WO2006042524A1

WO2006042524A1 - Rapid measurement of large path differences in birefringent materials without and with false colours

Info

Publication number: WO2006042524A1
Application number: PCT/DE2005/001864
Authority: WO
Inventors: Siegfried Kaufmann; Hannes Schache; Uwe Detlef Zeitner
Original assignee: Thüringisches Institut für Textil- und Kunststoff-Forschung e.V.
Priority date: 2004-10-20
Filing date: 2005-10-18
Publication date: 2006-04-27
Also published as: DE102004051247B3

Abstract

The aim of the invention is a method and device which may be used for automatic, non-contact and rapid measurement of large path differences in birefringent samples, whereby, even in the presence of false colours, the certainty (precision and unambiguity) and the rapidity (short periods) of the method can be regulated depending on the properties of the sample. Said aim is achieved, whereby, in addition to the basic constant measuring wavelength λ1, a variable auxiliary wavelength λ2 is introduced. Said auxiliary wavelength λ2 is compared to the measuring wavelength λ1 until the same order is simultaneously and repeatedly obtained for both wavelengths (same N+l). In one instance the separation of the wavelengths can for particular samples be set to greater than 10 nm and the certainty (precision and unambiguity) and rapidity of the method can be increased. In the other instance for very bright samples with particularly difficult false colours the separation of the wavelengths can be made less than 10 nm such that the path differences can be unambiguously recorded.

Description

FAST MEASUREMENT OF HIGH GAP DISTURBANCES OF DOUBLE BREAKING MEDIA WITHOUT AND WITH FALSE COLORS

The invention relates to a method for fast, automatic, non-contact, calibration-friendly and lichtstar¬ ken measurement of high path differences R of birefringent and transparent optical media or samples according to Senarmont with simultaneous or temporally successive digital Fourier analysis with multiple wavelengths, especially fibers, Filaments, films and other fabrics, both in the

Laboratory, on the microscope as well as online during the manufacturing process with fast or slow changes (ms or seconds). At the same time, a device for carrying out this method is also described. According to the method and the device, all transparent fibers, filaments, films and sheet formations can be examined while they are still in molten state during production or in the solid state and at rest. These samples can still be measured even if they show false colors in the white light.

The measurement of the path difference R of the mentioned samples is important because it results in the birefringence with the aid of the thickness D of these samples

Δn = R / D (1)

replacement blade which in many cases characterizes the orientation of the macromolecules in the fibers and films. This orientation determines technically and technologically important parameters, such as the tensile strength at break, the elongation at break and the modulus of elasticity of the samples mentioned. If Δn _{max is} the maximum possible birefringence, then a mean orientation angle α can be given which, in the case of fibers, means the angle between the cylinder axis and the central longitudinal direction of the macromolecules as follows:

2

Δn / Δn _max = 1 - - sin ² α. (2)

3

The birefringence Δn in the case of the fibers is always the difference of the refractive indices parallel and perpendicular to the fiber longitudinal axis, i.

Δn = ri _|| - ni. (3)

For the path difference R this applies analogously. In the case of optically uniaxial fibers with the diameter D, the path difference R is the difference of the optical paths nπ _* D and m _* D ₇ ie

R = (n | _| - m) _* D. (4)

In the first case (ny _* D) the electric vector of the light oscillates perpendicular to the propagation direction, but parallel to the cylinder axis of the fiber. In the second case, he swings vertically.

If the path difference R is smaller than the used vacuum light wavelength λ, then this area is called the area of the 1st order (N + 1 = 1, ie N = 0). If the transition difference lies between the wavelengths λ and 2λ, then this is the region of the second order (N + 1 = 2, ie N = 1), etc. One speaks generally of the optical order (N + 1).

[State of the art]

For automatic and non-contact determination of Gangun¬ difference in the field up to a wavelength of light λ is often the Senarmont method used (CH 342768 A, DE-AS 1097167, DE 4123936 Al, DE 4235065 Al and DE 19529899 Al), with monochromatic light is working.

