Method for determining at least two critical angles of total reflection in a liquid process medium

Abstract

The invention relates to a method for determining at least two critical angles of total internal reflection in a liquid process medium (10), - wherein a measurement beam (8) is emitted by a light source (1), said measurement beam comprising a main beam (9) and a number of sub-beams (S1, S2, ...), which have different angular differences with respect to the main beam (9), - wherein the measurement beam (8) is radiated into a measurement prism (3) via an illumination optical unit (2), - wherein the measurement prism (3) consists of sapphire, - wherein the measurement prism (3) has at least one measurement surface (31) at which the measurement beam (8) is brought into contact with the sample to be measured at an angle of incidence (αe) and comes into contact with the medium, and wherein the measurement beam (8) is deflected onto a detector (4) after coming into contact with the medium, wherein an intensity distribution formed around the angle of incidence (αe) of the main beam (9), which results from Fresnel's equation, is formed on the detector (4), wherein the light source (1) emits the measurement beam (8) as an unpolarised measurement beam (8) and the measurement beam (8) impinges on the detector (4) as an unpolarised measurement beam (8), wherein the spatial intensity distribution of the measurement beam (8) on the detector (4) is used to determine at least two limit angles of the total internal reflection (αkrit) of the process medium.

Description

[0001] Method for determining at least two critical angles of total internal reflection in a liquid process medium

[0002] The invention relates to a method for determining at least two critical angles of total internal reflection in a liquid process medium according to the preamble of claim 1, and a refractometer for determining at least two critical angles of total internal reflection in a liquid process medium according to the preamble of claim 11.

[0003] The field of the invention is refractometry, in particular process refractometers. In the latter, the measuring prism is, for example, inserted into a process line and the refractive index of a flowing sample is measured.

[0004] Refractometers known from the prior art operate on the principle of the critical angle of total internal reflection. Refractometers, which enable the measurement of refractive indices of liquids by determining the critical angle, have been known for over a hundred years. These refractometers are also known as Abbe refractometers, Wollaston refractometers, or Pulfrich refractometers. A divergent beam of light is directed onto a flat interface between a measuring prism and the medium to be measured. Due to partial or total reflection at this interface, the intensity of the reflected beam is modified depending on the angle of incidence. Using suitable optics, the angular distribution of the intensity can be displayed on a sensor with spatial resolution, usually a line or area detector. A relatively sharp light-dark boundary is then imaged on the detector.The position of this boundary on the detector is a measure of the critical angle, from which the refractive index of the sample is subsequently calculated. In principle, all common process refractometers known from the prior art are similarly designed. The diameter of the process connection of these instruments is typically around 50 mm.

[0005] US Patent 5442435 A (COOPER STEPHEN RW, ZHANG GUANGYU) dated August 15, 1995, discloses a refractometer sensor for determining the composition of a fluid, wherein light incident from a light source is reflected at two interfaces with the fluid in a measuring prism immersed in the fluid, and the reflected light is measured in a simple detector. The composition, density, temperature, etc., of the fluid is inferred from the proportion of reflected light.

[0006] German patent application GB 1216383 A (OPTIQUE PREC ELECTRONIQUE ET M) dated December 23, 1970, also discloses a refractometer of the type described in the present patent application, wherein the incident light is reflected in a measuring prism at several interfaces with the fluid under investigation. The angle of incidence of the light is the same at each interface, which leads to signal amplification.

[0007] CN 109187431 A (BEIJING BORUISI COMMERCE AND TRADE CO LTD) dated January 11, 2019, also deals with a refractometer for measuring the refractive index of a liquid according to the present invention. Here, too, the incident light is reflected several times at interfaces with the liquid in a measuring prism; however, this primarily serves to reduce measurement errors caused by bubbles in the liquid.

