Photometric System Design and Performances

If unsupported by appropriate diagnostic data, Gaia would yield immense numbers of positions and velocities of objects whose astrophysical nature would be unknown. The core science case requires measurement of luminosity, effective temperature, mass, age and chemical composition for the stellar populations in the Galaxy and its nearest neighbours. These quantities can be derived from the spectral energy distribution of the stars, i.e. through high-quality multi-band photometry. The current proposals for broad and medium band photometric systems, their capabilities for the astrophysical parametrization of the stars and hence the expected precision of the stellar populations characterization are reviewed.