Cinematography-Theory-And-Practice

The advantages are: between takeThe disadvantages are:The first dimmers invented were resistance units. They operate byplacing a variable resistance in the power line. The excess energy isburned off as heat. The major problem with resistance dimmers isthat they must be loaded to at least 80% of their capacity in orderto operate. If you put a 5K on a 10K dimmer, for example, thedimmer will only reduce the output a small amount; you won’t beable to dim all the way down. This sometimes necessitates the use ofa dummy load to operate.The next generation of dimmers were autotransformers. They operateas variable transformers and alter the voltage. They do not haveto be loaded to capacity, and they don’t generate excessive heat, butthey do operate only on AC. Known as variacs (for variable AC), theyare used, particularly in the 1K and 2K sizes, although they are bulkyand heavy.For larger applications, silicon-controlled rectifiers are used.Known as SCRs, these are electronic devices that are small in size,quiet, and relatively low cost. They can be controlled remotely bydimmer boards, which means that the dimmer packs themselves canbe near the lights, which saves cabling; only a small control cableneeds to run from the board to the dimmer packs. The remote cablecarries a control voltage of 0 to 10 volts and can be run up to 200feet. The signal is multiplex and can run up to 192 dimmers.SCR dimmers do not alter the voltage; they work by clipping thewaveform so that the voltage is heating the filament for a shortertime. The result of this is that the output cannot be read with anordinary voltage meter, only a VOM (volt-ohm meter), which readsroot mean square voltage (RMS) such as the will work.Figures 16.6 and 16.7. An exampleof greenscreen used to fill in awindow on a set. On the left, noticethe tracking marks on the greenscreen,which are essential if there isany camera movement at all, includinga tilt up or down. On the right isthe final result after the backgroundplate has been composited in.technical issues315