Object length measuring apparatus

Abstract

The aim of the invention is to coordinate as much as possible the two actually conflicting targets of high resolution or high image quality and minimum data rate when measuring the length of an object, in view of which the invention proposes a device (1) for determining the length of an object (6) by optical coherence tomography, comprising an interferometer having a light source (2), a sample arm (4) and a reference arm (5), the light emitted by the light source (2) can be split by means of a light splitter (3) such that a first light (4a) can be guided on the sample arm (4) in a reciprocating direction and a second light (5a) can be guided on the reference arm (5) in a reciprocating direction, the first light interfering with the retroreflected second light (4a, 5a), an evaluation unit (8) being provided for detecting and processing a signal of the first light (4a) interfering with the second light (5a), a length switching unit (13a, 13b) is arranged in the beam path of the sample arm (4) and / or in the beam path of the reference arm (5), which length switching unit (13a, 13b) changes the beam path length of the light (4a, 5a) through the length switching unit (13a, 13b), characterized in that at least one length switching unit (13a, 13b) switches the beam path length from a first value to a second value in a time-alternating manner.

Description

Technical Field

[0001] The invention relates to a device according to the subject-matter of claim 1. Background Art

[0002] The term "Optical Coherence Tomography" (often abbreviated as OCT) refers to an imaging method.

[0003] This method can be used to obtain two-dimensional and three-dimensional images of light scattering structures. In this method, light with a certain bandwidth is usually split into two beams in a beam splitter. The first beam falls on the sample or object to be tested, and the second beam passes through a reference path.

[0004] The light reflected from the sample or object interferes with the reference beam. Using the interference signal, the sample can be analyzed with depth resolution, i.e. at the depth of the optical axis of the first partial beam, with the aid of a so-called A-scan.

[0005] In addition, the first partial light beam can be used to perform planar or transverse scanning on the sample to acquire an OCT image.

[0006] In this context, patent document WO2012 / 104097A1 discloses a profile image recording method, in which a path length switching unit is used. The path length switching unit changes the optical path length of the sample beam and / or the reference beam of the interferometer, so that depth profile images of different depths of the sample can be generated. The optical path length is changed by redirecting the optical path to a different geometric path.

[0007] In order to measure the length of the eyeball (i.e., the length from the cornea to the retina) with the full resolution of OCT (i.e., resolution less than 10 μm), a deeper measurement depth of about 40 to 45 μm is currently required. In this case, the entire depth is measured with full resolution. Another approach is to reduce the resolution, i.e., the OCT bandwidth.

[0008] Using the full bandwidth or resolution of OCT would result in extremely high data rates that would be difficult or impossible to handle or process, and the technical effort and associated costs would be correspondingly high. However, the downside of reducing the resolution is that the quality of the measurement results is compromised and the image quality is significantly reduced. Summary of the invention

[0009] In view of this, the object of the present invention is to reconcile as far as possible the two practically conflicting goals of high resolution or high image quality and lowest data rate when measuring the length of an object.

[0010] In order to achieve the above object, the present invention provides the features described in claim 1.

[0011] According to the invention, the optical path length is switched from a first value to a second value in a temporally alternating manner by at least one path length switching unit, so that substantially only two structures required for measuring the length of an object (eg a human eye) are detected.

[0012] First, it was recognized that a drawback of the aforementioned methods is that when measuring the human eye, most of the acquired data sets cover the relatively irrelevant region of the vitreous body, which generally does not provide any information relevant to measuring the length of the eyeball.

[0013] In this context, it is also recognized that the solution in patent document WO2012 / 104097A1 needs to be extended so as to actually hide the relatively irrelevant area of ​​the vitreous body. According to the present invention, the optical path length switching unit described in the above document is used to switch back and forth between imaging the cornea area and the retina area. The inherent image depth of the OCT system may be much smaller than the length of the eyeball.

[0014] The present invention achieves for the first time the measurement of the length of the eyeball with extremely high accuracy of about 10 μm, while optimizing the measurement speed, signal-to-noise ratio (SNR) and image quality, and keeping the data rate at the minimum level required for OCT with a relatively small image depth (about 10 mm).

