RU2425621C2 - Device for elimination of speckle-modulation effect in measurement of eye aberrations by laser aberrometre and laser aberrometre - Google Patents

Abstract

FIELD: medicine.

SUBSTANCE: group of inventions relates to medical equipment, namely, laser aberrometres and devices for elimination of speckle-modulation effect in measurement of eye aberrations by laser aberrometres. Device contains transparent plate and drive. Plate is located between point source and human eye. Drive ensures plate movement in direction, transverse to direction of point source direction. Plate surface is made in form of random two-dimensional profile, whose characteristic size is smaller than laser beam diameter. Laser aberrometre contains said device, system of changing shape of wave front of irradiation outgoing from eye in form of wave front sensor, and system of laser aberrometre control. Control system contains computer for processing data obtained from wave front sensor, restoration of map of eye aberrations, data storage and laser aberrometre control.

EFFECT: application of group of inventions will make it possible to increase accuracy of measuring eye aberrations due to temporary averaging of speckle-modulation for time of matrix photodetector integration.

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Description

The invention relates to medical equipment and relates to improving the design of ophthalmic devices for measuring aberrations of the human eye - aberrometers used in clinical medical practice. Presented as an invention, the device is intended to increase the accuracy of measurements with a laser aberrometer and is a significant part of it.

To determine visual acuity in clinical practice, tables of symbols and paintings are used, the size of which (for a given distance is usually 5 m) corresponds to different angular sizes on the retina. An angular size of 1 minute corresponds to a vision unit (20/20 in the English literature). By interviewing the subject, the smallest size of distinguishable characters is established, which determines visual acuity (for example, if the size of distinguishable characters is 5 minutes, then visual acuity is 0.5). The selection of the best spherical cylindrical correction is carried out using test lenses or automated sets of such lenses (phoropters), while the correction results are controlled by the symbol tables. This process is rather laborious and time-consuming, especially in the presence of complex astigmatism (EI Kovalevsky. Ophthalmology. M: Medicine, 1995, p. 45-83). To accelerate the selection of correction, various methods of measuring refraction are used, for example, skioscopy, or automated refractometers. In this case, the initial parameters of corrective lenses are selected based on the readings of these devices. More advanced instruments for measuring the optical characteristics of the eye are aberrometers, which can measure not only refraction and astigmatism, but also aberrations of higher orders.

A device is known - a laser aberrometer, the design and principle of operation of which is described in the article "Objective measurement of wave aberrations of the human eye with use of a Hartmann-Shack wave-front sensor" Junzhong Liang, Bernhard Grimm, Stefan Goelz, Josef F. Bille ( JOSA A, Volume 11, Issue 7, 1949-July, 1994). This human eye aberration measuring device contains a point light source for illuminating the eye, which is projected onto the retina of the eye and creates a virtual reference source on it, the radiation of which is scattered by the retina, passes through the optical systems of the eye, while acquiring phase modulation, corresponding to the total optical aberrations of the eye, a system for measuring the shape of the wavefront of radiation emitted from the eye in the form of a Shack-Hartmann sensor, the output signal from which is fed to a device control system consisting of a computer that provides data processing, restoring the aberration map, data storage and device control according to operator’s commands.

The reasons that impede the achievement in the known device of the technical result indicated below, which consists in increasing the accuracy of aberration measurements (standard deviation of the measured wavefront from the true value), include the use of a point (in particular, laser) radiation source in it. The laser has a high spatial and temporal coherence of radiation. This leads to the appearance of speckle modulation of radiation scattered by the retina. The speckle field itself has both amplitude and phase modulation. When a speckle field passes through the optical system of the eye, it acquires additional phase modulation caused by the presence of aberrations in the optical system of the eye. A wavefront sensor, for example, of the Shack-Hartmann type, measures the profile of the resulting wavefront, i.e. total phase modulation. To measure the actual aberrations of the studied eye, it is necessary to somehow distinguish the phase modulation arising from aberrations from the phase modulation of the speckle field. In the known device, such a selection is carried out by averaging over time many statistically independent realizations of the speckle field. The change in implementation is due to reflex micromotion of the eye. As a result, the measurement time is quite large (1-2 seconds). Since in clinical practice, measurements must be taken during the time during which the patient’s eye remains stationary (1-10 ms), this method can only be of limited use.

