Aberrometer

Abstract

The invention relates to an aberrometer (8) for examining an eye (2) and for detecting wavefronts (7), the aberrometer comprising a device (9) having a light source for emitting light beams (1a, 1b) onto at least one scanning point (4a, 4b) of the retina (5) of the eye (2), wherein the device (9) comprises a detection unit (10) for detecting light beams (1a, 1b) reflected by the scanning point (4a, 4b) of the retina (5). The object of the invention is that of specifying an aberrometer by means of which aberrations can be detected particularly reliably and without problems. The invention is characterised in that the device (9) is designed as an OCT device for taking OCT images by means of optical coherence tomography, wherein the detection unit (10) records temporally successive scanning points (4a, 4b) and wherein a processing unit (11) is provided which is arranged in order to determine, from the optical data from the scanning points (4a, 4b), phase deviations in order to describe, by means of approximation on a model function, on the basis of the phase deviations, wavefronts (7) or wavefront deformations.

Description

[0001] Aberrometer

[0002] The invention relates to an aberrometer according to the preamble of claim 1.

[0003] Optical coherence tomography (OCT) is an imaging technique used to generate two- and three-dimensional images of light-scattering structures. In this technique, light with a specific bandwidth is typically split into two beams using a beam splitter. The first beam is directed at the sample under investigation, while the second beam passes through a reference path. The light reflected from the sample interferes with the reference beam. Signals from this interference allow for depth-resolved analysis of the sample, specifically along the optical axis of the first beam, using so-called A-scans.

[0004] The optical elements of the eye have the task of focusing incoming light onto the retina so that seen objects are sharply imaged. In reality, the refractive elements of the eye are not optimal, and objects are imaged distorted, impairing the individual visual perception. This is due to wavefront aberrations that arise when they pass through the optical elements of the eye. To improve visual perception, these aberrations must first be measured in order to correct them afterward. For this purpose, so-called aberrometers or wavefront analysis devices are used. Against this background, the invention aims to provide an aberrometer with which aberrations can be detected particularly reliably and easily.

[0005] The present invention solves the aforementioned problem by means of the features of claim 1.

[0006] First, it was recognized that an aberrometer is based on and can utilize OCT. It was then recognized that such an aberrometer is as immune to movement as possible because a phase-sensitive measurement is performed. Furthermore, it was recognized that, in a previously known technological method, as in other aberrometers, the fact that a sufficiently narrow light beam experiences only minimal aberrations from the eye's optical imaging system as it passes through the eye is exploited. The spot on the retina illuminated by the light beam therefore has the property that the path differences of all constructing light waves can be approximately neglected. This light is backscattered and detected from the entire available aperture, limited by the pupil, using a scanning OCT device.This allows an image of the approximate point spread function (PSF) of the imaging optical system to be generated, namely a stack of images from different depth layers.

[0007] Furthermore, it has been recognized that OCT technology enables these images to be complex-valued. Therefore, information can be obtained about the path length differences of the light for the individual constructed pixels. The aberrations of the eye are encoded here. Information from the pupil plane is often relevant. Therefore, wavefront propagation is common practice.

[0008] Furthermore, it has been recognized that a system can be technically constructed such that the object plane and the image plane are conjugate. Additionally, a system can be constructed such that the Fourier plane of the object plane is conjugate to the pupil plane of the eye under examination. In this case, using the so-called Fraunhofer approximation and a 2D Fourier transform, information about the wavefront can be propagated from the image plane to the pupil plane. Against this background, it has been recognized that, in a given technology, the complex image of the point spread function can be further transformed using a 2D Fourier transform. The aberrations are described here by a Zernike representation. However, for the approximation of the aberrations by Zernike synthesis in the pupil plane, the problem of phase wrapping must be solved.

[0009] Against this background, it has been specifically recognized that with currently known technologies, correct and precise aberrations can only be measured on stationary artificial eyes. The reason for this is that the eye of a living patient moves during the measurement process. The typical path length differences that the light travels on its way to the different pixels differ only by a few wavelengths – that is, by a few micrometers.

