JPH0323044B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a corneal topography measuring device that automatically measures the corneal topography of an eye, and particularly to a corneal topography measuring device suitable for being attached to a surgical microscope.

[Prior art and its problems] In recent years, cataract treatment by implanting an intraocular lens has been increasing. This surgery is performed by making an incision around the cornea, removing the cloudy crystalline lens, implanting an intraocular lens in its place, and then suturing the incision. , corneal astigmatism is known to occur.
For this reason, the degree of suturing is controlled while measuring the corneal shape during surgery.

A known device for this purpose is a manual keratometer built into a surgical microscope, but it has problems such as being complicated to operate and risking contamination of the surgeon's fingertips.

Furthermore, in recent years, so-called autokeratometers that automatically measure the curvature of the cornea have become widespread, and many patents have been filed showing the basic operating principle of these devices. For example, JP-A-58-54927, JP-A-61-85920, etc.

One problem with these schemes is that they have narrow alignment tolerances for measurements. Since the size of the marker formed on the cornea is about 3 mm, in order to image this marker on the light-receiving element, the alignment tolerance cannot be larger than the size of the marker. However, for alignment during surgery, it is necessary to move the measuring unit attached to the surgical microscope together with the surgical microscope, and it is not possible to have the patient consciously fixate on a fixed position during surgery. Therefore, alignment is extremely difficult when attaching these autokeratometers to a surgical microscope.

The second problem with ophthalmometers, which project an index onto the cornea and measure the corneal shape from the size of the projected image, is that the distance between the cornea of the eye to be examined, the index, and the imaging system (light receiving system) objective lens is very low. Since distance affects measurement accuracy, it is necessary to measure this distance accurately.

The size of the projected image changes depending on the distance between the cornea of the eye to be examined and the target. Regarding this point, a technique is known in which the projection optical system is a collimator optical system and the light flux emitted from the projection lens is made into a parallel light flux. , there is a disadvantage that the projection part becomes large.

Furthermore, it is known that even in an optical system that forms a projected image on a light-receiving element, the optical system magnification changes depending on the distance between the objective lens and the cornea of the eye to be examined, which causes measurement errors. This can also be improved to some extent by arranging a telecentric diaphragm at the image-side focal point of the substitute lens, as described above. However, in order to improve the operability of an off-salmometer that is attached to a surgical microscope, it is desirable to perform alignment using the observation system of the surgical microscope. In the case of observation using a surgical microscope, the focal length of the objective lens generally used in a surgical microscope is long, f = 175 to 200 mm, resulting in a deep depth of focus, and since the observation is performed using an aerial image, it may be difficult to measure due to changes in the operator's diopter. However, the focus position will be different, resulting in measurement errors.

As for the third problem, applications such as JP-A-56-66235 and JP-A-58-58025 to corneal shape measuring devices using two-dimensional position detection elements and image sensors are shown. However, when used attached to a surgical microscope, the illumination light from the observation system is projected onto the cornea, creating a bright illumination light image on the cornea.
This may cause a misunderstanding between the target image and the illumination light image, resulting in measurement errors.

SUMMARY OF THE INVENTION In view of the above-mentioned problems of the prior art, it is an object of the present invention to provide a corneal shape measuring device that is easy to align and has good measurement accuracy, particularly a corneal shape measuring device that is suitable for a surgical microscope.

[Means to be Solved] In order to achieve the above object, the corneal shape measuring device of the present invention includes an index projecting means for projecting a predetermined index onto the cornea of the eye to be examined, and an imaging optical system for capturing a corneal reflection image of the index. and an arithmetic means for calculating the corneal shape from the shape or position coordinates of the corneal reflection image imaged by the imaging optical system, an image extraction means for extracting the imaged corneal reflection image; a mask plate having a plurality of apertures disposed in the imaging optical system; an optical path selection means for opening and closing the optical paths passing through the apertures of the mask plate and sequentially selectively selecting the optical paths; calculation means for calculating the amount of deviation between the device and the subject's eye in the optical axis direction from the amount of deviation between the corneal reflection images that have passed through the optical path;
It is characterized by comprising a device calculated by the calculation means and a correction means for correcting the shape or position coordinates of the corneal reflection image based on the amount of deviation of the eye to be examined.

