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HMI PAR Operation Questions


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#1 Steve London

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Posted 29 July 2008 - 12:01 AM

I use tungsten lights constantly but don't get a chance to use HMI lights enough to be sure how to operate them properly in every respect. I know there are people here who use them daily so a little help please.

With these questions I have in mind an LTM Cinepar 1200W, the most common HMI fixture I encounter. Some of my questions are probably generic and not specific to the Cinepar.

1. The system draws 12.5 Amps in operation but I have been told it momentarily pulls twice that when first started. True? Does it make a difference whether the magnetic or electronic ballast is used?

2. Does it matter if the start button (on head or ballast) is held down for a few seconds as opposed to being momentarily pressed?

3. Is the light ever used with the safety glass in place but no additional beam control lens, as a very narrow spot?

4. Is there an easy way to distinguish the medium, large and extra large lenses from one another? What is the typical terminology for these lenses? I hear the extra large beam lens referred to as stippled, for example.

5. On the fixture's ears are several slots. The barndoors go in front, of course, but does the lens go next to the safety glass or next to the barndoors, and why?

6. Is heat absorbing gel ever used to prevent burning a color correction or diffusion gel clipped to the barndoors or on a separate frame close to the light? How close can a gel be?

7. Is there any problem moving a burning light if it is done carefully?

8. Does the beam control lens always have to be removed from the fixture if the fixture is going to be removed from the light stand and moved to a new light stand on the set?

9. Are the lenses always removed using leather gloves and pliers such as a Leatherman?

10. Is the light packed on the lighting cart with or without the barndoors on and vertically or horizontally? I usually place the fixture vertically with the barndoors on and hold it in place with a 15 or 35 lb sand bag leaning against it.

11. How long after being struck before the light is at full brightness and a stable color temperature?

12. How long before the lamp is typically replaced even though still burning?

13. How much does the color temperature shift with lamp life?

14. Why are PAR HMIs more popular than Fresnels?

15. What have I not asked that is important to know?

Thanks a million. I'll put your responses to work.
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#2 Matthew Parnell

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Posted 29 July 2008 - 12:57 AM

1. The system draws 12.5 Amps in operation but I have been told it momentarily pulls twice that when first started. True? Does it make a difference whether the magnetic or electronic ballast is used?

It takes a lot of power to strike at HMI, much more than starting up. What immediately follows the strike is different between typical Magnetic and Electronic ballasts.

Once struck an electronic ballasts pull will immediately decrease then increase as the lamps intensity increases.

A magnetic ballasts pull will slowly drop from the striking pull until the lamp reaches full intensity and the ballast reaches its operational pull.

As a further note, as with most discharge lamps, the ballast is not entirely efficient. So if you have a 1.2kw head, your ballast will probably pull 1.6kva to run it. Most ballasts have a power factor somewhere on it that allows you to calculate how much the system is going to pull.


2. Does it matter if the start button (on head or ballast) is held down for a few seconds as opposed to being momentarily pressed?


It depends on the head/ballast and how the switch mechanism works. Typically it doesn't hurt, but on some older ballasts you could be stopping the strike from releasing.



3. Is the light ever used with the safety glass in place but no additional beam control lens, as a very narrow spot?


Yes.

4. Is there an easy way to distinguish the medium, large and extra large lenses from one another? What is the typical terminology for these lenses? I hear the extra large beam lens referred to as stippled, for example.

The can be named regionaly, but typically you have a series of spot lenses, a stipple(named this because of its stippled appearance) and fresnel lenses.


5. On the fixture's ears are several slots. The barndoors go in front, of course, but does the lens go next to the safety glass or next to the barndoors, and why?


The safety glass in all of the pars that i have used is typically built into the front of the fixture directly in front of the lamp and reflector. On the ears there is one slot for the lens, and a slot for wires and a slot for the barn doors. The lenses will typically only be able to fit into the slot that is correct for them.


6. Is heat absorbing gel ever used to prevent burning a color correction or diffusion gel clipped to the barndoors or on a separate frame close to the light? How close can a gel be?


