Method and device for determining the refractive index of a wedge-shaped test object

Abstract

The invention relates to a method for determining a refractive index of an optical test object (100), which has a first surface (102) and a second surface (104) arranged at a wedge angle (ω) to the first surface (102), the method comprising the following steps: detecting a first reflection light beam (401), a second reflection light beam (402) and a third reflection light beam (413), wherein the first reflection light beam (401) represents a light beam reflected at the first surface (102) and wherein the second reflection light beam (402) represents a light beam reflected at the second surface (104) and wherein the third reflection light beam (413) represents a light beam reflected at a mirror element (420), determining a first angle (a) between the first reflection light beam (401) and the second reflection light beam (402) and determining a second angle (ß) with the aid of the third reflection light beam (413), and calculating (315) the refractive index (n) using the first angle (a) and the second angle (ß).

Description

[0001] Method and device for determining the refractive index of a wedge-shaped test specimen

[0002] The present approach relates to a method for determining a refractive index of an optical test specimen, a device and a measuring system.

[0003] Various approaches are known for measuring the refractive index of optical prisms with small prism angles or optical wedges. These can detect a back reflection on the inner side of the prism or wedge using an autocollimator. For example, the refractive index of the wedge material can be determined using Snell's law of refraction, given a known angle of incidence and apex. The methods described in the prior art have the common feature that the apex angle is determined prior to the measurement using a goniometer, which increases the overall measurement effort.

[0004] Disclosure of the invention

[0005] Against this background, the present approach presents a method for determining the refractive index of an optical test specimen, a device, and a measuring system according to the main claim. Advantageous embodiments emerge from the respective subclaims and the following description.

[0006] The advantages achievable with the presented approach are that the refractive index of a wedge-shaped test specimen can be measured without knowing the wedge angle or the apex angle, which enables a simple measurement with little time expenditure and significantly simplifies the measurement process.

[0007] A method is presented for determining a refractive index of an optical test specimen, wherein the test specimen comprises at least a first surface and a second surface arranged at a wedge angle to the first surface. The method comprises a step of detecting a first reflection light beam, a second reflection light beam, and a third reflection light beam. The first reflection light beam represents a light beam reflected from an outer side of the first surface, the second reflection light beam represents a light beam reflected from an inner side of the second surface, and the third reflection light beam represents a light beam reflected from a mirror surface of a mirror element. The mirror element is interpreted as being arranged on a side of the test specimen opposite the light source emitting the light beam.The method further comprises a step of 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 also comprises a step of calculating the refractive index using the first angle and the second angle.

[0008] Using the method presented here, for example, a refractive index measurement can be carried out on an at least partially wedge-shaped lens or another optical element, for example a prism-shaped one. Accordingly, the test object can also be referred to below as a wedge or a prism. In one method step, reflections of a light beam emitted by a light source are recorded. Using these recorded reflected light beams, a first angle α is determined. This angle is enclosed by two light beams reflected from different test object surfaces. Depending on the orientation of the test object, in one embodiment the angle α can be determined using only the light beam reflected from the first test object surface. In such an embodiment, the orientation of the test object should be monitored using an additional sensor.The first reflection can occur, for example, on the surface of the test specimen, which is first hit by the incident light rays. The second reflection can occur on an inner side of the test specimen, which is opposite the first surface of the test specimen and includes the wedge angle of the test specimen with it. In addition, a second angle ß is determined. This is the angle between the incident light rays and light rays that are reflected by a mirror, which can be located below the test specimen, for example, and propagate through the test specimen a second time. The position of the incident light rays should be determined prior to the measurement, without the test specimen in the beam path, by reflection from the mirror. The mirror should be positioned approximately orthogonal to the incident light rays.However, if the test piece is mounted for rotation, which is the preferred measurement configuration, the aforementioned reference measurement without the test piece can be omitted. The angle ß can then be determined directly from the diameter of the impact circle on the camera. In the calculation step, the refractive index n of the test piece material is finally calculated. For example, this can be determined from the two angles using the following relationship: αn = n. A A i l r r — a-ß - n A ir denotes the refractive index of the surrounding medium.

[0009] According to one embodiment, the method may comprise a step of emitting a bundle of collimated light beams from the light source toward the first surface. For example, the light beams may be emitted by a light source configured, for example, as an LED and collimated, for example, using a collimator. For example, during the emission of the light beams, the light source or the collimator may be arranged sequentially with the test object, or with a holder holding the test object, and with the mirror element. Advantageously, this allows the test object to be optimally illuminated, and reflected light beams of maximum intensity can be generated.

