ISO/TR 9241-312:2020
(Main)Ergonomics of human-system interaction — Part 312: Readability of electrophoretic displays
Ergonomics of human-system interaction — Part 312: Readability of electrophoretic displays
This document provides an overview of recent research on readability of electrophoretic displays. It also provides information for evaluating readability of electrophoretic displays and defining the context of their use.
Ergonomie de l'interaction homme-système — Partie 312: Lisibilité des écrans électrophorétiques
General Information
Standards Content (Sample)
TECHNICAL ISO/TR
REPORT 9241-312
First edition
2020-02
Ergonomics of human-system
interaction —
Part 312:
Readability of electrophoretic displays
Ergonomie de l'interaction homme-système —
Partie 312: Lisibilité des écrans électrophorétiques
Reference number
©
ISO 2020
© ISO 2020
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ii © ISO 2020 – All rights reserved
Contents Page
Foreword .v
Introduction .vi
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Literature review on readability and legibility for electronic paper displays .2
4.1 General . 2
4.2 Readability for electronic paper displays . 2
4.3 Legibility . 3
5 Overview . 3
6 Readability evaluation for EPD under 14 levels of illumination conditions .4
6.1 General . 4
6.2 Evaluation condition . 4
6.2.1 Equipment . 4
6.2.2 Participants . 4
6.2.3 Illumination condition . 4
6.2.4 Task (Evaluation methods) . 6
6.3 Experimental results . 6
6.4 Discussion . 7
7 Proposing a baseline setup for readability using VAS evaluation .9
7.1 General . 9
7.2 Experimental condition . 9
7.2.1 Equipment . 9
7.2.2 Participants . 9
7.2.3 Illumination condition .10
7.2.4 Task (Evaluation methods) .10
7.3 Experimental results .10
7.4 Discussion .10
8 Verification of the minimum illuminance for readability of an EPD .11
8.1 General .11
8.2 Experimental condition .11
8.2.1 Equipment .11
8.2.2 Participants .11
8.2.3 Illumination condition .11
8.2.4 Task (Evaluation methods) .11
8.3 Experimental results .11
8.4 Discussion .13
9 Contribution of character sizes to the readability of mobile devices .13
9.1 General .13
9.2 Experimental condition .13
9.2.1 Equipment (specimen) .13
9.2.2 Participants .13
9.2.3 Illumination condition .14
9.2.4 Task (Evaluation methods) .14
9.3 Experimental results .14
9.4 Discussion .15
10 Difference in readability of the contrast ratio of mobile devices .15
10.1 General .15
10.2 Experimental condition .16
10.2.1 Equipment .16
10.2.2 Participants .16
10.2.3 Illumination condition .16
10.2.4 Task (evaluation methods).16
10.3 Experimental results .16
10.4 Discussion .19
11 Effects of long-term reading on visual functions and subjective symptoms .20
11.1 General .20
11.2 Experimental condition .20
11.2.1 Equipment .20
11.2.2 Participants .20
11.2.3 Illumination condition .20
11.2.4 Task (Evaluation methods) .20
11.3 Experimental results .21
11.4 Discussion .22
12 Evaluation of readability for tablet devices by the severity of cataract cloudiness .22
12.1 General .22
12.2 Experimental condition .23
12.2.1 Equipment .23
12.2.2 Participants .23
12.2.3 Illumination condition .23
12.2.4 Evaluation methods . .23
12.3 Experimental results .23
12.4 Discussion .25
13 Summary .25
14 Context of use for electrophoretic displays .26
Annex A (informative) Standardization of electronic displays .31
Bibliography .34
iv © ISO 2020 – All rights reserved
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
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 collaborates 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 approval 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 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 can be in the Introduction and/or
on the ISO list of patent declarations received (see www .iso .org/ patents).
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and
expressions related to conformity assessment, as well as information about ISO's adherence to the
World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT) see www .iso .org/
iso/ foreword .html.
This document was prepared by Technical Committee ISO/TC 159, Ergonomics, Subcommittee SC 4,
Ergonomics of human-system interaction.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www .iso .org/ members .html.
A list of all parts in the ISO 9241-300 series can be found on the ISO website.
Introduction
Electrophoretic technology has led to the development of reflective e-paper displays (EPD) that have
fundamentally different optical characteristics compared to emissive display devices, such as backlit
liquid crystal displays (LCD) or organic light emitting diode displays (OLED). EPD are used in reading
devices, also known as e-readers. See Annex A for more information on the standardization of electronic
displays.
