Combinatorial reactive co-sputtering using Al, Si and Sm targets in an Ar + O 2 atmosphere, resulted in Sm doped SiAlO thin films with a wide Sm concentration- and Si:Al composition gradient. By combining position dependent EDX spectra and laser excited emission spectra, ternary phase diagrams were constructed that directly show the relation between Sm emission intensity, index of refraction, thickness and composition. Using this approach, the Sm 2+ and Sm 3+ emission intensity ratio was controlled towards films with predominantly Sm 2+ emission, which is most favorable for luminescent solar concentrator (LSC) applications. The optimum Sm 2+ efficiency was reached when the Al content was about equal to the Sm content. When the Si:Al ratio decreases, the Sm 2+ emission intensity strongly drops to almost zero. However, sputtering without Al resulted in no Sm 2+ emission intensity at all. The excitation and emission properties of Sm 2+ in the optimized thin films, especially the ratio between the 4f→4f and 5d→4f emission that is sensitively susceptible to the co-ordination polyhedron, closely resembles that of Sm 2+ doped crystalline powders with the same composition. This strongly suggest that the Sm 2+ ions in our amorphous films are coordinated in the same way. A homogeneous thin film on float glass clearly shows the light concentration effect of the red Sm 2+ emission. Due to an unexplained low Sm 2+ absorption of our films, even the optimized thin films do not luminesce brightly. @en