[0008] Due to its favorable properties, sapphire is frequently used as the sample-contacting material in process refractometers. One disadvantage of this crystalline material is its birefringence. Unpolarized light propagating through a sapphire component is split into an ordinary component, with the electric field oscillating perpendicular to the crystal axis, and an extraordinary component, with the electric field oscillating parallel to the crystal axis. These two components are referred to as the ordinary and extraordinary beams. The propagation speed within the crystal, and thus the effective refractive index of the crystal, differs for the two partial beams. In a refractometer based on measuring the critical angle at an interface, two different critical angles would therefore occur, corresponding to the ordinary and extraordinary beams, respectively.extraordinary beam. In the prior art, a polarization filter is typically installed in the beam path to suppress the extraordinary beam. The crystal axis of the measuring prism is then oriented so that it is perpendicular to the plane of the beam path; the polarization direction is usually parallel to this plane (p-polarization). The effect of birefringence is thus (almost) completely eliminated and subsequently does not need to be considered during evaluation. Most often, polarizing plastic film filters are used to select a polarization direction. However, this solution has the disadvantage that these films are problematic with regard to the long-term stability of the measuring system, as their maximum permissible continuous operating temperature is below 80°C.In process refractometers that are used for many years at high temperatures, a change in the polarization filter can lead to long-term drift and therefore distort the measurement or render it unusable.

[0009] It is therefore an object of the invention to enable the measurement of the refractive index in a process refractometer that exhibits no measurement drift and maintains consistent measurement quality over a long period. Furthermore, it is an object of the invention to increase the measurement accuracy or resolution at the detector and to provide a refractometer with a simple and robust design.

[0010] This problem is solved by the characterizing features of claim 1. According to the invention, it is provided that the light source emits the measuring beam as an unpolarized measuring beam and that the measuring beam strikes the detector as an unpolarized measuring beam, wherein the spatial intensity distribution of the measuring beam on the detector is used to determine at least two critical angles of total internal reflection of the process medium.

[0011] With the features according to the invention, it is possible to use a sapphire measuring prism without a polarization filter. Eliminating the polarization filter reduces manufacturing costs. Furthermore, the absence of a component prevents scratching, soiling, or degradation over time, thus reducing operating costs for the refractometer.

[0012] Furthermore, the information content in the intensity distribution is greater, which provides additional possibilities to assess the quality of the signal and to detect abnormal operating conditions - such as the formation of a coating on the measuring surface - and to provide the user with corresponding information.

[0013] Furthermore, the polarization filter represents an additional component in the prior art and incurs additional costs, such as higher component prices, more complex design measures for mounting and fixing it, and additional handling. Since plastic is generally softer and therefore more susceptible to damage compared to the glass or sapphire material of the measuring prism, the handling of the polarization filter, especially its cleaning, must be particularly careful. Eliminating the polarization filter thus results in a refractometer that is significantly easier to manufacture and also more cost-effective in its construction.

[0014] Particularly advantageous embodiments of the method are defined in more detail by the features of the dependent claims:

[0015] The evaluation can be particularly simple in one method by determining the critical angle of total internal reflection by forming the first derivative of the intensity distribution of the measuring beam at the detector, whereby at least two maximum values ​​of this first derivative are determined, whereby at least two critical angles are calculated from the position of the maximum values, whereby at least two refractive indices of the process medium are determined from the at least two critical angles, and whereby the refractive index of the process medium is calculated by forming an average value from the individual refractive indices.

[0016] Optionally, to determine the critical angle of total internal reflection, the second derivative of the intensity distribution of the measuring beam at the detector can be calculated, whereby at least two zeros of this second derivative are determined, from the positions of the zeros at least two critical angles are calculated, from which at least two refractive indices of the process medium are determined, and from which the refractive index of the process medium is calculated by taking an average of the individual refractive indices. Calculating the zeros in the second derivative provides a particularly reliable method for detecting the critical angles, with which the refractive index of the process medium can be easily determined for unpolarized light and without a polarizing filter.

[0017] Preferably, the light source is an LED, in particular a "phosphor converted" type LED.

[0018] The measurement accuracy is further increased by adjusting the position, in particular via the pixel position of the detector, of the partial beams of the measuring beam incident on the detector to the respective angle of incidence of the partial beam on the measuring surface.

