Lamp rotation

For lamps operated in the horizontal position, 180 degrees rotation at half-life is recommended. This allows the deposition of vaporized electrode material in the bulb to be more evenly deposited on the bulb envelope and not blacken the bulb body on one side – which can ultimately cause overheating of the envelope and lamp failure. After rotation, many times the arc becomes unstable. Factors causing this include convection currents, magnetic stabilization and electron emission preference from the cathode tip. The lamp may stabilize by running at maximum current for a period of time and then lowering the current.