Why don't metal halide lamps emit light?

Occasionally flashing, indicating that the ballast is not broken, it is the starter. There are two kinds of starters, one is capacitor starter, and the other is metal halide lamp starter. Capacitance can be detected by instruments, but the decrease of capacitance is unacceptable. The starter of metal halide lamp can not be detected by instrument, but can be checked by alternative method.