The first term in this equation for Ê T is simply the transmitted wave altered by an attenuation factor . The second term is the original object wave multiplied by an amplitude factor ; this term represents the virtual holographic image of the object . The third term is the conjugate object wave . In of f-axis holography , the real image formed by this wave is weak , lies out of the field of view , and does not make a significant contribution to the imaging process . However , for Gabor’s original in-line holography , this term represented an objectionable twin image which overlapped and obstructed viewing of the desired image . An important contribution of the Leith and Upatnieks of f-axis reference scheme was the elimination of this twin image .