JP7480552B2 - Ophthalmic device and axial length calculation program - Google Patents

Description

The present disclosure relates to an ophthalmic device and an axial length calculation program for obtaining the axial length of a subject's eye.

Ultrasonic and optical interference axial length measuring devices are used for intraocular lens prescription applications.

Ultrasonic axial length measuring devices are contact type, and measurements are taken by touching the probe to the cornea.

In optical interference type axial length measurement devices, two methods are currently in use: the TD method (time domain method) and the SS method (swept source method). The TD method uses a coherent light source, while the SS method uses a wavelength swept light source (see Patent Documents 1 and 2).

JP 2012-224621 A JP 2019-063044 A

In recent years, the prevalence of myopia, particularly among young people, has increased significantly in all countries. As myopia progresses with an increase in axial length, the risk of blindness increases, making it a major social issue. In this context, evaluation of myopia progression based on axial length has attracted attention. In order to conduct appropriate examinations for young people, it is desirable for axial length measurement devices to be widespread not only in cataract treatment facilities, but also in more facilities (hospitals, schools, etc.).

However, because ultrasonic axial length measurement devices are contact-type, the examiners who can use them are limited, and they place a heavy burden on the examinee. In addition, optical interference type axial length measurement devices are expensive, including the light source. Therefore, the cost of the devices can be an obstacle to their widespread use in facilities.

In response to this, the inventors of the present application have developed a new method for measuring axial length and an apparatus configuration for implementing the new method.

This disclosure has been made in consideration of the above circumstances, and has as its technical objective the provision of an ophthalmic device and an axial length calculation program that have a novel method for measuring axial length and a novel device configuration.

The ophthalmic device according to the first aspect of the present disclosure comprises a first optical system that projects measurement light onto the fundus of the test eye and acquires the ocular refractive power of the test eye based on fundus reflected light of the measurement light; a cross-sectional imaging optical system that captures a cross-sectional image of the anterior segment at a cut surface to acquire anterior segment information, which is information about the shape of the anterior segment , the anterior segment information relating to a cut surface on which the optical axis of the first optical system is located; and an axial length acquisition means that acquires the axial length of the test eye based on the ocular refractive power on the cut surface related to the cross-sectional image and the anterior segment information relating to the cut surface based on the cross-sectional image .

The axial length calculation program according to the second aspect of the present disclosure is executed by a processor of an ophthalmic computer to cause an ophthalmic device to execute a first acquisition step of acquiring ocular refractive power of the test eye measured based on fundus reflected light of measurement light projected onto the fundus of the test eye by a first optical system , a second acquisition step of acquiring anterior segment information indicating the shape of the anterior segment based on a cross-sectional image of the anterior segment at a cut surface on which the optical axis of the first optical system is located, the anterior segment information relating to the cut surface , and an axial length acquisition step of acquiring the axial length of the test eye based on the ocular refractive power on the cut surface relating to the cross-sectional image and the anterior segment information relating to the cut surface based on the cross-sectional image.

1 is an external view showing a schematic configuration of an ophthalmologic apparatus according to an embodiment; FIG. 2 is a schematic diagram illustrating the configuration of an optical system of an ophthalmic apparatus. 1 is a diagram showing a cross-sectional image of an anterior eye segment captured by a cross-sectional photographing optical system; FIG. 2 is a side view of the measurement unit, illustrating the positional relationship between the target projector and the optical axis of the cross-section photographing optical system. FIG. 4 is a perspective view of a measurement unit for explaining the positional relationship between a target projector and the optical axis of a cross-section photographing optical system. 4 is a flowchart for explaining the operation of the device. FIG. 11 is a schematic diagram for explaining a method for deriving the axial length by ray tracing. 1 is a diagram showing the refractive power in each meridian direction when SPH=-5D, CYL=-2D, and AXIS=30°.

"overview"
The following describes embodiments of the present disclosure. Items grouped in <> below may be used independently or in association with each other. For example, in a certain embodiment, a plurality of items may be appropriately combined. Also, for example, an item described with respect to a certain embodiment may be applied to other embodiments.

"First embodiment"
First, an ophthalmic apparatus and an axial length calculation program according to the first embodiment will be described. In the first embodiment, the ophthalmic apparatus and the axial length calculation program obtain the axial length of the subject's eye based on the ocular refractive power of the subject's eye obtained via a first optical system and anterior segment information obtained via a second optical system.

<Device Configuration>
The ophthalmic apparatus according to the first embodiment includes at least a first optical system, a second optical system, and an arithmetic and control unit. The arithmetic and control unit corresponds to the axial length acquisition unit and the control unit in the embodiment. The ophthalmic apparatus may additionally include a fixation target presenting optical system.

The axial length calculation program is executed by the calculation control unit. For convenience, unless otherwise specified, in the following description of the embodiment, it is assumed that the axial length calculation program is executed in an ophthalmic device (an example of an ophthalmic computer).

<First Optical System>
The first optical system is used to obtain the ocular refractive power of the subject's eye. The first optical system projects measurement light onto the fundus of the subject's eye. The ocular refractive power is obtained based on fundus reflection light of the measurement light. The first optical system may be, for example, a measurement optical system of an objective ocular refractive power measurement device such as an autoreflex or a wavefront sensor.

In this embodiment, the measurement light from the first optical system is infrared light. However, this is not necessarily limited to this and may be visible light.

<Second Optical System>
The second optical system is used to obtain anterior segment information, which is information regarding the shape of the anterior segment.

In the first embodiment, anterior eye information regarding the cut surface may be acquired via the second optical system. In this case, the optical axis of the first optical system is positioned on the plane of the cut surface.

The second optical system may be a cross-sectional imaging optical system such as a Scheimpflug optical system, or may be another optical system. When a Scheimpflug optical system is used as the second optical system, it is necessary that the projection optical axis of the illumination light in the second optical system and the projection optical axis of the measurement light in the first optical system are arranged coaxially.

