DE19810980A1 - Measuring distance between optical boundary surfaces using interferometer - Google Patents

Abstract

A reference signal is produced, having a frequency (fr) which is a factor of at least two greater than the frequency (fmMax) of the measurement signal, where fMmax corresponds to the maximum possible rate of change of the wavelength difference. The reference signal frequency is used as a clock signal for digitizing the measurement signal. Position values of the optical boundary surfaces are determined from the digitized measurement signal. An Independent claim is also included for an apparatus for measuring the distance between optical boundary surfaces.

Description

The invention relates to a method for measuring distances between optical interfaces, in which light of a short coherence length and a fixed wavelength λ is radiated into an interferometer arrangement, the optical path length difference of the interferometer arms is changed, a signal from the resulting interferences a frequency f _m dependent on the rate of change v of the path length difference is obtained and measured values for the positions or the distances of the boundary surfaces are determined from the combination of the measurement signal with a reference signal. The invention further relates to arrangements for performing this method.

From the patent specification WO 92/19930 an arrangement for Acquisition of structures and distances known that a Mi chelson interferometer and a light source of short Kohä limit length, the radiation of which as measuring light into the In terferometer arrangement is coupled. In the measuring arm of the Mi chelson interferometer is the one to be examined Object, while in the reference arm a sliding reflector is provided to change the optical path length.

To determine distances or to record structure structures of the test object, the reflector is moved and that resulting interference signal using an optoelectro African transducer detected. The interference signal has the Frequency f = 2v / λ and is amplitude modulated, where λ is the Wavelength and v the rate of change of the op table path length in the relevant interferometer arm ten.

Due to the short coherence length of, for example,  a superluminescent diode that emits light Interference signal only when the distance of the Re to the beam splitter of the interferometer is equal to that Distance between an optical interface of the measurement object tes and the beam splitter.

An interference signal is therefore an evidence of that Presence of an optical interface in the measurement object; out the positions of the reflector at which interference signals occur, are therefore conclusions on positions of Interfaces possible. The determination of these positions of the Reflector is therefore the purpose of this arrangement. Out the position values can be calculated from the Ab levels between the interfaces can be determined.

Caused by the low reflectivity of the measurement send interfaces is the intensity of the to be detected the interference signals, however, very low. These are au also overlaid with noise. Should the interference si gnale selected with reasonable electronic effort be, for example, by narrowband electronic Filtering for a fixed signal frequency must be the same remaining speed v when changing the opti path length or the adjustment of the reflector ahead be set, as also short-term speed swan cations lead to signal frequencies that are outside of the allowed filter bandwidth, so that the measurement signal is not or at least cannot be detected exactly.

Arrangements have been developed with which the Uniformity of the adjustment speed of the reflector and thus improves the detectability of the signals that should. For example, according to WO patent 96/53100 the change in the optical path length with the help of a ro ting reflector provided. This ensures that  the optical path length becomes linear over a larger area changed.

From DE 195 04 444 an arrangement is known in which if a Michelson interferometer with changeable Path length difference is provided and the radiation one Light source of short coherence length as measuring light in the inter ferometer arrangement is coupled. Different from the previous one State of the art described here, however, is in addition to Interference signal obtained a reference signal that at Moving the reflector changes its frequency as well like the measurement or interference signal.

A grating encoder is used to generate this reference signal used with a movable scale grid, which with the Device for changing the optical path length differences limit, such as an adjuster coupled with a reflector Oil device is rigidly connected. The grating sensor, the is in the beam path of a light source is a separate photoelectric receiver downstream, on the output a sinusoidal signal is available that is the same Has frequency like the measurement or noise Interference signal. This sine signal is called Refe limit signal used. Measurement and reference signal are according to De modulation in two channels phase-shifted by 90 ° dinate-transformed, the envelope of the amplitudes mo dulated measurement signal, which is proportional to the interference con is determined, and this ultimately as a function of displacement position reached in case of interference becomes.

The disadvantages of this solution are the complex Analog electronics that many operational amplifiers require changes, all in terms of gain and offset must be coordinated. It is also an accurate one  Matching the frequencies of the grating signal and the Measurement signal to an integer multiple required.

Another disadvantage arises from the fact that the Grenzfre sequence of the low preceding the coordinate transformation passes at high travel speeds, which leads to As a result, the bandwidth in the entire arrangement increases takes and the signal to be evaluated very badly from the rough can be separated.

