Radiation:
Laboratory
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Radiation: |
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Light transmission was also examined in
the laboratory using samples taken from ice cores. Sections taken
from an ice core are placed inside the white core jacket and the top
surface of the core is illuminated with normally incident broadband
light. A fiber optic cable is mounted in the housing beneath the
jacket, where light transmitted through the core is monitored
spectrally. The fiber is coupled to an Ocean Optics
spectrophotometer, where the signal is detected. The incident light
is monitored by using a second core jacket in “reference mode” with
no ice.
The core jacket can be used in two
modes. One mode measures single 13 cm long sections of a core. Each
section is assumed to have optical properties homogenous in space.
The other mode uses a “layer stripping” approach. A core segment as
long as 40 cm can be placed in the core jacket and its transmission
measured. Obvious changes in the appearance of the ice are used to
guide a segmenting process. Sections of core are cut from the top
according to this segmentation. The core jacket itself is segmented,
allowing for 2-cm increments to be removed from the sample. Each
time a layer is removed, the optical transmission is recorded.
Finally, the bottom-most 13 cm thick section is used and its
transmission recorded. The results from this measurement can be
reconstructed to piece together the full vertical profile of
scattering in an extended section of ice core.
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The plot on the right shows a
transmission spectra for a 13 cm thick section. Since the ice sample
is not horizontally infinite, but rather has finite diameter, a 2D
radiative transfer model is used to interpret the observed spectral
transmission. The model simulates the effects of the finite core
diameter, the optical properties of the interior walls of the core
jacket, the refractive boundaries at the top, base, and sides of the
ice and the transparent floor that supports the ice sample.
Inferences from the model result in determination of a quantitative
measure of the scattering within the core.
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