DETERMINATION OF THE CORRECTION FOR FAR-SIGHTEDNESS

Abstract: In a method for determining a needed reading correction, in particular for possible measurements of reading glasses, a pernickety test is used for determining the pernickety performance. After the accommodation of an eye has been established, the above reading correction is determined directly from this accommodation and the pernickety performance.

Description: DETERMINATION OF THE CORRECTION FOR FAR-SIGHTEDNESS The present invention relates to a method for determining a reading correction, in particular for possible measurements for reading glasses, using a pernickety test for determining the pernickety performance. As referred to herein, the pernickety performance is the inverse value of the smallest detail size that can just be observed at a specific distance. The pernickety performance is therefore an indication of the just discernible detail size of a sign or a series of signs as a function of the reading distance and is expressed in m "1 . The just discernible detail size as a function of the distance shows a minimum value at a specific distance from the eye. This minimum distance value will often depend on the degree of presbyopia; according as a person grows older, accommodation will be more difficult for him/her, and the distance at which this minimum occurs will come to be more and more remote from the person. The determination of the pernickety performance by means of a pernickety test is extensively described in an article by J.J. Vos and J. Boogaard, De TNO-priegel test, Tijdschrift for Sociale Gezondheidszorg, 62 (1984), No. 13, pp. 531-534, and has long since been used by, inter alia, medical examiners. In practice, this has the result that in the event of a specific degree of presbyopia a person in question will be referred to an optometrist or optician where measurements for correction glasses (reading glasses) can be taken. This is done by the use of a set of trial frames to determine the power of the glasses . Because in particular a medical examiner normally does not have a set of trial frames available, it is necessary for him to refer a person in question to an optometrist or optician. Consequently, this procedure is rather laborious. The object of the invention is therefore to provide an efficient and reliable determination of a reading correction, in particular for possible measurements for reading glasses, which reading correction determination is possible without the use of a set of trial frames and without the necessary assistance of other persons. According to the invention, this object is achieved by establishing the accommodation of an eye and determining the above reading correction from this accommodation in combination with the pernickety performance. The invention is therefore based on the insight that only two parameters are necessary for determining a reading correction, namely the accommodation D 0 , this is the inverse value of the smallest distance at which focusing is possible without glasses, and the pernickety performance P 0 . The reading correction can be established for each eye separately or for both eyes together. In the first case, always one eye has to be covered, and the method according to the invention has to be carried out for each eye separately. In the second case, accommodation and pernickety performance are determined for both eyes simultaneously and equally. According to the invention, the needed reading correction D b is substantially determined by the relation ob A, = E> -1 P 0 in which D 0 is the accommodation of an eye, P 0 is the pernickety performance and P ob is the target value of the pernickety level obtained by means of reading glasses. If the requirement is imposed that every person with reading glasses has to show the same value for the pernickety performance, then P ob is to be regarded as a constant applicable to all persons. In practice, it has been found that not everyone needs the same reading power. According to a further aspect of the invention, it is favorable if the target value of the pernickety level can be selected differently for a limited number of categories of persons. These may, for instance, be persons supposed to read either only extra large signs or also signs of average size or, additionally, extra small signs. In the last case, one out of three possible values which P ob can assume may optionally be used and entered into the above relation as a constant. If, for instance, the standard set is that in any case a person must be able to read signs of average size and therefore to see with glasses just a detail of about 0.08 mm, so that P ob = 13 mm "1 , while without glasses a pernickety performance of P 0 = 8 mm "1 is established at a distance of 0.5 m, so that the accommodation D 0 = 2 dioptries, then it applies that D b = 1.3 dioptries . If such a person must in any case be able to read extra small signs, in which a value P ob = 16 mm "1 is used, then D b = 2 dioptries and the person therefore needs more powerful reading glasses. On the other hand, if it is sufficient for the person to read large signs, in which P ob = 10 mm "1 , then D b = 0.5 dioptries and less powerful reading glasses are therefore sufficient for this person. The invention not only relates to a method for determining a reading correction but also to a device for using this method. According to the invention, this device is characterized in that a pernickety testing device is present as well as distance measuring means for determining the distance at which a pernickety test is carried out, while furthermore memory means are present to enable the determination of the above reading correction from the pernickety performance determined by means of the pernickety test and the above distance. In a simple embodiment, the memory means may consist of a table on paper, on the basis of which the needed reading correction can be read by means of the pernickety performance and accommodation determined by the pernickety testing device and the distance measuring means. To this end, the pernickety testing device comprises a