[Embodiment of the present invention] First, the measurement principle that is the premise of this embodiment will be explained. For ease of understanding, a case will be explained in which a telecentric diaphragm is arranged at the image-side focal position of the objective lens and a beam parallel to the optical axis is taken out.

FIG. 5 is a diagram explaining the measurement principle of the embodiment of the present invention, and shows a corneal reflection image formed on the cornea when an annular index is projected onto the cornea from a predetermined distance. . When an annular index (not shown) is projected onto the cornea, if the cornea is spherical, the radius a
A circle is formed. If the cornea is a toric surface, the major axis
An ellipse with b ₁ and short axis b ₂ is formed. Here, the point on the circle
Assume that (A) and (B) correspond to ellipsoids (A′) and (B′). Furthermore, it is assumed that the ellipse is tilted by (θ) from the x-axis with the origin as the center.

Here, (x, y) of the change from (A) to (A')
The components are (ΔAx) (ΔAy) and (B) to (B′), respectively.
The (x, y) components of the amount of change to (ΔBx)
(ΔBy), the following relationship holds true.

ΔAx＝b ₁ cos ² θ＋b ₂ sin ² θ−a ...(1) ΔAy＝(b ₁ − b ₂ ) sin ² θ・cos θ ...(2) ΔBx＝(b ₁ − b ₂ ) sin θ・cos θ ... ...(3) ΔBy=b ₁ cos ² θ+b ₂ sin ² θ−a ...(4) From this, b ₁ , b ₂ , and θ can be expressed by the following formula.

b ₁ =ΔBy+ΔAx+2a ±√(−) ² +4 ² …(5) b ₂ =ΔBy+ΔAx+2a−b ₁ …(6) 2θ=sin2ΔBx/b ₁ −b ₂ …(7) Furthermore, symmetrical with (A′) The position of the origin O is determined by detecting the position of point (C') and determining the center between the two points (A') and (C').

As described above, the (x, y) coordinates of points (A), (B), and (C) within the reference circle are memorized in advance, and each ( By detecting the x, y) coordinates, the corneal shape can be measured.

Next, the relationship between the elliptical shape and the corneal toric surface shape will be explained with reference to FIG.

A collimated point light source is projected onto the cornea at an angle α to the optical axis O. If the distance of the image formed at this time from the light source is b ₁ , the radius of corneal curvature Rb ₁ in this cross section can be expressed by the following equation.

Rb ₁ =b ₁ /sin α/2 (8) Similarly, the corneal curvature radius Rb ₂ when an image at a distance b ₂ from the optical axis O is formed can be expressed by the following equation.

Rb ₂ = b ₂ /sinα/2 ...(9) By substituting equations (5) and (6) into equations (8) and (9), calculate the major axis Rb ₁ and the minor axis Rb _2. I can do it.

Note that if a telecentric optical system is not used as the detection optical system, the chief ray from the corneal reflected image is not parallel to the optical axis of the eye being examined, so the position of the detected image and the desired position of the image on the cornea will differ. , it is necessary to correct this in order to ensure accuracy. However, if the position of the aperture is adjusted, the error becomes very small, so even if this error is ignored, there is no problem in practice.

Incidentally, if the angle between the principal ray from the corneal reflection image and the optical axis is Δθ, the corneal curvature radius R is It can be easily calculated.

FIG. 1 is a layout diagram of an optical system of an embodiment based on basically the same measurement principle as described above.

The light emitted from the point light sources 1a, 1b such as light emitting diodes becomes a parallel beam of light by the collimating lenses 2a, 2b, and is projected onto the cornea of the subject's eye at an angle (α) with the optical axis, forming point light source images 3a, 3b. I can do it.