The invention is therefore based on the object that was known to further develop methods of the type described above, that a more precise detection of the measurement signal and thus a Increasing the measuring accuracy is possible.

According to the invention the object is achieved in that the reference signal is generated at a frequency f _r which is a factor k ≧ 2 above the frequency f _{mMax of} the measurement signal. Here f _{mMax is} the frequency that occurs at the maximum possible rate of change of the path length difference. The reference signal with the frequency f _r is used as the clock signal for digitizing the measurement signal.

This ensures that the digitization of the Measurement signal fluctuations in the adjustment speed of the Re be compensated for, so that even at high travel speeds a narrow-band filtering with high Accuracy of the measurement result is possible. Second is it is possible in this way to precisely detect the measurement signal animals. Because of this, he will have a higher measurement accuracy aims and also a reduction in circuitry for the electronic signal processing compared to the state technology achieved, since now a narrow-band filtering is possible.  

An advantageous embodiment of the invention is in that the digitized measurement signal in addressable Memory cells is stored. Stand in the memory cells then data corresponding to the amplitude of the measurement signal. From the location of the memory cells in which the envelope of these amplitude values has a maximum, the Po are now sitions of the interfaces.

For this purpose, a numerical digital filtering is carried out according to the invention, for example according to the function

y _N = a ₀ x _N + a ₁ x _N-1 + a ₂ x _N-2 +. . . + a _M-1 x _{N- (M-1)} - b ₁ y _N-1 - b ₂ y _N-2 -. . . - b _z y _Nz

with y _N the filter output value of the memory cell N, Y _N-1 the filter output value of the previously used memory cell N-1, x _N the measured value in the memory cell N, x _N-1 the measured value in the memory cell N-1 etc. and a ₀ to a _M-1 and b ₁ to b _z the filter coefficients.

Filtering with b ₁ = 0 to b _z = 0 provides for a special embodiment and thus for the function

y _N = a ₀ x _N + a ₁ x _N-1 + a ₂ x _N-2 +. . . + a _M-1 x _{N- (M-1)} .

After filtering, the memory cell in which the stored measured value x _N. . . x _{N- (M-1) has} a maximum. The position of the displaceable reflector at which an interference signal occurs is derived from the position of this cell within the memory. The distances between the optical interfaces in the object to be examined are inferred from various of the like positions.

The invention further relates to an arrangement for Measuring distances between optical interfaces preferably between different tissue layers of the human eye, with an interferometer arrangement, with changeable optical path length difference of a short co  pension light source for generating a measuring beam path with an adjustment device for changing the optical Path length difference, with a receiver to obtain a electronic measuring signal from the measuring radiation at Verstel the path length difference, with a reference signal transmitter and with a signal processing device.

It is provided according to the invention that a reference signal with a frequency f _{r is} available at the output of the reference signal transmitter, which is by a factor of k ≧ 2 above the maximum frequency f _{mMax of} the measurement signal, which depends on the maximum possible rate of change from Path length difference occurs and that the output of the reference signal generator is connected to the clock input of an A / D converter, while the measurement signal is present at the signal input of the A / D converter.

There are different design variants of reference signal transmitters conceivable, three of which are described below be written. Each variant includes a reference light source, a detector for receiving the reference light and a trigger downstream of the detector for rectangular form measurement of the reference signal. The exit of the trigger ent speaks to the output of the reference signal generator.

In a first variant of the reference signal generator in addition to the aforementioned assemblies optical elements for Coupling the reference light into the interferometer arrangement and their decoupling after a single pass provided by the interferometer arrangement.

In a second embodiment variant, the reference signal generator comprises, in addition to the aforementioned assemblies and the optical elements for coupling the reference light into the interferometer arrangement, reflective elements which cause the reference light to pass through the interference arm several times with a variable optical path length. The reflecting elements are, for example, mirrors arranged on the irradiation side on outer surfaces of the beam splitter of the interferometer arrangement, which reflect only the wavelength of the reference light and reflect the reference light poorly in the respective interferometer. The multiple pass results in a reference signal which is already a factor of k ≧ 2 above the frequency f _{m of} the measurement signal.

A third embodiment of the reference signal transmitter sees no coupling of the reference light into the Interferometer arrangement in front, but one in the beam path the reference light source arranged grating with a movable scale grid that with the adjustment device is firmly connected.

