Abstract: A halogen cycle incandescent lamp with a bulb of soft glass, the inner suce of the bulb being depleted of alkali ions to avoid a reaction between the halogen constituents of the filling gas and the alkali constituents of the bulb glass. The vacancies thus generated in the glass lattice may be filled by replacement ions such as Li, Mg, Ca and/or the bulb may be coated with a protective layer of a metal and/or semi-metal oxide such as SiO.sub.2, TiO.sub.2, B.sub.2 O.sub.3. The sodium ions are removed from the bulb glass by preferably hydrogen chloride or hydrogen bromide gas which acts on the bulb at a temperature between 500.degree. C. and the softening point of the glass. The vacancies are filled by bringing the inner surface of the bulb in contact with molten salts or solutions of salts of Li, Mg or Ca. The protective layer is applied by introducing a metal halide and/or semi-metal halide e.g., TiCl.sub.4 or SiCl.sub.4 into the bulb which forms metal oxide and/or semi-metal oxide e.g., TiO.sub.2 or SiO.sub.