The ISO 9241-300 series provides requirements from the viewpoint of human beings’ visual properties
and are organized by subjects.
Electrophoretic EPD were selected for the experiments reported in this document because of their
widespread use as electronic reading devices.
vi © ISO 2020 – All rights reserved
TECHNICAL REPORT ISO/TR 9241-312:2020(E)
Ergonomics of human-system interaction —
Part 312:
Readability of electrophoretic displays
1 Scope
This document provides an overview of recent research on readability of electrophoretic displays.
It also provides information for evaluating readability of electrophoretic displays and defining the
context of their use.
2 Normative references
There are no normative references in this document.
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at http:// www .electropedia .org/
3.1
visual analogue scale
psychometric response measurement scale
3.2
legibility
ability for unambiguous identification of single characters or symbols that may be presented in a non-
contextual format
[SOURCE: ISO 9241-302: 2008, 3.3.35]
3.3
readability
characteristics of a text presentation on a display that affect performance when groups of characters
are to be easily discriminated, recognized and interpreted
[SOURCE: ISO 9241-302: 2008, 3.3.38]
3.4
electronic paper display
EPD
electronic display that shows information by diffuse reflection and holds the image with low power
consumption
3.5
electrophoretic display
electronic paper display (3.4) which forms an image by rearranging charged pigment particles using an
applied electric field
4 Literature review on readability and legibility for electronic paper displays
4.1 General
[1]
A human action of reading is basically analysed by two subjective attributes, that is, readability and
[2]
legibility .
4.2 Readability for electronic paper displays
[3]
In 2006, Alex Henzen, et al. suggested that the EPD would provide a reader with “immersive
[4]
reading” . Another paper reported that the viewing distance for EPD was similar to that of VDTs at
[5]
around 500 mm, but greater than normal paper, at about 360 mm .
[6]
In 2007, An-Hsiang Wang reported on the visual performance for bending/curvature EPD. This
study indicated a future fashion of EPD, but require further exploration with progress of radiometric
measurements. Wang also reported on the reading comprehension of subjects under several ambient
[7]
illuminance conditions for electronic displays .
[8]
In 2009, there was a report on the difference of usability between EPDs and conventional books , but
it was difficult to generalize, considering the results were based on a group of 20 university students.
I-Hsuan Shen, et al. studied the visual performance and visual fatigue from EPD and found that a greater
[9]
illumination than 700 lx was necessary. Wang studied the effects of ambient illuminance on EPD and
concluded the following:
1) under lower illuminance of 50 lx, the conventional LCD with a transmissive mode was the only choice;
[10]
2) under higher illuminance of 500 lx, the EPD can perform as well as the conventional LCD .
[11]
In 2010, Wang studied the visual performance of those subjects who were advancing in age .
In 2011, Der-Song Lee, et al. investigated the effects of light source, ambient illuminance, character size,
[12]
and interline spacing on visual performance and visual fatigue for reader of electronic displays. H.C.
Wu studied the preferable viewing distance and character size for EPD and suggested that age factors
[13]
can be considered for EPD design and VDT guidelines .
In 2012, Wang examined the effects of text/background colour combinations under three levels of
[14]
ambient illuminance on the discriminating performance of young and elderly subjects. This e-paper
became a guide for the designers of colour EPD. Monika Pӧlӧnen, et al. evaluated eyestrain, visually
induced motion sickness, changes in visual functioning, user experience, and the essential optical
parameters of reading equipment for near-to-eye displays such as small size displays (of mobile phones)
[15]
and paper. The results indicated that reading from a hard copy was the most comfortable experience.
[16]
Wang, et al. investigated the effects of bending curvature EPD , but the sample size was too small to
estimate effectively the effects.
Eva Siegenthaler, et al. concluded that the image quality seemed crucial for reading against the
[17]
expectation of differential effects for reading between EPD and LCD. Siegenthaler, et al. also
[18]
analysed the reading behaviour between EPD and tablet LCD with an eye-tracking measurement.
The participants showed no difference in fixation duration, but there were significant differences
in reading speed in the proportion of regressive saccades under special artificial light conditions. C.
[19]
Connell, et al. studied the reading comprehension of subjects using EPD and tablet LCD. The results
indicated that the subjects read printed material faster than EPD and tablet.
