1. An apparatus for identifying defects within the volume of a transparent sheet, comprising: an illumination device that directs incident light onto at least a portion of a surface of the sheet to illuminate the sheet; and an image detector onto which the incident light that is backscattered from the sheet is directed, wherein the apparatus is adapted to generate interference patterns by superposition of components of the light backscattered from the sheet and, based on interference patterns, to generate at least two interference images under different capturing conditions in order to identify the defects by evaluating the at least two interference images.