Patent Application
20130021576
The present invention relates to an ocular optical system, and particularly to an ocular optical system of an apparatus for taking an image of an eye retina.
As an ophthalmologic apparatus for examining an eye retina, there is known a scanning laser ophthalmoscope (SLO) which obtains a two-dimensional image of a retina as a plane. In addition, there is also known an optical coherence tomography (OCT) which obtains a tomographic image of a retina in a noninvasive manner. Those apparatuses have been put into practical use for a long time. Those apparatuses scan a retina by a light beam in a two-dimensional manner using a deflector, and synchronously measure reflected and backscattering light so as to obtain a two-dimensional image or a three-dimensional image of the retina.
Spatial resolution of the obtained image in a plane direction (lateral direction) of the retina (hereinafter, referred to as lateral resolution) is determined basically by a spot diameter of the beam that scans the retina. In order to reduce the spot diameter of the beam condensed on the retina, only the diameter of the beam that enters the eye needs to be increased. However, curved surface shapes and refractive index homogeneity of cornea and lens that perform the refracting action in an eyeball are not complete, which causes high order aberration in a wavefront of the transmitted light. Therefore, even if a thick beam enters, the spot on the retina cannot be condensed to a desired diameter and, on the contrary, is expanded. As a result, the lateral resolution of the obtained image is decreased, and the S/N ratio of the obtained image signal is also decreased. Therefore, conventionally, it has been common to use incident light of a thin beam of approximately 1 mm diameter that is hardly affected by aberration of the eye's optical system, and to form a spot of approximately 20 μm diameter on the retina.
A structure that is generally and often used in the two-dimensional scan optical system such as SLO or OCT is the structure in which two one-dimensional scanner mirrors are disposed close to each other (hereinafter, referred to as a tandem type). PTL 1 discloses a structure in which galvano mirrors, each of which can rotate about a rotation axis, are disposed close to each other so that the rotation axes are perpendicular to each other, and the incident beam is deflected in the two-dimensional direction so as to enter the pupil of the eyeball via an imaging optical system and an objective lens. In addition, NPL 1 discloses a structure using a two-dimensional scanner having a similar structure, in which a thick beam of 3.68 mm diameter is formed as the incident beam of the eyeball, and a compensation optical system using a wavefront sensor and a variable shape mirror is also disposed for correcting influence of aberration of the eyeball optical system.
When the two scanning units are disposed close to each other, if one of the two scanning units is disposed at a position conjugate to the anterior ocular segment, another scanning unit is not disposed at the position conjugate to the anterior ocular segment. Therefore, the incident position of the measuring beam scanned by the another scanning unit in the anterior ocular segment may have a large deviation. In this case, for example, a light amount loss occurs due to vignetting generated by the iris, which is not preferred.
In order to solve the above-mentioned problem, the present invention provides an ocular optical system, which permits a measuring beam scanned by two scanning units disposed close to each other to enter an anterior ocular segment of an eye to be inspected and to irradiate a fundus, in which the optical system includes an optical unit which is disposed at a position of an intermediate image, which is optically conjugate to the fundus, and has a surface having different optical powers corresponding to scan directions of the two scanning units.
According to the present invention, an optical unit is provided which is disposed at a position of an intermediate image, which is optically conjugate to the fundus, and has a surface having different optical powers corresponding to scan directions of the two scanning units. Thus, it is possible to reduce the deviation of the incident position of the measuring beam in the anterior ocular segment due to that one of two scanning units disposed close to each other is disposed at a position conjugate to the anterior ocular segment while another scanning unit is not located at the position conjugate to the anterior ocular segment.
Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings.
Preferred embodiments of the present invention will now be described in detail in accordance with the accompanying drawings.
