so I just picked up a LTM 575w fresnel hmi and it works fine. very happy with it, outside of what it wieghs :/ . I need to get a spare bulb. it seems there are two types out there the $40 ones rated at 500hrs to be found on ebay, and the $80-$90 ones rated at 1000hrs found at wholesale bulb distributers and a few ebay sellers.

please, no speculation here, I'm looking for real hands on knowledge - is there any real difference between them in color rendition ( CRI ), hotstrike, color rendition during the normal burn and aging process. The bulb currently in the light is a Osram which should be good, but my camera is telling me its putting out 4300k when white balancing. doesn't look bad, but its not right. I'll assume the bulb is a real 4300k bulb, or that its just really old and losing its CRI and shifting warm.

I've never seen a 4200K HMI bulb. They are all normally in the 5600K to 6000K range and above--even some in the 7000 to 8000K range which are more meant for spotlight use.

I wrote an article comparing various bulbs recently--perhaps that will be a good starting point for you:

http://www.coollights.biz/wordpress/archives/41

What I've found on the cheaper bulbs has been somewhat disappointing. We're working with one Taiwanese company who's been making clone HMI bulbs for a while. They are sincerely trying to work out the little problems and I'm sure they will at some point, but for now, when we sell our HMI models it will be with Phillips, Osram or Sylvania bulbs. We've just found a few issues with the other ones. For instance, the bulbs have an electronic hum when you drive them at high frequency like our forthcoming ballasts will do--to avoid flicker issues. The name brand bulbs don't hum. We've also seen them burn out long before their life was supposed to be over too. We just don't think its worth the trouble at this time to use those bulbs. That's the two main issues I've seen. If you're using a magnetic ballast by the way, you won't have the hum issue with the cheaper bulbs--only on electronic high frequency ballasts.

In my article, I talk a bit about the different types, grades and color temperatures in the various bulb choices. In general you can use HMI if you need G22 hot restrike and the HSR/HSD type if you need the GX9.5 socket NHR type bulb. You'll find that most of the big names make these in 6000K and the smaller names like USHIO and Eiko make 5600K versions. Outside the USA, Sylvania makes their own HMI bulb and its also a 5600K. In the USA, they market Osram HMI and that's 6000K. Hope this helps.