Name: ISO 24509:2019 — Ergonomics — Accessible design — A method for estimating minimum legible font size for people at any age
SKU: ISO 24509:2019
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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

ISO 24509:2019 PDF

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