The optical transparency in function of wavelength of CaF2 crystals doped with Eu (denoted by CaF2:Eu) has been measured. The optical absorption occurs below λ=400 nm depending on the amount of Eu atoms doped. The short wavelength below 400 nm has been shifted to around λ=425 nm. As a result, the 425 nm wavelength increased and the light below 400 nm was cut through CaF2:Eu crystals. This scintillating effect has been found much increased by adding oxygen atoms in crystals. This is due to the distortion of electrical and structural symmetry in crystals by adding oxygen atoms in the form of oxide materials.