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1953-54 Theatre Catalog, 11th Edition, Page 99 (65)

1953-54 Theatre Catalog, 11th Edition
1953-54 Theatre Catalog
1953-54 Theatre Catalog, 11th Edition, Page 99
Page 99

1953-54 Theatre Catalog, 11th Edition, Page 99

on a motor driven control a continuous brightness and color change would be easy to obtain.

In the foyer, a lower brightness, more subtle color change would be an effective decorative feature. Such techniques have been accomplished in the past with filament lamps. This was relatively inefficient because it meant absorbing with filters some of the colors emitted by the lamp to leave the desired color. Fluorescent lamps, on the other hand, convert invisible energy into colored light directly at two to 26 times the efficiency.

The attraction panels on the marquee could also be lighted with a continuous color change for high attention value, plus excellent legibility.

Far-thinking theatre people are looking at the possibilities of keying the whole atmosphere of the theatre to the picture being played. Lighting will be a major consideration in this, as well as in our developments 'in theatre design already being put into practice. Dimming of fluorescent lamps is an important contribution to the art of lighting for box office appeal.

The dimming system is relatively simple. It requires a 236 volt supply to the control unit; from this unit, three wires are connected to the entire load of lamps and special single-lamp ballasts. The brightness selector consists of a small manually operated pottentiometer which can be located at the control unit, the master control panel or at any other convenient location. Several types of brightness controls can be designed to ht various applications. Automatic control can be accomplished with a selsyndriven potentiometer to provide control in either direction, or a push-button unit with individual fixed resistors can be arranged to provide any pre-selected brightness. Since the electronic control is independent of lamp load any number of lamps up to the capacity of the unit can be operated. This suggests the possibility of mixed color circuits which can be switched on or off at random without effecting the control or the dimming range.

The new dimming system does not vary lamp voltage; it controls the time during which voltage is applied. Changes in light output result from changes in the length of time that current flows through the lamp during each cycle. Sufhcient voltage is present over the entire dimming range lo initiate and maintain the arc. Fast starting takes place at any point within the dimming range.

The ballast currently used with the dimming control is designed to operate either switch start or "Rapid Start" 40 Watt lamps. Due to the advantages of the HRapid Start'l lamp and ballast, it is anticipated that the ballast will be designed exclusively for fiRapid Start'l when this lamp is available in all colors.


Methods for dimming filament lamps appeared soon after the lamp was introduced because a change in the applied voltage was the only fundamental requirement. Early methods included the Water rheostat, iron wire, rhcostait and the step control using tapped resistors. Improvements in the smoothneSS and


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efficiency of the control were gradually made with the introduction of variable autotransformers and thyratron tubes. The amount of light emitted is directly related to the temperature of the filament so with voltage control the range of brightness can be considered infinite.

Fluorescent Lamps

Due to the complex characteristics of electric discharge lamps, and associated auxiliary equipment, a relatively narrow range of brightness change is practical with line voltage control. Since the fiuorescent lamp has a negative resistance characteristic, it requires a ballast in series to control the current. For stable operation the voltage drop across the conventional reactor ballast should be in the order of the lamp voltage. When line voltage control is used to dim the fiuorescent lamp the voltage drop across the ballast dee creases with the current. For this reason only a limited decrease can be accomplished before the circuit becomes unstable and the lamp arc extinguishes. This generally occurs at a point where lamp brightness is still 50 percent of normal. Even with this limited range of two to one it is doubtful whether the lamp will start at the low brightness point because starting voltage is reduced in direct ratio.

In applications where it is desirable to have some control of lamp brightness it is possible to operate one lamp on two ballasts of diderent current rating. For example, a slimline lamp could be operated at 120, 200 and 320 milliamperes by switching between single lamp 120 and 200 milliamp ballasts or by operating the tw0 ballasts in parallel. Since fluorescent lamps become more efficient at low current levels, brightness changes are not in proportion to lamp current so this system produces steps of 100 percent, 75 percent and 50 percent lamp lumens.

One well known control method which has been used to some extent in Europe involves the use of saturable reactors for control of ballast impedance. This system involves a reactor in series with each lamp. The reactors control the a. c. power to the lamps but have d. c. windings connected in series or multiple across a special direct current power supply. The lamp starting voltage is held constant while lamp current is varied by changing the impedance of each reactor ballast through the manipulation of d. c. voltage.

Fig. 1. Operating voltage applied to the lamp circuit for the 90 degree conduction point.

If lamp cathodes are kept hot by means of separate preheat transformers, it is possible to get a wide range of lamp brightness (100 to one or more). This system has the disadvantage of rather complex, expensive wiring and a comparatively large and heavy ballast.

Cold cathode fluorescent lamps can be successfully dimmed by operating a number of lamps in series on a high voltage, high reactance transformer. Dimming is accomplished by varying the voltage to the primary of the transformer. This arrangement does not produce as wide a range of brightness as can be produced with hot cathode lamps, because of the relatively low maximum cold cathode lamp current. The high voltage required to start lamps in series makes it necessary to provide special lampholders to insure safety during maintenance. It also has the disadvantage of poor starting at very low levels of current since voltage available to start the lamps is inadequate.

Circuit and Lamp Requirements

When current supplied to hot cathode lamps is reduced, 3 point is reached where arc current is inadequate to keep the electrodes at a satisfactory electron emitting temperature. The emission for very low current is then supplemented by ion bombardment. This action strips the cathodes of active material and short lamp life results. In view of this, lamp electrodes must be heated continuously at a temperature which will produce sufficient electron emission at any point in the dimming range.

In order to provide smooth operation the circuit should produce sufficient voltage to start the lamps quickly at minimum as well as maximum brightness. Sufficient voltage must also be available to reignite the arc each half cycle to avoid flickering.

Theory of Operation

Control of the lamp brightness is accomplished by adjusting the interval of current conduction during each half cycle of the 60 cycle voltage. Two thyratrons are connected back to back so that the pair of tubes will conduct current for both polarities of the a. c. voltage. These tubes act as a synchronous switch which closes the current at a predetermined point in the a. c. voltage each half cycle and opens at the end of each half cycle. In this way voltage is applied to the lamp for any desired fraction of the cycle. Figure 1 shows the voltage wave applied to the entire lamp load when the interval of conduction is set for 900 or half of each cycle.

The abrupt rise in voltage at the, leading 0ng of applied voltage is used to insure resignition of the fluorescent lamp on each cycle. The ballast for each lamp contains a choke which is in series with the lamp to limit the lamp current. Across the lamp is a small capacitor which suppresses radio interference and resonates the sharp voltage rise at the leading edge of the applied voltage to a higher peak value to start and rc-ignite the, lamp on each cycle. The oscillogram in Figure 2 illustrates this voltage condition.

The point in the cycle at which the thyratron tube begins conducting is con
1953-54 Theatre Catalog, 11th Edition, Page 99