High Speed Photography of the Cathode—Ray TubeH. Goldstein and Paul D. Bales Citation: Review of Scientific Instruments 17, 89 (1946); doi: 10.1063/1.1770447 View online: http://dx.doi.org/10.1063/1.1770447 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/17/3?ver=pdfcov Published by the AIP Publishing Articles you may be interested in The finebeam cathoderay tube Phys. Teach. 22, 80 (1984); 10.1119/1.2341472 Exposure Meter for CathodeRay Oscilloscope Photography Rev. Sci. Instrum. 32, 1143 (1961); 10.1063/1.1717184 Positive and Negative Ions in CathodeRay Tubes J. Appl. Phys. 24, 427 (1953); 10.1063/1.1721297 Illumination of CathodeRay Oscillograph Screen for Photography Rev. Sci. Instrum. 19, 271 (1948); 10.1063/1.1741244 Note on the Photography of CathodeRay Oscillograms Rev. Sci. Instrum. 18, 925 (1947); 10.1063/1.1740884

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THE REVIEW OF SCIENTIFIC

INSTRUMENTS VOLUME 17, NUMBER 3 MARCH, 1946

High Speed Photography of the Cathode-Ray Tube*

H. GOLDSTEIN** AND PAUL D. BALES

Radiation Laboratory, Massachusetts Institute of Technology, Cambridge, J.,fassachusetts

(Received January 17, 1946)

Some techniques are described that have been developed for the periodic recording of single, fast traces on a cathode-ray tube at rates up to 4000 per second. The factors affecting the maximum writing speed are discussed, and it is shown that speeds as high as 70 cm/j.lSec. can be obtained without sacrifice of deflection sensitivity and using commercially available tubes and films. Several 16-mm cameras, adaptations of existing models, are described which permit photography of as many as 4000 traces per second. The film in these cameras moves continu­ously and at high speeds and cannot normally be projected as a moving picture. Where such projection is desired, a camera is employed which provides trigger pulses to initiate the tran­sients synchronously with the film speed. Finally, a technique is presented for placing consecu­tive identification numbers in each frame area.

A RESEARCH project of the Wave Propa­gation Group, Radiation Laboratory, Mas­

sachusetts Institute of Technology, reported elsewhere,! required the recording of single high speed transients that are repeated at rates up to 4000 per second. The techniques developed for this purpose are described here in the hope they may be of use to workers in the same or related fields. 2

There are two distinct and independent aspects to the problem. First, the proper combination of cathode-ray tube, optical system, and film, capable of recording the high writing speed (about 25 cm/JLsec.), must be selected. Secondly, means must be devised for taking as many as

* This paper is based on work done for the Office of Scientific Research and Development under contract OEMsr-262 with the Radiation Laboratory, Massachu­setts Institute of Technology.

** Now at Harvard University. 1 To be published shortly. 2 These techniques were developed independently of

similar work in England by T. Gold. As far as is known, he has not published a detailed report on his methods.

4000 such records in one second, entailing the design of a suitable high speed camera. These separate aspects are discussed in detail in the following sections.

I. MAXIMUM WRITING SPEED

Electrical engineers in the field of power dis­tribution have long been concerned with the recording of high speed transients. For such applications the high voltage, continuously evac­uated cathode-ray tube has become the standard instrument. Using accelerating potentials up to 95 kv and with the photographic plate directly exposed to the electron beam, writing speeds of 500 cm/ JLsec. are common3 and 6000 cm/ JLsec. has been attained.4 The necessary high deflection potentials, of the order of kilovolts, and the general inconvenience of a vacuum system rule

89

3 J. T. MacGregor-Morris and J. A. Henley, Cathode­Ray Oscillography (London, 1936).

• W. Rogowski, Arch. f. Elektrotech. 24, 563 (1930).

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90 H. GOLDSTEIN AND P. D. BALES

out the use of such tubes in most fields where the signal voltages are small.

