Overview
ISO 9342-2:2005 defines requirements for test lenses used to calibrate focimeters when measuring contact lenses. The standard specifies how test lenses are designed, manufactured and verified so that focimeter measurements (instrument-measured power) can be reliably converted to back vertex power - the conventional “power” of a contact lens expressed in dioptres (D, m‑1). ISO 9342-2 applies to focimeters that meet ISO 8598.
Key topics and technical requirements
- Purpose: Test lenses provide precise correction values to convert focimeter readings into back vertex power.
- Recommended spherical set: −20 D, −15 D, −10 D, −5 D, +5 D, +10 D, +15 D, +20 D.
- Optical material and quality: Preferably precision homogeneous white crown glass free of bubbles/striae within a 4 mm radius of the centre; other materials permitted if durability and reproducibility match the specification.
- Geometry and apertures: Spherical test lenses must have a free aperture ≥ 8 mm. Back-surface curvature should approximate common contact lens base curves (7–9 mm); the nominal value is 8 mm with a radius tolerance of ±0.1 mm.
- Centre thickness ranges: Design thickness ranges are specified by power class to ensure durability:
- Negative powers (−20 to −5 D): 0.5 to 1.5 mm
- Positive powers (+5 to +20 D): 1.0 to 2.0 mm
- Power tolerances: Stated back vertex power values must lie within ±0.03 m‑1 (±0.03 D) of the declared value.
- Reference wavelengths: Measurements may use the green mercury line (546.07 nm) or the yellow helium line (587.56 nm) per ISO 7944.
- Manufacture and uncertainty: Annex A outlines production methods, component tolerances (radii, thickness, refractive index, surface fit) and uncertainty analysis. Example: for a +10.0 D lens the combined standard uncertainty u_c ≈ 0.0076 D and expanded uncertainty U (k=3) ≈ 0.023 D.
Applications and users
- Calibration laboratories and manufacturers of focimeters and calibration equipment.
- Optical metrology labs and quality-control teams in contact lens and ophthalmic optics manufacturing.
- Regulatory and standards personnel verifying instrument traceability to international norms.
- Eye-care equipment service technicians who perform focimeter calibration and validation.
Related standards
- ISO 8598 - Focimeters (instrument requirements to which this part applies)
- ISO 7944 - Reference wavelengths (defines the wavelengths used)
- ISO 18369‑3 - Contact lenses - measurement methods (related measurement practices)
Practical takeaway: ISO 9342-2 provides a clear specification for the design, tolerances and uncertainty budgeting of focimeter test lenses so that contact-lens power measurements are accurate, reproducible and traceable.
Open preview (PDF)Optics and optical instruments — Test lenses for calibration of focimeters — Part 2: Test lenses for focimeters used for measuring contact lenses