International projectionist (Jan-Dec 1939)

CURRENT FRECL.72 SHUTTER INT FRECL48 (I l) (I 0 (2 .5) (.5 2) (I I) U w 1M1 0 80 60 0— 40 20 0 .15 .50 .75 I SHUTTER REVOLUTIONS 1.25 1.50 1.75 D-PULSATION INT. 25 value of 24.5 is shown in the first l/96th second, and a minimum value of 13.5 in the second l/96th second; therefore a shift of 90 degrees would result in a shift from maximum to a minimum light value, a difference of 81% exists in the relative light values. Thus, in the case of 50 cycles, while the increase and decrease of intensity is gradual, the rapidity of the pulsation cycles results in a definite beat upon the screen. Nothing can be done with either of the two commercial frequencies mentioned. There are no characteristics which lend themselves to synchronism in any way; in fact, of all the frequencies above 48 cycles they present the most difficulty as far as motion picture projection is concerned. Beginning with 60 cycles, arc current frequencies in steps of 12 have been selected for analysis in order to show the frequency and intensity of the resultant pulsations on the screen. Figure 6 shows a current frequency of 72 cycles superimposed on the same shutter frequency chart as was used in illustrating 50 and 60 cycles. The ratio of shutter opening time to half-cycle time is in the ratio of 96 to 144, or 2 to 3. The successive light values for each shutter opening in the order of 2.5, 2, 2.5, and 2, showing that the increase in intensity is from minimum to maximum once during each shutter revolution, thereby producing a pulsation of 24 cycles per second. Even though the phase be shifted 90 degrees, the same values will prevail. Fig. 6 shows the pulsation intensity to be 25% and the pulsation levels to be equal during alternate 48ths of a second. Successive 48ths of a second, however, indicate a difference of 25%. The graph shows that while the rapidity of the pulsations have doubled over those occurring at 60 cycles, the intensity of the pulsations is only one-half. As illustrated, 50-cycle current having a frequency differential of 2 beyond the primary shutter frequency of 48, with a resultant pulsation rate of 2; PULSATION FREflU 24 FIGURE 6 and 60 cycles having a differential of 12 and a resultant pulsation frequency of 12; and 72 cycles having a differential of 24 and a resultant pulsation frequency of 24, it might easily be assumed that as progression is made in the scale of current frequencies the same results would ensue, that is, that a pulsation frequency equal to the differential between the shutter frequency and the arc TABLE A Arc Pulsation Pulsation Frequency Frequency Intensity 50 cycles 2 81% 60 cycles 12 50% 72 cycles 24 25% 84 cycles 12 10% supply frequency, would continue indefinitely. If this were true it is obvious that at some point, perhaps in the neighborhood of 82 cycles, there would be a pulsation frequency so rapid as to be imperceptible. This is not the case, however. At an arc current frequency of 72 cycles the maximum pulsation frequency (24 cycles) is reached with a shutter frequency of 48. From a frequency of 72 cycles upward there is a FIGURE 7 gradual decrease in the resultant pulsation frequency, as is indicated by the analysis of results at 84 cycles. Figure 7 shows the results of superimposing 84-cycle current on the standard 48-cycle shutter graph. During the entire first revolution of the shutter the average light remains at 5.5; but during the entire second revolution the average light value drops to 5; after which the cycle is repeated, producing on the screen a 12-cycle pulsation, the intensity of which is only 10%. Thus, instead of the pulsation frequency increasing to 36, which is the differential between 84 and 48, the pulsation rate has reverted to 12, or the same rate as was obtained at a frequency of 60 cycles. The intensity of the pulsation instead of being 50%, as it was at 60 cycles, has decreased to 10%. Table A shows the results obtained thus far. It is evident that as the current frequency is increased, the resultant pulsation rate increases to a maximum of 24 per second at 72 cycles, and then decreases. However, the intensity of the pulsations has continually decreased from the 50-cycle arc supply current. Evidently the assumption that it might be possible to reach a point of imperceptibility through high pulsation frequency must be abandoned. However, there is a point where both the pulsation rate and the pulsation intensity is zero, and complete co-ordination exists between shutter interception frequency and the arc current frequency. Figure 8 shows an arc frequency of 96 cycles superimposed on the 48-cycle shutter chart. It will be noted that during each shutter opening as well as during each shutter interception a full cycle of hght occurs. Not only is the average light value the same but the intercepted light values are also equal to those passed through the shutter openings. This means that no matter what phase differential exists between the arc current and shutter openings, the light values during successive shut 1.5) (.5 4 CURRENT FREft.84 I) (/ 4 .5) {1.5 4) SHUTTER INT. FREa. 48 (2 3) (I 2 2) (2 2 I) (3 2) 60 60 .40 20 O .25 .5 0 .75 I 1.25 SHUTTER REVOLUTIONS 1.50 ■ 75 D -PULSATION INT. 10 PULSATION FRE3, 12 INTERNATIONAL PROJECTIONIST