<Example of anterior eye information>
The anterior eye information may include shape information of the optic body in the anterior eye. The anterior eye information may be an image of the anterior eye. The shape of the optic body in the anterior eye can be specified from the anterior eye information. For example, it is desirable that two or more of the corneal thickness, the anterior corneal radius of curvature, the posterior corneal radius of curvature, the anterior chamber depth, the crystalline lens thickness, the anterior crystalline lens radius of curvature, and the posterior crystalline lens radius of curvature can be specified based on the anterior eye information. Of course, these values that specify the shape of the optic body may themselves be the anterior eye information.

The anterior eye information acquired through the second optical system may include at least shape information of the crystalline lens. Furthermore, the shape of the measurement area measured by the first optical system may be identifiable from the anterior eye information acquired through the second optical system. The measurement area is the area that is the subject of measurement of the ocular refractive power by the first optical system.

<Fixation target presentation optical system>
The ophthalmologic apparatus may include a fixation target presenting optical system. The fixation target presenting optical system is an optical system that presents a fixation target to the subject's eye, and may be used when the first optical system and the second optical system each operate.

The fixation target presenting optical system in this embodiment may be capable of changing the fixation target presentation distance. Such a fixation target presenting optical system may be used to fogging the test eye when measuring the refractive power with the first optical system. The first optical system may also be used to add accommodation within the eye.

<Method of deriving axial length (axial length calculation program)>
In the first embodiment, the calculation and control unit obtains the axial length of the subject eye based on the eye refractive power and the anterior segment information.

The eye refractive power and anterior eye information may be obtained based on measurements or photographs. Alternatively, the information may be obtained by storing measurements or photographs taken by a separate device in a memory.

At this time, the axial length may be obtained based on the on-plane ocular refractive power, which is the ocular refractive power on the cross section, and anterior segment information related to the cross section. The axial length is more likely to be derived properly when the ocular refractive power and the information for identifying the shape of the anterior segment are information related to the same cross section. At this time, the axial length can be more accurately determined by using information identifying the shape of the measurement area by the first optical system as the anterior segment information related to the cross section.

Here, for example, the calculation control unit may derive the axial length by ray tracing calculation. In the ray tracing calculation, the position of the far point may be identified based on the on-plane ocular refractive power.

In the ray tracing calculation, when a light ray incident on a predetermined position of the anterior eye from a far point intersects with the optical axis after being refracted by a transparent body, the distance between the intersection point and the corneal apex is derived as the axial length. In this case, the ocular refractive power at the cut surface (on-surface refractive power) may be used instead of the spherical equivalent power that is generally used in the field of ophthalmology to identify the far point. This allows the position of the far point of the light ray passing through the cut surface to be more accurately identified. As a result, the axial length can be more accurately calculated. In this case, ray tracing calculation may be performed for each of the multiple light rays, and the axial length may be calculated as the result of the ray tracing calculation for each light ray. For example, the average value (or weighted average) of the axial lengths obtained by each ray tracing calculation may be calculated as the axial length of the test eye.

In the ray tracing calculation, the position of incidence of the light beam on the boundary surface of each transparent body and the change in angle at the boundary surface are determined taking into account the shape of the transparent body at the cross section identified from the anterior eye information. A more detailed explanation of the ray tracing method will be given in the examples below.

Furthermore, the calculation control unit may take into account the decentering of the optic body of the anterior segment when deriving the axial length from the ocular refractive power and anterior segment information related to the same cross-section. The decentering is determined based on the anterior segment information. As a result of taking into account the decentering of the optic body in the cross-section, the axial length can be calculated more appropriately. In this case, for example, a ray tracing calculation may be performed for each of a plurality of rays including at least the first ray and the second ray to calculate the axial length for each ray, and a final measurement value may be calculated based on the plurality of axial lengths. The first ray and the second ray are rays arranged on either side of the axial length on the cross-section.

<Specific Example of the Second Optical System: Sectional Photographing Optical System>
In the first embodiment, the second optical system may be a cross-sectional imaging optical system that captures a cross-sectional image of a cut surface set in the anterior segment of the subject's eye. The cross-sectional imaging optical system may be, for example, a Scheimpflug optical system or an OCT optical system. From the cross-sectional image, not only the shape of the boundary surface (surface) in each translucent body but also the distance between the boundary surfaces can be specified. As a result, the axial length can be obtained with higher accuracy. When the second optical system is a cross-sectional imaging optical system, the imaging range of the cross-sectional imaging optical system preferably includes from the anterior cornea of the subject's eye to at least the anterior crystalline lens. Needless to say, it is even more preferable if the imaging range includes from the anterior cornea to the posterior crystalline lens. Since the corneal thickness, the anterior corneal radius of curvature, the posterior corneal radius of curvature, the anterior chamber depth, the crystalline lens thickness, the anterior crystalline lens radius of curvature, and the posterior crystalline lens radius of curvature can be obtained without omission, the axial length can be obtained more appropriately.

When the second optical system is a Scheimpflug optical system, the projection optical axis of the illumination light needs to be arranged coaxially with the projection optical axis of the measurement light in the first optical system. The projection optical system of the Scheimpflug optical system may irradiate slit light as the illumination light. The irradiation area of the slit light is set as the cutting surface. The receiving optical system of the Scheimpflug optical system has a lens system and an image sensor arranged in a Scheimpflug relationship with the cutting surface. The receiving optical system is arranged with a receiving optical axis tilted with respect to the cutting surface.

Either visible light or infrared light may be irradiated as the slit light. Visible light is more likely to be scattered in the translucent body than infrared light. On the other hand, when infrared light is irradiated, the burden on the subject during imaging can be reduced.

In addition, when the second optical system is an OCT optical system, it may be possible to capture not only anterior segment OCT but also fundus OCT. In this case, it is not necessary that anterior segment OCT and fundus OCT can be captured at the same time, and anterior segment OCT and fundus OCT can be captured separately by switching a part of the OCT optical system.

