In an exposure light source for use in exposing a pattern onto a substrate, an exposure lamp substantially encloses only a single isotope of a specific metal. The single isotope emits far ultraviolet light which has a very narrow bandwidth when vaporized and excited in the exposure lamp. Preferably, the single isotope may have a nuclear spin equal to zero and is effective to make the bandwidth of the far ultraviolet light narrower. The single isotope may be determined in consideration of an abundance ratio of each isotope. The specific metal may be selected from a group consisting of mercury, lead, zinc, and cadmium.