[0010] Furthermore, in the emission step, the light beam can be emitted at an angle between 80 and 100 degrees, in particular at right angles, relative to the first surface of the test object. For example, the light beam or a plurality of collimated light beams can be directed orthogonally toward the outer side of the first surface. This offers the advantage that both the first angle and the second angle can be determined with high precision. Such an orientation represents only a special case for illustrative purposes. The method also works if the outer side of the first surface is not aligned orthogonally to the incident light rays.

[0011] Another advantageous embodiment of the approach presented here is one in which the test specimen is initially oriented such that the first and second surfaces are reversed in their orientation. The first angle α can now be measured without passing through the test specimen material using the first reflected light beam. There is no change for the second angle β. The refractive index n can now be determined, for example, using the following formula:

[0012] This slightly modified formula has the advantage that the measurement error for high refractive indices can be reduced, as will be explained later.

[0013] For this variant to work, it is helpful to know how much the side resting on the test piece table is tilted relative to the optical axis of the incident light beam. For this purpose, the measuring device is extended with a sensor that uses a light beam reflected from the underside of the test piece to determine its tilt, so that it can be calculated during the measurement.

[0014] According to a further embodiment, the method can include a step of aligning the mirror surface of the mirror element orthogonal to the incident light beam. For example, the mirror element can be formed with a planar, reflective mirror surface. In the alignment step, the playing surface can be aligned approximately or sufficiently orthogonal to the incident light beam, for example, parallel to the first surface of the test piece. A prior calibration (determination of the zero position) of the mirror without a test piece is necessary if the refractive index measurement is performed without rotating the test piece.

[0015] According to a further embodiment, the first reflected light beam, the second reflected light beam, and the third reflected light beam can be detected simultaneously in the detection step. For example, collimated light beams can be emitted toward the test object or the mirror element arranged behind the test object, emanating from the light source, and the reflections of these light beams can be detected simultaneously. This offers the advantage that the method presented here can be carried out in a time-saving manner.

[0016] According to a further embodiment, the steps of detecting, determining, and calculating can be performed repeatedly, wherein in the repeated detecting step, the reflected light rays are interpreted as reflections of different wavelengths. For example, the refractive index can be measured sequentially at several different wavelengths. For this purpose, the light source can be implemented polychromatically, for example as a white light LED. To adjust one or more measurement wavelengths, an autocollimator or the light source unit can have interchangeable optical filters, for example. The lens of the autocollimator can be designed such that it can be moved linearly in order to compensate for longitudinal chromatic aberrations. Advantageously, this allows the refractive index to be determined particularly precisely.

[0017] According to a further embodiment, the method can comprise a step of controlling a rotation of the test object about a rotation axis arranged parallel to the light beam. The rotating step can be performed simultaneously with the detecting step. Additionally or alternatively, the rotating step can also be performed simultaneously with a step of emitting light by a light source. For example, the test object can be rotated by means of a suitable holder about an axis which can run parallel to the incident, collimated light beams. This offers the advantage that the second angle can be determined directly from the diameter of a striking circle created by the rotation, without aligning the mirror element.

[0018] This method can be implemented, for example, in software or hardware or in a mixed form of software and hardware, for example in a control unit.

[0019] The approach presented here further provides a device configured to perform, control, or implement the steps of a variant of a method presented here in corresponding devices. With the aid of this device, the problem underlying the invention can be solved quickly and efficiently.

[0020] For this purpose, the device can have at least one computing unit for processing signals or data, at least one memory unit for storing signals or data, at least one interface to a sensor or an actuator for reading sensor signals from the sensor or for outputting data or control signals to the actuator, and / or at least one communication interface for reading or outputting data embedded in a communication protocol. The computing unit can be, for example, a signal processor, a microcontroller, or the like, wherein the memory unit can be a flash memory or a magnetic storage unit.The communication interface can be designed to read in or output data wirelessly and / or wired, wherein a communication interface that can read in or output wired data can output this data, for example, electrically or optically from a corresponding data transmission line or input it into a corresponding data transmission line.

[0021] In this case, a device can be understood as an electrical device that processes sensor signals and outputs control and / or data signals depending on them. The device can have an interface, which can be implemented in hardware and / or software. In a hardware implementation, the interfaces can, for example, be part of a so-called system ASIC, which contains a wide variety of functions of the device. However, it is also possible for the interfaces to be separate integrated circuits or to consist at least partially of discrete components. In a software implementation, the interfaces can be software modules that are present, for example, on a microcontroller alongside other software modules.

[0022] Furthermore, a measuring system for measuring the refractive index of an optical test specimen is presented, wherein the measuring system comprises a variant of the previously presented device. The device is designed to control a light source and, additionally or alternatively, a receiving element and, additionally or alternatively, a mirror element of the measuring system. Furthermore, the measuring system comprises the light source for emitting a light beam, the receiving element for receiving the test specimen, and the mirror element, wherein the mirror element is arranged on a side of the receiving element opposite the light source. The measuring system presented here offers the advantage that a method as described above can be optimally implemented.

