Overview
EN ISO 9342-2:2005 - Optics and optical instruments - Test lenses for calibration of focimeters - Part 2: Test lenses for focimeters used for measuring contact lenses (ISO 9342-2:2005) specifies the design, tolerances and manufacturing guidance for optical test lenses used to calibrate focimeters that measure contact lenses. The standard defines test-lens purpose (to determine correction values that convert instrument readings to back vertex power), materials and form-factor recommendations, and permissible power tolerances for reliable calibration.
Key topics and technical requirements
- Scope and applicability
- Applies to focimeters meeting the requirements of ISO 8598.
- Defines test lenses intended to convert measured powers to back vertex power (the conventional “power” of a contact lens, expressed in dioptres m−1).
- Design recommendations
- Test lenses should be made from homogeneous optical glass (white crown glass) free of bubbles and striae within a 4 mm radius of the aperture centre; other materials are permitted if they provide equivalent durability and reproducibility.
- Lenses must have a protective mount that does not obstruct focimeter measurement when placed on the lens support.
- Spherical test-lens geometry
- Recommended nominal back vertex powers: −20, −15, −10, −5, +5, +10, +15, +20 D.
- Free aperture: at least 8 mm.
- Back-surface curvature should approximate common contact lenses (about 7–9 mm); the recommended nominal back radius is 8 mm with a radius tolerance of ±0.1 mm.
- Centre-thickness design ranges are specified for each power to ensure durability (see standard Table 1).
- Performance tolerances
- Permissible tolerance on stated back vertex power: ±0.03 m−1 (±0.03 D).
- Annex A supplies production tolerances, fitting criteria (interference-ring fits), calculation formulae for paraxial powers and guidance on assessing expanded uncertainty.
Applications and users
- Who uses EN ISO 9342-2:2005:
- Manufacturers and calibrators of focimeters and ophthalmic test equipment.
- Contact-lens manufacturers and optical laboratories performing instrument verification and quality control.
- Calibration laboratories and regulatory bodies involved in ophthalmic equipment conformity.
- Practical uses:
- Producing and specifying standard test lenses for routine focimeter calibration.
- Verifying focimeter accuracy in converting measured power to back vertex power for contact-lens measurement.
- Supporting quality assurance and traceable calibration procedures in clinical and manufacturing environments.
Related standards
- ISO 8598 - Focimeters (applicability requirement)
- ISO 7944 - Reference wavelengths (for specifying measurement wavelengths)
- EN/ISO 9342-1 - Part 1: test lenses for measuring spectacle lenses
- ISO 18369-3 (referenced for typical contact-lens curvature)
This standard ensures consistent, reproducible focimeter calibration for contact-lens metrology and supports traceable optical measurements in ophthalmic practice and manufacturing.
