Cinematography-Theory-And-Practice

Figure 8.4. (above) The ballast actsas a transformer to provide operatingvoltage and also starting voltage,which can be as high as 20,000volts. It is also a current limiter.Figure 8.5. (right) An 18K HMI with aChimera softbox in use on a day exteriorlocation.Technical Issues, but suffice it to say here that the normal sine wave ofAC current leaves too many “gaps” in the light output that becomevisible if the camera shutter is not synchronized to its rhythm. Bysquaring the wave, these gaps are minimized and there is less chanceof flicker. This is especially important if you are shooting at anythingother than normal speed; high-speed photography in particularwill create problems. It is important to note that flicker can be aproblem in video also, just as with film cameras.Voltages as high as 12,000 VAC (volts AC) or more are needed tostart the arc, which is provided by a separate ignitor circuit in the ballast.This creates the power needed for the electric current to jumpacross the gap between the two electrodes. The typical operatingvoltage is around 200V. When a lamp is already hot, much highervoltages are needed in order to ionize the pressurized gap betweenthe electrodes. This can be from 20 kV to more than 65 kV (kilo-Volt). For this reason, some HMIs can not be restruck while theyare hot — which means you may have to wait for the light to coolbefore you can start it again. This can be a major hindrance whenthe whole film crew is waiting on it. Hot restrike, which generates ahigher voltage to overcome this resistance, is a feature on most newerHMIs. HMI bulbs can change color temperature as they age. It isimportant to check the color temperature of HMI bulbs periodicallywith a color meter to see if they need correcting gels to keep themconsistent with other lighting units in use on the project.18K and 12K HMIThe 18K and the 12K HMIs are the most powerful fresnel lights currentlyavailable. Like all HMIs they are extremely efficient in lightoutput per watt of input power. They produce a sharp, clean light,which is the result of having a very small source (the gas arc) whichis focused through a very large lens (Figure 8.2).These large lights are invaluable where very large areas are beingcovered or there is a need for high light levels for high-speed shooting.They are also a natural for sunlight effects such as sunbeamsthrough a window or any other situation where a strong welldefinedbeam is needed. They are also among the few sources (alongwith HMI PARs) that will balance daylight and fill in the shadowssufficiently to permit shooting in the bare sun without silks or reflec-cinematography132