the first optical system and / or an image-side main plane of the second optical system such that the image-side main plane of the first optical system is different from the image-side main 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 characteristics of an input light field using the telecentric interferometer, and an interferometer assembly including the telecentric interferometer and an illumination device. Background Art

[0002] Optical interferometers can be used to reconstruct the phase and / or amplitude of a light field. For example, this enables three-dimensional image reconstruction. In particular, the spatial position (e.g., position and / or structure) or local refractive index of an object or individual points of an object will be determined based on the intensity of the light field measured using a detector that has interacted with the object. In addition, there is an interest in measuring pure phase objects (i.e., objects that are not visible in conventional microscopy (sometimes also referred to as quantitative phase contrast microscopy)).

[0003] In a conventional optical system, there may be problems where the lateral alignment and / or resolution and / or axial resolution (i.e., depth of field) of the interferometer are limited, resulting in difficulties in image reconstruction. In addition, a conventional optical system may have chromatic aberration and / or monochromatic aberration (errors in imaging), which require expensive and complex correction.

[0004] U.S. Patent Application US 2017 / 242398A1 discloses a birefringent lens in an inline interferometer for microscopy. This design is optimized for microscopy with incoherent sources. U.S. Patent Publication US 2017 / 0329280A1 discloses an apparatus including a beam expander for generating an interferogram generated by an incoherent point source. This means that strong refractive lenses are used in the interferometer, the complex requirements brought about by inherent aberrations have not been solved, and the necessary achromatism and chromatic aberration balance for aberration correction between the two interferometer arms have not been solved.

[0005] US 9417610B1 discloses an interferometer that uses a concave mirror to generate a significantly defocused interferogram. This is why the overlap of the spots is not important in this publication. The purpose of the device is to image the source of phase aberration in the upstream optical system in the interferometer via the mirror so that the phase aberration generated in the measured image can be compensated. This applies to mutually incoherent sources. The arrangement (design) of the imaging elements in the interferometer is determined by the physical position of the phase perturbation layer in the middle of the path from the object to the front optical device. The interferometer disclosed in this publication is not telecentric.

[0006] In an interferometer, typically two central rays (i.e., along the axis of symmetry) are superposed on a detector. However, objects outside of symmetry (e.g., different from the central rays) may experience a quality degradation. In a conventional arrangement, the quality of the interference typically varies across the field of view. This is particularly important for coherent light sources, since for these sources all field components may interfere with each other. Among other things, the quality of the interference can be measured by the quality of the overlap (in the case of an image with one emission point source).

[0007] In a conventional interferometer, if the overlap of the spots (e.g., the light field or a part of the light field is an image of a point source) is inaccurate, interference is typically not observable with mutually incoherent sources (e.g., spots), since coherence is only given for light within the same source. However, interference of the spots can be achieved by defocusing (i.e., blurring the spots). This blurring exploits the property that the superposition of inaccurately superposed (i.e., laterally shifted) Gaussian spots still results in a Gaussian spot at the central position. This method benefits from the automatic suppression of the misaligned parts of the light field due to non-interference. However, this method may not be suitable for coherent light (e.g., for quantitative phase contrast), since inaccurate or approximate superposition will lead to new (but false) interference with other spots. Additionally, the actual spots in an optical system are not Gaussian spots, but are generated by Fresnel diffraction at the exit pupil of the optical system (see e.g., "Born Principles of Optics", M. Born and E. Wolf, Cambridge University Press, 8th printing, 2013, chapter 8.8). Therefore, the non-central superposition of these complex spots can lead to considerable evaluation problems for non-rotationally symmetric superpositions and interference patterns, resulting in artifacts. Conventional methods sometimes also suffer from the problem that the resolution of the interferometer is maximum near the focus. Therefore, it is generally advantageous to superpose the images of different spots near the focus in an interferometer, which means "as accurate as possible". Therefore, devices with precise overlap are needed, especially for applications with (partially) coherent light.

[0008] The technical importance of quantitative phase microscopy (i.e., self-interference of partially coherent light) is very high. Therefore, interferometers are sought that are suitable for both coherent light and light from mutually incoherent sources. Additionally, the combined evaluation of light with different degrees of coherence can allow new sampling possibilities.

[0009] Conventional interferometer setups are typically based on the principle of only approximately overlapping the light spots, i.e., the above accuracy for off-axis light spots cannot be achieved. This can already be recognized by using different refractive powers in the optical systems of the two interferometer arms without proper compensation via a lens outside the interferometer. In this case, the condition of telecentricity cannot be satisfied for the two arms of the interferometer. In such a case, telecentricity can only be achieved by means of appropriate compensation implemented by an appropriately optimized lens outside the interferometer. If these optimized lenses are not provided outside the interferometer, the entire system cannot achieve telecentricity in the sense of the present application. In particular, conventional systems may not have the compensation required to achieve telecentricity in the sense of the present application.

[0010] It may also be beneficial for the interferometer to be applicable to a wide spectral range, including wavelength multiplexing for achieving good visibility of the interference fringes, synthetic wavelength applications, or simply using all available light for measurement.

[0011] Furthermore, conventional optical systems may not be telecentric, especially not in the 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 does not use lenses / mirrors or only uses lenses / mirrors with very low refractive power in the arms of the interferometer of the device. SUMMARY OF THE INVENTION

[0012] According to some aspects of the present disclosure, a telecentric interferometer includes: a front-side optical device including 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-side optical device; a second interferometer arm that is part of a second optical system and is located on the image side of the front-side optical device; a detector that is located on the image side of both the first optical system and the second optical system; and a shifting unit that is located between the front-side optical device 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 the object-side principal planes located identically and the same object-side optical axis. The exit pupil of the front-side optical device is separated from the object-side principal plane by the object-side focal length. The shifting unit shifts the image-side principal plane of the first optical system and / or the image-side principal plane of the second optical system 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.

[0013] According to some aspects of the present disclosure, a method for determining characteristics of an input light field using a telecentric interferometer, the method comprising: propagating an input light field through an exit pupil of a front-side optical device; splitting the input light field into a first part and a second part, wherein the first part propagates along a first optical system and the second part propagates along a second optical system; using a shifting unit to shift an image-side principal plane of the first optical system relative to an image-side principal plane of the second optical system 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 an optical path length of a first interferometer arm is equal to an optical path length of a second interferometer arm; combining the first part of the input light field and the second part of the input light field into an output light field; and measuring an interference pattern of the output light field using a detector.

[0014] Those skilled in the art will recognize additional features and advantages upon reading the following detailed description and viewing the drawings. BRIEF DESCRIPTION OF THE DRAWINGS

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

[0016] The elements of the drawings are not necessarily drawn to scale relative to each other. Identical reference numerals indicate corresponding like parts. The features of the various illustrated examples may be combined unless they are mutually exclusive. The examples will be depicted in the drawings and will be described in detail in the following description.

[0017] Figure 1 A telecentric interferometer according to aspects of the present disclosure is schematically depicted.

[0018] Figure 2 The principle of a telecentric optical system is schematically illustrated.

[0019] Figure 3A and Figure 3B Aspects of a telecentric interferometer according to an example of the present disclosure are schematically depicted.

[0020] Figure 4 An interferometer according to aspects of the present disclosure is schematically depicted.

[0021] Figure 5A An interferometer according to aspects of the present disclosure is schematically depicted.

[0022] Figure 5B An interferometer according to aspects of the present disclosure is schematically depicted.

[0023] Figure 6A and Figure 6B Aspects of a telecentric interferometer according to an example of the present disclosure are schematically depicted. Detailed implementation manners

[0024] The following provides a detailed description with reference to the accompanying drawings, which form a part of the present disclosure, and in these drawings, specific examples of the interferometer and / or method are illustrated for illustrative purposes. The existence of additional examples will be self-evident.

[0025] Throughout the specification, whenever possible or useful and unless otherwise stated, the notations of the "Field Guide to Geometrical Optics" by John E. Greivenkamp (SPIE, 2004) (hereinafter abbreviated as "Greivenkamp") are used.

[0026] Hereinafter, the positional relationship refers to the position within the optical path of the interferometer. For example, if a first component is located "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 located "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 the direction perpendicular to the optical axis.

[0027] The optical axis can be considered to be defined by optical elements and / or optical components, that is, defined on a local basis. Each optical element with optical power has an optical axis, which is determined by the symmetry of the optical element. In a so-called "aligned" optical system including several optical elements, the optical axes of the optical elements are aligned, that is, coincident. A mirror has no optical power, and the system may have different optical axes before and after the mirror. If the light ray before the mirror located on the optical axis is also the light ray on the optical axis after the mirror, the system is aligned. Other optical elements without optical power can be treated similarly. In a so-called tunnel diagram, all the optical axes of the aligned system are drawn on one axis. The light ray traveling along the optical axis is called the central ray. When the system is aligned, the central ray passes through the entire optical system on the optical axis. The optical axis can also be called the symmetry axis or the z-axis.

[0028] Throughout the specification, an optical component (sometimes also referred to as an "optical element") may include at least one of the following or may be: a mirror, a concave mirror, a convex mirror, a beam splitter (e.g., a beam splitter cube), a lens, or a dielectric plate. Other optical components are also possible.

[0029] In the following, the geometric path length of an optical path refers to the geometric length of the optical path. In other words, the geometric path length is the Euclidean distance of the ray integral along the optical path between the starting point and the end point. The geometric path length between two optical components in the first interferometer arm or the second interferometer arm does not include the length between the principal planes of the two optical components, that is, only the length outside the region between the principal planes is counted. For this purpose, the central ray or the path length along the z-axis (symmetry axis) can be used, and the principal planes are determined for all individual optical components in the (first) optical system including the first interferometer arm by Gaussian reduction and / or for all individual optical components in the (second) optical system including the second interferometer arm by Gaussian reduction. In contrast, the term "total geometric path length" refers to the case where the region between the principal planes is not subtracted. Here and in the following, unless another reference point is explicitly specified, the geometric path length and the total geometric path length are always measured between the object and the detector. The optical path length is the total geometric path length multiplied by the refractive index of the object located within the optical path. For a refractive index that varies along the optical path, the optical path length is given by the line integral of the refractive index.

[0030] If not otherwise stated in the following, 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).

[0031] If an optical component is referenced only by one principal plane without further distinction, it is assumed that the first principal plane and the second principal plane coincide. A person skilled in the art of optics can generalize this to a finite principal plane split.

[0032] A telecentric interferometer is provided. Telecentricity in the object or image space requires that the chief ray be parallel to the axis in that space. As defined in Greivenkamp, the chief ray starts from the edge of the object (i.e., the point at the field of view boundary), passes through the center of the pupil (e.g., the entrance pupil), exits at the center of another pupil (e.g., the exit pupil), and defines the corresponding image height and position of the (optional) other pupils. Due to image-side telecentricity, even if the detection plane is deviated from its focal position (i.e., defocused), the apparent system magnification is constant. The image will be blurred, but the center of each spot remains at the same lateral position, and the size or magnification remains constant even if each individual spot becomes blurred. Image space telecentricity is independent of the position of the object or the detector. Therefore, an image of the same size is produced regardless of the distance between the lens and the object or the detector. In the present disclosure, for a general object in the field of view, the ray passing through the center of the pupil from the edge of the object can be referred to as the chief ray.

