Method and apparatus for determining the refractive index of a wedge-shaped specimen

Abstract

The present invention provides a method for determining the refractive index of an optical test object (100) having a first surface (102) and a second surface (104) disposed at a wedge angle (ω) relative to the first surface (102), comprising the steps of: detecting a first reflected light beam (401), a second reflected light beam (402), and a third reflected light beam (413), wherein the first reflected light beam (401) represents a light beam reflected from the first surface (102), and the second reflected light beam (402) represents a light beam reflected from the first surface (102). , representing the light beam reflected by the second surface (104), and a third reflected light beam (413) representing the light beam reflected by the mirror element (420); determining a first angle (a) between the first reflected light beam (401) and the second reflected light beam (402) and determining a second angle (β) using the third reflected light beam (413); and calculating (315) the refractive index (n) using the first angle (a) and the second angle (β).

Description

[Technical Field]

[0001] The present technique relates to a method, apparatus and measurement system for determining the refractive index of an optical specimen. [Background technology]

[0002] Various techniques are known for measuring the refractive index of optical prisms or optical wedges with small prism angles, which may use an autocollimator to detect back reflections on the inner surface of the prism or wedge. For example, the refractive index of the wedge material can be determined from a known angle of incidence and apex using Snell's law of refraction. All of the methods described in the prior art share the feature of using a goniometer to determine the apex angle in preparation for the measurement, which increases the overall effort required for the measurement. Summary of the Invention [Means for solving the problem]

[0003] Against this background, the present approach presents a method, an apparatus and a measurement system for determining the refractive index of an optical object according to the main claims. Advantageous embodiments emerge from the respective dependent claims and the following description.

[0004] A potential advantage of this approach is that the refractive index of a wedge-shaped specimen may be measured even if the wedge angle or apex angle is unknown, which may allow for simple measurements that take very little time or may greatly simplify the measurement process.

[0005] A method for determining the refractive index of an optical test object is presented, the test object including at least one first surface and one second surface disposed at a wedge angle relative to the first surface. The method includes detecting a first reflected light beam, a second reflected light beam, and a third reflected light beam. The first reflected light beam represents a light beam reflected from the outside of the first surface, the second reflected light beam represents a light beam reflected from the inside of the second surface, and the third reflected light beam represents a light beam reflected from a mirror surface of a mirror element. The mirror element is understood to be disposed on an opposite side of the test object from a light source emitting the light beam. The method further includes determining a first angle using the first reflected light beam and / or the second reflected light beam, and determining a second angle using the third reflected light beam. The method further includes calculating the refractive index using the first angle and the second angle.

[0006] Using the method presented herein, for example, refractive index measurements can be performed on at least partially wedge-shaped lenses or other optical elements, e.g., prism-shaped. The test object may therefore also be referred to as a wedge or prism hereinafter. In a method step, reflections of a light beam emitted by a light source are detected. A first angle α is determined using these detected reflected light beams. This angle is enclosed by two light beams reflected from different test object surfaces. Depending on the orientation of the test object, the angle α can be determined, in one embodiment, using only the light beam reflected from the first test object surface. In such an embodiment, the orientation of the test object is monitored using an additional sensor. The first reflection may occur, for example, on the test object surface first impinged by the incident light beam. The second reflection may occur on the inside of the test object opposite the first test object surface, together forming the test object's wedge angle. Furthermore, a second angle β is determined. This is surrounded by an incident light beam and a light beam reflected by a mirror, which may be positioned, for example, below the object under test, and propagating again through the object under test. The position of the incident light beam is determined by reflection by the mirror in preparation for the measurement, without the object under test being in the beam path. The mirror is here positioned approximately perpendicular to the incident light beam. However, if the object under test is rotatably mounted, which is the preferred measurement configuration, the above-mentioned reference measurement without the object under test can be omitted. The angle β can therefore be determined directly from the diameter of the hit circle on the camera.

[0007] Finally, in a calculation step, the refractive index n of the specimen material is calculated. For example, this can be determined from two angles using the following relationship:

number

[0008] According to one embodiment, the method may include emitting a collimated light beam bundle from a light source toward the first surface. For example, the light beam may be output by a light source formed, for example, as an LED, and may be collimated using, for example, a collimator. In this case, for example, during generation of the light beam, the light source or the collimator may be arranged in series with the specimen or a mount and mirror element that holds the specimen. In this way, the specimen can be optimally illuminated and a reflected light beam with maximum intensity can be generated.

[0009] Furthermore, in the emitting step, the light beam is emitted at an angle of 80 to 100 degrees, particularly a right angle, relative to the first surface of the test object. For example, the light beam or multiple collimated light beams can be directed at a right angle to the outside of the first surface. This has the advantage that both the first angle and the second angle can be determined with high precision. This arrangement represents only a special case for illustrative purposes. The method also works even if the outside of the first surface is not aligned orthogonally to the incident light beam.

