The latest editions of BS EN ISO 10322, Ophthalmic optics – Semi-finished spectacle lens blanks – Part 1: specifications for single-vision and multifocal lens blanks and –Part 2: specifications for progressive-power and degressive-power lens blanks have just been published. They will be of interest to prescription houses and practices with their own surfacing laboratory.
With the increase in free-form surfacing, progressive-power lenses are now mostly produced using single-vision semi-finished lens blanks. Hence lens blanks (the abbreviation used throughout the standards) are now classified by the form of the finished surface, not the intended use.
The reference method for measuring surface curvature and hence power is now by reflectometry and not measurement of the sag for single-vision and multifocal lenses or calculation from vertex power for progressive-power lens blanks. Both standards now have orientation requirements for polarizing lens blanks. For single-vision lens blanks, the marking is usually along the intended horizontal orientation. Some manufacturers confusingly mark along the vertical, for the sensible reason that such marks, often notches in the edge of the blank, will not encroach on the usable area of the lens since the vertical boxed lens size is always less than the horizontal size. Such vertical marking is to be clearly identified as such.
Minor changes have been made to the marking and information to be made available clauses.
The informative annex on surface and material defects now applies the ‘no defect that may impair vision’ to the whole area of the lens blank, not the restricted areas that were in the previous editions. The caveat of ‘occasional defects may be acceptable’ still applies.
Tolerances
The power tolerances apply to the refractive index of the material. Some instruments may not be programmable to read the powers and tolerances in this index, but may refer everything to a fixed index, eg the notional 1.53 frequently used in the US.
A new informative annex has been added to give the equation for converting tolerances from one refractive index to another.
The Scope of Part 1 now applies tolerances to lens blanks with aspherical surfaces. The tolerances for unwanted cylindrical power have been widened slightly for both spherical and aspherical surfaces after a survey of what was actually able to be produced. A focimeter may be used for measuring the addition power of fused multifocal lens blanks, and can be used for measurement of addition of all lenses if no reflectometry instrument is available.
The Scope of Part 2 has been widened to include degressive-power lenses, so the power tolerances now also apply to the near portion of moulded degressive-power lens blanks.
Ronald Rabbetts is chairman of BSI spectacles (frames and lenses) committee and member of the sunglasses committee.