WO1994013192A1

WO1994013192A1 - Visual acuity testing

Info

Publication number: WO1994013192A1
Application number: PCT/GB1993/002546
Authority: WO
Inventors: Frederick William Fitzke; John Xing-Wang Wu
Original assignee: Institute Of Ophthalmology
Priority date: 1992-12-14
Filing date: 1993-12-14
Publication date: 1994-06-23
Also published as: GB9226015D0

Abstract

A method of testing the visual acuity of a person uses an apparatus that is capable of displaying in different sizes either of two asymetric images that are substantially mirror images of one another. In the method, one of the two images is displayed by the apparatus at a first size and an input signal is passed into the apparatus indicating either which of the two images the person judges the image to be or that the person is unable to make such a judgement. The apparatus reads the input signal and, in the event that the signal indicates which of the two images the person judges the image to be checks the correctness of the indication. Another image is then displayed the size of which is dependent upon the correctness of the person's judgement regarding the previous image. The process is repeated a multiplicity of times to ascertain the minimum size of image that a person can see.

Description

Visual Acuity Testing

This invention relates to a method and apparatus for testing the visual acuity of a person. The invention is especially, but not exclusively, applicable to the screening of large groups of people in order to assess generally the level of visual acuity of that group and, therefore, to determine the level of priority that should be given to further testing for and/or treatment of visual function impairment of those people. There are at present many forms of visual acuity tests but they are not generally suitable for the purposes of screening a large number of people because the process of testing each individual is too time consuming. For example, there is the well known Snellen chart in which a person is presented with a plurality of lines of different letters and their visual acuity determined by asking them to read lines of letters of progressively reducing size. It has also been proposed to carry out visual acuity tests by displaying a character on a computer screen; whilst such tests have been found to have some advantages, they have also proved to take longer than a conventional Snellen test and to reguire considerable explanation to the person being tested before the test itself can be started. It is an object of the invention to provide a method and apparatus for testing the visual acuity of a person in a simple and quick way. According to the invention there is provided a method of testing the visual acuity of a person using an apparatus that is capable of displaying in different sizes either of two asymmetric images that are substan- tially mirror images of one another, in which method the following steps are taken:

(a) one of the two images is displayed by the apparatus at a first size,

(b) an input signal is passed into the apparatus indicating either which of the two images the person judges the image to be or that the person is unable to make such a judgement,

(c) the apparatus reads the input signal and, in the event that the signal indicates which of the two images the person judges the image to be, checks the correctness of the indication,

(d) the apparatus displays another image which is one of the two images, the size of the image being dependent upon the result of step (c) , and (e) steps (b) , (c) and (d) are repeated a multiplicity of times.

Because the person being tested is only ever presented with two different images and need only select from three answers, the test is very simple to explain. Furthermore, because the person being tested is answering only a very simple question, the answer can be very rapid so that even though steps (b) , (c) and (d) are repeated a multiplicity of times, the overall time taken for the test can be short.

It will be understood that the test will normally finish after completion of test (c) for the last time.

Preferably, the selection of which of the two images is displayed is determined by the apparatus on a random or pseudo-random basis. It is desirable that there should not be any discernible pattern in the sequence of images displayed and it is also desirable that the choice of which of the images is displayed is made by the apparatus rather than by an operator because that is quicker.

Preferably, the images are an E optotype and the mirror image of an E optotype. The use of an E optotype has certain advantages. It has conventionally been used in testing visual acuity. Also, the shape of the

E optotype is very clearly asymmetrical and it is simple for a person to understand that they are required to distinguish between the two mirror images. One image can be referred to as pointing to the right (the E optotype itself) and the other image can be described as pointing to the left (the mirror image of the E optotype). It is advantageous for the input signals to the apparatus to be supplied by pressing the arrow keys on a keyboard and it is readily understandable that the "right" arrow key should be pressed to indicate the image of the E optotype itself as that points to the right and the "left" arrow key should be pressed to indicate the mirror image of the E optotype as that points to the left. In the paragraph immediately above, it is assumed that the E optotype and its mirror image are displayed in their upright form so that the plane about which the images are mirrored is a vertical plane. That is advantageous because of the greater simplicity of indicating whether the image is pointing to the left or to the right, but it is possible to arrange the images so that they are mirror images about a horizontal plane (in the case of the E optotype, the character is simply rotated through 90°).