There are various methods to suppress the influence of speckle modulation when measuring eye aberrations. In particular, A.S. Goncharov, A.V. Larichev, Speckle Structure of a Light Field Scattered by Human Eye Retina, Laser Physics, 2007, V.I 7, N9, p.1157-1165 investigated methods based on spectral and time averaging. In particular, it has been shown that time averaging has a significant advantage.

So, for example, a technical solution is known (USA patent No. 6827444), which proposes to use a scanning mirror located in the conjugate plane of the pupil of the optical system in the path of laser radiation to quickly move the source image over a small area of the surface of the retina. This device allows you to get fast-changing speckle pattern implementations. A similar principle was used in another device (Russian patent RU 2268637), where an optical wedge is used for fast scanning. A common drawback of such technical solutions is the need to place the scanning device in the conjugate plane of the pupil of the eye, in addition, for effective statistical averaging at a short exposure time, a significant scanning frequency is required, which complicates the design of this node.

A well-known device is a laser aberrometer, the design and principle of operation of which are described in the article "Objective measurement of wave aberrations of the human eye with use of a Hartmann-Shack wave-front sensor" Junzhong Liang, Bernhard Grimm, Stefan Goelz, Josef F. Bille ( JOSA A, Volume 11, Issue 7, 1949-July, 1994.) This device for measuring aberrations of the human eye, containing a point source of light to illuminate the eye, which is projected onto the retina of the eye and creates a virtual reference source on it, the radiation of which is scattered by the retina, passes through the optical systems of the eye, acquiring phase modulation, with corresponding to the total optical aberrations of the eye, a system for measuring the shape of the wavefront of radiation emitted from the eye in the form of a Shack-Hartmann sensor, the output signal from which is fed to a device control system consisting of a computer that provides data processing, restoring the aberration map, data storage and device control according to operator’s commands.

Due to the use of a laser point source of radiation in this device, it has the drawback noted above - low measurement accuracy.

A device is known for generating a laser beam without speckle structure, used in ophthalmic diagnostics US 20030174755 (A1), containing a point light source, which can be used as a laser, a topographic phase plate and a drive that moves the plate in the direction transverse to the direction of the beam of the point source .

The problem to which the claimed invention is directed, is to create an accurate ophthalmic device with wide functionality that allows you to automatically measure aberrations of the human eye.

Device for eliminating the influence of speckle modulation when measuring eye aberrations with a laser aberrometer, containing a point source of light, a transparent plate located between the point source and the human eye, a drive that moves the plate in the direction transverse to the direction of the beam of the point source, the surface of the transparent plate is made in the form random two-dimensional profile, the characteristic size of a random two-dimensional profile is less than the diameter of the laser beam. A two-dimensional profile is a random distribution of surface heights. The characteristic size of a random two-dimensional profile (correlation radius) is smaller than the beam diameter. When passing through a plate with a random profile on the surface of the plate, the laser radiation acquires phase modulation due to the different thickness of the plate. The device uses direct phase modulation due to the plate thickness difference. This allows you to simplify the design of the device. When passing through a plate with a random profile on the surface of the plate, the radiation divergence increases slightly. As a result, the size of the focal spot (virtual source) on the retina of the eye increases slightly. This gives a positive effect, since the light intensity on the retina decreases, so the sensibility of the probe radiation for the subject becomes less. Such a system is safer for the patient. An increase in the size of the focal spot on the retina also leads to a decrease in the characteristic size of speckles in scattered radiation. When the plate moves in the transverse direction, various sections of a random two-dimensional profile pass through the radiation beam. This leads to a change in the phase modulation of the transmitted radiation and, as a consequence, to a change in the implementation of the scattered speckle field. All this reduces the influence of speckle modulation in the radiation measured by the laser aberrometer, which makes it possible to increase the accuracy of measurements and reduce the exposure time during measurements.