<Specific Example of the Second Optical System: Purkinje Image Acquiring Optical System>
Instead of the cross-sectional photographing optical system, a Purkinje image acquiring optical system may be applied as the second optical system. The Purkinje image acquiring optical system includes an index projector that projects a measurement index (called a pattern index) based on a certain pattern onto the anterior segment from the front surface facing the subject's eye, and a front photographing optical system that captures a Purkinje image based on the pattern index. In this case, a first Purkinje image (reflected image by the anterior surface of the cornea), a second Purkinje image (reflected image by the posterior surface of the cornea), a third Purkinje image (reflected image by the anterior surface of the crystalline lens), and a fourth Purkinje image (reflected image by the posterior surface of the crystalline lens) may be generated. It is possible to obtain the shape of the boundary surface corresponding to each Purkinje image based on the position information of each Purkinje image. However, the corneal thickness, anterior chamber depth, and crystalline lens thickness cannot be obtained from the position information of the Purkinje image alone. In other words, the distance between the boundary surfaces of the transparent bodies cannot be obtained. In contrast, the cross-sectional photographing optical system is more advantageous in obtaining the distance between the boundary surfaces. In addition, since the third Purkinje image (reflected image by the anterior surface of the crystalline lens) appears closer to the optical axis of the front imaging optical system than the first, second, and fourth Purkinje images, differences in the shape of the anterior surface of the crystalline lens are less likely to appear as differences in the appearance position of the third Purkinje image. Therefore, a cross-sectional imaging optical system whose imaging range includes from the anterior surface of the cornea of the examinee's eye to at least the anterior surface of the crystalline lens is more advantageous in terms of accurately acquiring shape information about the anterior surface of the crystalline lens than a Purkinje image acquisition optical system.

The pattern index projected from the index projector may be a ring-shaped pattern, a line, or another two-dimensional pattern formed by multiple points. For example, multiple point indices arranged on a circumference may be projected as a pattern index. Also, multiple patterns may be combined.

<Measurement control>
In order to properly obtain the axial length by the above method, it is desirable that the shape information of more types of translucent bodies is included in the anterior segment information. In this case, it is possible that at least the shape information of the crystalline lens is included in the anterior segment information. In this case, the ocular refractive power measured by the first optical system and the shape information of the crystalline lens included in the anterior segment information are inevitably affected by the accommodation in the eye. Therefore, in order to ensure the accuracy and reproducibility of the axial length, it is required to acquire the ocular refractive power and the anterior segment information, respectively, taking into account the state of accommodation in the eye.

In contrast, in this embodiment, the calculation and control unit may control the first optical system and the second optical system so that the eye refractive power and the anterior eye information are each acquired in a state where the intraocular accommodation is the same.

If the eye refractive power and anterior segment information are acquired under conditions where the intraocular accommodation is the same, each piece of acquired information is equally affected by accommodation, and the axial length can be calculated accurately.

At this time, the calculation control unit may control the timing of acquisition of the ocular refractive power and the anterior segment information so that the accommodation load provided to the subject's eye by the fixation target is the same between the time of acquisition of the ocular refractive power and the time of acquisition of the anterior segment information. The acquisition timing may be controlled in conjunction with the control of changing the presentation distance of the fixation target.

For example, the timing of acquisition of eye refractive power and anterior eye information may be controlled so that the fixation target is presented at the same position between the acquisition.

Also, for example, the calculation and control unit may synchronize the timing of acquiring the eye refractive power and the anterior eye information. Synchronization here does not necessarily require that the timing of acquiring each be completely simultaneous. For example, there may be a time difference between the timing of acquiring each to the extent that no significant difference occurs in the state of accommodation.

The calculation control unit may also add a cloud to the test eye by controlling the fixation target presenting optical system, and acquire both the eye refractive power and the anterior segment information when the test eye is in a non-accommodative state. Calculating the axial length based on the eye refractive power and the anterior segment information acquired in a non-accommodative state improves the accuracy and reproducibility of the axial length compared to calculating the axial length from the eye refractive power and the anterior segment information measured in an accommodative state.

As mentioned above, the second optical system used to obtain anterior eye information may be a Scheimpflug optical system. In this case, it is assumed that the anterior eye is irradiated with visible light as illumination light and a cross-sectional image of the anterior eye is captured. On the other hand, infrared light is used as measurement light to measure the eye refractive power.

When cross-sectional images are taken with the Scheimpflug optical system, relatively strong visible light is irradiated as the imaging light. In this case, the irradiation of strong visible light may startle the subject, which may result in a change in the alignment state.

In response to this, the calculation control unit may first execute the operation of acquiring eye refractive power, and execute the operation of acquiring anterior eye information at the timing when the operation of acquiring eye refractive power is completed. This suppresses alignment deviation between the time when the eye refractive power is acquired and the time when the cross-sectional image, which is the anterior eye information, is acquired. In addition, the order of execution of each operation at this time may be fog ⇒ acquisition of eye refractive power ⇒ acquisition of anterior eye information. In addition, as described above, the timing of completion of the operation of acquiring eye refractive power and the timing of execution of the operation of acquiring anterior eye information are approximately simultaneous, so that deviation is unlikely to occur between the time when the eye refractive power is acquired and the time when the cross-sectional image, which is the anterior eye information, is acquired for each of the accommodation state and the alignment state. As a result, the accuracy and reproducibility of the axial length are improved.

As described above, the ophthalmic device of the first embodiment measures axial length using a new method and a new device configuration that differ from conventional axial length measurement devices.

Here, the first optical system can obtain the ocular refractive power of the subject's eye, which is important in evaluating myopia. The ophthalmic apparatus of the first embodiment can obtain, in a single device, information that is important in evaluating myopia, namely ocular refractive power and axial length.

In particular, in the first embodiment, when the Scheimpflug optical system is used as the second optical system, the cost of the device is kept sufficiently low compared to an optical interference type axial length measurement device, while still easily achieving the measurement accuracy of the axial length required for monitoring the progression of myopia.

