CATHODE RAY SCANNING General Techniques RALPH H. MULLER Washington Square College of Arts and Sciences, New York University, and Central Research Laboratory, General Aniline and Film Corporation, Easton, Pa. Cathode ray scanning offers a simple and rapid means for the examination of complex patterns arising in spectroscopy and related problems. High positional accuracy is attainable, especially with electronic shaping of the pulses. An infinite number of sequences, codes, or pulse patterns can be produced. True photometric rendition of relative intensities involves no problems of much greater difficulty than ordinary photoelectric photometry.
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HE cathode ray oscillograph is a versatile tool for the rapid delineation of transient or recurrent phenomena. Although it is indispensable in the fields of television, radar, and general electronics, it has found extensive use in chemical research, especially in studying the kinetics of the internal combustion engine, in polarography, and in chemical spectroscopy. Ten years ago, i t was shown in the author’s laboratory ( 5 ) that the oscillograph may be used to provide an instantaneous spectrophotometer, and as such will function in emission as well as absorption. It is not too generally known that the oscillograph may also be employed as a scanning light source, and it is the object of this preliminary report to describe some of its uses in this manner. The cathode ray scanner was first used by Garman (1) as a means of simulating radar echoes and presenting them to a student pilot in a modified Link trainer. This training device attained a high degree of perfection a t his hands during the war, and it was later applied by the author (4) to the tactical problems of air navigation in which maps and other strategic landmarks could be presented on a radar ’scope. Important simplifications in this technique vere made by Hexem (2) during this period. These are essentially abridged television techniques, but their simplicity and general applicability offer useful aids to the rapid measurement of physical and chemical phenomena. While this technique has shown great promise in this laboratory in the interpretation of spectra and electron diffraction patterns, it seems advisable at this time to describe the technique in general terms and show what may be accomplished in these and related applications. The principle is best described in terms of the schematic diagram of Figure 1. The horizontal deflection plates of the “scanner” oscillograph are excited by a sawtooth sweep generator. This results in a horizontal line of light on the face of the scanner tube. The “line” of light is actually the trace of a fine spot of light which moves at uniform velocity from left t o right. (The return trace is electronically blanked out in modern ’scopes.) The same sweep generator drives the horizontal plates of the receiving osH cillograph, producing a trace which is moving in exact synchronism with the scanning trace. The light which is emitted by the scanner is made to pass through the object pattern, 0, by means of a lens system, as shown, or by placing 0 directly in contact a i t h the screen of the scanner tube. The light passing through the object pattern is focused on a photomultiplier tube. The latter develops a signal proportional t o the amount of light which prevails a t any instant, and this signal is continuously applied to the vertical deflection plates of the receiv-
ing tube. Deflections are produced along the y’ axis as a function of light intensity and along X ’ as a function of time, or, since a common sweep is employed, as a function of the original distance along x, and therefore along the object pattern. In the schematic diagram, A represents an intermediate amplifier or special wave modifying element, the function of which will be described later. In the rimplest case, as illustrated here, if the obje:t pattern consists of three simple lines, the pattern on the receiver ail1 show a steady image, in which the lines are correctly disposed along the x‘ axis, and each will have a height proportional to the density (or transparency) of the original. The degree to Tvhich the latter is true depends upon a number of factors which will be discussed later; the important point for the present is the accurate delineation of spacing. KO“picture” of the object is presented on the receiver, because there is no scanning in the vertical plane and no beam intensity modulation. In other 11-ords,this is not an example of television, but rather a kind of photoelectric photometry in which a cathode beam trace acts as a flying light source and the resulting photocurrents are plotted as vertical deflections on the receiving odlograph. I t is obvious, from the customary controls on oscillographs, that the received pattern can be expanded or stretched out by increasing the horizontal or vertical gain, and the \\-hole pattern can be shifted along either axis without destroying the relative proportions of the trace. I t is thus possible t o examine any portion of the pattern in greater detail. APPLICATIONS TO DENSITOMETRY
Although this scanning technique will reproduce the position of a series of lines at high speed and precision, a number of factors control the accuracy with which differences in intensity can be presented. They are (1) uniformity of screen brilliance along the scanning sweep, (2) linearity of response of the phototube,
Figure 1
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V O L U M E 19, NO. 2, F E B R U A R Y 1 9 4 2
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successfully used in the military applications of scanmng. By Electronic Comparison. This method is applicahle t o the oase in which the unknown and reference pattern were originally photographed side by side, or can be put into such position. They are both scanned by alternate sweeps as shown in Figure 2. Pattern A is scanned a t an ordinate level, yl, and on the next sweep, pattern B is scanned at level yl, and the process is repeated.in this order. The switching is accomplished by impressing a square wave switching pulse, S, on the vertical plates. If the two patterns are identical in all respects they will be completely superimposed, as in A , 1. Since this may involve some uncertainty, pattern B can he broken up into dots by modulating the intensity of the receiving tube beam by a series of sharp pulses during the interval in which pattern B is scanned. Pattern B alone will then appear as in 2. If the two are viewed simnlttmneously, the trace will appear as shown in 3. The bright dots produced by pattern B will he nnifarmly spaced only along essentially hariaontal parts of the trace because the writing speed is obviously much greater during vertical excursions of the trace. The successful superimposition of patterns assumes, of course, that the amplifier is direct current coupled or that direct current restoration circuits are provided in order that all vertical deflections shall he referred to B common base level. EXPERIMENTAL
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The essential iestures of this scheme have been checked Tvith the equipment shown in Figure 3, with which a large number of patterns have been studied and from which the design oi a precision instiurnent has been determined.
