Ultra-compact optical system for 3D imaging

Abstract

The invention relates to an optical system (1) for 3D imaging comprising at least the following components: an input aperture (2) having an optical axis (OA), the input aperture (2) being configured to collimate object light (10) from an object (S) under study in a first spectral range, the first and second meta-lens arrays (3, 4) being configured to detect the object light (10) in a second spectral range, and a detector (5) being configured to detect the object light (10) in the first spectral range. The object light (10) propagates at a first inclination angle (beta) with respect to an optical axis (OA) of the input aperture (2) to the first meta-lens array (3), the first meta-lens array (3) being configured and arranged to focus a first portion (101) of the object light (10) and to remain a second portion (102) of the object light (10) unchanged, and wherein the second portion (102) of the object light (10) is at a second inclination angle (beta) with respect to an optical axis (OA) of the input aperture (2). The second metasurlens array (4) is configured and arranged to collimate the focused first portion (101) and to transmit the second portion (102) constantly such that the first portion (101) and the second portion (102) form a second tilt angle (beta ') with respect to their respective propagation directions, from each other, and strike the detector (5) to form an interference pattern, where the first portion (101) and the second portion (102) form a second tilt angle (beta') with respect to each other. The second tilt angle (beta ') corresponds in size to twice the first tilt angle (beta).

Description

Technical Field

[0001] The present invention relates to an optical system for 3D imaging as claimed in claim 1 . Background Art

[0002] Optical 3D imaging systems are known in the art. These systems evaluate holographic interference patterns to determine the three-dimensional position, surface structure, or composition of an object. These holographic systems typically use a scanning beam (called the object beam) and a reference beam (called the reference beam). These beams converge in a holographic unit, where the temporal and spatial coherence of the light creates an interference pattern on a detector. Various evaluation methods can be used to infer wavefront information, thereby obtaining 3D information about the object being scanned by the object beam.

[0003] However, these holographic systems have several drawbacks. They often require moving components, take up a large installation space, have relatively poor spatial resolution, or have complex and costly manufacturing processes.

[0004] Despite this, these systems remain important, particularly in minimally invasive medicine. 3D imaging applications are now also finding their way into mobile phones. In this field, completely different systems are used that are not based on holographic principles. Summary of the Invention

[0005] Therefore, an object of the present invention is to provide a 3D imaging system to overcome the above disadvantages.

[0006] The problem solved by the invention is solved by the system according to claim 1 .

[0007] Advantageous embodiments of the invention are given in the dependent claims and are described below.

[0008] Accordingly, an optical system for 3D imaging is provided, which has at least the following components:

[0009] - an input aperture with an optical axis,

[0010] - a first metalens array and a second metalens array,

[0011] - detector,

[0012] wherein the input aperture is configured to collimate object light from an object under investigation within a first spectral range, in particular when the input aperture is equipped with a lens and the object light originates from a focal plane of the input aperture, and to fix the object light in a predefined polarization state consisting of two mutually conjugated polarization states, wherein the object light propagates to a first metalens array at a first tilt angle, β, relative to an optical axis of the input aperture, wherein the first metalens array is configured and arranged to focus a first portion of the object light, the first portion comprising a first polarization state of the two mutually conjugated polarization states, and to leave unchanged a second portion of the object light, the second portion comprising The second polarization state of the two mutually conjugated polarization states, in particular, maintains a collimated state, wherein the second metalens array is configured and arranged behind the first metalens array along the propagation direction of the object light, for collimating the focused first portion and transmitting the second portion unchanged, in particular, maintaining the collimated state, so that after passing through the first and second metalens arrays, the first portion and the second portion each have the same wavefront curvature, in particular, are both in a collimated state, and form a second tilt angle β' with respect to their respective propagation directions, β'=2*β, and impinge on the detector to form an interference pattern. The second tilt angle corresponds to twice the magnitude of the first tilt angle, and three-dimensional position information of the object region of the object under investigation can be generated based on the interference pattern.

[0013] By using a meta-lens, the system according to the present invention successfully provides a holographic imaging system that solves the above-mentioned problems.

[0014] Metalenses or metalens arrays can be manufactured on a flat surface, without the need for complex grinding processes to create a curvature radius, as is the case with traditional lenses. However, more importantly, metalenses can be manufactured in a specific way to perform different optical operations or exhibit different properties depending on polarization, wavelength, or angle [1].

[0015] Polarization is particularly important in this context because it can be controlled across wavelengths. Ling Li et al. [2] describe how a single metalens can change its focal length depending on polarization. Depending on the design of the metalens, it can also operate over a broadband spectral range without changing its focusing properties. This is fundamentally different from diffractive structures such as gratings or holograms, which inherently diffract light depending on wavelength. Metalenses achieve these properties in part because their structural dimensions are smaller than the optical wavelength and significantly smaller than the size of a typical camera pixel of 2μm-5μm. In summary, the state of the art shows that metalenses can be designed with numerical apertures of NA>0.5 and optical bandwidths greater than 100nm in various color bands within the red, green, and blue (RGB) color range.

[0016] In particular, the input aperture may comprise an optical component, such as a lens or a plurality of lenses arranged in a lens arrangement, such as an objective lens. Alternatively, the input aperture may also be free of lenses and comprise only a pinhole aperture which, when the object to be investigated is at a sufficiently large distance from the pinhole aperture (e.g., in the range of about 100 mm to 500 mm), images the object light incident from the object to be investigated into the system and achieves a sufficient degree of collimation of the object light.

[0017] Furthermore, the input aperture has an optical element configured to impose a predefined polarization state on the object light coming from the object under investigation. For example, such an optical element can be a polarizer.

[0018] In the context of this specification, the term "input aperture" specifically refers to the area in front of the first meta-lens array, which means that the input aperture does not necessarily refer only to the opening of the system. The term "input aperture" can also cover all optical components and elements of the system that are arranged in front of the first meta-lens array in the direction of object light propagation.

[0019] The term "meta-lens array" particularly refers to an arrangement of a plurality of meta-lenses, wherein the optical axes of the individual meta-lenses are substantially parallel to each other. In particular, the optical axes of the individual meta-lenses are parallel to the optical axis of the input aperture.

[0020] The dimensions of a single metalens, such as its diameter, are known in the art and, in particular, may range from a few millimeters, for example, 0.2 mm to 10 mm.

[0021] In particular, each metalens in the metalens array may correspond to at least one focal length, which may depend on, for example, the polarization state of the incident object light.

[0022] Furthermore, the focal length of the metalens, and thus the focal length associated with the first and / or second metalens arrays, may also be wavelength dependent.

[0023] According to one embodiment of the present invention, the focal lengths of all metalenses in the first metalens array are the same.

[0024] According to another embodiment of the present invention, the focal lengths of all metalenses in the second metalens array are the same.

[0025] According to another embodiment of the present invention, the average focal lengths of the first metalens array and the second metalens array are the same.

[0026] The term "average focal length" particularly refers to the focal length associated with a respective metalens array, which focal length is particularly obtained by averaging the focal lengths of all metalenses arranged in the array, particularly wherein the average focal length corresponds to the average value.

[0027] According to one embodiment of the invention, the detector comprises a camera having a plurality of light-sensitive pixels, wherein the light-sensitive pixels are configured to record at least object light from a first spectral range. In this way, the detector can be used to record interference patterns.

[0028] The recorded interference pattern can then be transmitted in the form of data to an evaluation unit associated with or included in the system, which generates three-dimensional information or a three-dimensional representation of at least the investigated object or a region thereof from the transmitted data.

[0029] In the context of this specification, the term "collimated" and related terms should be understood to mean that, at least for one wavelength, and in particular for a certain wavelength or spectral range, the light has only minimal wavefront curvature. For deviating wavelengths, the convergence or divergence of the light beam or corresponding wavefield generally increases with wavelength. Within the context of the present invention, these deviations from the ideal collimation state due to dispersion are also included in the meaning of the term "collimated." The term "collimated" also includes deviations due to calibration and system tolerances.

[0030] In particular, if the system has no lens at the input aperture, then as understood by those skilled in the art, the explanation in the previous paragraph still applies, that is, the collimation does not need to be perfect, i.e., the light is imaged into the system in a slightly diverging or converging manner.

[0031] In particular, it should be noted that the system is also designed to image and / or process non-collimated object light on the detector in a similar manner. In particular, in the context of the present description, the system utilizes non-collimated light beams to generate depth information, i.e., 3D information about the object under investigation. The description of the invention based on collimated light beams is particularly intended only to clarify the relative positions and functions of the various components of the system, but is not intended to exclude the recording and / or processing of light beams that are not collimated by the input aperture.

[0032] In particular, within the scope of the present invention, the term "collimated" and related terms are also understood to mean that the light beam converges and / or diverges to a degree that reaches a divergence or convergence angle ξ, which is in the range 0°<ξ≤2*β max In the range of max is the maximum first tilt angle that the system can image or record.

[0033] Alternatively or additionally, in the context of this specification, the term "collimated" and related terms may also be understood to mean that the beam diameter (herein understood as the beam bundle) has a minimum divergence angle at its narrowest point. This definition of a collimated beam is applicable in the field of wave optics description of laser beams and can be similarly applied to the present system. This definition applies in particular to input apertures having at least one lens.

[0034] According to the invention, the first tilt angle may be measured relative to the propagation direction and the optical axis of the input aperture.

[0035] In terms of the wave representation of the object light, this means that the plane wavefront of the collimated object light after passing through the input aperture has an angle of 90° + β with respect to the optical axis of the input aperture.

