US20170292678A9

US20170292678A9 - Assembly for producing a plurality of beam bundles

Info

Publication number: US20170292678A9
Application number: US15/038,081
Authority: US
Inventors: Stefan Erfinder FRANZ
Original assignee: Jenoptik Optical Systems GmbH; Nova Measuring Instruments Ltd
Current assignee: Jenoptik Optical Systems GmbH; Nova Ltd
Priority date: 2013-11-20
Filing date: 2014-11-20
Publication date: 2017-10-12
Anticipated expiration: 2034-11-20
Also published as: WO2015074757A1; DE112014005283A5; IL245740A0; WO2015074757A8; US10113711B2; US20160305632A1; IL245740B

Abstract

The invention relates to a mirror assembly (1) for producing a plurality of beam bundles (K1, K2, . . . Kn) from the beam of a light source (L), wherein the plurality of beam bundles comprises at least one first beam bundle (K1) having a first main beam direction (SR1), a second beam bundle (K2) having a second main beam direction (SR2), and preferably further beam bundles (K3 . . . Kn) having further main beam directions, which mirror assembly comprises the following features: a first mirror segment (1 a) having a first focal point (F1), which first mirror segment converts a first partial region of the beam (S1) of the light source into the first beam bundle (K1), a second mirror segment (1 b) having a second focal point (F2), which second mirror segment converts a second partial region of the beam (S2) of the light source into the second beam bundle (K2), and preferably further mirror segments (1 c) having further focal points (F3 . . . Fn), which further mirror segments convert further partial regions of the beam of the light source into further beam bundles (K3 . . . Kn), wherein the back side of the mirror segments has a curvature having the radius R_s, which curvature is concentric to the light source.