[0023] According to one embodiment, the measuring system can have a collimator comprising the light source, wherein the collimator can comprise a plurality of interchangeable optical filters for filtering the light beam. For example, the measuring system can comprise an autocollimator for illuminating the test object and for detecting the reflections occurring on the respective test object surfaces. The light source and a reticle, for example, can be arranged in the illumination arm of the autocollimator. The light source can be polychromatic, for example, and implemented as a white light LED. The autocollimator or the light source unit can have interchangeable optical filters to advantageously enable setting of one or more measurement wavelengths. Additionally or alternatively, the lens of the autocollimator can be designed such that it can be moved linearly in order to compensate for longitudinal chromatic aberrations.

[0024] According to a further embodiment, the mirror element can have an adjustable aperture stop. For example, the mirror can have an additional, adjustable aperture stop for beam limitation. This offers the advantage that, by changing the intensity of the reflected light rays, the contrast of the detected reflections can be adjusted. Furthermore, the reflectivity of the mirror surface can advantageously be of a comparable magnitude to the reflectivity of the test object surface, so that the reflections on the individual surfaces can be detected without adjusting the camera's integration times.

[0025] According to a further embodiment, the receiving element can be mounted for rotation. For example, the receiving element can optionally be mounted for rotation about an axis of rotation running parallel to the optical axis of the autocollimator, whereby a tilt between the axis of rotation of the test specimen holder and the mirror surface, or a tilt of the mirror relative to the optical axis of the autocollimator, can advantageously be mathematically compensated. Alternatively, the test specimen can also be placed on a test specimen table. In this embodiment, the autocollimator would advantageously be located below the test specimen and the mirror above the test specimen, and the wedge would ideally be placed with one of its long sides on the test specimen table.

[0026] According to a further embodiment, the light source and the mirror element can be rotatable about a common axis. For example, as an alternative to rotating the test specimen holder, the autocollimator can also be rotated together with the mirror about a corresponding common axis. The test specimen together with the holder can remain stationary. This variant can also advantageously be used to mathematically compensate for a tilt between the rotation axis of the test specimen holder and the mirror surface or a tilt of the mirror relative to the optical axis of the autocollimator. Without rotating the holder element, the mirror surface should be aligned towards the autocollimator in a preparatory step, or a residual error regarding the tilt of the mirror surface relative to the optical axis of the autocollimator should be determined. This preparatory step is carried out without the test specimen.According to a further embodiment, the receiving element can be designed to receive a liquid and additionally or alternatively a solid. For example, the measuring system can advantageously also be used to measure the refractive index of a liquid. For this purpose, the liquid to be measured should be stabilized in a suitable manner. For example, the liquid can be clamped between two plane-parallel plates in such a way that it takes on the shape of a wedge. The measurement can thus advantageously be carried out analogously to the measurement of a solid. In this configuration, the test object should remain stationary and the autocollimator including the mirror should be rotated. Furthermore, care must be taken to ensure that the plane-parallel plates are very flat.

[0027] Also advantageous is a computer program product or computer program with program code that can be stored on a machine-readable carrier or storage medium such as a semiconductor memory, a hard disk memory or an optical memory and is used to carry out, implement and / or control the steps of the method according to one of the embodiments described above, in particular when the program product or program is executed on a computer or a device.

[0028] Examples of the approach presented here are shown in the drawings and explained in more detail in the following description. It shows:

[0029] Fig. 1 is a schematic representation of an optical test specimen according to an embodiment;

[0030] Fig. 2 shows a schematic representation of an optical test specimen and a measuring method;

[0031] Fig. 3 is a flowchart of a method for determining a refractive index of an optical test specimen according to an embodiment;

[0032] Fig. 4a is a schematic representation of an embodiment of an optical test specimen; Fig. 4b is a schematic representation of an embodiment of an optical test specimen;

[0033] Fig. 4c is a schematic representation of an embodiment for measuring a refractive index of an optical test specimen;

[0034] Fig. 4d is a schematic representation of an embodiment for measuring a refractive index of an optical test specimen

[0035] Fig. 4e A diagram showing the change in the refractive index versus a ratio of the two angles α and β;

[0036] Fig. 5 is a flowchart of a method for determining a refractive index of an optical test specimen according to an embodiment;

[0037] Fig. 6a is a schematic representation of an embodiment of a measuring system for measuring a refractive index of an optical test specimen;

[0038] Fig. 6b is a schematic representation of an embodiment of a measuring system for measuring a refractive index of an optical test specimen;

[0039] Fig. 7a shows an exemplary camera image of detected reflections with a test object rotation (azimuth) of 0°;

[0040] Fig. 7b shows an exemplary camera image of detected reflections with a test object rotation (azimuth) of 180°; and

[0041] Fig. 8 is a schematic cross-sectional view of an embodiment of a receiving element for receiving a liquid test specimen.