Preferably, in the event that in step (c) the input signal read by the apparatus provides a correct indica¬ tion of which of the images is being displayed, the next image displayed is of a size smaller than the image displayed immediately before. Similarly, it is preferable that, in the event that in step (c) the input signal provides an incorrect indication of which of the images is being displayed, the next image displayed is of a size greater than the image displayed immediately before. Changing the size of the image in an appropriate direction each time according to the input signal reduces the time taken for the whole test and assists in arriving as quickly as possible at an image substantially at the limit of the visual capability of the person being tested.

The size of image displayed in step (a) is preferably intermediate the ends of the range of sizes of images that can be displayed. By starting the test in the middle of the range of sizes, the duration of the test is again reduced.

Preferably, there are first and second phases of testing and the relationship between the size of image displayed in step (d) and the result of step (c) is different in the second phase from the first phase. Preferably, the increase in size of image displayed in step (d) , in the event that in step (c) the input signal provides an incorrect indication of which of the images is being displayed, is greater during the first phase of testing than during the second phase of testing. Furthermore, it is preferable that there are a multi¬ plicity of predetermined sizes of images and that during the second phase of testing, the size of image displayed in step (d) , in the event that in step (c) the input signal provides an incorrect indication of which of the images is being displayed, is one size greater, whereas during the first phase of testing the size of image displayed in step (d), in the event that in step (c) the input signal provides an incorrect indication of which of the images is being displayed, is a plurality of sizes greater, preferably at least five sizes greater. The steps referred to above are especially advantageous and important in shortening the time of a test and improving the reliability of the test. It is important to dis¬ courage a person being tested from guessing and it is therefore advantageous that during the first phase of testing an incorrect answer results in a much enlarged image. On the other hand, having completed the first phase of testing, the purpose of the second phase is to determine as accurately as possible the smallest image that the person being tested can correctly identify and, in order to do that, it is desirable that the change in image size following an incorrect answer is small.

There may be a multiplicity of predetermined sizes of image, there being a smallest size followed by a succession of other sizes each being approximately the same multiple of the size of the preceding image. It is advantageous for the sizes of the images to be evenly split on a logarithmic scale rather than an ordinary scale.

The test is preferably carried out with the person being tested within 2 of the display. Although visual acuity tests are conventionally carried out with the subject significantly further from the display, we have found that for the kind of tests for which the present invention is especially advantageous, a distance of less than 2 m is entirely satisfactory. With such a short distance, the sizes of the images to be displayed are reduced and therefore the size of the apparatus for displaying the images can be reduced.

In carrying out the method of the invention, the input signals may be provided directly by the person being tested or there may be an operator who converts the answer from the person being tested into an input signal into the apparatus. An advantage of the input signals being provided directly by the person being tested is that it enables a self-assessment to be carried out without any other person being present, whilst an advantage of having an operator is that the time spent on the test is again reduced because there is less explanation to give the person being tested.

The images are preferably displayed on a liquid crystal display and the smallest images may be of only one or two pixels in width. Such images are easily generated and are clear and stable to view.

The method of the invention can be carried out using a notebook computer, the screen of the computer providing a display and the keyboard of the computer or a mouse providing an input device. The method of the invention may be carried out in conjunction with another screening test. In a paper entitled "Laptop computer perimetry for glaucomascreen- ing" , by Wu X., Wormald R. , Fitzke F. , Poinoosawmy S. , Nagasubramanian S. and Hitchings R. and published in Invest Ophthal Vis. Soc, 32 (suppl): 810, 1991, there is described a movement sensitivity screening test which has proved very useful. A possible cause of a poor response in such a test, however, is a poor visual acuity and there is therefore advantage in carrying out the test of the present invention alongside the movement sensitivity screening test.

According to the invention there is also provided an apparatus including: a display capable of displaying in different sizes either of two asymmetric images that are substantially mirror images of one another, an input device for signalling either which of the two images a person judges the image to be or that the person is unable to make such a judgement, control means for receiving a signal from the input device and generating an output signal to control the display, the size of the image being altered in dependence upon the signal received from the input device.