[0036] The predefined polarization state of the object light after the input aperture comprises two polarization states that are conjugated to each other. These two polarization states may in particular be two linear polarization states, in particular a p-polarization state and an s-polarization state.

[0037] Alternatively, the two polarization states may also represent right-handed and left-handed circular polarization states.

[0038] It should be noted that the predefined polarization state is in particular a superposition state of the two conjugated polarization states.

[0039] It should be noted that if object light with a predefined polarization state is incident on the first metalens array, due to its optical properties, the first metalens array will focus the object light with the first of the two conjugated polarization states, specifically onto the focal plane associated with the first metalens array. Conversely, due to its optical properties, the first metalens array transmits the object light with the second of the two conjugated polarization states essentially unchanged. In other words, for the object light with the second polarization state, the first metalens array does not increase its convergence or divergence, but rather behaves essentially like an optically transparent medium without diffraction properties, that is, like a neutral optical medium, such as a homogeneous glass plate.

[0040] Therefore, the object light incident on the first metalens array is separated into a first portion and a second portion, the first portion consisting of the object light in the first polarization state and the second portion consisting of the object light in the second polarization state.

[0041] Ideally, this separation is in a ratio of approximately 1:1, that is, the object light is split into two parts of equal intensity.

[0042] The two portions of the object light are then incident on a second metalens array having optical properties that are identical to, or at least similar to, those of the first metalens array.

[0043] Specifically, the second metalens array is arranged so that it recollimates the first portion and transmits the already collimated second portion substantially unchanged. Similarly, with respect to the second portion, the second metalens array essentially acts like a penetrating neutral optical medium, as previously explained for the first metalens array.

[0044] In particular, the system is designed, for example, by means of corresponding optical components, such that the angle at which the first portion is incident on the first metalens array is reversed relative to the first tilt angle as it propagates through the second metalens array. In this way, the first portion and the second portion form a second tilt angle relative to each other. The second tilt angle can be measured relative to the propagation direction of the first portion and the second portion, or alternatively but equivalently, as the angle between the wavefronts of the first portion and the second portion.

[0045] Due to the design according to the present invention, the second tilt angle is twice the first tilt angle, especially when the focal lengths associated with the first metalens array and the second metalens array are the same.

[0046] The first and second parts are superimposed behind the second meta-lens array, forming an interference pattern on the detector. Based on this interference pattern and its analysis, three-dimensional information about the object area under investigation can be generated.

[0047] The object area includes one or more illumination areas illuminated by the object light, wherein the illumination area on the object under investigation is substantially circular with a diameter ranging from 1 mm to 50 mm.

[0048] The term "focusing" or related terms particularly refers to setting the wavefront curvature, which causes the corresponding light beam or light wave cluster to converge, that is, to be imaged as a minimum beam diameter (focus) at a certain position in space.

[0049] For example, by optical scanning, in particular by relative displacement of the system with respect to the object under investigation, complete three-dimensional information about the object under investigation can be generated. In addition, or alternatively, the system can simultaneously image, record and analyze multiple object regions of the object under investigation.

[0050] It should be noted that the object light reflected from the object at a certain distance from the input aperture cannot be collimated by the input aperture, but has different wavefront curvatures, thereby generating divergent or convergent object light.

[0051] This situation usually occurs when the input aperture consists only of a pinhole aperture rather than a lens, because the pinhole aperture has no corresponding focal length. However, if the object light is incident on the pinhole aperture from a sufficiently long distance, the pinhole aperture will make the object light highly collimated, or in other words, have a low enough divergence, so that the object light is considered collimated within the context of the present invention.

[0052] However, even if the object light is non-collimated within the scope defined in this specification, the system will image and record this beam of light based on physical principles and can include it in the corresponding evaluation to obtain 3D information of the object area of the object under investigation (for example, the surface of the object under investigation).

[0053] In any case (whether collimated or non-collimated), the second portion of the object light passes through the first meta-lens array and the second meta-lens array unchanged according to the above principles. The same is true for the first portion focused by the first meta-lens array, wherein the first portion does not fall on the focal plane associated with the first meta-lens array, but rather falls in front of or behind it, and wherein the second meta-lens array generates the first portion with a wavefront curvature that is modified accordingly based on the wavefront curvature.

[0054] These originally uncollimated first and second parts also form an interference pattern on the detector and can be evaluated accordingly in order to obtain three-dimensional information about the object region.

[0055] According to another embodiment of the present invention, it is provided that the first part and the second part of the object light are respectively linearly polarized perpendicular to each other, in particular s-polarized and p-polarized, and in particular wherein the predefined polarization state is a linearly polarized polarization state formed by superposition of the first part and the second part.

[0056] Linearly polarized light is relatively easy to generate. Furthermore, if the polarization is linear along one direction, the polarization state can be relatively easy to determine relative to its polarization. In contrast, distinguishing between circular and elliptical polarization states can be more difficult. Multiple optical elements are configured to separate conjugate linearly polarized light or manipulate them differently, such that linear polarization of the first and second portions can be advantageous.

[0057] According to yet another embodiment of the present invention, the system comprises a polarization-dependent beam splitter, in particular a polarization-dependent beam splitter cube, between the first metalens array and the second metalens array, wherein the system further comprises the following components:

[0058] - a first reflector, in particular wherein the first reflector is planar

[0059] - a reflector array comprising a plurality of reflective retroreflectors,

[0060] a first λ / 4 element arranged between the polarization-dependent beam splitter and the first mirror,

[0061] a second λ / 4 element arranged between the polarization-dependent beam splitter and the reflector array,

[0062] wherein the polarization-dependent beam splitter is arranged relative to object light incident from the first meta-lens array so that a first portion is reflected by the beam splitter and a second portion is transmitted through the beam splitter, wherein the reflector array is arranged to reflect the first portion back in a direction toward the beam splitter and guide it to the second meta-lens array, wherein the first reflector is arranged to reflect the second portion back in a direction toward the beam splitter and guide it to the second meta-lens array, in particular, due to the polarization states being reversed by the respective λ / 4 elements, the back-reflected first portion and the back-reflected second portion propagate through the polarization-dependent beam splitter toward the second meta-lens array, in particular, wherein the first portion and the second portion each pass through the first or second λ / 4 element twice, such that the first portion and the second portion have reversed polarization states after passing through the respective λ / 4 elements for the second time.

[0063] In other words, the polarization-dependent beam splitter is arranged relative to object light incident from the first meta-lens array so that a first portion is reflected by the beam splitter and a second portion is transmitted by the beam splitter, wherein the reflector array is arranged on one side of the beam splitter, and the first portion from the first meta-lens array and reflected by the beam splitter propagates toward this side, wherein the first portion incident on the reflector array is reflected back in a direction toward the beam splitter and directed to the second meta-lens array, wherein the first reflector is arranged on a side of the beam splitter opposite to the first meta-lens array, that is, on a side of the beam splitter from which the second portion from the first meta-lens array and transmitted through the beam splitter propagates, wherein the first reflector reflects the incident second portion back in a direction toward the beam splitter and directs it to the second meta-lens array, in particular, due to the inverted polarization states, the back-reflected first portion and the back-reflected second portion pass through the polarization-dependent beam splitter toward the second meta-lens array, and the polarization-dependent beam splitter is particularly configured to reflect one of the two portions of the object light while transmitting the other portion.

[0064] A λ / 4 wave plate, for example, can be used as a λ / 4 element. In other words, it can serve as an optical delay element with different refractive indices for different polarization directions. This allows the polarization state of the object light to be altered. According to the present invention, when the first and / or second portion passes through the optical element twice, each portion assumes its conjugate polarization state. For example, two passes through the λ / 4 element convert the s-polarization state to the p-polarization state, and vice versa.

[0065] As a result, each portion of the object light reflected from the polarization-dependent beam splitter propagates toward the second meta-lens array.

[0066] When using this configuration, it should be noted that the focusing properties of the second metalens array must be designed according to the polarization properties of the object light so that it can collimate the focused first portion. In other words, if no further optical elements are arranged before the second metalens array (which can convert the polarization states of the first and second portions back to the initial polarization states assigned after the input aperture), the focusing properties of the second metalens array should in each case point to a conjugate polarization state compared to the first metalens array.

[0067] According to yet another embodiment, the number of retroreflectors included in the reflector array is the same as the number of metalenses in each of the first metalens array and the second metalens array.

[0068] According to yet another embodiment of the present invention, it is proposed to arrange a first λ / 2 element between the polarization-dependent beam splitter and the second meta-lens array, wherein the first λ / 2 element is configured to invert the polarization states of the first and second portions, in particular, so that the polarization states of the first and second portions again correspond to the polarization states of the first and second portions of the object light after passing through the input aperture.

[0069] The first λ / 2 element is arranged in particular in such a way that it passes in the propagation direction of the first and second part only after the first and second part have propagated twice through the beam splitter or the beam splitter surface of the beam splitter, respectively.

[0070] This embodiment makes it possible to use a second metalens array that is identical to the first metalens array, but this second metalens array does not need to have opposite characteristics with respect to polarization as in the previous paragraph, but rather has the same characteristics with respect to polarization as the first metalens array.

[0071] In particular, the first λ / 2 element is a λ / 2 wave plate.

[0072] This enables the system to be produced in a cost-effective and simplified manner.

[0073] According to a further embodiment of the invention, it is proposed that the system comprises an actuator arrangement configured to set the position of the first mirror and / or the reflector array such that a phase is set between the wavefront associated with the first portion and the second portion.