Laser aberrometer containing a device for eliminating the influence of speckle modulation when measuring eye aberrations, containing a point source of light, a transparent plate located between a point source and the human eye, a drive that moves the plate in the direction transverse to the direction of the beam of the point source, a wave shape measuring system the front of the radiation that has left the eye in the form of a wavefront sensor, a laser aberrometer control system containing a computer for data processing, receiving Mykh from sensor wavefront aberrations of the eye card recovery, storage and management of the laser aberrometer transparent plate surface is in the form of a random two-dimensional profile, the characteristic dimension of a random two-dimensional profile of less than the diameter of the laser beam. When passing through the surface of a plate with a random profile deposited, the laser radiation acquires phase modulation. In this case, the divergence of radiation increases slightly. As a result, the size of the focal spot (virtual source) on the retina of the eye increases slightly. This gives a positive effect, since the light intensity on the retina decreases, the sensibility of the probe radiation for the subject becomes less, this makes the system safer for the patient. An increase in the size of the focal spot on the retina leads to a decrease in the characteristic size of speckles in scattered radiation. When the plate moves in the transverse direction, various sections of a random two-dimensional profile pass through the radiation beam. This leads to a change in the phase modulation of the transmitted radiation and, as a consequence, to a change in the implementation of the scattered speckle field. All this reduces the influence of speckle modulation in the radiation measured by the laser aberrometer, which makes it possible to increase the accuracy of measurements.

The wavefront shape measuring system comprises a camera, the drive moves the transparent plate at a speed such that during the exposure of the camera of the wavefront shape measuring system, many speckle field realizations change, sufficient for statistical averaging. With this design, the phase modulation associated with the speckle field will be suppressed due to statistical averaging. This will reduce the exposure time and simplify the design of the laser aberrometer.

The technical result of the proposed technical solution is the creation of a device that allows you to measure eye aberrations with high accuracy, which allows you to reduce the exposure time during the measurements, which has a relatively simple design, safe for the patient.

Figure 1 shows the principle of operation of the device to eliminate the influence of speckle modulation.

Figure 2 presents the structural diagram of the claimed device.

Figure 3 presents a schematic optical diagram of the device.

During operation of the device, the plate moves in the transverse (relative to the beam) direction. When passing through the surface of a plate with a random profile (Figure 1), the laser radiation acquires phase modulation. In this case, the divergence of radiation increases slightly. As a result, the size of the focal spot (virtual source) on the retina of the eye increases slightly. This gives a positive effect, since the light intensity on the retina decreases, so the sensibility of the probe radiation for the subject becomes less. An increase in the size of the focal spot on the retina also leads to a decrease in the characteristic size of speckles in scattered radiation. When the plate moves in the transverse direction, various sections of a random two-dimensional profile pass through the radiation beam. This leads to a change in the phase modulation of the transmitted radiation and, as a consequence, to a change in the implementation of the scattered speckle field. If the speed of the plate is such that during the exposure of the radiation wavefront measurement system there is a change in the set of speckle field realizations, then the phase modulation associated with the speckle field will be suppressed due to statistical averaging.

The following is information confirming the possibility of carrying out the invention with the implementation of this purpose.

A device for eliminating the influence of speckle modulation when measuring eye aberrations with a laser aberrometer contains the following structural blocks (figure 2):

1) point source of light;

2) a transparent plate with a random two-dimensional profile deposited on its surface and a drive ensuring its movement in the transverse direction;

3) a measurement system in the form of a wavefront sensor (Shack-Hartmann type);

4) refraction compensation system;

5) control system (computer and (or) microprocessor controller).

The inventive ophthalmic device for measuring aberrations of the human eye is built on the basis of the Shack-Hartmann wavefront sensor, which has proven itself in clinical practice. Naturally, other devices can also be used as a wavefront sensor: for example, a curvature sensor: Paul M. Blanchard, David J. Fisher, Simon C. Woods, Alan H. Greenaway “Phase-Diversity Wave-Front Sensing with a Distorted Diffraction Grating (Applied Optics, Vol. 39 Issue 35 P.6649, 2000).

The design and principle of operation of the device is as follows (see figure 3).

