As part of my lighting hobby, like many of us, I look on eBay for MH lamps. While purchases of such lamps are relatively infrequent, they occasionally happen. Like we all know, most eBay sellers are not professionals, and they make a description however they want, sometimes with great detail, sometimes just a picture of the lamp sleeve, etch, etc.

I'm half-looking (no rush to decide what, if, when...) for general purpose MH lamps and find the colour temperature value very difficult to determine. The vast majority of general purpose MH is either 3000K or 4000K (approximatively). Specialty lamps are usually very well indicated for colour temperature etc. My preference would be the 4000K temperature. Do manufacturers knowingly omit such designation on the etch, sleeve and box ? Maybe there is some other code to find the information ? I'm confused and a bit irritated by this. Normally, CDM lamps are well indicated, it is mostly the good old fashioned quartz MH, probe start that aren't well labeled.

If manufacturers omit the information from the etch, the sleeve and the box, how do retailers know which lamp to sell to a customer that asks for a specific colour ?

Most US probe-start have the same color temperature (4000-4200K). Thats why it is omitted often.

With European lamps "/N" or "/NDL" in the etch means 4000..4500K

Almost all probe start MH lamps are 4000K-range, but with probe start, the color tends to turn bluer with age, so a lamp that started off as 4000K might end up being close to 6000K by the time it gets near EOL. Pulse-start MH lamps tend not to "color shift" as badly. Also, I think the reason 3000K PSMH lamps are possible is the higher pressure in the arc tube (someone correct me if I'm mistaken) with different chemical combinations. Most PSMH (including CMH) lamps are available in 3000K or 4000K while probe start MH is generally 4000K (and usually the color temperature is not specified).

Both pulse start adn probe start halides are available in other colors-just look on the grow lamp and aquarium supply websites.Its the halide mixture that determines the color and not wether the light is pulse started or probe stated.On the packages of GE especially-they state the color of the probe start GE halides turn to a warmer color as the lamp agers.On a 175W Halarc lamp I have-they are opposite-the color becomes cooler as the lamp ages.The 175W Halarcs I have have a pulse start igniter built in the lamps base-you use it on standard 175W ballasts for probe start lamps.From Venture lighting you can order lamps that are saturated colors-like blue,green,magenta,yellow.These types of lamps are VERY expensive.

Sylvania Metalarc lamps turn bluer as they age. Same for Philips. I don't have any GE MH lamps (and from what I hear I'm not missing anything lol) so I wasn't sure how those aged. I always loved the wacky color shifts of probe start MH lamps, especially when run vertically (when horizontal they usually blow up before color shifting very much). In gym class I'd see some turn a nice hot-pink color, others green out and get dim like MV, others turn blue like a daylight fluorescent, and some get really dim and flickery and run a halogen yellow-white color. PSMH doesn't do that as much and CMH to even much less of an extent. CMH has beautiful color but man are those lamps expensive!

OK, so this is a summary of what I think I learnt from all replies :

We're talking mainly North American lamps here.

Probe start quartz MH in the 175-1000W range, are usually 4000K unless otherwise stated (no mark means 4000K). I know and have seen some quartz probe start MH in 3000K.

Quartz pulse start MH lamps 100W and smaller are usually 4000K unless otherwise stated, just like their larger siblings. However, due to the typical use of these smaller ones, 3000K lamps are more popular than they would be in larger wattages.

Ceramic pulse start MH are always marked and were not part of the problem initially posted.

You said nothing about the PSMH lamps larger than 100W...

Quartz PSMH lamps (50 to 1000W) are usually available in 3000K or 4000K just like the CMH counterparts, not just 4000K. Only the standard MH lamps are default 4000K.

Both pulse start adn probe start halides are available in other colors-just look on the grow lamp and aquarium supply websites.Its the halide mixture that determines the color and not wether the light is pulse started or probe stated.On the packages of GE especially-they state the color of the probe start GE halides turn to a warmer color as the lamp agers.On a 175W Halarc lamp I have-they are opposite-the color becomes cooler as the lamp ages.The 175W Halarcs I have have a pulse start igniter built in the lamps base-you use it on standard 175W ballasts for probe start lamps.From Venture lighting you can order lamps that are saturated colors-like blue,green,magenta,yellow.These types of lamps are VERY expensive.

