Letter: The outer limits
Author: Ronald Rabbetts, chairman, BSI spectacles committee
My thanks go to the authors of the article on UV protection by spectacle lenses (Optician, September 21) and for details of the potential problems in the 380nm to 400nm wavelength band.
I am very pleased that Zeiss has been able to extend its UV protection in clear spectacle lenses up to 400nm. The confusion over claims for ‘absorbs all UV’ on whether the upper limit for UV-A limit is taken as 380 or 400nm has been longstanding.1 Even now, there is strong objection by some members in industry to changing the upper limit of the UV-A band to 400nm, even for the purposes of occupational eye protection.
Past attempts to define UV400 in the ISO vocabulary standards for both spectacle lenses and eye and face protection were also rejected, culminating in a project to write an ISO technical report, 20772 Ophthalmic optics – Spectacle lenses – Short wavelength visible solar radiation and the eye, which should be published soon. This discusses further some of the points made by Dr Laughton et al in the recent article.
The Australian and New Zealand standards organizations have modified ISO standards to adopt the 400nm upper limit, something that we are not allowed to do. The 380nm upper wavelength limit for UV-A taken for spectacle lenses and sunglasses probably originated in the days of glass lenses, since these are generally very poor at absorbing the longer wavelengths of UV-A.
Even clear CR39 lenses transmit only about 7% of UV-A if calculated up to 380nm, 15% if calculated up to 400nm. If the ICNIRP UVR hazard weighting is ignored, as it should be for UV-A, these values rise to about 19% and 45% respectively; wearing spectacles is better than not from this point of view.