Figure 2 The oscillographon the left, which was used &sascanner, is a DuMont Model 175A with s low-persistence blue screen, the smctml characteristics of which are ideally suited to the R.C.A. (3) linearity n i the intermediate amplifier stages, and (4)linear response in the receiving oscillograph. There are other factors xbich affectthe over-all performance of this scanning system and which control the extent of faithful reproduction. The scanning spot must be extremely small and sharply focused. All amplifiers involved in the process must be capable of handling a wide range of frequencies. In the terms of television practioe, they must be “broad hand”, for, even if the scanning rate is only several hundred per second, any sharp discontinuities in the pattern will produce transient signals, the effective periodr of which may he several megaoycles. The general utility of this scheme may now he discussed. If a spectrogram or electron diffraction pattern is scanned, we shall ohtain a steady image on the receiver screen which is in effect a densitometer record. The spacing of the lines is exact, but the relative intensities are subject to the factors which have just been mentioned. Unless the desired informe tion can be obtained from the mreen by inspection, it will still he necessary to photograph the trace, for a permanent record, or for more careful study. However, if i t is desired t o compare this pattern with a reference standard, this is easily accomplished in two ways:
By Optical Comparison. In this method the observer views the pattern on the receiver screen and simultilneausly see8 an image of a reference pattern which is reflected towar$ him from a halfsilvered mirror inclined a t 45 t o the tube axis. Little or no parallax error is involved if the optically projected reference pattern is as far away from the mirror a6 the mirror is from the tube face. Under these conditions, the optical and electronic images are in the same plane. This scheme was
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mounted in this case, and a single woll-inbulated*lead connects t,his svstem with the 1000-volt Dower SUDDIYwhich amears on the t i p of the oscillograph case. A s i d e [ well-shieldid cable brings the output signal to the receiving oscillograph on the right. This is a DuMont Model 208, providing a green trace. The mnlt--r---i d i e r hnusine also contains 8. small vienina Dort from which the operator can-view the scanning trace. duiing operation, this port is closed with a lighetight cap. I n these studies, the test pattern, in the form of a photogra.ptphic negative, was fastened in ~
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Figure 3
76
ANALYTICAL CHEMISTRY
Figure 4
I n Figure 7. a, is shown a typical negative pulse as received from the photomultiplier. If this is used to drive a pentode tube to cutoff, a positive clipped pattern similar to b will he produced. If this signal, iu turo,~is1r.d through a wry small cnpicitor and large resirtw gnnmil,, ~ h v na diflcrcntinrcd signal will appear acru3s iliu resistor. h a n n r the nnnrnrnnlp of c. Tlicrc triecers. one positive and bne ne&tive, 'can be used to generate microsecond pulses, both of whch are positive or negative as desired, d . For t1.e final comparison n ilrluyed rriggtw generator . position ran be rovidcd which produces H siuglr *harp p u l ~ thc of whip[ can bc set in nt iiuv vnrinblv time. 1.. from li r t f m u c e starting time of la. The ex&t location, t., bf 'this interpolating pulse is determined by a direct current potential which controls the delay generator. It may he set in exact coincidence with either of the pattern triggers as observed on the oscillograph,,or if no oscillograph is to he used at all. the measured and t i m n e trigger.; mny h6 regiatercd by a simpie coinriJrner riwuit. 'I'hZ scqucnco oi operations shown in Figure 7 rcquircs somc cxplanntion in eonnertinn with it3 w e for orcrision meamrmwnt8. .~ If the original pulse a, arises from a symme$cal line, the object of the measurement;s to find its exact center. That information is d>tainnhb only in the final stages oi tho olwratioii, e bdrausz the width of the s uare wave, b, h n d distmcr Iheiwcrn the rcsulti n r triccers. c a n 8 d . h a 3 no soeeixl sicnifirnnrr. I i the ddsv trGgerye, is used to bnd the mean between the two triggers, 2, I
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Figure 5
direct contact with the face of the cathode ray tube. I n general, this method of mounting the test pattern is not good optical practice. aud the resolution is inferior to that obtainable with optical projection as shown in Figure 1. Consequently all {races shown in this paper lack the degree of sharpness wIhch can be attained by the projection method. To avoid extensive changes in the internal connections required to provide a common sweep, the individual sweep circuits of the two oscillographs were each carefully adjusted to 60 cycles
was connected directly to the receiving oscillograph,'the ve&cal input terminal of which contains a 0.5-mf. coupling capacitor.