[0019] Advantageously, the measuring beam can be guided through an imaging optic before striking the detector, the imaging optic being preferably designed as a telecentric optic. In the method according to the invention, the intensity distribution is simply recorded by designing the detector as a line or area detector, wherein each pixel of the detector is assigned an angle of incidence onto the measuring surface of the respective partial beam striking the detector. The detector is preferably designed as a line or area detector, wherein the distances between each pair of pixels are assigned angular differences between partial beams.

[0020] Small deviations of the crystal axis can lead to significant deviations in the measured intensity distribution. Therefore, the crystal axis of the measuring prism is arranged perpendicular to the beam path of the measuring beam. Thus, the inventive method makes it possible to obtain a reliable evaluation even if the crystal axis exhibits deviations of 5° or more.

[0021] To easily detect whether the process medium deposits or has deposited a coating on the measuring surface of the measuring prism, coating detection is advantageous. In the method according to the invention, the determined intensity distribution is therefore compared with an ideal intensity distribution or a previously measured intensity distribution, and a measure of the coating formed on the measuring surface of the measuring prism is derived from this comparison.

[0022] Furthermore, the invention aims to create a refractometer with which critical angles of total reflection in a liquid process medium can be determined using a method according to the invention.

[0023] This problem is solved by the characterizing features of claim 11. According to the invention, the light source emits the measuring beam as an unpolarized measuring beam, and the beam path of the measuring beam is free of polarization filters.

[0024] The refractometer according to the invention has a cost-effective design, since no additional polarization filters are required, and enables a robust and simple measurement of the refractive indices or the critical angle of total reflection in process media.

[0025] A preferred and cost-effective refractometer is provided when the light source is an LED, in particular a phosphor-converted type LED. Advantageously, the detector is designed as a line or area detector, wherein the pixels of the detector are each assigned angular differences of the partial beams incident on the detector, based on the angle of incidence onto the measuring surface and / or their spacing.

[0026] In order to easily focus the measuring beam, it can be provided that an imaging optic is arranged in the beam path of the measuring beam in front of the detector, wherein the imaging optic is in particular designed as a telecentric optic.

[0027] The measuring prism preferably has beam-guiding surfaces opposite each other with respect to the axis of the measuring prism, so that the measuring beam can be brought into contact with the sample to be measured in the measuring surface. This allows for a simple design of the refractometer with convenient arrangement of the other components.

[0028] Advantageously, the illumination optics are designed in such a way that an intermediate image of the light source can be generated in the area of ​​the measuring prism, wherein in particular the illumination optics with the measuring prism are designed in such a way that the intermediate image of the light source lies exactly on the measuring surface in contact with the medium.

[0029] Further advantages and embodiments of the invention will become apparent from the description and the accompanying drawings.

[0030] The invention is shown schematically in the drawings below with reference to particularly advantageous, but not limiting, embodiments and is described by way of example with reference to the drawings:

[0031] Fig. 1 shows a first embodiment of the refractometer according to the invention in a schematic view, Fig. 2 shows a diagram with three intensity profiles, Fig. 3 shows the first derivative of the signal of an unpolarized intensity distribution, Fig. 4 shows the second derivative of two intensity profiles, Fig. 5 shows the change in the intensity distribution upon formation of a coating, Fig. 6 shows a schematic view of a measuring prism, and Fig. 7 shows intensity profiles with deviations in the crystal axis.