[0010] It has also been recognized that a method for capturing an image requires a dense rasterization of the image field. This necessitates more than ten thousand pixels. The eye movements during scanning are therefore typically many times greater than the structure being detected, thus destroying the information about the structure that is being measured. It has also been noted that while there are publications proposing fast swept-source OCT systems, and a 1.6 MHz system is known, it has also been specifically recognized that these systems are associated with problems.

[0011] One fundamental problem, for example, is that such a fast system has a very poor signal-to-noise ratio (SNR). For a 1.6 MHz OCT system, the SNR from the retina should only be around 20 dB, even for nearly emmetropic (normally sighted) eyes not affected by cataracts. This is because the constantly illuminated and backscattered light is sampled as an image. This results in approximately 30 dB of signal loss compared to a conventional OCT method. The phase noise then reaches several hundred milliradians.

[0012] It has also been recognized that the currently achieved signal strength is, based on experience, insufficient to measure aberrations with adequate accuracy. Furthermore, such a fast OCT system can only be used effectively at its full sampling rate if the scanning setup is also sufficiently fast. It has also been recognized that the measurement process must be completed within a few milliseconds if aberrations are not to be distorted by eye movements. If dense scanning is required simultaneously, the scanners must be capable of very rapid acceleration.

[0013] It has also been recognized that even with a very fast, state-of-the-art OCT system, data acquisition is not fast enough to accurately measure aberrations despite eye movement. Furthermore, it has been recognized that digitizing and processing OCT data is very challenging. Finally, it has been recognized that currently no OCT-based aberrometer or wavefront analyzer is known that can solve the aforementioned problems.

[0014] It has been recognized according to the invention that an aberrometer must be provided with which motion-induced artifacts can be largely eliminated, which completely destroy the information from a signal.

[0015] It was then recognized that the reconstruction of a point spread function must be independent of the random (stochastic) random walk of accumulated phases in order to be robust against noise. According to the invention, an OCT aberrometer was therefore developed with which scanning OCT systems can be supplemented as universally as possible and without major or expensive hardware modifications, particularly for realizing a wide range of light source refresh rates. With a suitable configuration, especially one using a light source switching frequency of a few tens of kHz, this advantageously results in a measurement rate of over 100 Hz.

[0016] This is achieved by an aberrometer for examining an eye and capturing wavefronts, which includes a device with a light source for emitting light rays onto at least one scanning point on the retina of the eye, wherein the device has a detection unit for capturing the light rays reflected from the scanning point on the retina. The device is designed as an OCT device for acquiring OCT images using optical coherence tomography, wherein the detection unit acquires scanning points sequentially and wherein a processing unit is provided which is configured to determine phase derivatives from the optical data of the scanning points in order to describe wavefronts or wavefront deformations based on the phase derivatives by approximating them to a model function. Thus, an approximation of phase derivatives is performed.

[0017] The processing unit could be configured to determine second phase derivatives from the optical data of the sampling points, in order to describe wavefronts or wavefront deformations using an approximation to a model function based on these second phase derivatives. In principle, it is also possible to use the second phase derivative for the approximation itself. In this way, the dynamic range of the measurements can be extended by evaluating the second derivatives / curvatures of the wavefront.

[0018] The processing unit could determine at least one complex phase derivative and / or at least one path length difference of the respective light rays or the respective light source to the respective sampling point, based on at least two sampling points acquired sequentially in time and space. No image of the point spread function (PSF) is generated for reconstruction. Instead, the complex phase derivative is determined along the sequentially acquired sampling points. The determined phase derivatives are then used to determine the coefficients of a suitable basis by means of a fit approach.

[0019] Against this background, the processing unit could determine the phase derivatives of at least two consecutive A-scans in order to describe wavefronts or wavefront deformations based on these phase derivatives by approximating them to a model function. Crucially, in a fit approach, the phase derivatives are only required from at least two temporally and spatially adjacent sampling points. Within this short timeframe, namely the time interval between two A-scans, this phase information is not destroyed by eye movements. In contrast, in a reconstruction approach, the phase is reconstructed not only from temporal neighbors but also from spatial neighbors, which are not necessarily temporal neighbors. A multiple of the aforementioned time elapses between the acquisition of these neighbors. During this time, a signal is destroyed by movement.

