JPS63181735A - Apparatus for measuring shape of cornea - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an ophthalmometer that automatically measures the corneal shape of an eye.

[Prior Art] 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. 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 fingertips of the surgeon.

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 principles of these devices. For example, JP-A-58-54927, JP-A-61-85920, and the like.

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 approximately 3Mn, in order to image this marker on the light receiving element, the alignment tolerance cannot be set larger than the size of the marker. However, for alignment during surgery, it is necessary to move the measurement unit attached to the surgical microscope together with the surgical microscope, and it is not possible to set a fixation target for the patient during surgery or have him or her fixate it. 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 telecentric optical system and the light flux emitted from the projection lens is a parallel light flux. The disadvantage is that the portion 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 telescopic diaphragm at the image-side focal point of the objective lens, as described above. However, in order to improve the operability of an off-salmometer attached to a surgical microscope, it is desirable to perform alignment using the observation system of the surgical microscope. When performing focusing, the focal length of the objective lens commonly used in surgical microscopes is long, f = 175 to 200 m, so the depth of focus is deep, and since the observation is performed using an aerial image, it may be difficult to adjust due to changes in the operator's diopter. The focus position will be different and a measurement error will occur.

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 pickup devices 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, which can lead to misidentification of the finger edge image and the illumination light image, resulting in measurement errors. This may cause

[Object of the Invention] In view of the above-mentioned problems, an object of the present invention is to provide an ophthalmometer that is not affected by illumination light flux, etc. and is easy to align, particularly an ophthalmometer suitable for attachment to a surgical microscope. .

[Summary of the Invention] In order to achieve the above object, the present invention provides a projection means for projecting an index onto the cornea of an eye to be examined, an imaging optical system for forming an image of the index on an image sensor, and a projection means for projecting an index onto the cornea of an eye to be examined; An imaging optical system for guiding an image onto an image sensor, means for digitizing a video signal from the image sensor and storing it in an image memory, and means for extracting only images necessary for corneal shape measurement from the stored video signal. It is characterized by having means for calculating the corneal shape from the positional relationship of the extracted images.

[Example of the present invention] First, the measurement principle that is the premise of the present invention will be explained.

FIG. 5 is an explanatory diagram illustrating the measurement principle of the embodiment of the present invention, and is assumed to represent a corneal reflection image formed on the cornea when an annular index is projected onto the cornea. When an annular index (not shown) is projected onto the cornea, a circle with radius a is formed if the cornea is spherical. When the cornea is a toric surface, an ellipse with major axis b; and minor axis b2 is formed. Here, it is assumed that points (A) and (B) on the circle correspond to points (A') and (8') on the ellipse. Furthermore, it is assumed that the ellipse is tilted by (e) from the X-axis with the origin as the center.

Here, (x) of the amount of change from (A> to (A').

y) component respectively (ΔAX>(ΔAy>, the (x,,y) component of the amount of change from (B) to (B) respectively (ΔB
X)(ΔBy), the following relationship holds true.

ΔA x=b+co? e+b, Ls l ne
-a ・ (1) ΔAV=(bl-bl)s
i ne −cO8e−(2)ΔBx= (bl−b
2) sinO−cosθ・(3)Δ5y=b+s
I ne+b2cO5θ-a-(4) From this, bl
Sb2, θ can be expressed by the following formula.

(5) Find the position of the origin O by finding the center between the two points (A') and (C').

Above, each (x, y) of points (A), (B), and (C) on the reference circle
In addition to memorizing the coordinates in advance, each point (A') (B') (C') formed by the cornea of unknown shape (
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. 6.

A point light source collimated at an angle α with respect to the optical axis O is projected onto the cornea. If the distance from the optical axis of the image formed at this time is bl, then the corneal curvature radius R in this cross section
bl can be expressed by the following equation, and similarly, the radius of corneal curvature RbJ when an image at distance b2 from the optical axis 0 is formed can be expressed by the following equation.

By doing so, the major axis Rb1, the minor axis Rb2 . can be found.

FIG. 1 is a layout diagram of an optical system according to an embodiment of the present invention based on this measurement principle.

The light emitted from the point light sources (1a), (1b) such as light emitting diodes is turned into a parallel light beam by the collimating lenses (2a) (2b), and is projected onto the cornea of the subject's eye at an angle (α), forming a point light source image ( 3a) (3b) can be done.

Similarly, light emitted from a point light source 7C (not shown) located at a position rotated 900 degrees around the optical axis is a point light source image (3
Create C).

