9
Figure 1. Regardless of lens composition, the glasses tested were effective in blocking UVB radiation (280-315 nm).
The glasses were oriented with the outside of the eyewear lens touching the integrating sphere. Eyewear orientation, proximity to the integrating sphere, and measurement location had little or no observed effect on the consistency of the measurements.
Regardless of the composition of the lenses (glass or plastic, with or without different coatings to provide colour and remove polarisation), the eyewear had similar blocking effi ciency of light in the UVB region with very low (~0.5%) transmission across the entire UVB range (Figure 1). This level of blocking extended throughout the UVA region up to ~380 nm, where the lab safety glasses began transmitting light (Figure 2). This was not unexpected as these glasses were not designed to block UV.
Figure 3. The Vis-NIR blocking effi ciency of the more expensive eyewear options was very high, but even the least expensive glasses performed reasonably well.
However, the more expensive sunglasses were the most effective in fi ltering visible wavelengths, which might be worthwhile for people who require polarisation (light from the sun is unpolarised) or spend a lot of time in bright sunshine. Those considerations aside, even the inexpensive sunglasses helped protect from damaging UV rays.
Verifying the Sun Protection
Factor of Sunscreen A person’s tolerance to sun exposure varies signifi cantly, and depends on skin type. Exposure before burning may be as short as just fi ve minutes for a person with fair skin and hair, or far longer for someone with dark skin and hair. Applying sunscreen extends a person’s exposure time depending on the sun protection factor (SPF). For example, for someone who burns at 5 minutes of unprotected exposure, an SPF of 20 should increase the person’s ability to avoid sunburn by a factor of 20 minutes, giving them 100 minutes of sun protection. (Regardless of skin type and SPF level used, dermatologists recommend reapplying sunscreen every two hours).
In the past, the sun protection factor was measured using human subjects. Small patches of skin on the back of volunteers were exposed to increasing amounts of UV light, some areas protected with the sunscreen under test, some patches left unprotected as a control. The ratio of the exposure times that led to the fi rst sign of reddening with and without sunscreen then gave the sun protection factor for the sunscreen. Not only is this method potentially painful and subject to variability in volunteers’ skin, it is also diffi cult to fi nd volunteers for this type of testing now that the dangers of UV radiation are common knowledge. A spectrometer is a pain-free, accurate and objective way to quantify SPF.
Figure 2. All eyewear samples tested demonstrated effective blocking of UVA wavelengths (315-400 nm) up to 380 nm.
All of the sunglasses had increased transmission of light starting at ~400 nm with varying degrees of blocking through the Vis-NIR range (Figure 3). Interestingly, blocking across the Vis-NIR range had a similar profi le for all the sunglasses measured with the best blocking (lowest transmission) across the visible range by the more expensive sunglasses. This suggests that the sunglasses share similar coatings or materials to provide better blocking in the visible region.
Based on this experimental setup, the eyewear designed to absorb UVA and UVB radiation did so effectively, regardless of price.
Determining SPF Spectrally
Sunscreen works by absorbing or refl ecting UV light before it reaches the skin. White pigments, such as titanium dioxide used in high-SPF sun blockers, work in refl ection mode. Most other ingredients, such as benzophenones, absorb a portion of the UV radiation. Of interest for the measurement of the sun protection factor is the percentage of the UV light that is transmitted and still reaches the skin.
To test this, a general purpose UV-Vis spectrometer (200-850 nm) with focusing lens was combined with sample holders and an integrating sphere for the experimental setup. A deuterium light source simulated natural UV light.
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