10
Figure 4. A miniature spectrometer confi gured for transmission measurements was used to test sunscreen samples applied to surgical tape on microscope slides.
The sample holders were prepared with surgical tape (to receive a more even distribution of sunscreen), numbered, and weighed (to later determine the amount of sunscreen applied). The transmission through the blank sample holders was determined using a separate, marked, microscope slide with surgical tape (but no sunscreen) as the reference. As this sample preparation method would scatter the incoming UV light, an integrating sphere was placed after the sample to collect all of the transmitted and scattered light (Figure 4).
A thin layer of sunscreen was applied to the sample holder, scraped to create a thin, even layer, then weighed again and left to dry for 60 minutes. After this drying period, the transmission through the scattering reference slide was set again as the 100% reference to account for any changes in the lamp output over time. Finally, the transmission through the actual samples with applied sunscreen was measured. A total of eight spectra were recorded for each sample and then averaged, to account for any measurement-to-measurement variation.
Each transmission spectrum (Figure 5) indicates what percentage of the incoming UV radiation reaches the skin. However, not all UV wavelengths are created equal; some are more dangerous than others, depending on the amount of incoming sunlight and the susceptibility of the skin to sunburn as a function of wavelength. For example, radiation between 300-320 nm is the most dangerous of the rays reaching the Earth’s surface, which explains why sunscreens often predominantly absorb UV light in this region.
Calculating SPF
In our experiments, SPF is calculated by taking the ratio of the known total sunburn danger without sunscreen compared to the reduced sunburn danger as a function of the sunscreen’s transmission blocking effi ciency. The results are summarised in Table 2.
Table 2. SPF Determination for Six Sunscreen Samples Sample ID Brand
Notes
2 3 4 5 6
7
A A A A B
Regular Regular Regular Baby
Hard to spread
C Store brand
SPF
Claimed 20 30 50 50 45
30
SPF
Measured 79 99
193 48 32
35
Overall, most of the sunscreens tested in this experiment offered the claimed sun protection. In fact, the SPF determined for the regular sunscreens of brand A (Samples 2, 3 and 4) exceeded their SPF claims by a factor of 3 to 4. However the ‘Baby’ sunscreen from the same brand (Sample 5) fell short of its claimed SPF. The other sample that failed to meet the claimed SPF (Sample 6) was noted as diffi cult to spread evenly, which appears to have negatively affected its protection benefi ts.
Summary: Stay Safe in the Sun
Just because summer is ending, doesn’t mean that the dangers of UV exposure go away. The UV index, the measure of skin- damaging ultraviolet radiation at a particular location and time, can remain high even when it’s not particularly sunny or hot. Ground-based refl ection from environmental factors like sand or snow can also have a strong effect on UV index. As we head into fall and winter, protect your eyes and skin outdoors, confi dent in the fact that spectroscopic characterisation of transmission properties can confi rm that sun protection claims are reliable.
Figure 5. Transmission spectra for the sunscreen samples were averaged over eight measurements to account for variations in the spectrometer, sampling devices and UV source output.
Interested in publishing a Technical Article?
The modular spectrometers and accessories described here can be applied to similar types of transmission measurements of automobile windows and other products designed, in part, for UV protection. Also, the SPF setup described here could be used to measure SPF factors of fabrics and other materials in a manner that is both robust and reproducible.
Contact Gwyneth on +44 (0)1727 855574
or email:
gwyneth@intlabmate.com
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