[0042] In the following description of advantageous exemplary embodiments of the present invention, the same or similar reference numerals are used for the elements shown in the various figures and having a similar effect, whereby a repeated description of these elements is omitted. Fig. 1 shows a schematic representation of an optical test specimen 100 according to an exemplary embodiment. The test specimen 100 shown here is designed merely as an example as a wedge-shaped prism and comprises a first surface 102 and a second surface 104 arranged at a wedge angle α to the first surface 102. The wedge angle α can also generally be referred to as the apex angle α. When the test specimen 100 is illuminated with a light beam 110 from the first surface 102, the light can be deflected in its general path by the prism or wedge. The deflection δ of the light beam 110 depends on the refractive index n and the wedge angle α.The deflection 8 can be represented by the following formula:.

[0043] 8 = (n - n A ir) ro

[0044] Fig. 2 shows a schematic representation of an optical test specimen 100 according to an exemplary embodiment. The test specimen 100 shown here corresponds to or is similar to the test specimen described in the previous figure. The measurement method shown in this figure corresponds to the prior art, wherein a back reflection on the inner side of the prism / wedge can be detected using an autocollimator 200. With this method, the refractive index of the wedge material can be determined using Snell's law of refraction with a known angle of incidence i and apex angle λ.

[0045] The state-of-the-art measurement method illustrated here has the disadvantage that the apex angle A or the wedge angle co is determined prior to measuring the refractive index using a goniometer, which increases the overall measurement effort.

[0046] Fig. 3 shows a flowchart of a method 300 for determining a refractive index of an optical test specimen according to an exemplary embodiment. The method presented here is applied to a test specimen as described in the preceding figures, assuming at least one first surface and a second surface arranged at a wedge angle to the first surface.

[0047] The method 300 comprises a step 305 of 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 an outer side of the first surface, the second reflected light beam represents a light beam reflected from an inner side 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 interpreted as being arranged on a side of the test object opposite the light source emitting the light beam.

[0048] The method 300 further comprises a 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.

[0049] In addition, the method 300 includes a step 315 of calculating the refractive index using the first angle and the second angle.

[0050] The method 300 described here is based on the finding that the refractive index of a wedge-shaped test specimen can be measured without the wedge angle or the apex angle having to be known, which eliminates the need for a multi-stage measurement and significantly simplifies the measurement process.

[0051] Figures 4a and 4b each show a schematic representation of an embodiment of an optical test specimen 100 on which a method for determining a refractive index, as described in the preceding Figure 3, can be carried out. Figure 4a illustrates the determination of a first angle α, and Figure 4b illustrates the determination of a second angle β.

[0052] In one embodiment, during the measurement, the test object 100 is illuminated with collimated light, which is illustrated in the illustration shown here by a light beam 110. In an advantageous embodiment, the first surface 102 of the test object 100, i.e., a test object surface, is oriented orthogonally to the incident light rays, only by way of example.

[0053] In a first measurement step, shown in Figure 4a, a first angle α can be determined. This is arranged between a first reflection light beam 401 and a second reflection light beam 402, wherein the first reflection light beam 401 represents the light beam 110 reflected on an outer side 405 of the first surface 102 and the second reflection light beam 402 represents the light beam 110 reflected on an inner side 410 of the second surface 104. In other words, the first angle α is enclosed by two light beams reflected from different test specimen surfaces 102, 104. The first reflection occurs on the test specimen surface that is first hit by the incident light beams. The second reflection occurs on an inner side of the test specimen 100, which lies opposite the first test specimen surface and encloses the wedge angle α with it.

[0054] In one embodiment, the first angle a can be calculated using the following formula: a * «Air = 2 n <Jj

[0055] In a second measurement step, shown in Figure 4b, a second angle β can be determined. This angle is arranged between the incident light beam 110 and a third reflected light beam 413, wherein the third reflected light beam 413 represents a light beam reflected by a mirror surface 415 of a mirror element 420. In the illustration shown here, the mirror element 420, which can also be simply referred to as a mirror, is arranged on the side of the second surface 104 of the test piece 100 such that the mirror surface 415 is aligned parallel to the first surface 102 and opposite a light source 225 emitting the light beam 110. Furthermore, the incident light beam 110 runs orthogonally to the mirror surface 415.Accordingly, the second angle ß is an angle that is enclosed between the light rays reflected by a mirror, which in the illustration shown here is located below the test specimen 100, and propagating a second time through the test specimen, and light rays that are reflected by said mirror without the test specimen being in the beam path. However, the method also works when the first test specimen surface 405 and the mirror surface 415 are neither parallel to one another nor orthogonal to the incident light beam 110. In other words, the angle ß is enclosed between the light beam 413 and a reference reflection light beam, which is created at the mirror surface 415 without the test specimen being in the beam path.The reference reflected light beam is not shown here for reasons of clarity, since if the mirror is aligned orthogonally to the incident light beam 110, it coincides with the incident light beam 110. In the preferred application, in which the test specimen is rotated, a reference measurement without a test specimen is not required. As the test specimen rotates, the reflected light beam 413 creates a beating circle on the detection surface. The angle ß can be calculated directly from the diameter of this beating circle. This relationship will be explained in more detail later in connection with Fig. 7. In summary, it can be said at this point that the angle ß can be determined using the reflected light beam 413.

