Overview
EN ISO 8429:1996 (ISO 8429:1986) specifies the geometry and conventions for a graduated dial scale used in ophthalmic optics and optical instruments. The standard defines a consistent angular coordinate system and marking conventions for scales, reticles and other display means used to indicate axis, orientation or angular values on ophthalmic equipment. It is referenced in national adoptions (e.g., SIST EN ISO 8429:2000) and falls under ICS 11.040.70 (ophthalmic equipment).
Key topics and requirements
- Coordinate system consistency: The same coordinate system applies to right and left eye references; it is described as seen by a person viewing the eyes/instrument.
- Zero axis and direction: The zero axis is horizontal. Where numbering runs 0°–360°, the zero point is the right-hand portion of the horizontal axis and angular values increase counter-clockwise.
- Angle reference points: 90° corresponds to the vertical axis. Conventions differ by use:
- When defining cylinder axis or principal curvature meridians, angular values are expressed as a number between 0° and 180°.
- When defining prism base orientation, angular values are expressed between 0° and 360°, with the point where the prism base touches the circle marked.
- Polar coordinates and scale design: The standard uses polar coordinates for scale and reticle layout and provides guidance on how angular orientation should be depicted for lenses, corneal surfaces and related optical components.
- Intended elements: Graduated dials, angular reticles, axis markers and related display means in ophthalmic measuring instruments.
Practical applications
- Designing and calibrating ophthalmic instruments (keratometers, lensmeters, phoropters, slit-lamp attachments) that require angular scales.
- Specifying scale and reticle layouts for devices that indicate cylinder axis, prism base, or principal curvature orientation.
- Ensuring consistent axis notation across spectacle lens designs, contact lens specification, and clinical reporting.
- Providing a common reference for instrument manufacturers, test labs and device calibrators to avoid misinterpretation of angular readings.
Who uses this standard
- Ophthalmic and optical instrument manufacturers
- Lens designers and laboratories
- Clinical optometrists and ophthalmologists involved in instrument use and prescription interpretation
- Calibration and conformity assessment bodies
- Standards committees and technical writers in ophthalmic optics
Related standards
- Part of the ISO technical corpus for ophthalmic optics; associated with other ISO/CEN standards on spectacle lenses, contact lenses and ophthalmic instruments (see ICS 11.040.70 for related references).
Keywords: EN ISO 8429, ISO 8429, graduated dial scale, ophthalmology, ophthalmic instruments, coordinate system, cylinder axis, prism base orientation, reticle design, polar coordinates.