Keywords

1 Introduction

In recent years, display technology has become widespread and highly developed. People are able to read or view high-quality content on e-readers and tablet devices. There are various methods of displaying texts on tablet devices which can affect readability. In an environment where illuminance is high, such as the outdoors, readability is reduced for liquid crystal display terminals such as the iPad because of the glare caused by the reflection that is likely to occur. On the other hand, the readability of e-book readers is less likely to decrease when the environmental illuminance is high because this display system is similar to paper. However, since e-book readers do not have a light source, readability is reduced in an environment with low illuminance. In order to solve such problems, manufacturers have inserted front lights into e-book readers.

In general, elderly people often have difficulty with reading because their eye lenses become cloudy with age and of presbyopia.

In this study, we focused on the effects of contrast ratios between characters and background of mobile devices in terms of readability. We carried out experiments with a reading test to evaluate how different age groups evaluated e-books under various conditions of illuminance.

2 Method

2.1 Subjects

The subjects for this study included 107 healthy males and females between the ages of 15 and 78 years (mean: 46.9, SD: 15.5). We classified the subjects into three groups. Those individuals up to 44 years of as were placed in the “Younger” group, those who were 45–64 years old placed into the “Middle-age” group, and those who were 65 years old or older were in the “Elder” group. The subjects who usually wore glasses or contact lenses used them for the experiments. We obtained informed consent from all subjects and approval for the study from the Ethical Review Board of the Graduate School of Information Science at Nagoya University (Table 1).

Table 1. The age groups of the subjects
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2.2 Experimental Design

We carried out the experiment using an illumination box in a darkened room. The headrest for the subject’s forehead, which was on the illumination box, was kept at a visual distance of 40 cm. We carried out the experiment under conditions of 754 lx using a 6500 K LED light source with a fluorescent lamp that maintained the same color and temperature (Fig. 1).

Fig. 1.
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Experimental setting

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We used two types of e-paper, the Amazon Kindle Paperwhite (released in 2013) [1] and the Sony Reader (released in 2013) [2], as well as plain paper (with the text printed on PPC paper of 69 % whiteness). Below, we will refer to the Paperwhite as PW, the Sony Reader as SR and the paper as Paper. The PW had an EPD (Electrophoretic Display) with front light, and the SR used a traditional reflective EPD. Since two of the electronic devices had different colored frames around their screens, we covered each frame with white Kent paper. Each device was raised to the same height as the subjects on the mounting board.

2.3 Task Design

The experimental task required subjects read silently the texts displayed on the devices. We used a random alphanumeric text. We used a PDF file for displaying the text, and a unified font size displayed on each device. The font type was Courier, and the font size was set at a uniform 8 pt. There were 30 characters per line, and 14 lines on each page. The display format conformed to those used for evaluation of electronic display devices in the International Organization for Standardization (ISO) [3]. Figure 2 shows an example of the text that subjects read.

Fig. 2.
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Example of text

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We determined the contrast ratio for each display as the difference between the black text and the various changes in the shades of the colors for the different backgrounds. We calculated the contrast ratio from the measured values of the brightness of the background and the text. We set the contrast ratio at three levels by using three background colors in the grey level in which were numbered as 51 (dark gray), 153 (light gray) on the e 256 step scale between 0 (black) and 255 (white). We labelled the contrast ratio for, “Low” for the grey level of 51, “Medium” 153 and “High” at 255. We used 0 (black) for the text color. Table 2 shows the contrast ratio of each device.

Table 2. Contrast ratio of each device
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We had subjects read the text silently from the top left. After reading, we had subjects evaluate the readability of text for case. This trial was carried out a total of nine times for each device and each contrast ratio. The order of device and contrast ratio was random for each subject.

We used subjective evaluations as an index for comparing the readability of the devices. After reading, we had subjects evaluate the texts using a visual analog scale (VAS). We converted the values of the VAS into a 100-point scale for analysis of the subjective evaluations (Fig. 3).

