Considering in more detail the structure and function of each of the several layers of the target 12 shown in FIG. 1, a layer 26 of a conducting material such as rhodium or Tic, a form of tin oxide, may have a thickness of from about 6 to about 25 Angstroms so as to be light transparent for permitting substantially unimpeded light from a scene to reach a photoconductive layer 28.