[0015] The path length switching unit can switch the first value of the optical path length to the second value after a defined time interval, and then switch the second value back to the first value after the time interval or another time interval has passed. This can automatically change the optical path length and thus achieve reproducible measurements.

[0016] The path length switching unit can repeatedly change the value in a predetermined time period, preferably periodically and repeatedly change the value at a defined frequency. By specifying the time period as a measurement cycle and / or specifying the frequency, the signal-to-noise ratio can be variably adjusted and optimized.

[0017] In this context, the frequency range may be 1 to 1000 Hz. Within this frequency range, the data rate can be handled smoothly.

[0018] The difference between the optical path length of the first value and the second value can be variably adjusted depending on the length of the object to be examined to be measured, wherein the expected length can be input as an external parameter into the evaluation unit or control unit. This allows the device to be calibrated.

[0019] The evaluation unit can determine the object structures from the measured and processed signals, which are detected each time the first value and the second value are set, wherein the spatial distance between the structures can be determined by the evaluation unit and output as the object length. In this way, for example, the cornea and the retina can be recorded as structures of the human eye and the length of the eyeball can be determined using the distance between them.

[0020] Preferably, the path length offset between the two optical paths or optical path lengths is selected to cover all relevant eye lengths. When the device is put into operation, the offset is accurately calibrated. By rapidly alternating between the two positions and subsequently detecting the corneal and retinal positions in the OCT signal, the total length of the eyeball can be inferred if the offset is known.

[0021] An adjustable telescope can be set up so that the structure to be detected remains in focus. This improves the signal-to-noise ratio (SNR) in the area of ​​the structure. Specifically, the imaging of the two imaging paths can be adjusted by separate optics so that the focus of the corneal imaging is in the corneal area. The retinal imaging is designed so that the focus is in the retinal area. This ensures an optimal signal-to-noise ratio (SNR) for both areas. In order to further optimize the signal-to-noise ratio, an adjustable telescope can also be used to compensate for possible visual defects of the eye so that the retina is always in focus.

[0022] The evaluation unit can distinguish the signal conjugate complex plane from the signal real plane. This allows the data rate to be reduced. In order to reduce the data rate, the signal conjugate complex plane (CC) can be distinguished from the signal real plane, in particular but not limited to the retinal scanning area - similar to full-frequency OCT.

[0023] In this context, a numerical phase correction can be performed using the evaluation unit in order to determine the signal plane of the structure of the object to be examined. The numerical phase correction can be used to determine in which plane (complex or real) the retinal signal is located and thus its actual distance from the reference arm. This can be achieved by applying conjugate real and complex phase corrections to each measurement and comparing the two signals with each other. This makes it possible to break the Hermitian symmetry of FD-OCT, which can make it difficult to determine the exact optical length between two image areas.

[0024] A dispersive element, preferably an optical fiber, may be arranged in the reference arm or the sample arm. If sufficiently strong dispersion is introduced into the OCT interferometer or device, strong amplitude differences will appear in the signal after Fourier transformation, because the signal will be severely distorted if it does not come from the phase vector matching plane. If the interferometer itself hardly exhibits any dispersive properties, optical elements with dispersive properties can be inserted to fully amplify this effect. This can be achieved, for example, by using a section of dispersive optical fiber in the reference arm or the sample arm.

[0025] In a method for determining the length of an eyeball, a device of the type described herein may be used, wherein at least one path length switching unit switches the optical path length from a first value to a second value in a time-alternating manner, so that at the first value, light is transmitted or focused to the cornea region, and at the second value, light is transmitted or focused to the retina region. In this way, the length of the eyeball can be reliably determined.

[0026] In this context, a local position of the cornea is detected in the first value and a local position of the retina is detected in the second value, wherein the length of the eyeball is determined by the distance between these positions.

[0027] By evaluating several or a series of corneal and retinal position measurements, eye movements during the measurement can be recorded and the evaluation results can be used to correct for corneal and retinal detection errors. This allows compensation for eye movements to improve measurement accuracy when determining the length of the eye.