[0033] In the case of the interferometer described herein, telecentricity (image side) may in particular mean that the optical system of the first interferometer arm (Gaussian reduction of the components of the first interferometer arm) has the same focal length as the optical system of the second interferometer arm (Gaussian reduction of the components of the second interferometer arm). The two interferometer arms may also be referred to as the "interfering parts" of the interferometer. The interferometer provided herein can be easily implemented and allows for the superposition of interference images with perfect image-side telecentricity in the paraxial Gaussian approximation. Furthermore, it is possible that the choice of the positions of the optical elements is independent of the object or object-related properties such as aberrations, thus fulfilling the strict image-side telecentricity condition.

[0034] In an image-side telecentric system, different chief rays pass through the exit pupil of the front-side optical device at the pupil center and become rays parallel to the optical axis at the image side of the optical system or the detector. This may be the case for both the first and the second interferometer arms of the interferometer. Thus, a virtual point source at the center of the exit pupil can generate a plane wave on the detector (in the paraxial Gaussian approximation). That is, for the chief rays passing through the two interferometer arms, all these points may have the same phase difference. Therefore, a virtual point source at the center of the exit pupil can generate a plane wave on the detector. That is, for the corresponding chief rays, all object points may have the same phase difference when the chief ray passes through the two interferometer arms. The central ray may be the chief ray of the object on the axis. Different chief rays may have the same phase difference, which is equal to the phase difference of the central ray.

[0035] At least according to some aspects of the present disclosure, a telecentric interferometer may be applicable only to a single wavelength (e.g., one color) or multiple wavelengths (e.g., the entire visible spectrum) (e.g., without readjustment). For this purpose, using focusing achromatic optical elements may not be sufficient, but the path difference between the arms of the interferometer must be corrected for chromatic aberration according to the setup (which is another type of achromatism), and / or even the fringe spacing may be kept chromatically constant by appropriate correction.

[0036] Throughout this disclosure, the term "image space telecentric system" may also refer to or even be used as a synonym for an image space telecentric lens or an image space telecentric lens system, which is part of an interferometer and provides telecentricity characteristics. 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 at least one other optical component may be different. 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 be different at least within the first interferometer arm and the second interferometer arm.

[0037] The first interferometer arm and the second interferometer arm may be provided by a beam splitter that splits an incident light field into two light fields. Hereinafter, if not otherwise explicitly stated, the terms "light field" and "light beam" may be used interchangeably (i.e., a "light field" is a "light beam", and vice versa). A light field (or light beam) includes a plurality of light rays. The first light field of the two light fields may propagate along the first interferometer arm, while the second light field of the two light fields may propagate along the second interferometer arm. A light field (or light spot) may have a principal ray, which may be defined as the ray passing through the center of the exit pupil of the front-end optical device. After propagating along 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., in a Michelson-type configuration), or the beam combiner may be a separate optical component (e.g., in a Mach-Zehnder-type configuration).

[0038] For example, the beam splitter and / or the beam combiner may include or may be a beam splitter cube, a prism, a semi-transparent mirror, or a thin film or dielectric component. Depending on the application, chromatic aberration effects are compensated or corrected to the necessary degree. Those skilled in the art will understand that any optical component suitable for splitting and / or combining light fields can be used as the beam splitter and / or the beam combiner.

[0039] The beam splitter that defines the two interferometer arms may be part of the incident optical device of the interferometer. The beam combiner may be part of the exit optical device of the interferometer. If the beam splitter is the same as the beam combiner, the beam splitter may be part of the incident optical device and the exit optical device.

[0040] The first optical system and the second optical system may have the same object-side focal length and the same Gaussian image distance. Accordingly, the term "object-side focal length" ("Gaussian image distance") denotes 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 also referred to as the "object plane" on the object side and the "image plane" on the image side). It is possible that the first optical system and the second optical system also have the same image-side focal length and the same image-side Gaussian image distance.

[0041] The first optical system and the second optical system may have the same-positioned object-side principal planes and the same object-side optical axis. The "object-side optical axis" may be the optical axis originating from the front-side optical device. The same focal lengths (also referred to as "optical focal lengths" or "front focal lengths") and the same object-side principal planes of the first optical system and the second optical system enable the overlap of the image-side telecentric beams. If the distances between the exit pupil and the principal plane are not the same, the chief rays of any object point passing through the exit pupil at an angle higher than 0° at the center will hit the principal planes of the first optical system and the second optical system at different lateral distances from the axis. This would mean that the chief rays of the two interferometer arms may not have the same ray axis, and thus they will not overlap with each other at the ray center. In this scenario, which may exist in a conventional interferometer, no interference signal or only a small interference signal will be measured, which may have a negative impact on the resolution of the interferometer.

[0042] 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., located 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 (sometimes also referred to as the "rear focal length" of the first optical system) and the image-side focal length (sometimes also referred to as the "rear focal length" of the second optical system) of the second optical system may be equal. For example, the image-side conjugate plane of the first optical system is located at the position of the image of the object originating from the path via the first interferometer arm. Similarly, the image-side conjugate plane of the second optical system is located at the position of the image of the object originating from the path via the second interferometer arm.

[0043] The telecentric interferometer may include a detector. The detector is located on the image side of both the first optical system and the second optical system. For example, the detector may be or may include a CMOS sensor having a plurality of pixels. The detector may be arranged such that for a given object position, it is close to the conjugate plane of the first optical system and the conjugate plane of the second optical system, and the conjugate plane of the second optical system 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 this scenario, neither the image from the first interferometer arm nor the image from the second interferometer arm is in focus on the detector, but both images are slightly out of focus.

[0044] 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 system may be aligned. In this case, the interferometer may be a fully centered optical system. Alternatively, the image-side optical axis of the first optical system may also be laterally shifted relative to the image-side optical axis of the second optical system. This lateral shift may also be referred to as "lateral shear". The lateral shear may be selected such that adjacent light spots still at least partially overlap. It may be useful for coherent light sources, where even different light spots may interfere with each other. Hereinafter, a light spot may be a light field or a part of a light field, which is an image of a point source. For example, the lateral shear may be achieved by slightly rotating a physical optical component in one of the pupils located in the pupil of the system. In an embodiment of the present disclosure, the shear may be set to zero or to a finite value by adjustment.

[0045] In at least some examples, the telecentric interferometer includes front-side optics (sometimes also referred to as "front-end optics"). The first optical system and the second optical system may be located on the image side of the front-side optics. In the use of the interferometer, the object to be studied with the interferometer may be placed on the object side of the front-side optics. The front-side optics includes an exit pupil. The exit pupil may be mapped to infinity by the first optical system and / or the second optical system. It is possible that both the first optical system and the second optical system satisfy the image-side telecentric condition, but the image-side principal planes of the two optical systems may be different.

[0046] The exit pupil of the front-side optics may be located at the object-side focal length from the object-side principal planes of the first optical system and the second optical system. This position of the exit pupil allows for providing a telecentric system (e.g., an image-space telecentric system), for example, for the two arms of the interferometer. The image-space telecentric interferometer provides a telecentric property at least on the image side of the optical system.

[0047] The telecentric interferometer may include a displacement unit. The displacement unit is located between the front-side optics and the detector. For example, at least parts of the displacement 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 include a part of the displacement unit; for example, both the first interferometer arm and the second interferometer arm may include a part of the displacement unit. Alternatively, the displacement unit may be located only within the first interferometer arm or the second interferometer arm.

[0048] The displacement unit displaces 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 displacement unit displaces the image-side principal plane of the first interferometer arm and / or the image-side principal plane of the second interferometer arm. The displacement unit can be adjusted such that the displacement results in the image-side principal plane of the first optical system being different from the image-side principal plane of the second optical system, and the optical path length of the first interferometer arm being equal to the optical path length of the second interferometer arm. The optical paths of the first interferometer arm and the second interferometer arm can be determined separately for the central ray along the axis of symmetry of the system. In particular, the optical path can be determined between the corresponding starting point of the ray at the object point and the corresponding end point of the ray at the detector. Since the detector cannot be in the same image plane of the two arms (because they are different), the mapping from the object to the detector is not aberration-free, and rays different from the central ray may have a phase difference. This may be the reason for the appearance of the interference pattern.

[0049] The object-side principal plane of the first optical system is the same as (e.g., identically positioned to) the object-side principal plane of the second optical system. However, the principal planes on the image side of the first system and the second system may be different. For example, by displacing a mirror in one of the arms of the interferometer, the adjustment of the optical path equality can be achieved. In this case, the mirror can be part of the displacement unit.

[0050] In some examples, the first interferometer arm and / or the second interferometer arm may have a finite focal length, as long as the corresponding systems in the arms are constructed from optical elements with a finite focal length. In other words: The parts of the first optical system and / or the second optical system located within the first interferometer arm and / or the second interferometer arm respectively have a finite focal length (using Gaussian reduction). In contrast, if the part of the first optical system within the first interferometer arm and / or the part of the second optical system within the second interferometer arm only includes dielectric plates, the first interferometer arm and / or the second interferometer arm respectively have an infinite focal length. As long as the interferometer arm includes one or more optical elements with an absolute refractive power higher than zero, the magnitude of the absolute focal length of the corresponding optical element divided by the beam diameter (or the diameter of the exit pupil of the front-side optics) is greater than 10, or 20, or 30, or 50, or even greater than 100. It is possible that the first interferometer arm and / or the second interferometer arm only include elements with a negative refractive power, i.e., respectively include concave lenses or convex mirrors and plane mirrors.

[0051] The positions of the pupil, the principal plane, and the optical elements can be given by the interferometer setup, but the position of the image plane can depend on the position of the object. Considering the object, the position of the detector (which may not be counted among the optical elements) is appropriately selected. Focusing in the usual sense is not necessary. The object can be a set of inhomogeneities, disturbances, or scattering centers (also referred to hereinafter as "disturbances"). A disturbance is an object that disturbs or scatters the rectilinear propagation of light. In optical theory, all such objects can be considered as sources of spherical waves (or equivalent electromagnetic modes), which can then be imaged by the optical system. The light field of the disturbance can also be referred to as a "speckle". The imaging distance refers to these disturbances and the corresponding images.

[0052] The shift of the shift unit can correspond to the defocusing of the first interferometer arm (and / or the first optical system) relative to the second interferometer arm (and / or the second optical system). Here, for the first and second interferometer arms, the focal length and the distance of the corresponding conjugate plane from the corresponding principal plane can be the same. "Defocusing" in this context refers to the situation where the detection plane and the image plane (conjugate plane) do not coincide. In contrast, in an optical system without defocusing, the detection plane and the image plane usually coincide. Since the shift (or defocusing) of the first and second interferometer arms is carried out in different ways, the term "defocus the first interferometer arm relative to the second interferometer arm" (or vice versa) is used. When describing the optical system by the Gaussian equation, this can mean that the specified distances of the first and second interferometer arms to the principal plane are the same. For the first and second interferometer arms respectively, this may be the case for the object, the image, and the pupil of the system. These two design criteria (regarding the image and pupil positions) can be satisfied simultaneously.

[0053] The shift can change the axial position of the first light field propagating through the first optical system (e.g., the first part of the input light field) relative to the second light field propagating through the second optical system (e.g., the second part of the input light field). The lateral position of the first light field relative to the second light field is generally not changed by the shift. However, if the first and second light fields experience a relative lateral shift (so-called "lateral shear") brought about by the shift unit (or another optical component of the interferometer), then for example, for a physical component in one of the pupils located in the pupil, this lateral shift can be compensated, for example, by a slight tilt of at least one of the mirrors in the first and / or second interferometer arms and / or the mirrors of the interferometer.