[0032] Figure 1 shows a first schematic embodiment of the invention. The refractometer is designed to determine at least two critical angles of total internal reflection in a liquid process medium 10 and comprises a light source 1, which is a phosphor-converted LED. A measuring beam 8 is emitted from the light source 1. The measuring beam 8 consists of a number of partial beams S1, S2, ... and a main beam 9. The partial beams S1, S2, etc., each have an angular deviation from the main beam 9. (Figure 1) The measuring beam 8 emitted by the light source 1 is focused or pre-shaped by components of the illumination optics 2, in this embodiment two lenses. From the illumination optics 2, the focused measuring beam 8 enters a measuring prism 3. The measuring prism 3 has a prism angle α. PThe measuring prism 3 contains two beam guide surfaces 33, which are spaced apart from each other and inclined relative to each other. The measuring prism 3 borders the sample 10 to be measured with a measuring surface 31. The measuring beam 8 comes into contact with the medium at the measuring surface 31, i.e., at the interface, and is thus partially reflected at the measuring surface 31. The measuring beam 8, or the individual partial beams S1, S2, ... of the measuring beam 8, enter the measuring prism 3 via a beam entry surface 35 and then exit the measuring prism 3 via a beam exit surface 36 after contacting the medium at the measuring surface 31.

[0033] Upon exiting the measuring prism 3, the measuring beam 8, or rather the portion of the measuring beam 8 that did not enter the sample 10, is directed via an imaging optic 5 onto a detector 4. The angular dependence of the intensity resulting from total or partial reflection of the beams is transformed into a spatial dependence by the lens of the imaging optic 5. A Fresnel curve is then formed at the detector 4 (see Fig. 2). The intensity edges, or the Fresnel curve, are then used to determine the critical angle of total internal reflection α. krit The refractive index of sample 10 can then be determined using Snell's law of refraction. According to Snell's law of refraction, the critical angle of total internal reflection α is given by... krit depending on the refractive index

[0034]

[0035] The following relationship exists between the measuring prism 3 and the refractive index n2 of the medium or sample 10 to be examined at the measuring surface 31:

[0036] a krit = arcsin (n2 / n1)

[0037] This ratio is used when determining the refractive index of a medium n Me The medium was used. In order to perform this evaluation, the critical angle of total internal reflection must be precisely determined.

[0038] If the measuring prism 3 is made of a birefringent material, such as sapphire in the present invention, the intensity profiles change in that, depending on the polarization state of the beam relative to the crystal axis, the ordinary or extraordinary refractive index of the material comes into play (Fig. 2). The diagram in Fig. 2 shows a p-polarized, an s-polarized, and an unpolarized beam path p+s. The intensity I (y-axis) is plotted against the position at the detector 4 (x-axis) in each case. Without polarization filters and using sapphire as the measuring prism 3, the Fresnel curve exhibits a profile with two edges or discontinuities (p+s profile Fig. 2), which result from the superposition of the birefringent properties of the measuring prism 3.According to the invention, in contrast to the prior art, no polarization filter is used, which means that the intensity distribution at detector 4 has two points of discontinuity. Accordingly, in the method according to the invention, a new algorithm is used to evaluate the intensity distribution, with which the position of these points is determined and the unknown refractive index of the medium n is derived from this. Me dium or sample 10 is calculated.

[0039] The intensity distribution recorded at detector 4 is then evaluated by calculating the first derivative of the intensity distribution (Fig. 3). The intensity curve, in prisms actually measured in experiments, shows at least two maxima in the first derivative (Fig. 3). The position of the two discontinuities corresponds to the two local maxima of the first derivative of the intensity distribution, since both the s- and p-polarized intensity distributions have only one discontinuity and these do not overlap (Fig.

[0040] 2) From the position of the maxima X X2, the respective critical angle a can then be determined. krit 1 , a krit,2 can be determined. Using Snell's law of refraction, two refractive indices n1 and n2 of the medium or sample 10 are then obtained. The actual refractive index of the medium n Me dium is then calculated using the individual refractive indices n n2 by averaging.

[0041] Alternatively, instead of determining the location of the maximum values ​​X1 and X2 of the first derivative, the second derivative can be calculated and then the at least two zeros NN2 of this second derivative determined (Fig. 5). Since the zero crossings or zeros NN2 of the second derivative are better suited for interpolating the discrete intensity distribution, these are optionally used instead of the maximum values ​​X1 and X2 of the first derivative to determine the location of the respective critical angles a. krit 1 , a kriti 2used. Optionally, a smoothing FIR filter is used to calculate the second derivative, minimizing the influence of noise on the measured intensity distribution. With the calculated respective critical angles a krit 1 , a kriti 2 The respective refractive indices are then calculated as described above.

