Recently published is BS EN ISO 12312-2: 2015 Eye and face protection – Sunglasses and related eyewear Part 2: Filters for direct observation of the sun. This gives the requirements for filters to use during an eclipse. The transmittance requirements are very similar to those in BS EN 1836: 2005 Personal eye-equipment – Sunglasses and sunglare filters for general use and filters for direct observation of the sun, except that a single overall range of luminous transmittance of between 0.0032 per cent and 0.000061 per cent is given rather than being divided into the five different shades or categories that is in EN 1836. For any particular luminous transmittance, the UV-A and infra-red radiation requirements are the same in both standards, though the UV-B requirement is now defined in terms of a weighted average over the wavelength range rather than a maximum spectral value.
Use of filters
Many years ago when there was an eclipse in the UK, there was concern over whether metal-coated filters would be safe if there were tiny perforations in the film. This is now quantified, with ‘such filters shall not exhibit more than one pinhole defect not greater than 200µm in average diameter within any 5mm diameter circular zone’, and a test method is specified. Annex A gives information on the use of filters for direct observation of the sun, a comparison of their properties with those of welding filters, and retinal safety calculations. Annex B is a user’s guide, abridged from a paper in a NASA document. This also gives advice on filters for photography and use with telescopes. Sadly, the next partial solar eclipses visible in the UK will be years away in 2021 and 2022.
With the publication of this standard, all the content of EN 1836 is now covered by BS EN ISO standards, although EN 1836 has a three-year overlap period with its replacement ISO Standard: BS EN ISO 12312-1: 2013 Sunglasses for general use. Hence, after next spring, new models of sunglasses coming onto the European market should all be tested to the BS EN ISO standards and not to EN 1836.
Some minor revisions of Part 1: Sunglasses for general use are in progress. The most significant revision is concerned with the darkest category, luminous transmittance from over 3 per cent up to or equal to 8 per cent, where temporal protection is required, by a design having either a high face form angle or side-shields. The dimensions for this currently apply to models designed for a PD of 64mm, ie adult styles. It is intended to extend these dimensions to cover the PD range down to any model designed for a PD greater than 54mm. This will then apply to sunglasses designed for teenagers, but probably not those for young children where PD adopted for testing is the 54mm. The reason for this is that research will have to be done to evaluate what the measurements should be. The French government has specified, probably on the basis of providing the best UV protection, that in France children’s sunglasses must be this darkest category (that is 4 per cent). These will be needlessly dark except in very bright sunshine. There is an opposing opinion that such dark lenses or filters may work against the emmetropisation process, and hence the avoidance of myopia. The amended standard will be published this month.
Spectacle frames
An amendment to BS EN ISO 8624: 2013 Ophthalmic optics – Spectacle frames – Measuring system and terminology, will be published in the next few weeks. This adds a definition of angle of side into the standard, although a diagram showing the angle is already included in BS EN ISO 7998: 2005. The main purpose of the addition was to clarify the term, ‘pantoscopic angle’ for the US. This is used in the rest of the world to mean the angle between the plane of the spectacle lens and the vertical (the ‘as-worn pantoscopic angle’ in BS EN ISO 13666: 2012 Ophthalmic optics – Spectacle lenses – Vocabulary). In the USA, the term ‘frame pantoscopic angle’ will be allowed instead of angle of side. The author has seen an information sheet for measuring patients for individualised progressive-power lenses where the angle of side was measured, not the as-worn pantoscopic angle. There is a small change in the angle compared with that in BS 3521 Part 2: 1991 – Terms relating to ophthalmic optics and spectacle frames Part 2. Glossary of terms relating to spectacle frames, in that the line relating to the side is taken from the middle of the joint to the point on the lower edge of the side where it is assumed to rest on the top of the ear, not from the bottom of the joint. It is also specified in relation to the vertical centreline of the lens shape, so for a frame that has a markedly upswept shape towards the joint, the frame-measuring rule must be placed near the centre of the lens shape, and not against the rim close to the joint; the latter position will work well, however, with current lens shapes.
Members of the College of Optometrists can get copies of relevant British Standards through the Library section of the College’s website.
Ronald Rabbetts is chairman of the BSI spectacles (frames and lenses) committee and member of the sunglass committee