Hereinafter, a reason why a deviation occurs in the incident position of the measuring beam in the anterior ocular segment is described in detail. In recent years, a technology for compensation optics, which compensates for even high order wavefront aberration using an active optical element, is in the actual use and is being applied also to fundus measurement. In this technology, wavefront aberration generated due to characteristics of the subject itself or variation of measurement environment is measured sequentially by a wavefront detector, and is compensated by a wavefront aberration compensator such as a variable shape mirror or a space optical modulator. An example is reported in which a thick beam of approximately 7 mm diameter is introduced into an eyeball using the above-mentioned technology, and is condensed to approximately 3 μm diameter that is close to a diffraction limitation on a retina by wavefront compensation, with the result that an SLO image or an OCT image having high resolution is obtained. When a thick incident beam is formed in this way, there is a point to be noted in view of a structure of the optical system. The point is described with reference to
If a pupil incident beam diameter d2 is approximately 1 mm, it is easy to set a pupil magnification (lateral magnification) β of an ocular optical system 2, which is a ratio of the pupil incident beam diameter d2 to a scanner incident beam diameter d1, to a value smaller than one as a reduction system. Therefore, the pupil position shift amount ΔZ2 can be reduced to a negligibly small value. For example, when the scanner incident beam diameter d1 is 4 mm, and the pupil incident beam diameter d2 is 1 mm, the lateral magnification β is d2/d1=0.25. Therefore, a longitudinal magnification α=β2 is 0.0625. In this case, even if the distance ΔZ1 between scanner rotational axes is 10 mm, the pupil position shift amount ΔZ2 can be reduced to 0.625 mm. If the beam diameter is 4 mm, it is sufficient that the scanner mirror diameter be approximately 6 mm to 7 mm. The scanner that satisfies the necessary mirror diameter, deflection angle, and drive frequency can be easily obtained.
On the other hand, in order to thicken the pupil incident beam diameter d2 in the above-mentioned ocular optical system 2, the lateral magnification β needs to be increased, or the scanner incident beam diameter d1 needs to be increased. However, if the lateral magnification is increased to satisfy β>1 as in the former method, the pupil position shift amount ΔZ2 is increased. If d1=4 mm and d2=6 mm are satisfied, α is (6/4)2=2.25 so that the pupil position shift amount ΔZ2 becomes as large as 20 mm or larger (here, refractive power of the anterior ocular segment is not taken into account for simple description). When ΔZ2 is increased as described above, firstly an off-axis light beam is vignetted by the iris so that a desired spot diameter cannot be formed on the retina. As a result, amounts of light projected to the retina, and reflected and backscattering light from the retina are all decreased. In addition, a distortion may occur in the scanned area on the retina, so that image quality is deteriorated.
Further, when the compensation optics is used, it is necessary that the pupil of the eyeball, the wavefront sensor, and the wavefront compensator be optically conjugate. However, in the example illustrated in
In addition, even if the scanner incident beam diameter d1 is increased as in the latter method, d1 needs to be enormously increased as the reduction optical system. As a result, the scanner mirror is also increased so that the scanner interval d1 needs to be also increased. Finally, it is very difficult to set the pupil position shift amount ΔZ2 to a value smaller than 1 mm.
In order not to generate such a pupil position shift amount ΔZ2, it is considered to adopt a structure of the ocular optical system 2 in which a relay optical system 201 is disposed between the scanners as illustrated in
Hereinafter, the ocular optical system according to the present invention is described based on embodiments with reference to the drawings.
A pupil magnification β of this ocular optical system is two times. When the diameter of the beam 6 entering the x scanner mirror 11 is 3 mm, the diameter of the beam entering the iris 41 is 6 mm. Therefore, when the focal length of the eyeball optical system is 22 mm and the wavelength of the beam is 850 nm, a beam spot diameter on the retina 42 is approximately 4 μm.
The ocular optical system includes a lens unit 21 having a focal length of f1 (>0) (corresponding to a first optical element in the embodiments) and a lens unit 22 having a focal length of f2 (>0), which have a relationship satisfying f2=2·f1. In this case, the x scanner mirror 11 and the eyeball pupil 41 are disposed at positions that are optically conjugate by the ocular optical system 2. When the beam 6 deflected by the x scanner mirror 11 enters the eyeball 4 at a field angle, a principal light beam 52 thereof passes through the center of the eyeball pupil 41.