[0054] The first light field can be superimposed on the second light field after respectively propagating through the first interferometer arm and the second interferometer arm. The first spot of the first light field and the second spot of the second light field can interfere to output the common output spot of the light field. The first spot of the first light field and the second spot of the second light field can originate from the same input spot of the input light field. Therefore, the input light field can overlap with its defocused self, thereby generating an interference pattern that allows the characteristics (e.g., phase or amplitude) of the input light field to be determined. Due to the defocus design, the two light fields that interfere can have the same beam divergence angle with the same beam opening angle and / or the same curvature. For a light field or a spot, the divergence angle can be measured at the beam waist (i.e., at the position with the minimum beam radius), and represents the incident and divergence angles (beam divergence angle) of the light field at that point.

[0055] The defocus effect in an interferometer (especially a reference - beam - free interferometer) is explained in detail in U.S. Patent Application Publication No. US 2022 / 034645A1. This document also explains the extraction of the characteristics of an input light field using an interferometer including a defocus unit. The disclosure of the evaluation and general concept of the interferometer in the said U.S. Patent Application Publication is incorporated herein by reference. The present disclosure utilizes a concept similar to the defocus (shift) in U.S. Patent Application Publication No. US 2022 / 034645A1, but in combination with a novel telecentric design.

[0056] In addition to imaging errors, in some aspects of the present disclosure, compensating for chromatic aberration variations in the path length may be beneficial. Therefore, using lenses with high refractive power in the interferometer arms may be disadvantageous because the associated errors may be unavoidable. In the case where there are lenses with high refractive power in the interferometer arms, image components outside the center of the optical system may not interfere as desired. Conventional interferometers including lenses with high refractive power can only be used for incoherent light sources.

[0057] Using an interferometer with chromatic aberration compensation allows the interferometer to be used for coherent light fields (obtaining quantitative phase differences) as well. Interferometer errors have a particularly strong impact on coherent light because the electric field is included in the measurement - related terms from the two interferometer arms in a quadratic manner. In this case, the evaluation can be carried out according to the method of Patent Publication No. US10823547B2.

[0058] According to some examples, the shifting unit includes at least one weak lens located in one of the first interferometer arm or the second interferometer arm. For example, the shifting 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 shifting unit may further include a strong lens located on the image side of the first weak lens and / or the second weak lens. For example, each of the lenses may be a conventional lens or a mirror lens. It is possible that the strong lens compensates for the different refractive effects of the first weak lens and the second weak lens, such that the two optical systems thus image the exit pupil of the front-end optical device in a telecentric manner.

[0059] The weak lens (e.g., the first weak lens and / or the second weak lens) may have a finite focal length. The absolute focal length of the weak lens (i.e., its absolute value) may be higher than zero and lower than infinity. The strong lens may also have a finite focal length. The absolute value of the focal length of the strong lens may be higher than zero and lower than infinity. The weak lens (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. Individually or in combination, the focal length of the weak lens (e.g., the first weak lens and / or the second weak lens) divided by the absolute value of the beam diameter may be at least 10, or 20, or 30, or 50, or 100, or 150. Generally, the weak lens and / or the strong lens (e.g., in the shape of a regular lens or a mirror) may be only convex or only concave, or concave in one arm and convex in the other arm.

[0060] In some examples, the weak lens (e.g., the first weak lens and / or the second weak lens) may be separated from the strong lens by the focal length of the strong lens. It is possible that both the first weak lens, which is part of the first optical system, and the second weak lens, which is 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 have no effect on the focal length of the doublet system (i.e., the combination of the weak lens and the strong lens). However, the weak lens may have an effect on the axial position of the image-side principal plane of the said doublet system. The weak lens can thus shift the image-side principal plane in different ways for the first optical system and the second optical system.

[0061] The strong lens may be located outside the first interferometer arm and the second interferometer arm, for example, downstream of the first interferometer arm and the second interferometer arm. For example, the strong lens may be part of the first optical system and the second optical system. That is, the strong lens may not be part of any of the interferometer arms but part of the two optical systems. Thus, the strong lens may be a common strong lens of the first optical system and the second optical system. The strong lens may be part of the output optical device. The output optical device may be located between the interferometer arm on the one hand and the detector on the other hand.

[0062] The first weak lens and / or the second weak lens may be separated 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 an absolute object-side focal length greater than that of the strong lens. For example, the absolute object-side focal length of the first weak lens and / or the second weak lens may be at least one order of magnitude greater than the object-side focal length of the strong lens. In other cases, the absolute focal length of the weak lens may be greater than 2 times, or 3 times, or 5 times, or 10 times the absolute focal length of the strong lens. The optical power of the weak lens may be relatively weaker compared to the strong lens.

[0063] In some examples, the shifting unit includes a first weak lens in the first interferometer arm and a second weak lens in the second interferometer arm. The focal length (e.g., object-side focal length) of the first weak lens and the focal length (e.g., object-side focal length) of the second weak lens may be equal in magnitude but opposite in sign. For example, the first weak lens is a converging lens and the second weak lens is a diffusing lens, or vice versa. Individually 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 magnitude of the object-side focal length of the strong lens may be smaller than the focal lengths of the first weak lens and the second weak lens.

[0064] It is possible that the first weak lens and / or the second weak lens and / or the strong lens includes a plurality of optical elements that produce the properties of the above-mentioned focal lengths. Using a combination of lenses as the first weak lens and / or the second weak lens and / or the strong lens instead of using only a single lens may allow adjustment of the optical properties of the lens system. For example, geometric and / or chromatic aberrations can be suppressed by using a combination of lenses.

[0065] The weak lens has a weak optical power and thus only has a small monochromatic aberration effect. If monochromatic optical aberrations interfere with the image-side telecentricity and if they are different for the first interferometer arm and the second interferometer arm, they may be mainly relevant in the described system.

[0066] In at least some examples, the telecentric interferometer is configured in a Michelson-type configuration. In this configuration, the beam splitter that defines the two interferometer arms can also be used as a beam combiner. Each interferometer arm may be traversed twice by the light field traveling inside the interferometer arm.

[0067] In some examples, the interferometer is configured in a Michelson-type configuration, and the displacement unit includes a first weak lens and a second weak lens. Additionally, the displacement unit includes a strong lens. The first weak lens can be a concave mirror or a plane mirror, and the second weak lens can be a convex mirror, or vice versa. Thus, the focal lengths of the first weak lens and the second weak lens can have opposite signs. The magnitudes of the absolute values of the focal lengths of the first weak lens and the second weak lens can be the same.

[0068] In some examples, the displacement unit of the telecentric interferometer includes a dielectric plate. The dielectric plate can be located in the first interferometer arm or the second interferometer arm or both, where the dielectric plates can be different. It is possible that the displacement unit includes an additional dielectric plate located in the other interferometer arm. For example, the dielectric plate is located in the first interferometer arm and the additional dielectric plate is located in the second interferometer arm, or vice versa. The dielectric plate can have no converging or diffusing properties and / or can (if considered as an optical system) have a focal length approaching infinity. In other words: the dielectric plate can be not a lens. The incident surface of the dielectric plate can extend substantially parallel to the exit surface of the dielectric plate.

[0069] The refractive index and / or thickness of the dielectric plate can be selected such 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 path length of the first interferometer arm is different from the geometric path length of the second interferometer arm. The optical path length from the object to the detector can be measured. The detector can 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 path lengths from the object to the respective 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 incident surface and its exit surface. For example, the refractive index of the dielectric plate can be changed by changing the material of the dielectric plate.

[0070] The dielectric plate can be composed of a single dielectric material, or can include several dielectric plates and / or dielectric layers, where at least two of the several dielectric plates are composed of different dielectric materials. The plurality of dielectric plates and / or dielectric layers can be stacked together, and / or there can be a space between at least two of the dielectric plates and / or dielectric layers.

[0071] For example, a dielectric plate may be located in the first interferometer arm. The geometric path length of the first interferometer arm is selected such that it is different from the geometric 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 may be positioned slightly offset (e.g., in the axial direction or z-direction) compared to the mirror in the second interferometer arm. In the absence of any dielectric plate, the optical path lengths of the two interferometer arms will also be different in this configuration. The dielectric plate (e.g., the material and / or thickness of the dielectric plate) is selected such that it compensates for this optical path length difference between the two interferometer arms that occurs when no dielectric plate is inserted into the first interferometer arm. The dielectric material (or, in the case of multiple dielectric plates and / or layers, the dielectric materials) may be selected such that this compensation can be achieved for several different wavelengths. However, except for the principal plane splitting that should be considered when determining the geometric path, the geometric path length is not changed by the dielectric plate. These two effects, principal plane splitting and the change in optical path length, result in the desired shift effect in this way: for the first and second interferometer arms, the same optical path length has different geometric path lengths.

[0072] The difference between the geometric path length of the first interferometer arm and the geometric path length of the second interferometer arm is the optical path length that has been compensated by the dielectric plate. This is also the case if the dielectric plate is located in the second interferometer arm or if both interferometer arms include dielectric plates, mutatis mutandis. In the latter case, the dielectric plates of the two interferometer arms are different in terms of refractive index, dispersion, and / or thickness.

[0073] According to some examples, the alignment of the interferometer may include an alignment step (e.g., a final alignment step) that includes adjusting the axial mirror position in at least one of the interferometer arms such that there is no longer any optical path length difference between the first and second interferometer arms. This alignment step may also be used in combination with other examples described in this specification (e.g., for a shift unit that includes a lens). For example, in the case where the shift unit includes a dielectric plate, alignment can be achieved for multiple wavelengths despite the dispersion in the dielectric material. The alignment step of adjusting the optical path length by adjusting the position of the mirror may not be available in a so-called common path setup.

[0074] According to at least one example, the telecentric interferometer further includes an incident optical device. The incident optical device is located between the front-side optical device on one hand and the first interferometer arm and the second interferometer arm on the other hand. The incident optical device can be part of both the first optical system and the second optical system. The incident optical device 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 incident optical device can be used to adjust the first optical system and the second optical system to the exit pupil of the front-side optical device. In some examples, the incident optical device includes at least one of the following: a relay lens, a barrel lens, or an achromatic barrel lens. The incident optical device can include a beam splitter that defines two interferometer arms.

[0075] An interferometer (e.g., two interferometer arms, including a beam splitter and / or a beam combiner) can include a number of optical components. At least one of the optical components (e.g., a mirror) can be positioned in a different spatial location in the first optical system than the optical components of the second optical system. This enables the adjustment of the optical path length by spatially shifting one of the optical components. By moving, for example, a mirror, the optical path length difference between the two interferometer arms can be made zero. In this example, the interferometer avoids a common-path setup where the two paths of the beam splitter are implemented by polarization filters. Conceivably, a birefringent material can be used to achieve such path adjustment, such as a liquid-crystal-based device. This may allow setting the path length difference between two polarizations, but the compensation by the dielectric layer of the device is affected by dispersion. Compensating this dispersion is very complex and expensive. Therefore, the device is only suitable for a narrow wavelength range. Thus, the use of a common-path interferometer can be excluded.

