I was wondering since both a pulse start metal halide and high pressure sodium have ignitors are the bulbs interchangeable?
Not recommended - although i have not tried. The pulse start uses a stronger pulse to get going than the HPS. You might try visiting the Venture site, venturelighting - it is quite informative really. That's where I get my pulse start halides from and the ignitor. A 575 W PSMH required a remote ignitor very close to the lamp, otherwise the pulse is attenuated by the lamp cord, whereas HPS is fine with just a standard ballast setup. There are some ballasts that may be good for regular MH and HPS, but not pulse start as far as I know.