To generate a reference signal frequency, which lies by the factor k ≧ 2 above the frequency f _{m of} the measurement signal, a frequency multiplier can be provided in the signal path between the detector and the trigger in the first and third embodiment variants.

The frequency multipliers can, for example, from one Circuit arrangement exist in which the signal path of the Re reference signal is branched into two paths, the Re reference signal via a first path in which a 90 ° Phase shifter, a comparator and a monoflop in series are connected to a first input of an OR Building block and via the second path in which a comparator and a monoflop are provided at the second input of the OR module is present. The output of the OR block corresponds to the output of the frequency multiplier.  

This makes it possible to generate reference signals that in the above-described method according to the invention, the digitali sation of the measurement signal can be used as a basis. Before Part of this is a monomo as the reference light source to provide de-laser diode.

In an alternative embodiment of the invention, instead of a reference value transmitter with a frequency multiplier, a reference light source can be provided, the frequency of which is already at least a factor k = 2 higher than the frequency f _{m of} the short-coherent light source for generating the measuring beam path. In this case too there is a detector for receiving the reference light and a trigger connected after the detector.

It is within the scope of the invention that in the reference signal Via a position sensor is provided, which with the Ver actuator is related. With that, starting out from a given position when adjusting the Path length in the corresponding interferometer arm Start signal for the A / D converter or for digitization tion of the measurement signal can be obtained. This happens when the reflector, with its adjustment the optical path length is varied in an interferometer arm, this predetermined Position happens.

Furthermore, the signal processing device should be one addressable memory and an evaluation unit sen and in the evaluation unit a calculation circuit for nu merischer filtering of the digitized measurement signal existing be that.

For the purpose of pre-filtering, the receiver for obtaining an electronic measurement signal from the measurement radiation when adjusting the path length difference can be followed by a broadband pre-amplifier, the upper limit frequency of which corresponds to the frequency f _mMax and the lower limit frequency of which corresponds to the frequency f _mMin .

In a further advantageous embodiment, it is provided hen that the evaluation unit has an output device for the measurement result is subordinate. In the output facility can be a display for measured value display and / or an Er result printer available.

The method according to the invention is described below with the aid of meh Other design variants for the reference signal gain be explained. In the accompanying drawings gene:

Fig. 1 voltage the principle diagram of the present invention Anord,

Fig. 2 shows a first variant of the reference signal extraction,

Fig. 3 shows a second variant of the reference signal extraction,

Fig. 4 shows a third variant of the reference signal extraction,

Fig. 5 is a circuit variation of the frequency multiplier,

Fig. 6 The cut of an ideal measurement signal,

Fig. 7 The ratio of measurement and reference signal.

In Fig. 1, a beam splitter 1 , a fixed reflector 2 and a movable reflector 3 are shown as assemblies egg ner interferometer arrangement; the reflectors 2 , 3 can be designed, for example, as triple prisms. In the interferometer arrangement, the light of a superluminescent diode 5 fe ster wavelength λ is irradiated as an illumination or measuring beam path via a further beam splitter 4 .

Via a diaphragm 6 and a flat plate 7 , a control portion of the illuminating light is directed onto a photodiode 8 , while the rest of the beam beam splitter 9 and a λ / 4 plate 10 reach the patient's eye 11 .

The measurement light reflected from the patient's eye 11 is imaged via the polarization beam splitter 9 and an imaging system 12 , which is represented by two lenses, into an observation plane in which the eye 15 of an observer is located. In front of the eye 15 with a further beam splitter 13 part of the measuring light on a Photoemp catcher 14 , preferably an avalance photodiode, is directed.

The signal outputs of photoreceiver 14 , photodiode 8 and a reference signal generator 20 are connected to the inputs of a signal processing 17 which is connected on the output side via a control unit 19 to the superluminescent diode 5 and to an adjusting device 16 for the displaceable reflector 3 . Also connected to the signal processing 17 is an output device 18 for the measurement results, for example equipped with a display and / or a printer.

With the arrangement shown, a measurement signal can be received in a known manner from the photoreceiver 14 , the frequency f _m of which is dependent on the speed v at which the change in the difference in the optical path length takes place in both interferometer arms. The difference in the optical path length is changed by adjusting the position of the displaceable reflector 3 , which is mechanically coupled to the adjusting device 16 for this purpose.

