A device for measuring the length of an object

Abstract

In order to achieve the objective of achieving as high a resolution or image quality as possible while keeping the data rate as low as possible when measuring the length of an object, which are essentially contradictory goals, a device (1) for measuring the length of an object (6) when performing optical coherence tomography comprises an interferometer having a light source (2), a sample arm (4), and a reference arm (5), and light emitted from the light source (2) can be split by a beam splitter (3), so that a first light (4a) can be directed on the sample arm (4) in a forward direction and a backward direction, and a second light (5a) can be directed on the reference arm (5) in a forward direction and a backward direction. The optical path length switching unit (13a, 13b) is arranged in the beam path of the sample arm (4) and / or the beam path of the reference arm (5), and changes the optical path length of the light (4a, 5a) passing through the optical path length switching unit (13a, 13b), and at least one of the optical path length switching units (13a, 13b) changes the optical path length from a first value to a second value alternately in time.

Description

[Technical Field]

[0001] The present invention relates to a device according to the preamble of claim 1. [Background technology]

[0002] The term "optical coherence tomography (usually abbreviated as OCT)" is understood to be an imaging method.

[0003] This method allows obtaining two-dimensional and three-dimensional images from light-scattering structures. Typically, light of a given band is split into two partial beams in a beam splitter. The first partial beam is incident on the sample or object under examination, while the second partial beam passes through a reference section.

[0004] The light reflected by the sample or object interferes with the reference beam, and the interference signal can be used to examine the sample with depth resolution, i.e. at the depth of the optical axis of the first partial beam, by means of a so-called A-scan.

[0005] Additionally, the first partial beam can be scanned across the surface of the sample or across the sample in the lateral direction to obtain an OCT image.

[0006] Against this background, WO 2012 / 104097 A1 discloses a method for recording slice images in which an optical path length switching unit is used to change the optical path length of a sample beam and / or a reference beam of an interferometer so as to generate depth slice images at different depths of the sample. The optical path length is changed by redirecting the beam paths along different geometric paths.

[0007] Currently, to measure the length of the eye, i.e., from the cornea to the retina, with maximum OCT resolution, i.e., a resolution of less than 10 μm, requires a correspondingly large measurement depth of approximately 40–45 mm, in which case the entire depth is measured with maximum resolution. Alternatively, the resolution, i.e., the OCT bandwidth, can be reduced.

[0008] Using the maximum OCT bandwidth or resolution results in very high data rates. Processing or processing such data rates is difficult or impossible. The technical effort and associated costs are correspondingly high. However, reducing the resolution has the disadvantage of reducing the quality of the measurement results, resulting in a significant degradation of image quality. Summary of the Invention [Problem to be solved by the invention]

[0009] The problem addressed by the present invention is therefore to achieve as much as possible a balance between the essentially conflicting goals of high resolution or image quality and the lowest possible data rate when measuring the length of an object. [Means for solving the problem]

[0010] The present invention solves the above problem by the features of claim 1.

[0011] According to the present invention, in order to detect substantially only two structures necessary for measuring the length of an object such as a human eye, the optical path length is changed alternately in time from a first value to a second value using at least one optical path length switching unit.

[0012] First, the aforementioned methods were recognized as having a disadvantage in that the majority of the data sets acquired during measurements of the human eye cover relatively unimportant areas of the vitreous, which generally do not provide information relevant to the measurement of ocular length.

[0013] Against this background, it was also recognized that the teaching of WO 2012 / 104097 A1 needed to be extended, so to speak, to mask out less important areas of the vitreous. According to the present invention, the optical path length switching unit described in the above document is used in order to alternate between imaging the corneal area and the retinal area, in which case the imaging depth inherent in the OCT system can be made substantially smaller than the length of the eye.

[0014] The present invention makes it possible for the first time to measure eye length with an extremely high accuracy of approximately 10 μm while simultaneously optimizing measurement speed, signal-to-noise ratio (SNR), and image quality, while keeping data rates as low as required for OCT with a relatively small imaging depth (approximately 10 mm).

