WO2012025349A1

WO2012025349A1 - Aberration-corrected microscope

Info

Publication number: WO2012025349A1
Application number: PCT/EP2011/063320
Authority: WO
Inventors: Carsten Glasenapp
Original assignee: Carl Zeiss Ag
Priority date: 2010-08-25
Filing date: 2011-08-02
Publication date: 2012-03-01
Also published as: DE102010039746A1; DE102010039746B4

Abstract

The invention relates to an aberration-corrected microscope (M), comprising a microscope objective (1) for imaging an object (2) in a microscope optical path along an optical axis A and an aberration-correcting correction element (7), wherein the correction element (7) is arranged downstream of the microscope objective (1) in the imaging direction and lies close to an aperture diaphragm (P) of the microscope objective (1), wherein the correction element (7) has a first and a second plate (9, 10) each having two plate surfaces, wherein a first of the plate surfaces (11) of the first plate (9) has the surface topography of an at least fifth-order odd polynomial and a first of the plate surfaces (12) of the second plate (10) is designed to be complementary thereto.

Description

Aberration-corrected microscope

The invention relates to an aberration-corrected microscope having a microscope objective for imaging an objective in a microscope beam path along an optical axis and an aberration correcting correction element.

Spherical aberrations are known to occur in microscopes, among other things due to varying refractive indices of the objects or varying thicknesses of a coverslip covering the object. It is known to manually correct spherical aberrations by interfering with the objective. However, these procedures are usually not suitable for automatic operation of the microscope. Although one could think of equipping the mechanical intervention on the lens with a corresponding drive, highly accurate mechanisms must be accommodated in the smallest possible space inside the microscope objective. In addition, then every lens would have to be corrected, which in turn brings effort.

There are therefore known in the art approaches to make the correction outside of the lens. For example, EP 0859259 A2 describes a zoom group consisting of a plurality of spherical surfaces, which is arranged between the microscope objective and the tube lens of the microscope. However, this causes long light paths through the zoom group, which leads to further field errors.

DE 19782060 B4 shows an interferometer with catadioptric imaging system with extended numerical aperture range. DE 10049296 A1 discloses an optical arrangement and a method for deflecting light rays.

The invention is therefore based on the object of specifying a microscope which can be corrected with little expense, in particular with respect to aberrations of higher orders. In addition, the correction should be adjustable depending on the application in order to be able to correct aberrations caused by the sample or its cover glass.

The object is achieved by an aberration-corrected microscope having a microscope objective for imaging an object in a microscope beam path along an optical axis and an aberration correcting correction element. The correction element is the Subsequent to the microscope objective in the imaging direction and is located near an exit pupil of the microscope objective. The correction element comprises a first and a second plate, each having two plate surfaces. One of the disk surfaces of the first disk has the surface topography of an at least 5th order odd polynomial. A plate surface of the second plate is designed to be complementary. These surfaces are therefore aspheres of a certain shape.

The arrangement near the pupil makes it possible to keep field errors small. Particularly preferred is an arrangement between the pupil and the exit lens of the microscope objective. In general, the term "close to the pupil" is understood in the sense defined in DE 10200701 9812, and there in paragraph [0010].

By "higher order aberrations" are meant wavefront distortions with at least r ⁴ dependence or corresponding Zernike polynomials.

In order to achieve a simpler manufacturing, it is expedient to form the flat opposite the aspherical plate surfaces plate surfaces. In order to keep multiple reflections in the beam path low, it is expedient to design the plane surfaces at an angle, that is not perpendicular, but obliquely (but not parallel) to the optical axis.

The microscope according to the invention can be adjusted for aberration correction particularly easy depending on the application, if both plates are equipped with an adjustment drive, which shifts the plates relative to each other and is controlled by a control unit for setting the aberration correction. When moving, make sure that the center of gravity of both plates remains on the optical axis. This is preferably achieved in that the shift vectors are different only in sign, d. H. same amount and opposite direction.

The quality of the correction, in particular the desired correction of higher aberration orders, it is preferable to form the quality of the surfaces of the aspheric Platenflächen better than lambda / 10, preferably equal to or better than lambda / 20, where lambda is the shortest wavelength used in the microscope input.

The microscope according to the invention is independent of the currently used microscope objective with a single correction element. Such microscopes are usually provided with objective changers, which penetrate one of several microscope objectives in the microscope beam path. If the correction element is assigned to the objective changers, then it can be brought into the microscope beam path by anyone with the objective changer Microscope lenses for aberration correction. Combining this design with the already mentioned Versteilantrieb for the plates, can be in the control of a control unit automatically adjust the required for the currently placed in the microscope beam microscope objective aberration correction without the user has to act for this purpose. The microscope can also be equipped with an aperature to determine the image quality. This can be an additional sensor, such. As a wavefront sensor, his or (better) are obtained from the microscope image itself. With the image quality evaluation criterion, the aberration correction can preferably independently adjust the state of best focus on the microscope in a feedback loop.

It is understood that the features mentioned above and those yet to be explained below can be used not only in the specified combinations but also in other combinations or alone, without departing from the scope of the present invention.

