An optical device for assessing optical depth in a sample illuminated by polarized radiation from a source include two radiation guides having their end portions arranged for capturing reflected radiation from the sample. A detector measures two polarizations of the reflected radiation, and two intensities of the reflected radiation in the two radiation guides, respectively. A processor is configured to calculate two pectral functions, which are indicative of single scattering events in the sample. The processor is further configured to calculate a measure of the correlation between the two spectral functions so as to assess whether the single scattering events originate from substantially the same optical depth within the sample. Thus, the causal relation between the two spectral functions can be used for assessing whether the single scattering events giving rise to the two spectral functions come from substantially the same optical depth within the sample.