Experimental setup of computational ghost imaging. The beam expander is used to expand the transverse section of the light beam from the laser source. The Spatial Light Modulator (SLM) is controlled by a computer to generate illumination patterns of light spots with random central positions. After demagnified by the 4f-system, the light spots illuminate the object (group 4, element 4 portion of USAF resolution chart), and the transmitted light is collected by a bucket detector.