From the evaluation of the measurement signal obtained with the aid of the photoreceiver 14 and its combination with the reference signal emitted by the reference signal generator 20 , position values for the position of individual tissue layers of the patient eye 11 and from these position values, the distances between the tissue layers concerned are determined, as in the following with reference to FIG to be explained. 2nd

The received by the photoreceiver 14 , corresponding to the coherence length of the superluminescent diode 5, amplitude-modulated measurement signal with the frequency f _m = 2v / λ (v is the speed of the path length change in the interferometer arm when adjusting the reflector 3 ) is amplified in an amplifier 21 , in the A / D converter 22 converted into a digital signal and stored in an addressable memory 23 . The amplifier 21 is designed to be broadband for the frequencies f _mMax and f _mMin , where f _{mMax is} the frequency of the measurement signal at the maximum possible and f _mMin n is the frequency of the measurement signal at the minimum possible rate of change of the path length difference.

The stored measured values are called up by the evaluation unit 40 and, after the corresponding signal processing, are passed on as a result to the output device 18 or as instructions to the control unit 19 .

Due to the small coherence length of the superluminescent diode 5 occurs during the change in position of the reflector 3 evaluable interference signal always on, if the difference in distances between the reflectors 2 and 3 to the beam splitter 1 is equal to the distance between two optical interfaces in the patient's eye 11 (cf. Fig. . 1). The task of the measurement is therefore to determine the positions of the reflector 3 , at which interference signals occur, and from this to determine the distances between tissue layers in the patient's eye 11 .

However, since the intensity of the interference signals to be detected is very low and these are also superimposed with noise, in order to increase the measuring certainty, in addition to the short-coherent illuminating or measuring light of the superluminescent diode 5, the light of a reference light source 25 with a predetermined wavelength λ _r and compared to the measuring light of greater coherence length used as the basis for a reference signal, which he generates in the manner according to the invention and is linked to the measuring signal.

In the embodiment variant according to FIG. 2, a reference light source are in the Refe rence beeper 24 25 (Example, a single-mode laser diode), optical elements 26 for coupling the reference light via the beam splitter 1 in the interferometer, a detector 27, a frequency multiplier 28, a trigger 29 and a position indicator 30 are provided.

With this arrangement it is first achieved that a reference signal is recorded with the detector 27 , the frequency f _r 'changes during the position adjustment of the reflector 3 as well as the frequency f _{m of} the measuring signal. The frequency f _r 'of the reference signal is increased in the frequency multiplier 28 by at least a factor k = 2, after which a reference signal with the frequency f _r is available at the input of the trigger 29 . In the measuring arm of the interferometer there is an element (not shown) reflecting the light of the coherent laser diode, so that this light cannot fall into the patient's eye 11 and danger to the patient is avoided.

An example of the design of the frequency multiplier 24 is shown in FIG. 5. The signal path 31 for the sinusoidal reference signal is branched; in a first branch 32 the signal reaches a OR module 36 via a 90 ° phase shifter 33 , a comparator 34 and a monoflop 35 , while in the second branch 37 the signal reaches the OR module via a comparator 38 and a monoflop 39 36 arrives. In both branches 32 , 37 , the comparators 34 , 38 convert the sinusoidal signals into rectangular signals and the monoflops 35 , 39 emit a pulse on each falling and / or rising edge of a rectangular signal. At the output of the frequency multiplier 28 or at the input of the trigger 22 (see FIG. 2) there are four pulses for each period of the reference signal.

When a measurement is triggered by an operator, the control unit 19 outputs a start signal, by means of which the superluminescent diode 5 is switched on and, at the same time, the displacement of the reflector 3 over the intended travel path is initiated via the adjusting device 16 . The light reflected from the eye is formed on the photoreceptor 14 and the resulting interference signal is detected. The detected with the photoreceiver 14 interference signal is filtered by the broadband amplifier 21 and is then (because of the non-constant travel speed with fluctuating period) at the A / D converter 22 , because of the low coherence length, however, only if the path difference of the Light in the interferometer corresponds to the path difference occurring in the test object.

In Fig. 6 a section of an ideal, at the input of the A / D converter 22 applied interference measurement signal 54 is provided. The fluctuations in the frequency f _{m of} the signal are caused by the uneven travel speed. In comparison to this, the reference signal 55 with the frequency f _r ', which can be tapped at the output of the detector 27 , also has these frequency fluctuations. Because of the large coherence length of the reference light source, the reference signal 55, in contrast to the measurement signal 54, is received during the movement of the reflector 3 over its entire path length.