At the other extreme the maximum recording speed with the customary low voltage tube (2000 volts) and ordinary film (Super X) is around 2 cm/ J.!sec. 5 A step toward narrowing this gap was made in 1937 when RCA introduced the 912, a sealed-off tube running at accelerating voltages up to 15 kv, and with recorded writing speeds as high as 2S cm/ J.!sec. 6 However, the deflection sensitivity (one inch per 500 volts) was still much too small for many applications. In recent years the writing speed attainable at convenient signal voltages has been greatly increased by the intro­duction of post-deflection acceleration, more effi­cient phosphors, and more sensitive films.7 These advances have culminated in the 5RP "Multiband" tube developed by Dumont, in which the third anode potential is distributed along the length of the tube over as many as five electrodes. High over-all accelerating potentials, up to 25 kv, are thus possible without greatly affecting the deflection sensitivity. Thus, the deflection sensitivity, in volts per inch, when Eb2 =5 kv and Eb3=25 kv, is only 7S percent greater than when Eb2 = Eb3 = 5 kv. Writing speeds of 400 cm/ J.!sec. and higher have been successfully photographed. However, these tubes were experimental and only tubes commercially available at the time were used for the work described here.

1. Cathode-Ray Tube

Since the interval between successive transients was 3 milliseconds or less, a short persistence phosphor, such as the blue P5 screen, was neces­sary. The P5 phosphor is nominally made of calcium tungstate, but for some time tubes with this designation have often been manufactured with silver activated zinc sulfide, the blue com­ponent of the P7 phosphor. This screen has recently been given a separate RMA designation as P11. The spectrum of the P11 phosphor is similar to that of calcium tungstate but its photographic efficiency is about ten times greater, and the decay time to half energy is 10 micro-

6 R. W. Watson-Watt, The Cathode-Ray Oscillograph in Radio Research (1933).

6 H. P. Kuehni and S. Ramo, Elec. Eng. 56, 721 (1937). 7 R. Feldt, Electronics 17, 130 (1944).

seconds compared to S microseconds for P5. 8 In addition, the P11 is often contaminated with small quantities of the yellow componen't of the P7 screen which has a very long persistence. This is strikingly shown visually by suddenly moving the trace from its normal position on the tube. The weak phosphorescent image left behind will last for many seconds. This long time persistence is apparently too weak to affect the film although the effects of the intrinsic persistence are quite evident. Figure la shows some photographs taken with this screen on 16-mm film that was moving perpendicularly to the trace at a rate of 8 ft. per second. Figure Ib shows similar photographs taken with a different optical set-up and with the film moving about four times as fast. The fuzziness of the lower edge of the traces, as com­pared with the sharp upper edge, is clearly owing to persistence of the image.9 Measurement of the film shows that the intensity of the persistence has decayed to a low value in 20 microseconds, in substantial agreement with data from other sources. 8 The "trails" at the bottom of Fig. 2 are also the results of this short time phosphorence.

The tube brightness is a rapidly increasing function of total accelerating potential. The anode voltages should therefore be as high as possible without sacrifice of deflection sensitivity. As a suitable compromise we have used a 5CPll tube with the cathode at - 2500 volts, and the third anode at +3300 volts relative to the second anode. With these voltages the deflection sensi­tivity is 1.1 inches per 100 volts. Table I shows the effect on the maximum writing speed (ob­tained in a manner described below) when the third anode voltage is reduced by 1.6 kv.

2. The Optical System

The maximum writing speed is also a function of the characteristics of the optical system em­ployed, i.e., the lens aperture and magnification. The writing speed depends on these quantities according to the formula :10

where Vo is the maximum writing speed when the

8 Rev. Sci. Inst. 16, 260 (1945). 9 This fuzziness, which is striking in the original photo­

graph, is largely lost in reproduction. 10 R. Feldt, Electronics 17, 130 (1944).