[0010] Furthermore, certain embodiments of the techniques presented herein are advantageous in that the test object is initially oriented such that the first and second surfaces are swapped with respect to their orientation. The first angle α can therefore be measured using the first reflected light beam without passing through the test object, thereby causing no change in the second angle β. The refractive index n can then be determined, for example, using the following formula:

number

[0011] For the function of this variant, it is useful to know how strongly the surface placed on the test stage is tilted relative to the optical axis of the incident light beam. For this purpose, the measuring device is extended with a sensor, which determines the tilt using the light beam reflected from the lower surface of the test object and can therefore calculate it for the measurement.

[0012] According to another embodiment, the method may include a step of aligning the mirror surface of the mirror element so that it is orthogonal to the incident light beam. For example, the mirror element may be formed to have a reflective mirror surface that is planar. In the aligning step, the mirror surface may be aligned so that it is nearly or sufficiently orthogonal to the incident light beam for this purpose, in which case it may be aligned, for example, parallel to the first surface of the test object. If the refractive index measurement is performed without rotating the test object, a prior calibration (identification of the zero point) of the mirror without the test object is required.

[0013] According to another embodiment, in the detecting step, the first reflected light beam, the second reflected light beam, and the third reflected light beam can be detected simultaneously. For example, a collimated light beam can be emitted starting from the light source in the direction of the specimen or a mirror element arranged behind the specimen, and the reflections of these light beams can be detected simultaneously. This provides the advantage that the method presented herein can be performed in a time-saving manner.

[0014] According to another embodiment, the steps of detecting, determining, and calculating can be performed repeatedly, in which case the reflected light beam is interpreted as a reflection of different wavelengths in each repeated detecting step. For example, the measurement of the refractive index can be performed sequentially at several different wavelengths. For this purpose, the light source can be implemented as a polychromatic light source, for example, a white light LED. To set one or more measurement wavelengths, for example, the autocollimator or the light source unit can have exchangeable optical filters. The objective lens of the autocollimator can be designed so that it can be moved linearly and thus compensate for longitudinal chromatic aberrations. The refractive index can therefore advantageously be determined particularly precisely.

[0015] According to another embodiment, the method may include a step of actuating a rotation of the specimen around a rotation axis arranged parallel to the light beam. This rotating step may here be performed simultaneously with the detecting step. Additionally or alternatively, the rotating step may also be performed simultaneously with the emitting step by the light source. For example, the specimen may be rotated by a corresponding mount around an axis that may extend parallel to the incident collimated light beam. This provides the advantage that the second angle may be determined directly from the diameter of the hit circle generated by the rotation, without the need for a mirror element.

[0016] The method may be implemented, for example, in software or hardware or a mixture of software and hardware, for example in a control unit.

[0017] The approach presented herein further provides an apparatus designed to carry out, operate or implement the steps of the method variants presented herein in a corresponding instrument, by means of which the underlying objectives of the invention can be achieved quickly and efficiently.

[0018] For this purpose, the device may include at least one computing unit for processing signals or data, at least one memory unit for storing signals or data, at least one interface with a sensor or actuator for inputting a sensor signal from a sensor or outputting a data or control signal to an actuator, and / or at least one communication interface for inputting or outputting data embedded in a communication protocol. The computing unit may be, for example, a signal processor, a microcontroller, etc., and the memory unit may be a flash memory or a magnetic memory unit. The communication interface may be designed to input or output data wirelessly and / or wired. A communication interface capable of inputting or outputting data wired may output these data, for example, electrically or optically from a corresponding data transmission line, or input these data to a corresponding data transmission line.

[0019] A device can be understood in this case as an electrical device whose function is to process sensor signals and output control and / or data signals. The device can include an interface that can be designed in hardware and / or software. In the case of a hardware design, the interface can be, for example, part of a so-called system ASIC, which includes the various functions of the device. However, the interface can also be a separate integrated circuit or can be at least partially composed of discrete components. In the case of a software design, the interface can be, for example, a software module that resides together with other software modules on a microcontroller.

[0020] Furthermore, a measurement system for measuring the refractive index of an optical test object is presented, which measurement system includes a variant of the aforementioned device. The device is here designed to activate a light source of the measurement system, and additionally or alternatively a receiving element, and additionally or alternatively a mirror element. Furthermore, the measurement system includes a light source for emitting a light beam, a receiving element for receiving the test object, and a mirror element, the mirror element being arranged on the side of the receiving element opposite the light source. The measurement system presented here offers the advantage that the aforementioned method can be optimally implemented.