Obtaining axial length from multiple cross sections
In the above description, the axial length is calculated using anterior segment information related to one cut surface. However, this is not necessarily limited to this, and multiple pieces of anterior segment information may be obtained for each of multiple cut surfaces. In this case, the axial length may be derived for each cut surface using the above method. For example, the average value of the axial lengths obtained for each cut surface may be calculated.

In addition, a known method for photographing multiple cut surfaces in a Scheimpflug optical system is to rotate the light receiving optical system around the light projection optical axis, and this method may be used.

Second Embodiment
Next, a second embodiment will be described.

The ophthalmic apparatus according to the second embodiment further includes a third optical system in addition to the device configuration of the first embodiment. That is, the ophthalmic apparatus according to the second embodiment includes a first optical system, a second optical system, a third optical system, and an arithmetic control unit.

For configurations common to the first embodiment, the description of the first embodiment will be used as appropriate and details will be omitted. However, in the second embodiment, the second optical system is an anterior segment cross-sectional imaging optical system, and unless otherwise specified, will be described as a Scheimpflug optical system.

In the second embodiment, the third optical system has an index projector that projects a pattern index for measuring the corneal shape onto the anterior segment from the front side facing the test eye. Additionally, the third optical system may have a front imaging optical system that captures a corneal Purkinje image using the pattern index. The corneal Purkinje image may be captured as a frontal image of the anterior segment.

In the second embodiment, the calculation control unit may obtain the axial length based on the ocular refractive power and anterior segment information related to the cut surface, and corneal shape information based on the corneal Purkinje image.

Here, corneal shape information including at least the radius of curvature of the anterior surface of the cornea can be derived with higher accuracy from the corneal Purkinje image rather than from an anterior cross-sectional image captured by a Scheimpflug optical system. For this reason, for example, in the axial length calculation, some of the information related to the cornea in the anterior cross-sectional image may be replaced with corneal shape information derived from the corneal Purkinje image. Also, some or all of the anterior cross-sectional information may be corrected based on the corneal shape information. As a specific example of correction, the entire anterior cross-sectional image may be deformed so that the corneal shape in the anterior cross-sectional image matches the corneal shape based on the corneal Purkinje image, and the shape of each transparent body based on the deformed image may be used in the axial length calculation.

In this way, the axial length can be calculated more accurately by taking into account the corneal shape information derived from the corneal Purkinje image.

In the second optical system, which is a Scheimpflug optical system, the smaller the inclination of the optical axis relative to the cut surface (object surface), the easier it is to expand the imaging range in the depth direction. In other words, the smaller the inclination of the light receiving optical axis (imaging optical axis) of the second optical system relative to the cut surface (object surface), the more advantageous it is for the second optical system to image from the anterior surface of the cornea to the posterior surface of the lens. However, if the light receiving optical axis is tilted to an extent that it is possible to image from the anterior surface of the cornea to the posterior surface of the lens, there is a risk of spatial interference between the light receiving optical system of the second optical system and the target projector of the third optical system.

A more detailed explanation will be given. In keratometry, which is one of the methods for measuring corneal shape, one or more circumferential regions within the range of φ2 mm to φ4 mm of the cornea are usually used as the measurement region. In this case, the light beam forming the pattern index is projected at an angle ranging from 14° (corresponding to φ2 mm) to 29° (corresponding to φ4 mm) with respect to the eye axis.

On the other hand, when attempting to photograph the area from the anterior surface of the cornea to the posterior surface of the crystalline lens using the Scheimpflug optical system, which is the second optical system, it is desirable to make the angle between the light receiving optical axis of the second optical system and the cut surface smaller. Conventional ophthalmic Scheimpflug cameras are mainly used in anterior segment analysis devices, and the above angle is about 45° or larger because low distortion and high resolution are prioritized. In contrast, the above angle in the Scheimpflug optical system of this embodiment is desirably about 40° or less, with the priority being given to being able to photograph deeper. For this reason, the respective light beam emission positions of the index projector and the light receiving optical axis of the second optical system can be arranged on approximately the same circumference with respect to the visual axis. If the size of the optical elements of each part is then taken into consideration, the above-mentioned interference problem may occur.

In contrast, in the second embodiment, the index projector of the third optical system is positioned so as to avoid the light receiving optical axis of the second optical system. More specifically, the index projector of the third optical system may be positioned so as to avoid the normal direction of the cut surface of the anterior eye formed by the second optical system (at least the direction in which the light receiving optical system of the second optical system is placed). For example, if the normal direction is the up-down direction, the index projector of the third optical system may be positioned so as to avoid a position that is ±90° (the horizontal direction is 0°) with respect to the optical axis of the third optical system.

The pattern index projected from the index projector is formed to avoid positions in the ±90° direction (horizontal direction is 0°) with respect to the optical axis of the third optical system. The pattern index may have a pattern shape that is symmetrical with respect to the optical axis of the third optical system. For example, it may be a two-dimensional pattern formed by lines or multiple points. For example, multiple point indices arranged on a circumference may be projected as the pattern index. Multiple patterns may also be combined.

"Example"
Next, an example corresponding to the first and second embodiments will be described with reference to FIGS.

<Overall configuration of the embodiment>
First, referring to FIG. 1, a schematic configuration of an ophthalmologic apparatus 10 according to an embodiment is shown.

In this embodiment, the ophthalmic device 10 is a combination device of an objective eye refractive power measuring device (particularly, in this embodiment, an autorefractometer) and a Scheimpflug camera. In this embodiment, the ophthalmic device 10 is a stationary examination device, but is not necessarily limited to this and may be a handheld type.

As shown in FIG. 1, the ophthalmic device 10 has at least a measurement unit 11, a base 12, an alignment drive unit 13, a face support unit 15, a monitor 16, and a control unit 50.

The measurement unit 11 includes a measurement system and an imaging system that are used to examine the subject's eye. In this embodiment, the optical system shown in FIG. 2 is arranged.

The alignment drive unit 13 may be capable of moving the measurement unit 11 three-dimensionally relative to the base 12.

