Schematic diagram of the lensless ghost imaging system. BS: beam splitter. CCD1, CCD2: charge-coupled device camera (Mintron MTV-1881EX). The ground glass disk is placed in three different positions: (a) behind the object, (b) in front of the object, and (c) in front of CCD2. x₁, x₂ and y are the transverse coordinates on the test plane, the reference plane and the object plane, respectively.