Similarly, light emitted from a point light source 1c (not shown) located at a position where the point light source is rotated by 90 degrees about the optical axis forms a point light source image 3c (not shown).

The imaging lens 4 is arranged at a position where the imaging surface of the imaging element 7 and the point light source images 3a, 3b, 3c are conjugate,
A point light source image is formed on the imaging surface of the image sensor. The optical path is changed by an aperture mask 5 which is rotationally driven by a rotary solenoid 6.

FIG. 4 shows an example of the shape of the aperture mask.

9 is an open aperture and 8 is a closed aperture.

FIG. 7 is a block circuit diagram of an electrical system according to an embodiment of the present invention.

The image sensor 7 is a TV camera 21 using a CCD.
It is. There is no particular problem with this TV camera as long as it outputs standard TV signals, but a CCD TV camera is advantageous because it is small and has no image distortion. The signal from the TV camera is a composite video signal that includes a synchronization signal, but considering that the horizontal and vertical synchronization signals must be separated later, it is better to use a TV camera with separate outputs that are externally synchronized. .

The video signal from the TV camera 21 requires a synchronization separation circuit 22 if it is a composite video signal. Here, the video signal and synchronization signal are separated.
The separated synchronization signal is separated into a horizontal synchronization signal and a vertical synchronization signal by a horizontal/vertical synchronization separation circuit 28. These synchronization signals are stored in the frame memory 26.
It is used to generate frame memory addresses when writing video signals to.

Further, the video signal is amplified to an appropriate amplitude by an amplifier 23, and then inputted to an A/D converter 24 and digitized. The digitized image data is stored in the memory at the frame memory address generated by the frame address generator 29 based on the synchronization signal.

Further, the image data bus of the image data signal line after passing through the A/D converter and the read data bus of the image memory are respectively connected to the theoretical calculation unit 32, and inter-image calculations are possible. This inter-image calculation function first stores an image when the measurement point light source is off, and then subtracts it from the image data when the point light source is on, thereby eliminating the influence of observation illumination from the surgical microscope. The image data subjected to the inter-image calculation is compared with a preset brightness level by a comparator 33, and only the data of the bright portion of the image data is extracted. There are two types of data extracted here: one is a brightness value, and the other is a frame memory address, which indicates the position on the image sensor. These two pieces of data are stored in data memory 36. This processing makes it possible to extract a point light source image. The center coordinates of each point light source image are determined from the data memory 36 by well-known image processing, and the microcomputer circuit 37 performs calculation processing based on the measurement principle described above to determine the corneal shape of the eye to be examined.

Next, correction of errors in measured values when the position of the device and the eye to be examined are not at the predetermined positions will be explained with reference to FIGS. 2 and 3.

The basic idea of correcting errors in measured values due to changes in working distance is to find the amount of change in position of each point light source image obtained from images at two positions of the aperture mask, and then calculate the difference between this value and the distance between the two aperture masks. amount of change,
The amount of change in the working distance is determined and corrected from values such as the focal length of the imaging lens and the distance between the imaging lens and the imaging surface.

Figure 2 shows a specific concept for determining the working distance during measurement.

l is the reference working distance, l' is the working distance during measurement, H is the distance between the aperture masks, h is the size of the luminous flux circle during measurement,
f is the focal length of the imaging lens, m is the distance between the imaging lens and the imaging surface, and m' is the distance between the imaging lens and the imaging point at the time of measurement.

The working distance during measurement is determined as follows.

l'=m'×f/m'-f......(10) h=(m'-m)/m'×H...(11) From (11), m'=m・H/(H-h )...(12) Substituting equation (12) into (10), l'=m・H/(H-h)×f/m・H/(
H-h)-f...(13)

Furthermore, as mentioned above, in calculating the radius of corneal curvature R, since each Δθ between the optical axis and the principal ray of light from the corneal reflection image in Fig. 6 changes depending on the working distance. Calculate and use Δθ each time.

