NOVELTY - Wavelength converter comprises a silicate or metal-silicate of the hybrid organic-inorganic type, characterized by an X-ray diffractogram having reflections exclusively at angular values higher than 4.0 degrees of 2 theta , and characterized by an ordinate structure containing siloxane structural units (a) and possibly containing one or more elements T comprising elements belonging to groups IIIB, IVB, VB, and transition metals, with a silicon (Si)/(Si+T) molar ratio in the structure of higher than 0.3 and lower than or equal to 1.    USE - The silicate or metal-silicate of the hybrid organic-inorganic type is useful as a photoluminescent material, in a spectrum converter (claimed). The converter is useful in a solar device and a light system (all claimed).    ADVANTAGE - The converter optimizes the portion of solar spectrum which can be used by the cell for conversion to electric current, and prevents light with an excessively high energy from damaging the cell. The converter improves the performances of devices for the exploitation of solar energy, such as photovoltaic cells and photo-electrolytic cells. The converter improves the conversion efficiency of solar radiations. The converter allows an improvement in the light efficiency and color yield of the light system.    DETAILED DESCRIPTION - Wavelength converter comprises a silicate or metal-silicate of the hybrid organic-inorganic type, characterized by an X-ray diffractogram having reflections exclusively at angular values higher than 4.0 degrees of 2 theta , and characterized by an ordinate structure containing siloxane structural units of formula (a) and possibly containing one or more elements T comprising elements belonging to groups IIIB, IVB, VB, and transition metals, with a Si/(Si+T) molar ratio in the structure of higher than 0.3 and lower than or equal to 1.    R = organic aromatic group, preferably -C6H4-, -CH2-(C6H4)-CH2-, -C2H4-(C6H4)-C2H4-, -(C6H4)-(C6H4)- , -CH2-(C6H4)- (C6H4)-CH2- or -C2H4-(C6H4)-(C6H4)-C2H4-, when the aromatic group R of the structural unit (a) contains only one benzene ring, the substituents of the ring containing the silicon atoms are in position 1,4 and when the aromatic group R of the structural unit (a) contains a biphenyl group, the substituents of the biphenyl containing the silicon atoms are in position 4,4'.    An INDEPENDENT CLAIM is included for a light system comprising the converter, a light source and possibly a substrate.