[0074] This design makes it possible to adjust the relative phase between the first and second portions, thereby, in particular, avoiding the appearance of uniform light portions on the detector. In particular, the actuator arrangement is configured to adjust the position of the first mirror and / or the reflector array in such a way that a phase shift greater than 2π is possible. This has particular advantages for the color resolution of the system.

[0075] The actuator arrangement should be configured to move the position of the first mirror and / or the reflector array by a fraction of a wavelength of the first spectral range, in particular along the optical axis. In particular, for adjustment purposes, it may be advantageous if the actuator arrangement is configured to tilt the first mirror and / or the reflector array relative to the optical axis of the input aperture.

[0076] Furthermore, it may be advantageous if the actuator arrangement is configured to adjust the position of the reflector array perpendicular to the optical axis of the input aperture.

[0077] The actuator arrangement is used in particular to extend or shorten the optical path length of the first and / or second portion of the object light.

[0078] The actuator arrangement may be controlled by an external control unit associated with the system.

[0079] According to the invention of this embodiment, the actuator arrangement comprises at least one piezoelectric element. In particular, the actuator arrangement comprises at least one annular piezoelectric arrangement.

[0080] Although strictly speaking the piezoelectric element is the moving part, its relatively integrated design means that there is no particular risk of wear from exposed delicate mechanics, so the system can largely be considered to have no moving parts despite the presence of the piezoelectric element.

[0081] The use of piezoelectric elements contributes in particular to increasing the robustness of the system. Piezoelectric actuators can also be controlled and adjusted particularly accurately and precisely.

[0082] According to an alternative embodiment without a beam splitter, it is proposed that the system includes a transparent solid element, a first surface of which faces the direction of the first meta-lens array, and a second surface opposite to the first surface faces the direction of the second meta-lens array, in particular, wherein there are no selectively reflecting surfaces and selectively diffractive surfaces within the volume enclosed by the transparent solid element, and in particular, wherein the transparent element has a rectangular parallelepiped or plate shape.

[0083] This embodiment may be advantageous if a particularly compact design is desired along the mounting direction, for example along the optical axis of the input aperture.

[0084] The solid transparent element may be an ordinary glass plate, or an ordinary polymer plate, which is transparent in the first spectral range.

[0085] According to one embodiment of the invention, the solid transparent element is made of a material selected from glass, polymer or crystal.

[0086] In the embodiment using a beam splitter, the metalens arrays formed a 90° angle with each other, whereas in this embodiment, the metalens arrays are positioned exactly opposite each other and enclose the solid transparent element between them.

[0087] According to yet another embodiment of the present invention, the system comprises at least one liquid crystal configured to adjust a phase between wavefronts of object light associated with the first portion and the second portion, in particular, wherein the at least one liquid crystal is configured to change the phase between the wavefronts of the first portion and the second portion by a control module.

[0088] For example, in addition to the solid transparent element, a liquid crystal element can be disposed between the first and second metalens arrays along the optical axis of the input aperture. Alternatively, the solid transparent element can include, or be composed of, liquid crystal. In the latter embodiment, the liquid crystal should exhibit birefringence.

[0089] In the case of an embodiment using a beam splitter, the liquid crystal may be arranged on one side of the array where the first mirror or reflector is located.

[0090] In yet another embodiment, if the beam splitter is a beam splitter cube, one of the prisms constituting the beam splitter may contain liquid crystal, or be composed of liquid crystal.

[0091] Alternatively, both prisms forming the beam splitter can also include liquid crystals. In this way, the phase of the first and second components of the object light can be independently set relative to each other. Furthermore, the use of two liquid crystals results in a longer total optical path, enabling a greater phase shift between the first and second components.

[0092] Using liquid crystals to set the relative phase of the first and second parts makes it possible to create a system with no moving parts at all, making the system extremely robust.

[0093] According to one embodiment of the present invention (described in the previous paragraph), it is proposed that the transparent solid element comprises at least one liquid crystal, or consists of at least one liquid crystal element.

[0094] According to one embodiment of the present invention (which has been described in the previous paragraph and includes the polarization-dependent beam splitter cube according to at least one of the aforementioned embodiments), it is proposed that the polarization-dependent beam splitter includes a first prism and a second prism constituting the beam splitter cube of the beam splitter, wherein the first prism and / or the second prism includes at least one liquid crystal, in particular, wherein both the first prism and the second prism have liquid crystals.

[0095] According to one embodiment of the present invention, the focal plane of the first metalens array coincides with the focal plane of the second metalens array.

[0096] According to one embodiment of the present invention, a polarizer is arranged behind the second meta-lens array and in front of the detector in the propagation direction, and the polarizer is configured to change the polarization state of the first part and the second part so that the first part and the second part interfere with each other on the detector.

[0097] Such an embodiment enables a higher interference contrast to be achieved on the detector.

[0098] According to an embodiment of the present invention, it is proposed that the input aperture comprises a polarizer configured to bring the object light from the first spectral range into a predefined polarization state.

[0099] According to an embodiment of the present invention, it is proposed that the input aperture comprises at least one lens configured to collimate the object light.

[0100] According to an alternative embodiment of the invention, it is proposed that the input aperture comprises a pinhole aperture as imaging element, in particular wherein the input aperture does not comprise a lens or a refractive optical element for collimating the object light coming from the object under investigation.

[0101] According to one embodiment of the present invention, the system is configured to deflect the propagation direction of object light from a first spectral range incident on the system in a wavelength-dependent manner, so that the object light and the first part and the second part form, in addition to the first tilt angle, a wavelength-dependent angle with the optical axis.

[0102] For example, this can be achieved by designing a corresponding metalens for the first metalens array.

[0103] This embodiment can improve the color resolution of the system, since object light in the first spectral range is imaged onto different regions of the detector depending on the wavelength.

[0104] According to one embodiment of the present invention, it is proposed that the object light in the first spectral range consists of two or more non-overlapping wavelength ranges, and / or, wherein, the system is configured to filter the object light into two or more non-overlapping wavelength ranges to constitute the first spectral range, wherein there are gaps between the wavelength ranges, in particular wherein each of the gaps has a width of at least 50 nm, so that an interference pattern is generated on the detector for each wavelength range, and based on the interference pattern, three-dimensional position information and color composition of each wavelength range relative to the object area of the object under study can be obtained.

[0105] In particular, the first spectral range is divided into three primary colors: red, green and blue, which may correspond to the following wavelength ranges, for example:

[0106] The wavelength range of the first spectral range of the blue channel is particularly between 420 nm and 480 nm, the wavelength range of the first spectral range of the green channel is particularly between 520 nm and 565 nm, and the wavelength range of the first spectral range of the red channel is particularly between 630 nm and 680 nm.

[0107] According to one embodiment of the present invention, it is proposed that the object light includes at least one additional spectral range that is different from and does not intersect with the first spectral range, wherein the first meta-lens array and the second meta-lens array and the polarization-dependent beam splitter are transparent and optically ineffective, i.e., neutral, for light from the at least one additional spectral range, wherein the polarization-dependent beam splitter also includes a VPH (volume phase hologram) that is configured to diffract the light from the at least one additional spectral range in a polarization- and angle-dependent manner, and is transparent and optically ineffective, i.e., neutral, for light from the first spectral range.

[0108] The first spectral range and the at least one additional spectral range can occupy alternating wavelength ranges along the spectrum. In particular, with respect to the previous embodiment, for example, the additional spectral range can be arranged between the green channel and the red channel, in particular, it can be limited to the spectral range of 570 nm to 620 nm. Alternatively and / or in addition, the at least one additional spectral range can extend from the near-infrared range, that is, in particular from 700 nm or 800 nm up to the infrared range of more than 1300 nm. Since the VPH is arranged in particular in a Littrow arrangement and is designed for wavelengths from the additional spectral range, incident object light from the additional spectral range (also referred to as the second spectral range in the context of this specification) is diffracted at an angle of 90° towards the detector when it propagates along the optical axis of the input aperture and is incident on the VPH. In particular, the incident object light from the second spectral range is s-polarized when it is incident on the VPH.

[0109] In this embodiment, in particular, the reference light coupled into the beam splitter via the reference arm interferes on the detector with the object light coupled into the beam via the so-called object arm.

[0110] According to one embodiment of the invention, it is therefore proposed that the system is configured to guide reference light from the second spectral range, in particular through the reference arm, via the side of the beam splitter opposite the input aperture to the VPH, wherein the first mirror is transparent in particular to the reference light, i.e. in particular to the light from the second spectral range.

[0111] According to this embodiment, reference light (especially also s-polarized when incident on the VPH) can be collimated from the second spectral range on the side of the beam splitter opposite the input aperture and also sent to the VPH in a Littrow configuration (for this purpose, the first mirror must be transparent for the second spectral range). Here, the reference light is also diffracted at an angle of 90° in the direction opposite to the detector. Here, the reference light is reflected by a mirror or prism arrangement, wherein the mirror and / or prism arrangement forms an angle of more or less than 45° with the VPH, for example 45°±0.2°. As a result, the reflected reference light is at least partially transmitted at the VPH and interferes with the object light in the second spectral range. This interference pattern can be advantageously used to generate a higher spatial resolution along the optical axis of the input aperture. In particular, the additional spectral range consists of a plurality of non-intersecting spectral lines with very small linewidths, for example, in the sub-nanometer range, and extending, for example, to a range of 10 nm to 50 nm. Because the VPH diffracts each of these spectral lines slightly differently, this type of "optical comb" can be used to produce improved spatial resolution along the optical axis from the interference pattern produced on the detector.

