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How to light an outdoor night scene

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#1 Matin Akmal

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Posted 09 November 2014 - 03:13 AM

Hey guys, I am a film student, at my senior year and about to embark on my final film project for school. My short film project is a thriller where the protagonist has to uncover clues and find out where his mistress has disappeared to.

 

So in short, this scene is where we have our protagonist drive down a dirt track in the middle of a forest and continues on foot to the various clues. 

 

In depth, our protagonist is led to this location (forest) as the place where he'd find his mistress who has gone missing for the past few weeks. He drives down the dirt tracks, and stops when he sees a violet ribbon (recurring theme throughout the film) tied to a tire swing by a tree ahead of him. He got off on foot and whipped out his flashlight and walked towards the violet ribbon, he then sees another violet ribbon tied to a tree about 10 feet to his right and he walks towards it. He looks around for signs of his mistress, walking around the area by the tree until CREAK, he stepped on something wooden that is buried underneath the ground, he immediately dug frantically, fearing that his mistress could be underneath. He dug and dug until he uncovered a large wooden box, he opened it and his mistress lies dead inside. He freaks out.

 

 

The thing is, I can use the car headlights as backlight for a portion of the scene, but then we have the protagonist turn right and walk away from the headlights. There isn't any other light source around him whatsoever, except for the flashlight he has with him. This flashlight isn't omnidirectional though as it is coming from his mobile phone. My director wants the scene to be lit not too bright, but enough to see the facial features of the protagonist. So how do I light my subject if there are no natural light sources around to use as an excuse to place my lights (china lantern, arri fresnels)? I need advise, this is my first time lighting a night scene and I need all the help I can get, in terms of what kinds of light to use and how to light the area. Thanks Internet!


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#2 David Mullen ASC

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Posted 09 November 2014 - 11:00 AM

First question is why can't he have a real flashlight?  The LED light from a phone isn't going to be bright enough to shoot by unless you get some super-high sensitivity camera like the Sony A7S.  You're going to have to be shooting at 3200 ASA at least with a fast lens to have that phone light cast enough light on anything.

 

Or else you're going to have to fake everything, tape some bright LiteRibbon LED's to the off-camera side of the phone when you are pointed at what the phone is lighting, and even then it's not going to shine very far.

 

Re-write the scene so he can have a flashlight.  He's got a car, so why can't he have a flashlight in the glovebox?


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#3 Guy Holt

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Posted 09 November 2014 - 11:16 AM

So how do I light my subject if there are no natural light sources around to use as an excuse to place my lights (china lantern, arri fresnels)? I need advise, this is my first time lighting a night scene and I need all the help I can get, in terms of what kinds of light to use and how to light the area. 

 

I would start off by boosting the output of the one practical you have to work with, the phone light, by substituting it with a brighter source with a similar form.  You might try a Sunrei Search2 Xenon Headlamp.  If you take the head strap off, your actor will be able to cup it in their hand like a phone.  It operates off 3 AA batteries via a separate battery pack that can be hidden on your talent and it puts out a lot of light. Since it is also not omnidirectional and won’t light the face of the person holding it, I would also put your china lantern on the end of a painter’s pole and have someone walk just behind your actor holding the lantern slightly in front of them. The final ingredient is to edge light the trees as if by moonlight. On a big budget feature they would use a large source in a condor at a distance so that the trees would be evenly lit over the entire area from a single source.  On a low budget it is harder do without giving away that there are multiple sources. This is where your Arri Fresnels will come into play.

 

By meticulously feathering your Arri Fresnels in Flood you can create a consistent wash of light over a large area.  The trick is to place them only on one side of your set going off in the distance and run them high on stands. Set the heads at full flood and space them so that the edge of the beam of each light (the 50% drop-off point) is overlapped slightly so that it feathers into that of the next. Overlapping the beams in this fashion will create an even 100% intensity seamlessly from front to back of your set. To even the light output from one side of the set to the other, use a single net layered on a double net (the edge of the single net should extend 4-6” further than the edge of the double net) to feather the output of the bottom half of the fixtures. Given that your Arri Fresnels can’t run off batteries you will need a generator to power them.

