GB2177813A - Keratometer - Google Patents

Abstract

A keratometer has a pair of mires 2, 4 on opposite sides of an optical axis and illumination from the mires is directed onto a surface an optical characteristic of which is to be measured. A pair of optical doubling elements 18 are disposed in the path of the reflected illumination from the surface. The elements are rotatable to displace images of the mires in opposite directions transverse to the axis for measuring the chosen characteristic. With the rotation of the elements a position-responsive means 52 is arranged to rotate through an angle less than 360 DEG . The position-responsive means is provided with reading means 56, 58 actuating an input to an electrical circuit (66 Fig. 3) dependent on its angular position. The circuit has a memory store (66) for relating the angular position to values of the optical characteristics to be determined from the reflected images, and means 68 for displaying the optical characteristic values so determined. <IMAGE>

Description

SPECIFICATION Keratometer This invention relates to a keratometer, an optical instrument used to measure the cornea, in particular its radius of curvature, on which can also measure similar characteristics of a contact lens.

The cornea is essentially spherical and transparent, and acts as a refracting surface in the eye. Owing to its high surface finish and constant tear film it also acts as a convex mirror, creating a diminished, upright virtual image behind the corneal surface (as in a conventional convex mirror) of an object in front of the eye. The keratometer employs this principle, directing onto the surface to be measured an illuminated target, representing an object of known width, and measuring the image. If the distance of the target from the eye is large compared with the virtual image distance behind the surface, which is a function of the curvature of the surface, the relationship between the sizes of the target and the image is determined by the radius of curvature.Thus, by using a target of known size, the image size will provide a measure of the radius of curvature of the corneal surface.

As an example, in GB 1 514 802 a keratometer is described in which two mires or slitform beams defining a target are projected onto the cornea, thus effectively representing the opposite lateral limits (left and right) of an object. A part of the reflection of each mire from the surface of the cornea is deflected by an image doubling element which is adjustably controlled to bring the left-hand image of the mire representing the object right-hand limit coincident with the right-hand image of the mire representing the object left-hand limit, so that these two reflected images are brought into line in a viewing eyepiece. The adjustment to produce the required amount of deviation to have this effect is clearly related to the undeviated image size and is thus dependent on the radius of curvature.

The doubling element inevitably deflects both mires in the same direction so that the images being manipulated are both displaced laterally to one side of the optical axis of viewing, which can be inconvenient, and it is also known to employ two doubling elements which are interconnected to move in opposition to each other so as to produce equally and oppositely deflected images of the two mires, whereby one of the adjusted reflected images of each mire is limited to a central region of the field of view.

A less tractable difficulty in the use of known keratometers is, however, that the relationship between the radius of curvature and the amount of deflection required to bring the images of the mires together is not exactly linear. Also, some operators find it difficult to read the micrometer scales that are associated with the doubling device adjustment mechanism in known instruments to indicate the radius of curvature at any specific deflection setting. A further problem arises when a negative radius of curvature is to be maintained, such as the rear surface of a contact lens, when the optical geometry is slightly different and it is necessary to apply corrections to the micrometer scale that has been designed for the measurement.

According to the present invention, there is provided a keratometer having two laterally spaced mires on opposite sides of an optical axis for projecting illumination onto a surface to be measured, a pair of optical doubling elements in the path of the reflected illumination from said surface, means for rotating the elements through equal and opposite oblique angles of adjustment to the optical axis to displace images of the respective mires oppositely in a direction transverse to said axis, position-responsive means connected to the element-rotation means to be displaced in dependence to the adjustment of said elementrotation means over its operating range, reading means associated with said position-responsive means for actuating an input to an electrical circuit determined by the angular position of the position-responsive means, a memory store for said circuit to relate the angular position values read from said positionresponsive means to values of the optical characteristics to be determined from the reflected images, and means for displaying said optical characteristic values at desired angular positions of the doubling elements.

In a preferred arrangement, the position-responsive means takes the form of a rotary device which, advantageously, is arranged to be displaced by less than 360 in the adjustment of said element-rotation means over its operating range so that the device itself has a unique angular position of displacement for each position of adjustment. Other forms of position-responsive means, e.g. linearly displaceable, may be employed, however.