[0112] Similar holographic systems are known, but they employ, for example, conventional transmission diffraction gratings instead of VPHs, which however have disadvantageous properties with respect to the required performance.

[0113] It should be noted that, for the functioning of this embodiment, it is advantageous if the first reflector is at least partially transmissive with respect to light in the second spectral range.

[0114] Furthermore, it can be advantageous if the second λ / 4 element is in particular optically neutral, ie does not have a significant influence on the light.

[0115] Alternatively, a third λ / 4 element can be provided, for example, arranged in front of the first mirror, as seen from the direction of the incident reference light, that is, in particular, inaccessible to the object light in the first spectral range. The third λ / 4 element is configured to rotate the polarization of the reference light by 90° together with the second λ / 4 element, in particular so that the reference light is incident on the VPH in a favorable s-polarization state.

[0116] This is described in the following embodiment of the invention, according to which the system includes a third λ / 4 element, which is arranged on the side of the first mirror facing away from the beam splitter and is configured to work in conjunction with the second λ / 4 element to put the reference light in a predefined polarization state so that when the reference light comes from the first mirror and impinges on the VPH, it is linearly s-polarized.

[0117] Furthermore, it would be advantageous if the polarization-dependent beam splitter also does not have any optical effect on light in the second spectral range, that is, it is also optically neutral. Furthermore, it is desirable for the first and second metalens arrays to be optically neutral with respect to light from the second spectral range.

[0118] The reference light in the second spectral range can be provided by a reference light source, such as a laser, which can also be used as an object light source. Furthermore, the system can include a collimating lens for the reference light so that the reference light can be collimated and propagated toward the bulk VPH. A polarizer can also be provided to ensure that the reference light is incident on the VPH in an s-polarization state.

[0119] According to an improvement to the previous embodiment, there is provided: a system having a wavelength-selective prism arrangement between the reflector array and the beam splitter, which is configured to reflect light (in particular reference light of at least one additional spectral range diffracted by the VPH) towards the detector at a prism angle, and wherein the prism device is transparent and optically ineffective for light from the first spectral range.

[0120] As mentioned before, the prism angle is set such that the reflecting surface of the prism arrangement forms an angle with the VPH which is not equal to 45°, wherein the size of the formed angle is in particular greater than 45.2°.

[0121] This "tilted position" means that the reference beam reflected from the prism arrangement can pass through the VPH with sufficient transmission, so that sufficient reference light is available for interference with the object light in the second spectral range at the detector. By expanding the system in this way, the resolution along the optical axis can be improved to submillimeter levels, or even submicrometer levels. BRIEF DESCRIPTION OF THE DRAWINGS

[0122] Further features and advantages of the present invention are explained below with reference to the accompanying drawings of exemplary embodiments. In the drawings:

[0123] Figure 1 shows a schematic diagram of a first embodiment of the present invention;

[0124] Figure 2 A schematic diagram showing a second embodiment of the present invention having a color separation function;

[0125] Figure 3 A schematic diagram showing a third embodiment of the VPH of the present invention;

[0126] Figure 4 A schematic diagram showing a fourth embodiment of the present invention without a beam splitter. DETAILED DESCRIPTION

[0127] for Figure 1 A system 1 according to an exemplary embodiment of the present invention is shown in schematic form. The system 1 is configured for use in holographic imaging applications. In particular, the system 1 is suitable for three-dimensional color imaging.

[0128] Figure 1 A particular advantage of the system shown is its extremely compact design, in particular due to the fact that it does not require any moving parts or delicate mechanical components that could be susceptible to failure.

[0129] In the context of the present invention, Figure 1 The system 1 shown is also referred to as a compact 3D color module and is configured to capture the surface or the area below the surface of an object S (in this specification, alternatively referred to as an object) in three dimensions and display it when necessary.

[0130] To this end, the object S is illuminated, for example, in a point-like region, using an external light source 18, which may be included in the system 1 or provided separately. In the context of holography, the light 10 used to illuminate the object S under investigation is also referred to as object light 10. Natural ambient light can also serve as light source 18—even if it does not necessarily illuminate the point-like region of the object S.

[0131] After being illuminated, the object S reflects object light 10 through various processes such as scattering, reflection, or luminescence. The object light 10 originating from the object S is collected through the input aperture 2 of the system 1.

[0132] In the following content and part of this specification, the situation considered is that the object light S originating from the object S comes from or is close to the plane E of the object S, and this plane is located at the focal plane E of the input aperture 2. When the object light 10 comes from a plane near the focal plane E of the input aperture 2, the system 1 will respond accordingly based on the different bending forms of the wavefront of the object light 10 at the input aperture 2, which is well known to those skilled in the art.

[0133] The input aperture 2 is configured to collimate object light 10 originating from the object S under investigation, in particular from the focal plane E of the input aperture 2. It should be noted that the input aperture 2 may have a collimating optical element 2a, for example in the form of one or more lenses 2a. Figure 1 In the illustrated case, the input aperture 2 comprises a collimating lens 2a. However, the collimating optical unit 2a may also comprise only a pinhole aperture (not shown). If the pinhole aperture is small enough and / or the object distance is large enough, the incident object light 10 will also be collimated, or at least have a sufficiently high degree of collimation.

[0134] Regardless of the design of the input aperture 2, collimated light refers in particular to light having a substantially planar wavefront.

[0135] Furthermore, the input aperture 2 has an optical axis OA. Figure 1 The optical axis OA in the optical system extends at a central position and is perpendicular to the input aperture 2, or in other words, perpendicular to the collimating lens 2a. The collimating lens 2a is configured to collimate at least the object light 10 from the first spectral range. In addition, the collimating lens 2a can also be configured to collimate the object light 10 from the second spectral range. In this case, for example, the collimating lens 2a can include an achromatic lens, an apochromatic lens, or a superachromatic lens.

[0136] These figures use ray optics to represent light. This means that the wavefronts and curvatures of the wavefronts in the depicted beams are generally not shown. However, those skilled in the art will understand how the wavefronts of light are affected by the various optical components of system 1 and how this affects the direction of light propagation or, in other words, the properties of the light.

[0137] Figure 1 Two different beams 10a, 10b of object light 10 are shown. The first case relates to a first object light beam 10a originating from a region of the object under investigation S located on the optical axis OA of the input aperture 2. The second case relates to a second object light beam 10b originating from a region of the object under investigation that is laterally offset relative to the optical axis of the input aperture 2. For example, the term "laterally offset" may be used using a Cartesian coordinate system associated with the input aperture 2 (e.g., Figure 1The z-axis extends along the optical axis OA of the input aperture 2, while the x-axis and y-axis extend perpendicular to the z-axis, that is, extend transversely with respect to the optical axis OA. The terms "first" and "second" are used for distinction only and do not indicate order.

[0138] First object beam 10a is collimated by collimating lens 2a and then propagates parallel to the optical axis OA of collimating lens 2a. In other words, it forms an angle of 0° with the optical axis OA of input aperture 2. In the context of this specification, the angle formed by the collimated object beams 10, 10a, 10b with the optical axis OA is also referred to as the first tilt angle β.

[0139] The first tilt angle β is particularly defined by the angle between the optical axis OA of the input aperture 2 and the propagation direction of the collimated object light beam 10, 10a, 10b.

[0140] The second object beam 10b is also collimated by the collimator lens 2a and then propagates further, but at a first inclination angle β with the optical axis OA of the collimator lens 2a that is not equal to 0°. According to the laws of ray optics (in interaction with the input aperture 2), this first inclination angle β contains information about the lateral position on the object under investigation from which the associated object beam 10b originates.

[0141] A polarizer 15 is arranged after the collimating lens 2 a and is configured (and optionally arranged) to bring the object light 10 into a predefined polarization state. In particular, the polarizer 15 is configured in such a way that it brings the object light 10 into a predefined polarization state—at least for the object light 10 from the first spectral range.

[0142] It should be noted that the polarizer 15 can also be arranged in front of the collimator lens 2a. The incident object light 10 is first collimated by the collimator lens 2a downstream of the polarizer 15. It is also conceivable that the collimator lens 2a or the input aperture 2 itself has corresponding polarization characteristics.

[0143] The terms "rear" and "front" are to be understood in particular as an arrangement relative to the propagation direction of the object light 10, and less as referring to its geometrical order or arrangement, which may differ from a "purely optical" light path due to bending of the light path caused by beam splitters or reflectors.

[0144] In the present case, the polarizer 15 is arranged and configured (e.g., at a rotation angle of 45°) in such a way that, regardless of the direction of incidence or the first tilt angle β, the object light 10 has approximately equal proportions of s-polarized light and p-polarized light. This means that in this example, the predefined polarization state consists of a first polarization state comprising s-polarized object light and a second polarization state comprising p-polarized object light. It is assumed that those skilled in the art are familiar with the terms s-polarized light and p-polarized light as linear polarization directions that are conjugated to each other. The classification of s-polarized object light as the first polarization state and p-polarized light as the second polarization state can also be reversed, and this is done here merely for ease of explanation.

[0145] In general, the polarizer 15 can be configured to convert the incident object light 10 into object light 10 having two polarization states that are conjugated to each other. For example, these can also be left-handed and right-handed circularly polarized object light 10.

[0146] In the following, without loss of generality, the cases of s-polarization and p-polarization will be discussed. This is generally true because the object light originating from the object under investigation may also be unpolarized. In any case, even without the polarizer 15, the first meta-lens array will "see" this object light as the first part and the second part, because the unpolarized light also includes an s-polarization component and a p-polarization component.