visual acuity chart having series of signs depicted thereon, the sizes of which are shown smaller and smaller, and may be slidably provided on a measuring rule, which during the determination of the pernickety performance is kept in a fixed position with respect to the eye of a test person, in particular pressed against the chin of the test person. The pernickety testing device can easily be moved to the nearest position at which the visual acuity chart can just be observed sharply. By means of the measuring rule, the corresponding distance can be read. In another embodiment, the testing device, which of course also comprises a visual acuity chart, is fixedly arranged, and the distance measuring means are formed by a likewise fixedly arranged electronic sensor for determining the shortest distance to the testing device at which a test person still sharply sees specific details of the visual acuity chart. In this embodiment, therefore, the pernickety testing device is not moved, but the test person moves, namely until such a distance from the testing device that the person can still sharply observe specific details of the visual acuity chart. In yet another embodiment, the pernickety testing device comprises a visual acuity chart, which can, taken into the hand, be controlled by a test person himself/herself , while the distance measuring means are formed by an electronic sensor provided on the pernickety testing device for determining the shortest distance to the testing device at which a test person still sharply sees specific details of the visual acuity chart. In the embodiments described herein, the pernickety testing device can be rather simple. Possible is a visual acuity chart behind glass, which can be illuminated by a lamp placed behind. In the position in which the test person can just sharply see details of the visible or visualized signs, he/she will have to be able to read the signs. For a verification whether the test person has properly read the signs, he/she can either memorize the signs read or read them out while another person is verifying the signs; then this person, too, can read the adjusted distance. A more elegant solution, however, is obtained if the whole device according to the invention is electronic. In this embodiment, the pernickety testing device comprises a display unit on which signs read from a memory can be depicted, which display unit comprises means for identifying a depicted sign, while furthermore checking or verification means are present to establish whether the depicted signs have been identified correctly, in which case this process is repeated with other and/or smaller signs, and in case, optionally after repeated times, the signs are not identified correctly, to provide an electric signal indicating the pernickety performance. If desired, the pernickety performance may be depicted on the display unit. The means for identifying a depicted sign may simply be formed by knobs on the display unit, which are actuated by the test person, and via which he/she indicates which signs he/she has believed to identify. When an electric signal indicating the pernickety performance has been provided, the distance can be established again, and the reading correction can be determined on the basis of the above table. These last operations, however, can also be carried out electronically. To this end, the device according to the invention contains a further memory to which the signal indicating the pernickety performance is applied, as well as a distance measuring signal of an electronic distance sensor, on the basis of which signals from the last -mentioned further memory a signal indicating a reading correction is provided. Instead of using a table, a further memory is present, in which this table is included. This further memory may therefore be a PROM, in which for a number of values of the accommodation D 0 of an eye a number of values of the pernickety performance P 0 and a number of values of the corrected pernickety performance P ob the reading correction D b according to the relation is stored. The desired pernickety performance after correction can be adjusted previously, for instance by a selector switch. The electronic distance sensor is preferably formed by an ultrasonic sensor or an infrared laser sensor. The invention will now be explained in more detail with reference to the accompanying drawing. In this drawing: Fig. 1 is a diagrammatic view of a simple arrangement of the device according to the invention; Fig. 2 is a visual acuity chart which can be fitted in the pernickety testing device; Fig. 3 is a table in which the reading correction is expressed as a function of the pernickety performance and the accommodation; Fig. 4 is a diagram in explanation of the table of Fig. 3; Fig. 5 is a block diagram of an exemplary embodiment of a device according to the invention realized by means of electronics, and Fig. 6 is an exemplary embodiment of a device according to the invention. The arrangement of the device according to the invention, diagrammatically shown in Fig. 1, in its simplest form comprises a measuring rule 1 and a pernickety testing device 2 slidable thereon. By placing the measuring rule against the chin, the measuring rule is kept in a fixed position with respect to the eyes of the person whose eyes have to be tested. Although for determining a reading correction the person in question will possibly wear no glasses, this is absolutely no requirement. By keeping his own reading glasses on, the person can establish which extra reading correction or reading addition he/she needs. The pernickety testing device 2 is formed by a box having at the front a display on which are provided, for instance, the Landolt C signs shown in Fig. 2. In this embodiment, the box comprises a lamp for illuminating these signs, if desired. The test person first slides the pernickety testing device over the measuring rule until the shortest distance has been reached at which he/she still sees details of the signs sharply. It should be noted that, for the sake of convenience, it will hereinafter be assumed