When the reflector 3 passes through a predetermined position, the position indicator 30 in FIG. 2, for example a limit switch, gives a start signal to the trigger 29 , whereupon the trigger 29 begins with the output of the reference signal with the frequency f _r to the A / D converter. The ratio of the frequency f _m of the measurement signal 54 present at the signal input of the A / D converter to the frequency f _r of the reference signal 56 present at the clock signal input of the A / D converter can be seen in FIG. 7.

The superimposition of the measurement signal in the A / D converter with the reference signal of the frequency f _r leads to an A / D converter value being written into the memory 23 for each pulse derived from the reference signal, starting with the start pulse during the movement of the reflector 3 becomes. Each memory cell is thus assigned a location of the reflector 3 and the measured value x _{N is} in the memory cell N, the measured value x N-1 in the memory cell _N-1 , etc.

After driving through the entire travel path, the stored data are called into the evaluation unit 40 , in which a numerical digital filtering of the data is performed according to the function

y _N = a ₀ x _N + a ₁ x _N-1 + a ₂ x _N-2 +. . . + a _M-1 x _{N- (M-1)}

with y _N the filtered measured value and a ₀ to a _M-1 the filter coefficient of the filter of order M. The following applies to the filter coefficients:

at k = 0, 1, 2. . . (M-1). This operation corresponds to filtering the data at the center frequency f. The center frequency f becomes from the relationship

derived with λr the wavelength of the reference signal and λ _m the wavelength of the measurement signal. The factor 1/4 corresponds to four times the interpolation of the detected reference signal f _r 'for generating the reference signal f _r . The bandwidth Δf of the filter results from the coherence length of the measuring signal.

After the filtering, the memory cell is determined in which the envelope has a maximum of the stored measured value. From the position of this cell within the memory, the position of the reflector 3 is derived, in which an interference signal occurs. From various positions of this kind, as described above, the distances between the optical interfaces in the object to be examined are concluded.

Fig. 3 shows the embodiment variant of a reference signal generator 24 , in which a reference light source 41 , an optical element 42 for coupling the reference light into the interferometer arrangement, reflecting elements 43 and 44 , a detector 45 for the reference light coupled out of the interferometer arrangement and, the detector 45 nachge switched, a trigger 46 are provided. The output of the trigger 46 is identical to the output of the reference signal generator 24 and is present at the control input of the A / D converter 22 . Also included in the reference signal generator 24 is a position indicator 47 , the output of which is connected to the start signal input of the trigger 26 . A frequency multiplier can additionally be provided between detector 45 and trigger 46 .

The reference light reaches the splitting surface of the beam splitter 1 via the optical element 42 and is split there into two partial beam paths, one of which is directed in each case at the fixed reflector 2 and at the displaceable reflector 3 .

The first partial beam path of the reference light is thrown back from the reflector 2 to the beam splitter 1 and strikes the reflector element 43 arranged on the surface of the beam splitter 1 , impermeable to the wavelength of the reference light, from which it again to the reflector 2 and from there to the divider surface of the Beam splitter 1 is reflected, where the deflection finally takes place in the direction of the detector 45 .

The second partial radiation directed from the splitting surface of the beam splitter 1 onto the displaceable reflector 3 is reflected again by the reflector 3 onto the beam splitter 1 and is reflected there by the element 44 reflecting the reference light back to the reflector 3 . This partial radiation reaches the detector 45 from the reflector 3 through the beam splitter 1 .

The reflecting elements 43 and 44 can, for example, be designed as retroreflectors or mirrors.

The reference light thus passes through both interferometers twice and only then arrives at the detector 45 . Due to the multiple pass, the reference signal already has the multiple frequency of the measurement signal. Thus, a clock signal is present at the control input of the A / D converter, which can be used for digitizing the measurement signal, just as in the previously described embodiment variant according to FIG. 2.

The previously stated applies when the wavelengths at Measuring and reference light sources are the same or approximately the same.  

In contrast, becomes a reference light source with smaller waves length compared to the measuring light source used as it the measuring light source has an even higher frequency Reference signal can be achieved.