The closest you want a gel on a par is on the doors.

Heatshield can be used to stop burning of correction gels, but there needs to be an air gap between this and the correction, and any subsequent layers of correction or diffusion. In a lot of cases adding heatshild as a layer before you correction with no gap tends of make the gel melt/fade faster.


7. Is there any problem moving a burning light if it is done carefully?

If done with great care, it shouldn't be an issue for short moves(like when trimming the setup). But hiking the lamp from one end of a house to another is a no no.


8. Does the beam control lens always have to be removed from the fixture if the fixture is going to be removed from the light stand and moved to a new light stand on the set?

Yes, it should be, lenses on pars tend to be fairly fragile when hot. This can be hugely dangerous.


9. Are the lenses always removed using leather gloves and pliers such as a Leatherman?

Typically yes, far too hot otherwise. Most of the time grabbing them by the handle with suffice.


11. How long after being struck before the light is at full brightness and a stable color temperature?

Typically 2-6mins depending on a lot of factors.(bubble age, ballast, head, etc etc etc)


14. Why are PAR HMIs more popular than Fresnels?

Pars are smaller, lighter and have more punch than fresnels.

Typically as a direct source fresnels are more popular because of their controlled and consistant beam, as we tend to diffuse lights a lot, pars with their extra punch are the way to go.
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#3 Chris Keth

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Posted 29 July 2008 - 01:04 AM

1. The system draws 12.5 Amps in operation but I have been told it momentarily pulls twice that when first started. True? Does it make a difference whether the magnetic or electronic ballast is used?

I don't know if the draw is double but it does spike a good bit upon striking. Both types of ballast will do this.

2. Does it matter if the start button (on head or ballast) is held down for a few seconds as opposed to being momentarily pressed?

Not sure what you mean here. Doesn't it just have on/off switches on head and ballast like an Arri?

3. Is the light ever used with the safety glass in place but no additional beam control lens, as a very narrow spot?

I've never used nor seen anyone use one without any lens in place. I suppose you might do it on a night ext or something if you needed a lot of punch but not much shape.

4. Is there an easy way to distinguish the medium, large and extra large lenses from one another? What is the typical terminology for these lenses? I hear the extra large beam lens referred to as stippled, for example.

The wider the lens is, the smaller the little dots on the lens will be. The spottier lenses will be kind of long rectangular facets. I generally will just ask for a stipple, wide, medium, or spot lens. Someone out there probably has clever names for them.

5. On the fixture's ears are several slots. The barndoors go in front, of course, but does the lens go next to the safety glass or next to the barndoors, and why?

I have had people tell me that the lens should go directly behind the doors and that wire should go behind that. Then I've had other people say the opposite. I tend to think that the wire should go behind the lens. The reason being that the wire will cut the beam more evenly when the rays are essentially parallel rather than after they have been spread out by the lens.

6. Is heat absorbing gel ever used to prevent burning a color correction or diffusion gel clipped to the barndoors or on a separate frame close to the light? How close can a gel be?

I've never had gels burnt up by a 1200 when they're on the doors. I don't think I'd get them much closer, though.

7. Is there any problem moving a burning light if it is done carefully?

If you're just moving it a few feet, I wouldn't bother killing it. If you're moving further, I would be safe and kill and then restrike it. HMI globes are expensive and restriking is faster than changing a globe or the whole head.

8. Does the beam control lens always have to be removed from the fixture if the fixture is going to be removed from the light stand and moved to a new light stand on the set?

I leave the lens in unless it's going to be changed anyway or if the little spring thing that holds it in is broken.

9. Are the lenses always removed using leather gloves and pliers such as a Leatherman?

Pretty much. I tend to just use gloves if the lenses have good strong handle-loops.

10. Is the light packed on the lighting cart with or without the barndoors on and vertically or horizontally? I usually place the fixture vertically with the barndoors on and hold it in place with a 15 or 35 lb sand bag leaning against it.

Sounds good to me. I'm not really en electric but that seems fastest.

