ISO 24509:2019

INTERNATIONAL ISO
STANDARD 24509
First edition
2019-10
Ergonomics — Accessible design —
A method for estimating minimum
legible font size for people at any age
Ergonomie — Conception accessible — Taille de police lisible
minimale pour les personnes de tout âge
Reference number
©
ISO 2019
© ISO 2019
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Published in Switzerland
ii © ISO 2019 – All rights reserved

Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Application conditions . 2
5 Estimation of minimum legible font size . 2
6 Correction by contrast effect . 3
Annex A (normative) Visual acuity data as a function of age and viewing distance .5
Annex B (normative) Luminance correction coefficient to visual acuity . 7
Annex C (informative) Calculation example of the minimum legible font size at a given
viewing condition . 8
Annex D (informative) Minimum legible font size calculated for variable age and viewing
distance at constant luminance levels for reference .10
Annex E (informative) Practical application to a group of people with a range of age and
viewing conditions .25
Annex F (informative) Application to non-alphabetical characters: Korean, Chinese and
Thai languages .27
Annex G (informative) Data and principle of the method for estimating minimum legible
font size .29
Annex H (informative) Scaling of legibility using minimum legible font size .35
Bibliography .37
Foreword
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This document was prepared by Technical Committee ISO/TC 159, Ergonomics, Subcommittee SC 4,
Ergonomics of human-system interaction.
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iv © ISO 2019 – All rights reserved

Introduction
Characters are widely used almost every time displays, documents, and other media for visual
information are involved. Accessibility to printed or displayed text is one of the critical issues in this
age of mass information, especially for older people. However, methods for designing and evaluating
the legibility of characters have not been well established yet.
Most of the problems with legibility are concerned with appropriate font size to read text in various
conditions. This problem is worse for older people whose visual acuity decreases with age, especially
at near viewing distances. Provision of legible characters to older people at any viewing condition is
becoming more important to enhance safety and comfort in their social activities.
The legibility of text has two major problems. One is concerned with legibility of single characters
and the other one is for words and sentences where inter-character spacing or inter-line spacing is
additionally investigated. The former one is the basic problem of legibility and can be extended to the
legibility for words or sentences.
While there exist many factors that affect legibility of single characters, a limited number of critical
factors can be identified as vision-related ones, which include age of the viewer, viewing distance,
luminance and contrast. A method for estimating legible font size using these critical factors can
be developed and generally applied to a wide range of practical cases. There can exist other critical
factors concerning with physical aspects of presenting characters such as display characteristics and
[1]
their environments , but these effects can be evaluated once the basic method based on human vision
factors is established.
The legibility for people who have pathological disorders like low vision are not addressed in this
document due to a lack of scientific resources as well as due to large individual differences in visual
abilities among people with different types of impairments.
[2]
This document is based on principles of accessible design from ISO/IEC Guide 71 and on data from
[3]
ISO/TR 22411 .
INTERNATIONAL STANDARD ISO 24509:2019(E)
Ergonomics — Accessible design — A method for
estimating minimum legible font size for people at any age
1 Scope
This document provides a method for estimating minimum legible font size for single characters, but not
for words or sentences, in self-luminous or reflected mode, used in documents, products labels, signs,
and displays for people at any age and in any viewing condition in which viewing distance, luminance
and contrast are varied.
This document applies designing and evaluating legibility of single characters for people at any age
who have no pathological disorders in their eyes, but not for people with visual impairments such as
low vision. The application is specifically directed to, but not limited to, the cases of printed materials
where fixed font size is used.
Applicability of the method to other languages is given in Annex F.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements of this document. For dated references, only the edition cited applies. For
undated references, the latest edition of the referenced document (including any amendments) applies.
ISO 24502, Ergonomics — Accessible design — Specification of age-related luminance contrast for
coloured light
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 acuity
V
capacity for seeing distinctly fine details that have very small angular separation
any of a number of measures of spatial discrimination such as the reciprocal of the
value of the angular separation in minutes of arc of 2 neighbouring objects (points or lines or other
specified stimuli) which the observer can just perceive to be separate
[SOURCE: CIE S 017/E:2011]
3.2
viewing distance
D
distance between the eye and the object to look at
Note 1 to entry: Expressed in meters in this document.
3.3
luminance correction coefficient
coefficient for correcting the effect of luminance on visual acuity (3.1) relative to the acuity at the
100 cd/m level
3.4
minimum legible font size
character size, expressed in unit of point, for a single character under which the character is not legible
with less than 80 % probability at the specific viewing condition
Note 1 to entry: The unit of point is defined in this document as 1 pt = 0,351 4 in, which is used mainly in the US
and Japan. Another definition is 1 pt = 1/72 inch = 0,352 77 in, mainly used in the EU. The difference is too small
to have any influence on legibility.
Note 2 to entry: Minimum legible font size is a baseline of legibility and not at the comfort level of legibility (see
Annex H)
4 Application conditions
For the application of the method for estimating the minimum legible font size, the following viewing
conditions shall be specified:
a) age: the age of a person shall be from 6 years old to 90 years old;
b) viewing distance: the viewing distance shall be between 0,2 m and 100 m;
2 2
c) luminance: the luminance shall be between 0,01 cd/m and 3 000 cd/m .
NOTE Luminance means the luminance of the surrounding area, not of the font, for the positive font
type where a character looks darker than the background. In case of negative font type, where a character
looks lighter than the surrounding area, luminance means that of the character.
d) contrast: the contrast, positive or negative, shall be between 0 % and 100 %, defined by Michaelson
Contrast (see NOTE in Clause 6).
5 Estimation of minimum legible font size
The minimum legible font size shall be calculated by the following procedure. Calculation examples and
application are given in Annexes C, D and E.
a) Select a visual acuity, V , at the luminance level of 100 cd/m for a specified observer’s age and for a
specified viewing distance from Annex A.
b) Select a luminance correction coefficient, k, for a specified luminance, L (in cd/m ), of the viewing
condition from Annex B, and correct visual acuity to the one under the specified luminance using
Formula (1).
Vk= V (1)
where
V is a visual acuity under the specified viewing condition;
k is a luminance correction coefficient;
V is the visual acuity at the luminance of 100 cd/m .
c) Calculate a size factor, S, using Formula (2).
2 © ISO 2019 – All rights reserved

SD= V (2)
where
S is the size factor;
D is the viewing distance (m);
V is the visual acuity under the specified viewing condition.
d) Calculate the minimum legible font size, P (in points), using Formula (3) with the size factor and
min
coefficients given in Table 1 (see Annex G).
Pa=+Sb (3)
min
where
P is a minimum legible font size (in points);
min
S is the size factor;
a and b are the formula coefficients given in Table 1.
Table 1 — Formula coefficients a and b in Formula (3)
Font t
...