[0042]

[0043] and n2, and from this the refractive index of sample 10 or of the process medium is then determined by averaging. Using the positions determined from the intensity distribution and the corresponding refractive indices n! and n2 of the birefringent measuring prism 3, two refractive index candidates n! and n2 of the medium are calculated. These agree particularly well with the actual refractive indices of the p- and s-polarized intensity distributions due to the inventive method. The final refractive index of the medium n MeThe average of these two candidates is used. The difference can optionally be used for reliability testing.

[0044] The maximum curvature of the intensity distribution, the maximum of the absolute second derivative, is a measure of measurement quality. A decrease in this quality factor can be used as an indication of deposit formation on the measuring prism 3 in the beam path of the measuring beam 8, for a measuring prism 3 without a polarization filter or a refractometer without a polarization filter. As shown in Fig. 6, a deposit can accumulate on the measuring surface 31 of the measuring prism 3 over time due to the process medium. This leads to a decrease in the intensity distribution. To assess the measurement quality, the measured intensity distribution, represented in Fig. 6 by the solid intensity distribution, can therefore be compared with an idealized intensity distribution, represented in Fig. 6 by the dashed intensity distribution.The coating leads to a parallel shift in the intensity distribution, whereby the degree of shift can be used as a measure of coating formation.

[0045] As shown in Fig. 4, a coating on the measuring surface also leads to a lowering of the maxima or a shift of the zero crossings N N2 in the second derivative of the intensity distribution. In Fig. 4, the dashed line represents an idealized intensity distribution and the solid line represents an intensity distribution influenced by the coating. Should the quality deteriorate to such an extent that only one zero crossing NT can be detected, this is interpreted as p-polarized by the inventive method, and the refractive index and from this the critical angle a are calculated from only one point. kritThis refractive index is then compared with the "idealized" refractive index, and a measure of the measurement quality is derived from this. Optionally, this measure can also be used to provide the user of the refractometer with recommendations for action, such as cleaning the measuring surface 31.

[0046] The quality factor can be determined, for example, as shown in Fig. 4, using several factors. The height of peaks 11, I2, I3, I4 (Fig. 4) of the second derivative of the pixel intensity at detector 4 can be easily determined. If the intensity distribution curve begins to change due to coating, this is measured at least in the fall-off of the edge in the region of intensity 1 in the direction of the critical angle, from which the quality index can then be calculated. Alternatively or additionally, one of the upper and / or lower peak intensities, the distance between the zero crossings, the height of the curve to the right before the first break-off edge, or the position of the flank of the antiderivative can be used to determine the measure of measurement quality.

[0047] When recording the intensity distribution at detector 4, a map of the reflected intensity is created across the location. In refractometers according to the invention, detector 4 is preferably configured as a line or area detector. Each pixel of detector 4 is assigned an angle of incidence α. e1 , a e2... at the measuring surface 31 of the measuring beam 8 incident on the detector 4, or of the individual partial beams S1, S2, ... of the measuring beam 8. Thus, the position of the individual partial beams S1, S2, ... of the measuring beam 8 at the detector 4 is assigned to the respective angles of incidence of the partial beams S1, S2, ... This assignment is generally made using a third-order polynomial that takes into account the actual arrangement of the measuring prism 3 in the refractometer. Angular deviations of the crystal axis of the measuring prism 3 result in deviations in the position of the critical angles or the intensity distribution (Fig. 6, Fig. 7). To eliminate or reduce these, the built-in measuring prism 3 is optionally subjected to adjustment. For this purpose, a measurement is carried out with at least two media or several media with known refractive indices.When intensity profiles are measured, the refractive indices of the known media are recorded for the measuring prism 3 used and used for adjustment or, if necessary, calibration. This adjustment primarily eliminates manufacturing tolerances of all components, such as the measuring prism 1, the lenses, and the mechanics that define the alignment of the components relative to each other.