11. How long after being struck before the light is at full brightness and a stable color temperature?

Couple minutes. It'll usually be at temp by the time you have aimed it and put schmutz on it.

12. How long before the lamp is typically replaced even though still burning?

Not sure on that one.

13. How much does the color temperature shift with lamp life?

I don't think the color temperature shifts much. They get magenta, I believe.

14. Why are PAR HMIs more popular than Fresnels?

They're really versatile and have more punch to them. You can also put a fresnel in them and get much of the look of a fresnel fixture.

15. What have I not asked that is important to know?

You're pretty thorough. Maybe check out a table of HMI-safe shutter speeds and angles. With a magnetic ballast, some shutter speed/angle combinations will get a visible flicker, since HMIs aren't continuous sources.

Thanks a million. I'll put your responses to work.


Edited by Chris Keth, 29 July 2008 - 01:05 AM.

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#4 Andrew Koch

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Posted 29 July 2008 - 04:41 AM

A couple of other things:

When using an electronic ballast, there is often a delay when striking the lamp, Once you push the igniter button, let go and wait. It can take as long as 10 seconds sometimes. Don't keep pressing the button, this just cancels it out and you will get no strike. If it takes much longer than this, then there is a problem.

Different ballasts have different draw. I had a 1.2K that drew exactly 16.67 Amps (same as a 2K). A gaffer I know has worked with ones that have drawn a whopping 18-19 Amps. Some ballasts have the amperage written on the ballast (I'm not sure how accurate they are). To play it safe, give a 1.2K it's own 20Amp circuit.

Color Temperature varies more on smaller units like the 575 and 1.2Ks. Since the globes are handmade, it is much harder for the smaller globes to be as consistent as the bigger ones because more precision is required. Many of these smaller units will be much cooler than 5600K. At checkout, get a color temp meter (if you don't have one, you can usually rent one for about 20 dollars, but this is no guarantee that it will be calibrated), and find out the color temp for each unit. Then label each light on place where it won't get too hot and melt off or gunk up the light. This way you will have an idea of how much correction gel to put on each light.

If you are shooting where it is a bit cold, don't just shut off the HMI. The extreme temperature change can cause the lens to crack. Put a scrim in front of the lens for about 15 minutes before shutting of the lamp. The hot scrim will keep the lens warm and allow it to cool off slowly. This is also a reason to put the lens in the slot closest to the globe. I have never heard of putting scrims behind the lens, I was always told to do it the other way, but that does not necessarily mean it is the rule. I would be curious to hear other's thoughts on this.

The head cable for 575s and 1.2Ks look almost identical, but the keys are oriented differently. Make sure you have them properly labeled. Often the rental house will color code them. Yellow for the 1.2Ks and Green for the 575s, Red for 2.5K, Blue for 4K (It might be different depending on location).

Some electronic ballasts will have 2 inputs, each for a different wattage. Both of these are hot, so make sure you cover the one that is not in use.

A great book that covers all of this in detail is Set Lighting Technicians Handbook by Harry C. Box.
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#5 J. Lamar King

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Posted 29 July 2008 - 04:57 AM

Couple of notes about 1200's and bigger PAR's to expand on what was said before...

2. Just press and release the button. On the occasional ballast you made have to hold it. Others you will press and release and there will be a delay before the strike. Try not to press it again too quickly.

3. Rarely but you can certainly use it without a lens in place.

4. The technical terms for the lenses are Super-wide (stipple or Fly's-eye), Wide, Medium, Spot and fresnel. The fresnel is roughly equivalent to a wide I believe.

5. On most 1200's you can get the scrims in front of the lens, some you have to put them there. On a bigger unit like an LTM 12K put the lens in the center slot and scrim behind because you can barely fit two in front of the lens the other way round and they sit right against the hottest part of the glass.

7. Most HMI's 1200 and up you leave burning while moving them in the same setup. Avoids hot re-strike problems though 1200's typically aren't that bad to restrike. 4K's will usually be on a combo and one electrician can safely move the unit while burning if he knows how. Bigger lights are usually on super-cranks or roadrunners with wheels.

