Contains research objectives and reports on one research project

Edgerton, Harold E.

Logan, B. F., Jr.

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IX. STROBOSCOPIC RESEARCHProf. H. E. EdgertonB. F. Logan, Jr.RESEARCH OBJECTIVESTo gain an insight into the production and utilization of electronically generatedpulses of light is the goal of our research effort. Part of the work concerns studies ofthe physical mechanisms by which light can be obtained, including spectral distribution,flash duration, peak light, delay time, efficiency, and so on. In addition, the design offlash equipment for specific scientific measurement assignments is carried through.A. CHARACTERISTICS OF THE FT-17A FLASHTUBEThe experimentally determined electrical and light-producing characteristics of alarge quartz xenon-filled flashtube (FT-17A) are reported here. This tube was origi-nally developed at M.I.T. with technical help from the General Electric Company, LampDivision, as a source for airborne night photography (1,2) in 1942.Similar tubes (General Electric Company types FT-623 and FT-617) are now in useas sources for color photography in studios. The FT-623 has a sealed-in glass cover toreduce noise; the FT-617 is open so that air can be forced through the tube for cooling.The flashtube described in this section (FT-17A) differs from the FT-623 andthe FT-617 flashtubes by an additional turn of quartz tubing. Therefore, the resultsreported in this paper will differ somewhat from those from commercially availabletubes.Figure IX-1 shows the FT-17A flashtubes. The active spiral consists of about 40inches of 5/8-inch (outside diameter) quartz tubing formed into a 2. 5-inch (outsidediameter) 6-turn spiral and filled with 20 cm of xenon gas. Massive iron electrodes withsintered tungsten-barium-nickel slugs serve as electrodes. An outer glass tube is usedas a cover and support for the quartz spiral. Attached to the end of the tube is a bakelitemember that holds contact pins for terminals and connectors. Two of the five connectorsform a safety circuit to disconnect the power in case the tube is withdrawn from thesocket. The other three terminals are the anode, cathode, and external starting elec-trode. Either terminal of the tube can be used as cathode, since the electrodes areidentical.The FT-17A tube was used in a special model of the Airforce type D3 night photog-raphy equipment with 4800 mfd at 4000 volts (38, 400 watt sec). Actually two suchdevices were used in a B-24 airplane, thus totaling a stored energy of 76, 800 watt sec.The FT-17 tube (5 turns instead of 6) was used in other flash units as follows:Navy type N-2 3800 volts 300 mfd 2150 watt secAirforce type D-2 4000 volts 600 or 1200 mfd 4800 or 9600 watt secAirforce type D-3 4000 volts 7200 mfd 57, 600 watt sec(standard type)-34-Fig. IX-1Photograph of the FT-17A flashtube.RVR6 E VOLTS-NV%-TK--PTRIPVOLTAGE TOCURRENT TO OSCILLOSCOPEOSCILLOSCOPEFig. IX-2Experimental circuits used for measuring thecharacteristics of the FT-17A flashtube.-35-TIME - j sec.TIME u secTIME - j sec.Fig. IX-3Data from oscillograms showing light, voltage,and current as a function of time.-36-w04I--JEzCLwaw(IX. STROBOSCOPIC RESEARCH)1. Light-Time CharacteristicsThe light output of the flashtube was measured as a function of time, with the tubeoperated at a constant initial voltage (4kv) and with the discharge capacitance as thevariable parameter. The light output was measured normal to the axis of the tube andthe results recorded (Fig. IX-3) in horizontal candlepower (hcp).A 929 phototube (blue-sensitive S-4 surface) was used in conjunction with a cathode-follower amplifier to deflect the beam of a cathode-ray oscillograph Dumont A/R RangeScope proportional to light intensity with a single-shot sweep tripped by a voltage fromthe primary of the flashtube ignition coil. A camera recorded the trace, thus measuringthe light output as a function of time.The phototube and oscillograph were calibrated directly by the use of a flashtube,FT-214, whose output was known under predescribed operating voltage and capacitanceconditions. The normal FT-214 has a peak hcp output of about 2.6 million candlepower,when flashed from 100 mfd at 2000 volts. Differences in deflection calibration for thestandard lamp and the large quartz tube were effected by varying the distance from thelamp to the phototube and by the use of filters. The total light output was calculatedby multiplying the hcp by 10, a factor due to geometry that differs slightly from thefactor 47r, which applies for a symmetrical source.26C1200 288 576 p-SECI I I I I I I IINITII 2 AL 4 5 6 7INITIAL VOLTAGE (KILOVOLTS)Fig. IX-5Light (top) current (bottom) as a function oftime with 0. 54-mh series inductance. FT-17A,4000 volts, 300 mfd.8 9Fig. IX-4Starting-time delay of theFT-17A flashtube.-37-Ii--(IX. STROBOSCOPIC RESEARCH)The circuit connections used for measuring current and voltage and light in the flash-tube are shown in Fig. IX-2. Figure IX-3 shows the information that was measured fromthe oscillograms of voltage, current, and light as a function of time for the FT-17A.It was observed that the instantaneous output as a function of time during the firstpart of the flash showed a time delay in starting. Experimentally, it was found that thisdelay was less when a high voltage was used. A plot of the delay time from the start ofthe sweep is shown in Fig. IX-4. It should be remembered that there is an inherentdelay in the spark coil, which is also included in the delay time.The time delay of the starting of the light may be attributable to the time requiredfor the gas to become ionized or heated. The results of Fig. IX-4 indicate that theproduct of initial voltage squared by the delay time, after subtracting the spark delay,is about constant. Apparently the flashtube does not become an efficient producer oflight until a given minimum amount of energy has been discharged into the gas.The effect of a series inductance in the discharge circuit was next studied. A 0. 