[0147] The object light 10 having the predefined polarization state just described now impinges upon the first metalens array 3 according to the present invention. The first metalens array 3 comprises a plurality of metalenses 30 arranged in an array. According to the present invention, the first metalens array 3 is arranged and configured such that it focuses a first portion 101 of the object light having a first polarization state, i.e., at least increases the wavefront curvature, thereby converging the object light, while allowing a second portion 102 of the object light having a second polarization state to pass through substantially unchanged. This means that if the object light has already been collimated by the input aperture 2, the second portion 102 remains collimated after passing through the first metalens array 3.

[0148] In the context of the metalens arrays 3 and 4, the term "unchanged" specifically refers to the fact that the wavefront of the second portion 102, or the wavefront in the second polarization state, ideally passes through the metalens arrays 3 and 4 completely unchanged, such that the wavefront of the second portion 102, or the wavefront in the second polarization state, remains unchanged before and after the metalens arrays. Those skilled in the art will appreciate that slight variations in the wavefront may occur due to imperfections in the metalens arrays 3 and 4. This is intended to be reflected in the term "substantially." In the context of this specification, a synonymous term is "optically neutral."

[0149] In particular, the focused first portion 101 is focused in such a way that it is focused on the focal plane 3B associated with the first metalens array 3 .

[0150] The polarization-dependent beam splitter 6 is arranged behind the first metalens array 3, in particular, in front of the focal plane 3B of the first metalens array 3. Figure 1 In the illustrated example, the polarization-dependent beam splitter 6 is a polarization-dependent beam splitter cube. The beam splitter cube 6 includes two prisms, for example, a first prism 6A and a second prism 6B, which are connected at their bottom surfaces and define a beam splitter surface 6F along the bottom surface. The beam splitter surface 6F is angled 45° with respect to the optical axis OA of the input aperture 2, or in other words, with respect to the optical axis OA of the first metalens array 3.

[0151] The first object beam 10a including the first and second portions, and the second object beam 10b also including the first and second portions 101 and 102, now impinge on the first prism 6A of the beam splitter 6 and pass through it to the beam splitter surface 6F.

[0152] The object light 101 in the first polarization state of the first object light beam 10a and the second object light beam 10b is reflected at the beam splitter surface 6F, while the object light 102 in the second polarization state is transmitted therethrough.

[0153] Next, let us first consider the object beam of the first portion 101, which has been focused by the first metalens array 3, that is, is in the first polarization state after the first metalens array 3 and is therefore reflected at the beam splitter surface 6F. This light continues to propagate through the first prism 6A and then impinges on the first λ / 4 wave plate 9a, which is arranged on the surface of the beam splitter cube 6, for example.

[0154] The first λ / 4 wave plate 9a changes the polarization state of the first portion 101, in this example, from linear polarization to circular polarization.

[0155] Behind the first λ / 4 wave plate 9a, a reflector array 8 is arranged perpendicular to the (correspondingly warped) optical axis OA of the input aperture 2. The reflector array 8 includes a plurality of retroreflectors 80 arranged in an array, which are configured to reflect light in the direction of its origin, largely independent of the direction of incidence of the incident light 101. Furthermore, the reflector array 8 is arranged adjacent to or parallel to the focal plane 3B of the first metalens array 3, offset from the focal plane 3B.

[0156] The number of retroreflectors 80 included in the reflector array 8 is the same as the number of metalenses 30 included in the first metalens array 3 , and the number of retroreflectors 80 included in the reflector array 8 is also the same as the number of metalenses 40 included in the second metalens array 4 .

[0157] The first portion 101 reflected back by the reflector array 8 now passes through the first λ / 4 wave plate 9a again, which changes the polarization state of the first portion 101 so that the polarization state of the first portion 101 now corresponds to the second polarization state of the object light after the input aperture 2 - in this example, therefore, after the first portion 101 passes through the λ / 4 wave plate 9a a total of two times, the polarization state of the first portion is changed from s-polarized light to p-polarized light.

[0158] Next, the first portion 101 passes through the first triangular prism 6A again and strikes the beam splitter surface 6F. Since the polarization state has changed, the first portion 101 is now transmitted. The first portion 101 then passes through the second triangular prism 6B and strikes the second metalens array 4, which is arranged on the beam splitter 6, opposite the reflector array 8.

[0159] Now let's look at the second portion 102 of the object light. The second portion 102 of the object light, i.e., the portion having the second polarization state, passes through the second triangular prism 6B of the beam splitter 6 after the beam splitter surface 6F, and then passes through the λ / 4 wave plate 9b, which is disposed, for example, on the side of the beam splitter cube 6 opposite the first metalens array 3.

[0160] The second λ / 4 wave plate 9b changes the polarization state of the second portion 102, in this example, from linear polarization to circular polarization. After the second λ / 4 wave plate 9b, the plane mirror 7 is arranged perpendicular to the optical axis OA of the input aperture 2 and reflects the second portion 102 of the object light back. The reflected second portion 102 of the object light now passes through the second λ / 4 wave plate 9b again, which changes the polarization state of the second portion 102 of the object light again, so that the polarization state of the second portion 102 of the object light now corresponds to the first polarization state of the object light after the input aperture 2. Therefore, after the second portion 102 of the object light passes through the second λ / 4 wave plate 9b twice, the polarization state of the second portion 102 of the object light changes from p-polarization light to s-polarization light.

[0161] The second portion 102 reflected by the first reflector 7 then passes through the second triangular prism 6B and is incident on the beam splitter surface 6F again. At the beam splitter surface 6F, the second portion 102 is reflected due to a change in its polarization state. Therefore, the second portion 102 continues to pass through the second triangular prism 6B and, like the first portion 101, is incident on the second metalens array 4.

[0162] The second metalens array 4 has substantially the same characteristics as the first metalens array 4 and comprises a plurality of metalenses 40 arranged in an array. However, unlike the first metalens array 3, in this example, the second metalens array 4 has the characteristic of maintaining the polarization state of the light in the second portion 102 unchanged, particularly with respect to collimation, and is capable of collimating the light in the first portion 101. This means that if the object light from the input aperture 2 is already collimated and propagates toward the first metalens array 3, then the light in both the first portion 101 and the second portion 102 will be collimated after the second metalens array 4.

[0163] An analyzer 14 is positioned behind the second metalens array 4 to match the polarization states of the first portion 101 and the second portion 102, so that the light beams from the first portion 101 and the second portion 102 interfere with each other. For example, the analyzer 14 is positioned at an angle of 45°, so that the matched first portion 101 and the second portion 102 have the same polarization direction.

[0164] Arranged behind the analyzer 14 is a detector 5 configured to record the interference of the first and second portions 101, 102 of the object light. The detector 5 can be, for example, a camera. The interference of the first and second portions 101, 102 of the object light forms an interference pattern on the detector 5, which can be evaluated by an evaluation unit (not shown) to generate three-dimensional image information.

[0165] The following should be noted regarding object light 10, which forms a first tilt angle different from 0° with respect to optical axis OA. Due to the special arrangement of the optical components of system 1 according to the present invention, the first and second object beams 101, 102 of the object light entering system 1 at a first tilt angle β, i.e., the object beams that form the first tilt angle β with respect to optical axis OA after input aperture 2, are then reflected at a second tilt angle β' by reflector array 8 or plane mirror 7. This second tilt angle β' is twice the first tilt angle β and is achieved by the combination of plane mirror 7 and reflector array 8. While first portion 101 has an incident angle β after reflection from reflector array 8, second portion 102 has an exit angle -β after reflection from plane mirror 7. These angles sum to twice the first tilt angle, i.e., 2β, which corresponds to the second tilt angle β'.

[0166] The advantage of this arrangement is that the lateral resolution of the system 1 is improved compared to other systems. In addition, such a system 1 does not include any moving precision mechanical parts, which makes the system 1 extremely robust and allows for a very compact design.

[0167] The system 1 according to the present invention enables a person skilled in the art to calculate the wavefront of the object light based on the local wavefront angle of each metalens in the second metalens array, or in other words, based on the interference frequency of the interference pattern generated on the detector, which in turn allows to infer the z-direction deviation from the focal plane of the input aperture, that is, in addition to the lateral spatial resolution, it is also possible to generate image information related to the z position of the area of the object under investigation.

[0168] As an alternative to the optical properties of the second metalens array 4 , it is conceivable to design the second metalens array 4 to be identical to the first metalens array 3 , that is, to be completely identical in terms of its optical properties with respect to polarization states.

[0169] In this case, a λ / 2 wave plate 11 (indicated by a dotted line) is arranged behind the beam splitter cube 6 on the side where the second metalens array 4 is located, and in front of the second metalens array 4. The λ / 2 wave plate rotates the polarization states of the first portion 101 and the second portion 102 by 90°, so that they again correspond to their originally predefined polarization states. The embodiment using two identical metalens arrays 3 and 4 has the advantage that the system 1 and the metalens arrays 3 and 4 can be manufactured at a relatively low cost.

[0170] In a further advantageous embodiment (for at least one embodiment, Figure 1 ), the system 1 includes means for changing the phase of the light waves of the first portion 101 and / or the second portion 102 of the object light. This prevents interfering direct current or uniform light signals from appearing on the detector 5. To this end, the phase in the first portion or the second portion is typically achieved by slightly changing the light path (also called the light path length).

[0171] The change of the optical path can be achieved by actually geometrically extending the path of the first portion and / or the second portion, or by changing the refractive index of the medium through which the first portion and / or the second portion passes.