Description

The invention relates to a mirror assembly for producing a plurality of beam bundles from the beam of a light source as per the features of claim 1 and a process for manufacturing such an assembly as per the features of claim 11 and an optical assembly for displaying the light source as advantageous application of the mirror assembly as per claim 10.
The prior art discloses assemblies for producing a plurality of beam bundles from a light source. In this prior art an off-axis paraboloid generates a collimated light bundle, from which with plane mirrors parts or partial areas can be deflected in other directions.
These solutions are not adequate for providing the increasing demands of such assemblies, in particular lighting systems which at the same time have a small footprint, have minimal manufacturing and material costs and which are characterised by the least possible number of optical elements. The installation space relief is necessary for example to arrange more optical elements for other optical functions. By way of example these other optical elements can be switchable screens and/or switchable polarizers. Such optical elements are used in so-called ellipsometers. Ellipsometry is a measuring method of material research and surface physics by which dielectric material properties and the layer thickness of thin layers can be determined.
Ellipsometry can be applied for examining various materials, for example organic or inorganic samples (metals, semi-conductors, insulators and also liquid crystals). Ellipsometry determines the change in the state of polarisation of light during reflection (or transmission) on a sample.
For such use the change of the polarisation state by additional mirrors also has a disruptive effect. It is also necessary with the measuring method of ellipsometry that several measuring beams, for example three measuring beams, have to be recombined very precisely on a sample, for example a wafer or other object to be illuminated. But static (temperature drift, mechanical drift) and dynamic (vibrations) angle errors of each additional mirror further reduce the angle error of all optical elements. The service life of an optical system is further defined via a drop in transmission by all optical elements. This decrease is caused in particular in the case of short wavelengths by molecular contamination of mirror layers. The service life becomes all the greater the fewer optical elements contribute to this process. The aim of the invention therefore is to provide an assembly for producing a plurality of beam bundles, which overcomes these drawbacks. This task is solved by an assembly as per the features of claim 1. Preferred embodiments are the subject matter of the respective dependent claims.
In a fundamental idea of the invention a mirror assembly for producing a plurality of beam bundles from the beam has a light source, whereby the plurality of beam bundles of at least a first beam bundle with a first main beam direction, a second beam bundle with a second main beam direction and preferably more beam bundles with more main beam directions has the following features: a first mirror segment with a first focal point, which converts a first partial area of the beam of the light source into the first beam bundle and a second mirror segment with a second focal point which converts a second partial area of the beam of the light source into the second beam bundle and preferably more mirror segments with more focal points, which convert more partial areas of the beam of the light source into more beam bundles, whereby the back side of the mirror segments has a curvature with the radius R_s, which is concentric to the light source.
In a preferred embodiment the mirror segments are configured as parabolic segments and the first parabolic segment converts the first partial area of the beam of the light source into a first collimated beam bundle and the second parabolic segment converts the second partial area of the beam of the light source into a second collimated beam bundle and the third parabolic segment converts a third partial area of the beam of the light source into a third collimated beam bundle. A light beam can easily be converted by this embodiment into a plurality of collimated beams or beam bundles, whereby these beams or beam bundles have a different main beam direction.
The word parabolic segment is also generally understood in this context as a mirror as a mirror which is corrected for a special optical task. A paraboloid converts the light of a point source ideally into a collimated beam. For other applications this mirror can have another form: with an ellipsoid the light of a point source can ideally be depicted in a point. With a spherical mirror a similar task can be solved as with a paraboloid or an ellipsoid. If the requirements for correction are less, both can also be done with spherical mirrors. The inventive idea can be even applied to plane mirrors and mirrors with other correcting properties.
In a preferred embodiment the mirror segments are configured as spherical segments and the first spherical segment converts the first partial area of the beam of the light source into a first beam bundle with a modified convergence and the second spherical segment converts the second partial area of the beam of the light source into a second beam bundle with modified convergence and the third spherical segment converts the third partial area of the beam of the light source into a third beam bundle with modified convergence.
In another preferred embodiment the mirror segments are configured as ellipsoid segments and the first ellipsoid segment converts the first partial area of the beam of the light source into a first focused beam bundle and the second ellipsoid segment converts the second partial area of the beam of the light source into a second focused beam bundle and the third ellipsoid segment converts the third partial area of the beam of the light source into a third focused beam bundle. A light beam can easily be converted into a plurality of light points by this embodiment, whereby these light points can be shown at various places.
In a more preferred embodiment are the mirror segments as plane mirror segments configured and the first plane mirror segment converts the first partial area of the beam of the light source into a first divergent beam bundle and the second plane mirror segment converts the second partial area of the beam of the light source into a second divergent beam bundle and the third plane mirror segment converts the third partial area of the beam of the light source into a third divergent beam bundle.
