Optical grade crystals of lithium fluoride (LiF) have been developed for high level (50 MGy) and high temperature (250°C) dosimetry at GSF, based on optical density (OD) measurements. The measuring accuracy however was limited by a relatively high coefficient of variation (20%) for the interspecimen scattering of response. A method is described to improve precision by individual probe calibration at a relatively low dose level (1 kGy) by using F-centres only. It is shown that the calibration factors are applicable at high doses, where other centres are involved. By this procedure, the coefficient of variation for interspecimen scattering can be reduced to ≤ 5%. Thermoluminescence (TL) and trace element analysis was performed to find an explanation for the interspecimen scattering. While no correlation could be found between optical density and trace element content and between optical density and TL, a strong correlation was observed between optical density and fluorescence.