The invention relates to targets for an X-ray transmission tube (9); to a high efficiency, high excitation energy X-ray transmission tube; to combinations of the targets and high efficiency X-ray transmission tubes; and applications for utilizing such X-ray tubes. The target comprises two or more different thin foils (1) or at least two foils of the same material but different foil thickness on separate areas of a substantially planar substrate which is substantially transparent to X-rays. The target may also comprise at least two different foils (2, 3) layered sequentially one of the other, wherein X-rays are produced when an electron beam impinges the foil closest to the source fo the electron beam; wherein the energy of the electron beam is selectively changed to produce X-rays of a least one preselected energy characteristic of at least one of the foils.