  Light concentrators for the solar neutrino experiment Borexino and the
Counting Test Facility (CTF) have been developed and constructed. They increase
the light yield of these detectors by a factor of 2.5 and 8.8, respectively.
Technical challenges like long term stability in various media, high
reflectivity and radiopurity have been addressed and the concepts to overcome
these difficulties will be described. Gamma spectroscopy measurements of the
concentrators show an upper limit of 12e-6 Bq/g for uranium and a value of
120e-6 Bq/g for thorium. Upper limits on other possible contaminations like
26Al are presented. The impact of these results on the performance of Borexino
and the CTF are discussed and it is shown that the design goals of both
experiments are fulfilled.