The apparatus may include random or pseudo-random means for the selection of which of the two images is displayed. By way of example, certain embodiments of the invention will now be described with reference to the accompanying drawings, in which:

Fig. 1 is a perspective view of the apparatus embodying the invention, and Figs. 2A and 2B are diagrams showing the progress of the first and second phases of a test exemplifying the invention. The apparatus shown in Fig. 1 comprises a display 1, a control device 2 and an input device in the form of a keyboard 3. The control device 2 receives signals from the keyboard 3 along line 4 and generates an output signal to control the display 1. The display 1 is a liquid crystal display (LCD) and is capable of displaying either an E optotype (an E optotype being the capital letter E dimensioned as is customary for a visual acuity test, for example on a Snellen chart) or the mirror image in a vertical plane of an E optotype as shown in Fig. 1. Furthermore the apparatus is capable of displaying either the E optotype or its mirror image in a multiplicity of different sizes, comprising a smallest size followed by a succession of other sizes each being approximately the same multiple of the size of the preceding image. In one particular example, the step between each size is approximately 0.1 log.

In order to test a person's visual acuity, the subject is seated in front of the display 1 at a distance of, for example, 100 cm from the display 1. The sight of each eye is tested separately by covering the other eye of a subject during a test. The test begins with one of the two images being displayed on the screen at a size in the middle of the range of sizes that can be displayed. Each time an image is displayed the control device 2 determines on a random basis which of the two images

(E optotype or mirror image thereof) is to be displayed.

The subject is then required to make one of the following three responses:

A. Identify the image as an E B. Identify the image as the mirror image of an E

C. State that he is unable to identify the image.

A keyboard operator, who is present during the test, then enters the response on the keyboard 3 using the arrow keys. The "right" arrow key 7 is used for response A, the "left" arrow key 8 is used for response B and either of the "up" and "down" arrow keys 9, 10 are used for response C. The control device 2 receives the input signal from the keyboard 3 and in the event of responses A or B checks the correctness of the response and causes the display of a different size of image. If the response is correct a new image, selected at random, and one size smaller than the previous image is dis- played, whereas, if the response is incorrect, a new image, again selected at random but several sizes bigger, for example, seven sizes bigger, is displayed. The test is then repeated. In the event of response C, a new image, selected at random, and one size bigger than the previous image is displayed.

The subject is encouraged not to guess and when the optotype becomes too small to be seen, the subject should make response C. Once there have been a predetermined number, for example seven, correct responses (response C being regarded as a correct response) , the first phase of the test is complete and the second phase begins. The transfer from the first phase to the second phase is not one of which the subject need be aware; the only change that takes place is that in the second phase, if the response is incorrect, the new image that is displayed is only one size bigger instead of several sizes bigger. The size of image chosen at the beginning of the second phase is one size smaller than the image correctly identified at the end of the first phase.

The second phase continues until the last five responses of the subject comprise three correct responses separated by two response C's. The three correct responses will all have been made in respect of the same size of image and it is that size of image that is used to calculate the visual acuity of the subject's eye. The test is then complete.

If a subject does not guess, then a test of one eye can be carried out in about 30 seconds. If a subject does guess, then the duration of the test is of course increased.

In an embodiment of one particular example of the invention where the subject is tested at 100 cm, there are 15 sizes of image ranging from about 0.2 mm to about 7 mm in width.

One particular example of a test will now be described with reference to Figs. 2A and 2B which show, respectively, the first and second phases of the test. In each drawing Tn shows the approximate size and orientation of the E optotype that is displayed in the nth stage of the test and Rn shows which of the responses A, B or C described above is made in test Tn. The E optotypes are not drawn to scale but their relative sizes are shown approximately to scale.

In the particular example of the first phase of the test shown in Fig. 2A the second response (R2) of the subject is incorrect as a result of which the next E optotype display (T3) is 7 sizes bigger than the previous one (T2). It will be seen that the first phase of the test ends after 7 correct responses, response C being regarded as a correct response. The second phase of the test shown in Fig. 2B begins with an image one size smaller than the image correctly identified at the end of the first phase. In the second phase of the test, response R4 is incorrect but merely results in an image one size bigger being displayed. Thereafter there are 3 correct responses separated by 2 response C's resulting in the completion of the test. The correct responses R5, R7 and R9 are all made in respect of the same size of image and it is that size of image that is used to calculate the visual acuity of the subject's eye.

The testing procedure and apparatus described above is especially suitable for use in screening a large group of people to make an assessment of the visual function impairments of the group as a whole. A decision regard- ing further testing and/or treatment of the group can be made in the light of the results of the tests, either carried out on their own or in combination with some other test, for example, a visual field test.