[0015] The optical path length switching unit can change the optical path length from a first value to a second value after a predetermined time interval, and then change it back from the second value to the first value after the time interval or a further time interval has elapsed, thereby enabling automatic changes in the optical path length and enabling reproducible measurements.

[0016] The optical path length switching unit can repeatedly alternate between values over a preconfigurable period, preferably periodically at a predetermined frequency, which can be pre-set as a measurement period and / or pre-set as a frequency, thereby variably adjusting or optimizing the signal-to-noise ratio.

[0017] Against this background, frequencies can be in the range of 1 to 1000 Hz, which provides a data rate that can be handled without problems.

[0018] The difference between the first and second values of the optical path length can be variably adjusted based on the length of the inspected object to be measured, and the predicted length can be input as an external parameter to the evaluation unit or control unit, thereby allowing the device to be calibrated.

[0019] The structures of the object can be identified by the evaluation unit from the acquired and processed signals, and the structures can be detected when the first and second values are set, respectively, and the spatial distance between each structure can be identified and output by the evaluation unit as the length of the object, so that, for example, in the case of a human eye, the cornea and retina can be detected as structures and the distance between them can be used to identify the length of the eye.

[0020] The optical path length offset between the two optical paths or path lengths is preferably selected to cover all relevant eye lengths. When the device is activated, the offset is precisely calibrated. After rapidly switching between the two positions, by detecting the positions of the cornea and retina in the OCT signal, the total eye length can be inferred if the offset is known.

[0021] An adjustable telescope can be installed to keep the image focused on the structure being imaged. This improves the signal-to-noise ratio (SNR) in the region of the structure. Specifically, imaging in the two imaging paths can be adapted by individual optics so that the corneal region is focused in the case of corneal imaging. Retinal imaging is designed to focus on the retina region, thereby achieving the best signal-to-noise ratio (SNR) in both regions. To further optimize the SNR, an adjustable-focus telescope can be used to correct for refractive errors in the eye and keep the focus always on the retina.

[0022] The evaluation unit can distinguish the complex conjugate plane of the signal from the real plane of the signal, thereby allowing the data rate to be reduced. To reduce the data rate, in particular (but not exclusively) in the area of retinal scanning (as in full-range OCT), the complex conjugate (CC) plane of the signal can be distinguished from the real plane of the signal.

[0023] Against this background, it may be possible to perform numerical phase corrections using an evaluation unit to identify the plane in which the signal of the structure of the object being imaged is located. The numerical phase correction identifies the plane (complex or real) in which the retinal signal is located, and thus the real distance of the retinal signal from the reference arm. This can be achieved by applying a real phase correction and a complex conjugate phase correction to each measurement and comparing the two signals with each other. This breaks the Hermitian symmetry of FD-OCT, which makes it difficult to determine the intrinsic optical distance between two image regions.

[0024] A dispersive element, preferably a guided optical fiber, can be placed in the reference or sample arm. If sufficiently strong dispersion is introduced into the OCT interferometer or OCT setup, signals that do not originate from a plane coinciding with the phase vector will be strongly distorted, resulting in large amplitude differences after the signal is Fourier transformed. If the interferometer itself exhibits little dispersion, a dispersive optical element can be inserted to make the effect sufficiently large. For example, this can be done by using a piece of dispersive fiber in the reference or sample arm.

[0025] The device as described herein can be used in a method for measuring ocular length, wherein at least one optical path length switching unit alternately changes the optical path length from a first value to a second value, whereby at the first value the light is directed or focused onto a region of the cornea and at the second value the light is directed or focused onto a region of the retina, thereby enabling reliable determination of ocular length.

[0026] Against this background, the first value can be used to detect the local position of the cornea, the second value can be used to detect the local position of the retina, and the distance between these positions can be used to determine the length of the eye.

[0027] By evaluating multiple measurements or a series of measurements of the corneal and retinal positions, eye movement during measurement can be detected and the evaluation results can be used to correct for errors in corneal and retinal detection, thereby correcting for eye movement and improving measurement accuracy when measuring eye length.

