Front optical unit for optical element, in particular objective lens, and objective lens and microscope

Abstract

The invention relates to an optical element. The latter has a plano-convex basic shape along the optical axis, comprising a first portion and a second portion, each portion extending around the optical axis, where the portions have different refractive indices from each other. In this case, the planar side of the first part faces or should face the object to be imaged and the convex side of the second part faces or should face the image plane, as a result, at the planar side of the first part, the radiation to be captured is collected and diverted into the optical element, and due to the action of the optical element, the radiation to be captured is captured. The rays of captured radiation are guided in a beam path. The optical element is characterized in that a third portion is present between the first portion and the second portion; wherein the third portion has a concavo-convex form, and the portions are directly adjacent to each other. The refractive index of the third portion is greater than the refractive index of the second portion, and the refractive index of the second portion is greater than the refractive index of the first portion. The invention also relates to an objective lens and a microscope.

Description

Technical Field

[0001] The invention relates to an optical element, in particular to a front optical unit of an objective lens, an objective lens and a microscope. Background Art

[0002] System-related aberrations occur when imaging using an optical system and are detrimental to the unsophisticated imaging of an object into the image plane. One of these aberrations is called field curvature; when it occurs, the flat object plane is not imaged into the equally flat image plane, but is transferred into the image plane as an "image shell". Typically, this results in blurring, which increases from the center of the image (center [of the field of view]) to the edges (edges [of the field of view]) and which requires a continuous updating of the focus. As a result, not all image data of the object in the object plane can be imaged sharply into the image plane at the same time.

[0003] Since flat detectors, such as CCD or CMOS chips, are often used in modern optical systems, such aberrations are particularly disadvantageous in this case.

[0004] The field curvature is determined to a large extent by the properties of the optical components utilized in the beam path, and in this case in particular by the properties of the front optical unit of the objective used to capture the rays. Furthermore, if the intention is to use a front optical unit with a high numerical aperture, the field curvature correction is very complicated.

[0005] The prior art has disclosed optical elements, in particular front optical units, in which two parts with different refractive indices are combined with each other. For example, DE 10 2013 203 628 A1 describes a front lens group consisting of a plane parallel plate and a hyperhemisphere. The two optical components are bonded to each other in contact so that no additional optically effective transition occurs and are managed without a bonding agent or adhesive between the components.

[0006] For example, other technical solutions related to the reduction of field curvature can also be found in US2006 / 0082896 A1 and US2018 / 0113279 A1. Here, a hemispherical lens is combined with a super hemisphere in the front optical unit. Summary of the invention

[0007] The invention is based on the object of proposing an option for reducing aberrations, in particular field curvature, which is improved compared to the prior art.

[0008] This object is achieved by an optical element, in particular by a front optical unit of an objective. The optical element has a plano-convex basic shape along the optical axis of the optical element. A first part and a second part of the optical element are formed, each part extending around the optical axis, in particular extending around the optical axis in a rotationally symmetrical manner. In this case, the parts have different refractive indices from one another.

[0009] The planar side of the first part faces the sample space, in which the object to be imaged (sample) is arranged or can be arranged. The convex side of the second part faces the image space or can be made to face the image space; for example, further optically active elements (such as optical lenses, filters, diaphragms and / or detectors) are located in the beam path in the image space. At the planar side of the first part, the detection radiation to be captured can be collected and diverted into the optical element, and due to the action of the optical element, the rays of the captured detection radiation can be guided in the beam path.

[0010] The optical element according to the invention is characterized in that the third part is present between the first part and the second part, wherein all three parts are directly adjacent to each other. The refractive index of the third part is greater than the refractive index of the second part, and the refractive index of the second part is greater than the refractive index of the first part. Within the meaning of this specification, these parts are directly adjacent to each other if there is no space between them, in particular no air gap or liquid gap. In contrast, a thin layer of adhesive or bonding agent may optionally be present between the parts.

[0011] The first portion is plano-convex, whereas the third portion has a concave-convex shape and may be in the form of a meniscus lens, for example.

[0012] Here, meniscus or meniscus lens is understood to mean an optical lens whose side faces are curved in a common direction. Preferably, the radii of the two side faces relate to a common virtual center.

[0013] It has been found that adding a third portion having a high refractive index relative to the other two portions advantageously further reduces the occurring field curvature. In this case, the invention accepts that the optical element according to the invention has a more complex structure and is more difficult to produce than is the case with solutions according to the prior art.

