Journal of the Society of Motion Picture and Television Engineers (1950-1954)

pulse is amplified and applied to the 2D21 thyratron. This tube can be regarded as a one-kick blocking oscillator. Its plate circuit contains an autotransformer that produces an output pulse 3 /xsec wide and 4000 v in amplitude. This pulse of high voltage is used to ionize Jthe gas in the flashtube. The power supply unit, driven by a standard 117-v a-c line, is composed of selenium rectifiers in a full-wave circuit, delivering 150 v d-c at approximately 2 amp. This d-c output is inadequate for the requirements of the flashtube if used as d-c, but since it is wanted for only 830 //sec it can be readily peaked into an adequate supply. This is accomplished by LI and Cl of Fig. 1. These two components constitute a resonant circuit of a frequency of 30 cycles/sec. It discharges its energy through the flashtube at a peak current jof 70 amp and a peak voltage of 500 a-c, Iplus the power supply's 150 v d-c. The thyratron, 5545, is fired simultaneously with the flashtube; and the discharge of Cl completes its circuit through the 5545. LI, however, applies a negative bias to the plate of the 5545 after 830 jusec. The 5545 is thus cut off, and the cycle of events associated with one flash of the flashtube is thereby completed. The protective circuit of Fig. 1 , consisting essentially in the 12AT7 tube, operates through a plate circuit relay to open the line to the selenium rectifier. The tube is normally biassed to cut off; however, if the grid voltage of the 5545 thyratron drifts positive from its nominal value of —17, the 12AT7 conducts and its plate circuit relay is energized. The filter capacitors of the selenium rectifier circuit provide the 60-cycle a-c used for driving the intermittent motor. The power available is ample for that purpose; and, since the phase of this voltage is dependent on the firing of the flashtube, motor and light are always locked together. In effect, the motor is controlled by a remotely located syn chronizing generator. (When a still is projected, an inductance-resistance network is substituted for the motor, providing the same load on the filter as if the motor were running and thus keeping the flashtube current unchanged.) The second harmonic distortion in the filter-capacitor waveform is not of significant amplitude in this operation. Capacity coupling is used between motor and filter. Results Obtained The spectrum and the low duty cycle of the flashtube make its output a cold light. A spectral analysis of the output of the FT231 flashtube shows that it has its main peak at about 4600 A. A second peak, which loses its pulsed character, rises in the infrared region, mainly because the electrodes operate at an incandescent temperature. To prevent this infrared light from reaching the iconoscope, a blue-green filter is used. Curiously, despite the fact that the filter introduces a loss of approximately 20% of the total light energy-, there is normally an increase in the signal output from the iconoscope when the filter is introduced. The same result follows if the filter is used with an incandescent lamp. A number of theories have been advanced to explain this phenomenon, but at the moment we do not know of any explanation that is entirely satisfactory. Corning 9780 and 9788 are the two filters in current use. Comparison of the light output from this projector using the FT231 flashtube with that delivered by a 1000-w incandescent operating through a shutter is difficult to draw because of lack of satisfactory standards. The Joint SMPTERTMA Film Equipment Committee, TR4.8, is working toward a solution. They are at present considering, but have not as yet determined upon, a filtered light meter calibrated in Iconoscope Exposure Units. That is, just as foot-candles are measured by a detector having a spectral sensitivity similar to Putman and Lederer: Synchro-Lite Projector 387