In 2013, Po-Chun Chang, et al. investigated the effects of ambient illuminance and light source on the
reading performance of 100 participants as well as visual fatigue as they read three types of reading
[20]
tasks on an EPD and paper text. This paper showed that the reading speed depended on ambient
illuminance. Simone Benedetto, et al. studied the effects of display technology on visual fatigue over
an average period of 10 days. Their evaluation was to measure the blinks per second of each subject
and the visual fatigue scale. The results suggested that reading on tablet LCD triggered higher visual
[21]
fatigue compared to both EPD and paper .
2 © ISO 2020 – All rights reserved
In 2014, M. Miyao started to investigate readability of EPD compared to LCD and printed paper under
[22] to [30]
various ambient illuminance levels . The advantage of the experiment was to employ more than
100 participants and to analyse the results by elaborate statistical processing.
[31]
In 2015, S. Matsunami investigated the readability of EPD for aging under various illuminance levels .
4.3 Legibility
By definition, legibility is closely related to typeface design. The typeface design went through changes
from type-casting of lead block to outline font of font data on a computer. The outline font is scalable and
enables desktop publishing (DTP) with increasing display resolution. In the past, bitmap font existed
on a low-resolution display but was not scalable. With the beginning of DTP, the idea of page layout
on a computer emerged in the 1980s, although page layout was originally omnipresent in the printing
industry and publishing world from the age of movable type. Page layout has clearly become conscious
of readability, as proved by the large sales of books in the field of printing and publishing business.
In the field of visual information processing, it is commonly believed that there is a unified concept:
shape perception is explained by spatial frequency. Every shape responding to visual stimulus can be
described as a composite function with a various sinusoidal wave by Fourier analysis of its contrast
[32]
function. Elements of the contrast function are contrast sensitivities to various sinusoidal waves .
[33]
There has been a great deal of research in this fields .
5 Overview
This document explains the following 7 evaluations and results related to readability of EPD:
1) readability evaluation for EPD under 14 levels of illumination conditions (Clause 6);
2) proposing a baseline setup for readability using VAS evaluation (Clause 7);
3) verification of the minimum illuminance for readability of an EPD (Clause 8);
4) contribution of character sizes to the readability of mobile devices (Clause 9);
5) difference in readability of the contrast ratio of mobile devices (clause 10);
6) the effects of long-term reading on visual functions and subjective symptoms (Clause 11);
7) evaluation of readability for tablet devices by the severity of cataract cloudiness (Clause 12).
Equipment used in these reports:
1)
— 6-inch ILU-EPD: Kindle® Paperwhite (2012 model);
1)
— 9,7-inch EPD: Kindle® DX;
1)
— 9,7-inch backlit LCD: iPad3® (2012 launched) model: A1416.
1)
Kindle® is used due to its widespread use as an electronic reading device.
1) Kindle and iPad are trademarks of products supplied by Amazon and Apple, respectively. This information
is given for the convenience of users of this document and does not constitute an endorsement by ISO of these
products.
6 Readability evaluation for EPD under 14 levels of illumination conditions
6.1 General
The readability of EPD with and without integrated lighting unit (ILU) was compared to liquid crystal
display (LCD) with backlight and printed paper to evaluate the contributions of built-in front- or
backlights on the readability under different ambient illuminance levels from 10 lx to 8 000 lx. The
comparison was carried out under a wide range of illuminance levels. Readability was evaluated using
[22]
short English words .
6.2 Evaluation condition
6.2.1 Equipment
a) 6-inch ILU-EPD
b) 9,7-inch EPD
c) 9,7-inch backlit LCD
d) conventional paper as a reference (whiteness 69 % copy paper)
6.2.2 Participants
a) Number: 110
b) Gender: male (56), female (54)
c) Age: from 19 to 86 [mean: 45,7, standard deviation (SD): 17,8]
6.2.3 Illumination condition
Ideally, it would be best to measure the readability under natural surroundings but those are not stable
and they vary through time. Artificial stable lighting circumstances were created for 110 participants
to make several statistical comparisons. For immersive reading, a small compartment was developed to
produce stable illumination conditions. The compartment was set on a desk in a dark room for optical
measuring. Its structure is shown schematically in Figure 1. Its light source was D65 by certified
fluorescent lights (6 500 K). The illumination level was adjusted incrementally based on sophisticated
electronic circuits. Relations between set values and actual measured values of illuminance are shown
in Table 1.
