Apparatus for Subjectively Measuring the Refractive Power of an Eye

1. Field of the Invention

This invention relates to an apparatus for subjectively measuring the refractive power of an eye.

2. Description of the Prior Art

The method of measuring the refractive power of the eye is grouped into the subjective examination and objective examination. Generally, however, objective examination is carried out to obtain the aim of the refractive power of the eye and finally, the refractive power of the eye is determined by the subjective examination.

In the conventional subjective examination, the refractive power of the eye has usually been determined with visual acuity as the standard. That is, a visual acuity measurement has been carried out by the use of a visual acuity examination mark and the refractive power of the eye has been determined with the measured visual acuity as the standard. In some cases, it has been difficult to judge from which visual acuity the refractive power of the eye should be obtained. This is because it cannot always be said that the best visual acuity is the best refractive power. This is known as the problem of over-rectification. Also, the visual acuity measurement itself has been carried out by comparing objects before and after in terms of time, and therefore, it has been very difficult for the examinee to judge which of the objects before and after can be seen better, causing fatigue of the examinee.

It is a first object of the present invention to provide an apparatus for subjectively measuring the refractive power of an eye which can measure the refractive power of the eye to be examined independently of the visual acuity of the eye and which forces no difficult judgments upon the examinee during measurement.

It is a second object of the present invention to provide an apparatus for measuring the refractive power of an eye which can measure the refractive power of the eye simply by the examinee judging whether two images have become coincident with each other.

The invention will become more fully apparent from the following detailed description of an embodiment thereof taken in conjunction with the accompanying drawings.

FIG. 1 shows the construction of an embodiment of the present invention.

FIG. 2 shows a first diaphragm, a device for rotating it and a device for displaying the direction of the astigmatism main diametral line.

FIG. 3 is a plan view of a second diaphragm.

FIG. 4A shows a usually observed image of an examinee having an astigmatic eye.

FIG. 4B shows the image when two images are partially coincident with each other.

FIG. 4C shows the image when two images are completely coincident with each other.

FIG. 4D shows the image when the direction of the diaphragm opening is coincident with the direction of one astigmatism main diametral line of the examinee.

FIG. 5 shows an example in which an optical device for rotating a diaphragm 4 and a diaphragm 7 in synchronism with each other is added to the arrangement of FIG. 1.

FIG. 6 shows a moving device for a focusing prism and a device for displaying the refractive power.

FIG. 1 shows the optical system of an embodiment of the present invention. The optical system has an optical path extending from E.sub.1 to 8, as explained hereinafter.

In the reflection optical axis of an obliquely disposed half-mirror 1 (the reflection optical axis being part of the optical path), there are provided, at successive positions along the optical path, a first imaging lens 2, a reflecting mirror 3, a first diaphragm 4 having a pair of openings 4a and 4b (FIG. 2) formed symmetrically with the optical path, a second imaging lens 5, a focusing prism 6, a second diaphragm 7 formed with a quasi-rectangular opening 7a (FIG. 3) having a protruding central portion, and a light source 8. The first imaging lens 2 renders a predetermined position at one side of lens 2 (the cornea of the examinee's eye is set at this position) conjugate with the first diaphragm 4 at the opposite side by alignment. The first diaphragm 4, as shown in FIG. 2, is rotatable about the optical axis by rotating a rotatable knob 42 having a gear 41 meshing with a gear 40 formed on the outer periphery of the first diaphragm. The amount of rotation of the knob 42 is read by an angle reading device, for example, a position encoder or a potentiometer. In FIG. 2, a rotary encoder 43 having a rotatable disc secured to the knob 42 is provided and the angle read by this encoder 43 is applied as input to a converter circuit 44 for converting said angle into a degree of astigmatism, whereby the degree of astigmatism is displayed. Also the focusing prism 6, as shown in FIG. 6, is movable to change the length of the optical path between diaphragm 4 and 7 (as indicated by the arrow in FIG. 1) by rotation of a pinion 61 meshing with a rack member 60 integral with the focusing prism 6, and by the movement of the focusing prism 6, the effective position of the second diaphragm 7 relative to the first diaphragm 4 can be varied. The amount of movement of the focusing prism 6 is displayed by converting the output of a rotary encoder 62 coupled to the pinion 61 into a frequency. The opening 7a formed in the second diaphragm 7 is such as shown in FIG. 3, for example, and the optical path is so disposed as to pass through the center of the opening 7a, and the second diaphragm 7 is rotatable about the optical path by a rotating mechanism similar to the diaphragm 4 shown in FIG. 2.

