An illuminating device is made from a transparent hollow envelope that encloses a filler gas. The envelope has inner and outer surfaces. The illuminating device has a coating of a fluorescent material deposited on the inner surface of the transparent envelope where the fluorescent material emits light having a visible component and an ultraviolet component; and a titanium dioxide film having first and second opposing surfaces where the first surface of the titanium dioxide film is formed on the outer surface of the transparent envelope. A light transmittance of the titanium dioxide film is at least 50% for light having a wavelength of 550 nm, and the light transmitted from the fluorescent material through the transparent envelope and the first surface of the titanium dioxide film to the second surface thereof causes photocatalytic activity to be generated on the second surface of the titanium dioxide film.