[0056] Alternatively, the test specimen can also be illuminated from below. In this case, the mirror is located above the test specimen.

[0057] The second angle ß can be calculated using the following formula: ß * n A ir = 2 (n - n A ir) M

[0058] Furthermore, in one embodiment, the measurement is designed so that all reflections can be detected simultaneously, so that both angles α, β can be measured simultaneously.

[0059] The refractive index of the test material can now be determined from the two angles a, ß using the following relationship: an = n A Ai i r r — - a-ß

[0060] In this context, it should be noted that no absolute angle measurement is necessary. A calibration factor of the angle measuring system is eliminated during the measurement, since an angle is present in both the numerator and denominator of the formula.

[0061] The test specimen 100 is designed merely as an example so that the wedge angle OJ has a value of < 1 ° or < 0.5 °.

[0062] A further advantage of the measuring method described here is that no calibration of the measuring system is required, since only the ratio of the two angles α and β is decisive for the measurement of the refractive index.

[0063] Fig. 4c shows a schematic representation of an embodiment for measuring a refractive index of an optical test piece 100 on which a method for determining a refractive index, as described in the previous Figure 3, can be carried out. In contrast to the representation in Figures 4a and 4b, the test piece is oriented so that the side 104 points upward, so that the incident light beam 110 is now reflected at the angle α on the side 104. The angle α can now be determined using the first reflected light beam 401 via the following relationship: t - 260

[0064] The relationship described above applies to the angle ß. For this exemplary embodiment to function, the test specimen surface 102 must be oriented as perpendicular as possible to a reference axis, e.g., the optical axis of the incident light beam 110. If a tilt angle θ exists between the surface 102 and a reference axis, e.g., the optical axis of the incident light beam 110, it should be determined. This tilt angle can then be calculated during the measurement.

[0065] This tilt angle θ can be determined, for example, by a sensor (not shown in Fig. 4c), which emits a sensor light beam 430 onto side 102 of the test object 100 and detects a corresponding sensor reflection light beam 435 on an opposite side and determines its direction. From the knowledge of the directions of the sensor light beam 430 and the sensor reflection light beam 435, the tilt angle θ and / or the position of the test object 100 relative to this sensor can then be determined or detected. If the side 102 of the test object 100 is aligned at an angle of 90° to the optical axis of the incident light beam 110, it can be assumed that α = 2-αθ applies to the first angle. For the second angle β, which represents or maps the angle of the light transmitted through the test object 100, a relationship of β = 2-(n-nA)-αθ / nA applies. If both equations are solved for n, the following relationship results:

[0066] Fig. 4d shows a schematic representation of an embodiment for measuring a refractive index of an optical test specimen 100 on which a method for determining a refractive index as described in the previous Figure 3 can be carried out. In contrast to the procedure according to the representation in Fig. 4c, a rotation 440 of the test specimen 100 is now also taken into account. As a result, the relationship for the first angle α and the second angle β is a = 4-αθ and β = 4-(n-nA)-αθ / nA The rotation of the test specimen, regardless of its orientation, causes a factor of 2 to be included in the formulas for the angles α and β. The rotation has no influence on the calculation of the refractive index. From a measurement technology perspective, however, it is advantageous because it increases the amount of data, particularly when a full circle is measured.Adjustment errors in the measuring system, which lead to measurement errors in the angles, can also be more easily detected and corrected by shifting the impact circle.

[0067] Fig. 4e shows a diagram 450 illustrating the change in the refractive index compared to a ratio v of the two angles α and β in order to explain the background of the measurement configurations. In this diagram 450, the ratio v of the two angles α and β with v = β / α is plotted on the abscissa, and the refractive index q of the test object 100 at a refractive index of air r)Air is plotted on the ordinate. A first graph 455 denotes a determination of the refractive index q of the test object 100 using the embodiment of Figures 4a and 4b, and a second graph 460 denotes the determined refractive index q of the test object 100 using the embodiment according to Figures 4c and 4d. The measurement error for the refractive index correlates with the slope of the respective curve. The procedure presented according to Figures 4c and 4d can thus help to reduce or minimize the effect of measurement errors, especially with high refractive indices.