[0021] According to one embodiment, the measurement system may have a collimator including a light source, and the collimator may include multiple exchangeable optical filters for filtering the light beam. For example, the measurement system may include an autocollimator for illuminating the test object and detecting reflections occurring on each test object surface. The light source and reticle may be arranged, for example, in the illumination arm of the autocollimator. In this case, the light source may be, for example, polychromatic and may be implemented, for example, as a white light LED. The autocollimator or light source unit may advantageously have exchangeable optical filters to allow one or more measurement wavelengths to be set. Additionally or alternatively, the objective lens of the autocollimator may be designed so that it can be linearly moved, thereby compensating for longitudinal chromatic aberrations.

[0022] According to another embodiment, the mirror element can have a settable aperture stop. The mirror can have an additional settable aperture stop, for example, for beam limitation. This has the advantage that the contrast of the detected reflections can be matched to one another by changing the intensity of the reflected light beam. Furthermore, the reflectivity of the mirror surface can advantageously be comparable to the reflectivity of the test object surface, so that reflections at individual surfaces can be detected without adjusting the camera integration time.

[0023] According to another embodiment, the receiving element may be rotatably mounted. For example, the receiving element may optionally be rotatably mounted around a rotation axis extending parallel to the optical axis of the autocollimator, thereby advantageously allowing a tilt between the rotation axis of the specimen mount and the mirror surface or a tilt of the mirror relative to the optical axis of the autocollimator to be compensated for by a computer. The specimen may alternatively be mounted on a specimen stage. In this embodiment, the autocollimator is advantageously positioned below the specimen and the mirror above the specimen, and the wedge is best placed so that one of its long faces rests on the specimen stage.

[0024] According to another exemplary embodiment, the light source and the mirror element can be rotatable around a common axis. For example, instead of rotating the specimen mount, the autocollimator can also be rotated together with the mirror around a corresponding common axis. Here, the specimen can remain fixed in place together with the receiver. With this variant, tilt between the rotation axis of the specimen mount and the mirror surface or tilt of the mirror relative to the optical axis of the autocollimator can advantageously also be compensated for by a computer. If the receiver element is not rotated, in a preparation step, the mirror surface is aligned towards the autocollimator or a residual error in the tilt of the mirror surface relative to the optical axis of the autocollimator is determined. This preparation step is performed without a specimen.

[0025] According to another embodiment, the receiving element can be designed to receive a liquid and additionally or alternatively a solid. For example, the measurement system can also advantageously be used to measure the refractive index of a liquid. For this purpose, the liquid to be measured is stabilized in a suitable manner. The liquid can be fixed, for example, between two plane-parallel plates so that it assumes the shape of a wedge. Therefore, the measurement can also advantageously be performed on solids. In this arrangement, the test object remains fixed in place, and the autocollimator rotates together with the mirror. Furthermore, an extremely high flatness of the plane-parallel plates can be ensured.

[0026] Also advantageous is a computer program product or computer program, which may be stored on a machine-readable carrier or a memory medium such as a semiconductor memory, a hard drive memory or an optical memory, and which comprises a program code which is used to perform, implement and / or activate the steps of the method according to one of the above-described embodiments, in particular when the program product or program is executed by a computer or device.

[0027] Exemplary embodiments of the techniques presented herein are illustrated in the drawings and explained in detail in the following description. [Brief explanation of the drawings]

[0028] [Figure 1] 1 illustrates a schematic diagram of an optical specimen in accordance with an exemplary embodiment; [Figure 2] 1 shows a schematic diagram of an optical specimen and a measurement method. [Figure 3] 1 is a flowchart of a method for determining the refractive index of an optical specimen, according to an example embodiment. [Figure 4a] 1 shows a schematic diagram of an exemplary embodiment of an optical specimen; [Figure 4b] 1 shows a schematic diagram of an exemplary embodiment of an optical specimen; [Figure 4c] 1 shows a schematic diagram of an exemplary embodiment of measuring the refractive index of an optical specimen; [Figure 4d] 1 shows a schematic diagram of an exemplary embodiment of measuring the refractive index of an optical specimen; [Figure 4e] 1 shows a diagram illustrating the change in refractive index with respect to the ratio of two angles α and β. [Figure 5] 1 illustrates a flowchart of a method for determining the refractive index of an optical specimen, in accordance with an example embodiment. [Figure 6a] 1 shows a schematic diagram of an exemplary embodiment of a measurement system for measuring the refractive index of an optical specimen; [Figure 6b] 1 shows a schematic diagram of an exemplary embodiment of a measurement system for measuring the refractive index of an optical specimen; [Figure 7a]1 shows an exemplary camera image of a reflection detected when the test object has a rotation (azimuth angle) of 0°. [Figure 7b] 1 shows an exemplary camera image of the detected reflection when the object is rotated (azimuth) by 180°. [Figure 8] 1 shows a schematic cross-sectional view of an exemplary embodiment of a receiving element for receiving a liquid specimen. DETAILED DESCRIPTION OF THE INVENTION

[0029] In the following description of preferred exemplary embodiments of the present invention, the same or similar reference numerals are used for elements that appear in different drawings and function similarly, and repeated descriptions of these elements will be omitted.