The face support unit 102 is used to fix the subject's face in front of the measurement unit 11. The face support unit 102 is fixed to the base 12 and supports the subject's face.

The calculation control unit (also called a processor, hereafter simply referred to as the control unit) 50 is responsible for the overall control of the ophthalmic device 10. It also processes various test results acquired via the measurement unit 11.

<Optical system>
Next, the optical system in the ophthalmic apparatus 10 will be described with reference to FIG.

As an example, the ophthalmologic device 10 includes a measurement optical system 100, a fixation target presenting optical system 150, a frontal imaging optical system 200, cross-sectional imaging optical systems 300a and 300b, and a target projection optical system 400. It also includes BEEP splitters 501, 502, and 503 that split and combine the optical paths of each optical system.

<Measurement optical system>
The measurement optical system 100 objectively measures the ocular refractive power of the subject's eye E. For example, the values of SPH: spherical power, CYL: cylindrical power, and AXIS: astigmatism axis angle may be acquired as the measurement results of the ocular refractive power.

The measurement optics 100 has a projection optical system 100a and a light receiving optical system 100b.

The projection optical system 100a has at least a measurement light source 111, and projects a spot-shaped measurement light onto the fundus of the test eye E through the center of the pupil P or the corneal apex of the test eye E. In this embodiment, infrared light is used as the measurement light. However, this is not necessarily limited to this, and the measurement light may be visible light. The measurement light source 111 may be an SLD light source, an LED light source, or another light source.

In this embodiment, a prism 115 is placed on the common path of the projection optical system 100a and the light receiving optical system 100b. The prism 115 is rotated around the optical axis L1, causing the projection light beam on the pupil to be eccentrically rotated at high speed. As an example, in this embodiment, the projection light beam is eccentrically rotated in a region on the pupil of φ2 mm to φ4 mm. This region becomes the measurement region for the eye refractive power in this embodiment.

The light receiving optical system 100b has at least a ring lens 124 and an image sensor 125. As shown in FIG. 2, the measurement optical system 100 may also have other optical elements such as lenses and diaphragms. The light receiving optical system 100b extracts the measurement light beam reflected from the fundus in a ring shape through the periphery of the pupil. The ring lens 124 is disposed at a pupil conjugate position, and the image sensor 125 is disposed at a fundus conjugate position. The ring image formed on the image sensor 125 via the ring lens 124 is analyzed to derive the ocular refractive power.

As described above, in this embodiment, the measurement light is rotated eccentrically at high speed on the pupil, so that an analysis process is performed on the output image from the image sensor 125 based on an exposure time that is sufficiently long relative to the rotation period, or on an additive image of image data sequentially output from the image sensor 125, to derive the ocular refractive power. In this embodiment, at least the values of SPH: spherical power, CYL: cylindrical power, and AXIS: astigmatism axis angle are obtained as the results of the analysis process.

<Fixation target presentation optical system>
The fixation target presenting optical system 300 presents a fixation target to the subject's eye E. The fixation target is presented on the optical axis of the measurement optical system 100. The fixation target presenting optical system 300 is used to fixate the subject's eye. It is also used to apply fogging and accommodation load to the subject's eye. For example, the fixation target presenting optical system 300 includes at least a light source 151 and a fixation target plate 155. The fixation target plate 155 can be moved along the optical axis by a drive unit 155a. This allows the presentation distance (presentation position) of the fixation target to the subject's eye E to be changed.

<Frontal shooting optical system>
The front photographing optical system 200 captures a front image of the anterior segment of the subject's eye E. For example, the front photographing optical system 200 includes an image sensor 205 and the like. As the front image, an observation image of the anterior segment may be acquired. The observation image is used for alignment and the like. In addition, an index image (pattern index image) based on a pattern index projected from the index projection optical system 400 onto the cornea of the subject's eye is photographed by the front photographing optical system 200.

<Cross-section imaging optical system>
The cross-section photographing optical systems 300a and 300b are used to photograph a cross-sectional image of the anterior eye. The cross-section photographing optical systems 300a and 300b include an irradiation optical system 300a and a light receiving optical system 300b. The irradiation optical system 300a irradiates the anterior eye with slit light coaxial with the projection optical axis (optical axis L1) of the measurement light of the measurement optical system 100. The irradiation optical system 300a includes a light source 311 and a slit 312. In this embodiment, the slit light, which is the illumination light, is visible light. For example, a visible light source that emits blue light may be used as the light source 311.

In this embodiment, the cross section through which the slit light passes in the anterior segment is called the "cutting surface." The cutting surface is the object surface of the cross-sectional imaging optical systems 300a and 300b. In FIG. 2, the opening of the slit 312 has the horizontal direction (the depth direction of the paper in FIG. 2) as its longitudinal direction. Therefore, in this embodiment, the horizontal plane (XZ cross section) including the optical axis L1 is set as the cutting surface. In this embodiment, the cutting surface is formed at least between the anterior surface of the cornea and the posterior surface of the crystalline lens.

The light receiving optical system 300b includes a lens system 322 and an image sensor 321. In the light receiving optical system 300, the lens system 322 and the image sensor 321 are arranged in a Scheimpflug relationship with a cutting surface set in the anterior segment. That is, the optical arrangement is such that the extension planes of the cutting surface, the principal plane of the lens system 322, and the imaging surface of the image sensor 321 intersect at a single intersection line (single axis). A cross-sectional image of the anterior segment (see FIG. 3) is acquired based on a signal from the image sensor 321.