At the same time, the ocular optical system includes a cylindrical lens 3 (corresponding to a second optical element in the embodiments) disposed in a vicinity of an intermediate image position that is optically conjugate to the retina, between the lens units 21 and 22. Here, the “intermediate image position” is a position on the optical axis of the ocular optical system. This cylindrical lens 3 is disposed so as to have a refractive power only in a yz cross sectional direction and no refractive power in an xz cross sectional direction. In this case, it is supposed that there is a beam scanned in the y direction in the yz cross section. Here, a cylindrical surface 31 of the cylindrical lens 3 is set so as to agree substantially with the intermediate image position, namely the position where the beam is condensed. Thus, the beam scanned by the cylindrical surface 31 so as to pass off the optical axis (as illustrated in
In order to set a value of ΔZ2 to zero, the cylindrical surface 31 only needs to substantially agree with the intermediate image position. A relationship between the position where the cylindrical lens is disposed and the focal length of the cylindrical lens for satisfying the above-mentioned relationship is described with reference to
Here in the yz cross section, the object point of the lens unit 21 is the y scanner reference point SC12, and a distance between the y scanner reference point SC12 and the scanner side principal point L21O is s21 (=−(f21−Δz1)<0). Here, the sign is positive in the direction from the scanner to the eyeball pupil side. Therefore, the distance between the principal point L21I on the eyeball pupil side and the image point position of the lens unit 21 in this case is S22, which satisfies the equation below.
1/s22=1/s21+1/f21 (equation 1)
Here, when the CYL is disposed at a distance of −s3 (s3<0) from the principal point L21I on the eyeball pupil side of the lens unit 21, in the same manner, the position of the image point of the CYL is S32, in which the distance from the CYL satisfies the equation below.
1/s32=1/(s22+s3)+1/fCYL=1/[1/{1/s21+1/f21}+s3]+1/fCYL (equation 2)
In order that the image point of the lens unit 22 finally agrees with the same position as the image point in the xz cross section, namely the point at a distance of f22 from the principal point position L22I on the eyeball pupil side of the lens unit 22, s32=∞ only needs to be satisfied. Therefore, a value of fCYL only needs to be selected so that the right side of the equation given above becomes zero. Therefore, the following equation holds.
f
CYL=−1/{1/s21+1/f21}−s3 (equation 3)
According to this equation (3), supposing f21=50 (mm), f22=100 (mm), Δz1=10 (mm), and s3=−50 (mm), for example, fCYL only needs to be set to 250 mm. Further, if each of the optical elements is a mirror, it is supposed that the principal points on the scanner side and on the eyeball pupil side agree with each other.
In order to set a value of ΔZ2 to zero, the cylindrical surface 31 only needs to substantially agree with the intermediate image position. In order to prevent deterioration of the beam or decrease in light amount due to a flaw or dust on the surface of the lens, the cylindrical surface 31 and the intermediate image position may be separated from each other in such a range that astigmatism is not generated conspicuously. In this case, it is desirable that the cylindrical surface be closer to the intermediate image position than any other surface having optical power.
As described above, it is possible to reduce the pupil position shift amount ΔZ2 that is generated in the structure illustrated in
A specific example to which this fundamental structure of the ocular optical system is applied is illustrated in
Note that, the eyeball 4 is also referred to as an eye to be inspected in the present invention.
The collimated beam 6 formed by a light source and a collimator lens (not shown) is scanned by the x scanner 11 and the y scanner 12 in a two-dimensional manner, and is led to the viewer's eyeball 4 by the ocular optical system. Here, a distance between both scanners is set to 8 mm, and a diameter of the incident beam is set to 3 mm. The lateral magnification β of the ocular optical system is two times. The scan angle of the scanner mirror is 3.4 degrees in each scan direction. Therefore, the scan angle on the scanner side is 6.8 degrees, and the scan angle on the eyeball pupil side is 3.4 degrees in each scan direction. As a result, an area of 1.07 mm×1.07 mm on the retina 42 is scanned. In addition, the diameter of the beam entering the eyeball pupil 41 is 6 mm, and the spot diameter on the retina 42 is approximately 4 μm. Note that, the eyeball is assumed as an ideal lens here, too. With this structure, when the light source is modulated in synchronization with the scanning by the scan mirrors, the viewer can recognize a two-dimensional image.
