HMI Lighting

[videoproduction]

Lighting is everything! Whether it's natural sunlight, subdued ambience, perfect practicals or complex Hollywood style lighting, the image owes all of its power to light. An expensive but required type of light in most higher-end video production projects is HMI lighting. For pure lumen power, HMI's are hard to beat. They can be temperamental, flickery and a pain to move but when you need them there is no substitute.

HMI's come in many different types but the main differences come down to a few simple things. Ballasts. Newer, more expensive ballasts are electronic. Older, cheaper ballasts are magnetic. Electronic ballasts are lighter, more energy efficient and can be flicker-free which makes a big difference when using different frame rates. Magnetic ballasts are bigger, heavier, use more power and can lead to big issues when not shooting at 1/48 or 1/60 shutter speeds. The choice is simple if you have the money but magnetic ballasts can work just fine in certain situations.

Lens/Globe - The basic two types are Fresnel and Par. Focusable Fresnel heads which provide less punch but an-all-in one unit that are adjustable to a finer degree are good in certain situations where a little less light is needed and you don't want to carry around lenses. Par heads are better for a wider degree of adjustment by swapping out lenses from an external case and an often times more powerful overall output. If I had to choose one head I would most definitely take a Par.

A unique aspect of HMI's that make them significantly better than tungsten sources in many situations is that they are naturally balanced to sunlight. This means that the light they emit is between 5000 and 6000 degrees Kelvin and will match direct sunlight. Adding full or 3/4 CTOrange gels to an HMI to match tungsten lighting at 3200 Kelvin will only take away 2/3 of a stop but adding full CTBlue gels to tungsten lighting will take away 2 stops from a source that is already far less powerful per watt. If you are adding supplemental daylight colored lighting then HMI's are definitely the way to go.

In the last 10-15 years technology has given us a couple more options to compete with HMI's but at this point they are still limited with their uses. LED's and Fluorescent lights such as KINO FLO's are amazing lighting tools but they do not have the same sort of 'punch' that HMI's do. Fluorescents are inherently softer but have less throw than HMI's while LED's of the spot variety can muster a decent amount of throw it will still take an expensive array of LED's to compete with even a 20 year old, used HMI.

If you've never used an HMI I urge you to go to a store that sells them, rent one from a rental house or check out one from a friend. They are amazing lights and despite their complexities they offer something no other instrument can.

 

[documentarytube]

I've heard of HMI lighting and I must say, I've been meaning to try them out for a long time now. I've heard so many great things about them and this just more benefits to it.

thanks for this info, it helps me out greatly.

 

[NEJuicer]

... Electronic ballasts are ... more energy efficient ... Magnetic ballasts ... use more power... The choice is simple if you have the money but magnetic ballasts can work just fine in certain situations.

Unfortunately, in regard to ballasts it is not this cut and dry. Unless the electronic ballast is Power Factor Corrected (most ballasts under 6kw are not PFC) it will in fact draw more power than a comparably sized magnetic ballast. Take the 1200W Power Class for example: Power Factor Correction (PFC) is very new in 1200W HMIs and so you will not find it in most ballasts that size. In fact, I wouldn’t be surprised if you were not be even familiar with Power Factor Correction in HMI ballasts.

Since Power Factor Correction (PFC) is not mandated in this country, as it is in Europe for any electrical device that draws more than 75W, we are pretty much ignorant of Power Factor and effect that poor Power Factor can have on a distribution system. However, any film technician familiar with large HMI heads will be quite familiar with Power Factor and Power Factor Correction (PFC.) That is because after a false start back in the 90s, all major manufacturers now include PFC circuitry in HMI ballasts in the 6-18kw range. They do so by necessity. The early line of Lightmaker electronic ballasts were nick named by film electricians “Troublemaker” ballasts because they were not Power Factor Corrected and proved that PFC circuitry was absolutely necessary in large ballasts to reduce heat and returns on the neutral, and to increase ballast reliability (beware, some are still kicking around ebay). But, because of the added cost, weight, and complexity of PFC circuitry, ballast manufacturers in the US still only offer PFC circuitry as an option in medium-sized 2.5-4kw ballasts. And, until very recently manufacturers did not offer PFC circuitry in HMI ballasts smaller than 2.5kw in the US (in the EU PFC circuitry in mandatory in all HMI ballasts sold.)

Part of the reason was that PFC circuitry did not offer a huge advantage when plugging into house power. A typical 1200W Power Factor Corrected electronic HMI ballast will draw 11 Amps at 120 Volts verses the 19 Amp draw of a non-PFC electronic ballast. While not a huge advantage when plugging into house power, the added efficiency of a PFC 1200 ballast can make a huge difference when powering a lighting package off of a portable generator. For example, when you consider that a Kino Flo Parabeam 400 draws only 2 amps, the 8 Amp difference between using a PFC 1200W electronic ballast and standard non-PFC 1200W electronic ballast, can mean the difference between running four additional Parabeam 400s on a portable generator or not – I think you would have to agree that is a major boost in production capability and pertinent to any one using a portable generator as their principle source of set power. Unfortunately, it is still the case that almost every 575 - 1200 W electronic ballast that you will find in a rental house or for sale used in North America will be a non-PFC electronic ballast.