It will be understood that the particular example of apparatus and method described above is only one of many suitable forms. The individual elements of keyboard, control device and display may advantageously be integrated into a single device which may be a notebook computer. Instead of an operator entering responses on the keyboard, the subject whose sight is being tested may operate the keyboard.

Claims

Claims :

1. A method of testing the visual acuity of a person using an apparatus that is capable of displaying in different sizes either of two asymmetric images that are substantially mirror images of one another, in which method the following steps are taken:

(a) one of the two images is displayed by the apparatus at a first size,

2. A method according to claim 1, in which the selection of which of the two images is displayed is determined by the apparatus on a random or pseudo-random basis.

3. A method according to claim 1, in which the images are an E optotype and the mirror image of an E optotype.

4. A method according to any preceding claim, in which the plane about which the images are substantially mirror images is a vertical plane.

5. A method according to any preceding claim, in which, in the event that in step (c) the input signal read by the apparatus provides a correct indication of which of the images is being displayed, the next image displayed is of a size smaller than the image displayed immediately before.

6. A method according to any preceding claim, in which, in the event that in step (c) the input signal provides an incorrect indication of which of the images is being displayed, the next image displayed is of a size greater than the image displayed immediately before.

7. A method according to any preceding claim, in which the size of image displayed in step (a) is inter¬ mediate the ends of the range of sizes of images that can be displayed.

8. A method according to any preceding claim, in which there are first and second phases of testing and the relationship between the size of image displayed in step (d) and the result of step (c) is different in the second phase from the first phase.

9. A method according to claim 8, in which the increase in size of image displayed in step (d) , in the event that in step (c) the input signal provides an incorrect indication of which of the images is being displayed, is greater during the first phase of testing than during the second phase of testing.

10. A method according to claim 9, in which there are a multiplicity of predetermined sizes of images and in which during the second phase of testing, the size of image displayed in step (d), in the event that in step (c) the input signal provides an incorrect indication of which of the images is being displayed, is one size greater, whereas during the first phase of testing the size of image displayed in step (d) , in the event that in step (c) the input signal provides an incorrect indi¬ cation of which of the images is being displayed, is a plurality of sizes greater.

11. A method according to claim 10, in which the plurality of sizes greater is at least five sizes greater.

12. A method according to any preceding claim, in which there are a multiplicity of predetermined sizes of image, there being a smallest size followed by a succes¬ sion of other sizes each being approximately the same multiple of the size of the preceding image.

13. A method according to any preceding claim, in which the images are displayed on a liquid crystal display.

14. A method of testing the visual acuity of a person, the method being substantially as herein described with reference to and as illustrated by the accompanying drawings.

15. A method of screening a multiplicity of persons in order to make an overall assessment of the visual acuity of the multiplicity of persons considered as a group, the method involving testing the visual acuity of each person in turn using a method according to any preceding claim.

16. An apparatus for use in testing the visual acuity of a person, the apparatus including: a display capable of displaying in different sizes either of two asymmetric images that are substantially mirror images of one another, an input device for signalling either which of the two images a person judges the image to be or that the person is unable to make such a judgement, control means for receiving a signal from the input device and generating an output signal to control the display, the size of the image being altered in depen¬ dence upon the signal received from the input device.

17. An apparatus according to claim 16, in which the apparatus includes random or pseudo-random means for the selection of which of the two images is displayed.

18. An apparatus according to claim 16 or claim 17, in which the two images displayed are an E optotype and the mirror image of an E optotype.

19. An apparatus according to any one of claims 16 to 18, in which the plane about which the images are substantially mirror images is a vertical plane.

20. An apparatus according to any one of claims 16 to 19, in which the apparatus has first and second phases of operation and the alteration of the size of the image in dependence upon the signal received from the input device is different from one phase of operation to the other.

21. An apparatus according to any one of claims 16 to 20, in which the input device is a keyboard including a pair of arrow keys, one key of the pair being used for signalling that a person judges that one of the two images is being displayed and the other key of the pair being used to indicate that a person judges the other of the two images is being displayed.

22. An apparatus according to claim 21, in which the keyboard includes another pair of arrow keys, one or both keys of the other pair of arrow keys being used for signalling that a person is unable to judge which of the two images is being displayed.

23. An apparatus according to any one of claims 16 to 22, in which the display is a liquid crystal display.