9. Be aware that all 4K PAR's of any brand have the recessed lens (crap) design. Really hard to get that lens out when its sitting on a combo, very hard if it is sticked up. The trick with 4K's is to go up a latter so you can grab the lens from above the beam. Rookie mistake is to do it from the ground usually resulting in a burning sleeve because you reached across the lens. Arri 1200 PAR lenses are of the same design. When you pull hot lenses and put them in the box. Leave the box open for it to cool. Shutting the hot lens inside the box often results in a cracked lens.

10. You can pack it away face down, lens removed, it's fine. However don't put a hot light away like this as it can warp the reflector and shorten the socket life.

12. I don't really know the life of the globes either though I'll tell you that most brand new globes I come across start life in the 6100-6150K area. Then work their way down to around 5600K fairly quickly. When they are going bad they get blue, like if you measure one at 8000-10,000K, good bet it is going and going quick. Reject it.

13. Again not much until it's going bad.

When smaller lamps fail to strike the microswitch is usually the culprit. Switch off the ballast open up the face and turn the microswitch plunger counter clockwise a bit. Close the face and that will usually get it to fire. If you are renting several 1200's an easy color temp check is to fire them all at the same time and point them at an even colored floor or wall. It's easy to see which ones are too blue or too magenta and sometimes too green.
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#6 Steve London

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Posted 30 July 2008 - 01:38 AM

Thank you for all this solid and helpful information. I am very grateful for your knowledge and time.
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#7 Hans Engstrom

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Posted 30 July 2008 - 02:54 PM

If you are shooting where it is a bit cold, don't just shut off the HMI. The extreme temperature change can cause the lens to crack. Put a scrim in front of the lens for about 15 minutes before shutting of the lamp. The hot scrim will keep the lens warm and allow it to cool off slowly. This is also a reason to put the lens in the slot closest to the globe. I have never heard of putting scrims behind the lens, I was always told to do it the other way, but that does not necessarily mean it is the rule. I would be curious to hear other's thoughts on this.


My experience is that you don´t have to put in a scrim on a 1.2k hmi, not even on a 6k only on big units (12k and 18k). Please correct me if I´m wrong but I never had any problems with smaller HMIs and I live in the cold part of the world.
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#8 Michael Collier

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Posted 30 July 2008 - 05:15 PM

This winter I had some productions where we used HMIs in the cold, down as far as -25 (f). From my experience I can tell you that you don't need scrims when shutting down those lights (up to 1200 par). They never even got warm. The only pain of shooting that cold is the header cables freeze up and refuse to be coiled, and you can't put the header cable on certain head units with gloves on...not good when touching metal could mean instant frost bite.
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#9 J. Lamar King

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Posted 31 July 2008 - 02:16 AM

My experience is that you don´t have to put in a scrim on a 1.2k hmi, not even on a 6k only on big units (12k and 18k). Please correct me if I´m wrong but I never had any problems with smaller HMIs and I live in the cold part of the world.


In my years in the biz I've seen probably a hundred lenses from smaller units crack during cooling. 4K's being the worst. Watch out for cool humid air at night.
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#10 Hans Engstrom

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Posted 31 July 2008 - 10:46 AM

In my years in the biz I've seen probably a hundred lenses from smaller units crack during cooling. 4K's being the worst. Watch out for cool humid air at night.


The winters here are really cold so the air is dry as all humidity become ice crystals. That might be why I haven´t had any problems with cracking lenses during our cold winters.
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#11 J. Lamar King

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Posted 01 August 2008 - 03:30 AM

The winters here are really cold so the air is dry as all humidity become ice crystals. That might be why I haven´t had any problems with cracking lenses during our cold winters.


Yeah it's actually kind of weird because I've noticed that cracked lenses seem to happen all at once. Like you'll have a rash of them on one show then not have any for months and months. So I'm guessing it's humidity and air temp or something or maybe it's just a huge swing in temps from a hot day to a cold night.
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