A good way to test the efficiency of organic
sunscreens would be to monitor the absorption of
light by the material. A good way to do this
would be to coat transparent substrates with the
same amount of different types of sunscreen.
These substrates can then be placed into a
spectrophotometer, which can measure the amount of
light transmitted through the sunscreen. An ideal
sunscreen would transmit all visible light (so you
don't look purple or green, or whatever),but would
not allow any UV light to transmit through. To
conduct a thorough analysis, one could also
characterize the chemical content of the sunscreen
after the transmittance testing to see if any
dangerous compounds are generated during the test.
This can be done using mass-spectrometry or
infra-red spectroscopy. Another test might also
evaluate the ability of the sunscreen to stay
effective while you are swimming. In this case,
the transparent substrate covered in sunscreen
could be submerged underwater to see how easily it
can be washed off.
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