In another preferred embodiment the mirror segments are configured as free-form mirror elements and the first free-form mirror element converts the first partial area of the beam of the light source into a first specially corrected beam bundle and the second free-form mirror element converts the second partial area of the beam of the light source into a second specially corrected beam bundle and the third free-form mirror element converts the third partial area of the beam of the light source into a third specially corrected beam bundle.
In a more preferred embodiment the assembly also comprises a spherical shell segment or a cylindrical shell segment with the radius R_s, in which the mirror segments are arranged, whereby the individual mirror segments are connected to the spherical shell segment or cylindrical shell segment, in particular non-positively and/or positively. Such an embodiment makes it particularly easy to adjust the individual mirror segments.
In another preferred embodiment the mirror segments are parts of a common female part, whereby the female part has a working distance f relative to the light source at a point FP, with a vertex radius R_p, a back-side radius of curvature is R_s and a maximal thickness is D_max and the equation is D_max+R_p/2=R_s and the volume of the individual mirror segments is less than the volume of the female part.
The rear (back) side of the mirror segments is configured in particular as a spherical shell segment whenever beams are to be generated as defined and selectable in two spatial directions. In this case more special outer contours, straight guide edges for example, can make the adjustment easier.
If the beams to be produced span only one common plane, then the rear side of the mirror segments can also be a cylindrical shell segment which is set in a suitable load cylinder and adjusted. Then the outer contours are not need for fixing rotation of the segments about the optical axis.
An inventive design of the mirror verso enables advantageous use in the manufacture of multi-plane mirrors. Prisms are generally known for producing several beams from the light of one source by aperture division. Setting of the angle however is achieved as per the classic process of glass processing. Using the spherical or cylindrical rear side allows the angle to be adjusted precisely if necessary, as preferred for the short term. With a corresponding design the choice of angle can also first be in the specific application and then be ascertained permanently or detachably.
An advantage of the invention in general is also in the choice of paraboloids, ellipsoids or other freeform surfaces in that the known geometries of optical components can be used and producing several beams from one compact part requires no special technology for manufacturing mirrors.
It is clear to those skilled in the art that the inventive assembly can also be used in the opposite beam direction. In the embodiment with the parabolic segments several beam bundles from different directions can be focused on one point.
The invention also claims a process for manufacturing a mirror assembly for producing a plurality of beam bundles, whereby the process comprises the following features:
providing a first mirror segment, a second mirror segment and preferably more mirror segments;
providing a light source for sending out a beam;
arranging the mirror segments such that the first mirror segment converts a first partial area of the beam of the light source into a first beam bundle with a first main beam direction and the second mirror segment converts a second partial area of the beam of the light source into a second beam bundle with a second main beam direction and if needed more mirror segments convert more partial areas of the beam of the light source into more beam bundles with more main beam directions, whereby the back side of the mirror
segments has a curvature with the radius R_s, which is concentric to the light source.
In another preferred embodiment the method comprises the step of providing a first mirror segment, a second mirror segment and if needed more mirror segments and the step of arranging the mirror segments following additional steps of:
providing a female part with a working distance f relative to the light source on a point FP, with a vertex radius R_p, a rear-side radius of curvature R_s and a maximal thickness D_max satisfying a first equation R_p/2+D_max=R_s and a second equation R_p=2f gilt;
dividing the female part into at least a first, a second and if needed more mirror segments, whereby the volume of the first and second and if needed of more mirror segments is smaller than the volume of the female part;
providing a spherical shell segment or a cylindrical shell segment with a radius R_s, in which the mirror segments can be laid;
arranging the first mirror segment in the spherical shell segment or cylindrical shell segment;
arranging the second and if needed of more mirror segments in the spherical shell segment or cylindrical shell segment, whereby the arranging comprises rotation R of the second and if needed of more mirror segments about the point FP such that the second partial area of the beam of the light source is converted into a second beam bundle with a second main beam direction and if needed more partial areas of the beam of the light source are converted into more beam bundle with more main beam directions.
In another preferred embodiment the step of dividing the female part comprises dividing it into third segments, whereby at least a first, a second and a third mirror segments are cut out of the three third segments in each case. Such dividing can also save on material.
In another preferred embodiment the beam bundles are measured by a receiver and the step of rotation of the second and if needed of more mirror segments is conducted by way of measuring signals of the receiver. The corresponding angle between the main beam directions can be adjusted by means of measuring.
In another preferred embodiment the mirror segments have an outer contour with straight edges. In another preferred embodiment the mirror segments have straight edges which lie flush or parallel in pairs so that these edges can be used as a guide in the step of rotation in the spherical shell segment or cylindrical shell segment.
In another preferred embodiment the straight edges of mirror segment pairs enclose a defined angle of preferably 90°, which can also be shown in a device for assembling.
It is understood that the abovedescribed embodiments can be depicted in a unique position or in combination. A preferred embodiment comprises an ‘and/or’ link between a first feature and a second feature, so the outcome is that the embodiment has both the first feature and the second feature and as per another embodiment either the first feature only or the second feature only.

