HMI: how not to die

[Eric Sanderson]

Just want to emphasize Ryan's point about not touchign the bulbs. I was on a set for a short a few days ago and one of the bulbs in our HMI exploded. Nobody hurt, but if there hadn't been a lens in front of it, could have been a different story. Shooting glass into an actor's face is never a good idea.

[mkfotos.com]

Not even when it's very, very hot glass?

[Jon Starr]

When you have an HMI on, I know it's best to keep it on for the bulb. However, say you go for your hour lunch, you don't need to move it for the next shot, and you don't need to turn it off, what's the best thing to do? Do you keep it on or turn it off and re-strike it after lunch? I've heard the former (leave it on) but I just want to confirm this.

[dwiener]

When you have an HMI on, I know it's best to keep it on for the bulb. However, say you go for your hour lunch, you don't need to move it for the next shot, and you don't need to turn it off, what's the best thing to do? Do you keep it on or turn it off and re-strike it after lunch? I've heard the former (leave it on) but I just want to confirm this.

I have heard that striking the bulb is equal to anywhere from 1 to 3 hours o lamp life. I try to avoid turning off the lamps for anything less than an hour. Also even though an instrument may clim to do a hot restrike, chances are it won't when you need it to. Also if working outside in a possible wet environment you might want to look into putting everything on gfci's.

[Zephyrnoid]

This Post Needs To Be Made STICKY!!!
Just one little FYI aside. "HMI bulbs are supposed to be 5600k" not true. They can be any color and I prefer 4300k. HMI is actually a branded version of HID. They are the same exact Metal Halide type bulbs. HMI was coined by Osram and became synonymous with 'motion pictures'. Your advise about circuit draw is SPOT on! I would suggest if this goes sticky, that you insert a side-bar explaining how to calculate load on a circuit using the tried and true formula for calculating Amps drawn versus Amps available. Just saying 1.2KWatts per circuit is too imprecise. Finally. NEVER tamper with the Ballast or wires leading out of the ballast to the bulb assembly- when the unit is still 'Live'. The ballast converts input into 25,000V bursts ;) Thanks for laying this out so lucidly!

1) Never touch the globe with your bare hands, the oil on your skin will cause the bulb to further rise in operating temperture and possibly explode.
2) Make sure everything is off when you connect the header cable to ballast, and ballast to wall. Turn power switches on, in appropriate order to the light head.
3) Allow the HMI about 7 min to get to full strength and correct color temperature.
4) HMI bulbs are supposed to be 5600k, but are handmade and vary with lifetime, so it would be wise to have a color meter, if available. Although they are supposed to be daylight balanced, I find most of the time they are upwards around 6000-6500k... it will be rare that the bulb will actually be 5600k, and even then rare that multiple HMI's will be the same. I know some gaffers who will go to the lighting/grip rental house days before a shoot and test each HMI for this reason. They usually only take the heads they want from the rental house, which are heads that either match each other or are 5600k. Since you may not get that luxury, I'd have some small strengths of CTB, CTO, PlusGreen and MinusGreen on standby. If you don't have a color meter, this process becomes forsaken or incredibly difficult.
4.5) I mention MinusGreen and PlusGreen, because as HMI's are not tungsten/carbon based lighting tools, they can have a color spike in 'tint' or the magenta/green spectrum. Some HMI's can give some nasty color spikes. Be prepared.
5) There are two types of ballasts, both need to stay as dry as possible. Even in dry climates, or nice days, grips will put a small half apple box under a ballast to keep any water, dirt, and other things that may come in contact with the ballast on the ground, clear and dry. If using electronic ballasts in the bright outdoors, I like to have a small c-stand and a 2x3 solid to keep it in the shade and cool. Those things are little computers.
6) When allocating power on home circuits or balancing loads on generators, remember that HMI's pull a very heavy initial load when starting up. Don't strike too many HMI's at the same time if they are on the same line. Most HMI's are 1.2k's and above, so if you are in a household, it's typically one per circuit... however many smaller HMI's exist, and you may have them. So if you have a 1.2k HMI and a 650 tungsten unit on a house circuit, don't be surprised if during power up, you trip the circuit. It would be better to make sure the 650 was off, strike the HMI and in a min or two, power on the 650w.
7) HMI's are not dimmable in the sense you can intercept their power line with a hand squeezer or variac dimmer... You can't. If dimmable, they will have their own dimmer on the ballast. I've heard some of the very new HMI's don't do this, but from all the ones I've worked with, dimming an HMI does the opposite to color temperature as it does to tungsten units. The light will rise in color temperature as you dim the light down. Usually the dimmers on HMI ballasts don't have an incredible range, perhaps dimming a simple one stop difference.
8) When you turn off an HMI, unless you strike it immediately, you will most likely need to wait a period of around 5 min for the unit to be able to re-strike.
9) HMI's are not continuous lighting units. They illuminate a scene by very fast pulsating bursts of light. We cannot see this very well, but our cameras can. Only very specific frame-rates/shutter angles may be used on cameras before you start to see the flickering in your image. Look carefully, because sometimes it's hard to see on set when you have 1000 other things on your mind. If shooting with a magnetic ballast, your choices will be less than a flicker free electronic ballast, which despite the name, only offers a handfew more options before you still get that flicker. Don't worry though, people shot films for decades with standard magnetic ballasts and they did just fine! Most average shutter angles and frame-rates are safe.
10) Keep what ever glass lens is on the light, on the light. Not only can HMI bulbs occasionally (very rare) explode upon death, but HMI's without the lens, gives harmful UV rays that can give your talent a tan and a headache. Don't open-eye the light.

That's all I can think of.

[Zephyrnoid]

Ryan and h-munster, these are two of the best posts ever.

I agree! + 1 on combining them into a Lighting Safety Sticky or something.