[0030] FIG. 1 shows a schematic diagram of an optical test object 100 according to an exemplary embodiment. The test object 100 shown here is formed, by way of example only, as a wedge-shaped prism and includes a first surface 102 and a second surface 104 disposed at a wedge angle ω relative to the first surface 102. The wedge angle ω can also be generally designated as an apex angle A. When the test object 100 is illuminated using a light beam 110 from the first surface 102 side, the light can be deflected in a general path through a prism or wedge. The deflection angle δ of the light beam 110 here depends on the refractive index n and the wedge angle ω. The deflection angle δ here is represented by the following equation: δ=(nn Air )ω

[0031] 2 shows a schematic diagram of an optical test object 100 according to an exemplary embodiment. The test object 100 shown here corresponds to or is similar to the test objects described in the previous figures. The measurement method shown in this figure corresponds to the prior art, where back reflections at the inner surface of a prism / wedge can be detected by an autocollimator 200. In this method, the refractive index of the wedge material is determined from a known angle of incidence i and apex angle A using Snell's law of refraction.

[0032] The prior art measurement method presented here has the drawback that in preparation for measuring the refractive index, the apex angle A or the wedge angle ω is specified using a goniometer, which increases the overall measurement effort.

[0033] 3 is a flowchart of a method 300 for determining the refractive index of an optical specimen, according to an example embodiment. The method illustrated here also applies to the case of a specimen described in the previous figures, which is assumed to have at least one first surface and a second surface disposed at a wedge angle relative to the first surface.

[0034] The method 300 includes detecting 305 a first reflected light beam, a second reflected light beam, and a third reflected light beam, where the first reflected light beam here represents a light beam reflected off the outside of the first surface, the second reflected light beam here represents a light beam reflected off the inside of the second surface, and the third reflected light beam here represents a light beam reflected off a mirror surface of a mirror element, where the mirror element here is understood to be located on an opposite side of the object to a light source emitting the light beams.

[0035] The method 300 further includes the step 310 of determining a first angle between the first reflected light beam and the second reflected light beam, and determining a second angle using the third reflected light beam.

[0036] Additionally, the method 300 includes a step 315 of calculating the refractive index using the first angle and the second angle.

[0037] The method 300 described herein is based on the observation that the refractive index of a wedge-shaped specimen can be measured without knowing the wedge angle or apex angle, thereby eliminating the need for multi-step measurements but greatly simplifying the measurement process.

[0038] Figures 4a and 4b respectively show schematic diagrams of an exemplary embodiment of an optical specimen 100 on which the method for determining the refractive index as previously described in Figure 3 can be implemented. In Figure 4a the determination of the first angle α is shown, and in Figure 4b the determination of the second angle β is shown.

[0039] In one exemplary embodiment, during measurement, the test object 100 is illuminated, by way of example only, using collimated light, which is illustrated in the diagrams shown here by light beam 110. In one advantageous embodiment, the first surface 102 of the test object 100, i.e., the test surface, is oriented, by way of example only, orthogonal to the incident light beam.

[0040] In the first measurement step shown in FIG. 4a, a first angle α can be determined. This angle is located between the first reflected light beam 401 and the second reflected light beam 402, where the first reflected light beam 401 represents the light beam 110 reflected by the outer side 405 of the first surface 102, and the second reflected light beam 402 represents the light beam 110 reflected by the inner side 410 of the second surface 104. In other words, the first angle α is enclosed by the two light beams reflected by the different test object surfaces 102, 104. The first reflection here occurs at the test object surface that the incident light beam first strikes. The second reflection here occurs at the inner side of the test object 100 opposite the first test object surface and forming a wedge angle ω therewith.

[0041] In one exemplary embodiment, the first angle α may be calculated using the following formula: α * n Air =2nω

[0042] In the second measurement step shown in FIG. 4 b, a second angle β can be determined. This angle is located between the incident light beam 110 and the third reflected light beam 413, which represents light reflected by a mirror surface 415 of a mirror element 420. In the illustrated example, the mirror element 420, which may also be simply referred to as a mirror, is positioned on the side of the second surface 104 of the test object 100, opposite the light source 225 that outputs the light beam 110, so that the mirror surface 415 is parallel to the first surface 102. Furthermore, the incident light beam 110 extends perpendicular to the mirror surface 415. Therefore, the second angle β is the angle enclosed by the light beam reflected by a mirror positioned below the test object 100 in the illustrated example and propagating again within the test object, and the light beam reflected by the mirror without the test object in its beam path. However, this method works even if the first test object surface 405 and the mirror surface 415 are not aligned parallel to one another and orthogonal to the incident light beam 110. In other words, the angle β is that enclosed by the light beam 413 and a reference reflected light beam that occurs at the mirror surface 415 without the test object being positioned in the beam path. The reference reflected light beam is not shown here for clarity because it would coincide with the incident light beam 110 if the mirror were aligned orthogonal to the incident light beam 110. In preferred applications where the test object is rotated, a test object-free reference measurement is not necessary. As the test object is rotated, the reflected light beam 413 generates a hit circle at the detection surface. The angle β can be calculated directly from the diameter of this hit circle. This relationship is explained in more detail below with respect to FIG. 7. In summary, at this point, it can be said that the angle β can be determined using the reflected light beam 413.