[0020] The processing unit could use Zernike polynomials as a basis to determine their coefficients and thus describe a wavefront or wavefront deformation. Zernike polynomials are advantageously used as a basis because even with a very small basis (<10 elements) they can describe ocular wavefronts with sufficient accuracy to determine the necessary aberrations.

[0021] An adjustable optical unit could be provided to compensate for the spherical equivalent of the eye. Essential for the successful implementation of the approach described here is an adjustable optical system capable of compensating for the spherical equivalent of the eye under examination.

[0022] No additional wavefront sensors may be required. Advantageously, ocular wavefronts can be measured at video rate without the need for conventional wavefront sensors, which require additional hardware beyond an OCT setup. Existing OCT setups can be easily and cost-effectively upgraded using the new technology described here.

[0023] The processing unit could not generate an image of a point spread function. Advantageously, no image of a point spread function is generated for reconstruction. A major advantage of the approach described here is that only a fraction of the necessary sampling points are required to reconstruct the point spread function (PSF). The fit approach does not require dense scanning of the image field to generate an image. For the fit approach described here, no dense sampling is necessary.

[0024] The aberrometer could have an A-scan measurement rate in the range of 1 kHz to 10,000 kHz. The acquired scans are advantageously used not only to measure an underlying wavefront, but also to determine the positions on the retina where the necessary information about the backscattered light can be found. For each complete scan of the retina, a new scanning position is and must be calculated to accurately and reproducibly determine the aberrations.

[0025] Compared to commercially available aberrometers and the underlying technologies, the invention described here has the advantage that OCT devices can be extended to measure wavefront aberrations of the eye. These extensions are technically simple, compact, and inexpensive. Due to the high repetition rate of the measurements, it may be possible to observe dynamic processes that change aberrations in the eye, for example, during accommodation. It would also be conceivable to observe the dynamics of the tear film. Furthermore, it is possible to accurately determine the astigmatism of in-vivo eyes using the technology proposed here.

[0026] The processing unit could detect aberrations, especially higher-order ones. The processing unit can computationally determine any form of aberration. It is also conceivable to evaluate higher-order aberrations.

[0027] The drawing shows

[0028] Fig. 1 schematically shows two wavefront analyses based on two cross-sectional images of human eyes, performed with classical wavefront sensors, and

[0029] Fig. 2 schematically shows an aberrometer for wavefront analysis, which includes an OCT device.

[0030] Fig. 1 shows in the top view that, according to the prior art, when detecting aberrations, light rays 1 or light waves are sent into the eye 2', refracted at the lens 3', and focused onto a scanning point 4' on the retina 5'. From there, the light rays 1 or light waves are reflected and detected at various detection points 6' using classical wavefront sensors (not shown). Although depicted differently, an incident light ray into the eye 2' is very thin and does not fill the pupil. Fig. 1 shows in the top view a wavefront 7' of the reflected light. Based on the detected wavefront 7', a so-called wave map 7'a can be generated, which in the top view of Fig. 1 is ideally a flat sheet because, ideally, no aberrations are present. The ideal lens 13' focuses every parallel incident light ray 1 within the pupil 13' onto the same focal point, namely the scanning point 4'.

[0031] The lower view of Fig. 1 shows that the wavefront 7, and thus the wave map 7a, is curved and wavy because the lens 3 would focus parallel incident light rays 1 at different locations within the pupil 13 onto different focal points. A wave map 7 ultimately represents the wavefront deformation as a function of the location within the pupil 13 of the human eye 2.

[0032] Fig. 2 shows an aberrometer 8 according to the invention for examining an eye 2 and for detecting wavefronts 7, comprising a device 9 with a light source for emitting light rays 1a, 1b onto at least one scanning point 4a, 4b of the retina 5 of the eye 2, wherein the device 9 has a detection unit 10 for detecting the light rays reflected from the scanning point 4a, 4b of the retina 5. The device 9 is designed as an OCT device for acquiring OCT images using optical coherence tomography.

[0033] The detection unit 10 records sampling points 4a, 4b successively and a processing unit 11 is provided which is designed to determine phase derivatives from the optical data of the sampling points 4a, 4b, ....4n in order to describe wavefronts 7 or wavefront deformations on the basis of the phase derivatives by means of an approximation to a model function.