4 © ISO 2020 – All rights reserved
Dimensions in millimetres
Key
1 lighting system
2 EPD
Figure 1 — Compartment structure
Table 1 — Illuminance value table
Measured value
Set value
lx
10 13,47
20 22,73
50 51,60
100 101,4
150 151,4
200 176,3
300 261,7
500 516,7
750 787,7
1 000 1 042
1 500 1 591
2 000 1 983
5 000 4 670
8 500 8 017
6.2.4 Task (Evaluation methods)
1) The participants read aloud short English words in 9-point (3,18 mm in height) Times New Roman
font, black-on-white background, shown on a display for 15 s as shown in Figure 2.
Figure 2 — Example of contents
2) While they were reading, their reading speed was measured as the number of words they could
read in 15 s. The viewing distance between the eyes and the device was measured during reading.
3) After reading, the participants evaluated readability for each display by using the visual analogue
scale (VAS) shown in Figure 3. They converted the VAS to points between 0 and 100. If they felt that
readability was the worst, they marked the left edge of the scale as 0. If they felt that readability
was the best, they marked the right edge as 100. It can be considered that VAS = 50 is appropriate
for a split decision line (an allowable limit of readability) in this subjective assessment.
Key
1 worst
2 best
Figure 3 — Visual analogue scale (VAS)
6.3 Experimental results
VAS is a subjective assessment for readability. Figure 5 shows the number of words read aloud by the
participants and Figure 6 shows the viewing distance for each display. After reading, the participants
evaluated readability for each display by using VAS shown in Figure 4.
6 © ISO 2020 – All rights reserved
6.4 Discussion
1) Participants' evaluation of readability (Figure 4)
ILU-EPD can provide readability under low illuminance conditions in comparison with EPD,
backlit LCD, and conventional paper text. Under conditions of illuminance of less than 300 lx, the
participants evaluated ILU-EPD significantly higher than EPD. The ILU has a profound effect on
readability under low illuminance conditions. However, under conditions of illuminance of more
than 750 lx, the participants evaluation of ILU-EPD was worse than EPD. This is an interesting
result to investigate in detail in the future.
Key
X illuminance, lx 1 ILU-EPD
Y subjective evaluation 2 EPD
3 LCD
4 paper
Figure 4 — Participants' evaluation
2) Reading speed (Figure 5)
Differences in reading speed were observed at illuminance levels below 200 lx. Here, the lowest
reading speed was with the EPD. The use of an ILU considerably improved reading speed with EPD.
Below 200 lx, the order of the 4 devices in terms of reading speed (Figure 5) was almost the same as
in terms of subjective readability (Figure 4). At illuminance levels above 300 lx, the reading speeds
converged to approximately 30 words in 15 s. These results did not correspond to the subjective
evaluation, but indicated the minimum illuminance for comfortable reading can be 300 lx.
Key
X illuminance, lx 1 ILU-EPD
Y reading speed, in words per 15 seconds 2 EPD
3 LCD
4 paper
Figure 5 — Reading speed
3) Viewing distance (Figure 6)
In terms of the viewing distance as an objective index of evaluation, it was supposed that the
viewing distance would be shorter as the text becomes more difficult to be read. The results did
not correspond to the subject’s evaluation, but indicated that the comfortable viewing distance for
reading can be 45 cm.
The above results showed that significant differences exist between ILU-EPDs and EPDs without
ILU. The ILU improves readability.
8 © ISO 2020 – All rights reserved
Key
X illuminance, lx 1 ILU-EPD
Y viewing distance, mm 2 EPD
3 LCD
4 paper
Figure 6 — Viewing distance
7 Proposing a baseline setup for readability using VAS evaluation
7.1 General
This study focused on the correlation between the subjective evaluation of readability using a VAS, and
reading efficiency measured as reading speed in words per 15 seconds. The methodology for measuring
subjective readability and reading speed was described in Clause 5. The correlation analysis included
data from illuminance levels between 10 lx and 5 000 lx. A baseline for readability was established at a
[23]
VAS value of 45 instead of 50 .
7.2 Experimental condition
7.2.1 Equipment
a) 6-inch ILU-EPD
b) 9,7-inch EPD
c) 9,7-inch backlit LCD
d) conventional paper as a reference (whiteness 69 % copy paper)
7.2.2 Participants
a) Number: 110
b) Gender: male (56), female (54)
c) Age: from 19 to 86 (mean: 45,7, SD: 17,8)
7.2.3 Illumination condition
General illumination conditions including illumination light source and viewing geometry were as
explained in 6.2.3. The illumination conditions were between 10 lx and 5 000 lx.