[0076] In some examples, the telecentric interferometer can include an exit optical device that is located between the first interferometer arm and the second interferometer arm on one hand and the detector on the other hand. The exit optical device can be part of both the first optical system and the second optical system. For example, the exit optical device can include a beam combiner that combines the light fields propagating through the interferometer arms. In some examples, the exit optical device can include a strong lens as part of a shifting unit. The exit optical device can be configured to adjust the magnification, the image-side focal length, and / or the position of the principal plane of the first optical system and the second optical system to allow sufficient focusing on the detector.

[0077] The light has a linear phase in its propagation direction (e.g., the z-direction). Near the beam waist, there is an additional phase effect, namely the so-called Gouy phase. The change in the Gouy phase near the focus extends over a few Rayleigh lengths in the z-direction. For an optical field, the Rayleigh length is defined as follows: at a distance of one Rayleigh length from the beam waist, the area of the optical field doubles (image blurring). In the image-side telecentric interferometer disclosed herein, the interference can be measured such that their respective fields can be measured near the respective foci of the first interferometer arm and the second interferometer arm. For this purpose, the interference can be measured separately for the detector positions between the two image planes (principal planes) of the first interferometer arm and the second interferometer arm. For a detector position exactly at the center position, the two Gouy phases of the two fields can be exactly opposite, i.e., it can have a double effect on the interference. However, except for the global phase (provided that the propagation direction is orthogonal to the detection plane), the linear phase may not have an effect on the interference. This can be the case for an image telecentric setup.

[0078] According to some aspects of the present disclosure, the spacing between the principal planes of the two interferometer arms divided by the Rayleigh distance can be less than 2, or 5, or 10, or 20, or 30, or 50, or 100. The Rayleigh length or Rayleigh distance refers to the optical field leaving the exit optical device and can be determined for a point source for this purpose and can be determined for the optical field incident on the detector.

[0079] The image-side telecentric interferometer according to the present disclosure can have overlapping principal rays parallel to the optical axis in the image space of the two interferometer arms for each object point (spot). Thus, for the two interferometer arms, the principal rays can also hit the detector at the same point. The quality meeting this requirement can be measured by the (lateral) alignment error and / or the Gouy error.

[0080] (The lateral) alignment error can be the lateral distance at the intersection point of the principal rays of the first interferometer arm and the second interferometer arm in the detection plane (assuming symmetry between two conjugate planes, determined for a typical object distance for the application) divided by the spot size (i.e., the focus) caused by the minimum diffraction of the spot image on one of the principal planes in the principal plane. The lateral alignment error is typically determined at the edge of the field of view. In some examples, the interferometer according to the present disclosure can have an alignment error lower than 0.2, or lower than 0.5, or lower than 1, or lower than 2, or lower than 5, or lower than 10, or lower than 15, or lower than 20, or lower than 30, or lower than 50.

[0081] The Gouy error can be determined as follows. The chief ray in each of the first and second interferometer arms for the light spot can enclose a corresponding angle greater than zero with the system axis ( = symmetry axis). The larger of the two angles of the two interferometer arms is used to determine the path extension of the inclined path between the image (conjugate) planes of the first and second interferometer arms according to geometric rules. This extension results in a shift of the Gouy phase between the principal planes and can be avoided by good telecentricity, where "good" can mean that the chief ray hits the detector at an angle of 90°. The path extension divided by the Rayleigh length is called the "Gouy error". The Gouy error is determined at the edge of the field of view. In some examples, the 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.

[0082] 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 can include a correction unit. The correction unit is adapted to correct the chromatic aberration change of the optical path length in the first optical system and / or the second optical system caused by the shifting unit. Such a chromatic aberration change can especially be caused by the shift of the optical path length.

[0083] In contrast, an achromatic lens is corrected to provide the sharpest possible image for a wide range of wavelengths. In particular, it can be beneficial if the focal length does not depend on the wavelength. A sharp image means the lowest possible wavefront error. This involves the difference in path length with respect to a reference wavefront. For a common achromatic lens, for example, the path length of the chief ray itself does not remain constant. In particular, this is the case when both the first optical system and the second optical system include different achromatic lenses. For such a case, due to the inevitable dispersion in the dielectric medium of the lens, the path length difference between the central ray of the first optical system and the central ray of the second optical system can depend on the wavelength. Therefore, in addition to the achromatic property, the achromatic lens can also benefit from a correction unit for the path of the central ray. Thus, even when the path length is adjusted by moving an optical element in one of the interferometer arms (e.g., by changing the position of a mirror), it can be difficult to make the path length difference zero for all wavelengths. Therefore, for example, for the central ray (which can correspond to the center of the telecentric interference light spot), different wavelengths have different relative phase positions with respect to each other. The visibility of the interference is significantly reduced until the interference becomes invisible. If this is the case in the setup, a correction unit is recommended.

[0084] The aberration compensation (sometimes also referred to as chromatic aberration compensation) in the lenses and correction units described herein generally involves different aspects. For example, an aberration-corrected lens (e.g., an achromatic doublet) can cause a wavelength-dependent path length difference between two interferometer arms, which can be corrected by the correction unit. However, the correction unit can be adapted to correct such aberration effects in the difference between the path length of the first interferometer arm and the path length of the second interferometer arm (as measured, for example, for the central ray of the first interferometer arm and the central ray of the second interferometer arm). Such aberration effects can be caused by the displacement unit and / or by additional optical components of the first optical system and / or the second optical system.

[0085] The focal length of the correction unit can be close to infinity. In other words: The correction unit can have no diverging or converging properties. The correction unit can have translational invariance in a direction perpendicular to the optical axis. In other words: The correction unit can have no diverging or converging properties. The correction unit can also transversely correct the light field propagating through the first optical system and / or the second optical system such that the light field that travels 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.

[0086] In some examples, the correction unit can include or consist of at least one dielectric plate (e.g., a dielectric plane plate). The at least one dielectric plate can 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 can include at least one dielectric plate, where the optical flats of the dielectric plates in the interferometer arms are different 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, their thickness). Using at least one dielectric plate as the correction unit can allow for the correction of the chromatic aberration path length effect.

[0087] In some examples, the correction unit can include or consist of at least one optical flat that is slightly tilted with respect to the optical axis. The at least one optical flat can 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 can include at least one optical flat. Alternatively or in combination, the correction unit can include or consist of a first prism and a second prism that are oriented opposite to each other such that there is only a small air gap between the tilted surfaces. The respective second surfaces in the beam paths of the first prism and the second prism are parallel to each other. The entire system can act like a dielectric plate, the thickness of which can be changed by laterally moving the prisms towards each other.

[0088] If the interferometer includes a strong lens and a weak lens as described above, a telecentric setting can be achieved, where only the weak lens is located in one of the two interferometer arms (or the corresponding weak lens is located in either of the two interferometer arms). For example, if the weak lenses are concave mirrors or convex mirrors respectively, there is no chromatic aberration effect in the interferometer that changes the optical path difference between the first interferometer arm and the second interferometer arm. In this case, chromatic aberration correction can be omitted, and the interferometer can be without a chromatic aberration correction unit.

[0089] In some examples, the interferometer includes a chromatic aberration 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 within the chromatic aberration range. For example, the chromatic aberration correction unit can be part of the correction unit or can be different from the correction unit. The interferometer can be chromatically corrected for measuring light having a spatial coherence length that is 1, 2, 5, 10, 25, 50, 100, 500, or 1000 times the wavelength of the light.

[0090] The inventors of the present application have found that by combining the shift unit and the correction unit, the telecentric interferometer can be designed in a simplified manner compared to other systems. According to the requirements of the application band, the spectral width of the necessary chromatic aberration correction is determined. For a simple setup (LED or superluminescent LED as the illumination device of the interferometer, or measuring photoluminescence), a dielectric flat mirror is suitable; for a wider range, an achromatic design with a curved mirror is possible. In this case, the chromatic aberration correction unit can be omitted. The highest requirements are achieved using chromatic aberration spot correction (a chromatic aberration balanced shift unit). All methods result in a system with image-side telecentricity of the two interferometer arms and equality of the paths of the central rays.

[0091] The measured interference pattern of the interferometer may include interference patterns, interference rings, and / or interference fringes. These structures are in different phase positions at different wavelengths, while the central ray (parallel to the axis of symmetry or z-axis) has the same phase position (zero interference) according to the chromatic aberration correction (correction unit) of the described and implemented path lengths. The different chromatic aberration 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, the "2π" phase cycles of the interference can be counted, and this number determines the preferred temporal coherence length of the light. For example, if the maximum phase difference in the phase interleaved phase images is 4π, then at least two wavelengths of temporal coherence may be required in the measured light. The placement of the principal planes (and thus, the conjugate planes) may result in a phase shift of the rays that are not the central ray. Away from the zero-order interference, the position of the fringes may be wavelength-dependent (referred to as "spot chromaticity" in the present disclosure). The principal planes (and thus, the conjugate planes) can be shifted according to the wavelength (e.g., by using a weak chromatic aberration lens) such that the central ray has an invariant zero interference, but the fringes shift with the wavelength in the direction opposite to the spot chromaticity by the variable principal planes, making the entire interference pattern stationary. This not only allows for the correction of the chromaticity of the path length, but also allows for the correction of the spot chromaticity, and thus also measures white light with a coherence length of, for example, one wavelength without a filter. A shift unit having this property is referred to as a chromatic aberration balanced shift unit. It may be desirable to correct the weak chromatic aberration lens by the chromatic aberration correction unit.

[0092] In some examples, the correction unit includes at least two dielectric elements having different refractive indices. Individually or in combination, the shift unit includes a dielectric plate that is also part of the correction unit.

[0093] In some examples, the shift unit may include at least one optical component having a focal power (e.g., refractive power). The optical component having a focal power can be a curved mirror and / or a lens (e.g., a weak lens and / or a strong lens). Hereinafter, the term "lens" refers to both refractive lenses and curved mirrors. The Gaussian reduction of all the optical elements in the first interferometer arm and / or the Gaussian reduction of all the optical elements in the second interferometer arm, including the correction unit (which typically has a zero focal power) and the lenses (having a focal power) in the first interferometer arm and the second interferometer arm, may have a finite focal length (i.e., a focal power different from zero) in this case.

[0094] In an alternative example of an interferometer, the first interferometer arm and / or the second interferometer arm may not have a beam expander, for example, a strong dielectric lens, a strong concave mirror, a strong convex mirror, or a combination of these optical components. A beam expander is an afocal system (Keplerian telescope or Galilean telescope). A strong optical component is an optical component with an F-number (absolute value) less than 50, or 30, or 20, or 10, or 5 or 2. For example, the portion of the displacement unit within the first interferometer arm and / or the second interferometer arm may not have such a beam expander. In particular, the displacement unit may be different from the beam expander. For example, if the displacement unit includes a weak lens and a strong lens, the weak lens may be located within one of the interferometer arms and may be different from a true afocal system (which excludes afocal systems such as mirrors or dielectric plates here), and the strong dielectric lens may be located outside the interferometer arm. The afocal system in this context is a telescope, such as a Keplerian telescope or a Galilean telescope. The weak lens may be located in the interference part of the system (i.e., the first interferometer arm and / or the second interferometer arm), and the strong lens may be located outside the interference part. For example, in the absence of countermeasures, a strong dielectric lens used as a beam expander (afocal system) may cause relevant monochromatic and chromatic aberrations that may have to be corrected (if the lens is located in the interferometer part). Additionally, chromatic aberration correction (with respect to the optical path length) may have to be performed. Due to the large number of optical components involved, such corrections are usually complex and expensive. For these and other reasons, such a setup with a beam expander in the interference part has several disadvantages compared to a compact setup according to aspects of the present disclosure. For the same reason, the interferometer arms generally may not include strong optical lenses or compound lens systems. However, the interferometer may include such lenses located outside the interferometer arms. A lens is considered strong in this context if its focal length is shorter than 8 times, 6 times, 4 times, 2 times, 1.5 times, 1.0 times, or 0.5 times or 0.2 times the maximum linear dimension of the system. The system size 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 start at a first position in the optical path, where the beams of the first interferometer arm and the second interferometer arm experience different positions or different optical transformations (e.g., through a lens). The interferometer arms terminate at the position where the optical paths of the two beams of the first interferometer arm and the second interferometer arm overlap for interference.