[0028] The retina or its associated signals can be positioned in the real plane or real image portion of the OCT image, or in the complex conjugate plane or complex conjugate image portion of the OCT image, and the trajectories of the cornea and retina movement can be evaluated or signals from the cornea and retina can be phase analyzed to measure the eye length.

[0029] Images of the cornea and retina can be displayed and / or displayed in real time on a monitor, allowing the images to be evaluated and analyzed.

[0030] The devices described herein are capable of performing all of the method steps described herein individually or in combination.

[0031] The devices described herein can be used for ocular length measurement, axial length measurement, biometry, and fundus length measurement. [Brief explanation of the drawings]

[0032] [Figure 1] 1 shows a schematic diagram of an apparatus having an optical path length switching unit. [Figure 2] 1 shows a schematic diagram of imaging of the corneal region and imaging of the retinal region. [Figure 3] 1 illustrates various lens configurations for focusing purposes. DETAILED DESCRIPTION OF THE INVENTION

[0033] 1 shows an apparatus 1 having an interferometer. The apparatus 1 comprises a light source 2 and a beam splitter 3, which splits light from the light source 2 into a sample beam on a sample arm 4 and a reference beam on a reference arm 5.

[0034] The sample beam reflected by the sample 6 (specifically, the eye) and returned as return light 4a interferes with the reference beam reflected by the mirror 7 and returned as return light 5a.

[0035] An evaluation unit 8 evaluates the signals of the interfering beams or lights 4a, 5a and generates depth slice images therefrom.

[0036] To generate a depth slice image, the sample beam is directed by a deflection unit 9 to different lateral positions on the sample 6. These positions define a measurement area 6a.

[0037] If necessary, the optical unit 10 allows the sample beam to be focused to a depth in the sample 6 .

[0038] The depth slice images are taken at a depth 11 in the sample 6. The depth 11 can be defined independently of the position or movement of the mirror 7 and independently of the distance 12 between the device 1 and the sample 6 by optical path length switching units 13a and / or 13b located in the beam paths of the sample arm 4 and / or reference arm 5.

[0039] In this regard, Figure 1 represents an apparatus 1 for measuring the length of an object 6 when performing optical coherence tomography, comprising an interferometer having a light source 2, a sample arm 4 and a reference arm 5, wherein the light emitted by the light source 2 can be split by a beam splitter 3 so that a first light 4a can be directed in a forward and backward direction on the sample arm 4 and a second light 5a can be directed in a forward and backward direction on the reference arm 5, and the first and second backward light 4a, 5a can be caused to interfere.

[0040] An evaluation unit 8 is arranged for acquiring and processing signals of the interfering first and second light 4a, 5a, and optical path length switching units 13a, 13b are arranged in the beam path of the sample arm 4 and / or the beam path of the reference arm 5 to change the optical path lengths of the light 4a, 5a passing through the optical path length switching units 13a, 13b, respectively.

[0041] At least one of the optical path length switching units 13a and 13b changes the optical path length from a first value to a second value alternately over time.

[0042] At least one of the optical path length switching units 13a, 13b changes the first value to the second value after a predetermined time interval, and changes the second value back to the first value after the time interval or a further time interval has elapsed.

[0043] The optical path length switching units 13a and 13b alternately change the values over a pre-determinable period, preferably periodically at a predetermined frequency, which is in the range of 1 to 1000 Hz.

[0044] The difference between the first and second values of the optical path length can be variably adjusted based on the length of the inspected object 6 to be measured, and the predicted length can be input as an external parameter to the evaluation unit 8 or the control unit 8a.

[0045] Structures of the object can be identified by the evaluation unit 8 from the acquired and processed signals, said structures being detectable upon setting of the first and second values, respectively.

[0046] The spatial distance between each structure can be determined and output as the length of the object 6 by the evaluation unit 8 .

[0047] An adjustable telescope is provided as the optical unit 10 to keep the structure to be photographed in focus.

