Polarized Patterns in the Forest Canopy and Undergrowth — ASN Events

Polarized Patterns in the Forest Canopy and Undergrowth (#606)

Parrish Brady 1 , Molly Cummings 1
  1. University of Texas at Austin, Austin, TX-TEXAS, United States

Polarization patterns are produced by the scattering of light off of various forms of matter and are therefore both ubiquitous in natural environments and information rich. As such, the study of polarization vision, signaling, and camouflage are important topics in ecology.  However, unlike other environments with highly structured polarization patterns (e.g. the open ocean), little quantitative work has been done to characterize the polarization environments of forest canopies.  Understanding the structure of polarization patterns in the forest canopy can inform how polarization sensitive animals manage their polarization contrast with the background and potentially navigate in that environment. To measure polarization patterns in the canopy we use a custom division of time full stokes polarimeter and data logging compass and inclinometer.  This allows simultaneous measurements of the polarization signatures from vegetation and animals with the accompanying environmental angular information.  The light field structure surrounding the objects is also measured.   Simulations were developed to use the solar elevation compass and inclinometer data, ray tracing techniques, and Fresnel reflection physics to compare measured and simulated components of the vegetative polarization light field.  In direct sunlight the vegetative polarization patterns predictively follow the relative position of the sun.  But within the canopy and the forest floor where there is little direct sunlight or during cloudy conditions the polarization patterns, while still very significant, are much harder to predict. The polarization patterns in these environments are determined by the brightest sources of light and their respective solid angle.  We measure three test subjects (lepidoptera, odonata, and scarabidae) in situ for contrasting measures in these environments.  We find that the lepidoptera and odonata tested had contrasting degree of linear polarization but similar angle of linear polarization to the environment.  Scarab reflections showed contrasting angle of linear polarization with similar degree of linear polarization.