- Tungsten: As the electrode material, tungsten is likely to sputter on the surface of the tube. It is the black coating near the end of the tube.
- Na: When Na diffuses into the arctube, the tube will get dark. This may occur near the center of the tube, which has the highest temperature.
- Na outside of the arc tube: If the bulb gets dark, it is likely that Na has leaked from the tube and reacted with the glass, leaving brown stains.
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