[0048] The evaluation unit 8 distinguishes the complex conjugate plane of the signal from the real plane of the signal. The evaluation unit 8 can be used to perform a respective numerical phase correction in order to identify the plane in which the signal of the structure of the object 6 being imaged is located.

[0049] A dispersive element, preferably a guiding optical fiber, may be located in the reference arm 5 or the sample arm 4, but is not shown here.

[0050] FIG. 2 shows a schematic representation of a method for measuring ocular length, in which an apparatus 1 as described above is used and at least one optical path length switching unit 13a, 13b changes the optical path length alternately in time from a first value to a second value, thereby directing or focusing light onto an area of the cornea 14 at the first value and directing or focusing light onto an area of the retina 15 at the second value.

[0051] 2, the left column schematically shows the setting of a first value for detecting the area of the cornea 14, and the right column schematically shows the setting of a second value for detecting the area of the retina 15. The arrows indicate the difference between the optical path length values, i.e., the optical path length offset.

[0052] The first value detects the local position of the cornea 14, the second value detects the local position of the retina 15, and the distance between the positions determines the length of the eye.

[0053] By evaluating multiple measurements or a series of measurements of the position of the cornea 14 and retina 15, eye movement during the measurement can be detected and the evaluation results can be used to correct errors in the detection of the cornea 14 and retina 15.

[0054] The retina 15 or a signal associated therewith can be positioned in the real plane or real image portion of the OCT image, or in the complex conjugate plane or complex conjugate image portion of the OCT image, and the movement trajectory of the cornea 14 and retina 15 can be evaluated or the signals from the cornea 14 and retina 15 can be phase analyzed to measure the length of the eye.

[0055] Images of the cornea and retina are displayed in real time on the monitor 16 .

[0056] Specifically, the eye length is measured using the device 1 as follows.

[0057] Two optical paths are realized by at least one optical path length switching unit 13a, 13b.

[0058] The first optical path, represented by a first value, has the same optical path length just in front of the patient's cornea 14 ("DC position") as the optical path length of the reference arm 5. This can be achieved with a suitable lens whose focal point is located in the region of the cornea 14.

[0059] The second optical path, represented by a second value, has an optical path length such that the DC position is in the region of the retina 15 of an eye of typical length.

[0060] In Figure 2, the OCT imaging region is shown as a square, the central dividing line shown partially in dotted lines corresponds to the DC position, the minus sign indicates the complex conjugate plane, and the plus sign indicates the real plane.

[0061] The top square represents the standard eye condition, the middle square represents the short eye condition, and the bottom square represents the long eye condition.

[0062] The offset shown by the double arrow is the difference in optical path length between the two beam paths preset by the mechanisms of the optical path length switching units 13a and 13b, particularly preset mechanically.

[0063] To minimize the effect of patient movement on the measurement results, measurements must be alternated between the corneal 14 position and the retina 15 position at high speed.

[0064] The optical path length switching units 13a, 13b allow this to be done in milliseconds. Ideally, a series of alternating positions is measured, from which the eye / measuring device movement trajectory can be inferred and residuals corrected.

[0065] During use of the device 1 described herein, the retina 15 can be positioned in the complex conjugate image portion or the real image portion.

[0066] There are two ways to assign eye length and therefore to obtain the correct measurement.

[0067] One option is to check the trajectories of the cornea 14 and retina 15, which have the same meaning if the retina 15 is also in the real image part, and the opposite meaning if the retina is in the complex conjugate image part. The other option is based on a phase analysis of the signals.

[0068] To further optimize signal yield at the retina 15, a focusing telescope can be utilized to optimally focus the retina 15 for various eye lengths.

[0069] For this purpose, a lens configuration can be realized that provides a constant pupil filling factor, and therefore always works with the maximum numerical aperture for different eye lengths in order to further optimize the SNR, so that, of course, the shorter the eye, the higher the SNR.