Preferably, the memory store contains values for a plurality of different optical characteristics related to the adjusted positions of said position-responsive means and control means are provided to display said characteristic values selectively for particular settings of the doubling means.

An embodiment of the invention will be described in more detail by way of example with reference to the accompanying schematic drawings, in which: Fig. 1 illustrates the optical system in a keratometer according to the invention, Fig. 2 is a layout of mechanical elements of the keratometer, Fig. 3 is a block diagram of the electronic circuit of the keratometer, and Fig. 4 illustrates a setting as seen in the field of view of the optical system of Fig. 1.

In the optical system of Fig. 1, a target is formed by two mires, 2, 4, preferably coloured red and green respectively to avoid confusion in use, in the form of relatively narrow rectangular cross-section beams, with their longer axes perpendicular to the plane of the figure. The illumination from the mires is projected onto the cornea of an eye 6, from which it is reflected, a virtual target image 8 lying in the image plane behind the reflecting front surface 10 of the cornea corresponding to the image of an object, the width of which is represented by the distance between the mires. The image is viewed by a microscope comprising an objective 12 and an eyepiece 14, a reference graticule 16 being placed at the front of the eyepiece. In these respects the optical system is similar to that shown in GB 1 514 802.

Disposed between the eye and the microscope objective 12 are a pair of transparent doubling plates 18 arranged one above the other, each in the path of the reflected illumination from the eye. The two plates are pivoted on a common axis 20 intersecting the optical axis 22 of the system and directed parallel to the main axes of the mires which are disposed symmetrically to the optical axis.

The pivoting of the two plates is interlinked so that they always lie at equal and opposite angles to the optical axis and they can be jointly rotated in opposite directions, as will be described in more detail below.

As represented in Fig. 1 the doubling plates 18, which are simply optical flats, are already set at the angle which align the appropriate images of the mires so as to provide a reading from the instrument. That is to say, the image of the reflected light from the red mire 2 has been split into two beams by the oppositely angled doubling plates, with the illustrated beam 32a of the red mire image emerging from one of the plates along the optical axis 22 and the beam 32b emerging from the other of the plates is displaced to be further from the optical axis, while an equal and opposite effect has been achieved with the reflected beams 34a, 34b from the green mire. The two outer images 32b, 34b are not used for measurements.

It may be mentioned here that the mires are given a significant width, especially because the proportion of luminous flux intensity reflected will be low (less than 4%). Since only the outer boundary defining the target "edge" is significant the profile of each mire may be configured to identify the relevant side boundary. This is indicated in Fig 4, where the straight side boundaries are used for measurement and the opposite, notched side boundaries are ignored. Fig. 4 shows the view through the eyepiece 14 at the point at which the instrument has been adjusted to give a reading. The hatched areas represent the red mire images and the similar unhatched areas the images of the green mire.It can be seen that manipulation of the doubling plates has been carried out so that the two inner images 32a, 34a have their opposed straight edges, representing opposite edges of the target, touching each other at the centre of the graticule 16~that is to say the viewed target image has been apparently reduced to zero width by the angular setting of the doubling plates.

Referring to Fig. 2, the illuminated mires 2, 4 and the doubling plates 18 are illustrated in plan. The illumination for the mires may be provided by diodes giving an emission of a suitable hue, but of a higher intensity of illumination is required it can conveniently be obtained using fibre optic light guides to carry the light from a more remote source. The doubling plates 18 are attached to links 36 which have a common pivot 38 on a rod 40 of square cross-section held non-rotatably in the frame 40 of the instrument. Screwed partly into the rod 40 is a threaded portion 44 of a spindle 46 which is retained rotatably but is axially fixed in the instrument frame 42 and can be turned by an operating knob 48 on the exterior of the instrument. The rotation of the spindle drives the rod 40 axially and pivots the two doubling plates 18 equally and oppositely.Locked onto the spindle 46 is a pinion gear 50 which meshes with a much larger gear-wheel 52 on a parallel shaft 54.

The gearing is such that over the full range of angular movement of the doubling plates the larger gear-wheel rotates through slightly less than 360 .

An encoding disc 56 is also fixed to the shaft 54 and comprises apertures that define a binary-coded indication of the angular position of the shaft over its range of movement.