The invention will be explained in more detail for example with reference to the accompanying drawings, which also disclose characteristics essential to the invention. Show it:

1 is a schematic representation of a microscope objective with subordinate correction element,

Fig. 2 is a schematic representation of the input of a microscope, in which the structure of

Fig. 1 is used, and

Fig. 3 is a schematic representation of a correction element used in the microscope of Fig. 2.

FIG. 1 shows a microscope objective 1 of a microscope M, which is shown schematically in FIG. The microscope objective 1 comprises a plurality of individual lenses, as is known for such microscope objectives. In the microscope M, the microscope objective 1 images a sample 2, which is located under a cover glass 3. In this case, a tube 4 coincides with the microscope objective 1, which tube comprises a tube lens 5 and with which the microscope objective 1 images the sample 2 onto a detector 6. The mapping to a detector is of course only optional, of course, a conventional tube insight can be used.

Near the exit lens of the microscope objective 1, a correction element 7 is arranged. It is located, in particular, near the exit pupil P of the microscope objective 1, and preferably between the plane of the pupil P and the exit lens of the microscope objective. The correction element 7 consists of two plates 9 and 10, which are equipped with a drive 8. The drive 8 is like the detector 6 connected to a control unit C of the microscope M via unspecified and shown in dashed lines in Figure 2 lines.

Figure 3 shows enlarged the correction element 7. There it can be seen that the plates 9 and 1 0 are curved aspherically at their mutually associated surfaces. The curvature of the aspherical surface 10 corresponds to the curvature of an odd polynomial, and the aspherical surface 12 is designed to be complementary thereto.

For aberration correction, the plates 9 and 1 0 are shifted relative to each other, as shown in Figure 3 by double arrows, which illustrate a relative displacement 13, is located. A displacement along an axis perpendicular to the optical axis A causes an optical correction in an axis perpendicular to the direction of displacement. In FIG. 3, a shift in the x-axis is shown by way of example, and a correction denoted by 14 in FIG. 3 then takes place in the axis y perpendicular thereto.

If one considers the polymer function, which satisfies the asphere 1 1 and the asphere 12 complementarily, by the equation W (x, y) = ax ³ + by ³ , the measure of the relative shift 13 determines only the height of the polynomial function a and b, but not their shape.

However, the correction element 7 has a more complex surface topography and manipulates the wave front, which is emitted in the imaging of the lens 1 to the tube lens 5. By appropriate choice of the surface polynomial with at least 5th order and setting the relative displacement 13 can now for higher aberrations wavefront error, which z. B. caused by a misplacement of the refractive index of the sample 2 or a different thickness of the cover glass 3, be compensated. In order to correct higher orders of aberrations, it is provided that the surface topography not only satisfies a simple cubic form, ie a polynomial of exclusively third order, but that the polynomial also takes into account other odd orders in the surface topography, in particular the fifth order, and preferably also the first order and mixed orders in which one of the two parameters (x or y) occurs in an odd power. This can be done particularly expediently if an odd fifth-order surface polynomial of the form f (x, y) = cx ₅ × ⁵ + cy ₅ y ⁵ + cx ₃ × ³ + cy ₃ y ³ + cxi x + cy y y + cx ₄ ix ⁴ + cy y ₄ + iy ⁴ x ₃₂ x ³ y cx ² + cy y ₃₂ ³ x ² + cx ₂ + cy ₂ ² y ix iy ² x is used. The parameters cx _n and cy _n determine the shape of the wavefront, which is generated by the correction element 7. The exact values of the parameters are determined by a numerical adaptation. Also, even higher order odd polynomials can be used.

The aspherical surfaces 1 1 and 12 are preferably formed with the surface quality lambda / 20, wherein sometimes lambda / 10 may suffice. The microscope M further comprises an objective changer 15, which may be formed, for example, as a known objective revolver. As can be seen from FIG. 2, the correction element 7 is located between the objective changer 15 and the tube lens 5. Thus, it can perform an aberration correction for all objectives, which can be introduced into the microscope beam path of the microscope M by the objective changer.

Preferably, the objective changer 15 is provided with a drive 8 and is also controlled by the control unit C.

In the control unit C, a data memory is then provided, in which the control state of the correction element 7 is stored for different microscope objectives. The controller C can then automatically control the correction element 7 so that its position matches the currently effective microscope objective 1. Optionally, an image quality value can be determined and optimized by controlled adjustment of the correction element. For this purpose, a regulation, that is to say an iteration, can be used.

The aspheric surfaces 1 1 and 12 generate a rotationally symmetric wavefront with an r ² dependence - ie the shape of a parabola. This is achieved when the above function, which describes the surface shape of the surfaces 1 1 and 12, contains only the powers 1 and 3. Since the effect of the aspheres equals the derivative of the function, an action follows in the powers 0 and 2. (1 and r ² ) - this is the effect of a parabola or Zernike coefficient Z4 which describes a focus or defocusing.