[0043] Alternatively, the specimen can be illuminated from below, in which case the mirror is positioned above the specimen.

[0044] The second angle β is calculated, by way of example, using the following formula: β * n Air =2(nnAir )ω

[0045] Furthermore, the measurement is designed in one exemplary embodiment such that all reflections can be detected simultaneously, thereby allowing both angles α, β to be measured simultaneously.

[0046] Therefore, the refractive index of the test material can be determined from the two angles α and β using the following relationship:

number

[0047] Note that in this regard, an absolute angle measurement is not necessary: ​​the measurement does not omit the calibration coefficients of the angle measurement system, since the angle is included in the numerator and denominator of the equation.

[0048] The test object 100 is here designed, by way of example only, so that the wedge angle ω has a value of <1° or <0.5°.

[0049] Another advantage of the measurement method described herein is that no calibration of the measurement system is required, since only the ratio of the two angles α and β is decisive for the refractive index measurement.

[0050] Figure 4c shows a schematic diagram of an exemplary embodiment for measuring the refractive index of an optical test object 100, which can implement the method for determining the refractive index as previously described in Figure 3. Unlike the illustrations of Figures 4a and 4b, the test object is oriented with face 104 facing upwards, and therefore the incident light beam 110 is reflected at face 104 at an angle α. The angle α can be determined using the first reflected light beam 401 from the following relationship: α=2ω

[0051] The above relationship also applies to the angle β. For this exemplary embodiment to work, it is not necessary for the test object surface 102 to be oriented as orthogonal as possible to the reference axis, e.g., the optical axis of the incident light beam 110. If there is a tilt angle δ between the surface 102 and the reference axis, e.g., the optical axis of the incident light beam 110, it is determined. This tilt angle can then be calculated for the measurement.

[0052] The tilt angle δ can be determined, for example, by a sensor (not shown in FIG. 4c) that emits a sensor light beam 430 onto the surface 102 of the test object 100 and detects a corresponding sensor reflected light beam 435 on the opposite side to determine its direction. Thus, the tilt angle δ and / or the position of the test object 100 relative to the sensor can be determined or detected based on the known directions of the sensor light beam 430 and the sensor reflected light beam 435. If the surface 102 of the test object 100 is aligned at an angle of 90° with respect to the optical axis of the incident light beam 110, it can be assumed that α=2·ω applies to the first angle. For the second angle β, which represents or maps the angle of transmission of light through the test object 100, β=2·(nn Α )·ω / n Α holds. When both equations are solved for n, the following relationship is obtained:

number

[0053] 4d shows a schematic diagram of an exemplary embodiment for measuring the refractive index of an optical test object 100, which allows the method for determining the refractive index as previously described in FIG. 3 to be implemented. Unlike the exemplary procedure of FIG. 4c, here the rotation 440 of the test object 100 is also taken into account. Therefore, the relationship between the first angle α and the second angle β is: α=4·ω and β=4·(nn Α )·ω / n ΑWhen the specimen is rotated, a factor of 2 is incorporated into the formula for the angles α and β, regardless of the specimen orientation. The rotation does not affect the calculation of the refractive index. However, this is methodologically advantageous, as the amount of data thus increases, especially when the entire hit circle is measured. Therefore, adjustment errors in the measurement system that lead to angle measurement errors can also be recognized and more easily corrected by shifting the hit circle.

[0054] To provide some context for the measurement setup, Fig. 4e shows a diagram 450 illustrating the variation of the refractive index with respect to the ratio v of the two angles α and β, where v = β / α is plotted on the horizontal axis, and the refractive index of air η Air The refractive index η of the test object 100 at η is plotted on the vertical axis. A first graph 455 represents the determination of the refractive index η of the test object 100 using the embodiment of FIGS. 4a and 4b, and a second graph 460 represents the refractive index η of the test object 100 determined using the exemplary embodiment according to FIG. 4c or 4d. The measurement error of the refractive index correlates with the slope of the respective curve. The presented procedures according to FIGS. 4c and 4d can help to reduce or keep small the effect of measurement errors, especially for high refractive indices.

