Patent ID: 7837969

Claim:
A method of making a highly uniform, low-stress calcium fluoride single crystal, said method including tempering the calcium fluoride single crystal and said method comprising the steps of: a) heating the calcium fluoride single crystal at a temperature increase rate of less than 18 K/h to a holding temperature between 1050° C. and 1350° C. to temper the crystal; b) holding the calcium fluoride single crystal at said holding temperature reached during the heating of step a) for a holding time of at least 65 hours; c) during the holding of step b), keeping static radial temperature gradients within the crystal less than 0.013 K/cm and static axial temperature gradients within the crystal less than 0.07 K/cm; d) after the holding of step b), cooling the calcium fluoride single crystal to a limiting temperature from 600° C. to 900° C. with a cooling rate that is at maximum equal to 0.5 K/h; and then e) cooling the calcium fluoride single crystal to temperatures below said limiting temperature with a cooling rate that is at maximum equal to 3 K/h.