A stackable rack may comprise: at least two rails, a cross-section of each rail having a shape resembling a staircase, first step portion of which represents a first surface upon which a reflector can be disposed; and a second step portion of which represents a second surface which can support another rail. A method of making a narrow band x-ray filter may comprise: providing a substrate; and stacking one or more reflection units in succession upon the substrate, each reflection unit including a rack (such as mentioned above) and a reflector held by the rack. An apparatus to produce a substantially narrow band x-ray beam may include such a filter. An apparatus to make an x-ray image of a subject may include: the apparatus to produce a substantially narrow band x-ray beam, e.g., as set forth above, and an x-ray detector arranged to receive the narrow band x-ray.