[0048] Furthermore, the refractometer and / or the evaluation are optionally normalized. Air as the medium results in an intensity distribution at the detector that exhibits discontinuities far outside the measuring range. This profile or distribution is also referred to as the "aerial image." The aerial image is optionally acquired for each refractometer before adjustment, and the intensity distribution measured as the aerial image is stored in the refractometer's internal memory. Subsequently, each measured intensity distribution can be divided by this aerial image, resulting in normalized intensity profiles. In the investigations underlying the invention, a sapphire measuring prism 3 was used, whose crystal axis (c-axis) was oriented perpendicular to the beam path. Optionally, the measuring prism 1 can also have other orientations, which can then be taken into account during adjustment.

[0049] In the refractometer, the measuring prism 3 is inserted into the refractometer housing in such a way that the wetted measuring surface 31 is immersed in or can be wetted by the sample 10 under investigation. The other components of the refractometer are enclosed within the housing in a fluid-tight manner, protecting the sample 10. For process refractometers, the assembly can be installed in fluid-carrying process lines, e.g., using standard flanges.

[0050] The assignment of pixels or their position at detector 4 to the angles and positions can also be achieved, for example, by calibration with various known refractive indices or by means of an optical simulation, such as inverse ray tracing. Thus, the position of the rays S1, S2, ... incident on detector 4 is always assigned to the partial rays S1, S2, ... incident on detector 4 at the specified angle or position.

[0051] As an alternative to designing the light source 1 as a phosphor-converted LED, it can also be designed as another type of light source 1 used in refractometers that emits unpolarized light. Any divergent light source can be used for the light source 1. LEDs are a logical choice because they are compact, inexpensive, and efficient. Yellow LEDs based on AllnGaP (aluminum indium gallium phosphide) are preferred in refractometers. However, these exhibit a relatively high dependence of the emission wavelength on temperature (approximately 0.1 nm / °C), which can affect the stability of the measurement. Therefore, in the present invention, a phosphor-converted amber LED is preferably used, which behaves more favorably in this respect (approximately -0.01 nm / °C).

Claims

Patent claims 1. Method for determining at least two critical angles of total internal reflection in a liquid process medium (10), -whereby a measuring beam (8) is emitted from a light source (1) which comprises a main beam (9) as well as a number of partial beams (S1, S2, ...) which have different angular differences with respect to the main beam (9), -whereby the measuring beam (8) is directed into a measuring prism (3) via an illumination optic (2), -where the measuring prism (3) is made of sapphire, -whereby the measuring prism (3) has at least one measuring surface (31) at which the measuring beam (8) is directed at an angle of incidence (a e ) is brought into contact with the sample to be measured and comes into contact with the medium, and wherein the measuring beam (8) is deflected after contacting the medium onto a detector (4), wherein a beam is formed at the detector (4) by the angle of incidence (a e) of the main beam (9) forms the intensity distribution which results from the Fresnel equations, characterized in that the light source (1) emits the measuring beam (8) as an unpolarized measuring beam (8) and the measuring beam (8) as an unpolarized measuring beam (8) strikes the detector (4), wherein the spatial intensity distribution of the measuring beam (8) on the detector (4) is used to determine at least two critical angles of total internal reflection (a krit ) of the process medium is used.

2. Method according to claim 1, characterized in that, to determine the critical angle of total internal reflection (a krit ) the first derivative of the intensity distribution of the measuring beam (8) at the detector (4) is formed, - where at least two maximum values ​​(X X2) of this first derivative are determined, - where at least two critical angles (a) are determined from the position of the maximum values ​​(X X2). krit ,i, a krit,2) be calculated, - wherein from the at least two critical angles (a krit ,i, a krit ,2) at least two refractive indices (n n2) of the process medium are determined, and - where the refractive index of the process medium (n Me dium) is calculated by forming an average value from the individual refractive indices (n n2).