For gait differences greater than one wavelength λ, the Soleil-Babinet method is used (DE 4123935 Al and DE 4235065 Al), which works with white light. There are polymers and other substances which show an abnormal color sequence of the interference fringes. The compensation strip 0th order is then no longer neutral black. In many cases, then, it can not be decided between two adjacent strips which strip is the compensation strip. Often it is then neither and an exact indication of the birefringence is missing. One way to overcome this difficulty is to make an oblique cut and to count the orders in the monochromatic light. Of course, such a method is no longer identical with an automatic and, above all, non-contact and fast measuring method. Other patents (US Pat. No. 5,929,993, EP 225 590, GB 808 505, US Pat. No. 6,697,161, DE 19 953 528, EP 597 390), which also offer corresponding solutions for moving media, are not suitable for media with false colors measure or regulate because they only work at one wavelength. According to DE 3 129 505, the measuring range is even limited to half a wavelength.

Patents with several wavelengths do not deal with the convergence of the different orders at different wavelengths (US Pat. No. 4,973,163). Only by the constancy of the optical order in very small wavelength ranges, which can be up to 10 nm and smaller, in part, as shown below, false colors can be detected. Although the possibility of varying the wavelengths is present in US Pat. No. 5,406,371, it can not at the same time be realized for the quantities designated as measuring and auxiliary wavelengths in the present patent.

A way out in determining the path difference in the presence of false colors here is the Senarmont method, but applied with two wavelengths.

For path differences R> λ the minimum of the difference of the path differences of the two beams of different wavelengths is determined (DE 4306050 A1). The disadvantage of this method, however, is that it is not fast enough because of the time-consuming determination of the minimum, i. no high sampling frequencies are possible. In addition, the accuracy is limited because of the shallow maximum. However, high sampling frequencies are necessary in order to prevent the rapid production of special phenomena, for example, in the production process. to detect the shoulder (neck point) during tendon spinning or during the stretching process.

In addition, high sampling frequencies are required in quality testing, e.g. a monofilament is unwound from the bobbin and the orientation along the thread at high take-off speed is to be determined.

An elegant method for solving these problems is the use of the Senarmont method with at least two wavelengths in very many angular positions of the analyzer (DE 198 19 670 A1). If this last-mentioned method is used to measure the transition of shiny and, for example, 15 .mu.m thick fibers with minimal light scattering, the simultaneous (simultaneous) measurement of the light intensities at at least two wavelengths in very many angular positions (eg 10) of the analyzer is very difficult. because the scattered light also spreads to many recipients. This described method is therefore too faint and thus ultimately too inaccurate. The patented method and apparatus (DE 103 10 837 A1) for the automatic measurement of stress birefringence on transparent bodies is unsuitable for the determination of optical path differences R> λ, since measurements are carried out at only one measuring wavelength. In addition, the analyzer is mechanically rotated in this case.

A described achromatic Senarmont-

Kompensationseinreichtung (DE 40 32 212 Al) is used to measure optical path differences of optically anisotropic objects in an extended spectral range with significantly reduced systematic measurement error. In this case, it is possible to measure at different measuring light wavelengths by means of a rotatable analyzer, but no possibility of determining optical path differences R> λ is described. A more recent patent (DE 102 45 407) describes a method in which the Senarmont method is carried out simultaneously or chronologically successively using discrete Fourier analysis (DFA) with preferably three angular positions and at least two measuring wavelengths. In the Senarmont method with simultaneous DFA are used for scanning coming from the object to be measured

Signal preferably three angular positions simultaneously realized and requires at least three optical fiber with upstream λ / 4 plate and three polarizers. In the Senarmont method with time-successive DFA, only one light control cable, a photoreceiver and an electrical amplifier. As a result, the light intensity with respect to the above-mentioned Senarmont method increases with very many angles of the analyzer. During calibration, only the linearity, zero and maximum intensities need to be considered. The former variant (simultaneous DFA) is faster than the second variant (temporally successive DFA). In the latter case, however, the material cost is lower, the light intensity higher and the calibration requirements are particularly minimal.

To carry out the test method, a device is used in which the birefringent sample is in the direction of light between a polarizer and a λ / 4 plate. Polarizer, sample and λ / 4 plate are irradiated by a white light source with monochromate filter or a laser.