 

In open woods, you will need to run the generator at a distance so that you don’t pick it up in your audio tracks.  For that reason I would suggest you use one of the modified 7500W Honda  EU6500s with a transformer/distro.  The EU6500s are extremely quiet to begin with, and since the transformer/distro will compensate for line loss by boosting the voltage, the generator can be put 300-400’ away from set where it won’t be heard. Maintaining full line level on set with the transformer/distro will also maintain the color temperature and brightness of your Arri Fresnels which would otherwise dim and become very warm from the voltage drop over a long cable run.. For more details on how use transformer/distros  with portable generators, see my white paper on the use of portable generators in motion picture production

 

Guy Holt, Gaffer, ScreenLight & Grip, Lighting Rental & Sales in Boston 


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#4 Matin Akmal

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Posted 12 November 2014 - 12:23 PM

 

I would start off by boosting the output of the one practical you have to work with, the phone light, by substituting it with a brighter source with a similar form.  You might try a Sunrei Search2 Xenon Headlamp.  If you take the head strap off, your actor will be able to cup it in their hand like a phone.  It operates off 3 AA batteries via a separate battery pack that can be hidden on your talent and it puts out a lot of light. Since it is also not omnidirectional and won’t light the face of the person holding it, I would also put your china lantern on the end of a painter’s pole and have someone walk just behind your actor holding the lantern slightly in front of them. The final ingredient is to edge light the trees as if by moonlight. On a big budget feature they would use a large source in a condor at a distance so that the trees would be evenly lit over the entire area from a single source.  On a low budget it is harder do without giving away that there are multiple sources. This is where your Arri Fresnels will come into play.

 

By meticulously feathering your Arri Fresnels in Flood you can create a consistent wash of light over a large area.  The trick is to place them only on one side of your set going off in the distance and run them high on stands. Set the heads at full flood and space them so that the edge of the beam of each light (the 50% drop-off point) is overlapped slightly so that it feathers into that of the next. Overlapping the beams in this fashion will create an even 100% intensity seamlessly from front to back of your set. To even the light output from one side of the set to the other, use a single net layered on a double net (the edge of the single net should extend 4-6” further than the edge of the double net) to feather the output of the bottom half of the fixtures. Given that your Arri Fresnels can’t run off batteries you will need a generator to power them.

 

In open woods, you will need to run the generator at a distance so that you don’t pick it up in your audio tracks.  For that reason I would suggest you use one of the modified 7500W Honda  EU6500s with a transformer/distro.  The EU6500s are extremely quiet to begin with, and since the transformer/distro will compensate for line loss by boosting the voltage, the generator can be put 300-400’ away from set where it won’t be heard. Maintaining full line level on set with the transformer/distro will also maintain the color temperature and brightness of your Arri Fresnels which would otherwise dim and become very warm from the voltage drop over a long cable run.. For more details on how use transformer/distros  with portable generators, see my white paper on the use of portable generators in motion picture production

 

Guy Holt, Gaffer, ScreenLight & Grip, Lighting Rental & Sales in Boston 

 

Thanks for your input really helped a lot! Told my director about the issue regarding the flashlight and we're no longer using the light from the phone anymore. We are using 1 6.5kW generator for the shoot and I am planning to get 3-4 Arri lights. but I do have one last question though, do I get the tungsten or the HMIs? Again thanks for your advise it has helped tremendously in planning out the scene.


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#5 Guy Holt

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Posted 12 November 2014 - 12:29 PM

 

Thanks for your input really helped a lot! Told my director about the issue regarding the flashlight and we're no longer using the light from the phone anymore. We are using 1 6.5kW generator for the shoot and I am planning to get 3-4 Arri lights. but I do have one last question though, do I get the tungsten or the HMIs? Again thanks for your advise it has helped tremendously in planning out the scene.

 

 

If you want a cool moonlight effect for the deep background I would go with HMI lights if you can afford them. Their native color temperature (5500K) will be closer to what you want for cool moonlight and so you won't lose a lot of output to gels (full CTB on a tungsten light will cut its' output by 70%.) When using HMIs on portable generators it matters not only what type of ballast the HMIs use but also the type of generator they are used on.  What make and model HMIs do you have access to and what make and model 6.5kw generator will you be using?