[0070] FIG. 3 shows an example of such a lens configuration. [Explanation of symbols]

[0071] 1 device 2 light source 3 Beam splitter 4 Sample Arm 4a Outgoing and returning light 5. Reference arm 5a Forward and return light 6 samples (eye) 6a Measuring area 7. Mirror 8 Evaluation Units 8a Control unit 9 Deflection Unit 10 Optical unit 11 Depth 12 Distance between device 1 and sample 6 13a, 13b Optical path length switching unit 14 Cornea 15 Retina 16 monitors

Claims

1. An apparatus (1) for measuring the length of an object (6) when performing 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 a beam splitter (3) or a circulator, so that a first light (4a) can be guided in a forward direction and a backward direction on the sample arm (4) and a second light (5a) can be guided in a forward direction and a backward direction on the reference arm (5); The first and second return light beams (4a, 5a) can be made to interfere with each other; an evaluation unit (8) for acquiring and processing signals of the interfering first and second light (4a, 5a), optical path length switching units (13a, 13b) are arranged in the beam path of the sample arm (4) and / or the beam path of the reference arm (5), and change the optical path lengths of the light (4a, 5a) passing through the optical path length switching units (13a, 13b), respectively; An apparatus characterized in that at least one optical path length switching unit (13a, 13b) changes the optical path length alternately in time from a first value to a second value.

2. 2. The device according to claim 1, wherein the optical path length switching unit (13a, 13b) changes the first value to the second value after a predetermined time interval, and changes the second value back to the first value after the time interval or a further time interval has elapsed.

3. 3. The device according to claim 2, characterized in that the optical path length switching units (13a, 13b) repeatedly alternate between values over a preconfigurable period, preferably periodically at a predetermined frequency.

4. 4. The device of claim 3, wherein the frequency is in the range of 1 to 1000 Hz.

5. 2. The device according to claim 1, wherein the difference between the first and second values of the optical path length is variably adjustable based on the length of the inspected object to be measured, and the predicted length can be input as an external parameter to the evaluation unit (8) or the control unit (8a).

6. 6. The device according to claim 5, wherein structures of the object can be identified by the evaluation unit (8) from the acquired and processed signals, the structures can be detected when the first value and the second value are set, respectively, and the spatial distance between each structure can be identified and output by the evaluation unit (8) as the length of the object (6).

7. 10. The apparatus of claim 1, further comprising an adjustable telescope for maintaining focus on the structure being imaged.

8. 2. The device according to claim 1, characterized in that the evaluation unit (8) distinguishes the complex conjugate plane of the signal from the real plane of the signal.

9. 9. The device according to claim 8, characterized in that the evaluation unit 8 can be used to carry out respective numerical phase corrections in order to determine the plane in which the signals of the structures of the object (6) being photographed are located.

10. 2. The device according to claim 1, characterized in that a dispersive element, preferably a guiding optical fiber, is arranged in the reference arm (5) or in the sample arm (4).

11. A method for measuring an ocular length, comprising using an apparatus (1) according to any one of claims 1 to 10, and wherein at least one optical path length switching unit (13a, 13b) changes the optical path length alternately in time from a first value to a second value, so as to direct or focus light onto a region of the cornea (14) at the first value and to direct or focus light onto a region of the retina (15) at the second value.

12. 12. The method of claim 11, wherein the first value detects a local position of the cornea (14), the second value detects a local position of the retina (15), and the distance between the respective positions determines the length of the eye.

13. 13. The method according to claim 12, characterized in that the movement of the eye during the measurement is detected by evaluating multiple measurements or a series of measurements of the position of the cornea (14) and the retina (15), and the evaluation results are used to correct errors in the detection of the cornea (14) and the retina (15).

14. 12. The method of claim 11, wherein the retina (15) or a signal associated therewith is positioned in a real plane or real image portion of an OCT image or in a complex conjugate plane or complex conjugate image portion of an OCT image, and the cornea (14) and the retina (15) are evaluated for their movement trajectory or the signals from the cornea (14) and the retina (15) are phase analyzed to measure the eye length.

15. 14. The method according to claim 13, characterized in that the images of the cornea (14) and the retina (15) are displayed and / or displayed in real time on a monitor (16).

Images