  For the first time, lead fluoride is used as a fast and compact material in
electromagnetic calorimetry. Excellent optical and mechanical properties of the
pure Cherenkov crystals are necessary for the A4 collaboration to perform a
measurement of the nucleon's strange form factors. Visible scattering and
absorption centers as well as surface damages have been investigated to
characterize the quality of more than one thousand crystals. Besides,
transmittance measurements have been performed on all crystals to reveal
absorption bands produced by intrinsic or impurity related point-structure
defects. As a consequence, 89 crystals had to be replaced by the Chinese
manufacturer SICCAS.
