Last year, British Standards published an ISO Technical Report; PD ISO/TR 20772:2018 Ophthalmic optics – Spectacle lenses – Short wavelength visible solar radiation and the eye. Although not explicitly stated, the reason for writing the report was to try to evaluate whether or not radiation in the wavelength band 380 to 400nm and just above was likely to be particularly detrimental to the eye. 380nm is the wavelength chosen as the upper wavelength limit for UV-A for spectacle lenses and sunglasses while nearly all other applications use 400nm, eg medical physics, physics and Wikipedia (https://en.wikipedia.org/wiki/Ultraviolet). As a consequence, many sunglasses are labelled UV400, while Zeiss Vision has recently claimed that all its resin lenses have been treated to absorb up to 400nm.1,2
Section 4
Section 4 of the report gives a history of the possible reasons for the choice of 380nm for this upper limit for spectacle lenses and sunglasses. As a consequence, claims such as UV400 have become common, or even UV420, especially on sunglasses. An attempt was made some years ago in the ISO committees responsible for terminology for spectacle lenses and for sunglasses to define a blocking wavelength that described a cut-off wavelength below which the spectral transmittance was less than x% where x% was between 1% and 4%, while a note mentioned that this could be used to control claims such as UV400.
There was resistance to including a definition for either UV400 or blocking wavelength with a mention of UV in an ISO standard. The latest Australian/New Zealand standard for sunglasses3 has a definition for UV400 that is very similar to the cut-off wavelength with a value of 5% for x, and this may be the justification for Zeiss’ UV400 claim.
The report purposely omits discussion of the potential effects of ultraviolet radiation below 380nm on the eye since this wavelength range is both acknowledged to cause damage and is outside the scope of the report.
Section 5
Section 5 describes the passage of sunlight through the atmosphere, and introduces the concepts of Air Mass or the path length taken by radiation. Thus, Air Mass 1 is when the sun is directly overhead, and to an approximation, Air Mass 2 is when the sun is 30° above the horizon. The spectacle lens transmittance standard, BS EN ISO 8980-3, and the general purpose sunglass standard, BS EN ISO 12312-1, both take Air Mass 2 from a 1940 engineering paper as the spectral distribution of energy across the UV spectral range.
The section continues by describing the protection given by the eyebrows, and mentions Sasaki’s work4 which showed that in countries such as Britain, when, if facing the sun, the UV radiation on the eye peaks mid-morning and mid-afternoon since the brows no longer protect the cornea. Reflections from the ground, pupil reactions and the height of the upper lid relative to the pupil are discussed, as is the peripheral light focusing or Coroneo5 effect where light from the temporal side can be focused by the cornea onto the nasal limbus. The section concludes by detailing the absorption of the radiation at 380, 390 and 400nm by the components of the ocular media.
Section 6
Section 6 continues the theme with a graph plotted from data in a CIE report giving the spectral transmittance of the human eye at one year of age and then at 10 years to 100 years at 10-year intervals. The eyes of children up to about age 10 transmit a few percent of UV radiation around 320 to 330nm. The radiation hazards to the eye sections discuss blue light phototoxicity, mentioning that the solar radiance at midday can be 100 times higher than that of artificial lighting, while the proportion of blue light in the standard illuminant D65 that represents daylight is about 25% to 30% compared with less than 5% in incandescent illumination. Various epidemiological studies of damage to the retina and their mechanisms are discussed in a couple of sub-sections, while temporary after-effects of viewing a laser at 405nm are mentioned.
Although not mentioned in the report, the project group researching the subject could not find any significant evidence for damage to the eye caused by radiation in the 380nm to 400nm zone. This may be because there were no convenient radiation sources, eg lasers, in this waveband, though if used, they would have given information on acute damage rather than long term damage, such as the damage from a welding flash (arc eye) rather than cumulative damage over years.
The use of blue-light blocking coatings and treatments on lenses is currently a discussion point, particularly in their appropriateness for viewing computer, tablet and smart phone screens. Wisely, this report does not offer an opinion, but the function of the intrinsically photosensitive retinal ganglion cells (ipRGCs) using melanopsin associated pigment in controlling the pupil light reflex, melatonin synthesis, etc, are mentioned. Hence the report suggests that it might be sensible to reduce spectacle lens transmittance in the 400nm to 455nm range while maximising the transmittance at wavelengths above 475nm where the ipRGCs are sensitive.
The side effects of blue light absorption on clear lenses may result in a yellow discolouration unless this is masked by adding a faint blue dye that results in a faintly grey lens, while on tinted lenses, blue light absorption may affect traffic signal light detection.
The project group researching the subject could not find any significant evidence for damage to the eye caused by radiation in the 380nm to 400nm – acute exposure was not considered
Section 7
The 2004 and 2013 guidelines on protecting the eye from optical radiation by the International Commission on Non-Ionizing Radiation Protection (ICNIRP), mentioned in Section 7, show that for the adult eye, the blue light hazard function, B(λ), with its maximum at around 440nm would be appropriate for limiting acute damage. There is currently no equivalent action spectrum for protecting the retina against long term damage as a cause of AMD.
Section 8
Section 8 discusses how optical materials filter radiation, mentions frames with a high face-form (wrap) angle to help protect the eyes and a very brief summary of spectrophotometry – for further details on this, the reader is referred to ISO 12311:2013. The report concludes with a nine-point bullet list summary of the report. It is followed by the references section which has 122 entries that could be a useful source for further reading.
The report is available from the BSI Shop (https://shop.bsigroup.com/SearchResults/?q=20772) while members of the College of Optometrists can download it through the library section of their website and members of ABDO can also obtain copies through their Association.
Ronald Rabbetts is the chairman of the BSI Committee responsible for spectacle lenses and frames.
References
1 Laughton, D, Hopkins, P and Wolffsohn, J. Ocular UV protection in practice. Optician, 21 Sept 2018, 28-31
2 Anon. Zeiss UV Protect lenses. Optician, 11 Jan 2019, 22-23.
3 AS/NZS 1067-1:2016 Eye and face protection – Sunglasses and fashion spectacles – Part 1: Requirements Standards Australia Limited, Sydney/Standards New Zealand, Wellington
4 Sasaki H., Sakamoto Y., Schnider C., Fujita N., Hatsusaka N., Sliney D.H, Sasaki K. UV-B exposure to the eye depending on solar altitude. Eye & Contact Lens, 37, No 4, 191-195, 2011
5 Coroneo M.T. Pterygium as an early indicator of ultraviolet insolation: a hypothesis Br J Ophthalmol, 77: 734-739, 1993