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H I G H S PEE D PH 0 TOG RAP H Y 0 F CAT HOD E - RAY TUB E 91

aperture is f: 1.0 and the magnification ratio is unity, and where V is the corresponding writing speed obtained when the lens aperture is given by f and the magnification (ratio of image to object size) is M. This relation is a consequence of the statement that the illumination on the film is proportional to the solid angle subtended by the lens at the screen and inversely proportional to the spot area on the film.

It is obviously advantageous to use as wide a lens aperture as possible, since depth of field is usually not important. If one works with 35 mm or smaller movie film, it is not difficult to obtain lenses with apertures between f: 1 and f: 2. We have employed an Eastman Kodakf: 1.6 50-mm lens with very satisfactory results. The small film size has the added feature that the magnification is small with consequent higher maximum writing speeds. This advantage is lost to some extent when the film is enlarged for measurement purposes.

3. Photographic Film

Tests were made on a number of films to de­termine roughly the maximum writing speeds for each emulsion. The films, which included some experimental types, were obtained from Eastman Kodak Company in 16-mm rolls.

In order to make these measurements, it must be possible to vary the beam writing speed continuously and ()ver wide limits. This was achieved in several ways. In the first method a rectangular pulse was applied to the deflecting plates through a video amplifier. The rise time was measured on a much expanded sweep to be

TABLE I. 5CP11. Anode voltages relative to cathode.

2nd anode

2500 2500

3rd anode

5800 4200

Relative maximum writing speed

1.0 0.37

about 0.15 microsecond. The amplitude of this pulse could be varied by means of an input attenuator without affecting the rise time ap­preciably. The horizontal sweep on the tube was initiated periodically by a trigger pulse at a recurrence frequency of 333 c.p.s. The individual sweeps were photographed with a high speed

..,...... "" a-30 cm/I'sec. b-35 cm/I'sec. c--60 cm/I'sec.

FIG. 1. Test objects for determining maximum writing speed; 5CPll, Eb2 2.5 kv, Eb3 5.8 kv, Recording Negative, Pan; 10 microsecond sweep.

camera described below while the pulse ampli­tude was being varied. In this manner a wide

. range of writing speeds was obtained on one record. The leading edge of the pulse was used as the test object. The maximum beam writing speed reached in this fashion was about 35 cm/ ,",sec. To achieve higher speeds a negative sawtooth pulse, obtained by firing a 2080 gas tube, was applied directly to the deflecting plates. The rise time was considerably less than .04 microsecond and could not be accurately meas­ured. This method was in fact calibrated by measurements on a film whose maximum writing speed was in the region also covered by the previous method. The maximum writing speed attainable with the sawtooth pulse was in excess of 150 cm/ ,",sec.

Figure la was taken using the first method on Recording Negative, Panchromatic, and corre­sponds to a writing speed of about 30 cm/ ,",sec. Figure lc was taken on the same type of film but with the faster sawtooth pulse. The slower por-

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92 H. GOLDSTEIN AND P. D. BALES

FIG. 2. 3.5 me/sec. timing wave; 5CPll, Eb2 2.5 kv, Eb3 5.8 kv, Recording Negative, Pan;!: 1.6.

tions of the trace are out of focus because of the presence of astigmatism when the focusing volt­age is set to show the fast rise clearly.

Some measurements were also made with the more usual sinusoidal trace. The output of a crystal controlled oscillator was applied directly to the plates through a parallel resonant circuit. The amplitude was conveniently varied by tuning the circuit off resonance. Figure 2 shows a photograph made by this method on Recording Panchromatic, Negative using a 3.5 me/sec. timing wave. The writing speed is about 40 cm/}.Isec.

Following FeldtI° the maximum writing speed was taken to correspond to a density of .1 above fog, measured by comparison with a set of lines of the same width as the trace. The density of

TABLE II. Maximum writing speed. 5CPll; Eb2=2.5 kv; Eb3=5.8 kv.

Film

Recording Negative, Pan

Aero Tri X, Pan

Recording Negative, Ortho Super XX Aero Tri X, Ortho

. Recording Negative Fine Grain, Ortho

Method used

6.4 me/sec. wave, saw­tooth pulse

sawtooth and rectangular pulse

rectangular pulse rectangular pulse rectangular pulse rectangular pulse

Vo

65 cm/I'sec.