When I lived on the MS Gulf Coast, pre-Katrina, the Grand Casino Biloxi Islandview Hotel had Blue (the maintenance man called it Indigo for the Sylvania lamps. I was able to get a used one) Venture lighting (and 2 Sylvania) 1000w lamps shining on the hotel.  After Katrina, the hotel was imploded due to extensive irreparable damage

Sylvania Metalarc lamps turn bluer as they age. Same for Philips. I don't have any GE MH lamps (and from what I hear I'm not missing anything lol) so I wasn't sure how those aged. I always loved the wacky color shifts of probe start MH lamps, especially when run vertically (when horizontal they usually blow up before color shifting very much). In gym class I'd see some turn a nice hot-pink color, others green out and get dim like MV, others turn blue like a daylight fluorescent, and some get really dim and flickery and run a halogen yellow-white color. PSMH doesn't do that as much and CMH to even much less of an extent. CMH has beautiful color but man are those lamps expensive!

You mentioned something about Gum class and that reminded me about the gyms in my schools.  They had cages over them, but were open.  I'm surprised none of them ever exploded.  2 of them were always on, and AFAIK, none of them ever exploded while I was in those gyms for 7 years total (repeated 6th grade due to constant goofing off and a bad math teacher who couldn't teach right to save her life)

My elementary school had GE glass reflector highbays (400W probe start) and they had clear glass lenses on the bottom. Never saw one explode in class but I'd seen exploded lamps contained inside the fixtures. My middle school had 400W MH low-bays with enclosed plastic lenses and metal reflectors but there were about a dozen or so spot-replacement fixtures which were 400W MH with the open bottoms and wireguards as you mentioned.

Open-lamp MH fixtures require the use of open-rated (protected) lamps but before ~2000 they weren't a requirement (open-rated lamp fixtures use pink sockets with a shroud around the center contact that allows only protected lamps to be used; they're called Exclusionary Mogul sockets). However, MH lamps are way less likely to explode running in a vertical position. Running horizontally, the odds of a rupture at EOL are much higher (plus life is shorter in general). Also, a probe start MH lamp is less prone to rupture at EOL than a PSMH. PSMH is under greater pressure. With a horizontal PSMH lamp, you can just about guarantee an explosion at EOL lol. With vertical probe MH it's not a likely occurrence.

At my place of work, which is where I went to high school, the gym had Stonco low bay fixtures with a plastic lens and 400W. Never heard of explosions, although the lamps were run until EOL. They were always spot replaced (the gym was never group relamped). They used mostly GE lamps, which would typically turn bright pink for about two days before surrendering. I remember the first EOL lamp they had, when the gym was only about 2-3 years old. That one was replaced with a coated lamp of some flavour, and it lasted from about 1996ish to 2012 when everything was changed to T8. I don't think it was MV because the restrike time was as long as the MH lamps.

I do remember some lamps running yellow for about 3-4 days when brand new, but that is typical of new probe start MH. The ballasts were all remote and installed on a wall in the ventilation room. They were very, very quiet. If the ventilation system was not running, and it no one was in the gym, you could hear a very faint hum in the room.

Now typically a gym is a dangerous place for any type of light fixture. There was a period of time when some factors made it so the various balls used were thrown with more force than necessary, they might even have been thrown at the lights on purpose, and quite often the lens/reflector bowl assembly became detached and the lamp was exposed. Usually no one complained and activities continued with the bare lamp exposed. Amazingly, not one lamp was broken. The repair was normally not done until I became aware and spoke sometimes quite loudly about it and its associated dangers. One time there were 2 or 3 fixtures taken apart in one afternoon. The electrician was called (you need a special licence to climb to such heights) and when he came, he couldn't fix them right away and said he would come back in about an hour. Before leaving, he went in the electrical room and shut off the breakers for the whole gym so no one would use it.

Wow, this thread is now way off subject !

Now back to the subject, are Sylvania probe start MH lamps more prone to "green out" than others ? I've seen installations of MH, especially recessed cans, where the occasional lamp was very green, and always wondered with which manufacturer is more likely to happen.

I've seen MH lamps go green sometimes but not sure if it's a particular manufacturer. Sylvania lamps tend to stay relatively white from what I've seen, though Sylvanias will become noticeably higher in Kelvin as they age. I have two (one new and one well-used) 250W MH Sylvania lamps and the new one is a warmer color and the used one is about as bright as a 250W MV and a colder blue color.

I've noticed newer MH lamps take awhile to initially strike too. Once they get broken in they start better but I've seen it with MH and PSMH. The fixture sits idle for a few seconds, sometimes up to a minute, and then the lamp comes to life. It's noticeable when a large installation is switched on at once. Some lamps start instantly while others don't. Must be variations in the gas fill. MH lamps are definitely the least consistent between brands and even among lamps from the same brand and type. HPS and MV lamps are more consistent in color and behavior.

I've noticed that delay in striking as well. In the chapel at my university, I once switched all of them on (cold start) and to my surprise, none came on. Thinking there was a power outage, I looked at the lights thinking they wouldn't come on. Then one by one, they struck and warmed up. It took about one minute to have them all strike. It was these fixtures.