[0068] Fig. 5 shows a flowchart of a method 300 for determining a refractive index of an optical test specimen according to one exemplary embodiment. The method 300 illustrated here corresponds to or is similar to the method illustrated in the preceding Figure 3, with the difference that the method 300 in the exemplary embodiment described here includes additional and / or optional steps.

[0069] In this embodiment, the detection step 305 is preceded by a transmission step 500. In this transmission step 500, a bundle of collimated light rays is emitted from a light source toward the first surface of the test object.

[0070] In this exemplary embodiment, in step 500 of emission, the light beam is emitted at a right angle to the first surface of the test object. In another exemplary embodiment, the light beam can also be emitted at an angle between 80 and 100 degrees to the first surface of the test object. Furthermore, in this exemplary embodiment, the method 300 includes an optional step 505 of aligning the mirror surface of the mirror element orthogonal to the light beam. Aligning the mirror surface is always necessary if rotation of the wedge-shaped test object or the autocollimator is technically impossible. The alignment step is performed without the test object in the beam path.

[0071] For example only, in this exemplary embodiment, the detection step 305 occurs only after the transmission step 500. Simultaneously with the detection step, a control step 510 is also performed in this exemplary embodiment. In control step 510, for example only, a rotation of the test object about a rotation axis arranged parallel to the light beam is controlled, so that the reflected light beams are detected in different rotational positions of the test object.

[0072] In step 305 of detection, the first reflection light beam, the second reflection light beam and the third reflection light beam are detected simultaneously, merely by way of example.

[0073] Furthermore, in one embodiment, the steps 305 of detecting, 310 of determining and 315 of calculating are performed repeatedly, wherein in the repeated step 305 of detecting, the reflected light rays are interpreted as reflections of different wavelengths.

[0074] In other words, the method 300 shown in this and the previous Figure 3 for measuring the refractive index of a wedge-shaped test specimen can be described as follows:

[0075] First, collimated light rays are emitted towards a first surface of the test object and, in an optional preparatory step, the mirror surface of the mirror element is aligned orthogonally to the incident, collimated light rays without the test object being in the beam path. The incident collimated light rays hit the first surface of the test object at an angle of approximately 90°, for example only. In one embodiment, the test object is rotated about an axis that runs parallel to the incident, collimated light rays. In this case, the step of aligning the mirror surface is omitted. Subsequently, first light rays reflected from the first surface of the test object are detected. In addition, second light rays reflected from the inner side of a second surface of the test object are detected, wherein the second surface encloses a wedge angle with the first surface.A first angle a is determined between the first and second reflected light rays.

[0076] Furthermore, third light rays reflected by a mirror arranged sequentially behind the second surface of the test piece in the beam direction are detected after they have propagated through the test piece again after being reflected by the mirror surface. A second angle ß is determined using the third reflected light rays. In one embodiment, the first, second, and third reflected light rays are detected simultaneously.

[0077] The refractive index of the wedge-shaped test specimen is then calculated from the first and second angles. In one embodiment, the refractive index measurement is performed sequentially at several different wavelengths.

[0078] Fig. 6a shows a schematic representation of an embodiment of a measuring system 600 for measuring a refractive index of an optical test object 100. The measuring system 600 comprises a light source 225 for emitting a light beam 110, a receiving element 610 for receiving the test object 100, a mirror element 420 and a device 620 which is designed to control a method for determining a refractive index of the optical test object 100, as described in the preceding Figures 3 and 5.

[0079] In one embodiment, the light source 225 is configured as part of an autocollimator 200 to collimate the light emitted by the light source 225 and direct it to the test object 100. The autocollimator comprises the merely exemplary polychromatic light source 225 and, optionally, a plurality of interchangeable optical filters in the beam path to filter the light emitted by the light source 225.

[0080] The mirror element 420 is arranged on a side of the receiving element 610 opposite the light source 225. In other words, the autocollimator, the test specimen holder, and the mirror are arranged sequentially one after the other. In this exemplary embodiment, the receiving element 610 is designed as a mechanical holder for the wedge-shaped test specimen 100 and is mounted for rotation only by way of example. Similarly, the autocollimator 200 with the light source 225 and the mirror element 420 are also rotatable about a common axis 630 in one exemplary embodiment.

[0081] By way of example only, the mirror element 420 is introduced into the beam path using a kinematic device and optionally includes an adjustable aperture stop. The surface of the mirror and the surfaces of the test piece can also be designed to have a similar degree of reflection. However, it is important to ensure that reflections arising on the test piece surfaces can be distinguished from reflections arising on the mirror surface in order to be able to assign the detected signals to the corresponding light rays. For this purpose, it is helpful to know the reflection coefficient of the test piece for the desired wavelengths. Using the adjustable aperture stop, the degree of intensity of the light rays reflected from the mirror surface can be adjusted, which is helpful for distinguishing the light rays detected by the detector.