[0172] exist Figure 1 In FIG. 1 , the phase adjustment is achieved by an actuator 12 which is able to translate the reflector array 8 at least along the optical axis OA of the input aperture 2 (even if warped due to the beam splitter cube).

[0173] For example, the actuator 12 may include an electrically controllable piezoelectric element. In particular, the piezoelectric element may be a piezoelectric ring element.

[0174] Thus, the actuator 12 is able to change the optical path of a first portion 101 of the object light, which comes from the input aperture 2 and is reflected by the beam splitter 6 in the direction of the reflector array 8. By shifting the reflector array 8 along the optical axis OA, the relative phase with respect to the second portion 102 of the object light can be set.

[0175] Alternatively or in addition, a further actuator 12 ′ may also be arranged on one side of the first mirror 7 and configured to move the mirror 7 at least along the optical axis OA of the input aperture 2 .

[0176] It is also possible that the actuator 12 controls the reflector array 8 and another actuator 12 ′ controls the first mirror 7 , so that the optical paths of both the first part 101 and the second part 102 of the object light can be changed.

[0177] Alternatively or additionally, it is possible to generate a refractive index change in one or both prisms 6A, 6B of the beam splitter cube 6, thereby being able to adjust the light wave phase of the first portion 101 and / or the second portion 102 of the object light (not shown in the figures). The refractive index can be changed by applying a voltage to the first and / or second prisms 6A, 6B. To this end, the prisms 6A, 6B should be made of materials familiar to those skilled in the art.

[0178] Alternatively, the phase change of one of the two polarization states can also be set by the first and / or second metalens arrays 3, 4. This can be achieved, for example, by phase shifter elements or by novel modifications of the metalens arrays themselves that allow the phase of the polarization direction to be changed by applying electrical or magnetic variables.

[0179] Regardless of how these phase shifts are generated, the system 1 is designed in particular to be able to achieve relative phase shifts exceeding 2π, so that, in addition to suppressing uniform light in the interference pattern of a single wavelength or wavelength range, the phase position can be used to separate different colors or wavelength ranges that are far apart. For example, a corresponding separation can be achieved by Fourier transformation, and this method is generally familiar to those skilled in the art.

[0180] based on Figure 1 , Figure 2 An extension of the invention is shown for all three primary colors (RGB) of system 1. Although phase shifts within a relatively large range (several 2π) have been described in the previous paragraphs, this may limit the dynamic range of the detector 5, since the signals of all primary colors exhibit a high degree of interference in the central region around the system optical axis OA (now relative to the detector) (and less interference at the edges), and thus undesirable signal amplification may occur. Therefore, Figure 2An embodiment is shown in which the three primary wavelengths are selectively slightly deflected in the region of the first metalens array 3 away from a central region around the optical axis OA. This can be achieved by using localized prisms, gratings, or volume Bragg gratings (not shown) that are superimposed on the actual lens effect of the first metalens array 3.

[0181] Alternatively or additionally, this characteristic can also be achieved by the first metalens array 3 itself. Figure 2 The wavelength-selective deflection of three different wavelengths (also referred to as three primary colors RGB in the context of this specification) by the first meta-lens array 3 is shown in the form of light beams (arrows 21 , 22 , 23 ).

[0182] Each of the three wavelengths / colors for each object point or each object region generates an interference pattern or interference pattern with a spatial frequency and direction on the detector 5. Figure 1 The described phase shift enables a person skilled in the art to separate the respective spectral image components, so that by evaluating the interference pattern on the detector 5 using an evaluation unit, 3D color image information with spatial resolution can be generated.

[0183] In order to improve the resolution of the system 1, especially the resolution along the optical axis OA, the system 1 can be improved by using a beam splitter 6 designed as follows. In this way, the system can achieve extremely high resolution along the z-axis, especially in the sub-micrometer range.

[0184] exist Figure 3 In the exemplary embodiment shown, the beam splitter cube 6 includes a transmission diffraction grating arrangement in the form of at least one volume phase holographic grating (VPH) 16 along the beam splitter surface. The VPH 16 is a volume phase holographic (transmission) grating generally known to those skilled in the art. In addition, the system includes a wavelength-selective prism arrangement 17, in particular a wavelength-selective dual prism arrangement 17, on the side of the beam splitter cube 6 opposite the detector 5.

[0185] It should be noted that, according to this example, the VPH 16 and the prism arrangement 17 are configured for the second spectral range, in particular, for the first spectral range of the object light to be transparent, that is, the wavefront of the first spectral range passes unchanged through the VPH 16 and the prism arrangement 17. In contrast, the prism arrangement 17 is configured such that light from the second spectral range is reflected at the prism surface 17F, while light from the first spectral range passes through the prism surface 17F and thus passes through the prism arrangement 17 unchanged.

[0186] The VPH 16 is also configured to diffract light from the second spectral range according to angle and wavelength, and allow light from the first spectral range to pass through it unchanged.

[0187] In contrast, the metalens arrays 3, 4, the polarizer 15, the λ / 4 wave plates 9a, 9b, and (if applicable) the λ / 2 wave plate 11, also behave as transmissive (in particular, optically neutral) and do not alter the wavefront and polarization state of the object light in the second spectral range. The system 1 utilizes light from the second spectral range to produce a particularly high spatial resolution, in particular along the optical axis.

[0188] and Figure 1 The optical path of the object light in the first spectral range shown in FIG is different. Figure 3 The optical paths of object light 20-1 and reference light 20-2 are shown, which in this example also lie in the second spectral range. Reference light 20-2, at least when incident on the VPH, is preferably also s-polarized. It is emitted by reference light source 19, such as a single-mode aperture, and collimated by collimating optical element 25. The optical axis of collimating optical element 25 coincides with the optical axis of input aperture 2. Reference light 20-2 then enters beam splitter cube 6 in this collimated state from the side of beam splitter cube 6 opposite input aperture 2. To achieve this, the flat first mirror 7, which reflects object light 10 from the first spectral range, must be transparent to reference light 20 from the second spectral range. This means that mirror 7 is at least a dichroic mirror and does not alter the wavefront of the reference light. The second λ / 4 wave plate 9a can also be configured not to change the polarization state of the reference light 20-2, or, by interacting with another phase delay element (e.g., the third λ / 4 wave plate 9c), can be configured so that the reference light 20-2 is s-polarized when incident on the VPH16.

[0189] The mode of operation of the VPH 16 when interacting with the prismatic arrangement 17 will now be described.

[0190] In the example shown, object light 20-1 from the second spectral range, collimated by input aperture 2, is incident on VPH 16 at an angle of approximately 45°. Since VPH 16 is in a so-called Littrow configuration and is optimized for the second spectral range, incident object light 20-1 from the second spectral range is diffracted by VPH 16 at an angle of approximately 90° (relative to incident object light 20-1), or approximately 45° (relative to beam splitter surface 6F) toward detector 5. Ideally, object light 20-1 from the second spectral range is linearly polarized, particularly s-polarized, when incident on VPH 16, because the diffraction efficiency of VPH is maximized at this time.

[0191] On the other hand, the collimated reference light 20-2 is also incident on the VPH 16 and is diffracted by it toward the prism arrangement 17, which is arranged on the side of the beam splitter 6b opposite to the detector 5. Here again, the VPH 16 is at a 45° position relative to the reference light 20-2, so that a Littrow configuration is also present here.

[0192] Reference light 20-2 propagates toward prism arrangement 17 and is reflected by reflective surface 17F of prism arrangement 17, which forms an angle α with the beam splitter surface and the VPH. In this case, angle α (not shown; alternatively, differential angle (prism angle) Δα = 45° - α) is not equal to 45°, but is greater than or less than 45°, and in particular, differs from 45° by more than 0.2°. As a result, the reference light diffracted from VPH 16 at an angle of 45° toward prism arrangement 17 is reflected by prism arrangement 17 back onto VPH 16, where reference light 20-2 is incident on VPH 16 at an angle different from 45°. However, as a result, at least a portion of reference light 20-2 reflected back from VPH 16 is not diffracted again toward reference light source 19, but instead passes through VPH 16 toward detector 5, where it interferes with diffracted object light 20-1 at an angle corresponding to twice angle α.

[0193] The core idea of this embodiment is that the beam splitter cube 6 has a VPH 16 along its beam splitter surface 6F, and the angle α formed by the VPH and the reflecting surface 17F of the prism arrangement 17 is not equal to 45°. As a result, when the reference light 20-2 is incident on the VPH 16 for the second time after being reflected by the prism arrangement 17, at least a portion of the reference light 20-2 is transmitted out of the VPH 16 in the direction of the detector 5 to a sufficiently high extent.

[0194] In particular, the light from the second spectral range is provided in the form of an optical frequency comb comprising a plurality of non-overlapping narrowband spectral lines, in particular having line widths in the sub-nanometer range.

[0195] Due to the properties of VPH 16, the multiple spectral lines are diffracted slightly differently in a wavelength-selective manner (see beams 20-1', 20-2'). This results in a dispersion separation of the spectral lines at detector 5, enabling high-precision spatial resolution along optical axis OA. Information about the z position of object S can be found, in particular, in the phase data of the individual spectral lines. Combined with information from the interference pattern in the first spectral range, such a system 1 is capable of determining high-resolution 3D color information of object S.

[0196] The second spectral range is usually in the near infrared or infrared range. That is to say, for example, in the range of 700nm to 900nm, or even in the range of 1300nm and higher. In the latter case, it is even possible to measure the part below the surface of biological tissue.