[0014] The ratio of the refractive indexes of the three portions is found to be, for example, in the range between 0.7 and 0.85 for the first portion and the third portion (n1:n3), and in the range between 1.1 and 1.3 for the third portion and the second portion (n3:n2).

[0015] There may be different ways of producing an optical element according to the invention. In one possible embodiment, the first part, the second part and the third part are each parts that are joined to each other along the optical axis. For example, this joining can be achieved by gluing, adhesive bonding or contact joining. This configuration is advantageously used if parts of the optical element are made of glass.

[0016] In another embodiment, the first part, the second part and the third part are formed in a monolithic body. For example, the optical element can consist of a single body, wherein the three parts are realized, for example, by means of a material in the optical element, which is locally influenced by means of targeted irradiation with radiation suitable in terms of its wavelength and intensity and is changed in terms of its optical properties, in particular in terms of its respective refractive power. For example, the material of the optical element according to the invention can be a plastic that is structured within the meaning of the invention, for example, by means of two-photon irradiation / 2-photon methods.

[0017] So that the optical element achieves the desired function, only the planar side faces of the first part should be used for collecting the detection radiation. Therefore, in an advantageous embodiment, the optical element can include a cover for the second part and / or the third part on the side facing the sample space. For example, such a cover can be realized by a coating or by a layer (aperture) made of a light-impermeable material. The relevant faces can also be constructed so as to prevent the entry of light to a large extent or completely.

[0018] If the optical element according to the invention is installed in an optical device, for example in an objective, the entry of unwanted light can also be prevented by structural measures, for example by suitable design of the objective and / or the housing of the optical device (for example a microscope comprising the objective).

[0019] For example, preventing unwanted light from entering can be aided by the planar side of the first part protruding beyond the third part in the direction of the sample space. This configuration facilitates the second part and / or the third part to be covered by a side wall of the housing or by a cover. Furthermore, this allows the planar side of the first part to rest tightly against the object to be imaged, while the other parts recede.

[0020] In a further embodiment of the invention, an annular cover which is opaque to the detection radiation to be captured may also be present between the parts, in particular in the peripheral region. For example, such a cover along the outer radius may be designed as a stop delimiting the beam path, for example in order to reduce or prevent undesired influences of the lens mount on the homogeneous radiation channel.

[0021] The invention is advantageously used in the field of microscopy, in particular in the field of fluorescence microscopy. Since the second part has a superhemispherical form and the concave side of the second part is joined at least in the region around the convex side of the third part, the frequent requirements for large numerical apertures in this context can be advantageously met. In this case, the joining around the convex side of the third part is advantageously at least to the extent that substantially all rays from the first part and the third part propagating in the beam path are collected and guided by means of the second part.

[0022] The optical element according to the invention may advantageously be arranged in an optical device, in particular in a detection beam path.The detection beam path is used to capture and direct radiation from an object as detection radiation for imaging purposes.

[0023] Advantageously, the optical element according to the invention is a front optical unit of an objective, in particular an immersion objective. Such an objective can be arranged in a microscope, in particular in its detection beam path. For example, such an objective can be a plane apochromat and a plane achromat, in which the longitudinal chromatic aberrations produced are corrected. This use is advantageously realized with an immersion medium, for example an immersion oil. The absolute value of the resulting field curvature can advantageously be kept below 2RE (RE=Rayleigh unit, see below). In order to achieve the desired large aperture, parts of the assembled optical element advantageously emerge from so-called spherical manufacturing. In this case, the optical lens is produced in the form of a sphere or a sphere part. For example, in this case, the spherical part can be a hemisphere. If the spherical part ends before reaching the diameter below the sphere, a so-called sub-hemisphere is obtained. If the spherical part extends beyond the diameter below the sphere, the spherical part is also called a super-hemisphere.

[0024] The advantage of the invention is the significant reduction of the field curvature occurring in the image plane. At the same time, as a result of the invention, it is easier to find the compromise to be found with regard to the correction of further aberrations such as spherical aberration, coma, astigmatism and distortion. Thus, the primary longitudinal chromatic aberration present can be fully corrected, while the longitudinal chromatic aberration of the secondary spectrum can be brought into the desired depth of focus due to the use of appropriately selected glasses.

[0025] The reduction in field curvature facilitates the use of flat detectors, improving the achievable image quality and facilitating a variety of applications. For example, counting specific viruses across the entire field of view becomes easier and more efficient.

