The second day of shooting involved dressing a school dining hall, and hammering in the 'no hunting sign'. Last night the shoot ended at 1:30 am, everybody is knackered as hell. The first thing I did was to rub mud and water and leaves into the no hunting sign. This made it look dirty and used.
'Hydrargyrum medium-arc iodide, or HMI, is a mercury-halide gas discharge medium arc-length lamp with a multi-line spectra emission. The name implies that hydrargyrum, an archaic term for Mercury (Hg), is held as a vapour mixed with other rare halides in a quartz-glass envelope with two tungsten-coated electrodes of medium arc separation.
Unlike traditional tungsten lighting units, HMIs use ballasts to regulate and supply electricity to the lamp head via a header cable. The lamp operates not by heating a tungsten-based filament, but rather by creating an electrical arc between two electrodes within the bulb that excites the pressurized mercury vapour and provides phenomenal light output with greater efficiency than tungsten-based lighting units. The efficiency advantage is near fourfold, with approximately 85-108 lumens per watt of electricity. Unlike tungsten bulbs where the gas is inert and solely for filler and recirculation, HMI bulbs rely heavily on the mercury vapour for light output, and the other metal halides mixed with the mercury to give it the spectral peaks in output wavelengths that bring it to approximately 5600 K, or the color temperature of noon sunlight.'
Its a powerful light so the electric energy would have large fluctuations, causing flickering and a change in colour, however this is usually controlled by the Ballast, which is the small box with wires coming out of it in the photo above. It basically controls the fluctuations in the energy flow to produce a cleaner light.The Ballast also acts as the igniter for the HMI, allowing a larger than normal burst of power to start up the light before resuming mains voltage and current.
'In order to power a HMI bulb, special ballasts act as an ignitor to initially start the arc, and then regulate it by acting as a choke. Two types of ballasts exist - magnetic and electronic (square-wave or flicker-free). Magnetic ballasts are generally much heavier and bulkier than electronic ballasts, yet can usually be obtained at lower cost. Standard magnetic ballasts exhibit the previously mentioned problems of flicker on film or video unless the camera being used is crystal-controlled, the camera is run at a specific frame rate that evenly divides into 120, and the line voltage is crystal-controlled at 60 Hz. If all three of these requirements are not met, a noticeable pulsing will be seen on the final image. Magnetic ballasts, however, are very simple devices compared to electronic ballasts. Essentially, a magnetic ballast is a large, heavy transformer coil that uses a very simple principle to generate the high startup voltages needed to create an arc in a cold lamp. Input power is routed to a choke coil connected between the main input and the lamp. The coil may be tapped in several places to provide for various input voltages (120 V or 240 V) and a high start-up voltage. Capacitors are also included to compensate for the inductance of the coil and improve the power factor. Because of the high amount of current running through the ballast, a low humming sound is often heard due to magnetostriction of the ballast iron laminations. Some magnetic ballasts have internal insulation around the coil allowing silent operation.'
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