that one eye is covered, so that the eyes are tested separately. Fig. 2 shows to the left of the rows with Landolt C's the corresponding pernickety performance and to the right the size of the gap in the Landolt C's. Suppose the test person, after this focusing at the shortest possible distance, has just been able to read from the signs shown in the pernickety testing device the fourth row of Landolt C's correctly, that is to say can just identify a gap size of 0.05 mm, then the pernickety performance P 0 has the value of 20 mm "1 . The corresponding distance from the pernickety testing device to the eyes can then be read from the measuring rule 1. The inverse value of this distance is the accommodation D 0 of the eye. If it is assumed that the corrected pernickety performance P ob has a known value, for instance the value of 13 mm "1 , then the size D b of the needed reading correction or reading addition will be determined on the basis of the measured values for P 0 and D 0 . This can be done on the basis of the chart shown in Fig. 3. This chart shows a matrix of D b -values, to the left of which is indicated the pernickety performance (pernickety score) P 0 , while above it, in the uppermost line, the minimum distance in meters at which the signs can still be seen sharply is indicated with the inverse values thereof below, that is to say the accommodation D 0 expressed in dioptries. The matrix in Fig. 3 gives the reading additions needed to reach with reading glasses a pernickety performance of P ob = 13 mm "1 . At this pernickety performance the matrix gives a reading correction of 0 over the whole distance range, that is to say that at such a pernickety score the test person needs no reading glasses at all . At the above corrected pernickety performance of P ob = 13 mm "1 the values in the matrix are determined by the relation Not all the values from the matrix, however, lead to a useful statement about a needed reading correction. The values in the matrix are therefore divided by two stepped lines and a horizontal dotted line into four areas (see Figs. 3 and 4) . On the left-hand bottom side is indicated the area with values not occurring in practice (impossible scores) . On the right-hand bottom side is indicated the area (no correction needed) within which no reading glasses are necessary. On the right-hand top side is indicated the area (low vision: see your optometrist) within which normal reading glasses are not sufficient for the test person, who is referred to an ophthalmologist, optometrist or the like. The remaining area (suitable for correction with reading glasses) indicates the power of recommended reading lenses. Fig. 5 is a block diagram of an electronic device according to the invention. This block diagram shows a memory 3, in which, for instance, the codes of the four different types of Landolt C's are stored in different sizes. When the device is started, the diverse codes can be read out by means of an addressing circuit 4, and the corresponding Landolt C's are depicted on a display 5 in rows or columns arranged to size. The test person brings the device at a specific distance at which he/she believes to be able to see a specific series of Landolt C's just sharply. By means of an actuating knob 6 belonging to each type of Landolt C these can be identified in sequence in a comparator circuit 7. If this identification has not proceeded correctly, then the Landolt C's are depicted on the display once again, and the identification process can be repeated. It is also possible that on a part of the display it is indicated that not all the Landolt C's have been identified correctly and that the identification process has to be repeated. To increase the accuracy and reliability, it could also be prescribed that the identification process must be repeated a fixed number of times, for instance three times. If the identification has eventually proceeded correctly, then a signal indicating the corresponding pernickety performance is applied via a line 8 to and recorded in a register 9. The block diagram further shows a memory 10 in the form of a PROM, in which memory for a number of values of the accommodation D 0 , the pernickety performance P 0 and the corrected pernickety performance P ob the reading correction is stored in accordance with the above-mentioned relation. The address part indicating the corrected pernickety performance and contained in the addressing circuit 12 is controlled from a selector switch on the display case via a line 11. The address part indicating the pernickety performance is controlled from the register 9 by the addressing circuit 13. The address part indicating the accommodation of the eye is controlled from a distance sensor 14 by the addressing circuit 15. The distance sensor 14 can be formed by an ultrasonic or an infrared laser sensor arranged at the top of the display case, so that the distance at which the test person keeps the display case can be determined constantly. When reading out, the memory 10 gives the desired power for the reading glasses . This can be depicted on the display and/or printed. Fig. 6 is a possible embodiment of the device according to the invention. The device is formed by a hand- operated electronic device to be held in the hand, at the top of which the distance sensor 14 is arranged. The case of the device is also the case of the display 5. On this case the required knobs and switches may be arranged. The invention is not limited to the exemplary embodiments described herein with reference to the figures but comprises all kinds of modifications, of course as far as falling within the scope of protection of the appended claims. In particular in the electronic embodiment of the device according to the invention, numerous variants are possible. All kinds of instructions facilitating the operation of the device to obtain a reliable result and to make the device more customer-friendly may be depicted on the display. The device is not limited to the testing by means of Landolt C's either; all kinds of words or different signs decreasing in size can be used.