[0197] Of course, it is also possible to provide a separate arrangement for the VPH 16 described in the preceding paragraphs, which arrangement comprises only the optical components required for the second spectral range, ie in particular the VPH 16 and the prism arrangement, which can then also be designed as a reflector.

[0198] Furthermore, the system 1 can be equipped with another VPH (not shown) which is optically active in a third, different spectral range and diffracts the incident light depending on wavelength and angle. In this way, even a third spectral range can be scanned using appropriate components and transmission properties of these components.

[0199] For example, it is possible to capture surface contours in the near-infrared range and subsurface structures in the infrared range, where the system operates in the visible (first spectrum) range using the above-mentioned metalens array mode of action.

[0200] Alternatively, the second spectral range and the first spectral range may be nested but not overlapping. That is, for example, the first spectral range includes three wavelength ranges corresponding to the characteristic wavelength ranges of the red, green, and blue color channels (or colors), and the second spectral range is within at least one wavelength range between these three wavelength ranges.

[0201] The wavelength range of the first spectral range of the blue channel is particularly between 420 nm and 480 nm, the wavelength range of the first spectral range of the green channel is particularly between 520 nm and 565 nm, and the wavelength range of the first spectral range of the red channel is particularly between 630 nm and 680 nm. Thus, the second spectral range can, for example, extend within a wavelength range of 505 nm to 515 nm and / or within a wavelength range of 570 nm to 625 nm, or from a wavelength range of 690 nm and above.

[0202] A significantly less complex implementation of the present invention is for example Figure 4 In this variant, the beam splitter cube can be omitted without abandoning the basic concept of the invention. Figure 4 An advantage of the embodiment shown is that an ultra-compact design can be achieved.

[0203] exist Figure 1 In , the side lengths of the beam splitter cube determine the dimensions in all three spatial directions. However, in Figure 4In the embodiment of the present invention, the dimension in the spatial direction perpendicular to the optical axis can be selected to be smaller. In place of the beam splitter cube, a transparent solid element is provided, wherein a first surface thereof faces the direction of the first metalens array, and a second surface opposite to the first surface faces the direction of the second metalens array. In particular, no selectively reflecting surface and no selectively diffractive surface are included in the volume enclosed by the transparent solid element. In particular, the transparent element is in the shape of a cuboid or a plate.

[0204] Thus, the element can be, for example, a glass plate or a polymer plate. On this transparent element, the first metalens array is arranged on a planar surface opposite to the second metalens array, while the second metalens array is arranged on a planar surface opposite to the first surface. The input aperture and the polarization state generated at the input aperture and applied to the object light have been combined. Figure 1 Described.

[0205] Metalenses can be easily fabricated with a numerical aperture of NA = 0.5. For a typical metalens diameter of 1 mm, the focal length to the metalens plane is 1 mm, which means that the thickness of the transparent element separating the metalens array is about 2 mm (for simplicity, the refractive index of the transparent element is not taken into account). This is significantly smaller than, for example, the height of a side of a beam splitter cube (e.g., 5 mm). Moreover, in this example, the design height, height, and lateral dimensions are independent of each other, which is particularly advantageous for mobile phone applications, where the height of the structure is extremely important, while the design dimensions along the lateral direction are not an issue.

[0206] This means that with a compact design in the z-direction (design height), a large-area detector can still be used, since the detector extends in the x- and y-direction.

[0207] For details on the optical path, in particular regarding the polarization state and its generation at the input aperture, please refer to Figure 1 .

[0208] Figure 4 Two different beams of object light are shown. The first case involves a first object light beam 31 originating from an area of the object under investigation that is located on the optical axis OA of the input aperture 2. The second case involves a second object light beam 32 originating from an area of the object under investigation that is laterally offset relative to the optical axis OA of the input aperture 2.

[0209] First object beam 31 is collimated by input aperture 2 and then continues to propagate parallel to the optical axis OA of input aperture 2. That is, it forms an angle of 0° with the optical axis OA of input aperture 2. In the context of this specification, the angle between the collimated object beam and the optical axis is also referred to as the first tilt angle β.

[0210] The first tilt angle β is particularly defined by the angle between the optical axis OA of the input aperture 2 and the propagation direction of the collimated object light beam.

[0211] The second object beam 32 is also collimated by the collimating lens 2a and then propagates further, but with a first inclination angle β that is not equal to 0° with the optical axis OA of the input aperture 2. According to the laws of ray optics, the first inclination angle β includes (together with the associated focal length of the input aperture) information about the lateral position of the object under investigation from which the associated object beam originates.

[0212] The input aperture 2 further comprises a polarizer 15, which is configured to bring the object light into a predefined polarization state. In particular, the polarizer 15 is configured in such a way that it brings the object light into a predefined polarization state - at least for object light from the first spectral range.

[0213] In the present case, the polarizer 15 is arranged and configured (e.g., rotated 45°) such that the object light has approximately equal proportions of s-polarized light and p-polarized light, regardless of its incident direction or first tilt angle β. This means that in this example, the predefined polarization state consists of a first polarization state comprising s-polarized object light and a second polarization state comprising p-polarized object light. The classification of s-polarized object light as the first polarization state and p-polarized object light as the second polarization state can also be reversed, but this is done here merely for ease of explanation.

[0214] In the following, the cases of s-polarization state and p-polarization state will be discussed without loss of generality.

[0215] The object light having the predefined polarization state just described now impinges on the first metalens array 3 according to the present invention. According to the present invention, the first metalens array 3 is arranged and configured in such a way that it focuses a first portion 31-1 of the object light having the first polarization state, while allowing a second portion 31-2 of the object light having the second polarization state to pass through the first metalens array 3 substantially unchanged.

[0216] In particular, the focused first portion 31 - 1 is focused in such a way that it is focused on the focal plane 3B associated with the first metalens array 3 .

[0217] This applies to the first object beam 31 and the second object beam 32 .

[0218] The second metalens array 4 is arranged such that the focal plane 4B associated with the second metalens array 4 coincides with the focal plane 3B of the first metalens array 3. Furthermore, the second metalens array 4 is configured to collimate the object light 31-1 having the first polarization state and thus focused on the focal plane 3B of the first metalens array 3, while transmitting a second portion 31-2 of the object light having the second polarization state substantially unchanged—the second portion 31-2 remains collimated after transmission.

[0219] For the first object light beam 31 having an angle of 0° with the optical axis OA, this means that it emerges from the second meta-lens array 4 at an angle of 0° and is incident on the detector 5 at this angle.

[0220] For the second object beam 32 propagating at a first tilt angle β not equal to 0° with respect to the optical axis OA, its first portion 32-1, after the first portion 32-1 having the first polarization state is appropriately focused and re-collimated, forms a second tilt angle β' with the second portion 32-2 of the second object beam 32, which has passed through the first and second meta-lens arrays 3 and 4 substantially unchanged. This second tilt angle β' is twice the first tilt angle β. Therefore, the first and second portions 32-1, 32-2 of the second object beam 32 are incident on a detector disposed behind the second meta-lens array 4 at this second tilt angle. In particular, a polarization analyzer 14 may be disposed in front of the detector 5 to match the polarization states of the first and second portions, particularly by rotating them by 45°, to produce better interference on the detector 5, thereby forming a better interference pattern.

[0221] In order to achieve relative phase adaptation of the wavefront with respect to the first and second parts, as combined Figure 1 As described, the solid transparent element may include liquid crystals (not shown) having different refractive indices for object light having a first and / or second polarization state, so that a phase relationship with respect to the wavefronts can be set that can be controlled by the liquid crystals. To this end, a control unit may be provided in the system.

[0222] Figure 4 The embodiments described in are particularly advantageous for metalens arrays 3, 4 having metalenses with relatively high numerical apertures, for example, numerical apertures greater than 0.4.

[0223] Regardless of the specific embodiment, the system 1 may have a laser source 18 configured to controllably illuminate the object under investigation, in particular certain regions of the object S, in order to generate a complete image of the object under investigation, for example by an optical scanning process.

[0224] To this end, a laser source 18 may be provided to sequentially emit different wavelengths, thereby sequentially irradiating the object under investigation with the different wavelengths, so that color information can be obtained from the sequential illumination.

[0225] Alternatively, the laser source may be configured to emit light from the first spectral range and the second spectral range simultaneously or with a time delay.

[0226] References

[0227] [1] Jangwoon Sung et al., “Advances in practical metasurfaces for holography and lenses”, Nanophotonics, Vol. 8, No. 10, 2019, pp. 1701–1718.

[0228] [2] Ling Li et al., “Multifocal non-interleaved metasurface lens with switchable polarization under visible light”, Laser & Photonics Reviews, vol. 15, no. 2100198, 2021.