Here, a focal length f of the cylindrical lens 3 (at wavelength of 587.56 nm) is set to 308.76 mm. As a result, the pupil position shift amount ΔZ2 in the maximum scan angle beam is reduced to approximately 0.2 mm, with the result that the pupil is formed appropriately in each cross section illustrated in
Hereinafter, lens data of this embodiment is shown. Here, X, Y, and Z are respectively different from coordinates x, y, and z on the retina that are coordinates with respect to the coordinates of the aperture stop position as the origin. The unit of the positions in X, Y, and Z directions is millimeters, and the unit of a rotation angle in each axis is degrees in the counterclockwise direction. Here, the surface closest to the intermediate image position is the flat surface of the cylindrical lens that has no optical power, and the cylindrical surface 31 is disposed at the position closer to the intermediate image position than any other surface having optical power.
Next, a second embodiment is described below in the case where a scan type retinal inspection instrument is supposed, which measures reflection light from the retina. In the case of the structure illustrated in
Therefore, in this embodiment, as illustrated in
The diverging beam 6 emitted from a light source 7 is collimated by a collimator lens 81 and reaches the cylindrical lens 3 via the x scanner 11, the y scanner 12, and the positive power lens 21. After that, the beam is led to the eyeball via a lens (not shown). The beam is condensed to the retina and is reflected, and then passes along the opposite optical path. After that, a part of the beam is reflected by an optical branching member 91 and is led to a light receiving sensor 93 via a lens 82 and a pinhole 92. Here, the retina and the pinhole 92 have a positional relationship optically conjugate to each other, so as to detect only the reflected and backscattering light from a very small point on the retina corresponding to the diameter of the pinhole 92. Note that, the optical path including the path folded by the scanner mirror is illustrated like a straight line for easy view.
In the structure illustrated in
In this embodiment, the cylindrical lens 3 is inclined in the direction with respect to the rotational axis that is the axis perpendicular to the generatrix of the cylindrical surface. However, the cylindrical lens 3 may be inclined with respect to the generatrix as the rotational axis. However, because the inclination of the normal of the reflection surface becomes small, the separation amount of the eliminated reflection ghost light is decreased so that the effect is also decreased.
A third embodiment is described with reference to
The beam 6 formed by a light source and a collimator lens (not shown) is first deflected by the x scanner 11 and the y scanner 12 in a two-dimensional manner. After that, the beam 6 is led to the eyeball 4 via the ocular optical system constituted of a spherical mirror 21 having positive power, a cylindrical mirror 32 having positive power, and a spherical mirror 22 having positive power. Here, the incident beam diameter to the x scanner 11 is 3 mm, the lateral magnification of the ocular optical system is two times, and the incident beam diameter to the eyeball is 6 mm. The spot diameter on the retina is approximately 4 μm. Note that, the xz cross section is illustrated from the spherical mirror 22 to the eyeball, and the yz cross section is illustrated from the y scanner 12 to the eyeball, for easy view.
In this case, the spherical mirrors 21 and 22 are disposed so that the x scanner 11 and the eyeball pupil 41 are optically conjugate to each other in the xz cross section. In addition, the cylindrical mirror 32 has optical power only in the yz cross sectional direction, and is disposed at the intermediate image position of the ocular optical system. Thus, the y scanner 12 and the eyeball pupil 41 are optically conjugate to each other also in the yz cross section without affecting the focused state.
The returning light from the lens surface, which is generated in the first embodiment using the cylindrical lens, is not generated in the case where the eccentric cylindrical mirror is used as in this embodiment. Therefore, it can be said that the use of the mirror as units for reducing the pupil position shift amount ΔZ2 for the surface having different powers in the cross sectional directions is advantageous in the instrument for inspecting the retina reflection light or the like.
