Laboratory measurements of optical beams in young sea ice

The optical properties of young sea ice (<20 cm thick) have been measured in the Naval Ocean Systems Center Ice Optics Lab. Measurements of the upwelling irradiance from the ice surface as a function of distance from the center of the beam spot ( r in cm) were performed. The irradiance distributions were transformed to find the modulation transfer function (MTF) and beam spread function (BSF) as a function of ice thickness. The BSF could be approximated by BSF( r, z ) = exp(− k B z ) exp(− r 2 /2 〈 r 2 〉), with k B = 0.404 cm −1 and 〈 r 2 〉 ½ = 0.105 z 1.41 cm, where z is ice thickness (in cm). Angular radiance above the ice surface, resulting from a laser beam entering the ice from below, was measured as a function of ice thickness for a specific growth condition. The angular field followed a function of L ( θ, z ) = L ( 0, z )cos n ( θ ), with n = 8.1 z 0.71 , while the peak value followed a Beer’s law decay of the form L(0, z ) = L (0, 0)exp(−0.514z).