24 cm/,..ec.

24 cm/,..ec. 23 cm/,..ec. 20 cm/I'sec. 8 cm/,.sec .

these lines was determined by means of a larger area, exposed simultaneously, which could be measured with a conventional densitometer. The background of the density standard was negli­gible . .In use it was superimposed on the test film and the height of the test object measured at which the fastest portion of the trace had the same density as the standard. In order to elimi­nate the effects of varying optical conditions, the writing speeds so measured were reduced to the standard conditions of unit lens aperture number and magnification rati(). The final measured values are given in Table II.

Table II shows that the most sensitive film is Recording Negative, Pan. There is no significant difference between the values for Super XX and Recording Negative, Or tho. The low writing speed indicated for Aero Tri X Pan in Table II was measured on an old sample. From films of transients taken with fresh stock, but which could not be subjected to quantitative measure­ments, it would seem that the writing speed, when new, is almost as high as Recording Neg­ative, Pan.

This deterioration of film sensitivity can be greatly decreased by storing the film until used

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H I G H S PEE D P HOT 0 G RAP H Y 0 F CAT HOD E - RAY TUB E 93

at 60°F and 60 percent relative humidity. These uniform conditions also help to maintain the sprocket dimensions, which is of considerable importance if the film is to be used at high speed. This is especially necessary if the Fastax Camera is employed, in order to avoid excessive film breakage.

Data on other comparable high speed emulsions are not given in Table II as they were not readily available in 16-mm sizes. From the results quoted in Feldt's work, using Super XX as a point of reference, it would seem that Agfa Fluorapid Blue and Agfa Triple S Ortho would give a writing speed as high or higher than Recording Negative, Pan.

The Eastman Kodak Research Laboratories have conducted a series of tests to determine the optimum developing procedure. These tests have shown that for almost all films the use of D-19 for 8-12 minutes at 72°F will lead to maximum contrast to the fog background. This background, due to chemical fog, may often be quite high but the exposures will still be usable. The Kodak X-Ray developer runs a close second to D-19. D-ll yields high contrast and low fog level as well as having a shorter developing time, but its keeping qualities do not approach those of D-19.

II. THE HIGH SPEED CAMERA

The transients it was desired to photograph were periodically initiated by trigger pulses at definite rates between 40 and 4000 per second. The short duration of the transients, about 10 microseconds, made it impracticable to use moving film to provide the time base. Therefore a conventional triggered sweep was employed in the cathode-ray oscillograph and the film moved fast enough to keep successive traces from over­lapping. It is not feasible to use ordinary movie cameras for this application. The shutter cycle in these cameras would not, in general, be synchro­nized with the initiation of the transient phe­nomena. Further, the jerky motion needed to keep the film stationary during exposure cannot be carried to high speeds exceeding SO to 60 effective frame areas per second. Nor is such a motion necessary. If the film travels continu­ously, even at the highest speeds, it will be effectively stationary during the short life time of the transient.

FIG. 3. Bolex model H-16 camera as modified for high speed work.

It has been found possible, however, to modify a number of 16-mm movie cameras, so that they are suitable at relatively low repetition rates. (16-mm cameras have been used throughout be­cause of their compactness and general availa­bility). These modifications consist of removing the shutter and claw mechanism, and driving the film continuously from an external high speed motor. I t has also been necessary to install additional guide channels and rollers mounted in precision bearings to carry the film around critical places. The Bolex Model H-16 camera has lent itself particularly well to these modifications. With a direct shaft coupling to the sprocket wheel there is an 8: 1 step up between the motor speed in revolutions per second and the film speed in frames per second. Quite high speeds can there­fore be reached without an intervening train of gears. The Eastman Cine Special camera, espe­cially useful because of its convenient reflex focusing, has also been successfully modified. In this case a direct drive was not practicable and a gear box was used to couple the drive motor to the single frame shaft.