[0028] The retina or its related signals can be located in the real plane or real part of the OCT image, or in the conjugate complex plane or conjugate complex part of the OCT image, wherein the length of the eyeball is determined by analyzing the movement trajectory of the cornea and retina, or the phase analysis of the signals from the cornea and retina is performed.

[0029] Images of the cornea and retina can be displayed and / or shown in real time on a monitor. This allows personnel to analyze and evaluate the images.

[0030] The apparatus described herein can perform all the method steps described herein individually or in combination.

[0031] The device described in this article can be used to measure eye length, measure axial length, measure biometrics and fundus length. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 A schematic diagram of a device having a range length switching unit is shown;

[0033] Figure 2 A schematic diagram showing imaging of the cornea region and the retina region is shown;

[0034] Figure 3 Various lens configurations for focusing are shown. DETAILED DESCRIPTION

[0035] Figure 1 A device 1 with an interferometer is shown. The device 1 has a light source 2 and a beam splitter 3 which splits the light from the light source 2 into a sample beam on a sample arm 4 and a reference beam on a reference arm 5 .

[0036] The sample light beam reflected from the sample 6 (ie, the eyeball) is returned as the return light 4a, and the reference light beam reflected from the mirror 7 is returned as the return light 5a. The return light 4a interferes with the return light 5a.

[0037] The evaluation unit 8 evaluates the signals of the interfering light beams or lights 4a, 5a and creates a depth profile image from the signals.

[0038] In order to generate a depth profile image, the sample beam is directed to different lateral positions on the sample 6 by means of a deflection unit 9. These positions define a measurement range 6a.

[0039] If necessary, the optical unit 10 can focus the sample light beam to a certain depth in the sample 6 .

[0040] The depth profile image is recorded at a depth 11 of the sample 6. The depth 11 can be determined independently of the position or movement of the mirror 7 and the distance 12 of the device 1 to the sample 6 by means of a path length switching unit 13a and / or 13b arranged in the beam path of the sample arm 4 and / or the reference arm 5.

[0041] Figure 1 A device 1 for determining the length of an object 6 by optical coherence tomography is shown, comprising an interferometer having a light source 2, a sample arm 4 and a reference arm 5, wherein light emitted by the light source 2 can be split by a beam splitter 3, so that a first light 4a can be guided along a reciprocating direction on the sample arm 4, and a second light 5a can be guided along a reciprocating direction on the reference arm 5, wherein the first light 4a and the reflected second light 5a can interfere with each other.

[0042] The evaluation unit 8 is arranged to detect and process signals of interference between the first light 4a and the second light 5a, wherein the path length switching units 13a, 13b are arranged in the optical path of the sample arm 4 and / or in the optical path of the reference arm 5, and are used to change the optical path length of the light 4a, 5a each time it passes through the path length switching units 13a, 13b.

[0043] At least one path length switching unit 13a, 13b switches the optical path length from a first value to a second value in a time-alternating manner.

[0044] At least one path length switching unit 13a, 13b switches the first value to the second value after a defined time interval and then switches the second value back to the first value after the time interval or another time interval has elapsed.

[0045] The path length switching units 13a, 13b perform alternating value changes within a predeterminable time period, preferably periodically at a defined frequency, with the frequency range being 1-1000 Hz.

[0046] The difference between the optical path length of the first value and the second value can be variably adjusted depending on the length of the object 6 to be measured, wherein the expected length can be input as an external parameter into the evaluation unit 8 or the control unit 8a.

[0047] The evaluation unit 8 can be used to determine the object structure based on the detected and processed signals, which signals can be detected during the setting of the first value and the second value.

[0048] The spatial distance between these structures can be determined by the evaluation unit 8 and output as the length of the object 6 .

[0049] An adjustable telescope is provided as optical unit 10 in order to keep the structure to be inspected in focus.

[0050] The evaluation unit 8 distinguishes the signal conjugate complex plane from the signal real plane. A numerical phase correction can be performed by means of the evaluation unit 8 in order to determine the signal plane of the structure to be examined of the object 6 .

[0051] A dispersive element, preferably an optical fiber, may be arranged in the reference arm 5 or the sample arm 4 (but not shown in this figure).

