The third editions of two British standards have recently been published, ISO 13666:2019, Ophthalmic optics – Spectacle lenses – Vocabulary and ISO 4007:2018, Personal protective equipment – Eye and face protection – Vocabulary. Because of some common membership of the groups revising these two documents, the wording for terms that are common to both are very similar, if not identical apart from the reference to lenses for 13666 and lenses or filters for 4007. Obviously, ISO 13666 is much more important for optometrists and dispensing opticians.
Thirty-one new terms have been added, while over 50% of terms and definitions have been revised. In some cases, this is as little as changing the order of synonyms for the term, to significant changes in the wording of definitions or notes to entry, but without change of meaning.
New introductions
The new terms introduced in ISO 21987 have been copied here. These include the umbrella term ‘power-variation lens’ that has been introduced both to simplify the writing of standards such as ISO 21987 on mounted lenses, and to enable lens manufacturers to group lens types such as progressive-power lenses, degressive-power lenses and lenses such as the accommodation relief lenses for non-presbyopes into adjacent places in their catalogues.
Progressive-power lenses have both a primary and a secondary reference point for focal power verification, thus giving the addition or variation power, while the other two types have only a single, primary, reference point thus implying that the variation power is not to be measured.
Other terms and definitions that have been introduced include the as-worn pantoscopic angle, faded and darkened states (for photochromic lenses), fused multifocal lens, ordered power and ordered prismatic effect, segment top and segment bottom (which were previously only in a note for segment extreme point position), solar blue-light transmittance and spherical equivalent power.
A diagram (see figure 1) has been added to illustrate the relationship between the prescribed, ordered, verification and measured powers. The term ‘measured power’ has sometimes been incorrectly used in manufacturer’s literature for the verification power.
Figure 1
To give greater similarity to the probable property measured by digital dispensing devices, the meaning of vertex distance has been altered so that it now refers to the horizontal distance between the back surface of the lens and the apex of the cornea, measured with the eyes in the primary position.
The plane of the lens shape is now defined as being the plane containing the vertical centreline parallel to the horizontal centreline of the individual lens based on the apex of the groove instead of being based on the plane tangential to the demonstration lens. This is likely to be closer to the back vertex of a mounted lens than the previous definition. The design reference points are where the manufacturer’s specifications apply, while the reference points are where the lenses are to be verified. For single-vision and most multifocal lenses, these are the same. The previous distinction for a multifocal lens, where the distance design reference point was usually the centre of the semi-finished lens blank (now called simply ‘blank’) while the distance reference point was usually the intended position of the optical centre of the distance portion after surfacing, has been removed – they are the design reference point or simply the reference points of the blank and the finished lens.
The term ‘as-worn’ corrected dioptric power has been replaced by the term verification power – this word explains its purpose better, and is clarified by a new definition. Shape magnification has been replaced by the more general spectacle magnification. The centration point position is clarified as referring to the ordered position of the centration point, not the mounted position.
Improved communication
Although terminology standards are written primarily to ensure the correct interpretation of the relevant standards, ISO 13666 was also intended to help in communications between lens manufacturers, prescription houses and dispensing opticians.
The second edition of ISO 4007 was published in 2012 to accompany the then new ISO sunglass standards, while this third edition includes the terms and definitions for the forthcoming occupational requirement and test method standards that are due out early next year. Compared with the previous edition, 52 new terms and definitions have been added, including those for mesh protectors.
The ISO committee writing the new occupational standards has decided not to use the term ‘ocular’ to describe the ‘window’ in eye protectors as in the European standard EN 166 but use ‘lens’, ‘filter’ or ‘mesh’. Although these new occupational protector standards will, sadly, continue to use the wavelength limits of 315nm to 380nm for UV-A, many of the UV transmittance terms also allow the upper limit to be 400nm, as is used in Australian/New Zealand standards and medical occupational organisations such as ICNIRP (International Commission for Non-Ionising Radiation Protection) and ACGIH (American Conference of Governmental Industrial Hygienists). As a consequence of these two possible upper limits for UV-A, a new transmittance term ‘mean 380nm to 400nm transmittance’ has been developed, which will have its own requirement in many of the filter transmittance requirements in the forthcoming standards.
The position of some of the terms within the standard has been adjusted, some obsolete terms removed, and over 100 definitions reworded to improve their clarity. Many of the terms that had previously been hyphenated are no longer, eg dark state for a welding filter, though blue-light remains hyphenated, perhaps to match the hyphen in UV-A?
Online definitions
The definition for any term used in an ISO standard may be looked up free on the online browsing platform: www.iso.org/obp/ui/#home. Click on the ‘Terms and definitions’ button before clicking on the search icon, and all definitions for that term will appear, whatever the field of use. If too many are listed, eg for ‘transmittance’, click on ‘Terms and definitions’ on the left of the screen, then on ISO/TC172/SC7 for ophthalmic optical terms, on ISO/TC94/SC6 for eye protection terms. Unless a diagram is restricted to a single term, it does not appear in this online programme.
Members of the College of Optometrists may access these standards free though the library section of the website. ABDO members also have free access to 25 different standards which they can view through the ABDO website by logging in and typing British Standards into the search bar on the site.
Ronald Rabbetts, chairman of the BSI Spectacles Committee.