[0055] 5 shows a flowchart of a method 300 for determining the refractive index of an optical specimen according to an exemplary embodiment. The method 300 shown here corresponds to or is similar to the method shown in FIG. 3 above, except that the method 300 includes additional and / or optional steps to the exemplary embodiments described herein.

[0056] In this exemplary embodiment, the detecting step 305 is preceded by a emitting step 500. In the emitting step 500, a collimated light beam bundle is emitted by a light source in the direction of a first surface of the specimen.

[0057] In this exemplary embodiment, the light beam is emitted at a normal angle to the first surface of the specimen in the emitting step 500. In other exemplary embodiments, the light beam may be emitted at an angle of 80-100 degrees to the first surface of the specimen.

[0058] Furthermore, the method 300 of this exemplary embodiment includes an optional step 505 of aligning the mirror surfaces of the mirror elements so that they are orthogonal to the light beam. Aligning the mirror surfaces is necessary whenever it is technically impossible to rotate the wedge-shaped specimen or the autocollimator. The aligning step is performed without the specimen in the beam path.

[0059] By way of example only, the detecting step 305 occurs in this exemplary embodiment only after the emitting step 500. Simultaneous with the detecting step, this embodiment also occurs with an actuating step 510. In the actuating step 510, for example, only rotation of the specimen about a rotation axis arranged parallel to the light beam is actuated, whereby the reflected light beam is detected at various rotational positions of the specimen.

[0060] By way of example only, in the detecting step 305, the first reflected light beam, the second reflected light beam, and the third reflected light beam are detected simultaneously.

[0061] Furthermore, in one exemplary embodiment, the detecting step 305, identifying step 310, and calculating step 315 are performed iteratively, and in the iterative detecting step 305, the reflected light beam is interpreted as reflections of different wavelengths.

[0062] In other words, the method 300 for measuring the refractive index of a wedge-shaped specimen described in this figure and the previous FIG. 3 can be described as follows.

[0063] First, a collimated light beam is emitted in the direction of a first surface of the test object, and in an optional preparation step, the mirror surfaces of the mirror elements are aligned so as to be orthogonal to the incident collimated light beam without the test object being placed in the beam path. The incident collimated light beam strikes the first surface of the test object at an angle of approximately 90°, here by way of example only. In one exemplary embodiment, the test object is rotated around an axis extending parallel to the incident collimated light beam. In this case, the step of aligning the mirror surfaces is omitted.

[0064] Thus, a first light beam reflected from a first surface of the test object is detected, and a second light beam reflected from within a second surface of the test object is detected, the second surface forming a wedge angle with the first surface, and a first angle α is determined between the first and second reflected light beams.

[0065] Furthermore, a third light beam reflected from a mirror disposed sequentially behind the second surface of the test object in the beam direction is detected after it reflects from the mirror surface and propagates again within the test object. The third reflected light beam is used to determine a second angle β. In one exemplary embodiment, the first, second, and third reflected light beams are detected simultaneously.

[0066] Thus, the refractive index of the wedge-shaped specimen is calculated from the first and second angles. In one exemplary embodiment, the refractive index measurements are taken sequentially at a number of different wavelengths.

[0067] 6a shows a schematic diagram of an exemplary embodiment of a measurement system 600 for measuring the refractive index of an optical specimen 100. The measurement system 600 includes a light source 225 for emitting a light beam 110, a receiving element 610 for receiving the specimen 100, a mirror element 420, and an apparatus 620 designed to control a method for determining the refractive index of the optical specimen 100, as described in previous figures 3 and 5.

[0068] In one exemplary embodiment, the light source 225 forms part of an autocollimator 200 that collimates light emitted from the light source 225 and directs it towards the test object 100. In this case, the autocollimator includes the light source 225, which is, by way of example only, polychromatic, and optionally a number of interchangeable optical filters in the beam path for filtering the light emitted by the light source 225.

[0069] The mirror element 420 is arranged on the opposite side of the receiving element 610 from the light source 225. In other words, the autocollimator, the specimen mount, and the mirror are arranged one after the other. The receiving element 610 is designed in this exemplary embodiment as a mechanical mount for the wedge-shaped specimen 100 and is rotatably mounted, by way of example only. Similarly, the autocollimator 200, which includes the light source 225 and the mirror element 420, is rotatable about a common axis 630 in one exemplary embodiment.

[0070] By way of example only, the mirror element 420 is introduced into the beam path using a kinematic device and optionally includes a configurable aperture stop. The mirror surface and the surface of the test object can also be designed to have similar reflectivity. However, it is important to ensure that the reflections occurring on the test object surface and the reflections occurring on the mirror surface are distinguishable so that the detected signals can be assigned to the corresponding light beams. It is useful to know the reflection coefficient of the test object for the desired wavelength. The configurable aperture stop can be used to adjust the intensity of the light beam reflected from the mirror surface, which helps to distinguish the light beams detected by the detector.