Depending on the type of filming you do, you may in fact be better served by an older magnetic ballast over a non- Power Factor Corrected electronic ballast. A 1.2kw electronic ballast draws 19amps (verses the 13.5 amps of a magnetic ballast) so it will always trip the common 15amp house circuit and will trip a 20 Amp circuit if there is something else, like a computer or light, on the same circuit. Where you can't always know what else is on the same circuit, or even if it is a 20 or 15 Amp circuit, a 1.2kw magnetic ballast drawing only 13.5 Amps is the safer bet since it can operate on a 15 amp circuit even with other things on the circuit. Non-Power Factor Corrected electronic ballasts are meant to be used on film sets where every circuit is 20 Amps and you know what is on the circuit because you are distributing the power yourself from a tie in or generator. If your style of shooting requires that you plug into wall outlets, you will be better served by a magnetic ballast.

But that is not the only benefit to using a magnetic ballast over a non-PFC electronic ballasts. If you don’t have access to the newest PFC electronic ballasts, the older magnetic ballasts are in fact cleaner running on portable gas generators than non-PFC electronic ballasts. The harmonic distortion created by non-PFC ballasts reacting poorly with the distorted power waveform of conventional AVR generators limited the number of HMIs you could power on a portable generator. The primary factors limiting the use of HMIs on portable generators has been the inefficient use of power by non-PFC electronic ballasts and the harmonic noise they throw back into the power stream. The adverse effects of this harmonic noise, can take the form of overheating and failing equipment, efficiency losses, circuit breaker trips, excessive current on the neutral wire, and instability of the generator’s voltage and frequency. For these reasons it has never been possible to operate more than a couple of 1200W HMIs on a conventional 6500W portable gas generator. Harmonic noise of this magnitude can also damage HD digital cinema production equipment, create ground loops, and possibly create radio frequency (RF) interference. The increasing use of personal computers, hard drives, and microprocessor-controlled recording equipment in production has created an unprecedented demand for clean, reliable power on set.

However, now that inverter generators, like the Honda EU6500is, do not require crystal governors to run at precisely 60Hz, magnetic ballasts offer a cost effective alternative to dirty non-PFC electronic ballasts because you can operate magnetic HMI ballasts “flicker free” on inverter generators. And as mentioned above, the smaller magnetic ballasts (575-2500W) offer the distinct advantage of being less expensive and draw less power (once they have come up to speed) than the commonly available non-PFC electronic equivalents (13.5A versus 19A for a 1.2kw.)

Of course there are downsides to using magnetic ballasts. One down side is that you are restricted to using only the safe frame rates and shutter angles. But, when you consider that every film made up to the early 1990s were made with magnetic HMI ballasts you can see that being limited to the safe frame rates is not all that restrictive. Another downside to magnetic ballasts is that you can’t load the generator to full capacity because you must leave “head room” for their higher front end striking load. When choosing HMIs to run off portable generators, bear in mind that magnetic ballasts draw more current during the striking phase and then they “settle down” and require less power to maintain the HMI Arc. By contrast, an electronic ballasts “ramps up”. That is, its’ current draw gradually builds until it “tops off.”

While older HMIs with magnetic ballasts are less expensive to purchase or rent, Power Factor Correction (PFC) makes the newest electronic ballasts worth the extra money when it comes to lighting with portable generators. The substantial reduction in line noise that results from using power factor corrected ballasts on the nearly pure power waveform of an inverter generator creates a new math when it comes to calculating the load you can put on a generator. In the past we had to de-rate portable gas generators because of the inherent short comings of conventional generators with AVR and Frequency governing systems when dealing with non-PFC electronic ballasts. The harmonic distortion created by non-PFC ballasts reacting poorly with the distorted power waveform of conventional AVR generators limited the number of HMIs you could power on a portable generator to 75% of their rated capacity (4200Watts on a 6500W Generator). But now, where inverter generators have virtually no inherent harmonic distortion or sub-transient impedance and power factor correction (PFC) is available in small HMI ballasts, this conventional wisdom regarding portable gas generators no longer holds true. Where before you could not operate more than a couple 1200W HMIs with non-PFC electronic ballasts on a conventional generator because of the consequent harmonic distortion, now according to the new math of low line noise, you can load an inverter generator to capacity. And if the generator is one of our modified Honda EU6500is inverter generators, you will be able to run a continuous load of up to 7500W as long as your HMI and Kino ballasts are Power Factor Corrected.

For more detailed information on HMIs I would suggest you read an article I wrote for our company newsletter on operating HMIs.

This article is cited in the just released 4th Edition of Harry Box's "Set Lighting Technician's Handbook" and featured on the companion website "Box Book Extras." Of the article Harry Box exclaims:

"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."​

The article is available online at http://www.screenlightandgrip.com/html/emailnewsletter_generators.html.


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