Reflectance difference spectroscopy (RDS) is a spectroscopic technique which measures the difference in reflectance of two beams of light that are shone in normal incident on a surface with different linear polarizations. It is also known as reflectance anisotropy spectroscopy (RAS). Reflectance difference spectroscopy (RDS) is a spectroscopic technique which measures the difference in reflectance of two beams of light that are shone in normal incident on a surface with different linear polarizations. It is also known as reflectance anisotropy spectroscopy (RAS).