A further variant to obtain a Re ference signal shown in FIG. 4. Here, in the reference signal generator 24, a reference light source 48, a grating encoder 49, a De Tektor 50, a frequency multiplier 51 and a the fre quency multiplier 51 downstream trigger 52 vorgese hen. The output of the trigger 52 corresponds to the output of the reference signal generator 24 and is present at the control input of the A / D converter 22 . A position indicator 53 is also provided here, the output of which is connected to the start signal input of the trigger 52 .

The grating sensor has a fixed as well as a movable grating, the movable grating (not shown in detail) is mechanically coupled to the adjusting device 16 and is adjusted synchronously to the reflector 3 when the path length difference is adjusted. This makes it possible to obtain a reference signal during the adjustment movement, the period of which changes in the case of a nonuniform adjustment speed as well as that of the measurement signal. Fluctuations in the period of the measurement signal, which can be attributed to irregularities in the speed when the reflector 3 is displaced, the reference signal frequency are equally impressed.

As described further above in the embodiment variant with reference to FIG. 2, the reference signal detected and output by the detector 50 is also used after the frequency change with the multiplier 51 , which can be configured as shown in FIG. 5, and after the rectangle has been formed with the trigger 52 Digitization of the measurement signal used.

Reference list

1

Beam splitter

2nd

Reflector, fixed

3rd

Sliding reflector

4th

Beam splitter

5

Superluminescent diode

6

cover

7

Face plate

8th

Photodiode

9

Polarization beam splitter

10th

λ / 4 plate

11

Patient eye

12th

Imaging system

13

Beam splitter

14

receiver

15

eye

16

Adjustment device

17th

Signal processing

18th

Output device

19th

Control unit

20th

Reference signal generator

21

amplifier

22

A / D converter

23

Storage

24th

Reference signal generator

25th

Reference light source

26

optical elements

27

detector

28

Frequency multiplier

29

Trigger

30th

Position detector

31

Signal path

32

branch

33

90 ° phase shifter

34

Comparator

35

Monoflop

36

OR block

37

branch

38

Comparator

39

Monoflop

40

Evaluation unit

41

Reference light source

42

optical element

43

,

44

reflective elements

45

detector

46

Trigger

47

Position detector

48

Reference light source

49

Lattice encoder

50

detector

51

Frequency multiplier

52

Trigger

53

Position detector

54

Interference measurement signal

55

Reference signal for _r

'

56

Reference signal for _r

Claims (16)

1. A method for measuring distances between optical interfaces, in which light of short coherence length and fixed wavelength λ is radiated into an interferometer arrangement, the optical path length difference of the interferometer arms is changed, from the resulting interference a measurement signal with a change in the velocity v of the path length difference obtained from the dependent frequency f _m and measuring values for the positions or the distances of the interfaces are determined from the combination of the measurement signal with a reference signal, characterized in that

• - that the reference signal having a frequency f _r is produced, the f by a factor k ≧ 2 the frequency of the measuring signal is _mMax wherein _mMax the maximum possible rate of change of path length difference f ent speaks
• - That the reference signal with the frequency f _r as a clock signal is based on a digitization of the measurement signal and
• - Position values of the optical interfaces are determined on the measurement signal digitized in this way.

2. The method according to claim 1, characterized in that the digitized with the frequency f _r measurement signal is stored in addressable memory cells, from the position of the memory cells in which the amplitude of the Meßsigna les has a maximum, he determines the positions of the interfaces and from the distances between the interfaces are derived from these positions. 3. The method according to claim 1 or 2, characterized net that the stored measurement signal of a numerical digital filtering. 4. The method according to claim 3, characterized in that a filtering is carried out according to the function
y _N = a ₀ x _N + a ₁ x _N-1 + a ₂ x _N-2 +. . . + a _M-1 x _{N- (M-1)} - b ₁ y _N-1 - b ₂ y _N-2 -. . . -b _z y _Nz
with y _N the filter output value of the memory cell N, y _N-1 the filter output value of the previously used memory cell N-1, x _N the measured value in the memory cell N, x _N-1 the measured value in the memory cell N-1 etc. and a ₀ to a _M-1 and b ₁ to b _z the filter coefficient of the filter. 5. The method according to claim 4, characterized in that the filtering with b ₁ = 0 to b _z = 0 and the resulting function
y _N = a ₀ x _N + a ₁ x _N-1 + a ₂ x _N-2 +. . . + a _M-1 x _{N- (M-1)}
is made. 6. Arrangement for measuring distances between optical interfaces, preferably coating different tissues of the human eye,