Advantageous embodiments of the present invention are explained in more detail hereinbelow with reference to the figures, in which:

FIG. 1 illustrates a schematic configuration of a first embodiment of an inventive assembly for producing a plurality of beam bundles;

FIG. 2 illustrates a schematic configuration of an inventive female part;

FIG. 3 illustrates a schematic representation of the process sequence for producing inventive parabolic segments;

FIG. 4 illustrates a perspective view of inventive parabolic segments;

FIG. 5 illustrates a further perspective view of inventive parabolic segments;

FIG. 6 illustrates a schematic configuration of an inventive ellipsometer;

FIG. 7 illustrates a schematic configuration of a second embodiment of an inventive assembly for producing a plurality of light points.

In the following description of favourable embodiments of the present invention identical or similar reference numerals are used for the elements illustrated in the various figures, which act the same, whereby repeated description of these elements is omitted.
FIG. 1 shows a schematic configuration of a first embodiment of an inventive assembly for producing a plurality of beam bundles. Such an assembly can be used for example in an ellipsometer, as will be described in more detail hereinbelow. The illustrated embodiment shows a so-called triple collimator. Such a collimator has three collimated beam bundles K1, K2 and K3, whereby these beam bundles are deflected by the inventive configuration into three different main beam directions SR1, SR2 and SR3.
The beam bundles K1, K2 and K3 originate from a common light source L, whereby the light source in this embodiment is an optic fibre, from which a light cone exits. The light cone exiting from the light source L, also designated as beam as per the invention, is reflected on three mirror segments 1 a, 1 b and 1 c. So a first partial area of the beam S1 is converted into a first collimated beam bundle K1, a second partial area of the beam S2 is converted into a second collimated beam bundle K2 and a third partial area of the beam is converted into a third collimated bean bundle K3.
With reference to FIG. 2 and FIG. 3 the manufacturing of the individual mirror segments 1 a, 1 b and 1 c will now be described in more detail.
The start point of manufacturing the individual mirror segments is an optical element designated according to the invention as a female part 1. The female part has a vertex radius R_p and a rear radius of curvature R_s. The maximal thickness of the female part is D_max, whereby the thickness reduces continuously to the edges of the female part. These equations apply to the female part:
D_max+R_p/2=R_s and (1)
R_p=2f, (2)
whereby f is the focal length of the female part and FP constitutes the focal point of the female part. The corresponding sizes are shown in FIG. 2. For a collimated beam bundle to be produced the mirror segments are designed as parabolic segments.
A solution as per the prior art for configuration of a group of three parabolic segments comprises adjusting each segment such that it fulfils its optical function individually and ensuring that the angle between the collimated beam bundles or respectively the preferred beam direction of the collimated beam bundles is produced correctly. For this a minimum number of degrees of liberty or respectively closely tolerated form elements is needed.
A light source must be in the focal point of all parabolic segments. For three parabolic segments three points must be brought to congruence. One segment simulates the point. Both other segments must be adjusted to the latter. A total of six degrees of liberty results for both other points with coordinates (x_i, y_i, z_i). The collimated beams are also to run in three beam directions. One parabolic segment simulates a direction. The other two segments must be adjusted to the latter. The beam direction can be represented depending on two angles alpha and beta giving that the equation SR=SR (alpha, beta). Four more degrees of liberty result for the two other beam directions so that a total of ten degrees of liberty is to be acknowledged and adjusted.
The invention is based on this prior art, in that it reduces the number of degrees of liberty and simplifies adjusting of the individual parabolic segments.
In a first procedural step the female part 1 is divided into parabolic segments 1 a, 1 b and 1 c. The individual parabolic segments are then arranged in a spherical shell segment or a spherical shell 2 with the radius R_s. The first parabolic segment with the focal point F1 sets the position of the light source L in this focal point. Rotation R of another parabolic segment about the common focal point FP results in a change in beam direction of the collimated beam bundles. Such a change in direction of the collimated beam direction is shown in FIG. 1. In the embodiment as per FIG. 1 a second parabolic segment lb and a third parabolic segment 1 c from a first position (dashed line) is rotated in a second position about the common focal point FP so that the beam directions SR2 and SR3 change relative to the beam direction SR1. Such a process in each case omits three degrees of liberty for the second parabolic segment 1 b and the third parabolic segment 1 c.
Strictly speaking, such a procedure guarantees via the spherical shell 2 only the degree of liberty in the z direction. The condition of rotation R about the common focal point FP must still be ensured. This condition is ensured by means of an adjusting device or respectively a device for assembling 7, as will be explained in more detail by way of FIG. 3 hereinbelow.
By way of FIG. 3 the process sequence for manufacturing an assembly for producing a plurality of collimated beam bundles will be specified in more detail. The starting point of the process is a female part 1, which as per this embodiment in a first procedural step is disassembled into three identically sized third segments 3 a, 3 b and 3 c. The female part is shown in FIG. 3 in a plan view. In the following only the third segment 3 a will be explained further, since the following procedural steps are identical for the other third segments.
In a second procedural step two straight cuts 4 a and 4 b are made on the third segment 3 a, which enclose an angle of 90°. In a third procedural step the three parabolic segments 1 a, 1 b and 1 c are excised and arranged in the spherical shell or the spherical shell segment 2. In a fourth procedural step the parabolic segments are shifted by means of the adjusting device 7 along the common edges 4 a, 4 b, so that the preferred angle or respectively the preferred beam direction SR1 and SR2 is set. Shifting the parabolic segments 1 b and 1 c corresponds to rotation R about the common focal point FP, as per FIG. 1.
FIG. 4 and FIG. 5 show the individual parabolic segments 1 a, 1 b and 1 c in the assembled state in the spherical shell 2 and also the optically used areas 10. The spherical shell 2 can also be connected to another base plate 3. Also, the component can have more form elements 11 a, 11 b and 11 c for mounting and more stabilising.
FIG. 6 shows a schematic configuration of an inventive ellipsometer as embodiment of an optical assembly. The ellipsometer is housed in a housing, not shown in more detail here. FIG. 6 clearly illustrates the whole beam path which starts out from a light source L. Partial areas of the beam are converted into collimated beam bundles K1, K2 and K3 by means of the inventive assembly 1. The beam bundles K2 and K3 are reflected on mirrors 5 a and 5 b and then deflected to a sample 6, for example a wafer. Specific information can be determined by the sample by means of the measuring radiation M1, M2 and M3 reflected on the sample 6.
FIG. 7 shows a schematic configuration of a second embodiment of an inventive assembly for producing a plurality of light points. Partial areas S1, S2 and S3 of a beam of a light source L are displayed in the light points LP1, LP2 and LP3 by means of ellipsoid segments 1 a, 1 b and 1 c.