The choice of a motor for the external drive presents a number of problems. The film must be brought up to its maximum speed gradually, for if the acceleration is too rapid the film invariably

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94 H. GOLDSTEIN AND P. D. BALES

FIG. 4'. Eastman Kodak Cine Special, as modified for high speed work, with special high voltage oscillograph.

breaks. It is desirable that a range of maximum speeds be available to best fit the particular repetition rate used. For these reasons it should be possible to change the motor speed con-

FIG. 5. Sweep length 10 microseconds, recurrence frequency 333 c/sec.

veniently and continuously. On the other hand it is helpful if the speed is not influenced by fluctuations in line voltage. These requirements are satisfactorily met with the Lee Governed SPeed motor manufactured by Lee Engineering and Manufacturing Company. Designed for 115 v, 60 cycles, the speed can be varied from 500 r.p.m. to 5000 r.p.m. by means of a carbon pile regulator mounted on the motor. I n addition the motor speed depends on the line voltage only when it is low, becoming roughly independent of it above 100 volts. Hence the motor regulator can be set to a maximum speed dependent on the repetition rate. A Variac in the external line is used to raise the motor speed gradually to the maximum.

Figure 3 is a photograph of the modified Bolex camera, showing the motor mounting. The slotted wheel mounted on the shaft is required in another connection described below. The camera is placed on a stand at a convenient height for the oscillograph, with the 'Variac and switches located on a panel of this mount, as illustrated in the figure. Figure 4 shows the Cine Special after modification and in operating position in front of the cathode-ray oscillograph. In this case the motor was mounted beneath the camera and the regulator dial can be seen on the panel on the left. The voltmeter indicates the Variac voltage across the motor. Figure 5 is an excerpt from a film made with this camera. The sweeplength is about 10 microseconds, and the interval between sweeps is 3 milliseconds.

These cameras are satisfactory up to speeds around 400 frame areas per second. To extend the range it is necessary to use cameras specifically

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H I G H S PEE D P HOT 0 G RAP H Y 0 F CAT HOD E - RAY TUB E 95

designed for high speed work. The Fastax, manu­factured by Western Electric Company is such a camera and has an upper limit of 4000 frames per second for 16-mm rolls. The film in the Fastax moves continuously, and in order that ordinary motion pictures can be taken a rotating prism is placed in the light beam to deflect the image in pace with the film motion. This arrangement is not desirable when photographing single tran­sients, and the prism should be removed. Fortu­nately this can be done without too great difficulty. The ground glass focusing arrangement was originally designed for high illumination levels, and a more direct focusing viewer is necessary for this type of work. With these modi­fications the Fastax has proved highly efficient and foolproof in operation. The maximum speed is conveniently controlled by a line Variac. The initial gradual acceleration is automatically pro­vided for by governors in the motors. The photographs in Fig. 1c were taken with the Fastax. A similar high speed camera manu­factured by Eastman Kodak (Model III) has also been successfully modified for this work but the removal of the optical shu tter is not recommended, unless th~ best shop facilities are available.

m. SYNCHRONOUS HIGH SPEED PHOTOGRAPHY

In general the speed of the film in frames per second will not be exactly the same as the repeti-

SLOTTED DISC

( ---0--jll~

32 CP AUTOMOBILE HEADLIGHT

SIOK

tion rate of the transients under study. Hence, films taken in this manner cannot be projected as an ordinary moving picture; the trace will appear to drift up or down the screen. This is not a handicap in many applications, as when measurements are to be made on individual traces. However, it is occasionally desired to project the film in order to observe any changes at slow speeds. A method was therefore devised by which the triggers initiating the transients are supplied by the camera itself, in synchronism with the film speed. A disk with eight equally spaced radial slots was mounted on the shaft of the driving motor of the Bolex Camera, as shown in Fig. 3. This wheel rotates between two housings, one containing a 32-cp automobile headlamp, the other a 918 photo-cell. Each time the slot passes between the housings a pulse of light strikes the photo-cell. The resulting elec­trical pulse is then amplified and used to fire a 2D21 miniature gas tube. The output trigger from this tube is usually sharp enough to employ as is, but additional sharpening could be provided if necessary. A diagram of the circuit, which was developed by W. H. Paulsen, is given in Fig. 6. The two tubes and associated components are mounted on a small chassis placed in the camera stand as shown in Fig. 3. Eight slots are cut in the disk because of the 8: 1 step up ratio in this camera, as previously alluded to. The phase of