The described configuration of the aspheres with higher odd powers than 3, 5 and 7 achieves the correction of rotationally symmetric wavefront errors of higher order, ie Zernike coefficients Z9, Z1 6, Z25 (corresponding to the effect r ⁴ , r ⁶ ,...) Are generated ,

The design of the surface shape is done in two steps: 1 . Analytical predetermination and

2. Numerical fine-tuning

The purpose of the correction element 7 is the correction of a wavefront error, which in high-NA lenses z. B. is induced by a mismatch of the object (eg, wrong cover glass thickness or incorrect refractive index of the sample). The wavefront error induced by this mismatch can be analytically determined by the sine condition

p: relative pupil radius

Az: Defocus

n: refractive index

k: wavenumber

describe.

The root can be developed as a series

_ N / A

n

1 4 4

Φ (Αζ, ρ, s) = nk Az (l - ^ s ² p ^{2 ■} - S p - s ⁶ p ⁶

Here, the individual terms of the series developments describe known rotationally symmetric wavefront errors: i. Exponent 0: offset

ii. Exponent 2: Defocus

iii. Exponent 4: Spherical aberrations

iv. Exponent> 4: Spherical aberrations of higher order

These terms are similar to the well-known Zernike polynomials w (p, s) = Z ₁ + Z ₄ + Z ₉ + Z ₁₆ + .... = ( _Zl - l + z ₄ (lp ² -l) + ζ ₉ (όρ ⁴ - 6p ² + l) + z ₁₆ (20p ⁶ - 30p ⁴ + I2p ²

- they differ only in the preliminary factor. Consequently, these can be related. This is helpful because optical systems are usually described with Zernike polynomials. There the correction element 7 should only correct spherical wavefront errors, but not defocusing, the defocus pattern need not be corrected - this can be done by refocusing in the microscope. In the simplest case, this means that the wavefront error described by the last equation

a. by refocusing the term with the exponent 2 and

b. is corrected with the correction element 7 with respect to all higher terms.

Consequently, the shape of the wavefront to be corrected can be described as follows:

N / A

The following example explains the effect again. The phase error of an objective of the type Zeiss Plan APO 63x NA = 1 .2 is μιτι in defocus by 50 μιτι in water up to 40 μιτι. The refocusing can be described by subtracting the Zernike coefficient Z ₄ from the wavefront. From a coefficient comparison of the last two equations, the factor Z ₄ can be determined analytically. This phase function must be realized by the correction element 7. The phase deviation of 5 μιτι is significantly lower than the total phase error - that comes the fertility clearly contrary.

Such are considered

1 . the influence of higher orders of the phase front by refocusing,

2. preservation of the symmetry of the phase front in polynomials of orders> 3 by appropriate optimization of the mixed terms (eg x ³ y ² ),

3. a color longitudinal and lateral chromatic aberration and

4. a field error.

An optimization gives for the former equation which describes the asphere, e.g. For example, the following parameters:

• cx1 = -1 .64e-6;

• cy1 = cx1;

• cx3 = 7.86e-4; • cx21 = -4.93e-7;

• cy21 = cx21;

• cy3 = cx3;

• cx5 = 2.92Θ-7;

· Cx41 = -4.18e-8;

• cx32 = 2.66e-7;

• cy32 = cx32;

• cy41 = cx41;

• cy5 = cx5

With this design, a performance (PSF) of 80% of a theoretically ideal lens is achieved. Without a correction, the value drops by 50 μιη to -30% at a defocusing.

Claims

claims

1 . An aberration-corrected microscope comprising a microscope objective (1) for imaging an object (2) in a microscope beam path along an optical axis (A) and having an aberration correcting element (7), characterized in that the correction element (7) is the microscope objective (1) is downstream in the imaging direction and near a pupil (P) of the microscope objective (1), wherein the correction element (7) has a first and a second plate (9, 10) each having two plate surfaces, wherein a first of the plate surfaces ( 1 1) of the first plate (9) has the surface topography of an odd polynomial of at least 5th order and a first of the plate surfaces (12) of the second plate (10) is complementary thereto.

2. A microscope according to Anspruchl, characterized in that the second plate surfaces (1 1, 12) of the plates (9, 1 0) are flat and are inclined to the optical axis (A).

3. Microscope according to one of the above claims, characterized in that both plates (9, 10) are connected to a Versteilantrieb (8), the mutual displacement of the plates transverse to the optical axis (A) and formed by a control unit (C) is set to adjust the aberration correction.

4. Microscope according to one of the above claims, characterized in that the quality of the surfaces of the first plate surfaces (1 1, 12) is better than lambda / 10, preferably equal to or better than lambda / 20, wherein lambda is the shortest used in the microscope beam path Wavelength is.

5. Microscope according to one of the above claims, characterized in that an objective changer (15) is provided, with which a plurality of microscope objectives can be brought into the microscope beam path, wherein the correction element (7) downstream of the objective changer (15) on the optical axis (A) so it's for everyone with the lens changer (15) corrected in the microscope beam microscope lenses (1) corrected for aberrations.

6. Microscope according to one of the above claims, characterized in that the correction element (7) between the pupil plane (P) and an exit surface of a last lens of the microscope objective (1).

7. A microscope according to claim 3, characterized in that the control unit (C) determines a quality value for an image generated in the microscope beam path and the Versteilantriebs (8) controls to optimize the quality value, preferably a control.