GB2152693A

GB2152693A - Optometer with astigmatism measurement

Info

Publication number: GB2152693A
Application number: GB08333524A
Authority: GB
Inventors: Ian Dennison Carter
Original assignee: TRANSPORT SECRETARY OF STA
Current assignee: TRANSPORT SECRETARY OF STA
Priority date: 1983-12-16
Filing date: 1983-12-16
Publication date: 1985-08-07
Also published as: GB8333524D0

Abstract

An optometer for the measurement of focussing error and for the detection and measurement of the characteristics of astigmatism giving an external view to allow the eye (2) under test to focus at a known distance, having a test image being a ring with fiducial marks (10c, 10d) superimposable upon the external view (3), said test image being mounted in an axially rotatable member (17) adjustable to provide first and second linear test images at right angles by adjustment of the optometer to a configuration at which the first and second focussing errors of an astigmatic eye may be determined, the radial position of the astigmatism being indicated by rotation of the test image to place the fiducial in the centre of the first or second linear test image. <IMAGE>

Description

SPECIFICATION Optometer with astigmatism measurement The present invention relates to optometers, particularly optometers having the means to measure astigmatism.

Astigmatism in an eye arises from aspherical curvatures of the refracting surfaces which prevent the focussing of light rays to a common point on the retina. The result of the aspherical curvatures of the refracting surfaces is that the overall eye focussing power may be resolved into two different focussing powers lying normally to one another.

In order to provide for the correction of astigmatism the optical characteristics of the astigmatic eye, that is the two focussing powers and the radial positions of those focussing powers must be determined.

Known optometers for the indication and measurement of astigmatism require the eye under test to be accurately located in relation to the optical path of the optometer, for instance by the use of a jaw rest and skull cramp. Also, known optometers for astigmatism measurement do not allow the eye under test an uninterrupted external view. This increases the time taken to obtain the required readings.

The inventive optometer system, usable to detect and measure astigmatism, may be sufficiently accurately located relative to the eye under test without the use of a jaw rest and skull cramp, allow the eye under test an uninterrupted external view and may be made both portable and personally usable.

According to the present invention there is provided an optometer for the measurement of focussing error and for the detection and measurement of the characteristics of astigmatism giving an external view to allow the eye under test to focus at a known distance, having a test image being a ring with a fiducial mark superimposable upon the external view, said test image being axially rotatable, the optometer being adjustable to provide first and second linear test images at right angles by adjustment of the optometei to a configuration at which the first and second focussing errors of an astigmatic eye may be determined, the radial position of the astigmatism being indicated by rotation of the test image to place the fiducial in the centre of the first or second linear test image.

Focussing of the test image over the range of target distances is accomplished by the test image being of small aperture relative to the eye lens aperture, giving the eye lens a wide depth of field of focus for the test image, but not for the external view.

An advantage of the inventive optometer system lies in the continuous view the eye under test has of the target placed at a known distance from the eye allowing rapid adjustment of the optometer to the configuration at which first and second focussing errors may be determined.

A diffusely transmitting screen or an opaque cap may be placed to interrupt the external view to allow the testing of empty field myopia.

The optometer system may be incorporated into a portable unit suitable for personal operation.

An embodiment of the invention will now be described with reference to the accompanying drawings, in which Figures 1A, B show schematically an optometer incorporating the invention; Figures 2A, B, C, D, E, F, G show the view presented to an astigmatic eye.

In Fig. 1A an optometric system indicated by the broad arrow 1 is shown in cross section. The eye 2 to be tested observes a target 3 through the combining cube 4. The combining cube serves to superimpose the optometric test image on the view of the target seen by the eye. The combining cube is rigidly mounted on the inner barrel 5 of the optometer and contained within the inner barrel is a Badal lens 6. The outer barrel 7 of the optometer is axially slidable on the inner barrel and is fixed radially by the peg 8 mounted on the inner barrel sliding in the cooperating slot 9 in the outer barrel. The demountable end 17 of the outer barrel contains a high intensity green light emitting diode 9 and power supply (not shown) which illuminates a translucent area 10 in the first opaque screen 11.The pinhole target 13 in a second opaque screen 14 is illuminated by light from the translucent area 10 which has passed through the collimating lens 12. The demountable end 17 of the outer barrel is located on the outer barrel 7 by means of an interlocking circumferential groove 18, on the barrel 7 and a ridge 19 on the demountable end 17. The demountable end is rotatable relative to the outer barrel 7, carrying the first opaque screen 11 with it. A measurement scale 20 is marked on the outer barrel to be read relative to a mark 21 on the demountable end 7.

A measurement scale 22 is marked on the outer surface of the inner barrel, to be read relative to the position of the end 23 of the outer barrel.

Fig. 1 B shows the view on section AA of Fig. 1A. The wall of the outer barrel 7 is shown dashed for clarity. The opaque screen 11 has the translucent area 10 in the form of two circular areas 1 or, 1 0B separated by areas 1 0C and 1 0D of the opaque screen 11.

The opaque areas 1 0C and 1 0D act as fiducial marks on the test image presented to the eye.

In operation the optometer with a target at infinity is held horizontally by the user with the optometer located relative to the eye by an eye surround (not shown) which rests on the user's face.

The view presented to an astigmatic eye during the course of measurement required to obtain the optical characteristics of the eye is shown in Fig. 2A, B, C, D, E, F and G.

In Fig. 2A the target 3 has superimposed upon it an ellipse 31 with the fiducial marks 1 0C and 1 OD appearing as dark regions in the circumference of the ellipse. Movement of the outer barrel 7 along the inner barrel 5 will cause the ellipse to collapse toward the major axis 32 and at the first focussing power measurement position as shown in Fig. 2B the ellipse collapses to a line 33 containing the fiducial marks 1 OC and 1 or. Rotation of the demountable end 17 relative to the outer barrel 7 will cause the fiducial marks 1 OC and 1 OD to coalesce as shown in Fig. 2C at the centre of line 33. At this position the radial position of the first of the focussing powers of the eye may be measured. Further movement of the outer barrel will cause the line to broaden to an ellipse and then to change to an ellipse with major axis normal to the line, as shown in Figs. 2D and 2E, and eventually the ellipse will collapse to a line as shown in Fig. 2F at which point the second focussing power may be determined. Rotation of the demountable end 17 relative to the outer barrel to bring the fiducial marks to the centre of the line as shown in Fig. 2G allows the measurement of the radial position of the second of the focussing powers, this being normal to the first measurement.

Claims

1. An optometer for the measurement of focussing error and for the detection and measurement of the characteristics of astigmatism giving an external view to allow the eye under test to focus at a known distance, having a test image being a ring with a fiducial mark superimposable upon the external view, said test image being axially rotatable, the optometer being adjustable to provide first and second linear test images at right angles by adjustment of the optometer to a configuration at which the first and second focussing errors of an astigmatic eye may be determined, the radial position of the astigmatism being indicated by rotation of the test image to place the fiducial in the centre of the first or second linear test image.

2. An optometer substantially as described herein with reference to the drawings.