The present invention relates to an optical device for assessing optical depth (D) in a sample (100) illuminated by polarized radiation (20) from a source (10). A first and a second radiation guide have their end portions (3Oa', 30b') arranged for capturing reflected radiation (25a, 25b) from the sample. A detector (40) measures a first polarization (P_l) and a second polarization (P_2) of the reflected radiation (25), and a first and a second intensity (I_1, I_2) of the reflected radiation (25a, 25b) in the first (30a) and the second (30b) radiation guide, respectively. Processing means (60) then calculates a first (f) and a second (g) spectral function, both spectral functions (f, g) being indicative of single scattering events in the sample. The processing means (60) is further arranged to calculate a measure of the correlation (C) between the first (f) and a second (g) spectral function so as to assess whether the single scattering events originate from substantially the same optical depth (D) within the sample. Thus, the causal relation between the first and second 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 (D) within the sample. The invention is particular advantageous for optically probing an epithelial layer of a patient.