[0082] In one embodiment, the device 620 is configured to control, merely by way of example, not only the output of light by means of the light source but also an exemplary movement of an objective of the autocollimator parallel to its optical axis. Additionally or alternatively, a rotation of the recording element 610 or a rotation of the autocollimator 200 and the mirror element 420 can be controlled.

[0083] Fig. 6b shows a schematic representation of an embodiment of a measuring system 600 for measuring a refractive index of an optical test piece 100. In contrast to the embodiment of the measuring system 600 shown in Fig. 6a, the measuring system 600 shown in Fig. 6b also includes a detection sensor 650 for the tilt angle δ, the result of which is taken into account or processed in the device 620, as described above. The measuring system 600 according to the illustration in Fig. 6b is thus expanded by the following features, such that the first angle α is determined using the first light beam reflected at the oblique edge of the test piece, and the measuring device is expanded by an additional sensor 650 (referred to here as a tilt sensor), which serves to detect a tilt of the side 102 with respect to the reference axis of the light beam 110 (optical axis).

[0084] In other words, the measuring system 600 presented here and the method that can be carried out with it can be described as follows:

[0085] The measurement system 600 comprises an autocollimator for illuminating the test piece 100 and for detecting the reflections generated at the respective test piece surfaces. A reticle, which can be configured, for example, as a pinhole or cross, is located in the illumination arm of the autocollimator. The detection arm comprises, 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 the light source unit can have interchangeable optical filters to adjust one or more measurement wavelengths. The lens 210 of the autocollimator can be designed such that it can be moved linearly in order to compensate for longitudinal chromatic aberrations.

[0086] The test specimen 100 is fastened in a suitable holder. This holder can optionally be mounted so as to be rotatable about an axis of rotation running parallel to the optical axis of the autocollimator, whereby any tilt between the rotation axis of the test specimen holder and the mirror surface or any tilt of the mirror relative to the optical axis of the autocollimator can be mathematically compensated. As an alternative to rotating the test specimen holder, the autocollimator can also be rotated together with the mirror about a corresponding common axis 630. Without rotating the test specimen holder, the mirror surface should be aligned towards the autocollimator in a preparatory step, or any residual error regarding the tilt of the mirror surface relative to the optical axis of the autocollimator should be determined. This preparatory step is carried out without the test specimen 100.

[0087] Alternatively, the test specimen 100 can also be placed on a test table. In this configuration, the autocollimator would be advantageously located below the test specimen and the mirror above it, and the wedge would ideally be placed with one of its long sides on the test table. The mirror can also be pivoted into or out of the beam path via a kinematic mount.

[0088] Furthermore, the reflectivity of the mirror surface should ideally be comparable to the reflectivity of the test specimen surface, so that the reflections on the individual surfaces can be detected without adjusting the camera's integration times. The mirror can optionally be equipped with an additional, adjustable aperture stop for beam limitation. This allows the contrast of the detected reflections to be adjusted by changing the intensity of the reflected light rays.

[0089] Fig. 7a shows an example of a first camera image of the three reflected light beams 401, 402, 413. The corresponding detection signals are labeled with reference numerals 701, 702, 713. In this example, the angle α can be determined from the distance between signals 701 and 702. To determine the angle β, the mirror would have to be aligned orthogonally to the optical axis of the autocollimator. Then, the angle β can be determined from the distance between signal 713 and a reference signal (not shown), which is acquired without a test object in the beam path.

[0090] Fig. 7b shows an example of a second camera image of the reflected light rays. In this case, the test specimen was 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 a circular path of a respective impact circle. As shown in the figure, the angle ß can be determined by connecting signals 713 and 713', which represents the diameter of a impact circle. Tilting the mirror merely ensures that the impact circle with diameter 713-713' is shifted linearly on the detector, without changing the diameter and thus the angle ß.

[0091] Fig. 8 shows a schematic cross-sectional view of an embodiment of a receiving element 610 for receiving a liquid test specimen 100. The receiving element 610 shown here corresponds to or is similar to the receiving element described in the previous Figure 6. In this embodiment, the receiving element 610 is designed to receive a test specimen 100, which is merely an example of a liquid. The measuring system described in the previous Figure 6 or the method described in the previous Figures 3 and 5 can also be used to measure the refractive index of a liquid. For this purpose, the liquid to be measured should be stabilized in a suitable manner, as shown in the figure shown here.For example, the liquid is clamped between two plane-parallel plates of the recording element 610 in such a way that it assumes the shape of a wedge. The measurement can thus be performed analogously to the measurement of a solid. In this configuration, the test specimen 100 should remain stationary, and the autocollimator and mirror should be rotated. Furthermore, the plane-parallel plates must be very flat.