A main body 10 holding the above-described optical system is disposed for three-dimensional movement on a base 11 by means of a well-known mechanism, for example, a stand 12 used in a slit lamp microscope or an eye fundus camera well known as an ophthalmologic instrument, and a fixing pedestal 13 for fixing the examinee's head thereon is secured to the base 11. Alignment between the examinee's eye E.sub.1 fixed to the fixing pedestal 13 and the main body 10 is carried out so that the diaphragm 4 becomes conjugate with the cornea of the examinee's eye E.sub.1. Various means therefor are conceivable, and for example, the design may be such that the intensity of the light source 8 is increased and the stand 12 is moved until the examiner who observes the examinee's eye E.sub.1 through the half-mirror 1 confirms that the image of the diaphragm 4 is formed on the surface of the cornea of the examinee's eye E.sub.1. Of course, focusing may be effected by movement of the first imaging lens 2 in the direction of the optical axis. However, where the stand 12 is one as described above, the first imaging lens 2 may be stationary. If the first imaging lens 2 is stationary, the main body 10 of the apparatus can be moved in the base 11 so that the cornea of the examinee's eye E.sub.1 comes to a position conjugate with the first diaphragm 4 by the first imaging lens 2.

In such a condition, when the examinee having an astigmatic eye is looking forward through the half-mirror 1, the image is shown in FIG. 4A, is observed by the examinee. (The portion indicated by hatching is light.)

This case is a condition in which the direction of the astigmatism axis of the examinee's eye E.sub.1 is not coincident with the direction of the opening in the diaphragm 4, namely, the most usual condition.

Therefore, the pinion 61 is first rotated to move the focusing prism 6 to change the length of the optical path between diaphragms 4 and 7. When the two images become partially coincident with each other as shown in FIG. 4B, the focusing prism is moved until the examinee responds. At that time, the amount of movement of the prism 6 corresponds to the refractive power of the examinee's eye E.sub.1. The refractive power is displayed by the converter circuit 63.

Subsequently, the diaphragm 4 and the diaphragm 7 are rotated and, when the two images become completely coincident with each other, the focusing prism is moved until the examinee responds (FIG. 4C). The then angle of rotation of the diaphragm 4 corresponds to the direction of the astigmatism main diametral line of the examinee's eye E.sub.1. The direction of the astigmatism main diametral line is displayed by the converter circuit 44.

Accordingly, through the above-described operation, the direction of one astigmatism main diametral line in the astigmatic eye E.sub.1 and the refractive power of the eye in this direction can be known.

Subsequently, the diaphragm 4 and the diaphragm 7 are further rotated by 90.degree. and thereafter, if the focusing prism 6 is moved until the examinee responds when the two images become coincident with each other, the refractive power of the eye for the direction of the other astigmatism main diametral line can be known from the then position of the focusing prism 6. This value is also displayed in a manner similar to what has been described above.

If the direction of the opening in the diaphragm 4 is coincident with the direction of one astigmatism main diametral line of the examinee's eye E.sub.1, the examiner can observe the image as shown in FIG. 4D and therefore, in this case, the coincident image as shown in FIG. 4C can be made visible merely by moving the focusing prism 6.

It will be seen that the movement of the focusing prism 6 may take place earlier than the rotation of the diaphragms 4 and 7 or the latter may take place earlier than the former, but it is very difficult to judge, by rotating the diaphragms 4 and 7 that the two images in the condition of FIG. 4A have been vertically arranged as shown in FIG. 4D, and therefore, it is preferable to design the apparatus such that the focusing prism 6 is first moved and then the diaphragms 4 and 7 are rotated.

Where the examinee's eye E.sub.1 is not an astigmatic eye, irrespective of the angle of rotation of the first diaphragm 4, the image of the diaphragm is never seen skewed as shown in FIG. 4A. Also, when the initial position of the focusing prism 6 lies at a position corresponding to zero diopter, the image of the diaphragm will always be seen spaced apart by an amount corresponding to the frequency thereof as shown in FIG. 4D.

In the foregoing description, the diaphragm 4 and the diaphragm 7 have been described as being rotated in synchronism with each other. Although the mechanisms therefor may be coupled together so that the diaphragms 4 and 7 are rotated in synchronism with each other by transmitting the rotation of the shaft 42 of FIG. 2 through an intermediate gear or the like, it is preferable to use optical processing. That is, if, as shown in FIG. 5, an image rotating prism 50 is inserted between the objective lens 2 and the diaphragm 4 and rotation of this prism 50 is utilized, the construction will become simpler.

The opening in diaphragm 7 forms a "mark" that is projected onto the eye to be examined. In the form shown, the mark is generally rectangular with a portion central to the length that is expanded in the direction of the width. The shapes of the openings in the diaphragms 4 and 7 are not restricted to the shown shapes, but may be other shapes, for example, circular shape, and the diaphragm 7 and the light source 8 may be replaced by marks which themselves emit light.

Further, the diaphragm 4 and the diaphragm 7 need not rotate together with each other, but at least the diaphragm 4 alone should rotate. The focusing prism 6 need not always be used to change the position of the diaphragm 7 relative to th diaphragm 4, but the diaphragm 7 may be made movable along the optical path. Still further, a wedge-shaped prism may be made insertable between the diaphragm 4 and the diaphragm 7 to vary the length of the optical path.

The half-mirror 1 may be one through which the examinee can see a long distance, whereby the examinee's eye is in relaxed condition and can see the image of the diaphragm 7 overlapped with a distant image.