Unlike the optical spectrum, x rays' short wavelength means that they do not refract very much through normal optical materials, but rather are absorbed or pass right through with only small changes in propogation. X-ray optics, therefore, seek to nudge the radiation in the right direction as opposed to the much simpler visible optics that can grab, bend, and literally twist light into any shape necessary. A polycapillary lens, also called a Kumakhov lens after its Russian inventor, is basically an array of hollow optical fibers. Arrays of these capillaries can actually redirect the x rays to a focal point. This scanning electron micrograph shows a single fiber approximately 0.5-mm wide.
Borosilicate glass is similar to the material used to clad visible and IR optical fibers.
The variance of the internal walls has to be controlled to within 10s of angstroms, which excludes the use of most metal with conventional manufacturing techniques. Other considerations enforced by the small critical angles and the desire to capture as much of the radiation as possible is the fiber's internal diameter. Multifiber components are mainly used for collimating diverse x-ray sources. External screens hold the fibers with their hundreds of channels. Internal diameters stay constant in this configuration, although they can be bent to 'focus' the photons as dictated by the application.