 

Guy Holt, Gaffer, ScreenLight & Grip, Lighting Rental & Sales in Boston


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#6 Matin Akmal

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Posted 12 November 2014 - 10:22 PM

 

 

If you want a cool moonlight effect for the deep background I would go with HMI lights if you can afford them. Their native color temperature (5500K) will be closer to what you want for cool moonlight and so you won't lose a lot of output to gels (full CTB on a tungsten light will cut its' output by 70%.) When using HMIs on portable generators it matters not only what type of ballast the HMIs use but also the type of generator they are used on.  What make and model HMIs do you have access to and what make and model 6.5kw generator will you be using?

 

Guy Holt, Gaffer, ScreenLight & Grip, Lighting Rental & Sales in Boston

 

 

I figured as much, didn't want to pile up on the budget as it is with the color gels. As for the HMI lights I'm looking at the ARRI M18 1200W/1800W HMI, while I'd be using a Honda 6.5Kw Generator to power them.


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#7 David Mullen ASC

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Posted 12 November 2014 - 11:42 PM

So now there's no flashlight at all?
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#8 Matin Akmal

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Posted 13 November 2014 - 12:41 AM

So now there's no flashlight at all?

No no, we substituted the phone light to a LED flashlight


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#9 David Mullen ASC

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Posted 13 November 2014 - 02:23 AM

Good, because what's nice about a bright flashlight is that it means that your surrounding moonlight can be dim and spotty since it's not providing the main source for seeing what's going on.

 

You might consider stringing a few paper lanterns with daylight LED bulbs in them for an ambient blue moonlight effect if there are a lot of trees. I've also used single-bank daylight Kino tubes mounted up tree trunks for a soft backlight for the foreground (when the background could be lit with a farther HMI on a condor.)

 

Other option if you are on a hill with trees is to smoke up the woods and backlight from below, behind the hill, and throw people into silhouette.  Shelly Johnson, ASC did this wonderfully in "The Wolfman" and the second "Percy Jackson" film.

 

wolfman1.jpg

 

wolfman2.jpg

 

The trick though is to have a long row of lights to get the spread, just a couple would look too sourcey.


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#10 Guy Holt

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Posted 13 November 2014 - 02:41 PM



I figured as much, didn't want to pile up on the budget as it is with the color gels. As for the HMI lights I'm looking at the ARRI M18 1200W/1800W HMI, while I'd be using a Honda 6.5Kw Generator to power them.

 

Arri M18s do not operate reliably on the Edison outlets of 6500W generators. Even though the receptacles are rated for 20Amps, it has been my experience that you cannot run them reliably because the current drawn by 1800W bulbs – 19.5A at 120V – is just too close to the threshold for comfort.  If there is any line loss from a long cable run, or increased resistance from an overheated plug end, the draw of the ballast climbs over 20 Amps and trips the breaker. Since cable plug ends are typically rated for only 15 Amps, more often than not they melt. The increased resistance that results from the heat causes the voltage to the ballast to drop and so it draws more power to maintain the 1800W load. In my experience the power drawn by the 1800W Baby Max is just too near the operating threshold of a 20A circuit for it to operate reliably. You could use the 30A/120V twist-lock receptacle on the generator but that limits you to running only one M18. Also 30A/120V twist-lock connectors aren’t very common in film distros so you may find it difficult to rent 30A extension cables.

 

I have found that the only reliable way to power M18s on portable generators is from their 240V circuit through a 240v-to-120v step down transformer. A transformer will convert the 240V output of the generator into a single large 120V circuit that is more than capable of powering the 19.5A load of a M18. If the generator is one of our modified 7500W Honda EU6500is generators, a transformer will convert the enhanced 7500W output of the generator into a single 60A/120V circuit that is capable of powering up to three M18s.

 

For more detailed information on using transformers with portable generators I would suggest you read an article I wrote 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. If you haven't yet read the article, or looked at it in a while, it is worth reading.  I have expanded it with a new section on paralleling Honda EU6500s or EU7000s for over 100A output. Of the article Harry Box states:

 

"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 news letter article on the use of portable gas generators in motion picture production.

