Plasmat lens further refines these achromats into triplets with the following design parameters: a high-index, doubly convex (DCX) lens cemented to a medium-index, doubly concave (DCV) lens cemented to a low-index meniscus lens. Up to this point, all refinements have maintained symmetry about the stop. The Plasmat further refines the Dagor form by uncementing the meniscus, allowing for placement away from the first two elements and removing the criterion of symmetry. In its most basic form, it is symmetrical and consists of two cemented groups of three lenses each. The innermost element in each group is a positive meniscus, the outermost is
Compound refractive lens the order of meters, by using many lenses in series. To minimize the absorption the lens must be made from a material such as aluminium, beryllium, or lithium. CRLs were first demonstrated in the mid-1990s by a group of scientists at the ESRF. They drilled holes in an aluminium block, and achieved focusing in two dimensions. For X-rays a concave lens focuses the X-rays because the index of refraction is slightly below unity. In a CRL of this type the walls between the cylindrical holes act as concave lenses for X-rays traveling perpendicular to the axis of the drilled cylinders.