Claims

1. A mirror assembly (1) for producing a plurality of beam bundles (K1, K2, . . . Kn) from the beam of a light source (L), whereby the plurality of beam bundles has at least a first beam bundle (K1) with a first main beam direction (SR1), a second beam bundle (K2) with a second main beam direction (SR2) and preferably more beam bundles (K3 . . . Kn) with more main beam directions, having the following features:

a first mirror segment (1 a) with a first focal point (F1), which converts a first partial area of the beam (S1) of the light source into the first beam bundle (K1) and a second mirror segment (1 b) with a second focal point (F2) which converts a second partial area of the beam (S2) of the light source into the second beam bundle (K2) and preferably more mirror segments (1 c) with more focal points (F3 . . . Fn), which convert more partial areas of the beam of the light source into more beam bundles (K3 . . . Kn), whereby the rear side of the mirror segments has a curvature with the radius R_s, which is concentric to the light source.

2. The mirror assembly as claimed in claim 1, characterised in that the mirror segments (1 a, 1 b, 1 c) are configured as parabolic segments and the first parabolic segment (1 a) converts the first partial area of the beam (S1) of the light source into a first collimated beam bundle (K1) and the second parabolic segment (1 b) converts the second partial area of the beam (S2) of the light source into a second collimated beam bundle (K2) and a third parabolic segment (1 c) converts the third partial area of the beam (S3) of the light source into a third collimated beam bundle (K3).

3. The mirror assembly as claimed in claim 1, characterised in that the mirror segments (1 a, 1 b, 1 c) are configured as spherical segments and the first spherical segment (1 a) converts the first partial area of the beam (S1) of the light source into a first beam bundle (K1) with a modified convergence and the second spherical segment (1 b) converts the second partial area of the beam (S2) of the light source into a second beam bundle (K2) with a modified convergence and the third spherical segment (1 c) converts the third partial area of the beam (S3) of the light source into a third beam bundle (K3) with a modified convergence.

4. The mirror assembly as claimed in claim 1, characterised in that the mirror segments (1 a, 1 b, 1 c) are configured as ellipsoid segments and the first ellipsoid segment (1 a) converts the first partial area of the beam (S1) of the light source into a first focused beam bundle (K1) and the second ellipsoid segment (1 b) converts the second partial area of the beam (S2) of the light source into a second focused beam bundle (K2) and the third ellipsoid segment (1 c) converts the third partial area of the beam (S3) of the light source into a third focused beam bundle (K3).

5. The mirror assembly as claimed in claim 1, characterised in that the mirror segments (1 a, 1 b, 1 c) are configured as plane mirror segments and the first plane mirror segment (1 a) converts the first partial area of the beam (S1) of the light source into a first divergent beam bundle (K1) and the second plane mirror segment (1 b) converts the second partial area of the beam (S2) of the light source into a second divergent beam bundle (K2) and the third plane mirror segment (1 c) converts the third partial area of the beam (S3) of the light source into a third divergent beam bundle (K3).

6. The mirror assembly as claimed in claim 1, characterised in that the mirror segments (1 a, 1 b, 1 c) are configured as free-form mirror elements and the first free-form mirror element (1 a) converts the first partial area of the beam (S1) of the light source into a first specially corrected beam bundle (K1) and the second free-form mirror element (1 b) converts the second partial area of the beam (S2) of the light source into a second specially corrected beam bundle (K2) and the third free-form mirror element (1 c) converts the third partial area of the beam (S3) of the light source into a third specially corrected beam bundle (K3).

