1、BSI Standards Publication Ophthalmic optics and instruments Correlation of optotypes PD ISO/TR 19498:2015National foreword This Published Document is the UK implementation of ISO/TR 19498:2015. The UK participation in its preparation was entrusted by Technical Committee CH/172, Ophthalmic optics, to
2、 Subcommittee CH/172/6, Ophthalmic instruments. A list of organizations represented on this committee can be obtained on request to its secretary. This publication does not purport to include all the necessary provisions of a contract. Users are responsible for its correct application. The British S
3、tandards Institution 2015. Published by BSI Standards Limited 2015 ISBN 978 0 580 85357 9 ICS 11.040.70 Compliance with a British Standard cannot confer immunity from legal obligations. This Published Document was published under the authority of the Standards Policy and Strategy Committee on 31 Dec
4、ember 2015. Amendments/corrigenda issued since publication Date Text affected PUBLISHED DOCUMENT PD ISO/TR 19498:2015 ISO 2015 Ophthalmic optics and instruments Correlation of optotypes Optique et instruments ophtalmiques Corrlation des optotypes TECHNICAL REPORT ISO/TR 19498 Reference number ISO/TR
5、 19498:2015(E) First edition 2015-12-15 PD ISO/TR 19498:2015 ISO/TR 19498:2015(E)ii ISO 2015 All rights reserved COPYRIGHT PROTECTED DOCUMENT ISO 2015, Published in Switzerland All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized otherwise in any
6、 form or by any means, electronic or mechanical, including photocopying, or posting on the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address below or ISOs member body in the country of the requester. ISO copyright office Ch. de Blan
7、donnet 8 CP 401 CH-1214 Vernier, Geneva, Switzerland Tel. +41 22 749 01 11 Fax +41 22 749 09 47 copyrightiso.org www.iso.org PD ISO/TR 19498:2015 ISO/TR 19498:2015(E)Foreword iv 1 Scope . 1 2 General requirements for optotypes . 1 3 Correlation of optotypes. 1 3.1 Selection of optotypes for correlat
8、ion testing 1 3.2 Test area 1 3.3 Presentation of the optotypes 1 3.4 Corrective lenses . 2 3.5 Test distance 2 3.6 Luminance 2 4 Assignment of an acuity score 2 5 Assessing the equivalence of two kinds of optotype . 3 6 Significanc e of the differ enc e betw een the tw o means . 3 Bibliogr aph y 4
9、ISO 2015 All rights reserved iii Contents Page PD ISO/TR 19498:2015 ISO/TR 19498:2015(E) Foreword ISO (the International Organization for Standardization) is a worldwide federation of national standards bodies (ISO member bodies). The work of preparing International Standards is normally carried out
10、 through ISO technical committees. Each member body interested in a subject for which a technical committee has been established has the right to be represented on that committee. International organizations, governmental and non-governmental, in liaison with ISO, also take part in the work. ISO col
11、laborates closely with the International Electrotechnical Commission (IEC) on all matters of electrotechnical standardization. The procedures used to develop this document and those intended for its further maintenance are described in the ISO/IEC Directives, Part 1. In particular the different appr
12、oval criteria needed for the different types of ISO documents should be noted. This document was drafted in accordance with the editorial rules of the ISO/IEC Directives, Part 2 (see www.iso.org/directives). Attention is drawn to the possibility that some of the elements of this document may be the
13、subject of patent rights. ISO shall not be held responsible for identifying any or all such patent rights. Details of any patent rights identified during the development of the document will be in the Introduction and/or on the ISO list of patent declarations received (see www.iso.org/patents). Any
14、trade name used in this document is information given for the convenience of users and does not constitute an endorsement. For an explanation on the meaning of ISO specific terms and expressions related to conformity assessment, as well as information about ISOs adherence to the WTO principles in th
15、e Technical Barriers to Trade (TBT) see the following URL: Foreword - Supplementary information The committee responsible for this document is ISO/TC 172, Optics and photonics, Subcommittee SC 7, Ophthalmic optics and instruments.iv ISO 2015 All rights reserved PD ISO/TR 19498:2015 TECHNICAL REPORT
16、ISO/TR 19498:2015(E) Ophthalmic optics and instruments Correlation of optotypes 1 Scope This Technical Report specifies a method of correlation between a given set of optotypes and the standard optotype (Landolt ring) formed and presented as specified in ISO 8596. All test methods are type tests and
17、 suitable equivalent test methods may be substituted. 2 General requirements for optotypes Each size of a set of optotypes is specified in terms of the size of the standard Landolt ring that is equally recognizable as determined according to the method of Clause 3. This means that the size of a set
18、of optotypes is the size of the nominal 1,0 acuity set magnified or minified by a multiple of the ratio 1,2589 (see ISO 8596:2009, Clause 4). 3 Correlation of optotypes 3.1 Selection of optotypes for correlation testing Sufficient grades or steps should be used to establish a frequency of seeing cur
19、ve for the standard optotype and the optotype being investigated. A recommended range of decimal acuity sizes to use is 2,0 to 0,4 in Log MAR steps of 0,1. 3.2 Test area The test area is circular with a diameter of 4 1. The surrounding field has a diameter of 15 1,5 and is illuminated homogeneously
20、so that it does not influence the measurement. The luminance of the surrounding field does not exceed that of the test area. 3.3 Presentation of the optotypes 3.3.1 When a measurement of visual acuity is made with the eight-position Landolt ring for purposes of correlation, 120 presentations are mad
21、e one ring at a time with the ring positions for successive presentations arranged in random order. The optotypes to be correlated are also presented one at a time in random order until a series of 120 presentations has been completed. In the 120 presentations, the different optotypes in each set ar