In the simultaneous DFA Senarmont method, the available light intensity is preferably divided into three channels with three polarizers in three angular positions (e.g., 0 °, 60 ° and 120 °) and then with a CCD line, CCD

Camera, diodes or measured by SEV's (multiplier), converted into electrical signals and evaluated. In the Senarmont method with time-successive DFA, the analyzer is rotated into preferably three different angular positions by means of pulse-controlled and rapidly rotating miniature servo drive and the resulting intensities are measured with or without optical cable and photoreceiver and from the results both the Senarmont angle as well as the path difference calculated. The assignment of the angular position and the light intensity takes place via the characteristic pulse belonging to each angular position.

The disadvantage of the invention is the too low spectral

Variability of the method in the case of a concrete object fibers, filaments, films and other birefringent surfaces). This is explained specifically as follows: For films without false colors, the distance of the two wavelengths of the two-wavelength Senarmont method could well be again greater than 10 nra, which would increase the security (accuracy and uniqueness) and speed of the process several times. Due to the rigid specification of the wavelength spacing of 10 nm, this is not the case. Here it would be favorable if the wavelength distance is variable.

The same problem occurs with filaments without false colors with the smallest diameter, where an accurate measurement at the given wavelength spacing of 10 nm is also not possible, because at the small wavelength separation, the measurement signals partly overlap each other and the noise increases.

Conversely, in the case of very bright objects with pronounced false colors, the wavelength spacing of 10 nm can again be too large. Then, e.g. the false colors are clearly identified only at a wavelength distance of 5 nm. Only then are the results clear. At the fixed distance of 10 nm, the measurement would be wrong.

OBJECT OF THE INVENTION

The object of the invention is the further development of the Senarmont method for the automatic, contactless and rapid measurement of high path differences R of double-break and transparent samples or objects in online operation, in particular of fibers, filaments, films and

Fabrics that can also show false colors in white light, where the spectral variability is guaranteed. This means that the wavelength spacing is not constant, but can be made variable. The integration times should be in the order of seconds or seconds. According to the invention, the object is achieved by introducing a variable auxiliary wavelength λ ₂ in addition to the actual constant measurement wavelength λ i. The last-mentioned auxiliary wavelength λ ₂ is as long as the measuring wavelength λi angegli¬ chen until the same order is present for both wavelengths (the same N + l). As a result, in a case for certain samples, the distance between the two wavelengths may be greater than 10 nm, and the safety (accuracy and uniqueness) and speed of the method increase. In the other case, in the case of very bright samples with pronounced, difficult false colors, the distance between the two wavelengths can even be made smaller than 10 nm, so that the false colors can be detected correctly.

The method and apparatus can be used for the automatic, non-contact and rapid measurement of the path difference of birefringent samples without ambiguity of the results, whereby the safety (accuracy and uniqueness) and speed (short period) of the method can be regulated depending on the properties of the sample , The method and the device are suitable for measurement in laboratory operation, for fast (simultaneous measurement) and slowly variable processes (time-successive measurement) with an integration time in the ms and seconds range and for tracking the change in the transition difference of microscopically small Samples with false color. [Examples]

1. Scheme of the procedure

(DMSV = Multicolor Discrete Senarmont Method = Simultaneous or Successive Coupling of Multicolor Senarmont Method [MSV] with Discrete Fourier Analysis [DFA])