Claims: Claims 1. A method for determining a needed reading correction, in particular for possible measurements for reading glasses ' , using a pernickety test for determining the pernickety performance, characterized in that the accommodation of an eye is established and the above reading correction is determined from this accommodation in combination with the pernickety performance. 2. A method according to claim 1, characterized in that the reading correction D b is substantially determined by the relation in which D 0 is the accommodation of an eye, P 0 is the pernickety performance and P ob is the target value of the pernickety level obtained by means of reading glasses. 3. A method according to claim 2, characterized in that the target value of the pernickety level can be selected differently for persons which are supposed either to read only extra large signs or also signs of average size or, additionally, extra small signs. 4. A device for determining a needed reading correction by the use of the method according to claim 1, 2 or 3, characterized in that a pernickety testing device is present, as well as distance measuring means for determining the distance at which a pernickety test is carried out, while furthermore memory means are present to enable the determination of the above reading correction from the pernickety performance determined by means of the pernickety test and the above distance. 5. A device according to claim 4, characterized in that the pernickety testing device comprises a visual acuity chart and is slidably arranged on a measuring rule, which during the determination of the pernickety performance is kept in a fixed position with respect to the eye of a test person, in particular pressed against the chin of the test person. 6. A device according to claim 4, characterized in that the pernickety testing device comprises a visual acuity chart and is fixedly arranged, and the distance measuring means are formed by a likewise fixedly arranged electronic sensor for determining the shortest distance to the testing device at which a test person sharply sees specific details of the visual acuity chart. 7. A device according to claim 4, characterized in that the pernickety testing device comprises a visual acuity chart, which, taken into the hand, can be controlled by a test person himself/herself , and the distance measuring means are formed by an electronic sensor arranged on the pernickety testing device for determining the shortest distance to the testing device at which a test person sharply sees specific details of the visual acuity chart. 8. A device according to claim 4, characterized in that the pernickety testing device comprises a display unit, at which signs read from a memory can be depicted, which display unit comprises means for identifying a depicted sign, while furthermore checking means are present to establish whether the depicted signs are identified correctly, in which case this process is repeated with other and/or smaller signs, and if, optionally after repeated times, the signs are not identified correctly, to provide an electric signal indicating the pernickety performance. 9. A device according to claim 8, characterized in that a further memory is present to which the signal indicating the pernickety performance is applied, as well as a distance measuring signal of an electronic distance sensor, on the basis of which signals from the last-mentioned further memory a signal indicating a reading correction is applied. 10. A device according to claim 9, characterized in that the further memory is a PROM, in which for a number of values of the accommodation D 0 of an eye a number of values of the pernickety performance P 0 and a number of values of the desired pernickety performance after correction P ob the lens power D b according to the relation = 1 Ob P. is stored. 11. A device according to claim 6, 7, 9 or 10, characterized in that the sensor is formed by a ultrasonic sensor or an infrared laser sensor.

Inventors: KOOI FRANK LEONARD, KOOI, FRANK, LEONARD
Applicants: KOOI FRANK LEONARD, TNO, KOOI, FRANK, LEONARD, NEDERLANDSE ORGANISATIE VOOR TOEGEPASTNATUURWETENSCHAPPELIJK ONDERZOEK TNO
Application year: 1999
Main classification: A61B3
IPC codes: A61B3, G02C13