 

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


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#11 Guy Holt

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Posted 13 November 2014 - 03:36 PM



 

You might consider stringing a few paper lanterns with daylight LED bulbs in them for an ambient blue moonlight effect if there are a lot of trees. I've also used single-bank daylight Kino tubes mounted up tree trunks for a soft backlight for the foreground (when the background could be lit with a farther HMI on a condor.)

 

When you use lights sources like LEDs, Fluorescents, HMIs, & CLF lamp banks, on generators it matters not only what type of generator you use but also what type of power supply the lights use.  All of these lights use Switch Mode Power Supplies (SMPSs) that if not power factor corrected will generate harmonic currents that will have a severe adverse effect on the power waveform of some generators, but not others, which is why I asked the make and model of 6500W generator you plan to use.  Honda has made four 6500W genertors (ES, EB, EX, EU models). The use of LEDs, Fluorescents, HMIs, & CLFs with non-power factor corrected power supplies on three out of the four models can lead to trouble.

 

For example,  under the best of circumstances a 1.2kw HMI & a Kino will only draw 16 Amps and you will have no problem operating them on a 20A circuit of a generator.  Under the worst of circumstances a 1.2kw HMI & a Kino will draw 23.6 Amps and you will have nothing but trouble operating them on the generator. Why the difference? Because it depends on whether the HMI & Kino ballasts are power factor corrected and whether the generator is an inverter generator or a conventional AVR generator. If not power factor corrected, Switch Mode Power Supplies can draw excessive amounts of power for the wattage of light they generate and generate harmonics that can have a severe adverse effect on not only the generator, but also other electronic equipment operating on the same power. There is a video on You-Tube by a Lighting Designer by the name of Kevan Shaw that illustrates just this.

 

 

CFL_vs_Gen_Dem.jpg

 

 

In his You-Tube Video, “Compact Fluorescent verses The Generator,” Kevan Shaw compares the effect of equal wattages of CFLs and Incandescent lights on a small portable generator. In his test, he first operates a 575W ETC Source Four Leko with Quartz Halogen bulb on an 850W two-stroke conventional gas generator without problem. However, when he tries to operate an equivalent wattage of CFLs (30-18W bulbs) the generator goes berserk. Only after turning off half the CFL Bulbs does the generator operate normally with a remaining load of 15 - 18W CFLs (270 W.) What accounts for the erratic behavior of the generator in this video under a smaller load of CFLs? It is a combination of the poor Power Factor of the CFL bulbs and the harmonic currents they generate.

 

Even though the 15 CFL bulbs have a True Power of 270W (15 x 18W = 270W ), the Watt indicator on Kevan's generator indicates that they draw twice that in Apparent Power (535W), or have a Power Factor of .5 (270W/535W =.504.) The fact that CFL bulbs consume double the energy (Apparent Power) for the 18 Watts of light (True Power) they generate, is only half the story here. Kevan Shaw’s video also clearly demonstrates the severe effect that leading power factor loads - like LEDs, CFLs, HMIs, & Fluorescents - can have on the governing systems of conventional AVR generators.

 

When Kevan turns off the 18W CFL bulbs one at a time until the generator stabilizes, he is not only demonstrating that 15 – 18W CFL bulbs has roughly the same Apparent Power (535W), according to the generator’s Watt meter, as a 575W incandescent light; but, also that the maximum Leading Power Factor load a 850W conventional generator can operate satisfactorily is 270 Watts (15 – 18W CFL bulbs). Looked at from another angle, 576 Watts of Apparent Power with a Leading Power Factor (16 - 18W CFL bulbs) overloaded the generator, while 575 Watts of Apparent Power with a Unity Power Factor (the 575W Quartz Leko) did not. What accounts for this difference? Since the load is almost the same (576 & 575 Watts of Apparent Power respectively), the only factor that can account for the generator going berserk with the equivalent load of CFL lights is the harmonic currents that they generate, that the Quartz Leko does not. Without a doubt, Kevan Shaw’s video is a clear demonstration of the adverse effect that harmonic currents have on the governing systems of conventional AVR generators (click on the image above to see the video.)