[0026] Another advantage of the present invention is that the numerical aperture is increased due to the structure. For example, it can be shown that, by the present invention, an initial numerical aperture of NA=1.42 (63 times magnification) for a conventional structure of an optical assembly can be increased to NA=1.49 by an embodiment of the optical element according to the present invention. Other embodiments of the present invention will allow numerical apertures greater than NA=1.5.

[0027] The invention also results in a need for less lens mounting, in particular when implemented in an objective. Thus, components requiring very precise manufacture and adjustment can be omitted. Furthermore, less installation space is required. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] The present invention will be explained in detail below based on exemplary embodiments, drawings and tables. In the drawings:

[0029] Figure 1 An exemplary embodiment of an optical element according to the invention and a schematic diagram of an object to be imaged are shown in the middle longitudinal section;

[0030] Figure 2 A schematic diagram of an exemplary embodiment of a microscope with an objective, whose front optical unit is formed by an optical element according to the invention, is shown in a middle longitudinal section;

[0031] Table 1 shows examples of dimensions of faces and parts of an embodiment of an objective lens having an optical element according to the present invention as a front optical unit; and

[0032] Table 2 shows examples of refractive index and field curvature of an exemplary embodiment of an objective lens having an optical element according to the present invention as a front optical unit. DETAILED DESCRIPTION

[0033] like Figure 1 As shown, an exemplary embodiment of an optical element E according to the present invention includes a first part 1 in the form of a hemisphere (plano-convex) as well as a second part 2 (concave-convex, super hemisphere) and a third part 3 (concave-convex, meniscus). Parts 1 to 3 are arranged directly adjacent to each other along the optical axis oA of the optical element E, that is, there is no air gap between them. A bonding material (such as an adhesive) may be present between the parts and does not eliminate the directly adjacent arrangement. The three parts 1 to 3 are formed rotationally symmetrically around the optical axis oA. Functionally, the optical element E has a plano-convex basic shape, in which the planar side of the first part 1 faces the object 4 to be imaged and the convex side of the second part 2 faces away from the object 4. The refractive indices n1, n2 and n3 of parts 1, 2 and 3 are respectively related to each other as follows: n3>n2>n1. The different refractive indices are explained by the density of points in parts 1, 2 and 3.

[0034] Due to the extreme ratios between the maximum paths of the rays in the respective portions 1 to 3 and the radius of curvature r (see also Tables 1 and 2), all portions 1 to 3 are advantageously produced by respective spherical lenses. Figure 2 ) has a super-hemispherical embodiment in order to meet the high numerical aperture requirements of optical element E.

[0035] For simplicity, here the microscope slide (eg cover glass), the sample space containing the object 4 and the image plane 8 (see Figure 2 ) are denoted by object 4. Object 4 has a thickness of, for example, 0.17 mm, a refractive index n (see Table 1) and a planar first face P1.

[0036] To put it simply, Figure 1 and Figure 2 Selected rays emanating from an object 4 are shown, which are collected by means of a first part 1 of an optical element E, diverted to a beam path S, for example a detection beam path S, and guided in the latter. The planar side faces of the first part 1 slightly protrude beyond the third part 3 in the direction of the object 4. At the first face P1, the collected rays enter the immersion medium 5, which forms the second face P2, and from there reach the first part 1 having a refractive index n1 at the third face P3. After passing through the first part 1, the rays reach the interface of the third part 3 (fourth face P4) and are refracted there according to the refractive index n3 of the third part 3. The same situation occurs after passing through the third part 3 at the interface with the second part 2 at the fifth face P5. The rays leave the optical element E at the sixth face P6, where they are refracted according to the refractive index n2. The outgoing rays are guided along the beam path S.

[0037] exist Figure 2 , the use of an optical element E as a front optical unit in an objective 6 is shown. The objective 6 is arranged, for example, in a detection beam path S of a microscope M, which is only shown here. The rays emerging partially divergently from the optical element E can be deflected, for example, by downstream optical components 7 (only shown here) of suitable shape and refractive power and can be collimated, for example, therewith, with the result that the rays are guided forward in a manner parallel to one another on the detection beam path S. Further optical components (not shown here), such as optical lenses, apertures, beam splitters, filters, etc., can be present in the detection beam path S and their overall effect is that the radiation is guided, for example, to a detector 8 (=image plane 8) arranged in the image plane 8 and is captured as image data therefrom.

