"TV Technology"
6 October 2010
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TV Technology
Television Lighting: “Commemorating the PAR Can”.
by Bill Klages
Commemorating the PAR Can
It was of great interest to me that two of last month's entertainment production periodicals featured articles celebrating the lowly, but certainly loved, PAR Can fixture. Both articles (one by Richard Cardena and the other by Rob Halliday, very talented friends) credited the legendary lighting designer Chip Monck with creating the popularity of this simple lamp in the live-performance concert world. It should be added that prior to this it was common practice in film - and television - to boost the voltage on the 500 watt, 2850°K lamp to make it equivalent to a 3200°K or 3400°K lamp to get nearly 50% more light output. Of course, the life of the lamp was greatly shortened. The availability of the 1000 watt, 3200°K, Par-64 in four beam configurations eliminated the need for the auxiliary boost unit and gained industry-wide acceptance.
Television Applications
In television, a great step forward was the realization that efficient wiring systems and methods were available from concert equipment suppliers, and that a few hundred of these very efficient fixtures could be employed for such brute-force applications as lighting audiences or to easily obtain multi-color washes. A few narrow-spot Par Cans would put out more beam lumens than a 5K Fresnel fully spotted at a long throw distance (say, one hundred feet). Things like six circuit multi-cable and bars with six Par heads provided great convenience.
Personal Recollection
However, one incident early on made me more conscious that a problem could exist even in such a simple load distribution scheme. I was lighting a beauty pageant in an arena in Honolulu. At the time there was very little equipment available in the islands and I had shipped a system from Los Angeles, much of it supplied by a concert rental house. I recall that for some reason I had lined a perimeter catwalk in the arena ceiling with a number of six light bars, wired with multi-cable and fed from dimmer racks located at floor level. When I started to focus, it was very apparent that there was a problem with voltage drop due to the lengths of the multi-cables. As you followed the cable path from bar to bar, the lamp color became more and more orange. Even though at the present time our attention may be diverted to the vexing challenge as to how to employ an LED fixture, I would like to review more basic issues.
The Basics
Cable has electrical resistance; the thicker the cable, the less its resistance. As the current flows to our lamp by an amount determined by the actual voltage across the lamp, there is a voltage loss due to the cable's resistance. The light output from an incandescent fixture is very sensitive to the applied voltage. As an illustration of this, a lamp intended to be used at 120 volts will have half the light at about 96 volts. This is only a 20-percent voltage drop. (Note: This would amount to about a 200°K lower color temperature. In the case of a photographic lamp that is specified as operating at 3200°K, it would then be operating at about 3000°K.) So it is important that if we are interested in making our system performance efficient, we should limit this voltage drop. But, we are limited to the wire size that is available to us by what the supplier has in inventory. In the case of multi-cable it is either #12 or #14 AWG (American Wire Gauge), #12 being thicker. A greater cross-sectional area has less resistance; hence, a lower voltage drop than #14 AWG. (Note: There is some #10 AWG multi-cable, but it's very rare.)
Voltage Drop Limit
Next, we have to set a realistic voltage drop that we will accept. Looking to the NEC (the National Electrical Code) for guidance, it suggests a 3% voltage drop limit. At 120 volts this would be 3.6 volts. At this voltage, our lamp will have approximately 90 percent of the light output at the proper voltage. I have provided a table that lists the maximum cable run that would be acceptable for the 3% limit under different possible alternatives. A 2KW load would have two Par-64s two-fered as a load while a 1Kw load would be one lamp. Note that the lengths are not very long.
About the Supply Voltage
In order to meet the requirement that we start with a feed of 120 volts to realize the criteria detailed above, the dimmer rack output must deliver 120 volts at each dimmer output when "full." The rub here is that there is also a voltage drop in each dimmer module due to a voltage loss through the solid-state device, an SCR in most cases, as well as a drop when current flows through the filter choke in each dimmer. To eliminate these voltage losses, the professional dimmer manufacturers provide that the voltage output of the dimmer is regulated to 120 volts - if the input voltage is greater than the sum of the dimmer's maximum voltage drop under rated load and 120 volts. It is good practice to raise the dimmer rack input voltage at the supply transformer, if possible, usually to the +10% tap. This will insure that dimmer's output voltage will be equal to the regulated amount of 120 volts. It goes without saying that this should be done by a qualified person. (Note: Actually, in the case of some of the more sophisticated dimmer modules, the voltage output can be regulated at greater than 120 volts - this is not recommended.)
Moving Lights and Arc Sources
It must be emphasized that we are talking about conventional incandescent fixtures. The majority of moving light fixtures have discharge lamps. In addition there are other fixtures that are not incandescent such as HMI and LED sources. In these cases, the individual power supply for the fixture is designed to adapt to a range of input voltages; in some instances, the range can be wide, from 100 to 240 volts. Under any of these voltages, the power supply will regulate to deliver the correct lamp operating voltage and current. Voltage drop, therefore, is not a consideration for such fixtures as long as the input voltage is within the operating range (and there is no limitation to the current that needs to be supplied). Some instruments that use an incandescent lamp are even equipped with self-regulating supplies.
Be Nice to Our Old Friend
So, treat your lowly Par Can with respect and get every drop of light out of it as possible, Keep the voltage drop as low as possible by locating your dimmer racks as close to the lamps as possible. Boost the input voltage to your dimmer rack and set your dimmer's regulated output to 120 volts. Use the largest wire size available and place only one lamp on each circuit.
Bill Klages would like to extend an invitation to all the lighting people out there to give him your thoughts at billklages@roadrunner.com.