Patent ID: 6855203

Claim:
A process of making a 157 nm transmitting barium fluoride optical crystal comprising: providing at least one barium fluoride crystal graphite crucible, said at least one barium fluoride crystal crucible comprised of a permeable graphite having a permeability greater than 4 cm 2 /s (measured according to the DIN 51935 Standard), providing a barium fluoride crystal feedstock having an oxygen contamination level no greater than 150 ppm by weight, loading said barium fluoride crystal feedstock having an oxygen contamination level no greater than 150 ppm by weight into said at least one barium fluoride crystal crucible with a graphite permeability greater than 4 cm 2 /s, melting said loaded barium fluoride crystal feedstock in said at least one barium fluoride crystal crucible with a graphite permeability greater than 4 cm 2 /s to provide a barium fluoride crystal melt contained in said at least one barium fluoride crystal crucible with a graphite permeability greater than 4 cm 2 /s, inside said at least one barium fluoride crystal crucible with a graphite permeability greater than 4 cm 2 /s growing a barium fluoride crystal from said barium fluoride crystal melt to provide a grown barium fluoride crystal contained in said crucible, said grown barium fluoride crystal contained in said crucible having a 157 nm internal transmission greater than 99%/cm, and removing said grown barium fluoride crystal from said barium fluoride crystal crucible with a graphite permeability greater than 4 cm 2 /s to provide a high 157 nm transmitting barium fluoride crystal with a 157 nm internal transmission greater than 99%/cm.