OUTPUT

(+)

APPROX

FIG. 6. Diagram of the synchronizer circuit.

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96 H. GOLDSTEIN AND P. D. BALES

FIG. 7. Set-up for placing individual frame numbers on film.

the trigger relative to the position of the frame area can be adjusted by slipping the disk on the motor shaft till exact synchronism is achieved. In normal operation the rotating disk is protected by a guard, not shown in the photograph.

IV. FRAME NUMBERING

In Fig. 5 it will be noticed that there are con­secutive numbers corresponding to each frame. By means of these numbers any given trace can be quickly identified. When a long series of frames must be measured, such a means of identification is almost a necessity. The method by which the numbers are placed on the film may therefore be of interest.

The high speed of the film during actual ex­posure precludes putting the numbers on the film

at that time. Instead the numbers are placed on the film before it is used in the high speed camera. A compact device has been developed to ac­complish this and is shown in Fig. 7. A solenoid­operated counter is photographed with an Eastman Cine Special camera driven by an ex­ternal motor at 4 frames per second. The counter is advanced one digit between exposures by means of a cam on the motor drive shaft which trips a microswitch. All of the frame area, except for the image of the counter, is covered by a mask to prevent exposure by stray light. The coun'ter is conveniently illuminated by two six-watt bulbs on a mount placed in front of it. The camera motor is automatically turned off by an electric timer when the film has run out. The film, after rewinding, is then ready for use in the high speed camera. A cover fits over the apparatus so that a darkened room is not necessary. Operation is thus practically automatic and requires very little attention.

ACKNOWLEDGMENT

The authors wish to thank the Eastman Kodak Research Laboratories for their kind help in making some experimental films available, and conducting the tests of various developing pro­cedures. They also wish to thank Mr. R. C. Babish for constructing the density standard.

Calendar of Meetings

March 21-22 Institute of Aeronautical Sciences. Cleveland. Ohio 27-30 American Association for the Advancement of Science,

St. Louis, Missouri

April 1- 4 American Society of Mechanical Engineers. Chattanooga.

Tennessee 3- 5 Society of Automotive Engineers. New Vork. New York 8-12 American Chemical Society. Atlantic City, New Jersey

12-13 American Physical Society, Southeastern Section, Atlanta, Georgia

17-19 American Society of Civil Engineers, Philadelphia. Penn-sylvania

18-20 American Philosophical Society, Philadelphia, Penn-sylvania

22-24 National Academy of Seiences, Washington, D. C. 25-27 American Physical Society, Cambridge. Massachusetts 26-27 American Mathematical Society, New York. New Vork 26-27 American Mathematical Society. Chicago. Illinois 28-May 1 American Ceramic Society. Inc., Buffalo, New York

May 6-10

10-11

27-29

June 2- 5

13-14

17-20

20-23

24-28

24-28

July 17-19

Society of Motion Picture Engineers. New York. New York

Acoustical Society of America. New York. New York American Geophysical Union. Washington. D. C.

American Society of Refrigerating Engineers, St. Paul, Minnesota

Institute of Aeronautical Sciences. Detroit. Michigan

American Society of Mechanical Engineers, Detroit. Michigan

Society for the Promotion of Engineering Education. St. Louis. Missouri

American Institute of Electrical Engineers, Detroit. Michigan

American Society for Testing Materials, Buffalo, New York

American Society of Civil Engineers, Spokane. Washington

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