Observations and estimations of solar radiation at ground level deal most frequently with global horizontal irradiance (GHI) while direct irradiance is crucial notably for Concentrated Solar Technology (CST) such as solar energy conversion systems: parabolic through, solar towers, parabolic dish or concentrated photovoltaic. Several global-to-direct irradiance conversion schemes are proposed in the literature. They are obtained by regression of an empirical parametric function with global and direct irradiance measurements made at few stations over a time period. These schemes are well suited to the climatic region where they have been designed, but not likely to other climatic regions. A new method is proposed. A general shape of the relationship between the clearsky index (Kc) and the ratio of the diffuse to global irradiance (fD) is derived from literature and analysis of several data sets of ground measurements. This analytical function needs two parameters. One is defined by the case of the overcast skies; the other is changing depending on the clear-sky conditions for the location and time under concern. A clear-sky model provides the Kc and fD for the clear-sky conditions. The new method was validated against ground measurements made by BSRN ground stations located in Carpentras, Sede Boqer and Tamanrasset. It reveals itself accurate compared to other methods. Indeed, for the data sets used, the bias amounts to 0%, -9% and 10%, the root mean square deviation (RMSD) to 16%, 19% and 21% and the correlation coefficient to 0.98, 0.96 and 0.95. The proposed method is interesting because it is flexible, adaptive and does not rely on empirical parameter.