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
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.
