Cinematography-Theory-And-Practice

BallastsAll HMIs require a ballast, which acts as a current limiter. The reasonfor this is simple: an arc is basically a dead short; if the current wereallowed to flow freely, the circuit would overload and either blowthe fuse or burn up. Early ballasts for HMIs were extremely heavyand bulky (200 pounds or more) because they contained current limitersthat consisted of heavy copper wire wound in a coil like a transformer.Fortunately, few of these remain in use.The invention of the smaller and lighter electronic ballast was amajor improvement. Electronic ballasts also allow the unit to operateon a square-wave (unlike the sine wave of normal alternating currentelectricity).The most significant new development in HMIs is the new flickerfreeballasts, which use square-wave technology to provide flickerlessshooting at any frame rate. With some units there is a penaltypaid for flicker-free shooting at frame rates other than sync soundspeed: it results in a significantly higher noise level. If the ballastscan be placed outside or shooting is MOS, this is not a problem. It isnot usually an issue as high-speed shooting rarely involves recordingaudio. For a more detailed discussion of flicker and how to avoid it,see the chapter Technical Issues.Header cables are the power connection from the ballast to thelight head itself. Many larger HMIs can only take two header cables;a third header will usually result in a voltage loss too great to get thelamp to fire up. Square-wave refers to the shape of the sine wave ofthe alternating current after it has been reshaped by the electronicsof the ballast. Flicker is discussed in more detail in the chapter onFigure 8.3. A combination of KinoFlo and tungsten units on a bluescreenefx set. (Photo courtesy ofKino Flo.)lighting sources131