Three revised British Standards relating to spectacle lenses were published last year. They are:
- BS EN ISO 21987 Ophthalmic optics – Mounted spectacle lenses.
- BS EN ISO 8980-1 Ophthalmic optics – Uncut finished spectacle lenses – Part 1: Specifications for single-vision and multifocal lenses.
- BS EN ISO 8980-2 Ophthalmic optics – Uncut finished spectacle lenses – Part 2: Specifications for power-variation lenses.
The three standards were revised by the same project group, with the mounted lens standard being taken as the lead. Significant effort was made to use the same verbal descriptions across all three standards, for example in measurement of the addition. More detailed instructions are included for those readers who may be less familiar with using focimeters, manual or automatic.
The new terms ‘power-variation lens’, ‘primary reference point’ and ‘secondary reference point’ have been introduced to simplify the instructions for power measurement in BS EN ISO 21987 and 8980-2. Instead of saying ‘the tolerances apply to the distance portion of a progressive-power lens and the near portion of a degressive-power lens’, these now say that ‘the back vertex power at the primary reference point of power-variation lenses shall comply with the tolerances in Table 2.’
Power-variation lenses include progressive-power lenses, degressive-power lenses and lenses such as anti-fatigue lenses that are primarily distance lenses but have a weak addition for pre-presbyopic users. It is not proposed that these terms should be used in discussion of lenses with patients, but they may also simplify manufacturers’ technical literature and the positioning of lens types within catalogues.
The term ‘position-specific single-vision lens’ applies to those lenses compensated for the as-worn position with complex surfaces which therefore have to be positioned precisely in front of the eye in order to give their optimum performance. While the power properties of these lenses are those for normal single-vision lenses, the positioning and prismatic imbalance tolerances are those for power-variation lenses.
The term ‘verification power’ has been introduced to describe the power calculated by a manufacturer that is expected to be found when checking the lens on a focimeter if the power has been compensated for the as-worn position. These powers have previously sometimes been called the compensated powers. The measured power is still that which is read on the focimeter.
Tolerances in ISO 21987 on mounted lenses
The tolerances in BS EN ISO 21987 generally remain unaltered from the first edition. If using the cumbersome method in clause 5.3.5 for the prismatic imbalance (relative prism error), the prism tolerances are now based on the highest principal power of the two lenses instead of the lowest.
This is because, for lenses with a very high astigmatic power, the prismatic tolerance allowed could correspond to a very tight positioning tolerance in the more powerful meridian. For example, a prescription such as R and L +7.00 DS/-3.00 DC x 90 has the lowest principal power of +4.00 D. In the vertical meridian, the prismatic imbalance allowed was 0.50 D. Hence, the +7.00 has to be positioned, by Prentice’s rule, to 0.5/7 cm = 0.5/0.7 = 0.7 mm. In the new edition, the tolerance is now that applying to a power of 7.00 D, ie 0.7 D or 1 mm error. If using the British preferred method that is in the informative Annex C, the tolerances remain unaltered.
The method in clause 5.3.5 is simple in concept, and probably very accurate if lenses are positioned robotically, but there are cumulative errors in the marking of the centration point on the mounted lenses and in placing these marks centrally on the focimeter’s lens support, reading any prism at these points and then calculating the prism imbalance.
The response to the GB experts on the project group has always been that the Annex C method could allow both lenses to have unwanted prism in the same direction, thus cancelling out so that the (mounted) optical centration matches the (ordered) centration distance. This is both nonsensical in that having 1∆ base out right and 1∆ base in left would not occur since the lenses would have been rejected as uncut finished lenses, and secondly lenses with such ‘yoked’ vertical prism are routinely used because of thinning prism in progressive-power lenses.
Sadly, the British experts have also been unable to persuade the group to accept tighter tolerances on prismatic imbalance for power-variation lenses. The argument is that, for example in Europe, many of these lenses are mounted in practices. Hence, the full tolerances in ISO 8980-2 should apply to the uncut finished lenses, and the mounting tolerance of ±1.0 mm horizontally and vertically (but with an over-rider that in the vertical the maximum error between the pair of lenses is 1.0 mm) is needed when mounting the lenses.