<Target projection optical system>
In this embodiment, the index projection optical system 400 has a plurality of point light sources 401. In this embodiment, each point light source 401 emits infrared light. However, it may be visible light. As shown in FIG. 4A and FIG. 4B, in this embodiment, the index projection optical system 400 is disposed in front of the measurement unit 11 as an index projector 410. The index projection optical system 400 projects a pattern index for measuring the corneal shape to the anterior segment from the front surface facing the test eye. In this embodiment, a pattern index consisting of four point images symmetrical with respect to the optical axis L1 is projected onto the cornea. The circumferential area onto which the pattern index is projected is the measurement area of the corneal shape by the index projection optical system 400 and the front photographing optical system 200. As an example, in this embodiment, when a corneal model eye having a predetermined radius of curvature is placed at a position of a predetermined working distance, each point image constituting the pattern index is projected onto a φ3 mm circumferential area of the corneal model eye. In this embodiment, the pattern index is composed of four point images, but the number of indices is not necessarily limited to this. The pattern index may be composed of three or more point images, or may include a linear index image or the like.

In this embodiment, the light receiving optical axis L2 in the cross-sectional imaging optical system 300 is located directly below the optical axis L1 (-90° direction). In order to achieve an imaging range from the anterior surface of the cornea to the posterior surface of the lens, the inclination of the light receiving optical axis (imaging optical axis) of the second optical system with respect to the cross section is sufficiently small, so that when the device is viewed from the side, the light beam of the pattern index projected from the index projection optical system 400 and the light receiving optical axis L2 are positioned close to each other (see FIG. 4A).

In FIG. 4B, the index projector 450, which is virtually placed (not actually placed) in the measurement unit 13, is shown by a dashed line. The index projector 450 projects a Mayering in the same circumferential area as the index projector 400. When the light beam of the pattern index and the light receiving optical axis L2 are in a close positional relationship, it can be seen that when attempting to project a ring-shaped pattern index such as a Mayering, the light receiving optical axis L2 will interfere with the index projector 450.

In contrast, the index projector 410 is positioned so as to avoid being below the optical axis L1. For example, FIG. 4B illustrates an example of an apparatus configuration in which a light source 401 and an angle adjustment mirror 402 (not shown in FIG. 2) are positioned at four locations on the upper right, lower right, upper left, and lower left of the optical axis L1. In this embodiment, a pattern index consisting of four point images symmetrical with respect to the optical axis L1 is projected onto the cornea. As a result of such an arrangement of the index projector 410, this embodiment achieves both wide-range imaging by the cross-sectional imaging optical system 300 and acquisition of the corneal shape by the Purkinje image.

<Alignment target projection optical system>
Further, the ophthalmic apparatus 10 includes an alignment light source 600. The alignment light sources 600 may be provided on the left and right sides. For example, a light beam is projected along a horizontal plane including the optical axis L1. In this embodiment, the alignment index projection optical system 400 and the light source 600 form an alignment index projection optical system. One of the index projection optical system 400 and the light source 600 projects diffused light, and the other projects parallel light. The working distance may be adjusted by moving in the forward and backward directions so that a corneal Purkinje image by parallel light and a Purkinje image by diffused light are captured at a predetermined ratio.

<Control operation>
Next, the control operation of the ophthalmologic apparatus 10 will be described with reference to the flowchart of FIG.

In this embodiment, the ophthalmic device 10 sequentially performs corneal curvature measurement, captures a cross-sectional image of the anterior eye segment, and measures eye refractive power, and the axial length is obtained based on the results of the measurements and capture.

First, the measurement unit 11 is aligned with the subject's eye E (S1). The examiner instructs the subject to place his or her face on the face support unit 15. Then, the presentation of a fixation target and the acquisition of an anterior eye observation image are started.

Then, for example, the subject's eye and the device are adjusted to a predetermined positional relationship based at least on an observation image of the anterior eye acquired via the front imaging optical system 200. More specifically, alignment in the XY directions is performed so that the optical axis L1 coincides with the corneal apex of the subject's eye E. Also, alignment in the Z direction is performed so that the distance between the subject's eye and the device is a predetermined working distance. At this time, an alignment index (not shown) may be projected onto the cornea, and the alignment may be adjusted based on the alignment index detected in the observation image.

Next, the corneal shape is measured (S2). A pattern index is projected from the index projector 410 (index projection optical system 400), and a corneal Purkinje image of the pattern index is captured by the frontal imaging optical system 200. Corneal shape information is obtained based on the corneal Purkinje image. Corneal shape information is derived based on the image height of the corneal Purkinje image. In this embodiment, at least the values of the corneal curvature, the degree of astigmatism, and the astigmatism axis angle are acquired as the corneal shape information.

Next, in this embodiment, the eye refractive power is measured (S3). For example, a preliminary measurement may be performed first, and then the main measurement may be performed.

In the preliminary measurement, the ocular refractive power of the test eye E is measured with the fixation target positioned at a predetermined presentation distance. During the measurement, the fixation target plate 155 may be positioned at an initial position that is an optically sufficiently distant distance from the test eye E and corresponds to the far point of the 0D eye. A ring image captured by the image sensor 125 based on the measurement light irradiated in this state is image-analyzed by the calculation control unit 50. As a result of the analysis, the value of the refractive power in each meridian direction is obtained. At least the spherical power in the preliminary measurement is obtained by performing a predetermined process on the refractive power in each meridian direction.

Then, the control unit 50 moves the fixation target plate 155 to the fogging start position where the subject's eye E is in focus, according to the spherical power of the preliminary measurement of the subject's eye E. This allows the fixation target to be clearly observed in the subject's eye E. The control unit 50 then moves the fixation target from the fogging start position, thereby adding fogging to the subject's eye E. This releases the adjustment of the subject's eye E.

This measurement is performed with the test eye E clouded. A predetermined analysis process is performed on the ring image captured of the test eye E clouded, to obtain the objective values of the test eye's SPH: spherical power, CYL: cylindrical power, and AXIS: astigmatism axis angle.

Next, a cross-sectional image of the anterior eye is captured (S4). At this time, the capturing operation is performed immediately after the main measurement of the eye refractive power is completed. For example, the operation of capturing the cross-sectional image may be performed triggered by the completion of the main measurement of the eye refractive power. In other words, immediately after the main measurement is completed, visible light is irradiated from the irradiation optical system 300a, and a cross-sectional image of the anterior eye segment formed on the image sensor 321 is obtained. Since the cross-sectional image of the anterior eye segment is captured immediately after the main measurement of the eye refractive power is completed, alignment deviation between the measurement of the eye refractive power and the capture of the cross-sectional image is reduced.