[0071] In one exemplary embodiment, the device 620 is designed, by way of example only, to output light by the light source, as well as to cause an exemplary movement of the objective lens of the autocollimator parallel to its optical axis. Additionally or alternatively, a rotation of the receiving element 610 or a rotation of the autocollimator 200 and the mirror element 420 can also be caused.

[0072] Fig. 6b shows a schematic diagram of an exemplary embodiment of a measurement system 600 for measuring the refractive index of an optical test object 100. Unlike the exemplary embodiment of the measurement system 600 shown in Fig. 6a, in the measurement system 600 shown in Fig. 6b, a detection sensor 650 for the tilt angle δ is also provided, the results of which are also taken into account or processed in the device 620, as described above. The measurement system 600 according to the example of Fig. 6b is therefore only extended by the following function: a first angle α is determined using a first light beam reflected from a beveled edge of the test object, and the measurement device is extended by an additional sensor 650 (herein referred to as a tilt sensor) used to detect the tilt of the surface 102 with respect to a reference axis (optical axis) of the light beam 110.

[0073] In other words, the measurement system 600 presented herein and the methods executable thereby can be described as follows.

[0074] The measurement system 600 includes an autocollimator for illuminating the test object 100 and detecting reflections occurring on the respective test object surface. A reticle, which may be designed, for example, as a pinhole or a cross, is positioned in the illumination arm of the autocollimator. The detection arm includes, for example, a CMOS or CCD camera. The light source 225 is, for example, polychromatic and implemented, for example, as a white light LED. The autocollimator or light source unit may have exchangeable optical filters for setting one or more measurement wavelengths. The objective lens 210 of the autocollimator may be designed so that it can be moved linearly and thus compensate for longitudinal chromatic aberrations.

[0075] The test object 100 is fixed to a suitable mount, which can optionally be mounted rotatably about a rotation axis extending parallel to the optical axis of the autocollimator, so that tilt between the rotation axis of the test object mount and the mirror surface or tilt of the mirror with respect to the optical axis of the autocollimator can be compensated for by a computer. Instead of rotating the test object mount, the autocollimator can also be rotated together with the mirror about a corresponding common axis 630. If the test object mount is not rotated, in a preparation step, the mirror surface is aligned toward the autocollimator or a residual error in the tilt of the mirror surface with respect to the optical axis of the autocollimator is determined. This preparation step is performed without the test object 100.

[0076] The specimen 100 can alternatively be mounted on a specimen stage. In this embodiment, the autocollimator is advantageously positioned below the specimen, the mirror is positioned above the specimen, and the wedge is best placed on one of the long sides of the specimen stage.

[0077] The mirror can also be pivoted into or out of the beam path via a motion mount.

[0078] Furthermore, the reflectivity of the mirror surface is advantageously comparable to that of the test object surface, allowing detection of reflections at individual surfaces without adjusting the camera integration time. Optionally, the mirror may have an additional configurable aperture stop for beam limitation. Therefore, by varying the intensity of the reflected light beam, the contrast of the detected reflections can be adjusted relative to one another.

[0079] 7a shows, by way of example, a first camera image of three reflected light beams 401, 402, 413. The corresponding detected signals are indicated by reference signs 701, 702, 713. In this example, the angle α can be determined from the distance of signals 701 and 702. In this case, to determine the angle β, the mirror would have to be aligned orthogonal to the optical axis of the autocollimator. Therefore, the angle β can be determined from the distance of signal 713 to a reference signal (not shown), which is detected without an object being placed in the beam path.

[0080] FIG. 7b shows, by way of example, a second camera image of the reflected light beam. In this case, the test object has been rotated by an angle (azimuth) of 180°. In addition to signals 701, 702, and 713, additional signals 701′, 702′, and 713′ are detected, which lie on the circular paths of the respective hit circles. As shown in the figure, the connection between signals 713 and 713′, which represent the diameter of the hit circle, can be used to determine angle β. The tilt of the mirror simply ensures that the hit circle with diameter 713-713′ is linearly shifted on the detector, without changing the diameter and therefore angle β.

[0081] Figure 8 shows a schematic cross-sectional view of an exemplary embodiment of a receiving element 610 for receiving a liquid specimen 100. The receiving element 610 shown here corresponds to or is similar to the receiving element previously described in Figure 6. The receiving element 610 is designed in this exemplary embodiment to receive a specimen 100 that is, by way of example only, a liquid.

[0082] The measurement system described above in FIG. 6 or the method described above in FIGS. 3 and 5 can also be used to measure the refractive index of a liquid. For this purpose, the liquid to be measured is stabilized in a suitable manner as illustrated in the diagram shown here. The liquid is here fixed in the shape of a wedge between two plane-parallel plates of the receiving element 610, for example. Measurements can therefore be performed similarly to measurements on solids. In this arrangement, the test object 100 remains fixed in place, while the autocollimator rotates together with the mirror. Furthermore, an extremely high flatness of the plane-parallel plates can be ensured.