Although all the errors are unlikely to add up, it is feasible for there to be a {4 mm error + 0.50∆} at the intended position of the prism reference points, which, for a +4.00 D lens, adds up to 2.1∆ of unwanted prism. In Britain, we would regard such a pair of lenses as the prescription house’s responsibility to reject and remake.
Objections have also been raised at the apparent inconsistency in that the tolerances are a mixture of positioning errors in mm or prismatic errors in prism dioptres. This simply shows the lack of familiarity with Prentice’s Rule, and the need for a prismatic tolerance for low powered lenses and a displacement tolerance for the higher-powered lenses, exactly as shown in figures 1 and 2 in the Standard.
Although applying only to spectacles with single-vision spherical powered lenses, clause 5.6 has been added govern the alignment of the plane of transmission for polarizing lenses, while 6.7, which is copied from ISO 8980-3 on transmittance specifications, provides the test method.
In Annex A, the area of inspection for defects now applies to the whole area of the lens. Small defects unlikely to impair vision may, as before, be accepted.
Description of test methods
The standards emphasise that if the power of the lens is adjusted by the manufacturer to provide the ordered power in the as-worn position, then the verification power and not the ordered power is the one both governing the tolerances to be applied and the one to which the tolerances apply. There is a typing error on the first line of clause 6.5 – the lens should be measured in plan view, not plane view.
Addition measurement for multifocal and power-variation lenses having two reference points remains unaltered in that the default measurement method is to place the surface with the segment or the power-variation on the focimeter’s lens support.
The description is, however, shorter than in the previous editions where the appropriate surface had to be identified as the front or the back surface, and then repeating the text in the front surface and back surface methods. Manufacturers remain free to specify a different reference surface, eg front surface even if the progression is on the back surface. The addition power may also be termed the variation power for power-variation lenses. Power-variation lenses with only a primary reference point, eg degressive-power lenses, do not have their variation power measured.
Annex C has been reworded, again to make it easier to understand for those less familiar with focimeter use.
Tolerances in ISO 8980 – 1 and 2 on uncut finished lenses
The cylinder axis tolerance for powers from 0.125 and including 0.25 D has been doubled to 14°. (The 0.125 step has been included for lenses with this power generated from compensation for the as-worn position, not with the expectation that this cylindrical power will be prescribed.)
The first edition of ISO 21987 and unchanged in the new edition has 16° for the mounted lens, thus allowing 2° for mounting. The axis tolerances for powers up to 1.50 D are based on the axis error that would induce a 0.12D unwanted astigmatic error. The 2° for powers over 1.50D is chosen as the sensible manufacturing limit. For the same reason, the axis tolerances in ISO 21987 are larger than those in the uncut standards to allow for mounting error, with no allowance for this on lenses of over 2.50 D.
Other changes in ISO 8980-1 and 2
The definitions for IOA and FOA focimeters have been deleted since they are now in the spectacle lens vocabulary standard, BS EN ISO 13666:2012 (a revised edition is expected either late this year or early next year). The clauses of size and thickness have been combined and clarified.
Conclusion
A copy of BS EN ISO 21987 should be in every practice or prescription house, preferably the new one although the first edition is better than nothing. Parts 1 and 2 of BS EN ISO 8980 are mainly of interest to prescription houses or those practices that undertake their own lens mounting (glazing seems an outdated word now that extremely few glass lenses are used). These standards are available from BSI Sales, while members of the College of Optometrists can obtain copies through the College Library. Members of the FMO should contact their organisation.
The ISO committee for writing these standards wishes to undertake a world-wide survey on how lenses are verified in prescription houses and practices. This is mainly aimed at power verification and prismatic imbalance measurement. This will be a survey on the web, and will become live later this year.
Ronald Rabbetts is a retired optometrist and chairman of BSI spectacles (frames and lenses) committee and member of the sunglasses committee.