Furthermore, in this embodiment, visible light is not irradiated before the cross-sectional image is captured, so that miosis is suppressed when the cross-sectional image is captured. As a result, cross-sectional images that capture the deeper parts of the anterior segment are more easily captured.

Next, the calculation control unit 50 calculates the axial length of the test eye based on the information or images acquired in each step S2 to S4.

In this embodiment, the axial length is derived based on ray tracing calculations on the cross-section.

As shown in FIG. 6, a light ray (e.g., light ray Lx in FIG. 6) entering the test eye from the far point FP is tracked, and the position of the intersection point where the light ray is refracted by each optical body of the test eye and intersects with the optical axis is determined. The distance between the determined intersection point and the corneal apex is derived as the axial length. Note that in this embodiment, for the sake of convenience, it is assumed that the refractive index of each optical body (cornea, aqueous humor, and crystalline lens) is constant and that there is no change in refraction within each. However, this is not necessarily limited to this, and the axial length may be derived taking into account changes in the refractive index within the optical body (e.g., changes in the refractive index between the inside and outside of the crystalline lens).

In addition, refractive index information on the refractive index of the translucent body may be obtained separately from the cross-sectional image, which is the anterior eye information, and the refractive index information may be used to derive the axial length. In other words, the refractive index of the translucent body based on the refractive index information may be further taken into consideration when obtaining the axial length. For example, the refractive index of the crystalline lens is known to change with age. Therefore, the device may have a calculation formula or lookup table in which the refractive index of the crystalline lens is associated with each age. In this case, the age is input and the refractive index according to the age is obtained. This refractive index may be used to perform ray tracing calculations.

In this technique, in addition to the position of the far point FP, the following parameters are utilized: The following parameters are obtained based on the Scheimpflug image and corneal topography information.
Ra: Radius of curvature of the anterior surface of the cornea Rp: Radius of curvature of the posterior surface of the cornea CT: Corneal thickness ACD: Anterior chamber depth ra: Radius of curvature of the anterior surface of the crystalline lens rp: Radius of curvature of the posterior surface of the crystalline lens LT: Lens thickness In addition, the position of the far point FP of the test eye relative to the corneal apex is obtained based on the measurement result of the eye refractive power. For example, if the test eye E has no astigmatism, SPH=-5D, and VD=12mm, the distance from the corneal apex to the far point FP is 12+1000/5=212mm. It is considered that the light from here forms an image on the fundus of the test eye. Note that the VD of 12mm is a fixed value indicating the distance between the corneal apexes assuming the wearing of spectacle lenses. VD may vary depending on the device.

In the widely used expression of ocular refractive power using SPH, CYL, and AXIS, SPH indicates the refractive power related to the strongest meridian (or weakest meridian), so it does not necessarily become an appropriate value in ray tracing on a cross section. For example, consider a case where SPH = -5D, CYL = -2D, and AXIS = 30°. In this case, if a horizontal cross section is obtained using the above optical system example, the refractive power on this cross section will be neither -5D nor -7D with CYL added.

In contrast, in this embodiment, the on-surface ocular refractive power, which is the ocular refractive power on the cut surface, is calculated, and the position of the far point FP is set based on the on-surface refractive power. Here, the refractive power at an arbitrary surface is expressed by the following formula.

P(θ) = S + C × [sin ² (θ - A)]
Here, θ is the angle with respect to the horizontal plane, with the horizontal direction being 0°. The cutting surface in this embodiment is a horizontal plane (θ=0°). Therefore, when SPH=-5D, CYL=-2D, and AXIS=30°, P(0°)=-5.5D is calculated (see FIG. 7). In this case, 12+1000/5.5=194 mm is the distance from the corneal apex to the far point FP on the cutting surface.

Here, a ray from the far point FP thus set is tracked. For example, a ray (e.g., ray Lx in FIG. 6) is guided from the far point FP toward a fixed position (as an example, a position of φ6 mm at the position of the pupil of the test eye (approximately 3 mm behind the cornea)). Note that setting the fixed position as φ6 mm at the position of the pupil of the test eye is merely an example, and can be changed as appropriate.

This light ray is first refracted at the front surface of the cornea. The intersection of the light ray and the front surface of the cornea is calculated based on the radius of curvature Ra of the front surface of the cornea, the position of the far point FP, and the light ray angle at the far point FP. Furthermore, the angle of incidence of the light ray at the intersection is calculated. The light ray that reaches the front surface of the cornea changes its direction at a refraction angle determined with respect to the angle of incidence based on Snell's law. In this way, the light ray is tracked sequentially at each transparent body boundary surface. At that time, various parameters (Ra, Rp, CT, ACD, ra, rp, LT) obtained based on the Scheimpflug image and corneal shape information are appropriately used to give the intersection of the light ray with each boundary surface. In this embodiment, the point at which the light ray intersects with the axis of the eye (here, the visual axis) after leaving the rear surface of the crystalline lens is finally obtained. The distance from the intersection to the corneal apex (here, the origin) is used as the axial length AL.

When the above-mentioned various parameters (Ra, Rp, CT, ACD, ra, rp, LT) are used in the ray tracing calculation, in this embodiment, at least the value based on the corneal Purkinje image of the pattern index is used for the radius of curvature Ra of the anterior cornea, and the values based on the Scheimpflug image are used for the remaining values. This is because, for the shape of the anterior cornea, the measurement accuracy based on the corneal Purkinje image is generally higher than that based on the Scheimpflug image. As described above, in this embodiment, at least the values of the corneal curvature, the degree of astigmatism, and the angle of the astigmatism axis are obtained as corneal shape information. The corneal curvature (curvature of the anterior cornea) at the cut surface can be obtained from these values using a method similar to that used to obtain the refractive power for the cut surface. The reciprocal of the obtained value is used as Ra.

