International projectionist (Jan-Dec 1939)

ter openings will be identical and an equal amount of light will always be A projected upon the screen. Fig. 8 shows the pulsation intensity to be zero B with a consequent pulsation level of zero. At 96 cycles the exact point of co-ordination of light pulsation values has been reached. At this point phase relation is of no consequence and absolute synchronism between the arc and shutter frequencies is wholly unnecessary. The arc frequency may vary either way from this point, within reasonable limits, without detrimental results; likewise the shutter frequency may vary to a certain extent, as can be deduced from the curve shown in Fig. 10. Here the comparative flatness at the point of contact with the zero line indicates such a low value of pulsation frequency and intensity in the immediate vicinity of the ideal fre CURRENT FREQ. 120 (I 2.5) iS 2 I) SHUTTER INT. FREQ.46 I I) (I I 2) (I 2 .5) (5 3.5 3.5 80 60 40 20 0 .25 .50 .75 I SHUTTER REVOLUTIONS 1.25 1.50 1.75 D PULSATION FIGURE 8 to be the values of pulsation frequency and pulsation intensity over a range of supply current frequencies of from 48 to 208 cycles, when the light is inter CURRENT FREQ. 96 0 -5) (• .5) (I SHUTTER INT.FREa.48 .5) (l .5) (I .5) (| 0— 80 60 40 20 0 .25 .50 ,75 I 1.25 SHUTTER REVOLUTIONS •kI X D PULSATION INT. O quency that deviation from that point is entirely practical, within limits. Figure 9 shows an arc frequency of 120 cycles superimposed on the 48-cycle shutter chart. At 120 cycles the ratio of shutter opening to half-cycle time is 96 to 240, or 2 to 5. The successive light values for each shutter opening are 4, 3.5, 4, and 3.5, showing an increase in intensity of 14.28% and a pulsation frequency of 24 per second, which, of course, is unsatisfactory for motion picture projection. Figure 10 shows what I have found loo PULSATION 1.50 FREGl. O FIGURE 9 1.75 $ 75 * 8 5: 25 \ . -/ *ul sat) on J»l 9/li. /ty VI s , — -fi >a/i at/ ">r> fee qui nc\ f 1 / / s i \ t i \ \ / / \ / / \ i \ / \ / i \ x i N \ / / V 1 \ i \ / 1 \ / \ i \ / / i \ i \ / / 1 \ > \ 1 V / 1 1 \ / \ '" \ / 1 ^ / \ , \ / 1 \ 1 jS* \ ■ — i / tf — cepted 48 times per second by a shutter having diametrically opposed blades and two light openings of substantially 90 degrees each. The pulsation intensity as illustrated is the result obtained when a value of 1 is given to the alternation acting upon the carbon facing the reflector, and .5 to the alternation acting upon the carbon facing away from the reflector. The vertical lines represent the frequency of FIGURE 10 24 X 20 ^ Ul "ft 48 72 96 120 144 |68 192 .Arc Su/ojoly Current Freouer?cy /» Cycles /oerSec. INT. 14.23 PULSATION FREO 24 the arc supply current; while the horizontal lines, numbered at the right, represent the light pulsation rate in terms of pulsations per second. The horizontal lines, numbered at the left, represent the pulsation intensity in terms of the percent of increase of the maximum intensity over the minimum intensity during one pulsation. The curve shown in dotted lines represents the pulsation frequency; and that shown as a solid line, the pulsation intensity. These curves were plotted from numerous graphs of various frequencies throughout the range shown, and were verified by actual experiments. These curves show that from 48 to 72 cycles the pulsation rate increases in direct proportion to the increase in current frequency; that from 72 to 96 cycles the pulsation rate decreases in proportion to the current frequency increase; that from 96 to 120 cycles the pulsation rate again increases in direct proportion to the increase in current frequency; and that thereafter the same cycle of increase and decrease in the pulsation rate will occur as the current frequency is increased by a number equal to the number of shutter openings per second. The intensity of the pulsations, however, continues to decrease from a maximum at 48 cycles to a minimum at 96 cycles as the frequency is increased. At 96 cycles, as shown, the pulsation intensity as well as the pulsation frequency is zero. Above 96 cycles the pulsation intensity again increases and again reaches a maximum at 144 cycles, whereafter another decrease occurs, reaching zero at 192 cycles, or the fourth harmonic, of the shutter frequency. It will be noted that at the third harmonic, 144 cycles, although the pulsation frequency is shown as zero, there is a pulsation intensity of 25%. This means that, if exact synchronism between the shutter speed and the current frequency is maintained an exact (Turn to Col. 1, next page) OCTOBER 193 9 13