[0092] If an embodiment includes an "and / or" link between a first feature and a second feature, this should be read as meaning that the embodiment according to one embodiment has both the first feature and the second feature and according to another embodiment has either only the first feature or only the second feature.

Claims

Patent claims 1. A method (300) for determining a refractive index of an optical test piece (100), wherein the test piece (100) is assumed to comprise at least a first surface (102) and a second surface (104) arranged at a wedge angle (co) to the first surface (102), wherein the method (300) comprises the following steps (305, 310, 315): Detecting (305) a first reflection light beam (401), a second reflection light beam (402) and a third reflection light beam (413), wherein the first reflection light beam (401) represents a light beam reflected on an outer side (405) of the first surface (102) and wherein the second reflection light beam (402) represents a light beam reflected on an inner side (410) of the second surface (104) and wherein the third reflection light beam (413) represents a light beam reflected on a mirror surface (415) of a mirror element (420), wherein the mirror element (420) is interpreted as being arranged on a side of the test object (100) opposite the light source (225) emitting 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); and Calculating (315) the refractive index using the first angle (α) and the second angle (β), characterized by a step (510) of controlling a rotation of the test object (100) about a rotation axis arranged parallel to the light beam (110), wherein the step (510) of rotating is carried out simultaneously with the step (305) of detecting.

2. Method (300) according to claim 1, comprising a step (500) of emitting a bundle of collimated light beams from the light source (225) in the direction of the first surface (102), in particular wherein in the emitting step (500) the light beam (110) is emitted at an angle between 80 and 100 degrees, in particular at right angles, with respect to the first surface (102) of the test piece (100).

3. Method (300) according to claim 2, wherein in the step (305) of detecting, a tilt angle (δ) is detected by which the second surface (104) is tilted relative to an incident direction of the light beam (110), and wherein in the step (315) of calculating, the refractive index is calculated taking into account the tilt angle (δ) 4. Method (300) according to one of the preceding claims, comprising a step (505) of aligning the mirror surface (415) of the mirror element (420) approximately orthogonal to the light beam (110).

5. The method (300) according to any one of the preceding claims, 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. Method (300) according to one of the preceding claims, wherein the steps (305, 310, 315) of detecting, determining and calculating are performed repeatedly, wherein in the repeated step (305) of detecting the reflected light rays (401, 402, 413) are interpreted as reflections of different wavelengths.

7. Method (300) according to one of the preceding claims, wherein in the step (500) of emitting the light beam (110) is emitted at an angle between 80 and 100 degrees, in particular at right angles, with respect to the first surface (102) of the test piece (100).

8. Device (620) which is configured to carry out and / or control the steps (305, 310, 315) of the method (300) according to one of the preceding claims in corresponding units.

9. Measuring system (600) for measuring a refractive index of an optical test piece (100), wherein the measuring system (600) has the following features: a device (620) which is configured to carry out the steps (305, 310, 315) of the method (300) according to one of the preceding claims in corresponding units to carry out and / or control, wherein the device (620) is designed to control a light source (225) and / or a receiving element (610) and / or a mirror element (420) of the measuring system (600), and wherein the device (620) is designed to control a rotation of the test object (100) about an axis of rotation arranged parallel to the light beam (110), wherein the rotation is carried out simultaneously with the detecting step; the light source (225) for emitting a light beam (110); the receiving element (610) for receiving the test object (100); and the mirror element (420), wherein the mirror element (420) is arranged on a side of the receiving element (610) opposite the light source (225).

10. Measuring system (600) according to claim 9, with an autocollimator (200) comprising the light source (225), wherein the autocollimator (200) comprises a plurality of exchangeable optical filters for filtering the light beam (110), in particular wherein the autocollimator (200) has at least one optical element (210) which can be moved parallel to the optical axis of the autocollimator (200).

11. Measuring system (600) according to one of claims 9 or 10, wherein the mirror element (420) has an adjustable aperture stop, and / or wherein the mirror element (420) can be pivoted into the beam path of the light beam (110).

12. Measuring system (600) according to one of claims 9 to 11, wherein the receiving element (610) is rotatably mounted.

13. Measuring system (600) according to one of claims 9 to 12, wherein the light source (225) and the mirror element (420) are rotatable about a common axis (630).

14. Measuring system (600) according to one of claims 9 to 13, wherein the receiving element (610) is designed to receive a liquid and / or a solid body.

15. A computer program configured to execute and / or control the steps of the method (300) according to any one of the preceding claims 1 to 7 when the computer program is executed on a computer or a device.

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