 

For the same reason that Kevan Shaw was not able to operate more than 270 Watts of CFL bulbs (15–18W bulbs) on his little 850W generator, you may not be able to operate a bunch of Kino and LED lanterns on a conventional 6500W AVR generator if the ballasts are not Power Factor Corrected (most LED power supplies and Fluorescent Ballasts are not.) The adverse effects of the harmonic currents that non PFC ballasts generate, so graphically demonstrated in Kevan’s video, limits the total amount of Leading Power Factor loads, as compared to Unity Power Factor loads, that can be reliably operated on conventional AVR generators.

 

Guy Holt, Gaffer, ScreenLight & Grip, Boston


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#12 Matin Akmal

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Posted 14 November 2014 - 04:35 AM

 

When you use lights sources like LEDs, Fluorescents, HMIs, & CLF lamp banks, on generators it matters not only what type of generator you use but also what type of power supply the lights use.  All of these lights use Switch Mode Power Supplies (SMPSs) that if not power factor corrected will generate harmonic currents that will have a severe adverse effect on the power waveform of some generators, but not others, which is why I asked the make and model of 6500W generator you plan to use.  Honda has made four 6500W genertors (ES, EB, EX, EU models). The use of LEDs, Fluorescents, HMIs, & CLFs with non-power factor corrected power supplies on three out of the four models can lead to trouble.

 

For example,  under the best of circumstances a 1.2kw HMI & a Kino will only draw 16 Amps and you will have no problem operating them on a 20A circuit of a generator.  Under the worst of circumstances a 1.2kw HMI & a Kino will draw 23.6 Amps and you will have nothing but trouble operating them on the generator. Why the difference? Because it depends on whether the HMI & Kino ballasts are power factor corrected and whether the generator is an inverter generator or a conventional AVR generator. If not power factor corrected, Switch Mode Power Supplies can draw excessive amounts of power for the wattage of light they generate and generate harmonics that can have a severe adverse effect on not only the generator, but also other electronic equipment operating on the same power. There is a video on You-Tube by a Lighting Designer by the name of Kevan Shaw that illustrates just this.

 

 

CFL_vs_Gen_Dem.jpg

 

 

In his You-Tube Video, “Compact Fluorescent verses The Generator,” Kevan Shaw compares the effect of equal wattages of CFLs and Incandescent lights on a small portable generator. In his test, he first operates a 575W ETC Source Four Leko with Quartz Halogen bulb on an 850W two-stroke conventional gas generator without problem. However, when he tries to operate an equivalent wattage of CFLs (30-18W bulbs) the generator goes berserk. Only after turning off half the CFL Bulbs does the generator operate normally with a remaining load of 15 - 18W CFLs (270 W.) What accounts for the erratic behavior of the generator in this video under a smaller load of CFLs? It is a combination of the poor Power Factor of the CFL bulbs and the harmonic currents they generate.

 

Even though the 15 CFL bulbs have a True Power of 270W (15 x 18W = 270W ), the Watt indicator on Kevan's generator indicates that they draw twice that in Apparent Power (535W), or have a Power Factor of .5 (270W/535W =.504.) The fact that CFL bulbs consume double the energy (Apparent Power) for the 18 Watts of light (True Power) they generate, is only half the story here. Kevan Shaw’s video also clearly demonstrates the severe effect that leading power factor loads - like LEDs, CFLs, HMIs, & Fluorescents - can have on the governing systems of conventional AVR generators.

 

When Kevan turns off the 18W CFL bulbs one at a time until the generator stabilizes, he is not only demonstrating that 15 – 18W CFL bulbs has roughly the same Apparent Power (535W), according to the generator’s Watt meter, as a 575W incandescent light; but, also that the maximum Leading Power Factor load a 850W conventional generator can operate satisfactorily is 270 Watts (15 – 18W CFL bulbs). Looked at from another angle, 576 Watts of Apparent Power with a Leading Power Factor (16 - 18W CFL bulbs) overloaded the generator, while 575 Watts of Apparent Power with a Unity Power Factor (the 575W Quartz Leko) did not. What accounts for this difference? Since the load is almost the same (576 & 575 Watts of Apparent Power respectively), the only factor that can account for the generator going berserk with the equivalent load of CFL lights is the harmonic currents that they generate, that the Quartz Leko does not. Without a doubt, Kevan Shaw’s video is a clear demonstration of the adverse effect that harmonic currents have on the governing systems of conventional AVR generators (click on the image above to see the video.)