In this manner, the axial length can be calculated by tracing rays that are directed toward a fixed position. However, the ray tracing method is not limited to the above method. For example, the point at which the image is formed from a distant point may be calculated by paraxial calculation. Also, the point at which the image is formed from a distant point may be calculated by taking into account a number of rays that enter the subject's eye at different positions. For example, ray tracing for a paraxial ray and a ray that is directed toward a fixed position other than the paraxial ray may be combined. When ray tracing for a number of rays is performed, the final measured value (calculated value) of the axial length may be the average value of the axial lengths calculated by each ray tracing (or a weighted average value).

The axial length may also be obtained by tracing rays directed toward the measurement region (φ2 mm to φ4 mm on the pupil) by the measurement optical system 100. For example, ray tracing may be performed on each of a number of rays directed toward the region of φ2 mm to φ4 mm on the pupil, and the average value of the axial length obtained by each ray tracing may be obtained as the calculation result. Ray tracing is performed under more appropriate conditions, making it easier to obtain the axial length with greater accuracy.

Note that a specified offset value may be added to the axial length value obtained in this embodiment. The offset value corrects the error between the calculated value and the actual measured value.

Ray tracing may also be performed by tracing rays that are emitted from a far point and pass through a circumferential region onto which a pattern index for corneal shape measurement is projected. This makes the conditions for ray tracing more appropriate, making it easier to obtain the axial length with greater accuracy.

The calculated axial length is displayed on the monitor 16. In this embodiment, the axial length is displayed along with at least one of the ocular refractive power (SPH, CYL, AXIS) and corneal shape information. If previous measurement results exist for the subject eye, the current measurement results may be displayed along with the previous axial length measurement results. For example, the measurement results may be displayed using a trend graph with age (date of measurement) on the horizontal axis and axial length on the vertical axis. Of course, the manner in which the measurement results are displayed is not limited to these.

<Example of transformation>
Although the present disclosure has been described above based on the embodiments and examples, it should be understood that the present disclosure is not limited to the above-described embodiments and may be modified in various ways.

For example, in the above embodiment, the various parameters (Ra, Rp, CT, ACD, ra, rp, LT) used in ray tracing were all actually measured values. However, this is not necessarily limited to this, and standard values (assumed values) may be used partially for the various parameters. The standard value may be an average value, or a value adopted in a predetermined optical model of the eye (e.g., the Gullstrand eye model, etc.). Multiple standard values may be prepared for at least one of age, sex, and region, and the examiner may be able to select which standard value to use to calculate the axial length.

In the above embodiment, for example, the axial length of the subject eye is obtained as a result of calculation based on the ocular refractive power of the subject eye and the anterior segment information. However, this is not necessarily limited to this, and the axial length may be obtained using a mathematical model trained by a machine learning algorithm. The mathematical model refers to, for example, a data structure for predicting the relationship between input data and output data. The mathematical model is constructed by training using a training data set. The training data set is a set of input training data and output training data. The input training data is sample data input to the mathematical model. For example, the input training data uses the ocular refractive power and anterior segment information of multiple subject eyes acquired in the past. The output training data is sample data of values predicted by the mathematical model. For example, the output training data uses axial length values of multiple subject eyes acquired in the past. The axial length value may be a measurement value acquired by an optical interference type or ultrasonic type axial length measuring device. The mathematical model is trained so that when certain input training data is input, corresponding output training data is output. In this embodiment, the calculation control unit may input the eye refractive power and anterior eye information to the mathematical model to obtain the axial length value of the subject eye as a predicted value.

In addition, for example, in the above embodiments and examples, anterior segment information including lens shape information may be acquired sequentially while the fixation target is moved to add fogging. This allows a change in the lens shape to be detected from the anterior segment information, and it may be confirmed whether fogging is being performed appropriately based on the shape change.

10 Ophthalmic apparatus 50 Control unit 100 Measurement optical system 300a, 300b Sectional photographing optical system

Claims (4)

a first optical system that projects measurement light onto a fundus of a subject's eye and acquires an ocular refractive power of the subject's eye based on fundus reflection light of the measurement light;
a cross-sectional photographing optical system that photographs a cross-sectional image of the anterior eye segment at a cut surface on which the optical axis of the first optical system is disposed in order to obtain anterior eye segment information, which is information about a shape of the anterior eye segment, and the cross-sectional image of the anterior eye segment at the cut surface is obtained;
an axial length acquisition means for acquiring an axial length of the subject's eye based on an ocular refractive power on the cut surface related to the cross-sectional image and anterior eye information related to the cut surface based on the cross-sectional image . a third optical system having an index projector for projecting a pattern index for measuring a corneal shape onto an anterior segment from a front surface facing the subject's eye, and capturing a corneal Purkinje image of the pattern index;
2. The ophthalmologic apparatus according to claim 1 , wherein the axial length acquisition means acquires the axial length based on the ocular refractive power, the anterior eye information, and corneal shape information based on the corneal Purkinje image. When executed by a processor of the ophthalmic computer,
a first acquisition step of acquiring an ocular refractive power of the test eye measured based on fundus reflected light due to measurement light projected onto the fundus of the test eye by a first optical system ;
a second acquisition step of acquiring anterior-eye-segment information indicating a shape of the anterior-eye segment based on a cross-sectional image of the anterior- eye segment taken along a cross-sectional surface on which the optical axis of the first optical system is disposed;
an axial length acquisition step of acquiring the axial length of the subject's eye based on the ocular refractive power on the cut surface related to the cross-sectional image and the anterior eye segment information related to the cut surface based on the cross-sectional image . In the first acquisition step, values of SPH: spherical power, CYL: cylindrical power, and AXIS: astigmatism axis angle of the subject's eye are acquired,
4. The axial length calculation program according to claim 3, wherein the axial length acquisition step acquires the axial length of the subject's eye based on the ocular refractive power on the cut surface related to the cross-sectional image converted from the values of SPH, CYL , and AXIS, and the anterior eye information.

Images