7.2.4 Task (Evaluation methods)
The participants had to evaluate the readability of the text (Figure 2) by using VAS as shown on Figure 3.
7.3 Experimental results
This was an evaluation of a VAS baseline. Figure 7 shows that the data were divided into two groups.
The first group falls below the baseline (group A). The second group exceeds the baseline (group B).
Figure 7 shows the scatter plot and linear regression line of each group. The red dots represent group A,
the blue dots represent group B. According to the analysis, the borderline between group A and group B
is approximately 45. Furthermore, the regression lines of each group show a consistent regression
curve, and the two lines intersect at about 44. From these analyses, the real baseline was found at
VAS = 45 not VAS = 50 and this baseline is useful for future investigations concerning the readability of
EPD by using VAS.
Key
X VAS A group A
Y reading speed, in words per 15 seconds B group B
Figure 7 — Scatter plot of reading speed (in words per 15 seconds) vs. VAS of readability, and
regression curve by k-means clustering (N = 1 533, K = 2)
7.4 Discussion
In this study, an experiment to evaluate the readability of EPD was carried out. Participant evaluations
were conducted using a VAS while also checking the participants reading speeds. It was found that it
is possible to establish a baseline at VAS value of 45 for future studies to indicate whether participants
reach a sufficient level of reading of a text.
10 © ISO 2020 – All rights reserved
8 Verification of the minimum illuminance for readability of an EPD
8.1 General
The aim of this experiment was to verify the minimum illumination for readability with EPD in low
illumination (300 lx or less). A participant’s readability score and viewing distance were used to
evaluate readability. Results show the 200 lx illuminance level as the minimum optimum limit of a
[24][30]
comfortable environment for reading with EPD .
8.2 Experimental condition
8.2.1 Equipment
a) 9,7-inch EPD (150 ppi)
b) 11,5-inch EPD (300 ppi)
c) 9,7-inch backlit LCD
d) conventional paper as a reference (whiteness 69 % copy paper)
8.2.2 Participants
a) Number: 130
b) Gender: male (69), female (61)
c) Age: from 17 to 85 (mean: 45,9; SD: 19,7)
8.2.3 Illumination condition
Illumination in 6 steps (10 lx, 20 lx, 50 lx, 100 lx, 200 lx, 300 lx). These illumination conditions including
illumination light source and viewing geometry were as explained in 6.2.3.
8.2.4 Task (Evaluation methods)
The text which the participants read is a fairy tale written in Japanese with 9-point (3,18 mm in height)
Shuei Mincho. There were 34 characters per line, and 13 lines in each text passage. The capability of
text interpretation is a level which a fifth grader (almost 11-year-old pupils) in an elementary school
can read. The participants read the text displayed on the devices. Subjective evaluations and viewing
distances were measured. The evaluation by the participants fell into 6 groups rated between 0 and 5.
0 indicates "very hard to read" and 5 indicates "very easy to read".
8.3 Experimental results
1) Participants assessment score (Figure 8)
The ratings on readability of the backlit LCD was high while the ratings for the EPD was lower.
This is because the contrast ratio (luminance contrast) of the EPD was low. Therefore, in this
experiment, they aimed to verify the practical allowable lower limit of environment illuminance for
reading the EPD in lower conditions of light (less than 300 lx).
Key
X illuminance, lx 1 LCD
Y subjective evaluation, rated between 0 and 5 2 paper
3 EPD (300dpi)
4 EPD (150 dpi)
Figure 8 — Subjective evaluation
2) Viewing distance (Figure 9)
As levels of illuminance increased, the participants moved their heads closer to the LCD. In
contrast, participants using the paper and the EPD, moved further away when levels of illuminance
were increased. Thus, it was suggested that paper and EPD had an advantage under a brighter
environment. In a dark environment, the LCD obtained a good rating for readability, but the
participants rated it poorer as illuminance increased. Conversely, the evaluation of the EPD and
paper recorded poor ratings in a dark environment but had improved ratings as the environment
became brighter. In totality, the viewing distance for all the media forms were similar under
between 200 lx and 300 lx illuminance.