3. Method according to claim 1, characterized in that, to determine the critical angle of total reflection (a krit ) the second derivative of the intensity distribution of the measuring beam (8) at the detector (4) is formed, - where at least two zeros (N N2) of this second derivative are determined, - where at least two critical angles (a) can be determined from the position of the zeros (X X2). krit 1 , a k rit, 2) be calculated, - wherein from the at least two critical angles (a krit 1 , a kriti 2) at least two refractive indices (n n2) of the process medium are determined, and - where the refractive index of the process medium (n Me dium) is calculated by forming an average of the individual refractive indices (n n2).

4. Method according to one of claims 1 to 3, characterized in that the light source (1) is an LED, in particular a “phosphor converted” type LED.

5. Method according to one of claims 1 to 4, characterized in that the position, in particular via the pixel position of the detector (4), of the partial beams (S1, S2, ...) of the measuring beam (8) incident on the detector (4) is assigned to the respective angle of incidence (a^a^ ...) of the partial beam (S1, S2, ...) on the measuring surface by adjustment.

6. Method according to one of claims 1 to 5, characterized in that the measuring beam (8) is guided through an imaging optic (5) before striking the detector (4), wherein the imaging optic (5) is in particular designed as a telecentric optic.

7. Method according to any one of claims 1 to 6, characterized in that the detector (4) is designed as a line or area detector, - where each pixel of the detector (4) is assigned an angle of incidence (a e ) is assigned to the measuring surface of the partial beam (S1, S2, ...) that each hits the detector (4).

8. A method according to any one of claims 1 to 7, characterized in that the detector (4) is configured as a line or area detector, wherein the distances between each pair of pixels are assigned angular differences between partial beams (S1, S2, ...).

9. A method according to any one of claims 1 to 8, characterized in that the crystal axis of the measuring prism (3) is arranged perpendicular to the beam path of the measuring beam (8).

10. Method according to one of claims 1 to 9, characterized in that the determined intensity distribution is compared with an ideal intensity distribution or previously measured intensity distribution and a measure for the coating formed on the measuring surface (31) of the measuring prism (3) is formed therefrom.

11. Refractometer for determining at least two critical angles of total reflection in a liquid process medium (10), in particular according to a method according to one of claims 1 to 10, comprising: - a light source (1) from which a measuring beam (8) is emitted, comprising a main beam (9) and a number of partial beams (S1, S2, ...) which have different angular differences relative to the main beam (9), - at least one illumination optic (2), at least one measuring prism (3) and one detector (4), - wherein the measuring prism (3) has at least one measuring surface (31) and is made of sapphire, - wherein the measuring prism (3) is arranged and designed such that the measuring beam (8) comes into contact with the process medium to be measured at the measuring surface (31), and wherein the measuring beam (8) can be deflected onto a detector (4) after coming into contact with the medium, characterized in that the light source (1) emits the measuring beam (8) as an unpolarized measuring beam (8) and wherein the beam path of the measuring beam (8) is free of polarization filters.

12. Refractometer according to claim 11 characterized in that the light source (1) is an LED, in particular a “phosphor converted” type LED.

13. Refractometer according to claim 11 or 12, characterized in that the detector (4) is configured as a line or area detector, wherein the pixels of the detector (4) have angles of incidence (a e ) on the measuring surface and / or its distances, angular differences of the partial beams (S1, S2, ...) incident on the detector (4) are each assigned.

14. Refractometer according to one of claims 11 to 13, characterized in that an imaging optic (5) is arranged in the beam path of the measuring beam (8) in front of the detector (4), wherein the imaging optic (5) is in particular designed as a telecentric optic.

15. Refractometer according to one of claims 11 to 14, characterized in that the measuring prism (3) has beam guide surfaces (33) opposite each other with respect to the axis of the measuring prism (3), so that the measuring beam (8) can be brought into contact with the sample to be measured in the measuring surface (31).

16. Refractometer according to one of claims 11 to 15, characterized in that the illumination optics (2) are designed such that an intermediate image of the light source (1) can be generated in the area of ​​the measuring prism (3), wherein in particular the illumination optics (2) with the measuring prism (3) are designed such that the intermediate image of the light source (1) lies exactly on the medium-contacting measuring surface (31).

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