1.2 HMI Par through Silk or Diff?

[Mikael Gustafson]

I'm lighting a 25ft stage next week and there are some power concerns so I'm going to be using two 1.2kW HMI Pars to light the stage from near the back of the room, one on either side about 45º to the stage. I'm wondering that kind of kick I'm going to get out of a 1.2 par going through a 42x42 silk armed out from its lollipop. Will I lose too much of the HMI's punch from say, 30 feet away? Would a frame of 216 or 416 be better than the silk? This will be to supplement the room's existing lighting and to nicely fill the talent on stage.

[Mark Kenfield]

Is it a 25ft² stage or is 25ft just the width of the stage? Also what's the sensitivity of your stock/camera?

 

Assuming you're using a modern 800 ISO camera, then two 1.2k HMIs through frames should provide plenty of stop for a 25ft² space.

[Mikael Gustafson]

Is it a 25ft² stage or is 25ft just the width of the stage? Also what's the sensitivity of your stock/camera?

 

Assuming you're using a modern 800 ISO camera, then two 1.2k HMIs through frames should provide plenty of stop for a 25ft² space.

 

The stage is 25 feet long and 12 ft deep. Native ISO will be 800ASA.

[Mark Kenfield]

Cool, then I think you'll be just fine with a couple of 1.2k PARs through 4x4s of 216 or 250.

[Albion Hockney]

Silks are pretty beat, they just kinda fill in around the shadow and keep shadow lines in tact. I'm sure somtimes there is a specfic use for them but generally diff is your friend.

 

 

Arri has a photo metric calcuator you can check out (just google arri photometric) and you can get an idea of what stop you get at that distance.

 

250 cuts about 1 stop of light and 216 cuts close to 2 stops of light.

 

 

I'd guess at 30ft away through 250 you'd be at something like a 2.8/4 at 800

Edited April 28, 2015 by Albion Hockney

[Stuart Brereton]

Silks are pretty beat, they just kinda fill in around the shadow and keep shadow lines in tact. I'm sure somtimes there is a specfic use for them but generally diff is your friend.

 

There are specific uses for all the rags we use. The specular quality of light through a silk is hard to match. I wouldn't say that silk is any less useful than diffusion.

 

To address the original question, at 30' away a 4x4 frame is not really going to provide soft light, just diffuse light. I'd be tempted to use something light, like 251 or 250, and then use something even lighter, like Opal as a double break for the close ups, if necessary.

[Guy Holt]

... there are some power concerns so I'm going to be using two 1.2kW HMI Pars ....

 

If you plan to plug the 1.2kw HMI Pars into wall outlets you want to make sure their ballast’s are Power Factor Corrected (PFC.) If not, they will surely trip a 15A circuit and maybe even trip a 20A circuit. The reason for this is that non-PFC HMI ballasts draw current in high amplitude pulses that include harmonic currents. I won’t go into detail here on the adverse effects that the harmonics generated by non-PFC electronic HMI ballasts can have, but anyone operating HMIs, Kinos, and even LEDs should make them selves acquainted with harmonics. (use this link for more details.)

 

How much current a HMI ballast will draw is indicated on the manufacturer’s nameplate. Let’s take a look at one of these nameplates, since deciphering them can be as difficult as deciphering Egyptian Hieroglyphs if you don’t know how to read them.

 

Manufacturer’s nameplate from an Arri 575/1200 Electronic Ballast specifying its’ electrical characteristics (learn how to read it.)

 

Above is the nameplate from an Arri non-PFC 575/1200 Electronic Ballast. The first thing to look for is the ballast’s Volt-Amperes (VA), which is called “Pmax” here. Calculated as the RMS voltage times the current measured at the input to the device, Volt-Amperes is the measurement of "Apparent Power" delivered to a load, which is different than Wattage. Wattage is the True Power dissipation of the lamp calculated by integrating the product of current through the lamp and voltage over time. These may sound like they would be the same (they are in the case of incandescent lamps), however one characteristics of HMI, Fluorescent, and LED power supplies is that some of the current flowing into them is not used in the generation of light. The relationship between True and Apparent Power is called the Power Factor (PF.) Since, the Wattage will always be lower than or equal to the Volt-Amperes, PF varies from 0.0 to 1.0.

 

As indicated on its’ nameplate, this ballast has an Apparent Power of 2290VA - which means it draws nearly twice the load of its’ 1200W output. Next to the Pmax, it also indicates that the Power Factor is .6 (cos@=.6) meaning the ballast has to draw 40 percent more power than it uses to generate lumens. The greater Apparent Power drawn by the ballast consists not only of high amplitude short pulses of current, but also harmonic currents that the ballast also draws.

 

The next thing to look for is the relationship between Voltage (U) and Current (I). As this nameplate indicates this ballast will operate with line voltages ranging from 90 to 125 volts (US), and 180 to 250 volts (European.) You will also notice that the current (I) the ballast draws varies with the supply voltage. That is because this ballast is a “Constant Power” ballast. With constant power ballasts, if voltage drops the ballast will draw more current to maintain a constant Apparent Power - 2290VA in this case. For example, according to the nameplate it will draw 18A of current (I) at 125 Volts (U) (2290VA/125V = 18.32A.) If the voltage drops to 90V, it will draw over 25 Amps (2290VA/90V = 25.44A.)

 

This is an important characteristic of this ballast that should be taken into account when used outside the studio on location. With an Apparent Power of 2290VA, this non-PFC constant power HMI ballast will operate very close to the threshold of a 20A circuit – too close to operate reliably unless precautions are taken. For example, according to its’ nameplate, it takes 19 Amps at 120 Volts to generate 1200 Watts of light (2290W/120V = 19A). If there is any line loss from a long cable run, the ballast will possibly draw over 20 Amps in order to compensate for the voltage drop. For instance, at 110V it will draw 20.8 Amps. To the problem of line loss, usually there is also increased resistance from an overheated plug end, which makes the voltage drop even further. Since most stinger plug-ends are only rated for 15 Amps they tend to overheat with 1200W non-PFC electronic ballasts. The increased resistance that results from the heat causes the voltage to the ballast to drop even further and so it will draw more power to maintain the 2290VA load. If the light is operating on a small generator, there will also be voltage drop on the generator because of the greater load. The voltage output of generators can drop 5-10 volts under load. At 105V, the ballst that drew 19A at 120V, will now draw 21.8 Amps and cause circuit breakers to overheat and potentially trip.

 

For more facts/tips about power/HMIs/safety, check out an article I wrote for our company newsletter on the use of portable generators in motion picture production. Harry Box, author of “The Set Lighting Technician’s Handbook” has cited my article in the just released Fourth Edition of the handbook. In addition, he has established a link to it from the companion website for the Fourth Edition of the Handbook, called “Box Book Extras.”

 

 

 

 

 

"Great work!... this is the kind of thing I think very few technician's ever get to see, and as a result many people have absolutely no idea why things stop working."

"Following the prescriptions contained in this article enables the operation of bigger lights, or more smaller lights, on portable generators than has ever been possible before."

 

 

Use this link for my FREE news letter article on the use of portable gas generators in motion picture production.

 

Guy Holt, Gaffer, SceenLight & Grip, Lighting and Grip Rental & Sales in Boston

Edited April 29, 2015 by Guy Holt