[0038] Will use Figure 1 and Figure 2 The effect of the invention is explained by way of example. The working distance of the front optical unit (optical element E) of the objective 6 with a large numerical aperture is determined by the thickness of the cover glass 4 and the layer thickness of the immersion medium 5 (see Table 1). The wavelength of the spectral line e selected as an example is 546 nm, and the numerical aperture NA in the object space is NA=1.49. The objective 6 produces a magnification of 63 times. The diameter of the object field is given as 0.397 mm.

[0039] In the context of the Seidel aberration, the deviation of the field curvature ("image shell") from a Gaussian image plane along the optical axis oA (z direction) can be described by summing over all surface contributions. The following holds true:

[0040] Field curvature = -915.8 [LLW] 2 ∑i[(n' i –n i ) / (r i n' i n i )] in units of RE (Rayleigh units), [1]

[0042] in

[0043] 1RE=nλ / NA 2 , where λ = 0.000546 mm (546 nm); NA = numerical aperture;

[0044] LLW light etendue=B max NA, where B max = maximum (object or) image height;

[0045] r i is the radius (of curvature) of surface i;

[0046] n i is the refractive index before refraction at face i; and

[0047] n i ' is the refractive index after refraction at face i.

[0048] Equation [1] can be applied to both object space and image space. In this process, in each case, the image height and the numerical aperture NA should be used in the same space.

[0049] Using the exemplary values ​​from Tables 1 and 2, equation [1] yields Field Curvature = -915.8 (12.5 x 1.48 / 63) 2 x0.0171=-1.4RE.

[0050] Table 1

[0051]

[0052]

[0053] ν e The Abbe numbers are described relative to the defined wavelengths 546 nm (e), 480 nm (F') and 644 nm (C').

[0054] Table 2

[0055] <![CDATA[(n' i -n i ) / (r i n' i n i )]]> noodle r n n’ Pittsval P3 flat 1.518 1.542 0.0000 P4 -1.6313 1.542 2.009 -0.0924 P5 -3.9816 2.009 1.635 0.0286 P6 -4.8000 1.635 1.000 0.0809 sum 0.0171

[0056] Reference numerals

[0057] 1 Part 1

[0058] 2 Part 2

[0059] 3 Part 3

[0060] 4Object / microscope slide / cover glass / sample plane / sample space

[0061] 5. Impregnation medium

[0062] 6 Objective lens

[0063] 7Optical components

[0064] 8 detectors

[0065] 9. Cover, aperture

[0066] E-Optics

[0067] M microscope

[0068] oA Optical axis

[0069] P1 First page

[0070] P2 Second side

[0071] P3 Third side

[0072] P4 Page 4

[0073] P5 Page 5

[0074] P6 Page 6

[0075] S beam path

Claims

1. An optical element (E), in particular a front optical unit of an objective lens (6), comprising: - a plano-convex basic shape along the optical axis (oA) of said optical element (E), - a first portion (1) and a second portion (2), said first portion (1) and said second portion (2) each extending around said optical axis (oA), wherein said portions (1, 2) have mutually different refractive indices; - and wherein the planar side of the first portion (1) faces or should face the object (4) to be imaged, and the convex side of the second portion (2) faces or should face the image plane (8), with the result that at the planar side of the first portion (1) the radiation to be captured is collected and diverted into the optical element (E), and the rays of the captured radiation are guided in the beam path (S) due to the action of the optical element (E); Features - a third portion (3) is present between said first portion (1) and said second portion (2); wherein said third portion (3) has a concave-convex form; - said parts (1, 2 and 3) are directly adjacent to each other, and The refractive index of the third portion (3) is greater than the refractive index (3) of the second portion (2), and the refractive index of the second portion (2) is greater than the refractive index of the first portion (1).

2. The optical element (E) according to claim 1, characterized in that The first part (1), the second part (2) and the third part (3) are components joined to each other along the optical axis (oA).

3. The optical element (E) according to claim 1, characterized in that The first part (1), the second part (2) and the third part (3) are formed as an integral body.

4. The optical element (E) according to any one of the preceding claims, characterized in that Only the planar side faces of the first portion (1) are implemented to collect the radiation.

5. The optical element (E) according to claim 4, characterized in that The planar side surface of the first portion (1) protrudes beyond the third portion (3) in the direction of the object (4).

6. An optical element (E) according to any one of the preceding claims, characterized in that The second portion (2) has a super-hemispherical form and engages around the convex side of the third portion (3) in the region.

7. An objective lens (6) having the optical element (E) according to any one of claims 1 to 6 as a front optical unit.

8. A microscope (M) having an objective (6) according to claim 7.

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