Light sources 58 direct radiation, e.g. visible or infra-red, through the binary-coded apertures onto the ends of fibre optic guides 60 in fixed positions on the opposite side of the disc and the guides carry the coded information to photo-sensors 62 mounted on a circuit board 64. As shown in Fig. 3, the electrical signals thus generated are fed to an EPROM 66 functioning as a code converter, translating the signal of the angular position of the shaft 54 into the appropriate optical characteristics of the associated angle of obliquity of the doubling plates 18. The EPROM outputs drive a visual display 68.

The gearing between the hand knob 48 and the encoding disc 56 may give about eight revolutions of the spindle 46 over the range of movement of the disc, so that the simplicity of the photo-electrical reading system does not impair the sensitivity of the manual adjustment mechanism. Another feature of the adjustment mechanism is that the pinion gear 50 is secured to the spindle 46 by a lock-nut 70 disposed immediately behind the knob 48 so that it is easily accessible from the exterior. It is therefore possible to calibrate the instrument by sighting onto a surface of known radius of curvature to give a matching read-out, then releasing the lock-nut and adjusting the doubling plates until the viewed images touch, and finally tightening the locknut to recouple the adjustment mechanism and the reading system which are now in a matched setting.

In a typical application to the measurement of the radius of curvature of the cornea, the measurement values may be required either in terms of radius of curvature (mm) or in terms of diopters. Series of both perameters are stored in the EPROM 66 and the printed circuit board 64 also carries a timing circuit 72 which automatically switches the EPROM so that at any setting of the doubling plates the two different parameters alternate every few seconds in the display.

As has already been mentioned, in the measurement of a negative radius of curvature, the values represented by a particular setting of the doubling plates cannot simply be represented by changing the sign of the positive radius of curvature value. The EPROM also contains a series of modified values associated with negative (concave) curvature; operating a changeover switch 74 on the instrument will cause the EPROM to feed these values to the display instead.

It may be noted that the construction described with reference to the drawings, including such features as the use of diodes as light sources, results in an instrument that is compact and easily portable.

Claims (10)

1. A keratometer having: (i) two mires laterally spaced on opposite sides of an optical axis for projecting illumination onto a surface to be measured, (ii) a pair of optical doubling elements in the path of the reflected illumination from said surface, (iii) means for rotating the elements through equal and opposite oblique angles of adjustment to the optical axis to displace images of the respective mires oppositely in a direction transverse to said axis, (iv) position-responsive means connected to the elementrotation means to be displaced in dependence on the adjustment of said element-rotation means over its operating range, (v) reading means associated with said positionresponsive means for actuating an input to an electrical circuit determined by the angular position of the position-responsive means, (vi) a memory store for said circuit to relate the angular position values read from said position-responsive means to values of the optical characteristics to be determined from the reflected images, and (vii) means for displaying said optical characteristic values at desired angular positions of the doubling elements.

2. A keratometer according to claim 1 wherein the position-responsive means is a rotary device arranged to be displaced through an angle of less than 3600 in the adjustment of the element-rotation means over its operating range.

3. A keratometer according to claim 1 or claim 2 wherein the memory store contains values for a plurality of different optical characteristics related to the range of adjustment of said doubling elements.

4. A keratometer according to claim 3 wherein control means are provided to display the values of said different optical characteristics selectively for particular settings of the doubling elements.

5. A keratometer according to claim 4 wherein, at chosen settings of the display means, automatic switching means are arranged to present the different optical characteristics alternately.

6. A keratometer according to any one of claims 3 to 5 wherein the memory store contains values and/or algorithms for characteristics related to both positive and negative radii of curvature of a surface to be measured.

7. A keratometer according to any one of the preceding claims wherein the mires are illuminated by diodes.

8. A keratometer according to any one of claims 1 to 6 wherein the mires are provided with illumination through optical light guides from a remote lighting source.

9. A keratometer according to any one of the preceding claims wherein releasable coupling means are provided between said rotational means for the doubling elements and said means for the rotation of said positionresponsive means, whereby to calibrate the positions of the doubling elements to the values at a setting of the position-responsive means.

10. A keratometer constructed and arranged for use and operation substantially as described herein with reference to the accompanying drawings.