[0095] In some examples, the telecentric interferometer includes a phase-shifting unit that is arranged in or downstream of at least one of the first interferometer arm or the second interferometer arm. For example, the phase-shifting unit may be arranged in the first interferometer arm and / or the second interferometer arm. Individually or in combination, the phase-shifting unit may be arranged between the beam combiner and the detector that combines the two interferometer arms. The phase-shifting unit may include at least one of the following: a movable mirror (e.g., in the first interferometer arm and / or the second interferometer arm), such as by a piezoelectric crystal, a circular polarizer, a linear polarizer, a quarter-wave plate (e.g., an achromatic quarter-wave plate), or a polarization-sensitive detector. With the phase-shifting unit, images with different relative phase shifts can be acquired, and the interference terms at two phase positions (e.g., the true phase position and the imaginary phase position) can be determined therefrom. Thus, the phase-shifting unit can modulate the zero optical path difference between the two interferometer arms by an angular value less than 180° (or less than 360°) or less than half a wavelength (or the whole wavelength). The goal may be to determine the complex interference term (also referred to as the "complex interferogram") from different images. The complex interference term IF(x, y) can be given by: IF(x, y) = conjugate(E2(x, y)) * E1(x, y) from different images. IF(x, y) is a complex quantity determined by evaluating the intensity images. E1(x, y) and E2(x, y) respectively represent the electric fields originating from the first interferometer arm and the second interferometer arm, expressed 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 light field measured in self-interference. Thus, the device according to the present disclosure may be equipped with a phase-shifting unit and an evaluation unit for determining the complex interferogram IF.

[0096] According to some aspects, the phase shift (using the phase-shifting unit) may be different from the shift of the principal plane (using the shifting unit). The phase shift may be performed to determine the complex interference term based on the detected intensity. The principal plane shift may be performed to obtain sufficiently different electric fields E1 and E2 in different interferometer arms.

[0097] A further aspect of the present disclosure relates to a method for determining the characteristics of an input light field using a telecentric interferometer according to the examples described herein. The characteristics of the input light field may be or may include at least one of the following: the phase of the input light field or the amplitude of the input light field. The input light field may originate from an object. For example, the input light field is a coherent field or a partially incoherent field or a fully incoherent field. The input light field may include a number of light rays. The input light field may be monochromatic or polychromatic. The central ray of the input light field may define the optical axis of the interferometer.

[0098] According to some examples, the method includes propagating an input light field through an exit pupil of a front-side optical device and splitting the input light field into a first part (e.g., a first light field, which may be denoted as E1) and a second part (e.g., a second light field, which may be denoted as E2), where the first part propagates along a first optical system (e.g., along a first interferometer arm) and the second part propagates along a second optical system (e.g., along a second interferometer arm). A central ray of the first (second) part may define an optical axis of the first (second) interferometer arm.

[0099] The method may further include shifting an image-side principal plane of the first optical system relative to an image-side principal plane of the second optical system by using a shifting 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 an optical path length of the first interferometer arm is equal to an optical path length of the second interferometer arm. Measure the optical path length between the object and the detector. Thereby, the first part as measured on the detector is defocused relative to the second part as measured on the detector.

[0100] In some examples, the first part and the second part are combined into an output light field. An interference pattern (e.g., a complex interference pattern) of the output light field may be measured by using a detector. The interference pattern may result from interference between the first part of the input light field and the second part of the input light field that is shifted relative to the first part. Thus, the input light field may be interfered with by itself after its shift (e.g., after defocusing). Thus, the telecentric interferometer may be a reference-beam-free interferometer.

[0101] Due to the shift of the principal plane, when interfering on the detector, the first light field and the second light field may have different propagation lengths, but the optical path lengths of the central rays of the two light fields are the same. Thus, the central rays may exhibit a zero optical path difference. Since the image is usually not aberration-free (or not in focus), other rays of the light field have a path difference, which itself manifests as a difference in interference. Thus, on the detector, the intensity of the interference pattern is measured. According to the measurement of different interference patterns at different phase positions (via, e.g., a phase-shifting unit), the complex interference pattern IF may be determined.

[0102] The first light field and / or the second light field can be constructed from a complex interference pattern. This can correspond to physically and / or mathematically propagating the first light field and / or the second light field back to the exit pupil (the so-called "backpropagation"), and from there taking the path of another interferometer arm (e.g., the second interferometer arm in the case of backpropagating the first light field, and the first interferometer arm in the case of backpropagating the second light field) to the detector. At the detector, this field corresponds to the field from the other interferometer arm. Thus, the first light field and the second light field can be correlated. Since the optical imaging ratios of the first light field and the second light field can be the same up to the respective principal planes, these fields can be the same (but at different positions) in this plane. Therefore, in order to bring these fields to the same position, for example, the first light field can be brought 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 achieved by making the central rays have the same optical path length at the superposition position. Similarly, this process can be performed for conjugate planes, where the principal planes are replaced by conjugate planes. For this case, the difference between the two fields lies only in the 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 backpropagate the optical path to the exit pupil and then advance again to the principal plane or conjugate plane. This significantly simplifies the analysis and evaluation.

[0103] However, the entire field of view can be correct only if the system is telecentric on the image side, because only in this case is the overlap exact and the propagation direction is at a 90° angle to the detection plane. Thus, the relationship between the first light field and the second light field can be represented by optical propagation over the distance of the principal plane split. Mathematically, this is a unitary transformation (also known as a mapping). The unitary transformation can be represented by a propagator matrix U (the "mapping U"). The propagation can be represented mathematically, for example, as the propagator of the Fresnel integral or the Helmholtz equation, or the Fresnel diffraction integral of the Huygens–Fresnel principle, or the Debye integral (see, for example, "Born's Principles of Optics", M. Born and E. Wolf, Cambridge University Press, 8th printing 2013, chapter 8.2).

[0104] Thus, the interferometer can be adapted to measure the properties of the light field (the complex interference pattern IF), and the interpretation of the properties can be based on the knowledge of the principal plane split (and / or the knowledge of the mapping [unitary transformation] from the first light field to the second light field).

[0105] In at least some examples, an interferometer (e.g., the length of an interferometer arm) may be calibrated before and / or during and / or after measuring an object using 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 the interferogram in 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 with the measured interferogram. The measured interferogram may differ from the reference interferogram in global phase (i.e., e iφ ). The measured interferogram may be calibrated to the fixed phase of the known reference interferogram. The calibrated interferogram may remain unchanged in global phase even if there is a small change in the optical path length of the interferometer.

[0106] Calibration may be performed using a calibration light source (e.g., a laser source). The calibration light source may be blocked during the measurement of the image of the object. However, in this case, an additional detector may be required.

[0107] To evaluate the complex interferogram IF, it may be advantageous to distinguish whether the measurement is performed using spatially coherent light or spatially incoherent light.

[0108] In the case of spatially coherent light, the process is described in detail in the patent publication US10823547 B2. The mapping U that maps the first light field onto the second light field introduced in the present disclosure corresponds to the propagator mapping U in US10,823,357 B2. To apply the method, it may be necessary to know the intensities of the first light field and the second light field (i.e., |E1| 2 and |E2| 2 ). The measurement of this quantity may be done, for example, by blocking one of the interferometer arms in the interferometer and only measuring the intensity originating from the other interferometer arm. Alternatively or additionally, in addition to the interferogram, a separate second camera for the intensity image may be integrated into the setup, 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 may be the quantity to be determined in quantitative phase microscopy. Thus, the device may be used as a quantitative phase microscope.

[0109] A different situation is when recording a complex interference pattern IF with incoherent light. This process is illustrated in International Patent Application Publication US2022 / 034645A1. This publication relates to a situation where, for a given object position, a beam (e.g., a spot) produces a complex point spread function with its own self-reference. The point spread function contains a part of the diffraction of light at the exit pupil of the front-side optical device (see, for example, "Born Principles of Optics" by M. Born and E. Wolf, Cambridge University Press, 8th printing 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. The image information can be reconstructed, for example, by refolding using the point spread function, as described in US2022 / 034645A1. Refolding is the mathematical inverse of folding. The point spread functions of different z positions of the object point are different. Therefore, different z positions can be distinguished in the image. Thus, this method allows for 3D reconstruction of the object. The processes of folding and refolding can be referred to as propagation. The corresponding functions are also called propagation operators.

[0110] According to at least some aspects of the present disclosure, the measured interference spots can be focused and defocused via a mathematical propagation operator. Thus, the interference image at the new focus position can be calculated electronically. For example, assuming any such focus position, the set of Gaussian-like interference spots IF(x,y) can be squared point by point. A new image IF′(x,y) = IF(x,y)² corresponding to the new interference pattern is formed. 2 In this image, each spot can be re-sharpened by squaring, i.e., giving a sharper spot when propagated to focus. Additionally, ghost spots can be generated between all different pairs of spots at the half-lateral distance. This ghost spot is the opposite of re-sharpening: the image is sharper but includes artifacts. The process of squaring is typically done at an arbitrarily selected electronic distance position. If the results are compared for different orthogonal distances, it can be seen that the phase of each squared single spot (referred to as a single spot) is different from different the phase of the superposition of spots (i.e., the ghost spot halfway). The different behaviors of the single spot and the ghost spot allow determining which image parts originate from artifacts on multiple different frames. Thus, the re-sharpened image can be artifact-free.

[0111] According to at least some aspects, a method for determining characteristics of an input light field may include generating at least two intermediate interference patterns from a measured interference pattern, such as at mutually different focal points and / or at different electron focusing positions. For example, the intermediate interference patterns may be generated electronically, such as by convolution of the measured interference pattern with a propagation kernel of a selected propagation distance. Then, the at least two intermediate interference patterns may be processed algebraically. For example, the algebraic method may include at least one of the following: subtracting the processed images from each other (e.g., using a linear per-pixel complex weighting function), summing the squares per pixel, and / or multiplying by a chirp function. Before or after processing, a filtering function may be applied to the at least two intermediate interference patterns, thereby creating a resulting image. For example, the filtering function may be a linear combination of the intermediate images.

[0112] The method steps (i.e., generating intermediate interference patterns, processing intermediate interference patterns, and applying a filtering function) may be iteratively repeated, such as at least twice. The resulting image created in a previous iteration loop may be used as the measured interference pattern in a subsequent iteration loop. For each loop, the algebraic method of the previous loop may be used, or new per-pixel manipulation rules may be applied. The result may be a clearer image, i.e., fine features may be better detected. The resulting image may be less noisy because it is averaged over several images. The z-component value of the resulting image may have changed (electronically).