22、e represented approximately the same number of times. 3.3.2 The number 120 is divisible by 2, 3, 4, 6, 8, 10, 12, 15, 20, 30, 40, and 60. Hence, with sets of optotypes having any of these numbers of different optotypes, it is possible for each optotype to be represented the same number of times in 1
23、20 presentations. The comparison is started with a grade of optotypes large enough to yield a frequency of seeing of 100 %. Measurements are made with both eight-position Landolt rings and the optotypes of the same size being correlated. When this has been completed, the procedure is repeated with s
24、maller and smaller sizes until the failure rate corresponds to the level of guessing of 0,125. The probability of guessing p of 0,125 results from the choice of eight different optotypes per acuity grade and the definition of p given in 4.2. Each optotype is exposed for 3 s with an interval of 4 s b
25、etween exposures. ISO 2015 All rights reserved 1 PD ISO/TR 19498:2015 ISO/TR 19498:2015(E) 3.4 Corrective lenses The observers are fully corrected to a visual acuity of 1,0 or better, if correction is necessary. 3.5 Test distance For correlation purposes only, the test is performed at a distance of
26、5 m 0,05 m between the subject and the optotype. 3.6 Luminance The background luminance of the test area as viewed by a patient (or subject) is 200 cd/m 2 50 cd/m 2and is the same for the Landolt ring as for the optotypes to be correlated. The difference between the background luminances of both tes
27、t areas does not exceed 10 %. The luminance of the optotypes themselves does not exceed 10 % of the background. 4 Assignment of an acuity score 4.1 If, before the end of the test, the observer is no longer able to recognize the test types, he/she is required to make a guess. The observer should not
28、be informed before the end of the test whether or not any mistakes were made. The number of errors per optotype size is recorded. From the raw data, an allowance for guessing is made and the frequency of seeing is assessed for each optotype size. 4.2 The frequency of seeing value, corrected for gues
29、sing, is given by Formula (1): 1(1) whereE is the number of right answers corrected for guessing;N is the number of presentations;R is the number of right answers;p is the probability of guessing (p is equal to the reciprocal of the number of different optotypes or directions in the set). 4.3 For th
30、e various grades, the frequency of seeing is plotted against the logarithm of the size of the critical details of the Landolt ring used as a standard. The points on the graph for each type of optotype are fitted with a cumulative frequency curve represented by the integral of the function that expre
31、sses the probability that the acuity grade selected is the threshold acuity grade. Any of the usual methods of fitting the cumulative frequency curve may be used. From the curves, the optotype sizes at which the frequency of seeing is 50 % can be estimated, representing the thresholds for the Landol
32、t ring optotype and the optotype being correlated. From these thresholds, the acuity scores can be derived. From the fitting frequency of seeing curves, find estimates of the sample standard deviations using the following procedure. Estimate the acuity values where the frequency of seeing curve take
33、s the value 16 % and the value 84 %. One half the difference of these two acuity values is the estimate of the sample standard deviation for the optotype acuity threshold probability function and is to be used in Clauses 5 and 6.2 ISO 2015 All rights reserved PD ISO/TR 19498:2015 ISO/TR 19498:2015(E
34、) 5 Assessing the equivalence of two kinds of optotype The measurements described in 3.3 are repeated with 10 or more observers having uncorrected visual acuity of 1,0 or better, or the observers are fully corrected to a visual acuity of 1,0 or better, if correction is necessary. The threshold value
35、s for each kind of optotype are averaged by finding the mean. If the difference, , between the two means differs by more than 0,05 log units, the two sets of optotypes cannot be said to be equivalent. If the difference of the threshold means is equal to or less than 0,05 log units, but the sample st
36、andard deviation of the tested optotype acuity threshold probability is more than 1,5 times the sample standard deviation of the Landolt ring acuity threshold probability, the tested optotype cannot be said to be equivalent to the Landolt ring. 6 Sig nificanc e of the differ enc e betw een th e tw o
37、 means The statistical significance of the difference between the two means, as found in Clause 5, is defined as the probability that the difference between the two means, as found in Clause 5, is indeed other than 0. This probability value is called the level of significance, and a value commonly u
38、sed for this level of significance is 5 %. Upon running the tests and finding the difference of the means, the difference is termed significant if the probability that there was indeed no difference is below the level of significance chosen. To test whether or not the difference of the means is sign
39、ificant to the chosen level, the following calculations are made: Calculate the sample standard deviation, S, of the probability function expressing the difference between the two means using Formula (2): (2) whereS L is the sample standard deviation of the Landolt ring acuity threshold probability
40、function;S t is the sample standard deviation of the tested optotype acuity threshold probability function;N is the number of presentations. The z-score, also termed the standard score, represents the number of standard deviations a measurement is from the mean. Use Formula (3) to calculate the z-sc
41、ore using the difference value, , found in Clause 5 and S in Formula (2): (3) Use the value of the z-score in Table 1 to see if it is greater than the chosen significance level. T a b l e 1 R e l a t i o n b e t w e e n z -s c o r e a n d l e v e l o f s i g n i f i c a n c e z-score 1,28 1,645 2,33
42、 2,58 Level of sig nif i c an ce 10 % 5 % 1 % 0,5 % ISO 2015 All rights reserved 3 PD ISO/TR 19498:2015 ISO/TR 19498:2015(E) Bibliogr aph y 1 ISO 8596:2009, Ophthalmic optics Visual acuity testing Standard optotype and its presentation4 ISO 2015 All rights reserved PD ISO/TR 19498:2015This page deli
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