7. The mirror assembly as claimed in any one of the foregoing claims, characterised in that the mirror segments are parts of a common female part (1), whereby the female part has a working distance f relative to the light source at a point (FP), with a vertex radius (R_p), a rear-side radius of curvature (R_s) and a maximal thickness (D_max) and the equation D_max+R_p/2=R_s applies and the volume of the individual mirror segments is smaller than the volume of the female part.

8. The mirror assembly as claimed in any one of the foregoing claims, characterised in that the assembly also comprises a spherical shell segment (2) with the radius R_s, in which the mirror segments are arranged, whereby the individual mirror segments with the spherical shell segment, in particular non-positively and/or positively, are connected.

9. The mirror assembly as claimed in any one of claims 1 to 7, characterised in that the assembly also comprises a cylindrical shell segment (2) with the radius R_s, in which the mirror segments are arranged, whereby the individual mirror segments are connected to the cylindrical shell segment, in particular non-positively and/or positively.

10. An optical assembly with a mirror assembly as claimed in any one of the foregoing claims, characterised in that the optical assembly also comprises optically depicting elements such as individual lenses, mirrors or lenses which are arranged in one or more of the beams and which can display the light source on a common point.

11. A process for manufacturing a mirror assembly as claimed in any one of the foregoing claims, whereby the process comprises the following features:

providing a first mirror segment (1 a), a second mirror segment (1 b) and preferably more mirror segments;

providing a light source (L) for sending out a beam (S);

arranging the mirror segments such that the first mirror segment (1 a) converts a first partial area of the beam (S1) of the light source into a first beam bundle (K1) with a first main beam direction (SR1) and the second mirror segment (1 b) converts a second partial area of the beam (S2) of the light source into a second beam bundle (K1) with a second main beam direction (SR1) and if needed more mirror segments convert more partial areas of the beam of the light source into more beam bundles (K3 . . . Kn) with more main beam

directions, whereby the rear side of the mirror segments has a curvature with the radius R_s, which is concentric to the light source.

12. The process as claimed in claim 11, characterised in that the step of providing a first mirror segment (1 a), a second mirror segment (1 b) and if needed more mirror segments and the step of arranging the mirror segments (1 a, 1 b, 1 c) comprises the following additional steps:

providing a female part (1) with a working distance f relative to the light source at a point (FP), with a vertex radius (R_p), a rear-side radius of curvature (R_s) and a maximal thickness (D_max), whereby a first equation is R_p/2+D_max=R_s and a second equation is R_p=2f;

dividing the female part in at least a first, a second and if needed more mirror segments, whereby the volume of the first and second and if needed of more mirror segments is smaller than the volume of the female part;

providing a spherical shell segment or a cylindrical shell segment with a radius R_s, in which the mirror segments can be laid;

arranging the first mirror segment in the spherical shell segment or cylindrical shell segment;

arranging the second and if needed of more mirror segments in the spherical shell segment or cylindrical shell segment, whereby the arranging step comprises rotation (R) of the second and if needed of more mirror segments about the point (FP), such that the second partial area of the beam of the light source is converted into a second beam bundle with a second main beam direction and if needed more partial areas of the beam of the light source are converted into more beam bundles with more main beam directions.

13. The process as claimed in claim 12, characterised in that the step of dividing the female part comprises disassembling into third segments (3 a, 3 b, 3 c) and at least a first, a second and a third mirror segment are cut out of the three third segments in each case.

14. The process as claimed in any one of claims 11 to 13, characterised in that the beam bundles (K1 . . . Kn) are measured by means of a receiver and rotation (R) of the second and if needed of more mirror segments is completed by way of measuring signals from the receiver.

15. The process as claimed in any one of claims 11 to 14, characterised in that mirror segments have an outer contour with straight edges (4 a, 4 b).

16. The process as claimed in claim 15, characterised in that the mirror segments have straight edges (4, 4 a, 4 b) which lie flush in pairs or in parallel so that these edges can be used as a guide in the step of rotation (R) in the spherical shell segment or cylindrical shell segment.

17. The process as claimed in claim 16, characterised in that the straight edges of mirror segment pairs enclose a defined angle of preferably 90°, which can also be shown in a device for assembling (7).