• - With an interferometer arrangement in which the optical path length difference is adjustable by changing the optical path length in one of the interferometer arms and in which a short-coherent light source ( 5 ) is provided for generating a measuring beam path of a frequency f _m ,
• - With an adjusting device ( 16 ) for changing the optical path length difference,
• - With a receiver ( 14 ) for obtaining an electronic measuring signal's from the measuring radiation when adjusting the path length difference,
• - With a reference signal generator ( 24 ) and with a signal processing ( 17 ), characterized in that
• - that at the output of the reference signal generator (24) Refe rence signal having a frequency f _r is available, f by a factor k ≧ 2 over the frequency is _mMax of the measurement signal, wherein f _mMax at the maximum possible change speed v of the path length occurs, and
• - That the output of the reference signal generator ( 24 ) is connected to the clock input of an A / D converter ( 22 ), while the measurement signal is present at the signal input of the A / D converter ( 22 ).

7. Arrangement according to claim 6, characterized in that the reference signal generator ( 24 ) are provided

• - a reference light source ( 25 ),
• - Optical elements ( 26 ) for coupling the reference light into the interferometer arrangement,
• - A detector ( 27 ) for receiving the reference arrangement coupled out of the interferome and
• - A trigger ( 29 ) connected downstream of the detector ( 27 ).

8. Arrangement according to claim 6, characterized in that the reference signal transmitter ( 24 ) are provided:

• - a reference light source ( 41 ),
• - at least one optical element ( 42 ) for coupling the reference light into the interferometer arrangement,
• the elements ( 43 , 44 ) reflecting the reference light and causing the reference light to pass through the interferometer arrangement several times,
• - A detector ( 45 ) for receiving the coupling out of the interferome reference arrangement and
• - A trigger ( 46 ) connected downstream of the detector ( 45 ).

9. Arrangement according to claim 6, characterized in that the reference signal generator ( 24 ) are provided

• - a reference light source ( 48 ),
• a grating sensor ( 49 ) arranged in the beam path of the reference light and having a movable scale grating which is related to the adjusting device ( 16 ),
• - A detector ( 50 ) downstream of the grating sensor ( 49 ) for receiving the reference light and
• - A trigger ( 52 ) which is connected downstream of the detector ( 50 ).

10. Arrangement according to one of claims 6 to 9, characterized in that in the signal path between the detector ( 27 , 45 , 50 ) and trigger ( 29 , 46 , 52 ) frequency multiplier ( 28 , 51 ) are provided. 11. The arrangement according to claim 10, characterized in that the frequency multiplier ( 28 , 51 ) consists of a circuit arrangement in which the signal path of the reference signal is branched into two paths ( 32 , 37 ), the first path ( 32 ) a 90 ° phase shifter ( 33 ), a comparator ( 34 ) and a monoflop ( 35 ) and the second path ( 37 ) contain a comparator ( 38 ) and a monoflop ( 39 ) and each of the two paths ( 32 , 37 ) an input of an OR component ( 36 ) is present. 12. Arrangement according to one of claims 6 to 11, characterized in that in the reference signal transmitter ( 24 ) a Po sitionsmelder ( 30 , 47 , 53 ) is provided, which is related to the adjusting device ( 16 ) and whose output with a start signal input Triggers ( 29 , 46 , 52 ) is connected. 13. Arrangement according to one of claims 6 to 12, characterized in that a single-mode laser diode is provided as the reference light source ( 25 , 41 , 48 ). 14. Arrangement according to one of claims 6 to 12, characterized in that a reference light source ( 25 , 41 , 48 ) is provided, the frequency of which is at least a factor k = 2 higher than the frequency f _{m of} the short-coherent light source ( 5 ) Generation of the measuring beam path. 15. The arrangement according to claim 6, characterized in that an addressable memory ( 23 ) and an evaluation unit ( 40 ) are provided in the signal processing ( 17 ) and in the evaluation unit ( 40 ) a computing circuit for numerical filtering of the digitized measurement signal and to search for maxima in the filtered measurement signal is present. 16. An arrangement according to one of claims 6 to 15, characterized in that the receiver (14) with a wide banding ger preamplifier (21) is connected downstream, the upper limit frequency of the frequency f _mMax and the lower limit frequency of the frequency f _Mmin corresponds, where f _mMax and f _{mMin are} the frequencies of the measurement signal which occur at the maximum possible and at the minimum possible rate of change v of the path length difference.