An X-ray imaging apparatus is disclosed that includes an x-ray detector. The x-ray detector includes a member configured to convert incident x-ray wavelength photons into emitted visible wavelength photons, a position for a material under test, an x-ray source, and a structure configured to perturb an x-ray energy spectrum, each lying on a common axis. The x-ray source is arranged to direct an x-ray energy spectrum along the common axis to impinge upon the member, the structure configured to perturb the x-ray energy spectrum, and positioned material under test. The structure lies between the x-ray source and the member to one side of the position for material under test intersecting the common axis, and the structure includes at least three adjacent regions, each region different to immediately adjacent regions and configured to perturb the x-ray energy spectrum differently.