[0052] Figure 2 A method for determining the length of an eyeball is schematically shown, in which a device 1 of the above-mentioned type is used, wherein at least one path length switching unit 13a, 13b switches the optical path length from a first value to a second value in a time-alternating manner, so that at the first value, light is transmitted or focused to the area of ​​the cornea (Cornea) 14, and at the second value, light is transmitted or focused to the area of ​​the retina (Retina) 15.

[0053] Figure 2 In FIG. 1 , the left column schematically shows a first value setting for detecting the cornea 14 region, and the right column shows a second value setting for detecting the retina 15 region. The arrows indicate the difference in optical path lengths of the values, i.e., the path length offset.

[0054] The local position of the cornea 14 is detected when the first value is used, and the local position of the retina 15 is detected when the second value is used, and the length of the eyeball is determined based on the distance between the positions.

[0055] By evaluating a large number or a series of cornea 14 and retina 15 position measurements, eye movements during the measurement can be recorded and the evaluation results can be used to correct errors in the cornea 14 and retina 15 detection.

[0056] The retina 15 or its related signals may be located in the real plane or real part of the OCT image, or in the conjugate complex plane or conjugate complex part of the OCT image, wherein the length of the eyeball is determined by analyzing the movement trajectory of the cornea 14 and the retina 15, or the phase analysis of the signals from the cornea 14 and the retina 15 is performed.

[0057] Images of the cornea and retina are displayed on the monitor 16 in real time.

[0058] Specifically, the eye length measurement is performed using the device 1 as follows:

[0059] Two optical paths are realized by at least one path length switching unit 13a, 13b.

[0060] The first value represents the first beam path, whose optical path length is the same as the optical path length of the reference arm 5 just in front of the patient's cornea 14 (“DC position”). This can be achieved by a suitable lens, so that the focus is in the area of ​​the cornea 14.

[0061] The second optical path is represented by a second value, and its optical path length is such that the DC position in the region of the retina 15 is exactly the typical length of the eyeball.

[0062] Figure 2 The OCT imaging area is marked with a box in the figure, the central dividing line (partially marked as a dotted line) corresponds to the DC position, the minus sign indicates the conjugate complex plane, and the plus sign indicates the real plane.

[0063] The upper frame shows the standard eye condition, the middle frame shows the short eye condition, and the lower frame shows the long eye condition.

[0064] The offset indicated by the double arrow is the difference in optical path lengths of the two optical paths, which is predetermined by the construction of the path length switching units 13 a , 13 b , in particular by mechanical means.

[0065] In order to minimize the influence of patient activities on the measurement results, it is necessary to quickly switch and alternately measure the positions of the cornea 14 and the retina 15 .

[0066] By means of the path length switching units 13a, 13b, this switching may be controlled in the millisecond range. Ideally, a series of alternating positions are measured. From this series of measurements the movement trajectory of the eyeball / measuring instrument can be inferred in order to correct any residual errors.

[0067] When using the apparatus 1 described herein, the retina 15 may be located in the conjugate complex part or the real part of the image.

[0068] There are two ways to complete the correlation and correctly determine the eye length.

[0069] This can be done by checking the movement trajectory between the cornea 14 and the retina 15. If the retina 15 is also in the real part of the image, the trajectories are in the same direction. If the retina 15 is in the conjugate complex part, the trajectories are in opposite directions. Alternatively, phase analysis of the signal can be performed.

[0070] To further optimize the signal output on the retina 15, a focusing telescope may be used to achieve optimal focusing on the retina 15 for different eye lengths.

[0071] To this end, a lens configuration can be used to achieve a constant pupil filling degree, so that different eye lengths can achieve the maximum numerical aperture, further optimizing the signal-to-noise ratio (SNR). Of course, in this way, shorter eyes can achieve a higher SNR.

[0072] Figure 3 An example of such a lens configuration is shown.

