Sun Screen Strength
A student in the science fair tried sun screen on light
sensitive plastic beads... they all turned color, indicating the screen
didn't work. Is this a valid test of the strngth of sun screen? Ideas?
What would be the effect if no-sun screen was used.
Would the beads change color to a larger extent?
What would happen with the beads using lower/higher SBF sunscreens.?
What is sun screen trying to filter out? All light, or just (invisible)
ultraviolet? What do the beads detect? Only ultraviolet, or visible light?
I think you can answer these questions, and from that answer your original
Richard Barrans Jr., Ph.D.
I assume that the beads were subjected to sunlight and they changed color.
These are some possible explanations for the stated observations.
The beads could be visible light sensitive and change color when exposed to
visible light. In that case, the observation does not prove anything.
The beads could be sensitive to both UV and visible light, in which case,
the sun screen could stop the UV and pass the visible. The beads would turn
color as a result of the exposure to visible light. No conclusion can be
made about sunscreen effectiveness.
If the beads are sensitive only to UV radiation, then we need to ask how
sensitive they are. Will they still show a change in color when exposed
to the fraction of the UV radiation that passes through the sunscreen?
If the beads are very sensitive, then even low levels of UV radiation could
saturate them (i.e., complete the color change). Sunscreens, even the
ones with high SPF (sun protection factor) of 30 or more, pass some UV
radiation and this may be sufficient to show a full color change to the
human eye (which is not a good light meter) .
Dr. Ali Khounsary
Advanced Photon Source
Argonne National Laboratory
Sun screen only attenuates light in certain wavelength regions. If the
light-sensitive beads respond to a wavelength range the sun screen is
not intended to block, then the test results don't say anything about the
effectiveness of the sun screen.
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Update: June 2012