[0113] Compared to the original data, the resulting image may be focused on a plane different from the detector plane. This may correspond to electronic focusing and refocusing. Thereby, aberrations in the optical devices used (e.g., in the object, front-side optics, incident optics, and / or first optical system and / or second optical system) may be compensated, and at least a part of the image may be reproduced more clearly.

[0114] According to at least some aspects, the method may include reshaping the measured interference pattern by electronic deconvolution of the measured interference pattern with a reshaping function. The reshaping may be done before generating the at least two intermediate interference patterns. The reshaping function may be selected so as to avoid steps in the intermediate interference patterns. For example, the diffraction effect of the exit pupil of the front-side optics may be removed in a separate step to facilitate subsequent processing. For this purpose, the diffraction pattern is determined for a specific focal position (or z-position). The result may be referred to as the reshaping function. The correction may be done, for example, by refolding. This corresponds to the inverse of the convolution. The method allows removal of the diffraction effect of the set of object points in the interferogram. This corrected interferogram consists of a superposition of Gaussian-like complex interference spots.

[0115] The method may benefit from the incoherence of resolvable object points with respect to each other. Thus, the measured interference pattern (interferogram) may be a superposition of complex point spread functions. That is, interference of isolated object points may be measured in this method.

[0116] When different points in the object field that can be resolved by the front optical device receive mutually incoherent illumination light, the incoherent illumination can be referred to as "ideally incoherent". According to some aspects of the present disclosure, some or all of the light rays in the light field originating from the object can be coherent with each other. In this context, a "coherent light field" can refer to a situation where light rays interfere, as long as the path difference is within the coherence length. The term incoherent light is used to denote a light field in which the light rays are only coherent when they originate from the same point source. In addition, for the coherent case, it is assumed that the structure of the light field is characterized by a point-like disturbance from which the radial scattered light field originates. If the term "imaging" is used in this case, this means imaging the point-like disturbance. It may not be possible to image a possible background field (e.g., a field from bright-field illumination in a microscope) using an interferometer according to aspects of the present disclosure. The point-like disturbance may cause light to be emitted in other directions or result in a situation where light is lost (such as a shadow). A perturbation in the light field from coherent illumination can be an elastic scatterer, i.e., the outgoing light field is in a complex phase relationship, and the light fields from different scatterers can interfere as long as the optical path difference is less than the coherence length. Compared with incoherent light, the interference relationship of coherent light is usually more complex. If the object is illuminated coherently, the object can only emit a coherent scattered light field. Coherence may be required such that all the disturbances that interfere within the interferometer are within the coherence volume of the illumination. In other words: within spatial and temporal coherence, the disturbances are usually illuminated coherently. Illumination can be carried out in such a way that the illumination light enters the interferometer and reaches the detector (bright-field illumination); or in such a way that the illumination light does not enter the interferometer (dark-field illumination).

[0117] According to at least some examples, the object is illuminated appropriately. In particular, the illumination can allow the object to emit coherent and / or incoherent light rays. Due to the optical path equality, in the coherent case, the coherence requirement is also lower. In this case, possible illumination devices for the interferometer are lasers, superluminescent LEDs, or LEDs. In the incoherent case, incoherent illumination is required, such as critical illumination or Köhler illumination. The illumination can be, for example, transmission, reflection, or EPI illumination. Photoluminescence can also be measured, i.e., illumination blocked spectrally. When different points in the object field that can be resolved by the front optical device receive mutually incoherent illumination light, the incoherent illumination is called "ideally incoherent". This situation can also be described by the following expression: "incoherent within the resolution".

[0118] A further aspect of the present disclosure relates to an interferometer assembly. The interferometer assembly may include a telecentric interferometer (e.g., a lateral telecentric interferometer) according to the examples described herein. The interferometer assembly may further include an illumination device. The illumination device may be adapted to illuminate an object with illumination light such that different object points of the object coherently reflect or transmit a portion of the illumination light. Alternatively, the illumination device may be adapted to illuminate an 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 distance corresponding to the resolution of the front optical device are illuminated incoherently and thus scatter light that is mutually incoherent.

[0119] The illumination may be such that the non-scattered or specularly reflected illumination light reaches the detector (so-called "bright field"). Alternatively, the non-scattered or specularly reflected illumination light may not reach the detector (so-called "dark field"). The illumination may be performed such that different parts of the object are illuminated at different times (so-called "structured illumination") and / or within different wavelength ranges. The latter may be measured with different detectors via a beam splitter or a color filter with wavelength resolution.

[0120] Reference Figure 1 The schematic diagram of

[0121] The telecentric interferometer includes a detector 35 and a front-side optical device ( Figure 1The exit pupil 42 (not shown in []) 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 and the second optical system 20. The detector 35 may include a plurality of pixels. The detector may be or may include a CMOS sensor (e.g., a CMOS sensor array).

[0122] 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. For both the first optical system 10 and the second optical system 20, the corresponding optical properties (e.g., object-side focal lengths 141, 142, object-side principal planes 142, 242, object-side optical axes 143, 243, etc.) reflect the optical properties of the entire optical system.

[0123] First, referring to the object sides of the first optical system 10 and the second optical system 20. The object-side principal plane 142 of the first optical system 10 is the same as (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 the same as the object-side focal length 241 of the second optical system 20. In the present example, the incident optical devices 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. According to the present disclosure, other designs are possible.

[0124] The exit pupil 42 is located on the object side of the first optical system 10 and the second optical system 20. In other words: The object to be imaged ( Figure 1 (not shown in []) may be located upstream of the exit pupil 42, and the exit pupil 42 may be located between the first optical system 10 and the second optical system 20 on the one hand and the object on the other hand. 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 way, a telecentric interferometer (especially an image-side telecentric interferometer) can be achieved. Thereby, the image of the exit pupil 42 is shifted to infinity in the image space, which corresponds to a telecentric configuration.

[0125] Now refer to the first optical system 10 and the second optical system 20. The incident optical device 31 may be or may include at least one of the following: a lens, a (curved) mirror, an optical lens, a barrel lens, a microscope objective, a telescope, or another beam shaping element. The incident optical device 31 is located upstream of the beam splitter 32. In some examples, the beam splitter 32 may be part of the incident optical device 31. The beam splitter 32 can split the input light field into a first light field and a second light field ( Figure 1 not shown in). The first light field propagates through the first interferometer arm 11, and the second light field propagates through the second interferometer arm 21.

[0126] The first interferometer arm 11 and the second interferometer arm 12 have the same optical path length but different geometric 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 is explained for the shift unit 12 of the first optical system 10. The following description also applies to the shift unit 22 of the second optical system 20 with the necessary modifications. Compared with the first optical system 10 without the shift unit 12, the shift unit 12 of the first optical system 10 can change (e.g., shorten or extend) the geometric path length of the first interferometer arm 11. This results in a shift of the image-side principal plane 152 of the first optical system 10. The geometric path length of the first interferometer arm 11 is further adjusted so as to compensate for any introduced shift or optical path length difference of the first interferometer arm 11 relative 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 shift unit 12 of the first optical system 10, the geometric path length is reduced by the length d compared to the geometric path length of the first interferometer arm 11 without the shift unit 12 (and / or adjust d for any refractive index n in the optical path). As an example, the geometric or optical path can be adjusted by moving at least one mirror in one of the first interferometer arm 11 and the second interferometer arm 21 of a two-path interferometer. Preferably, the two interferometer arms 11, 21 have separate paths and mirrors (i.e., cannot be a common 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 such 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, wherein the optical path length of the first interferometer arm 11 is equal to the optical path length of the second interferometer arm 21.

[0127] The first optical system 10 and / or the second optical system 20 may also include corresponding correction units 13, 23. The correction units 13, 23 are adapted to correct the chromatic aberration change of the optical path length of the corresponding optical systems 10, 20 caused by the displacement units of the corresponding optical systems 10, 20. For example, the correction units 13, 23 may be dielectric plates. The focal length of the correction units 13, 23 may be close to infinity. Different from the depiction in Figure 1 , the correction unit 13 of the first optical system 10 (for example, at least a part of the correction unit 13) may be a part of the displacement unit 12 of the first optical system 10. The same may be true for the correction unit 23 and the displacement unit 22 of the second optical system 20, mutatis mutandis.

[0128] After passing through the first interferometer arm 11 and the second interferometer arm 21 respectively, the first light field and the second light field are combined using a beam combiner 33, and the combined light field propagates through an output optical device 34. For example, the output optical device includes a lens that is used to correct the different focal lengths of the optical components in the first interferometer arm 11 and the second interferometer arm 21, so as to then form corresponding image planes of the first interferometer arm 11 and the second interferometer arm 21 near the detector 25.

[0129] Now 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 caused by the displacement units 12, 22. The detector 35 is 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, 254 are separated from the image-side principal planes 152, 254 of the corresponding optical systems 10, 20 by the Gaussian image distances 151, 251 of the corresponding optical systems 10, 20. The detector 35 is defocused for both the image from the first optical system 10 and the image from the second optical system 20, but the deviation from the focus is small enough to allow the analysis of the interference pattern between the first light field and the second light field.

[0130] Figure 2 Illustrates the principle of a telecentric optical system in the case of a simple optical system having 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 drawn as a thin lens, that is, in the drawings, the object-side principal plane coincides with the image-side principal plane. The object-side focal length of the lens 311 is the same as the image-side focal length of the lens 311, that is, the refractive index (the focal length 311f of the lens 311) is the same. The object-side optical axis is also the same as the image-side optical axis (optical axis 311a). The same optical medium is assumed in the object space and the image space.

[0131] To explain the effect of the telecentric system, inFigure 2 The left side depicts small arrows as exemplary objects. An optical field 61 having a first light ray 61a, a second light ray 62b, and a chief ray 61c originates from an object point of the object. A central ray 61d extends along the optical axis 311a (for better visibility, the central ray 61d is depicted as being slightly off-axis). In Figure 2 In the simplified illustration, the image-side optical axis coincides with the object-side optical axis. The optical field 61 passes through the exit pupil 42 of the front-side optical device 41. For simplicity, it is assumed that the two principal planes of the optical device coincide. The first light ray 61a and the second light ray 62b may correspond to the outer light rays (so-called marginal rays) of the optical field 61 that can pass through the exit pupil 42. The chief ray 61c intersects the optical axis 311a at the exit pupil 42. Only as an example, the arrowhead is used as the object point from which the chief ray 61c originates. For simplicity, the front-side optical device 41 is positioned such that the distance from the first principal plane of the front-side optical device 41 to the object (arrow) is the focal length 411f of the front-side optical device 41. The lens 311 is separated from the exit pupil 42 by the focal length 311f of the lens 311. The lens 311 may be a Gaussian reduction of a much more complex optical system. For simplicity of the drawing, it is assumed that the splitting of the principal planes disappears.

[0132] The optical field 61 passes through the lens 311. On the image side of the lens 311, the image position 313 is separated from the lens 311 by the focal length 311f. 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.

[0133] This corresponds to an image-side telecentric system. Telecentricity in the object space or the image space requires that the chief ray 61c be parallel to the axis in the object space or the image space, respectively. Thus, even if the object plane or the image plane deviates from its nominal position, the apparent system magnification is constant. The image will be blurred, but have the correct size or magnification.

