the technological developments that made them credible film lights and how their popularity spread over to Hollywood film sets.
I don’t know the particulars of where, when, and on what HMIs were first used, but I would be very surprised if they came about as a consequence of HID lamps being used on set first. The output and color rendering of HID lamps is and always has been not up the requirements of motion picture imaging. One distinguishing feature of HMI lamps that separates them from HID lamps is the precious metals incorporated in HMI lamps to generate a near daylight spectrum. HMI lamps are also designed with a comparatively short electrode gap in order to increase output, improve luminous efficacy, and color rendering. Their increased luminous efficacy places an increased load on the bulb wall which accelerates the devitrification of their quartz envelope; which, in turn, leads to increased lumen depreciation. In other words, their brilliance comes at the expense of lamp life. Over their relatively short life of 500-750 hrs, it is not uncommon for HMI lamps to lose 15-20 percent of their initial lumen output before they fail all together. For this reason, HMI lamps were never adopted for general illumination.
It took a while for manufacturers to work out all the bugs in HMI power supplies, so any discussion of “the technological developments that made them credible film lights” would have to include the advancements we have seen over the years in ballast design. Today you have a choice between magnetic and electronic ballasts; and to complicate matters even more, you have a choice between Power Factor Corrected electronic ballasts and non-Power Factor Corrected electronic ballasts.
Power Factor Correction (PFC) is relatively new to HMIs and adds considerably to their cost and so you will still not find it in smaller HMI ballasts. In fact, I wouldn’t be surprised if you were not even familiar with Power Factor Correction. 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 and power generation source. However, any veteran film technician who has worked in the industry for more than twenty years 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. 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 PFC circuitry was not incorporated in smaller HMIs was that it 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. Unfortunately, it is still the case that almost every 575 - 1200W electronic ballast that you will find in a rental house or for sale used in North America will be a non-PFC electronic ballast.
To this day older magnetic HMI ballasts offer a number of benefits 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 loads 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. As mentioned above 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.
Characteristic voltage waveform of a non-PFC electronic HMI ballast on grid power (left),
on power generated by a conventional AVR generator (middle),
and power generated by an inverter generator (right)
The adverse effects of this harmonic noise that can be seen in the middle oscilloscope image above, 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.
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.
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. And, since after ramping up, PFC electronic ballasts top off at a lower current draw than non-pfc electronic ballast, it is now possible to operate a 12kw HMI off of a couple of paralleling Honda EU6500 generators. And if that 12kw globe has one of the CAD designed ARRIMAX reflectors behind it, you have output comparable to that of an 18kw HMI Fresnel on Honda generators. Since there is nothing else that can run portable generators that can compare to an 18kw Fresnel, HMIs will continue to be the go to source when it comes to lighting day exteriors or the deep background of night exteriors.
Guy Holt, Gaffer,
ScreenLight & Grip Boston
Lighting Rental & Sales in Boston