After knowing that most high pressure sodium lamps have barium getters near the base of the lamp, I have noticed that some quartz metal halide lamps such as the GE/Tungsram Kolorarc and some Osram HQI-T lamps have them, but also some ceramic metal halide lamps such as some Philips MasterColor lamps and some Iwasaki Cera Arc lamps have barium getters.
Why were barium getters often added to certain quartz and ceramic metal halide lamps?