Ⅰ. Introduction

Recently, as the importance of smart education with information technology increases, readability of display also becomes more and more important in education.¹⁾ As results of several studies that suggested the effect of wearing progressive addition lenses (PALs) suppressed development of myopia,^2-4) PALs have frequently been prescribed in recent years for children and teenagers with myopia.

However, PALs have a narrower visual field than single focus lenses (SFLs),⁵⁾ which may cause degradation in legibility and then adecline in the students’ learning ability as well. In a previous study, one of the authors already pointed out the degradation in legibility of Korean letters (Han-Gul) when wearing PALs.⁶⁾

Letters have their own characteristics; therefore, it cannot be presumed that the results for Korean letters would be the same for another type of letters. For example, Korean letters are almost cubic and are put in type with almost the same width.⁷⁾ Meanwhile, the widths of English letters are not the same as one another when being typeset.⁸⁾ So it has to be tested whether wearing of PALs also affects legibility of other types of letters, similarly to Korean letters.

English and Chinese were chosen for the test letters because they are two of the most frequently used letters in the world. In this study, the legibility of English and Chinese letters were measured when readers wore PALs and also compared to the legibility when readers wore SFLs.

Ⅱ. METHODS

This study followed the tenets of the Declaration of Helsinki and was approved by the Public Institutional Review Board designated by the Korean Ministry of Health and Welfare. Every subject agreed to participate in this research by signing documents of written consent, after being informed about the nature and possible consequences of study participation.

Subjects

In total, 47 volunteers applied to participate in this study. Seven of them were excluded due to physical, mental, or ocular illness, or their experience of ophthalmic surgery (for example, LASIK). Seventeen of them were male and others were female. Characteristics of the subjects were shown in Table 1. Visual acuities of the subjects were corrected to 20/20 before reading time measurements, so as not to affect the measurements. All the subjects were Korean, but they had been taught English and Chinese letters for years from their elementary or middle school days.

Before measuring the reading time, simple explanations about PALs were given to the subjects because they did not have experience to wear PALs. The data were collected from neither subject in their 20s or old people wearing PALs. It is because this is not a study on people who are wearing PALs due to presbyopia but a study on children wearing PALs for suppression of the development of myopia.

Lenses specification

The PALs used in this study were the XL Trial Lenses from SOLA International Inc., with +1.00 D additions and the soft design. Their fitting heights are 14 mm, and widths of their intermediate zone are 6.5 mm at minimum.^9,10) Using the intermediate zone of PALs, the subjects were asked to read letters on visual charts displayed on the monitors described below.

The SFLs used in this study were also the Trial Lenses from SOLA International Inc. They had +0.50 D additions, which were the same as the average addition of the narrow intermediate zone of the PALs. The additions were for investigation of the effects of the narrow visual fields of PALs on legibility, subtracting the defocus effect of PALs.

Text Charts

The text charts utilized in this study were designed according to a previous study.⁶⁾ In Figs. 1 and 2 show examples of the English and Chinese text charts applied in this study. Their dimensions were 26.4 cm in width by 5.5 cm in height. They were made using Microsoft Power-Point (2010), and English or Chinese letters were arranged in six lines with 1.5 lines spacing. The letters were in Times New Roman font for English and PMingLiU font for Chinese, with 12-point letter size. The letters on the text charts were randomly arranged using the Random Sampling Function of Microsoft Excel.

The English letters ‘m’ and ‘s’ were selected for the English charts, and the Chinese letters ‘一’ and ‘觀’ were selected for the Chinese charts. Even among the English letters or the Chinese letters, the legibility of each letter differs from one another.¹¹⁾ One letter of high legibility and the other letter of low legibility were selected for both languages. The letters ‘m’ and ‘一’ were the letters of high legibility, while the letters ‘s’ and ‘觀’ were the letters of low legibility. Of the 4 selected letters, one letter was designated to be found in each text chart. The text charts were designed so that the designated letter appeared 20 times. Five text charts were made for each of the 4 selected letters (that is, 20 text charts in total), and they were randomly presented to the subjects to avoid the learning effect and systemic error.

The letter charts used is not a good representation of reading material in the real world as they are just random distribution of letters, not a real sentence. However, the subjects (Korean college students) are not educated to read Chinese sentence or text but Chinese letters only. So the charts that consist of randomly distributed letters had to be used for Chinese, and also for English, to keep consistency through this study.

Display

The visual charts were displayed on a MOTVMT-17D monitor made from NTNSC&C Inc., which was a liquid crystal display (LCD) including 17 inches in width, with a resolution of 1024ⅹ768, and 32-bit true colours.

Reading Time Measurement

There are many kinds of legibility measurement methods, such as the visibility method,^12,13) the defocus method,^13,14) the short-exposure method,¹⁵⁾ the blink rate method,¹⁶⁾ the distance method,¹⁷⁾ the speed of reading method,^7,13,17) and the measurement of eye movements.^18-20) Among them, the speed of reading method was chosen to measure legibility in this study. The speed of reading method evaluates the legibility of the text based on how much time it takes the subjects to read the text. The longer the reading time is, the worse the legibility is.

Study Design

The reading times were measured when the subjects wore the PALs and the SFLs. The sequence of wearing the PALs and SFLs was randomized. The reading times with PALs were compared to that with SFLs.