 

For the same reason that Kevan Shaw was not able to operate more than 270 Watts of CFL bulbs (15–18W bulbs) on his little 850W generator, you may not be able to operate a bunch of Kino and LED lanterns on a conventional 6500W AVR generator if the ballasts are not Power Factor Corrected (most LED power supplies and Fluorescent Ballasts are not.) The adverse effects of the harmonic currents that non PFC ballasts generate, so graphically demonstrated in Kevan’s video, limits the total amount of Leading Power Factor loads, as compared to Unity Power Factor loads, that can be reliably operated on conventional AVR generators.

 

Guy Holt, Gaffer, ScreenLight & Grip, Boston

 

May I know which model of the four is the safest decision? Because the one we're using is the Honda EU 6500iS


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#13 Phil Rhodes

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Posted 14 November 2014 - 09:13 AM

Guy, I think you need to be a little more careful here. Most (basically all) LEDs are run from some form of switch mode power supply, which are used on more or less everything that isn't an incandescent light bulb or some form of heater. Electronic fluorescent and metal halide ballasts (including movie-world HMI ballasts and the types used industrially) are also switch mode power supplies, as are the power supplies for cellphones, laptops (which can be comparatively high power), desktop computers, TVs and video monitors, battery chargers and frankly more or less all modern electronics

 

Some switch mode power supplies are better engineered than others and I am not blind to the faults of any of them, which include things you don't mention, such as RFI. Even so, this is no reason to specifically decry a particular piece of lighting technology. It is not an LED problem or a fluorescent problem, it's a bad quality electronics problem. If people stopped buying incredibly badly-made electronics from iffy far-eastern manufacturers, things would be different. I think that's a more useful fight.

 

I would be horrified and astonished to find that Kino-Flo were not shipping ballasts with adequate power factor correction and suppression of harmonics. Surely half the point of purchasing a branded option would be to ensure that sort of thing had been dealt with! This is making me feel a lot better about my jury-rigged options, which use Osram QTP ballasts (marked  γ=0.98) and run quite happily from modified-sine-wave inverters.

 

P


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#14 Guy Holt

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Posted 14 November 2014 - 10:55 AM



Guy, I think you need to be a little more careful here. Most (basically all) LEDs are run from some form of switch mode power supply, which are used on more or less everything that isn't an incandescent light bulb... I would be horrified and astonished to find that Kino-Flo were not shipping ballasts with adequate power factor correction and suppression of harmonics. Surely half the point of purchasing a branded option would be to ensure that sort of thing had been dealt with! 

 

Phil, from where you sit most brands have dealt with harmonic suppression because power factor correction is mandated by EU standards for any luminary over 75W (a lot of LED bulbs do not meet that criterion.) In the US we have no such requirement, which results in the fact that companies like ARRI and K5600 distribute different ballasts for the EU and US. The Joker 200, 400,  and 800 ballasts for the EU are PFC and the ballasts for the US are not PFC (I’m not sure about the 1600W Ballast). Because PFC circuitry added a $1000.00 to the cost of a 575/1200W HMI ballast, almost all 575/1200 ballasts in this country are non-PFC and a lot of 2.5/4kw ballasts are as well.  In the US, the power supplies for the Litepanel 1x1s and Sola Fresnels are not PFC. And until, the introduction of the DMX ballast just this year, all Kino ballasts in this country to drive T12 fixtures were not PFC (the 4, 2, and Single Bank fixtures as well as the Flat Head 80,  Image 80 and Image 85 fixtures.) What this means is that most of the lighting rental inventory in this country that would likely be used on a portable generator in use non-power factor corrected switch mode power supplies.

 

I agree a hundred percent. We are no longer in our parent’s linear world. To make matters worse, the power generation and electrical distribution systems developed then were much simpler and not designed to deal with the abundance of non-linear loads like the electronic HMI, Kino Flo, and LED power supplies that make up lighting packages today. The problem is being further compounded by the increasing prevalence on set of sophisticated electronic production equipment like HD cameras, computers, hard drives, and monitors which are not only sensitive to harmonic distortion, but are themselves sources of harmonic distortion. 



