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. They are an alternative to the KB mirror. 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. Their signature material is beryllium: a group at the Advanced Photon Source APS demonstrated the same lenses in lithium. These lenses have a direct counterpart in visible light.