The heads of the subjects were set up at 64 cm from the monitor, where the 12-point sized letters corresponds to 20/47 visual charts, following to the previous studies.^6,21 The heads were fixed to prevent the effect of head movement during reading. When the subjects wore PALs, the positions and angles of the PALs and the monitor were appropriately adjusted so that the subjects had to read the text charts through the narrow intermediate region of the PALs. On the other hand, there was no limitation of visual field when the subjects wore the SFLs.

The subjects were asked to read the given texts on the monitor, indicating where they were reading using a cursor. They were also asked to mark the designated letter of the chart by a mouse-click whenever they found it. This marking instruction was given to make the subjects sincerely read the texts. Fig. 3 shows an example of the text chart on which the designated letter ‘m’ was marked by mouse-click of a subject. The subjects were also asked not to read backward. By a second monitor displaying the same picture as a first monitor for the subject, it was verified whether the subjects sincerely followed the instructions or not during reading. Before reading, the subjects practiced their task to acclimatize a few times to the measurement set-up.

The reading times were measured and recorded when the subjects took to read the given text and to find the designated letter on the chart. The 4 selected letters were designated in a random sequence, so every subject had to read 4 text charts in series with a different designated letter. Break times of 1 minute were given between reading each text chart.

Statistical Analysis

For each of the 4 selected letters, the measured reading times were averaged over the 40 subjects. The average data were analysed by the paired t-test method by IBM SPSS statistics 18. The lens types (PALs or SFLs) were assigned as independent variables, and the average reading time were assigned as dependent variables.

Ⅲ. RESULTS

Table 2 shows the average reading times and the results of the paired t-tests for English letters. When subjects wore PALs, the average reading times were approximately 93 second for ‘ m’ and approximately 110 second for ‘s’. These were longer than the average reading times when wearing SFLs, which were approximately 88 second for ‘m’ and approximately 108 second for ‘s ’. The difference between the average reading times for ‘m’ was statistically significant, but the difference was not shown significant for ‘s’.

Table 3 presents similar results for Chinese letters. The average reading times when wearing PALs were approximately 61 second for ‘一’ and approximately 91 second for ‘觀’. These results were also longer than the reading times when wearing SFLs, which were approximately 54 second for ‘一’ and approximately 87 second for ‘ 觀’. The difference between the average reading times for ‘一’ was statistically significant, but the difference was not shown significant for ‘觀’.

For all the 4 designated letters, the average reading times when wearing PALs were consistently longer than those when wearing SFLs. The results for the letters ‘m’ and ‘一’ were statistically significant. Therefore, we could estimate from these results that it took longer to read the texts while wearing PALs than while wearing SFLs.

Based on the results in Tables 2 and 3, it revealed the consistent tendency for the reading times to increase when wearing the PALs meaning that wearing PALs degrades the legibility of English and Chinese letters.

Table 4 depicts similar results of the reading time measurements for the Korean letters, which are quoted from a previous report written by Ku and one of the authors of this work.⁶⁾The results in Table 4 were measured by the same method to this study except for differences in the type of letters and the subjects groups. The results in Table 4 show similar statistically significant increase in the average reading times, when wearing PALs. It also supports our hypothesis that wearing PALs degrades the legibility of letters.

Ⅳ. DISCUSSION

The legibility of letters might be lowered when readers wear PALs due to the narrow visual field of the intermediate zone of PALs.⁶⁾ The width of the intermediate zones used in these measurements was 6.5 mm in length and equivalent to 14° in angle,^10,21) while the width of the text charts was 26.4 cm in length and equivalent to 23.1° at distance of 64cm. The narrow visual field of the intermediate zone might hinder reading of wide text charts contributing to lower the legibility of the letters.

On the other hand, considering of the binocular vision, the horizontal visual field is supposed to be about 28° through the intermediate zone of the progressive lenses. In the actual experiment, it was possible to read the 26.4 cm-wide web document without head movement. The degradation of legibility may not solely be due to the narrow visual fields of progressive lenses in monocular vision. The degradation of legibility could also be due to imperfect binocular fusion, that is, two monocular images are not fully fused under binocular vision, caused by the narrow visual fields of progressive lenses.^21,22)

It was known that wearing PALs weakly suppresses the development of myopia;^2,4) therefore, PALs have been prescribed for school-aged children with short-sightedness for years. The suppression of myopia development could be a positive aspect of wearing PALs.

Along with the development of information technology, learning has been actively carried out not only by text books (through near part of progressive lenses) but also by PC displays (through intermediate zone of progressive lenses) more and more. It was revealed from this study that wearing PALs could degrade the legibility of letters due to the narrow visual field of PALS. This degradation of legibility would be a negative aspect of wearing PALs, especially for school-aged children.

It is well known that legibility affects readability, and low readability makes it harder to learn information from text.^13,17,23) Therefore, the degradation of legibility can result in lowered learning ability.²⁴⁾ Therefore, the progressive lens prescribed for the suppression of near-sightedness may lead to a deterioration in students' readability and learning ability in the education using PC displays. This negative aspect should carefully be considered when an optometrist or ophthalmologist prescribes PALs for school-aged children.

This study had several limitations. At first, the fixation of head movement was not a realistic situation for school-aged children. At second, the subjects in their early 20s were not people of the children’s age, but rather were several years older than them. Finally, these reading time measurements were performed using only one type of PALs, the SOLA XL trial lenses. The next study is in progress and includes more realistic situations regarding the limitations such as the head movement and the subject group issue, using various types of PALs. This study would investigate not only legibility but also readability and learning ability.