A compound refractive lens (CRL) is a series of individual lenses arranged in a linear array in order to achieve focusing of X-rays in the energy range of 5–40 keV.
The CRL gets its reasonably short focal length, on the order of meters, by using many lenses in series, hence reducing the curvatures of each lens to practical levels.
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.
In contrast, for visible light the index of refraction is larger than unity and focusing is done with a convex lens.
Scientists associated with the ESRF synchrotron have done much of the CRL's subsequent development, notably the parabolic CRLs pioneered by the Aachen group under Lengeler.