In Fig. 1, the scheme of the method is shown, after which the monochromatic radiation of the two light sources (1) and (2) with the two wavelengths λ _x and λ ₂ on the Tei¬ lerwürfel (4) together on the polarizer (5) , The light source (1) characterizes the measuring wavelength λi. The light source (2) characterizes the auxiliary wavelength X ₂ . The essence of the method is expressed in that the radiation of the auxiliary wavelength λ ₂ on the way to the Teiler¬ cube (4) passes through a device that can vary the Hilfswellen¬ length as required. For false-color-free samples, the distance to the measurement wavelength can be increased in the sense of increasing safety (accuracy and uniqueness) and speed of the measurement. For light-strong samples with false colors, the distance of the auxiliary wavelength to the measurement wavelength can be reduced to less than 10 nm, so that the uniqueness of the measurement is ensured. The birefringent sample (6) to be measured is illuminated via the polarizer (5) and fed to the λ / 4 plate (7). This converts the elliptically polarized light generated by the sample to be measured into linearly polarized light. The subsequent diffuser (8) is used to homogenize the light beam over the entire bundle cross-section. This is important because the subsequent analyzer (9) is equipped for three polarization directions in which the incident light intensities must be the same (polarization filter array or analyzer array). The position of the polarization directions is shown in Fig. 2, where specifically on the polarization directions of the polarizer (5), the sample (6), meant n _γ , the λ / 4 plate (7) and the analyzer (9) are shown (In Fig. 2, the position of the analyzer (9) means the home position).

The specific three polarization directions (including the basic position) of the analyzer (9) are shown in FIG. 3, where additionally the polarization positions of the polarizer (5), the λ / 4 plate (7) and the basic position of the analyzer (9) are indicated (Ii). In the "box" of Fig. 3 are still the other two analyzer positions (I ₂ and I ₃ ) can be seen. In terms of method, the now six existing radiations with the intensities I ₁ , I ₂ and I ₃ for the measuring wavelength λ 1 and the auxiliary wavelength λ 2 are transmitted to the photodetectors and amplifiers via a divider cube (10) without changing the polarization state. 12). In front of the photoreceptors (12), the measuring radiation passes through the filter (11), which is tuned to the wavelength λ i of the measuring radiation. The

By contrast, radiation with the auxiliary wavelength passes through the device for wavelength change (3), which is coupled to the first device (3). Both devices are tuned to the same wavelength (auxiliary wavelength X ₂ ). In the computer (12), the respective Senarmont angle, the order and the Gangunter¬ differences are calculated for the corresponding wavelengths. With the device for wavelength change (3) is the

Auxiliary wavelength λ _{2 is} changed until the same orders are present for both wavelengths [X ₁ and X ₂ ].

Then both wavelengths lie in the false color-free area. The calculated path differences are calculated correctly. As long as the order remains the same, the auxiliary wavelength can now be changed as desired. It can be enlarged and reduced in size. In the first case in favor of safety and speed of the procedure, in the second case to improve the uniqueness of the measurement results. The analyzer (9) is followed by ferrules, each with three optical cables, which strike a total of six photoreceptors (12) (three polarization directions are available for each wavelength after the analyzer (9)).

2. Method for a 2-1 / 4-λ plate (without false colors)

Table 1 summarizes the intensities Ii to I ₃ that result in the experiment at the different wavelengths and angular positions of the analyzer, which have been determined simultaneously (device according to FIGS. 1-5), and the calculated gear differences.

Table 1: Compilation of the measured values and results for the 2-1 / 4-λ plate (without false colors) for the different measuring and auxiliary wavelengths and the corresponding angular positions (0 °, 60 ° and 120 ° )

Explanatory notes to Table 1: λ = wavelength; Ii, I- ₂ and I ₃ = light intensities with different linear polarization directions; ε = Senarmont angle; R = retardation; (N + 1) = optical order.

For the example in the first two lines, the distance between the measuring and auxiliary wavelengths is 40 nm (550 and 590 nm). In the second example, the distance is 20 nm (550 and 570 nm) and in the last example 10 nm (550 and 560 nm).

The measurement is preferably carried out at 40 nm, because this can increase the safety (accuracy and uniqueness) and the speed of the method, as has been explained above.

