A radiation source apparatus for irradiating the inner walls of elongated cavities, especially for hardening of artificial resin laminates that can be radiation hardened by ultraviolet or infrared radiation, has a base body with at least one light source and multiple wheels by which the apparatus is guided inside the cavities. The multiple wheels are movably mounted on arms in a direction radial to the base body. At least one gas pressure spring is assigned to each arm, which pushes the arm from a radially proximal first position into a radially distal second position.