The radiation of a point light source 1 (for example, a semiconductor laser superluminescent diode) with a wavelength of 780 nm - 850 nm) passes through a transparent plate with a random two-dimensional profile 2 deposited on its surface and mounted on the actuator 3, which ensures its transverse movement, hits polarization dividing cube 4. When the plate is moved in the transverse direction, the source phase is randomly modulated. The nature and scale of this modulation depends on the statistical properties of the two-dimensional profile deposited on the plate. For best results, the average size of the inhomogeneities on the wafer surface should be less than the size of the radiation beam incident on it. The smallest size of the inhomogeneities is limited by the permissible divergence of the transmitted radiation. In a typical case, the divergence should not exceed 3-5 degrees. The laser radiation is polarized in such a way that it is completely transmitted by the dividing face of the cube (in figure 2 in the left direction). Then the radiation enters the telescopic system with the necessary increase of 5. After reflection from the rotary mirror 6, the radiation passes through the refraction compensator 7. The refraction compensator 7 (a telescope consisting of lenses 7a, 7d, with a movable prism 7b and a spectral beam splitter 7c) allows you to control the focus of the laser beam . Leaving the device, laser radiation enters the patient’s examined eye, focuses on the retina with the optical elements of the eye and creates a virtual reference source on it, the radiation of which is partially scattered on the retina and passes through the optical media of the eye in the opposite direction, acquiring phase modulation. The phase modulation of the beam emerging from the eye carries information about the total aberrations that characterize the optical system of the eye. This radiation passes through the already mentioned optical elements of the aberrometer in the opposite direction. However, since the radiation scattered by the retina is practically non-polarized, when passing through the polarizing beam splitter 2, one of the polarizing components is reflected from the dividing face and enters the telescope 8, which is necessary for pairing the entrance pupil with the plane of the lens raster 9 of the wavefront sensor 10.

The lens raster forms a picture in the form of a system of focal spots on a matrix of a standard CCD (charge-coupled device) or CMOS (complementary metal-oxide semiconductor) sensor camera (creates a set of images of a virtual source). The output signal from the matrix is transmitted to a computer, which restores the aberration map and generates control signals for the refraction compensator. Moreover, the measurement speed is determined by the speed of reading data from the camera to the computer and can reach 100 frames per second. The speed of movement of the plate 2 should correspond to the frame rate of the wavefront sensor. So, for example, if it is specified to reduce the influence of speckle modulation by 10 times, then 100 realizations of the speckle field should change during the exposure time of the camera.

The shift of the spots in the picture is proportional to the local slopes of the wavefront within the corresponding subaperture of the lens raster. By measuring these displacements, one can reconstruct the wavefront shape using the least squares method. Spot coordinates can be determined using the center of mass algorithm: [J. Lang, B. Grimm, S. Goels, J. Bill, "Objective measurements of the wave aberrations of the human eye using a Hartman-Shack wavefront sensor", J.Opt .Soc. Am. A, 11 1949-1957 (1994)]. The parameters of the lens raster are selected in such a way that when processing the picture of the Shack-Hartmann sensor, it is possible to restore the wavefront with an accuracy of 1/8 of the wavelength of the probe radiation. In this case, the wavefront shape can be represented by 36 Zernike polynomials.

The refraction compensator is necessary for correcting the spherical component of the distortions of the wavefront of optical radiation with a given amplitude.

The controller 11 and the computer 12 are necessary to control the device devices and process the received information.

Thus, the above information shows that when using the claimed invention, the following set of conditions:

- a tool embodying the claimed invention is intended for use in the medical industry, namely to perform automatic measurement of aberrations of the human eye;

- for the claimed invention in the form described in the independent claims of the claims, the possibility of its implementation using the means and methods described above or known prior to the priority date is confirmed.

Therefore, the claimed invention meets the condition of "industrial applicability".

Claims (3)

1. Device for eliminating the influence of speckle modulation when measuring eye aberrations with a laser aberrometer, containing a point source of light, a transparent plate located between the point source and the human eye, a drive that moves the plate in the direction transverse to the direction of the beam of the point source, characterized in that the surface of the transparent plate is made in the form of a random two-dimensional profile, the characteristic size of a random two-dimensional profile is less than the diameter of the laser beam. 2. Laser aberrometer containing a device for eliminating the influence of speckle modulation when measuring eye aberrations,
containing a point source of light,
a transparent plate located between a point source and the human eye,
a drive providing movement of the plate in a direction transverse to the direction of the beam of a point source,
a system for measuring the wavefront shape of radiation emerging from the eye in the form of a wavefront sensor,
a laser aberrometer control system comprising a computer for processing data received from a wavefront sensor, reconstructing an eye aberration map, storing data and controlling a laser aberrometer,
characterized in that the surface of the transparent plate is made in the form of a random two-dimensional profile, the characteristic size of the random two-dimensional profile is less than the diameter of the laser beam. 3. The laser aberrometer according to claim 2, characterized in that the wavefront shape measuring system comprises a camera, the drive moves the transparent plate at a speed such that, during the exposure time of the camera of the wavefront shape measuring system, a plurality of speckle field implementations change, sufficient for statistical averaging.

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