54-mh inductance was first used, and its effect on the shape of current and light-vs. -timeoscillograms is shown in Fig. IX-5. The inductive effect delays and smooths out thepeak, giving a longer "effective-light" duration. A much smaller inductance (approxi-mately 5 1ih) was then used, and it was noted that it smoothed out the small ripple in thecurrent and light-output rises.The peak light output as a function of voltage was measured with the dischargecapacitance as the parameter. The output of the phototube circuit was fed through acalibrated voltage divider (in order to obtain a multiplier) to the oscilloscope, and thepeak was measured with a scale on the scope face. This scale was calibrated with thestandard flashtube. The results plotted on log-log paper are straight lines and show thatthe peak light varies as the cube of the voltage (up to the working limit of the tube). SeeFig. IX-6.Next, the voltage was used as the constant parameter, and the peak light as a functionof capacitance was determined. The curves (Fig. IX-7) show that the peak light tendsto approach a limit as the capacitance is increased.In the course of taking the data for the peak-light curves, interesting double-peaking phenomena were observed in the range of 2000-3000 volts and 500-1000 [f. Inthis range a second peak appeared in the light-vs. -time oscillograms; and as the voltageor capacitance was increased, the second peak climbed until the original peak becameonly a ripple in the rise of the second peak. This is shown in the oscillograms ofFig. IX-8 and Fig. IX-9.A final curve, Fig. IX-10, gives the over-all efficacy of the FT-17A in lumens/wattas a function of voltage and capacity. This data was measured by J. A. Vitka, Jr., andW. W. Smith with a General Radio Integrating meter type 1501, as a thesis project (1949)in the Electrical Engineering Department, M.I.T.38-U1zzY.Iz.9INITIAL VOLTAGE (KILOVOLTS)Fig. IX-6Peak light as a function of initial voltage for several capacitors.The peak light appears to be a cube function of the voltage.4004005000 V4000 V3000 V2000 V0 500 1000 1500 2000C (p2500 3000Fig. IX-7Peak light as a function of capacitance.-39---~----1I2000 V -------I ---Fig. IX-8Light-time oscillograms (10 kc/sec timing wave) to show ripplesin light output. Capacity 287, 383, 481, 576, 850, 1400, 2510 mfd(peak light, 71 106 lumens). Voltage, 2500.Fig. IX-9Light-time oscillograms (10 kc/sec timing wave) to show ripplesin light output. Capacity 287, 383, 481, 576, 850, and 1400 mfd(peak light, 117 X 106 lumens). Voltage, 3000.-40-(IX. STROBOSCOPIC RESEARCH)8 10Efficacy ofFig. IX-10the FT-17A flashtube.The lumen-second output was calculated asQ = Wh D 2 10 lumen secwhere W h is the horizontal reading of the meter in candlepower seconds at a distanceof D feet from the meter to the lamp. The factor, 10, is one whose exact value dependsupon the geometry of the flashtube.The electrical input was calculated from the relationshipCE2 energy input in watt sec2WhD210n - lumen/wattCE /22. System Guide-Factor CalculationsThe "guide factor" of a photographic system composed of a closely spaced cameraand light can be stated asDf = bcps)where D is the light-subject distance in feet, f is the aperture of the lens, S is theexposure index of the film (ASA Z38.2. 1. 1947), C is a constant varying from 15 to 30,and bcps is the beam-candle-power-second output of the flash-lamp reflector combina-tion. (See ASA Z38. 2.6. 1948.)-41-2 4 6VOLTAGE (KILOVOLTS)(IX. STROBOSCOPIC RESEARCH)As an example, consider the FT-17A excited from C = 4800 mfd at 4 kv in a reflec-tor with a factor of 10 (increases the bcp by 10).2bcps CE 2 n 10 = 1, 500, 0002 10Note that n = 41 lumens/watt from Fig. IX-10, or n/10 = 4. 1 candlepower/watt.Assume S = 100, and C = 15, to give Df = 3170, the guide factor. Suppose an f2.5lens is used, then3170D - - 1260 ft2.5In actual use it was found that a much greater distance could be used for aerialreconnaissance, since a considerably underexposed negative could still give an adequatephotograph with useful intelligence.For example, this condition under favorable weather conditions has produced usablephotographs at 6000 feet and even higher.The back-scattered light from the beam of the flash is a very serious problem inaerial photography, since it creates an over-all fog on the film and thereby reduces thecontrast. Every effort must be made to obtain the maximum camera-to-lamp spacingon the airplane. For example, in the A-26 airplane, the camera was in the nose com-partment, while the lamp was mounted on the tail. Night aerial photography will begreatly improved when the spacing is increased.References1. H. E. Edgerton, Airborne photographic equipment, PSA Journal 13, No. 7, 439-440 (July 1947); Night aerial photography, Technology Review, pp. 273-278 (March1947).2. J. E. Burchard, Q.E.D., M.I.T. in World War II (John Wiley and Sons, Inc., NewYork, 1948), pp. 196-204.-42-

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