The individual calculations are shown below for the wavelengths 550 and 590 nm (Table, lines 1 and 2): For the wavelength λi = 550 nm (line 1), the equations for the Senarmont method with discrete result

Fourier analysis (DFA) of the Senarmont angle ε over ε ° = 28.648 ° arctg b / a + 45 ° (1 + a / | a |), a = Ii - 0.5 (I ₂ + I ₃ ) and b = 0.866 (I ₂ - I ₃ ) and with the numerical values of Table 1 (line 1) al = -99 and bl = -386.236 to εl ° = 37.81 ° «37.8 °. For the wavelength λ ₂ = 590 nm (line 2) (analogous to the calculations for the wavelength 550 nm) a 2 = -410 b 2 = -150.684 and ε2 ° = 10, 09 ° »10, 1 ° •

The integer optical order (reduced by 1) is given by

N = (1/180 °) * (εi ° * λi -ε ₂ ° * λ ₂ ) / (λ ₂ -X ₁ ) = (1/180 °) * (37.8 * 550-10, 1 * 590 ) / (590 - 550)

= 2 and thus the retardation follows for the measurement length of 550 nm to R ₁ = (εl / 180 + N) * λi = (37.8 / 180 + 2) * 550 = 1215.5 nm and for the auxiliary wavelength of 590 nm to

R ₂ = (ε 2 / l80) * λ ₂ = (10.1 / 180 + 2) * 590 = 1213.1 nm.

Exactly these two numerical values are given in Table 1 in the last column for the path difference in nm.

Because there are no false colors, similar values for the path difference result for the other combinations (wavelength spacing of 20 and 10 nm) in lines 3 to 6. That is, it repeatedly gives the same optical order (N + 1) and thus the calculated path differences are unique. So here is an unproblematic measurement. The advantage is that it is possible to work with the largest wavelength difference between the measuring and auxiliary wavelengths (of the variants indicated, this means the choice of 550 and 590 nm). This increases the safety (accuracy and uniqueness) and the speed of the measurement! The possible variation of the auxiliary wavelength is extremely advantageous in this sense!

3. Method for a false-color film

The situation is quite different when the foil arranged between the crossed polarizers (polarizer and analyzer) shows false-color, as shown in Table 2. Table 2: Compilation of the measured values and results which result for a plate with false colors for the different wavelengths and angular positions (0 °, 60 ° and 120 °) (measuring wavelength λi and auxiliary wavelength X ₂ always belong together)

Explanations: See Table 1

At a distance of 40 nm between the measuring and auxiliary wavelengths (550 and 590 nm), no meaningful measured values are yet to be obtained (negative gait differences do not normally exist). Only from 20 nm (550 and 570 nm) is the same optical order repeated. Thus, the gait difference can be clearly calculated and can be used for evaluation by the researcher or the quality engineer.

With the normal polarizing microscope (type: Laboval from Zeiss, measurement in white light, calculation for 550 nm), a path difference of 6769 nm was measured on the same sample. sen. The compensation stripe with the correct optical order was not recognized in the white light due to the presence of the false colors.

3. Apparatus for implementing the change in the aid

Wavelength for the multicolor Senarmont method with simulant DFA (DMSV)

The change of the auxiliary wavelength can be carried out with any wavelength-dispersive device, e.g. B. with a prism spectrometer, an optical grating or a metal interference filter. In FIG. 4 this is shown for a metal interference filter (21) whose surface normal lies in the optical axis (20). With the rotation of the stepping motor, the metal interference filter rotates to the changed auxiliary wavelength at which a redetermination of the optical order (N + 1) is made. The change of the position of the metal interference filter takes place until there is no change in the optical order (N + 1) repeatedly for the measuring and the auxiliary wavelength. Then only clear and therefore usable results will follow (see example 1 or 2).

Due to the mechanical rotation of the stepping motor, the time for determining the optical order is greater than Is. If the optical order lies after variation of the auxiliary

Wavelength fixed, then very short-period measurements in the ms range can be carried out with the above method (see Example 2 or 3). 4. Device for Discrete Multicolor Senarmont Method (DMSV) with Known Optical Order (N + 1)

If the optical order (N + 1) is known, the detection channel can be set up in a very uncomplicated manner, ie the measuring channel following the sample to be measured can be kept very small for measurements under production conditions (positions 7 to 12 in FIG. 1). , In the simplest case, the optical order (N + 1) can be entered and it is only necessary to evaluate three intensities at the set measuring wavelength. In Fig. 5, the λ / 4 plate (7) is behind a lens (13a), which is a spatial filter with the other lenses (13b) and (13c) and the slit (14), so that stray light (ZB room light) excluded can be. The responsible for the homogenization diffuser has been omitted in Fig. 5 for clarity. After the λ / 4 plate (7), the polarization filter array (9) is arranged, which is imaged with the lenses (13a) and (13b) on the CCD chip (16), all on the thread (15). are compiled. To evaluate the measurement signals of the three intensities I _x , I ₂ and I ₃ , the CCD chip (16) is connected to the computer and the evaluation unit. The type of evaluation was described in Examples 2 and 3.

