Ultraviolet Eye Safety: Rationale For Sunglasses

Ultraviolet (UV) protection for sunglasses is widely discussed in  specialized literature due to its association with cataracts. One of the aspects addressed in literature is that the use of sunglasses is critical, because they can lead to the opposite effect, if the lenses do not provide adequate UV protection [1– 4]. The main and natural mechanism is that, without the use of sunglasses and under sun exposure, miosis reduce the incidence of radiation and protect the inside of the eye. In addition, cornea and acqueous humor absorptions should  be considered. Sunglasses frame play an important role. This work is about the calculus of the solar UV influx through the pupil of the eye to rationale eye safety and ultimately standards revision.

The typical investigated situation is that of an individual, wearing sunglasses in the upright position, exposed to atmospheric conditions described as clear sky, that is, without clouds and pollutants. This individual can be anywhere on the globe, on any day and time of the year. In this scenario, the individual will keep his gaze fixed on the horizon (0° line of sight). We calculate the irradiance and radiant exposure for this vertical surface, simulating the spectrum of solar radiation [5–7].

We made this calculation for the latitude of the geographic center of São Paulo (–22°04'11.8'' S), Brazil, for each day of the year, from sunrise to sunset. In the calculation of the luminance of the sky element, we use the luminance distribution model from the literature [8]. In this work, we propose an improvement in the modelling using SMARTS2.7–9. Pupil diameter as a function of environment luminosity is calculated as described in literature. The relative influx of UV through the pupil, with and without the glasses was determined using the solar spectral irradiance incident on the cornea, spectral transmittance of the anterior chamber to the lens and the spectral transmittance of sunglasses lenses measured by a spectrophotometer.

The determined coefficients Af and Au are the pupil areas WITH and WITHOUT sunlight attenuating  lens, respectively, where Af>Au and is calculated as a function of the luminance sensitive by the eye. The initial approach for solving this problem determining the pupil diameter as a function of luminance. Thus, as a first result, we were able to calculate, for a given day of the year, the pupil diameter as a function of the time of day, as the sun light rays relatively move. The working range of interest for luminance is of daylight. We used 4 formulas to calculate the pupil diameter according to brightness sensed by the eye (avarage valeu). This lead to a rationale about pupil dilation and ultraviolet entering the eye considering eye safety.

It was determined the quantitative adaptation of  pupils to brightness throughout the day. At noon, the sun is closer to Zenith, the direct incidence of light on the pupil is lower if compared to mid-morning or afternoon, when the sun shines more directly into the eye. Thus, the pupil is larger at noon. These results combined with the transmittance spectra of sunglasses samples leads to determining how light attenuation provided by sunglasses affects the pupil diameter. We have been contributing over the last 27 years for sunglasses standards. We suggest ISO12312-1 to adequate UV limits for the standards (up to 400nm) and that frames geometry should be considered for eye UV protection as well. FAPESP 2021/12240-2 is our scientific sponsor.