[0134] Reference Figure 3A and Figure 3B The schematic diagrams of and explain in detail the principles of the displacement units 21, 22 of the telecentric interferometer according to aspects of the present disclosure. Figure 3A Schematically illustrates a simplified scheme of the displacement units 12, 22. The displacement units 12, 22 include weak lenses 51, 52 and a strong lens 53. The strong lens 53 is located in both the first optical system 10 and the second optical system 20, but outside the first interferometer arm 11 and the second interferometer arm 21 ( Figure 3A not shown). In some examples, the displacement unit 12 of the first optical system 10 may include a first weak lens 51 (e.g., located in the first interferometer arm 11), and the displacement unit 22 of the second optical system 20 may include a second weak lens 52 (e.g., located in the second interferometer arm 21). For simplicity of the drawing, Figure 3AOnly the first weak lens 51 and the second weak lens 52 are depicted as one component. However, the first weak lens 51 and the second weak lens 52 are different lenses, and they are located in different interferometer arms 11, 12.

[0135] 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. In the thin-lens approximation, the principal planes 511, 521, 531 of the first weak lens 51, the second weak lens 52, and the strong lens 53 are considered separately (without principal-plane splitting).

[0136] The first weak lens 51 and the second weak lens 52 may have different focal lengths. 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 have the same magnitude as the focal length of the second weak lens 52. For example, the first weak lens 51 is a diffusing lens and the second weak lens 52 is a converging lens, or vice versa. The focal lengths of both the first weak lens 51 and the second weak lens 52 may be greater than the object-side focal length 53f of the strong lens 53.

[0137] The Gaussian-reduced combination of the first weak lens 51 and the strong lens 53 may have a first image-side principal plane 513, and the Gaussian-reduced combination of the second weak lens 52 and the strong lens 53 may have a second image-side principal plane 523. The first image-side principal plane 513 and the second image-side principal plane 523 have different positions. In other words, the first image-side principal plane 513 and the second image-side principal plane 523 are shifted relative to each other. The first image-side principal plane 513 and the second image-side principal plane 523 may also both 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 magnitude of the shift of the shift units 12, 22. If both the first interferometer arm 11 and the second interferometer arm 21 include weak lenses (i.e., the first weak lens 51 and the second weak lens 52, respectively), then the shift units 12, 22 are located in both the first optical system 10 and the second optical system 20.

[0138] Figure 3B An example of the shift units 12, 22 combined with the first interferometer arm 11 and the second interferometer arm 21 of the interferometer is illustrated. The light field 61 from the object passes through a front-side optical device having an exit pupil ( Figure 3B not shown in the figure). The light field 61 then passes through the incident optical device 31 of the interferometer. Imaging through the incident optical device 31 generates a real or virtual image 314 ( Figure 3B(not shown in the figure). For example, the virtual image 314 can also be at infinity. The shifting units 12, 22 can be independent of the position of the intermediate image 314. The light field 61 is then used as a beam splitter 32 of the beam combiner 33 to be split into a first light field 611 and a second light field 612. 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 diffuses 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 the 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 the object-side focal length 53f of the strong lens 53.

[0139] Both the first weak lens 51 and the second weak lens 52 are positioned at the image of the exit pupil 421 through the incident optical device 31. Due to the combination of the first weak lens 51, the second weak lens 52, and the strong lens 53 (which are combined to act as a shifting unit), the first light field 611 is shifted (i.e., defocused) relative to the second light field 612. Therefore, the first image-side principal plane 513 of the Gaussian reduced 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 reduced combination of the second weak lens 52 and the strong lens 53. Then, the light beam is imaged onto the detector 35 through an optical device (not shown). Due to the shifting unit, the image-side conjugate plane 154 of the first optical system 10 including the first interferometer arm 11 is different from the image-side conjugate plane 254 of the second optical system 20 including the second interferometer arm 21. The detector 35 can 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 the 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 the Gaussian image distance 542 of the second optical system 10. In Figure 3B the example shown, the Gaussian image distances 541 of the first optical system 10 and 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 intermediate image 314 of the object ( Figure 3B314) and the distance between 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 consequence 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 described layout and focal power of the strong lens 53 do not depend on the local position of the intermediate image 314, but on the local positions of the weak lenses 51, 52.

[0140] Figure 4 Schematically depicts an interferometer according to various aspects of the present disclosure. The shift unit of the interferometer is as shown in Figure 3B That is, the shift unit includes a first weak lens 51, a second weak lens 52 and a strong lens 53. Figure 4 The interferometer shown in FIG. 4 includes a front optical device 41 including an exit pupil 42. In some examples, the front optical device 41 can be an external optical device, and Figure 4 The rest of the optics depicted are part of the apparatus.

[0141] The incident optical device 31 is located downstream of the front optical device 41. The beam splitter / combiner 32, 33 is located after the incident optical device 31 (see also the above Figure 3B ). The beam splitters / combiners 32, 33 define the first interferometer arm 11 and the second interferometer arm 21. The first interferometer arm 11 comprises a first weak lens 51 in the form of a concave reflector. The second interferometer arm 21 comprises a second weak lens 52 in the form of a convex reflector. The strong lens 53 is located downstream of the beam splitters / combiners 32, 33. Some additional optical devices (as an example only, in Figure 4 ) and some optional additional exit optics ( Figure 4 21 ) after the strong lens 53. The strong lens 53 may be considered as part of the exit optics of the two interferometer arms 11, 21. The detector 35 is located downstream of the strong lens 53. The detector 35 may be located between the conjugate planes 154 and 254.

[0142] Entrance optics 31, first interferometer arm 11 (ie its optics) and strong lens 53 are part of first optical system 10. Entrance optics 31 and strong lens 53 and second interferometer arm 21 (ie its optics) are part of second optical system 20.

[0143] Light field 61 from an object passes through front-side optical device 41 and exit pupil 42. The light field 61 is then imaged by incident optical device 31 into a (virtual) intermediate image 314. The light field 61 passes through beam splitters / combiners 32, 33 and is split into a first light field and a second light field, which pass through first interferometer arm 11 and second interferometer arm 21 respectively, and are then recombined into a common light field. The light field propagates through strong lens 53 and reaches detector 35. The combination of first weak lens 51, second weak lens 52 and strong lens 53 results in a shift of image-side conjugate plane 154 of first optical system 10 relative to image-side conjugate plane 254 of second optical system 20. The detector can be placed within the range therebetween.

[0144] Figure 5A An interferometer is depicted in accordance with aspects of the present disclosure. The interferometer includes a front-side optical device 41 having an exit pupil 42, an incident optical device 31 having a lens 311, a beam splitter 32, weak lenses 51, 52 having principal planes 511, 521 (e.g., first weak lens 51 or second weak lens 52), a beam combiner 33, a strong lens 53 having a principal plane 531, and a detector 35. Reference numerals 143 and 243 denote optical axes.

[0145] The focal length 311f of lens 311 of incident optical device 31 can be, for example, at least 150 mm and at most 250 mm, such as 200 mm. Lens 311 can be a converging lens or a diffusing lens, depending on the (optional) other parts of incident optical device 31 ( Figure 5A not shown in the figure). For example, lens 311 is a tube lens (e.g., an achromatic tube lens) or a relay lens. Incident optical device 31 maps exit pupil 42 of front-side optical device 41 to principal plane 511 of first weak lens 51 (in the case of first interferometer arm 11) and / or principal plane 521 of second weak lens 52 (in the case of second interferometer arm 21). Thus, principal plane 511 of first weak lens 51 and principal plane 521 of second weak lens 52 are located in image plane 421 of exit pupil 42 via incident optical device 31.

[0146] Beam splitter 32 is located 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 depicted in Figure 5A the figure, which represents either first interferometer arm 11 or second interferometer arm 21. The following description also applies to the other interferometer arm mutatis mutandis.

[0147] The interferometer arms 11, 21 include weak lenses 51, 52 - in the case where the interferometer arm is the first interferometer arm 11, the first interferometer arm includes the first weak lens 51, and in the case where the interferometer arm is the second interferometer arm 21, the second interferometer arm 21 includes the second weak lens 52. The weak lenses 51, 52 are depicted as diffractive lenses, but converging lenses are also possible. For example, the first interferometer arm 11 may include a diffractive first weak lens 51, and the second interferometer arm 21 may include a converging second weak lens 52, or vice versa. The weak lenses 51, 52 may for example have a focal length of at least 50 cm and at most 200 cm, for example at least 80 cm and at most 120 cm, for example 100 cm (positive in the case of a converging lens and negative in the case of a diffractive lens). The weak lenses 51, 52 may be (deliberately) chromatic aberration lenses.

[0148] A beam combiner 33 (which may be an optical component different from or the same as the beam splitter 32) combines the two interferometer arms 11, 21. Downstream of the beam combiner 33, a strong lens 53 is located in the interferometer. The 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 the focal length 53f of the strong lens 53. The focal length 53f of the strong lens 53 may for example be at least 5 cm and at most 30 cm, for example at least 10 cm and at most 20 cm, for example 15 cm. A detector 35 is located on the image side of the strong lens 53, between the image-side conjugate plane 154 of the first optical system having the first interferometer arm 11 and the image-side conjugate plane 254 of the second optical system having the second interferometer arm 21.

[0149] The light field 61 originating from an object ( Figure 5A not depicted in the figure) passes through the exit pupil 42. The light field 61 includes a first ray 61a, a second ray 61b, and a principal ray 61c. The first ray 61a and the second ray 61b are equidistantly spaced from the principal ray 61c. When the object point moves to the optical axis, the principal ray 61c becomes the central ray 61d. The central ray 61d is drawn slightly off-axis for better visibility. The first ray 61a and the second ray 61b are equidistantly spaced from the principal ray 61c.

[0150] The light field 61 propagates through the lens 311 and is split into two parts at the beam splitter 32. The chief rays 61c in the interferometer arms 11, 21 cross the optical axes 143, 243 at the positions of the weak lenses 51, 52. After passing through the first interferometer arm 11 with the first weak lens 51 and the second interferometer arm 21 with the second weak lens 52 respectively, these two parts are combined by 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 of the optical path length of the first interferometer arm 11 relative to the optical path length of the second interferometer arm 21. The conjugate plane of the first optical system with the first interferometer arm 11 is shifted by the first weak lens 51, but the telecentric nature of the interferometer is not affected. With the necessary modifications, this also applies to the second interferometer arm 21 and the second weak lens 52. After passing through the strong lens 53, the first ray 61a and the second ray 61b still remain equidistantly spaced from the chief ray 61c (indicated by the dotted arrows). The chief ray 61c extends parallel to the optical axes 143, 243.

[0151] Figure 5B An interferometer according to aspects of the present disclosure is depicted. Figure 5B The interferometer depicted in is similar to Figure 5A the interferometer depicted in. Therefore, the differences are mainly explained below. Compared with Figure 5A the interferometer of Figure 5B the interferometer of includes incident optics having 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 is also possible that the first lens 311 and the second lens 312 are diffusing lenses, or one of the first lens 311 and the second lens 312 is a converging lens and the other is a diffusing lens. Figure 5B The Gaussian reduction of the focal length 311f of the first lens 311 and the focal length 312f of the second lens 312 shown in can be similar to Figure 5A the focal length of the lens 311 shown in. For example, the Gaussian reduction of the focal length 311f of the first lens 311 and the focal length 312f of the second lens 312 can be at least 10 cm and at most 30 cm. The distance between the first lens 311 and the second lens 312 can be selected such that it is 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 tolerance of ±5% of the distance). The combination of the first lens 311 and the second lens 312 can be, for example, a Keplerian telescope or a Galilean telescope or a afocal system.