[0229] Reference Signs List

[0230] 1 System

[0231] 2 Input Aperture

[0232] 2a Collimating optical unit / lens / objective lens

[0233] 3 The First Metalens Array

[0234] 30 Metalens

[0235] 3B Focal plane of the first metalens array

[0236] 4 Second Metalens Array

[0237] 40 Metalens

[0238] 4B Focal plane of the second metalens array

[0239] 5 Detector

[0240] 6 beam splitters

[0241] First prism of 6A beam splitter

[0242] Second prism of 6B beam splitter

[0243] 6F Reflective Surface

[0244] 7 First reflector

[0245] 8 reflector array

[0246] 80 Retroreflector

[0247] 9a First λ / 4 element

[0248] 9b Second λ / 4 element

[0249] 11 λ / 2 element

[0250] 12,12' actuator arrangement

[0251] 13 Solid transparent components

[0252] 13-1 First side of solid transparent element

[0253] 13-2 Second side of solid transparent element

[0254] 14 Analyzer

[0255] 15 Polarizer

[0256] 16 VPH

[0257] 17 Prism arrangement, biprism

[0258] 17F Reflective Surface

[0259] 18 Object Light Source

[0260] 19 Reference light source

[0261] 25 Reference light collimating optical unit

[0262] 21,22,23 Red (21), green (22), blue (23) beams

[0263] 31 First Beam

[0264] 31-1 Part 1

[0265] 31-2 Part 2

[0266] 32 Second Beam

[0267] 32-1 Part 1

[0268] 32-2 Part 2

[0269] 10 Object Light

[0270] 101 Part 1

[0271] 102 Part 2

[0272] 10a First beam

[0273] 10b Second beam

[0274] 20-1,20-1' Object beam in the second spectral range

[0275] 20-2,20-2' Reference beam for the second spectral range

[0276] E Focal plane of the input aperture

[0277] OA optical axis

[0278] S The object being studied

[0279] x,y,z Cartesian coordinate system directions

[0280] Δα prism angle

[0281] β first tilt angle

[0282] β' second tilt angle

Claims

1. An optical system (1) for three-dimensional imaging, comprising at least the following components: - an input aperture (2) having an optical axis (OA), - a first metalens array (3) and a second metalens array (4), - detector (5), in, The input aperture (2) is configured to collimate object light (10) in a first spectral range from an object under investigation (S) and fix it in a predefined polarization state consisting of two mutually conjugated polarization states, wherein the object light (10) propagates to the first metalens array (3) at a first tilt angle (β) relative to the optical axis (OA) of the input aperture (2), wherein the first meta-lens array (3) is configured and arranged to focus a first portion (101) of the object light (10), the first portion comprising a first polarization state of two mutually conjugated polarization states, and to keep a second portion (102) of the object light (10) unchanged, the second portion comprising a second polarization state of two mutually conjugated polarization states, The second metalens array (4) is configured and arranged to collimate the focused first portion (101) and transmit the second portion (102) unchanged, so that the first portion (101) and the second portion (102) each have the same wavefront curvature after propagating through the first metalens array (3) and the second metalens array (4), in particular, the first portion (101) and the second portion (102) are each collimated and form a second tilt angle (β') with respect to their respective propagation directions, and strike the detector (5) to form an interference pattern, wherein the second tilt angle (β') corresponds in size to twice the first tilt angle (β), and three-dimensional position information of an object region of the object (S) under investigation can be generated based on the interference pattern.

2. The system (1) according to claim 1, characterized in that The first portion (101) and the second portion (102) of the object light (10) are each linearly polarized perpendicular to each other, in particular wherein the predefined polarization state is a linearly polarized polarization state consisting of a superposition of the first portion (101) and the second portion (102).

3. System (1) according to any one of the preceding claims, wherein The system (1) comprises a polarization-dependent beam splitter (6), in particular a polarization-dependent beam splitter cube, between the first metalens array (3) and the second metalens array (4), wherein the system (1) further comprises the following components: - a first reflector (7), - a reflector array (8) comprising a plurality of reflective retroreflectors (80), - a first λ / 4 element (9a) arranged between the polarization-dependent beam splitter (6) and the first reflector (7), - a second λ / 4 element (9b) arranged between the polarization-dependent beam splitter (6) and the reflector array (8), wherein the polarization-dependent beam splitter is arranged relative to the object light (10) incident from the first meta-lens array (3) so that the first part (101) is reflected by the beam splitter (6), while the second part (102) propagates through the beam splitter (6); wherein the reflector array (8) is arranged on one side of the beam splitter (6), and the first part (101) from the first meta-lens array (3) and reflected by the beam splitter (6) propagates toward the side; wherein the first reflector is arranged on the side of the beam splitter (6); The invention relates to a method for transmitting the light of the first metalens array (4) to a light emitting diode (LED) device comprising: a first reflector (7) and a second reflector (102) disposed on a side of a beam splitter (6) opposite to the first meta-lens array (3), wherein the first reflector (7) reflects the second portion (102) impinging on the first reflector (7) back in a direction toward the beam splitter (6) and toward the second meta-lens array (4), in particular wherein, due to the reversed polarization states, the back-reflected first portion (101) and the back-reflected second portion (102) propagate through the polarization-dependent beam splitter (6) in a direction toward the second meta-lens array.

4. The system (1) according to claim 3, characterized in that A λ / 2 element (11) is provided between the beam splitter (6) and the second meta-lens array (4), and is configured to invert the polarization states of the first portion (101) and the second portion (102).

5. The system (1) according to any one of claims 3 or 4, characterized in that The system (1) comprises an actuator arrangement (12) configured to set the position of the first mirror (7) and / or the reflector array (8) such that a phase is set between wavefronts associated with the first portion (101) and the second portion (102).

6. The system (1) according to any one of claims 1 or 2, characterized in that The system (1) comprises a transparent solid element (13), a first surface (13-1) of which faces the direction of the first meta-lens array (3), and a second surface (13-2) of which, opposite to the first surface (13-1), faces the direction of the second meta-lens array (4), wherein, in particular, there is no selective reflection surface and selective diffraction surface in the volume enclosed by the transparent solid element (13), and in particular, the transparent element is in the shape of a cube or a plate.

7. System (1) according to any one of the preceding claims, characterized in that The system (1) comprises at least one liquid crystal configured to adjust the phase between wavefronts associated with the first portion (101) and the second portion (102), in particular wherein the at least one liquid crystal (13) is configured to adjust the phase between the wavefronts of the first portion (101) and the second portion (102) via a control module.

8. The system according to claim 6 and 7, wherein: The transparent solid element (13) includes at least one liquid crystal, or is composed of at least one liquid crystal.

9. The system according to claim 7 and any one of claims 3, 4 or 5, characterized in that The polarization-dependent beam splitter (6) comprises a first prism (6A) and a second prism (6B) forming a beam splitter cube of the beam splitter (6), wherein the first prism (6A) and / or the second prism (6B) comprise at least one liquid crystal, in particular wherein the first prism (6A) and the second prism (6B) both comprise a liquid crystal according to claim 7.

10. System (1) according to any one of the preceding claims, characterized in that The focal plane (3B) of the first metalens array (3) and the focal plane (4B) of the second metalens array (4) coincide with each other.

11. System (1) according to any one of the preceding claims, characterized in that In the propagation direction, a polarizer (14) is arranged behind the second meta-lens array (4) and in front of the detector (5), and the polarizer is configured to match the polarization states of the first part (101) and the second part (102) so that the first part (101) and the second part (102) generate interference on the detector (5).

12. System (1) according to any one of the preceding claims, characterized in that The input aperture (2) comprises a polarizer (15) configured to bring the object light (10) from the first spectral range into a predefined polarization state.

13. System (1) according to any one of the preceding claims, characterized in that The input aperture (2) comprises at least one lens (2a) configured to collimate the object light (10) coming from the object (S) under investigation.

14. System (1) according to any one of the preceding claims, wherein The system (1) is configured to deflect the propagation direction of the object light (10) from the first spectral range incident on the system (1) in a wavelength-dependent manner, so that the object light (10) and the first portion (101) and the second portion (102) form, in addition to the first tilt angle (β), a wavelength-dependent angle with the optical axis (OA).

15. System (1) according to any one of the preceding claims, characterized in that The object light (10) in the first spectral range consists of two or more non-overlapping wavelength ranges, and / or, wherein the system (1) is configured to filter the object light (10) into two or more non-overlapping wavelength ranges to constitute the first spectral range, wherein there are gaps between the wavelength ranges, wherein for each wavelength range an interference pattern is generated on the detector (5), and based on the interference pattern, three-dimensional position information and color composition of each wavelength range relative to the object area of the object (S) under investigation can be obtained.

16. System (1) according to any of the preceding claims, provided that it is dependent on claim 3, characterized in that The object light (10) comprises at least one additional spectral range that is different from and does not intersect the first spectral range, wherein the first metalens array (3) and the second metalens array (4) and the polarization-dependent beam splitter (6) are transparent and optically inactive for light from the at least one additional spectral range, wherein the polarization-dependent beam splitter (6) further comprises a volume phase hologram (16), i.e., a VPH, which is configured to diffract light from the at least one additional spectral range in a polarization- and angle-dependent manner and is transparent and optically inactive for light from the first spectral range, in particular wherein the volume phase hologram (16) extends along a reflective surface (6F) of the beam splitter (6).

17. The system according to claim 16, wherein: The system is configured to direct the reference light from the second spectral range, in particular via a reference arm, via a side of the beam splitter (6) opposite the input aperture to the VPH, wherein the first mirror is transparent in particular for the reference light.

18. The system according to claim 17, wherein: The system (1) includes a third λ / 4 element (9c) which is arranged on the side of the first reflector (7) facing away from the beam splitter (6) and is configured to work in conjunction with a second λ / 4 element (9b) to put the reference light (20-2, 20-2') in a predefined polarization state so that when the reference light (20-2, 20-2') comes from the first reflector and strikes the VPH (16), it is s-polarized.

19. System (1) according to any one of claims 16 to 18, wherein The system (1) has a wavelength-selective prism arrangement (17) between the reflector array (8) and the beam splitter (6), which is configured to reflect light of at least one additional spectral range in the direction of the detector (5) at a prism angle (Δα), and wherein the prism arrangement (17) is transparent and optically inactive for light from the first spectral range.

Images