Another variant of the device with a known optical order (N + 1) simplified to FIG. 5 is indicated in FIG. 6, where the slit diaphragm has been removed to achieve higher intensities for poorly permeable media, in contrast to FIG. The lens (13a) was placed behind the λ / 4

Platelets (7) arranged, followed by the polarization grid array (9) follows. The lenses (13a) and (13b) image the array (9) onto the CCD chip (16). 5. Device for the Discrete Multicolor Senarmont Method (DMSV) with known optical order (N + 1) for filaments and other filaments

With a known optical order (N + 1), the arrangement described in item 4 can also be modified for threads and filaments. It must be noted that it is not possible to use a direct and straight beam path, as is the case with foils. To avoid extraneous light, the beam path must be angled. Because of the existing conditions in the spinning shaft, the entire arrangement must be accommodated in a thin measuring plate (6 - 8 mm), which is shown in gray in Fig. 7. In this case, we distinguish between an illumination beam path ((17) to (18)) and a measurement beam path ((13) to (19)). In between is the thread not drawn in and to be measured. From a base plate located outside with the monochromatic light source, the polarized radiation (position of the polarization direction see FIG. 2) is introduced into the measuring plate with a single-mode illumination fiber (17) and widened with the cylindrical lenses (18). This linearly polarized light acts on the thread, which in turn emits elliptical light on the basis of its anisotropic properties, which, via a lens (13), radiates onto the λ / 4.

Platelets (7) passed and there linearly polarized again, but with a different polarization direction. The diffuser (8) and the multimode fiber array (19) serve to uniformly detect the three light intensities with different polarization direction. With appropriately sensitive diodes, the light signals are conducted via ferrules to the photoreceivers with the computer-based evaluation. [REFERENCE LIST]

1. White light source with filter, LED or laser diode for the wavelength λi

2. White light source with filter or LED for wavelength X ₂

3. Device for wavelength change

4. splitter cube (without changing the polarization state of the radiation)

5. Polarizer 6. Sample (foil or other birefringent sample)

7. λ / 4 plate

8. Diffuser

9. Analyzer with three angular positions (polarization filter array or analyzer array). 10. Divider Cube (Without changing the polarization state of the radiation)

11. Wavelength filter for the measuring wavelength λi

12. Photo receiver for every 3 polarization positions and computer 13. Optical lenses (13a to 13c)

14. Slit diaphragm

15th C-mount

16. CCD chip

17. Single-mode illumination fiber 18. Cylindrical lenses as expansion optics

19. Multimode fiber array

20. Optical axis

22. Metal interference filter whose surface normal lies in the direction of the optical axis 22. Stepping motor whose drive axis is once perpendicular to the optical axis and at the same time represents the drive axis of the metal interference filter with which it is rigidly connected. 23. Direction of rotation of the metal interference filter

Claims

[Claims]

1. A method for the automatic, non-contact and quick measurement of high path differences of transparent and optically birefringent media according to Senarmont in on-line operation with simultaneous discrete Fourier analysis (DFA), in particular of fibers, filaments, films and fabrics, which in white light can also show false colors, characterized in that in addition to the actually constant measuring wavelength λi a variable

Auxiliary wavelength λ ₂ is used, which is changed while the measurement wavelength X _{1 is} adjusted until the same optical order is present for both wavelengths simultaneously and repeatedly (same N + 1) and thereby the "discrete multicolor Senarmont measuring method ( DMSV) "can be performed by simultaneously at the measuring wavelength λi and at the auxiliary wavelength λ _{2 of} the behind the polarizer, sample, λ / 4 plate and diffuser arranged analyzer with n angular positions and the corresponding photoreceivers, the n light intensities photoelectrically and, after amplification based on the relationships of the discrete Fourier analysis with stored light intensities, the Senarmont angles εχi and εχ ₂ and the corresponding _path differences R _λl and Rχ _{2 are} calculated, with the same optical order repeated

Correctness of the measurement results accepted and completed, while in the opposite case, the auxiliary wavelength λ _{2 of} the measuring wavelength λi further approximated and this process is repeated until the same optical order is repeated for both wavelengths.

