ISO 7921:2024

Overview

ISO 7921:2024 - "Ophthalmic optics and instruments - Near reading charts" specifies requirements, definitions and test methods for printed, projected and electronic near reading charts intended to assess near reading acuity under photopic (well-lit) conditions. The standard applies to the Latin alphabet (and can inform work on similar alphabets) and defines a clinical notation system based on logRAD (logarithm of reading acuity determination), x‑height and other common notations (M size, N size, decimal acuity, reduced Snellen).

Key Topics and Requirements

• Scope: High-contrast text charts for near reading acuity at a standardized reading distance of 40 cm; applies to printed, projected and electronic displays.
• Core definitions: logRAD, reading acuity angle, x‑height, M size and N size - all tied to the angular size of lowercase letters.
• Typeface and layout:
  • Use a typeface similar to Times New Roman (serif) or Helvetica (sans serif); standard font/spacing, no italics.
  • Line spacing ≤ 1.5×; spacing between different text sizes ≥ double the smaller line spacing.
  • Text should be continuous, linguistically simple excerpts; minimal use of uppercase, numbers and symbols.
• Size progression: Text sizes must progress geometrically (logarithmically) per ISO 3:1973 (R’10); charts must include logRAD 0.00 (x‑height 0.582 mm at 40 cm) and may include smaller sizes.
• Tolerances: x‑height permissible deviation ±5% (for logRAD ≥ −0.10) and ±10% (for logRAD ≤ −0.20).
• Electronic display requirements: Pixel pitch guidance (e.g., full vertical pixel size ≲ 0.08 mm for logRAD 0.00, ≲ 0.04 mm for logRAD −0.30) to avoid performance differences versus printed charts.
• Luminance and contrast:
  • Background luminance 80–200 cd/m² (recommended 80–120 cd/m²); white matt background for printed charts.
  • Text luminance ≤ 15% of background → contrast ≥ 85% (Weber formula).
• Verification & tests: All tests are type tests; conformity requires measuring devices with x‑height error ≤ 0.01 mm.
• Accompanying documentation: Manufacturer details, setup/use instructions, maintenance and illumination checks; Annex A covers clinical application and using charts at other distances.

Applications and Users

ISO 7921:2024 is intended for:

• Ophthalmic instrument and chart manufacturers designing near reading charts (printed or digital).
• Clinicians and eye-care professionals performing standardized near reading acuity assessments.
• Medical device and software developers of visual-acuity apps and electronic displays for clinical testing.
• Researchers comparing reading performance, validating chart equivalence across scripts, or developing cross-language assessments.
• Regulatory and conformity assessment personnel evaluating chart performance and documentation.

Related Standards

• ISO 3:1973 (preferred numbers)
• ISO 15004-1 (ophthalmic instruments - general requirements)
• IEC 60601-1 (medical electrical equipment - general safety)

Keywords: ISO 7921:2024, near reading charts, near reading acuity, logRAD, x‑height, M size, N size, reading acuity, electronic displays, luminance, contrast, ophthalmic optics.