[0034] The processing unit 11 determines, based on at least two successively recorded sampling points 4a, 4b, at least one complex phase derivative and / or at least one path length difference of the respective light rays 1a, 1b or of the respective light for detecting the respective sampling point 4a, 4b. Specifically, the processing unit 11 determines the phase derivatives of at least two consecutive A-scans in order to describe wavefronts 7 or wavefront deformations based on the phase derivatives by approximating them to a model function.

[0035] Processing unit 11 uses Zernike polynomials as an example basis to determine their coefficients and thus describe a wavefront 7 or wavefront deformation.

[0036] An adjustable optical unit 12 is provided to compensate for the spherical equivalent of the eye 2. No further conventional wavefront sensors, as described in Fig. 1, are provided.

[0037] Processing unit 11 does not generate an image of a point spread function.

[0038] The aberrometer described in Fig. 2 has an A-scan measurement rate in the range of 1 kHz to 10000 kHz.

[0039] Processing unit 11 detects aberrations, especially higher-order aberrations.

[0040] The processing unit 11 can display a wave map 7a, which represents the wavefront deformation as a function of the location within the pupil 13 of the human eye 2.

[0041] Reference symbol list:

[0042] 1, 1a, 1b Light beam

[0043] 2, 2' eye

[0044] 3.3' lens

[0045] 4, 4a, 4b, 4n sampling point

[0046] 5.5' Retina

[0047] 6, 6' Detection point

[0048] 7.7' wavefront

[0049] 7a, 7'a wave map

[0050] 8 Aberrometer

[0051] 9 OCT devices out of 8

[0052] 10 detection units of 8

[0053] 11 processing units out of 8

[0054] 12 optical units

[0055] 13, 13' Pupil

[0056] 14, 14' cornea

Claims

Patent claims 1. Aberrometer (8) for examining an eye (2) and for detecting wavefronts (7), comprising a device (9) with a light source for emitting light rays (1a, 1b) onto at least one scanning point (4a, 4b) of the retina (5) of the eye (2), wherein the device (9) has a detection unit (10) for detecting the light rays (1a, 1b) reflected from the scanning point (4a, 4b) of the retina (5), characterized in that the device (9) is configured as an OCT device for acquiring OCT images using optical coherence tomography, wherein the detection unit (10) sequentially acquires scanning points (4a, 4b) and wherein a processing unit (11) is provided which is configured to determine phase derivatives from the optical data of the scanning points (4a, 4b) in order to approximate a model function based on the phase derivatives to describe wavefronts (7) or wavefront deformations.

2. Aberrometer according to claim 1, characterized in that the processing unit (11) is configured to determine second phase derivatives from the optical data of the sampling points (4a, 4b) in order to describe wavefronts (7) or wavefront deformations on the basis of the second phase derivatives by means of an approximation to a model function.

3. Aberrometer according to claim 1 or 2, characterized in that the processing unit (11 ) determines at least one complex phase derivative and / or at least one path length difference of the respective light rays (1a, 1b) or of the respective light for detecting the respective sampling point (4a, 4b) on the basis of at least two sampling points (4a, 4b) recorded successively in time and space.

4. Aberrometer according to one of the preceding claims, characterized in that the processing unit (11) determines the phase derivatives of at least two successive A-scans in order to use the Phase derivatives can be described by approximating wavefronts (7) or wavefront deformations using a model function.

5. Aberrometer according to one of the preceding claims, characterized in that the processing unit (11) uses Zernike polynomials as a basis to determine their coefficients and thus describe a wavefront (7) or wavefront deformation.

6. Aberrometer according to one of the preceding claims, characterized in that an adjustable optical unit (12) is provided which compensates for the spherical equivalent of the eye (2).

7. Aberrometer according to one of the preceding claims, characterized in that no further wavefront sensors are provided.

8. Aberrometer according to one of the preceding claims, characterized in that the processing unit (11) does not generate an image of a point spread function.

9. Aberrometer according to one of the preceding claims, characterized by an A-scan measurement rate in the range of 1 kHz to 10000 kHz.

10. Aberrometer according to one of the preceding claims, characterized in that the processing unit (11) determines aberrations, in particular higher order.

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