[0073] Reference numerals

[0074] 1 Equipment

[0075] 2 Light Source

[0076] 3 Beam splitter

[0077] 4 Sample Arm

[0078] 4a Round trip light

[0079] 5 Reference arm

[0080] 5a Round trip light

[0081] 6 samples (eyeballs)

[0082] 6a Measuring range

[0083] 7 mirrors

[0084] 8 Evaluation Units

[0085] 8a Control unit

[0086] 9 Deflection unit

[0087] 10 Optical unit

[0088] 11 Depth

[0089] 12 Distance between device 1 and sample 6

[0090] 13a, 13b Length switching unit

[0091] 14 Cornea

[0092] 15 Retina

[0093] 16 Monitor

Claims

1. A device (1) for determining the length of an object (6) by optical coherence tomography, comprising an interferometer having a light source (2), a sample arm (4) and a reference arm (5), in, The light emitted by the light source (2) can be split by a beam splitter (3) or a circulator, so that the sample arm (4) can guide the first light (4a) in a reciprocating direction and the reference arm (5) can guide the second light (5a) in a reciprocating direction. The first light (4a) and the reflected second light (5a) can interfere with each other. An evaluation unit (8) is provided for detecting and processing a signal of interference between the first light (4a) and the second light (5a). A path length switching unit (13a, 13b) is arranged in the optical path of the sample arm (4) and / or in the optical path of the reference arm (5), and the path length switching unit (13a, 13b) changes the optical path length of the light (4a, 5a) passing through the path length switching unit (13a, 13b). It is characterized in that At least one path length switching unit (13a, 13b) switches the optical path length from a first value to a second value in a time-alternating manner.

2. The device according to claim 1, characterized in that The path length switching unit (13a, 13b) switches the first value to the second value after a defined time interval, and then switches the second value back to the first value after the time interval or another time interval has passed.

3. The device according to claim 1 or 2, characterized in that The path length switching unit (13a, 13b) periodically and repeatedly changes the value alternately within a predeterminable time period, preferably at a defined frequency.

4. The device according to claim 3, characterized in that The frequency ranges from 1 to 1000 Hz.

5. The device according to any one of the preceding claims, characterized in that The difference between the optical path lengths of the first value and the second value can be variably adjusted according to the length of the object to be examined to be measured, wherein the expected length can be input as an external parameter into the evaluation unit (8) or the control unit (8a).

6. The device according to any one of the preceding claims, characterized in that The evaluation unit (8) can determine object structures from the measured and processed signals, which are detected each time the first value and the second value are set, and the spatial distance between these structures can be determined by the evaluation unit (8) and output as the length of the object (6).

7. The device according to any one of the preceding claims, characterized in that An adjustable telescope is provided to keep the structure to be inspected in focus.

8. The device according to any one of the preceding claims, characterized in that The evaluation unit (8) distinguishes a signal conjugate complex plane from a signal real plane.

9. The device according to claim 8, characterized in that The evaluation unit (8) can be used to carry out a numerical phase correction in order to determine the signal plane of the structure to be detected of the object (6).

10. The device according to any one of the preceding claims, characterized in that A dispersive element, preferably an optical fiber, is arranged in the reference arm (5) or the sample arm (4).

11. A method for determining the length of an eyeball, using a device (1) according to any one of the preceding claims, wherein: At least one path length switching unit (13a, 13b) switches the optical path length from a first value to a second value in a time-alternating manner, so that when the first value is reached, light is transmitted or focused to a cornea (14) area, and when the second value is reached, light is transmitted or focused to a retina (15) area.

12. The method according to claim 11, characterized in that The first value is the detection of a local position of the cornea (14), and the second value is the detection of a local position of the retina (15), wherein the length of the eyeball is determined by the distance between the positions.

13. The method according to claim 12, characterized in that By evaluating several or a series of position measurements of the cornea (14) and the retina (15), eye movements during the measurements are recorded and the evaluation results are used to correct for detection errors of the cornea (14) and the retina (15).

14. The method according to any one of claims 11 to 13, characterized in that The retina (15) or its related signal is located in the real plane or real part of the OCT image or in the conjugate complex plane or conjugate complex part of the OCT image, wherein the movement trajectory of the cornea (14) and the retina (15) is evaluated to determine the length of the eyeball, or wherein the signals from the cornea (14) and the retina (15) are phase analyzed.

15. The method according to any one of claims 11 to 14, characterized in that The images of the cornea (14) and the retina (15) are displayed and / or shown in real time on a monitor (16).

Images