Characterization of bidirectional reflectance of a black material with a goniometer submerged in water

We present a new, to our knowledge, instrument for calibrated angular reflection measurements in water. The system measures the bidirectional reflectance of finite samples at various incident angles, both in water and in air. By submerging the instrument itself, the setup minimizes systematic uncertainties arising from reflection and absorption at the sample container interfaces. Total hemispherical reflectance factors, calculated by integrating measured bidirectional reflectance distributions, indicate a decrease in the reflectance of black Spectralon when immersed in water. These bidirectional measurements are further evaluated against theoretical models using parameterized fits.