Fig. 3.
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Visual Analog Scale (VAS)

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3 Results

3.1 Comparison of Each Device and Contrast Ratio

Figure 4 shows a graph of the subjective evaluations for each device according to the three contrast ratios. The vertical axis indicates the subjective evaluations, and the horizontal axis indicates the contrast ratio. After applying an ANOVA, there were significant differences in each device and each contrast ratio. Comparisons of the devices are shown by solid lines and comparisons of the contrast ratios are shown by broken lines. For all devices, the subjective evaluations decreased as the contrast ratios became lower. In particular, the evaluations of the Low contrast ratio were much lower than those of the Medium and High contrast ratios. Looking at each contrast ratio, evaluation of SR with a Medium contrast ratio was significantly lower than that of the other two devices.

Fig. 4.
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Subjective evaluations for each device

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3.2 Comparison of Each Age Group

Figures 5, 6, and 7 show the subjective evaluations by each age group divided according to each device. The vertical axis indicates the subjective evaluation, and the horizontal axis indicates the contrast ratio. After applying an ANOVA, we found significant differences between how ages rated each device. As a whole, the subjective evaluations decreased as the age of the subjects increased for the Medium and High contrast ratios.

Fig. 5.
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Subjective evaluations of each age group (PW)

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Fig. 6.
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Subjective evaluations of each age group (SR)

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Fig. 7.
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Subjective evaluations of each age group (Paper)

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4 Discussion

In this study, we evaluated the readability of e-paper and ordinary paper text. In the case of the Medium contrast ratios, the subjective evaluations of the SR were significantly lower than that of the other two devices (Fig. 4). This is explained by the fact that there was the boundary value of the contrast ratios that divided the evaluation between the contrast ratio of the SR and those of the other two devices. From a previous experiment, the contrast ratio of a display that can be used without discomforts was found to be a value of 5 or more [4]. On the other hand, the value of the Medium contrast ratio that we used in this experiment was lower than 5 for the SR, but greater than 5 for the other two devices (Fig. 8). From this, we considered the value of the contrast ratio of 5 to be a reference value for determining whether the readability of e-paper reached the recommended range. Also, in our previous experiment with similar tasks, it was suggested that a reference value for the subjective evaluation to determine whether the readability of e-paper has reached the recommended range be 45 on a 100-point scale [5].

Fig. 8.
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Contrast ratios (Red line shows the value of 5) (Color figure online)

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Figure 9 shows the value of the subjective evaluation of the Medium contrast ratio is less than 45 for the SR, but above 45 in the other two devices. Thus, we considered the value of the contrast ratio of 5 (Fig. 8) to correspond to the value of the subjective evaluation (VAS) of 45 with respect to readability.

Fig. 9.
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Subjective evaluations (Red line shows the value of 45) (Color figure online)

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A comparison of the age groups shows that the subjective evaluations of the Younger group was significantly higher than that of Elder for the Medium contrast ratios of the PW and SR. However, there were no significant differences in the ratings of the Paper text. The value of the contrast ratio for the Paper text was 6.62. This “Medium” value for paper is a value close to the contrast ratio of a newspaper [4]. Many elderly people are accustomed to this level of contrast ratio found in newspapers.

We found that the subjects tended to evaluate Paper higher than those of the other two devices for this experiment. For example, the subjective evaluations for Paper text were higher than those of e-paper devices in an environment of 750 lx. And this was consistent with the results of previous experiments [6]. Conventional paper has some advantages because of its various characteristics and long history [7].

5 Conclusions

In this study, we conducted a subjective experiment in order to compare and examine the differences in readability of mobile devices according to the contrast ratio of the text and background. For statistical processing, we divided subjects into three age groups. We used three devices in this experiment, Amazon Kindle Paperwhite, SONY Reader and plain paper text. We set the contrast ratios at three levels and asked subjects to evaluate the readability of each device using Visual Analog Scale.

From the experimental results, we found that there are positive and negative differences readability by the degree of the contrast ratio. Because the value of the contrast ratio of 5 corresponded to the value of the subjective evaluation (VAS) of 45, we think that the contrast ratio of 5 should be the criteria for determining whether e-paper has reached the recommended range of comfortable readability.

In terms of the age groups, the subjective evaluations decreased as the age of subjects increased for the Medium and High contrast ratios. We recognize that cloudiness in the eyes and cataracts reduces readability. For Low contrast ratios, the evaluations were low regardless of the device or age groups. Therefore, we feel that the values of Low contrast ratios are not suitable for any age.