EXAMINATION OF THE VISUALACUITY ON THE LCD
OPTOTYPE WITH WHOLE-LINE AND INTERPOLATION
METHOD
Tomáš Zajíček, Petr Veselý, Svatopluk Synek
Masaryk University, Faculty of Medicine, Brno, Komenskeho nam. 2, 602 00, Czech Republic
E-mail(s): zaj.tommy@gmail.com, 176573@mail.muni.cz, 1561@mail.muni.cz
Introduction: In human history visual acuity has been defined many times. One conventional definition, which also describes the resolution of the eye is as follows: “Visual acuity is the eye’s ability
to distinguish two points lying as close as possible." [1, p. 71] Resolution of two of values was points that lie at a distance from each other perceived below 1 arc minute is possible, because in the macula
there are the images of these two points separated by one cone. [1] It is necessary to realize that the patient´s visual acuity and even the minimum separabile does not reflect only on the state of macula
but also the whole refractive condition of the eye. The work focuses on the visual acuity value differences measured with whole–line and threshold interpolation method. It includes also a comparison of
measured values (between these two methods). Measurement was performed on LCD optotype in order to maintain the opportunity to repeat this study. The purpose of this work is to highlight the
differences between whole – line and threshold interpolation visual acuity scoring methods. It should be pointed the fact that the methods and measurements procedure were inspired by the optometric
practice. For this reason there was the optotype which is used in practice used for each method. In this period of time fairly comprehensive set measured that can be processed in various ways. In this
work there was chosen such processing methods which allow investigators to choose the appropriate method, according to the character of the measured values, available equipment and possibilities for
the investigation. These methods of measuring visual acuity will be evaluated and compared. It is necessary to remark that if there is a comparison of the results from different measuring methods and
optotype boards, the finally output could be affected by the conversion of the result to a common unit (logMAR).
Hypotheses:
First hypothesis: The value of visual acuity measured with
whole-line method on Snellen chart is not equal to the
value measured by interpolation method on ETDRS chart.
Second hypothesis: Interpolation method has better
repeatability, so it is more reliable than the whole-line
method.
Participants and methods:
Sample FNUSA: Number of
respondents was 36, in the range of
age from 22 to 88 years. Some
patients had artificial mydriasis, or
had an amblyopic eye, that is why
only 66 eyes were measured. The
measurements were performed only
with letter optotype characters. The
charts were presented with a LCD
optotype Smartchart LCD CP-400
made by Opto, which has 24-bit
display and a diagonal dimension of
19 inches. The testing distance was 4
meters, the background brightness
was constant (200 cd/m2).
Sample LF: Number of respondents was 33,
including 7 men 26 women (age 19-30). The
measurements were performed with letter
optotype characters and some with Landolt rings.
The measurement was carried out with LCD
optotype Smartchart CP-200 made by Opto,
which has 18-bit display and a diagonal
dimension of 15 inches. The testing distance was
5 meters, the background brightness was
constant (200 cd/m2).
Results: A total of 1034 measurements of visual acuity, were divided into different groups and evaluated.
First hypothesis
Snellen chart: The vision testing started with
the top of the chart and continued until a line
was reached where more than 60% of the
letters (eg, 2 of 3, 3 of 5) were read incorrectly
or the patient read all letters on the chart. The
visual acuity was scored using whole-line
scoring method, with the lowest value of the
line, where at least 60% of the letters (eg, 2 of
3, 3 of 5) were identified correctly.
ETDRS chart: The patient read the lines one by
one from the top. The testing continued to the
bottom until the patient made a complete line of
errors or read all letters on the chart. The visual
acuity was scored using interpolation method,
where the examiner calculated the value of visual
acuity in logMAR units.
Mean values of visual acuity [logMAR]
Sample Method of scoring
Average visual
acuity [logMAR]
FNUSA ETDRS-interpolation 0,0247
Snellen-whole-line 0,0454
LF ETDRS-interpolation -0,0184
Snellen-whole line -0,0431
Sample Method of scoring
Average character difference
[number of characters]
FNUSA ETDRS-interpolation 1,727
Snellen-whole-line 2,288
LF ETDRS-interpolation 2,279
Snellen-whole line 4,552
Sample FNUSA, box plot of visual acuity values [logMAR]
Sample LF, box plot of visual acuity values [logMAR]
Second hypothesis
Average character differences between the measurements
In a sample where only two measurements (FNUSA) were performed
the average is "repeatability" at ETDRS chart 1,7, at Snellen chart 2,3
characters. This represents 25% difference between the two methods.
In the second sample, where 5 repeated measurements were made
the difference grows up to 50% (mean difference at ETDRS chart is 2,3,
at Snellen chart 4,6 characters).
Sample LF - percentage distribution of the number of characters
differences according to the methods of measurement
Sample FNUSA-percentage distribution of the number
of characters differences according to the methods of
measurement
Conclusion: From the measured results implies that there is significant difference between the measurement of visual acuity with whole-line and interpolation method. It is also confirmed that the
interpolation method on ETDRS chart has a better repeatability than the whole-line method on the Snellen chart. Similar results describe authors of following studies [3,4,5,6,7,8,9]. In this study there
have been not described all the factors by which these two methods could be examined and compared (e.g. measuring length, intensity of the patient and examiner), so it would be premature to draw
absolute conclusions about determining the better method. However, on the base of results, it can be formulated some recommendations for practice:
•If comparing the patient's visual acuity measured by these different methods, you must pay attention to that the actual visual acuity will be different even if the measurements indicate the same value.
The worse vision is, the greater the deviation. (see hypothesis 1)
•If we wanted to compare the visual acuity measured by several different examinators in different offices, it is better to perform these measurements on ETDRS chart. It is more reliable and standardized in
practice than the Snellen chart. (see hypothesis 2)
•In the case of repeated measurements, such as long-term monitoring of the patient's visual acuity the interpolation method on the ETDRS chart is more efficient than the whole-line method to eliminate
errors. (see hypothesis2)
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