  Conventional energy filters for x-ray imaging are based on absorbing
materials which attenuate low energy photons, sometimes combined with an
absorption edge, thus also discriminating towards photons of higher energies.
These filters are fairly inefficient, in particular for photons of higher
energies, and other methods for achieving a narrower bandwidth have been
proposed. Such methods include various types of monochromators, based on for
instance mosaic crystals or refractive multi-prism x-ray lenses (MPL's).
Prism-array lenses (PAL's) are similar to MPL's, but are shorter, have larger
apertures, and higher transmission. A PAL consists of a number of small prisms
arranged in columns perpendicular to the optical axis. The column height
decreases along the optical axis so that the projection of lens material is
approximately linear with a Fresnel phase-plate pattern superimposed on it. The
focusing effect is one dimensional, and the lens is chromatic. Hence, unwanted
energies can be blocked by placing a slit in the image plane of a desired
energy. We present the first experimental and theoretical results on an energy
filter based on a silicon PAL. The study includes an evaluation of the spectral
shaping properties of the filter as well as a quantification of the achievable
increase in dose efficiency compared to standard methods. Previously, PAL's
have been investigated with synchrotron radiation, but in this study a medical
imaging setup, based on a regular x-ray tube, is considered.
