The present invention provides a transmission type X-ray tube and a reflection type X-ray tube. The transmission type X-ray tube comprises a target and a filter material. The target has at least one element which produces X-rays as being excited. The X-rays comprise characteristic Kα and Kβ emission energies of the element for producing images of an object impinged by the X-rays. The filter material through which the X-rays pass has a k-edge absorption energy that is higher than the Kα emission energies and is lower than the Kβ emission energies. The thickness of the filter material is at least 10 microns and less than 3 millimeters.