Studies of Scattering and Absorption in Light-Absorbing Materials

Sage Farrar, Fishers Island School, Fishers Island, NY; Harold Metcalf
and John Noe, Laser Teaching Center, Department of Physics and
Astronomy, Stony Brook University.

This summer I studied two different topics dealing with the scattering
and absorption of light in different materials. Both topics were
suggested by recent news stories, and both are related to familiar
situations or problems.

During the initial weeks of the Simons program I researched the
scattering of light in human tissue. My interest in this began when I
read about an FDA-approved acne treatment which involves exposure to
strong blue light from an array of LEDs (light emitting diodes).
Porphyrin compounds within the propionibacterium acnes (P. acnes)
bacteria that cause acne are photo-sensitive, and when exposed to the
blue LED light produce singlet oxygen which kills the anaerobic
bacteria. This treatment is remarkable in that only light is used,
without any other chemicals or drugs. In England, the same method of
treatment is used, but they use yellow light, claiming that it
penetrates the skin deeper. I wanted to test different colors of light
on human tissue to see which wavelength is absorbed the most, and
scattered least, as this might relate to the effectiveness of the
different treatments. While this topic is very interesting, designing
a suitable experiment that would give new and useful information is
not easy. For this reason this study was set aside when a more
straight-forward topic came up.

My second topic was suggested by a news story (New York Times, 22
July) about a Yale study that tested the efficiency of anti-glare
decals versus the dark greasepaint that football and baseball players
have traditionally used on their faces. The conclusion was that the
traditional greasepaint was more effective in reducing glare than the
newer decals.  It seemed that direct measurements of the actual
scattering and absorption properties of the different materials could
complement the Yale study, which was based on questionnaires.

My experimental setup was the same as that being used by an
undergraduate, Thomas Cummings, for a study related to laser
drilling. The setup allows a sensitive photodetector to be moved in a
semi-circle in front of the sample, at a constant distance. In my
initial measurements the light source was a near-infrared laser beam
(lambda=1.06 microns) incident perpendicular to the test
surface. Measurements were taken every five degrees from 0 to
180. Measured light levels were compared to readings from a "bright
white" diffuse test material (Spectralon). These initial
measurements showed that even the black portion of the decals
reflected about twice as much light as the greasepaint sample did (20%
versus less than 10%). The measurements are now being extended to
other wavelengths and angles of incidence. In the future I would like
to continue my earlier research on light scattering from human tissue
with medical applications.

This study was supported by the Simons Foundation. I would also like
to thank Richard Migliaccio for providing the photodetector and other
assistance.