The imaging lens 4 connects the imaging surface of the image sensor 7 and the point light source (1a) (
1b) and 1c are conjugated, and a point light source image is formed on the imaging surface of the imaging element. 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 frontage 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. There is no particular problem with this TV camera as long as it outputs a standard TV signal, but a C0D 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 horizontal and vertical synchronization signals must be separated later, a TV camera with external synchronization type separate type outputs is preferable.

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 used to generate a frame memory atomless when writing a video signal to the frame memory 26.

Further, the video signal is amplified to an appropriate amplitude by an amplifier 23, and then input to a converter 24 where it is 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.

Furthermore, the image data bus of the image data signal line after passing through the converter and the read data bus of the image memory are respectively connected to the logic operation unit 32, and inter-image operations 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 part of the image data is extracted. There are two types of data taken 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 process makes it possible to extract a point light source into a luminous flux. Data memory 36
The center coordinates of each point light source image are determined from the above, and the microcomputer circuit 37 performs calculation processing based on the measurement principle described above to determine the shape of the cornea of the subject.

Next, correction of errors in measured values 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. The amount of change in the working distance is determined and corrected from values such as the amount of change, 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.

1 is the standard working distance, 1' 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, and m is the distance between the imaging lens and the imaging surface. distance, m
l indicates the distance between the imaging lens and the imaging point during measurement.

The working distance during measurement is determined as follows.

Substituting equation (3) from (2) into (1) As one of the items for correcting measurement errors, α in equations (8) and (9) above changes due to the working distance in Fig. 6. Calculate and use 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 by 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 point light source image height. As the working distance moves from 1 to d, the image height on the imaging surface changes from h to h. Therefore, the following formula holds.

h completed -;→l-1m=hl Correct point light source image height h can be determined by correcting the quasi-working distance 1 and the working distance 1' during measurement.

The method described above can eliminate measurement errors even if the working distance differs.

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

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 performed using the remaining number of point light sources. It becomes possible to do so.

In addition, by using an image sensor and frame memory, flexible processing including inter-image arithmetic processing is possible.
For example, in this embodiment, measurement is performed using three point light sources, but it is now possible to measure not only point light sources but also indicators on a ring by simply changing the calculation processing program.

[Effects of the Invention] As explained above, according to the present invention, measurement is possible if there are at least three point light source images projected on the cornea, and alignment is easy by using an image sensor. This also makes it possible to eliminate measurement errors due to differences in working distance.

[Brief explanation of the drawing]

FIG. 1 is a layout diagram of an optical system according to an embodiment of the present invention, FIG.
FIG. 3 is an explanatory diagram illustrating correction of errors in measured values, FIG. 4 is a diagram illustrating an example of the shape of an aperture mask, FIG. 5 is an explanatory diagram illustrating the measurement principle of the embodiment of the present invention, and FIG. The figure is an explanatory diagram for explaining the relationship between the corneal projection image and the shape of the corneal toe link surface, and FIG. 7 is a block circuit diagram of the electrical system of the embodiment of the present invention. 4...Objective lens 5...Aperture mask 7...Image sensor 21...T
V camera 22... Synchronization separation circuit 26...
...Frame memory 32...Logic operation unit 33...Comparator circuit 36...
data memory

Claims (5)

[Claims] (1) a projection means for projecting an index onto the cornea of the eye to be examined; an imaging optical system for forming the index image on an imaging element;
A means for digitizing a video signal from an image sensor and storing it in an image memory, a means for extracting only images necessary for corneal shape measurement from the stored video signal, and calculating a corneal shape from the positional relationship of the extracted images. A corneal shape measuring device characterized by having a means for measuring the shape of a cornea. (2) The corneal shape measuring device according to claim 1, wherein the projection means is a projection means that projects at least three point images on the same circumference onto the cornea of the eye to be examined. . (3) A mask plate having two or more apertures and a shutter mechanism for selecting an optical path that makes one of the apertures effective are disposed in front or behind the imaging lens in the imaging optical system. ,
Claim 1, characterized by comprising means for measuring the distance from the image formed by each aperture to the eye to be examined, and correcting corneal shape measurement data based on this result.
The corneal shape measuring device according to item 1 or 2. (4) The calculation means is capable of calculating image data stored in the image memory with the measurement light source turned off and newly inputted image data with the light source turned on. A corneal shape measuring device according to items 1 to 3. (5) The corneal shape measuring device according to any one of claims 1 to 4, which is attached to a surgical microscope.