The beam emitted from the light source 7 propagates in a fiber 71 and is split by a fiber coupler 75 into fibers 73 and 74 at a predetermined ratio. The light that has propagated in the fiber 73 is emitted from an emission end on the observation arm side and is collimated by the collimator lens 81. Then, the light is sent to a reflection type wavefront compensator 50 via concave mirrors 53 and 54. In this case, the wavefront compensator 50 is the state of non-working flat surface. The reflected beam is reflected by concave mirrors 55 and 56, and enters the eyeball 4 via the scanners 11 and 12, and the ocular optical system 20. Here, the ocular optical system 20 is constituted of the spherical mirrors 21 and 22, and the cylindrical mirror 32. After that, the light is condensed to the retina by the anterior ocular segment optical system of the eyeball. Further, the reflected and scattering light propagates in the eyeball anterior ocular segment, the ocular optical system 20, the wavefront compensator 50, and the like along the incident optical path in the opposite direction, and then is split by a branching part 52 constituted of a beam splitter or the like at a predetermined ratio. Here, the reflected beam is detected by a Shack-Hartmann type wavefront sensor 51 so that wavefront aberration generated by the optical system of the eyeball 4 is measured. The measured wavefront aberration value is sent to a computer 13, in which a control value for the wavefront compensator 50 is calculated. After that, the wavefront compensator 50 is driven to cancel the aberration based on the control value. Here, the wavefront compensator 50 may be a variable shape mirror or a space optical modulator.
By the driven wavefront compensator 50, the wavefront of the illumination beam entering the eyeball has such characteristics that the aberration of the optical system of the eyeball is inverted. Therefore, the aberration is canceled and reduced in the eyeball, with the result that an image is formed on the retina in a good condensing state close to the diffraction limitation. On the other hand, the light reflected and scattered at the condensing point becomes a wavefront having aberration again when the light passes through the optical system of the eyeball 4. However, after propagating along the incident optical path in the opposite direction, the aberration is corrected by the wavefront compensator 50 in the same manner, and is connected appropriately to the fiber 73 via the collimator lens 81.
On the other hand, in the reference arm 102, the beam emitted from the end of the fiber 74 is collimated by the collimator lens 81 in the same manner, passes through a dispersion compensation glass 82, and is reflected by a turn back mirror 80. After that, the beam is connected to the fiber 74 again via the original optical path.
The individual beams that have propagated in the fibers 73 and 74 are combined by the coupler 75 and the combined beam propagates in the fiber 72 so as to be sent to the spectroscope 90. The light emitted from the fiber end is collimated by a lens 96 and is separated into individual wavelengths of light by a grating 98, which are then condensed on a one-dimensional sensor 99 by an imaging lens 97. The light intensities of the individual wavelengths detected by the sensor 99 are converted into electric signals and are sent to the computer 13. The signal is Fourier transformed after converting the wavelength into a wave number, so as to determine scattering intensity with respect to a position in the vertical direction. This is performed for each of the beam positions of scanning so that a tomographic image and a three-dimensional image are obtained. In this way, when the ocular optical system of the first, second, or third embodiment is used for the OCT, good performance can be obtained without increasing the entire size even if a thick beam is formed as the beam entering the pupil and the compensation optical system is disposed in the OCT. In other words, by using the ocular optical system of the first, second, or third embodiment for the OCT, a fundus image generating apparatus can be realized.
In this embodiment, the case where the ocular optical system is applied to the OCT is described, but needless to say, the same effect can be obtained even in the case where the ocular optical system is applied to the SLO
In addition, the present invention can also be realized by performing the following process. Specifically, the process includes supplying software (program) that realizes the function of the embodiments described above to a system or an apparatus via a network or various storage media so that a computer (or CPU, MPU, or the like) of the system or the apparatus reads and executes the program.
While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions.
This application claims the benefit of Japanese Patent Application No. 2010-094119, filed Apr. 15, 2010, which is hereby incorporated by reference herein in its entirety.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2010-094119 | Apr 2010 | JP | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/JP2011/059476 | 4/12/2011 | WO | 00 | 10/4/2012 |
