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Mechanism has been developed which permits the focusing of lamps entirely independent of the optical system of the machine. It has been demonstrated by actual test that more uniform results can be obtained in this way than can be obtained by focusing each individual lamp in the system by judging the appearance of the screen by the eye. There are certain modifications of this equipment which include a part or all of the features which have been outlined.
ELECTRICAL CONTROL EQUIPMENT
The lamps for motion picture projection are operated at extremely high filament temperatures. It is necessary, therefore, that the current supplied to the lamp be closely regulated as, otherwise, satisfactory lamp performance cannot be secured. If the current is reduced below that at which the lamp is rated the intensity of the light is diminished while if the current is increased above the rated value there is danger that the lamp may fail very shortly.
In order to keep the current at the desired value it is essential that an ammeter be included in the lamp circuit. Where the lamp can be supplied from an alternating current circuit it is possible to use a transformer equipped with an ammeter and a regulating device. The regulating device may consist of a switch which connects different taps of the secondary to the lamp in order to obtain the current control, or may consist of an adjustable reactance. The reactance method of controlling the current has the advantage of permitting very accurate adjustment of the current and voltage. This is of primary importance and the transformer is not a serious disadvantage. In fact under the worst conditions the power factor of the control equipment for incandescent lamps will be higher than the power factor of arc lamps with reactance coils which are often used. The power required by this lamp is less than the power required by the arc, so that existing circuits in the theatres will be ample to supply the new lamp and the voltage drop will be less than when the arc is used.
It is important that the rush of current through the lamp at starting be kept fairly low in order to protect the ammeter and reduce the danger of cracking the lamp seal at the point where the leading-in wires pass through the glass. The resistance of the lamp filament is very low when the filament is cool and as the filament is large there is an appreciable time before it heats up so as to limit the current. If no method is used to limit the starting current the initial rush is momentarily many times the normal lamp current.
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