about 1 ppma of oh in li doped crystals the alkali fluoride crystals of this invention can be colored by any additive or electrolytic technique as was expected the heavier alkali fluorides such as potassium fluoride and rubidium fluoride were the easiest and quickest to color the two heaviest alkali fluorides i e cesium and francium do not have a crystalline structure suitable for any practical use of their colored centers the following two examples illustrate an additive coloration and an electrolytic coloration of naf crystal prepared by the method of example i for additive coloration the crystals and about one tenth their weight of na metal were sealed at room temperature under 0 5 atm of argon in a vacuum annealed stainless steel tube the temperature is raised to 900 u00b0 c and is maintained at 900 u00b0 c for 16 hours then the crystals are cooled in 4 hours to room temperature for electrolytic coloration crystals were held in slowly flowing helium between a pointed tungsten electrode and a flat graphite anode with an electric field strength of 100 v cm applied the next examples illustrate the additive coloration of this invention the crystals had a hydroxyl ion concentration of 2 ppma kf crystals and about one tenth of their weight of k metal were sealed at room temperature under 0 5 atm of argon in a vacuum annealed stainless steel tube the temperature was raised to 700 u00b0 c and held for 16 hours then the temperature was reduced to room temperature in 4 hours the absorption spectra of the crystals colored in example ii were determined the results are shown and compared with commercially available naf crystals in fig1 curve a represents the absorption data.