[0152] In Figure 5BIn the interferometer shown, a light field having a first light ray 61a, a second light ray 61b, and a principal light ray 61c propagates through an exit pupil 42 of a front-side optical device 41. Thereafter, it propagates through a first lens 311 and a second lens 312. Different from the interferometer shown in Figure 5A where the light field is only parallel after passing through a strong lens 53, Figure 5B the interferometer shown in Figure 5A is configured such that the light rays 61a, 61b, 61c of the light field are parallelized by a combination of the first lens 311 and the second lens 312. Then, the parallelized light rays 61a, 61b, 61c propagate through a beam splitter 32, interferometer arms 11, 21 including a first weak lens 51 and / or a second weak lens 52, and a beam combiner 33. For the example depicted in Figure 5B in the interferometer depicted in

[0153] a reference Figure 6A and Figure 6B 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 of the geometric 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 remain equidistant from the principal light ray 61c. After passing through the strong lens 53, the principal light ray 61c extends parallel to the optical axes 143, 243. Finally, the light field is measured with a detector 35. Figure 6A and Figure 6B both show a part of an interferometer according to an example of the present disclosure. The interferometer includes an incident optical device 31 and a beam splitter 32 which also serves 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 Figure 6A and Figure 6B additional components not shown in

[0154] In the example shown in Figure 6A only the first interferometer arm 11 includes a dielectric plate (a first dielectric plate 361). In the example shown in Figure 6B both the first interferometer arm 11 and the second interferometer arm 21 include dielectric plates (a first dielectric plate 361 and a 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, more than one dielectric plate stacked together, where at least some of the dielectric plates may have different dielectric constants. The first dielectric plate 361 ( Figure 6A) or the first dielectric plate 361 and the second dielectric plate 362 have the function of the displacement unit of the interferometer.

[0155] The incident light field 61 is split by the beam splitters / combiners 32, 33 into a first light field 611 and a second light field 612. 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 Figure 6A and Figure 6B both, the first light field 611 propagates through the first dielectric plate 361. The first dielectric plate 361 causes a change in the optical path length and the geometric path length of the first light field 611. For example, if the displacement is not compensated, the first light field 611 is delayed relative to the second light field 612, or vice versa. In other words: if the displacement is not compensated, the first light field 611 may lag behind the second light field 612, or the first light field may lead the second light field 612. In Figure 6A the second light field 612 travels through the second interferometer arm 21 without being disturbed by any 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 path length than the first interferometer arm 11 (or, in the case where the first light field 611 leads the second light field 612 due to the shift, a shorter geometric path length).

[0156] In Figure 6B the second interferometer arm 21 also includes a dielectric plate (the second dielectric plate 362). The second dielectric plate 362 changes the optical path length and the geometric path length of the second light field 612. However, the second dielectric plate 362 is different from the first dielectric plate 361. Therefore, the change in the optical path length and the geometric path length of the first light field 611 is different from the change in the optical path length and the geometric path length of the second light field 612. This results in a change in the optical path length and the geometric path length of the first light field 611 relative to the second light field 612. To compensate for the relative change in the geometric path length, the geometric path length of one of the first interferometer arm 11 and the second interferometer arm 21 is selected to be longer than the geometric path length of the other of the two interferometer arms 11, 21.

[0157] 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 using the beam splitters / combiners 32, 33, and the combined output light field is propagated to the detector 35, where the interference pattern of the first light field 611 and the second light field 612 is measured. The interference is caused by the shift of the geometric path length of the first interferometer arm 11 relative to the geometric path length of the second interferometer arm 21, but the optical path lengths of the first interferometer arm 11 and the second interferometer arm 21 are equal.

[0158] Although specific examples have been illustrated and described herein, those of ordinary skill in the art will understand that various alternative and / or equivalent specific implementations may be substituted for the specific examples shown and described without departing from the scope of the present invention. This application is intended to cover any adaptations or variations of the specific examples discussed herein. Accordingly, the present invention is intended to be limited only by the claims and their equivalents.

[0159] It should be noted that examples of interferometers and / or methods and / or interferometer components as outlined in this document may be used independently or in combination with other examples disclosed in this document. Additionally, features outlined in the context of an interferometer or interferometer component also apply to the corresponding method, and vice versa. Moreover, all aspects of the examples of interferometers and / or methods and / or interferometer components outlined in this document may be combined arbitrarily. In particular, the features of the claims may be combined with each other in any way.

[0160] It should be noted that the specification and drawings merely illustrate the principles of the proposed method and system. Although not explicitly described or shown herein, those skilled in the art will be able to implement various arrangements that embody the principles of the present invention and are included within the spirit and scope of the present invention. Additionally, all examples and implementations outlined in this document are primarily and explicitly intended for explanatory purposes only to assist the reader in understanding the principles of the proposed method and system. Moreover, all statements providing the principles, aspects, and implementations of the present invention, as well as their specific examples, are intended to cover their equivalents.

Claims

1. A telecentric interferometer, the telecentric interferometer comprising: A front-side optical device, the front-side optical device including an exit pupil (42); A first interferometer arm (11), the first interferometer arm being part of a first optical system (10) and located on the image side of the front-side optical device; And A second interferometer arm (22), the second interferometer arm being part of a second optical system (20) and located on the image side of the front-side optical device; A detector (35), the detector being located on the image side of both the first optical system (10) and the second optical system (20); And A shifting unit (12, 22), the shifting unit being located between the front-side optical device and the detector (35); Wherein 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); Wherein the first optical system (10) and the second optical system (20) have the same-positioned object-side principal planes (142, 242) and the same object-side optical axes (143, 243); Wherein the exit pupil (41) of the front-side optical device is separated from the object-side principal planes (142, 242) by the object-side focal length (141, 241); and Wherein the shifting 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) such 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).

2. The telecentric interferometer according to the preceding claim, Wherein 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; Wherein the image-side conjugate plane (154) of the first optical system (10) is different from the image-side conjugate plane (254) of the second optical system (20); and Wherein 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).

3. The telecentric interferometer according to any one of the preceding claims, Wherein the image-side optical axis (153) of the first optical system (10) is the same as the image-side optical axis (253) of the second optical system (20).

4. The telecentric interferometer according to any one of the preceding claims, Wherein the shifting unit (12, 22) comprises: 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), and A strong lens (53) located on the image side of the first weak lens (51) and / or the second weak lens (52); wherein the first weak lens (51) and / or the second weak lens (52) is separated from the object-side principal plane (531) of the strong lens by the object-side focal length (53f) of the strong lens (53).

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

6. The telecentric interferometer according to any one of the preceding two claims, wherein the displacement unit (12, 22) includes the first weak lens (51) and the second weak lens (52); wherein the focal length of the first weak lens (51) and the focal length of the second weak lens (52) are equal in magnitude but opposite in sign, and / or wherein the first weak lens (51) or the second weak lens (52) is a concave lens or a convex mirror; and wherein the magnitude of the object-side focal length (53f) of the strong lens (53) is smaller than the focal length of the first weak lens (51) and the focal length of the second weak lens (52).

7. The telecentric interferometer according to any one of the preceding three claims, wherein the interferometer is configured in a Michelson-type configuration; wherein the displacement unit (12, 22) includes the first weak lens (51) and the second weak lens (52); wherein the first weak lens (51) is a concave mirror or a plane mirror, and the second weak lens (52) is a convex mirror.

8. The telecentric interferometer according to any one of the preceding claims, wherein the displacement unit (12, 22) includes dielectric plates (361, 362) located in the first interferometer arm (11) or the second interferometer arm (12); wherein the refractive index and / or thickness of the dielectric plates (361, 362) are selected such that: the optical path length of the first interferometer arm (11) is equal to the optical path length of the second interferometer arm (12), and the geometric path length of the first interferometer arm (11) is different from the geometric path length of the second interferometer arm (12).

9. The telecentric interferometer according to any one of the preceding claims, the telecentric interferometer further comprising: an incident optical device (31), the incident optical device being located between the front-side optical device on the one hand and the first interferometer arm (11) and the second interferometer arm (12) on the other hand; wherein the incident optical device (31) is part of both the first optical system (10) and the second optical system (20), and wherein the incident optical device (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. The telecentric interferometer according to any one of the preceding claims, the telecentric interferometer further comprising: An output optical device (34) located between the first interferometer arm (11) and the second interferometer arm (12) on one hand and the detector (35) on the other hand, wherein the output optical device (34) is part of both the first optical system (10) and the second optical system (20).

11. The telecentric interferometer according to any one of the preceding claims, wherein at least one of the first optical system (10) or the second optical system (20) includes a correction unit (13, 23), the correction unit being adapted to correct a chromatic aberration change in the optical path length in the first optical system (10) and / or the second optical system (20) caused by the displacement unit (12, 22), and wherein the focal length of the correction unit (13, 23) is close to infinity.

12. The telecentric interferometer according to the preceding claim, wherein the correction unit (13, 23) includes at least two dielectric elements having different refractive indices, and / or wherein the displacement unit (12, 22) includes dielectric plates (361, 362), the dielectric plates also being part of the correction unit (13, 23), and / or wherein the displacement unit (12, 22) includes at least one optical element having a refractive power, and at least one of the Gaussian reduction of all the optical elements in the first interferometer arm (11) or the Gaussian reduction of all the optical elements in the second interferometer arm (21), the displacement unit includes the correction unit (12, 22), having a finite focal length.

13. The telecentric interferometer according to any one of the preceding claims, the telecentric interferometer further comprising: a phase shift unit arranged in or downstream of at least one of the first interferometer arm (11) or the second interferometer arm (21); wherein the phase shift unit includes at least one of the following: 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 the characteristics of an input light field using the telecentric interferometer according to any one of the preceding claims, the method comprising: propagating the input light field through the exit pupil of the front-side optical device; dividing the input light field into a first part and a second part, wherein the first part propagates along the first optical system (20), and the second part propagates along the second optical system (20); using the displacement unit (12, 22) to displace the image-side principal plane (152) of the first optical system (10) relative to the image-side principal plane (252) of the second optical system (20) such 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); Combining the first portion of the input light field and the second portion of the input light field into an output light field; and Measuring an interference pattern of the output light field by means of the detector (35).

15. The method according to the preceding claim, the method comprising at least one of the following: (i) Generating at least two intermediate interference patterns from the interference patterns measured at different foci by convolution of the measured interference pattern with a propagation kernel of a selected propagation distance; (ii) Processing the at least two intermediate interference patterns algebraically; (iii) Applying a filtering function to the at least two intermediate interference patterns to create a resulting image.

16. The method according to the preceding claim, the method comprising Iteratively repeating steps (i) to (iii), wherein the resulting image created in a previous iteration cycle is used as the interference pattern measured in a subsequent iteration cycle.

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

18. An interferometer assembly, the interferometer assembly comprising A telecentric interferometer according to any one of claims 1 to 14, and An illumination device; wherein the illumination device is adapted to illuminate an object with illumination light such that different object points of the object: Coherently reflect or transmit a portion of the illumination light, or Incoherently scatter or transmit the illumination light within the resolution.

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