The invention relates to a system and method of detecting impurities in a cylindrically shaped transparent medium, wherein the cylindrically shaped transparent medium is illuminated with electromagnetic radiation, and the radiation having components emerging radially from the medium, and at least some of the components are received by a detector for detecting impurities of the medium. The components are detected at a multiplicity of relative angular positions around the symmetry axis of the cylinder, so as to form an impurity diagram that may be analyzed to detect and measure impurities in the medium.