British Kinematography (1950)

94 BRITISH KINEMATOGRAPHY Vol. 16, No. 3 accidentally at 5 kW. The grid is shaped in such a way that it can conveniently be mounted on the stand when the unit is used as a front light on the floor. The lamphouse weighs 100 lbs. and the grid 80 lbs. Spotlight Unit The optical system in the spotlight (Fig. 1) consists of the standard 20-in. Fresnel lens combined with a spherical metal mirror. The size of the lamphouse has been reduced considerably and a conical shape is adopted, which increases the gangway space on the spot rail. The spotlight resembles the floodlight in general appearance. In both units the lamp housing consists of an outer casing constructed from aluminium castings and sheet metal, and an inner removable chassis carrying the light source, optical system, high voltage impulse unit and other components. The lamphead weighs 173 lbs. as compared with 196 lbs. for the Type 170, while the overall dimensions are the same. A padlock on the lens ring prevents unauthorised access with consequent risk of injury to personnel by exposure to ultra-violet radiation, electric shock from the high voltage circuit, or explosion of the bulb which might be caused by sudden cold draughts. The bulb is mounted in spring-loaded holders. A red pilot light on each side of the top of the lamphouse is extinguished when the lamp is fully run up and is ready for use. The lens ring carries a shutter, and shutter control motor, on which are mounted barn doors. Light Control The shutter motor may be operated in synchronism with others on the set, or may be operated from the remote control box. Focus may be controlled either by a built-in motor or by a handle on the back of the spotlight. A dial reads " Percentage Spot." It is essential to prtvent the lamp from being struck on reversed polarity. A red disc bearing the legend " Reverse Input Leads " is illuminated if the polarity is incorrect. In addition, a rectifier renders the striking circuit inoperative, so that it is impossible to ignite the arc until the leads have been reversed. The hand controller for the spotlight is a small metal box of convenient shape for holding in the hand, or for mounting on a lampstand. It contains the following controls : — (1) On-Off switch. (2) " Full or Dim " switch to reduce the power in a 10 kW. lamp to 6 kW. (3) A spot and flood control switch with a central "off" position, to control the focusing motor. (4) A shutter motor controller. In the case of the floodlight, the remote controller contains only a single " On and Off" switch ; no provision is made for operating the 5 kW. lamp at reduced power. This remote controller, and that used on the Type 460 Spotlight, are interchangeable. When the lamp is switched on from cold, the pilot lights will extinguish after 3 to 4 minutes, showing that the lamp is giving its full light output. If the lamp is switched off after it has been running it may be operated again immediately without any delay. If it has been off for some time it will have cooled down so that time will be required for it to warm up again, depending on how long it has been switched off. It will cool down completely in 20 minutes and would then take 4 minutes to warm up again. If it has been off for only 5 minutes full light output will be obtained in 1 minute. A fall in supply voltage of 1 per cent, affects the light output from the compact source lamp by approximately 3 per cent., an amount similar to that of a carbon arc, but less than that of an incandescent lamp. Reduction of supply voltage has no appreciable effect on the stability of the discharge lamp.