The DIRC detector is successfully operating as the hadronic particle
identification system for the BaBar experiment at SLAC. The
production of its Cherenkov radiator required much effort in
practice, both in manufacture and conception, which in turn
required a large number of R&D measurements. One of the major
outcomes of this R&D work was an understanding of methods to
select radiation hard and optically uniform fused silica
material. Others included measurement of the wavelength
dependency of the internal reflection coefficient, and its
sensitivity to the surface pollution, selection of the radiator
support, selection of good optical glue, etc. This note
summarizes the optical R&D test results.