[0083] If an exemplary embodiment includes the conjunction "and / or" between a first feature and a second feature, this shall be interpreted to mean that the exemplary embodiment includes both the first feature and the second feature according to one embodiment, and either only the first feature or only the second feature according to another embodiment.

Claims

1. A method (300) for determining the refractive index of an optical test object (100), the test object (100) being assumed to include at least one first surface (102) and a second surface (104) disposed at a wedge angle (ω) relative to the first surface (102), the method (300) comprising the following steps (305, 310, 315): a step (305) of detecting a first reflected light beam (401), a second reflected light beam (402), and a third reflected light beam (413), where the first reflected light beam (401) represents a light beam reflected from the outside (405) of the first surface (102), the second reflected light beam (402) represents a light beam reflected from the inside (410) of the second surface (104), and the third reflected light beam (413) represents a light beam reflected from a mirror surface (415) of a mirror element (420), where the mirror element (420) is located on an opposite side of the test object (100) from a light source (225) that emits the light beam (110); determining (310) a first angle (α) using the first reflected light beam (401) and / or the second reflected light beam (402) and determining a second angle (β) using the third reflected light beam (413); calculating (315) the refractive index using the first angle (α) and the second angle (β); Including, Actuating (510) a rotation of the test object (100) about an axis of rotation arranged parallel to the light beam (110), the step (510) being performed simultaneously with the detecting step (305). The method (300) is characterized by:

2. 2. The method (300) of claim 1, further comprising a step (500) of emitting a collimated light beam bundle from the light source (225) in the direction of the first surface (102), and in particular, in the emitting step (500), the light beam (110) is emitted at an angle of 80 to 100 degrees, in particular at a right angle, with respect to the first surface (102) of the test object (100).

3. 3. The method (300) of claim 2, wherein in the detecting step (305), an inclination angle (δ) at which the second surface (104) is inclined with respect to the incident direction of the light beam (110) is detected, and in the calculating step (315), the refractive index is calculated taking into account the inclination angle (δ).

4. The method (300) of any one of claims 1 to 3, comprising the step of aligning (505) the mirror surfaces (415) of the mirror elements (420) substantially orthogonal to the light beam (110).

5. 4. The method (300) of claim 1, wherein in the detecting step (305), the first reflected light beam (401), the second reflected light beam (402) and the third reflected light beam (413) are detected simultaneously.

6. 4. The method (300) of any one of claims 1 to 3, wherein the steps of detecting, identifying and calculating (305, 310, 315) are performed iteratively, and in the iterative step of detecting (305), the reflected light beams (401, 402, 413) are interpreted as reflections of different wavelengths.

7. The method (300) according to any one of claims 1 to 3, wherein in the emitting step (500), the light beam (110) is emitted at an angle of 80 to 100°, in particular at a right angle, relative to the first surface (102) of the test object (100).

8. An apparatus (620) configured to implement and / or activate the steps (305, 310, 315) of the method (300) of any one of claims 1 to 3 in a corresponding unit.

9. A measurement system (600) for measuring the refractive index of an optical specimen (100), comprising: an apparatus (620) configured to implement and / or activate the steps (305, 310, 315) of the method (300) according to any one of claims 1 to 3 in a corresponding unit, the apparatus (620) being designed to activate a light source (225), and / or a receiving element (610), and / or a mirror element (420) of the measurement system (600) and to activate a rotation of the test object (100) around an axis of rotation arranged parallel to the light beam (110), the rotation being performed simultaneously with the detecting step; said light source (225) for emitting a light beam (110); the receiving element (610) for receiving the specimen (100); the mirror element (420) disposed on the opposite side of the receiving element (610) from the light source (225); A measurement system (600) comprising:

10. 10. The measurement system (600) of claim 9, comprising an autocollimator (200) including the light source (225), the autocollimator (200) including a plurality of interchangeable optical filters for filtering the light beam (110), and in particular the autocollimator (200) having at least one optical element (210) that is movable parallel to the optical axis of the autocollimator (200).

11. 10. The measurement system (600) of claim 9, wherein the mirror element (420) includes an adjustable aperture stop and / or the mirror element (420) is pivotable into the beam path of the light beam (110).

12. 10. The measurement system (600) of claim 9, wherein the receiving element (610) is rotatably mounted.

13. 10. The measurement system (600) of claim 9, wherein the light source (225) and the mirror element (420) are rotatable about a common axis (630).

14. 10. The measurement system (600) of claim 9, wherein the receiving element (610) is designed to receive liquids and / or solids.

15. A computer program arranged to perform and / or operate the steps of the method (300) of any one of claims 1 to 3 when said computer program is run on a computer or device.

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