2. The method according to claim 1, characterized in that only n = 3 angular positions for the analyzer of 0 °, 60 ° and 120 ° for measuring the three light intensities I ₁ to I _third and only assuming linearity, zero and maximum intensity, the senor angle ε being uniquely determined from the relationships ε ° = (90 ° / π) * arctg b / a + 45 ° (1 + a / | a | ) in angular degrees or ε = 0.5 * arctg b / a + n / 4 * (l + a / | a |) in radians, a = I ₁ - 0.5 * (I ₂ + I ₃ ) and b = 0.866 * (I ₂ - I ₃ ) and N = (1/180 °) * (εi ° * λi - ε ₂ ° * λ ₂ ) / (λ ₂ - λi) and the _path differences R _λi = (ε ° / 180 ° + N) * X ₁

R _λ2 = (ε / n + N) * λ ₂ .

If the same optical order (N + 1) is repeated, the correctness of the measurement results is accepted and the measuring process is completed, while in the opposite case the auxiliary wavelength λ ₂ of the measuring wavelength λ i is further approached and this process is repeated until it is repeated for both wavelengths the same optical order (N + 1) occurs.

3. The method according to claim 1 and 2, characterized in that in selected cases, the monochromatization of the measuring and auxiliary wavelength, generated with the device for changing the wavelength, takes place only once directly in front of the photoreceptors.

4. Apparatus for the automatic, non-contact and rapid measurement of high path differences of transparent and optically birefringent media according to Senarmont in on-line operation with simultaneous discrete Fourier analysis (DFA), in particular of fibers, filaments, films and fabrics, which in the white light can also show false colors, characterized in that for the beam path of the auxiliary wavelength both in front of the polarizer and after the analyzer array, a device for the wavelength change in the form of a prism, optical grating or rotatable metal interference filter is introduced, with which the auxiliary wavelength is changed.

5. The device according to claim 3, characterized in that the change of the auxiliary wavelength takes place discretely by ent speaking connection from a set of permanently installed monochromatic light sources.

6. Apparatus according to claim 3 or 4, characterized gekennzeich¬ net, that the change of the auxiliary wavelength discretely by selection with a narrow-band, wavelength-selective filter in the detection beam path from a brei¬ th illumination spectrum.

7. The device according to claim 3 to 5, characterized gekennzeich¬ net, that in known order (N + l) a detection channel is used, in which arranged behind the λ / 4-plate a dif fusor and the analyzer or polarization filter array are and that are mapped with an optical system, the three Licht¬ intensities different polarization direction on a CCD line or a CCD surface chip and evaluated by means of electronic image processing (EBV).

8. The device according to claim 3 to 6, characterized gekennzeich¬ net, that in known order (N + l) a detection channel is used, in which arranged behind the λ / 4-plate a dif fusor and the analyzer or polarization filter array and that with an optic, the three light intensities of different polarization direction after the analyzer or polarization filter array are led directly via photodetectors (photodiodes, photoelements, photoresistors) and amplification for the purpose of evaluation to the computer.

9. Apparatus according to claim 7, characterized in that the transducer, above-mentioned measuring plate, does not contain any active electronics.

10. Apparatus according to claim 7 and 8, characterized da¬ by that the supply of the measuring light in the Mess¬ value recorder, o.g. Measuring plate, and the transmission of the detected light to the photodetectors by means of optical waveguides and thereby the transducer, o.g. Measuring plate, is galvanically decoupled.

11. The device according to claim 6, characterized in that the polarization filter array is designed as a metal strip grid with sub-wavelength period.