Furthermore, correction of measurement errors due to changes in magnification of the imaging optical system due to changes in working distance is determined based on the distance between the imaging lens and the cornea, and the distance between the imaging lens and the imaging surface.

FIG. 3 is a diagram showing the basic concept of this correction.

h indicates the height of the point light source image on the imaging surface at the reference working distance. As the working distance moves from l to l', the image height on the imaging surface changes from h to _h1 ' to _h2 '.
Note that H is the distance between the aperture mask and the optical axis.
If the height of the image at the imaging point at this time is g, then the following equation holds true.

h/b=m/l g/b=m'/l'g=m'/l'b
b=m'/l'l/mhH±g/H±h'=m'/m∴h=l'/lh'±l'/lH(1-m/m' Therefore, h=l' /l (h ₁ + h ₂ )/2 Therefore, by correcting the point light source image height h at the time of measurement on the imaging surface by the reference working distance l and the working distance at the time of measurement l', the correct point light source image height can be obtained. You can find h.

By the method shown above, measurement errors can be eliminated even if the working distances are different.

The radius of corneal curvature calculated as described above is displayed on the display 38 and printer 39, and is also communicated 40 as data to other devices.

Although a collimator optical system is used as the projection means in the embodiments, it is not necessary to use a collimator optical system because measurement errors due to differences in working distance can be corrected.

Generally, a surgical microscope is equipped with a television camera for observation, so when the present invention is applied to a surgical microscope, it can be used as is, and there is no need to particularly arrange an optical system for observation.

In addition to improving measurement accuracy by increasing the number of point light sources, even if the number of point light source images decreases when the corneal surface of the subject's eye is irregular, calculations can be made using the remaining number of point light sources. It becomes possible to do this.

It is clear that the present invention can also be used in a corneal shape measuring device using a telecentric optical system as a detection system.

[Effects of the Invention] As described above, according to the present invention, errors caused by a difference in working distance between the eye to be examined and the apparatus can be corrected without moving the measurement optical system.

Further, since there is no need to consider problems caused by differences in working distance between the eye to be examined and the device, the optical system can be simplified, which is advantageous for assembly and maintenance of the device, and improves operability.

[Brief explanation of drawings]

FIG. 1 is a layout diagram of an optical system according to an embodiment of the present invention.
FIGS. 2 and 3 are explanatory diagrams for explaining correction of errors in measured values, and FIG. 4 is a diagram showing an example of the shape of an aperture mask.
FIG. 5 is an explanatory diagram for explaining the measurement principle of the embodiment of the present invention, FIG. 6 is an explanatory diagram for explaining the relationship between the corneal projection image and the angular toric surface shape, and FIG. 7 is an explanatory diagram for explaining the measurement principle of the embodiment of the present invention. FIG. 3 is a block circuit diagram of the system. 4...Objective lens, 5...Aperture mask, 7...
Image sensor, 21...TV camera, 22...Synchronization separation circuit, 26...Frame memory, 32...Logic operation unit, 33...Comparator circuit, 36
...Data memory, 37...Microcomputer circuit.

Claims (1)

[Scope of Claims] 1. An index projecting means for projecting a predetermined index onto the cornea of the eye to be examined, an imaging optical system for capturing a corneal reflection image of the index, and a corneal reflection image captured by the imaging optical system. A corneal shape measuring device comprising a calculation means for calculating the corneal shape from the shape or positional coordinates, comprising: an image extraction means for extracting the captured corneal reflection image; and a mask having a plurality of apertures disposed in the imaging optical system. a plate, an optical path selection means for sequentially selectively selecting an optical path by opening and closing an optical path passing through the aperture of the mask plate; A calculation means for calculating the amount of deviation between the device and the eye to be examined in the axial direction, and a correction means for correcting the shape or position coordinates of the corneal reflected image from the amount of deviation between the device and the eye to be examined calculated by the calculation means. A corneal shape measuring device characterized by: