ISO 24509:2019

Overview

ISO 24509:2019 - Ergonomics - Accessible design - A method for estimating minimum legible font size for people at any age - defines a reproducible, vision‑based method to estimate the minimum legible font size for single characters across a wide range of viewing conditions. It addresses legibility in both self‑luminous and reflected modes for use in documents, product labels, signs and displays where viewing distance, luminance and contrast vary. The method is targeted at people without pathological eye disorders (not for low‑vision populations) and applies primarily to fixed‑size printed materials; applicability to other languages is provided in Annex F.

Key technical topics and requirements

• Scope and limits: Applicable for ages 6–90 years, viewing distances 0.2 m–100 m, luminance 0.01–3000 cd/m2 and contrasts 0–100% (Michaelson contrast).
• Core inputs: viewer age, viewing distance, luminance, and contrast - identified as the primary vision‑related determinants of single‑character legibility.
• Calculation procedure:
  • Use normative visual acuity data at 100 cd/m2 (Annex A).
  • Apply a luminance correction coefficient (Annex B) to obtain acuity at the actual luminance (Formula (1)).
  • Compute a size factor from acuity and viewing distance (Formula (2)).
  • Convert the size factor to a minimum legible font size in points using formula Pmin = a + b·S with font‑type coefficients for serif and sans‑serif (Table 1 / Annex G).
  • Apply a contrast correction multiplier from Table 2 when contrast <100% (Formula (4)); multipliers increase as contrast decreases and differ for positive vs negative contrast.
• Supporting material: Informative annexes contain worked examples, tables for variable age/distance/luminance, language notes (Korean, Chinese, Thai), and principles behind the model.

Practical applications and users

• Who uses it:
  • Ergonomists, accessibility specialists, human‑factors engineers
  • Graphic and packaging designers, typographers, UI/UX designers
  • Signage and wayfinding planners, product compliance teams, regulators
• Typical applications:
  • Specifying minimum character sizes for product labels, packaging text, safety signs and public displays
  • Designing readable printed collateral and fixed‑font documents for mixed‑age audiences
  • Evaluating existing designs under different lighting, distance and contrast conditions to improve readability and safety

Related standards

• ISO 24502 - age‑related luminance contrast specification (referenced normative)
• ISO/TR 22411 - ergonomics of human‑system interaction (data source)
• ISO/IEC Guide 71 - accessible design principles referenced in the Introduction

Keywords: ISO 24509:2019, minimum legible font size, legibility, accessible design, visual acuity, viewing distance, luminance, contrast, ergonomics.