 

For instance, the self-excited AVR systems of conventional portable generators were not designed to operate with leading power factor loads. In AVR systems, the AC voltage generated is controlled by DC excitation of the electro-magnets of the generator's Rotor. The amount of DC excitation required is a function of generator load; or, put another way, the excitation required to maintain constant voltage increases with load. The type of load also affects the amount of excitation required. Lagging power factor loads (magnetic ballasts) require more excitation than a unity power factor load (Quartz Lights.) Leading power factor loads (electronic ballasts) require less excitation than unity power factor loads. 

Rudimentary AVR systems like those in portable generators are ill equipped to deal with leading power factor loads like electronic ballasts because the harmonic currents they generate create flux in the armature coils of the Stator that reacts additively with the Exciter flux in the field poles of the Rotor to increase saturation and produce a higher terminal voltage than called for a given load. Consequently, the AVR system responds erroneously to control voltage by reducing excitation. The end result is that the regulator goes to its minimum excitation capability while the additive excitation of the armature flux from the leading power factor causes the terminal voltage to continue to rise and not be controlled by the voltage regulator. 



 

Erroneous regulation of voltage is just one example of the more severe effect that leading power factor loads have on conventional AVR generators than do lagging power factor loads. In researching my  newsletter article I compared the characteristic voltage waveform distortion created by different lighting loads on different generators, and found that leading power factor loads also have a more severe effect on other production equipment operating on the same power.

Use this link for my newsletter article that explains the electrical engineering principles behind these issues and how to resolve them.

 

 

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


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#15 Guy Holt

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  • Gaffer
  • Boston

Posted 14 November 2014 - 03:50 PM

May I know which model of the four is the safest decision? Because the one we're using is the Honda EU 6500iS

 

When using HMIs, Kinos, & LEDs that are not power factor corrected, you want to use an inverter generator like the Honda EU6500is because the harmonics generated by these lights can have a severe adverse effect on the power waveform of conventional AVR generators like the Honda EB, EX, & ES 6500s. The harmonic noise these light sources generate will not have nearly as bad an effect on the cleaner power supplied by an inverter generator like the Honda EU6500is.

 

The middle oscilloscope shot below shows the severe adverse effect a non-PFC 1200W HMI Electronic ballast can have on the power waveform of a conventional portable generator. Given the large sub-transient impedance of conventional portable generators, even a small degree of harmonic noise being fed back into the power stream will result in a large amount of distortion in its’ voltage.

 

waveform_elec_ballast.jpg

Left: Grid Power w/ 1.2Kw Arri non-PFC Elec. Ballast. Center: Conventional AVR Power w/ 1.2Kw Arri non-PFC Elec. Ballast. Right: Inverter Power w/ 1.2Kw Arri non-PFC Elec. Ballast.

 

The adverse effects of the harmonic noise exhibited here, can take the form of overheating and failing equipment, circuit breaker trips, excessive current on the neutral wire, and instability of the generator’s voltage and frequency. Harmonic noise of this magnitude can also damage HD digital cinema production equipment, create ground loops, and possibly create radio frequency (RF) interference.

 

When your lighting package consists predominantly of non-linear light sources, like HMI, Fluorescent, & LED lights, it is essential to have Power Factor Correction circuitry (PFC) in the ballasts and to operate them on an inverter generator. The combination of improved power factor and the nearly pure power waveform (1-2 %THD) of the inverter generator creates clean stable set power (like that in the power waveform below right).

 

 

wwaveform_PFC-ballast.jpg

Left: Grid Power w/ 1.2Kw P-2-L PFC Elec. Ballast. Center: Conventional AVR Power w/ 1.2Kw P-2-L PFC Elec. Ballast. Right: Inverter Power w/ 1.2Kw P-2-L PFC Elec. Ballast.

 

For more details on what type of generator to use with non-PFC HMI, Fluorescent, & LED lights, read the article I wrote for our company news letter on the use of portable generators in motion picture production. The article is available online at http://www.screenlig...generators.html.

 

Guy Holt, Gaffer, ScreenLight & Grip, Lightng Rental and Sales in Boston


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rebotnix Technologies