12 © ISO 2020 – All rights reserved
Key
X illuminance, lx 1 LCD
Y viewing distance, mm 2 paper
3 EPD (300 dpi)
4 EPD (150 dpi)
Figure 9 — Viewing distance
8.4 Discussion
From the experiment, 200 lx can be a critical illuminance point. Furthermore, in a 200 lx environment,
the viewing distance for the EPD was similar to that of the backlight LCD and the paper. Thus, the 200 lx
illuminance level is considered the minimum optimum limit of a comfortable environment for reading
EPD. As for ILU-EPD, they can compensate for the disadvantages of EPD in lower conditions of light.
9 Contribution of character sizes to the readability of mobile devices
9.1 General
The readability of EPD and backlight LCD have been investigated. This study focused on the contribution
of character size to the readability. The study showed that significant differences existed among the
[25]
devices .
9.2 Experimental condition
9.2.1 Equipment (specimen)
a) 9,7-inch EPD
b) 9,7-inch backlit LCD
c) conventional paper as a reference (whiteness 69 % copy paper)
9.2.2 Participants
a) Number: 107
b) Gender: male (49), female (58)
c) Age: from 15 and 78 (mean: 47,6, SD: 15,4)
9.2.3 Illumination condition
The illumination was under 942 lx and viewing distance was kept as 40 cm. The other illumination
condition was as explained in 6.2.3.
9.2.4 Task (Evaluation methods)
The reading devices were placed in compartments on desks (Figure 1). There were 30 characters per
line in English, and 14 lines in each text passage, as shown in Figure 10. The character fonts were 8 point
(2,82 mm in height), 12 point (4,23 mm in height), and 16 point (5,64 mm in height) using the Courier
typeface. This display form conforms to those used in the evaluation criteria of a liquid crystal display
[34]
according to ISO 9241-304 .
The headrest for the participant’s forehead in the compartment was kept at a visual distance of 40 cm.
The participants began to read from the top left-hand corner. They recorded the number of capitals Ms
that each participant identified in the reading, and the total time the participants needed to read the
text was measured. After reading, they evaluated the readability by using VAS to convert into a scale
ranging from 0 to 100 points as explained in 6.2.4.
Figure 10 — Example of contents
9.3 Experimental results
Figure 11 shows the participants’ evaluation for each device. The participants evaluated the 8-point
(2,82 mm in height) font size significantly less than for the 12-point (4,23 mm in height) and 16-point
(5,65 mm in height) with the EPD, the LCD and the paper (p < 0,01). A significant difference was found
among the devices for 12-point (4,23 mm in height) between the EPD and the LCD (p < 0,05).
14 © ISO 2020 – All rights reserved
Key
X character sizes, point 1 EPD
Y subjective evaluation of VAS 2 LCD
3 paper
a
It is the significant difference in p < 0,05.
b
It is the significant difference in p < 0,01.
Figure 11 — Participants’ evaluation for each device
9.4 Discussion
The readability of the EPD and the backlight LCD were investigated by focusing on the contribution
of character size. There were statistically significant differences between the 8-point size characters
(2,82 mm in height) and the 12-point (4,23 mm in height) and 16-point (5,65 mm in height). This study
also showed that, statistically, significant differences exist among the EPD and the LCD at 12 points
(Figure 11). From the viewpoint of readability, the minimum font size is 8 points (2,82 mm in height)
because this is coincident with VAS = 45. The VAS of 12 points (4,23 mm in height) was 60,7 and the VAS
of 16 points (5,65 mm in height) was 65,2 of the EPD.
It is interesting that this result in the case of alphabetical font is close to that of Japanese fonts which
are used in Japanese paperback books or pocket books, although those typefaces are more complicated
than that of alphabetical fonts. This size for Japanese paperbacks is attributed to traditional efforts, that
is, trial and error, of making page layouts to satisfy the readers’ requirements for comfortable reading.
10 Difference in readability of the contrast ratio of mobile devices
10.1 General
In this study, the effects of the contrast ratios (luminance contrast) of characters and background
greyscale were focused on the readability of EPD. The experiments were carried out with a reading test
to evaluate the readability of EPD under various contrast ratios. Two types of EPD were used. By way of
[26]
comparison, conventional paper texts were used .
10.2 Experimental condition
10.2.1 Equipment
a) 6-inch ILU-EPD
b) 6-inch EPD
c) conventional paper as a reference (whiteness 69 % copy paper)
The participants used two types of EPD, that is, 212 dpi ILU-EPD and 150 dpi EPD. Texts on the
conventional paper printed by PPC were also used.
Since these three devices had different coloured bezel around their screens, each bezel was covered
with white Kent paper. Each device was put in the same position in the com
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