Krypton is used commercially as a filling gas for energy-saving fluorescent lights: Krypton Metal Halide Lamps (2006).
Unlike the lighter gases in its group, it is reactive enough to form ions and some chemical compounds; for example, krypton will react with fluorine to form krypton fluoride.
Krypton fluoride is used in some lasers: Krypton Fluoride Excimer Laser Utilizing Nitrogen Trifluoride As A Fluorine Donor (1977); Krypton ion lasers also exist: Krypton Laser (1972).
Radioactive krypton 85 was used during the Cold War to estimate Soviet nuclear production. The gas is a product of all nuclear reactors, so the Russian share was found by subtracting the amount that came from Western reactors from the total in the air: Process And Apparatus For Separating And Recovering Krypton-85 From Exhaust Gas Of Nuclear Reactor Or The Like (1975).
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All patent information has been obtained from Espacenet (European Patent Office).