Figure 6
A photograph of a typical trace is shown in Figure 4. I n this case the pattern consisted of a simple series of rectangles (Figure 5 ) eomprising code characters. The direct trace appears as a negative signal of the correct spacing, and the variations in the heights of the peaks indicate small differences in density, hut also some of the limitations of the scanning system. Figure 6 shows what may he accomplished by further treatment of the output signal. I n this case the negative signals from the multiplier were used to drive pentode to cutoff. The plate of the pentode was connected to the receiving oscillograph, and it will he noticed that the top of the trace is limited or clipped to produce well-dehed pulses more closely related to the original pattern.
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to Figure 7 Prim- signal b. Inverted end dipped c. Di5eerentiat.d d. PmitirsUiggsrpvlssin~=~~=d e. Delayed trigger for coin& denos setting
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ELECTRONIC TREATMENT OF VIDEO SIGNALS
The mere reproduction of a scanned trace on a receiving oscillograph is a relatively insignificant part of this technique. It is evident from the simple example of Figure 6 that the primary signal may be shaped and modified and put in a form more suitable for measurement. From what is briefly described below, it becomes evident that the receiving oscillograph can he eliminated entirely as far as the h a 1 measurements are concerned, or, a t least, retained only for the convenience of seeing what goes on.
Figure 8
V O L U M E 19, NO. 2, F E B R U A R Y 1 9 4 7 one can obtain an exact positional measurement. Manually, this is given by the mean of two values, t., and h,. Tracking circuits are available for positioning the 1inear.delay trigger exactly halfway between these markers automatically. The exact details of each of these operations and the circuits for their achievement are omitted here because they are adequately described in several sources (3, 6). These and still other circuit resources are available for the assimilation and measurement of the patterns without recourse to their visual presentation on an oscillograph screen. Figure 8 illustrates a case in which a series of lines was scanned, differentiated, and used to trigger a microsecond pulse generator The lines in the original pattern were spaced in a square law sequence and were not of uniform intensity. (For purposes of reproduction it mas necessary t o retouch this figure in order to reveal the very narrow lines.) This difficulty is ordinarily overcome by beam intensification. CONCLUSIONS
Brief mention has been made of double scanning, but in general, an elaborate series of patterns can be scanned from top to bottom, if after each sweep the scanning beam is depressed by an appropriate sweep increment along the vertical axis. bionlinear traces have important uses. For example, if a sine wave is phase-shifted and used to produce a circular trace on the
77 scanning oscillograph, a pattern may then be prepared in which a series of radial lines is drawn. As the circular sweep passes over these lines, a series of pulfies will be produced which are integral multiples of the original sine wave frequency. The use of polarizing film also presents the possibility of scanning anisotropic samples. These and other applications are currently under investigation with an improved high-precision instrument. ACKNOWLEDGMENT
These experiments were conducted in the Central Research Laboratory of the General Aniline and Film Corporation. For the facilities and technical assistance the author is greatly indebted to L. T. Hallett, F. A. Hamm, F. C. Snowden, and L. V. M eyers. LITERATURE CITED
(1) Garman, R. L., “M.I.T. Radiation Laboratory Series”, M.I.T. Radiation Laboratory Report 105-3, New York, McGrawHill Book Co. (to be published in 1947). (2) Hexem, J., in ( 1 ) . (3) Lewis, W. B., “Electrical Counting”, New York, Cambridge University Press, 1943. (4) Muller, R. H., in (1). (6) Miiller, R. H., and Garman, R. L., Mikrochemie, 21,302 (1936). (6) Puckle, 0. S., “Time Bases”,’New York, John Wiley I% Sons Co., 1943.
Rapid Determination of Small Amounts of Carbon Monoxide Preliminary Report on the NBS Colorimetric Indicating Gel MARTIN SHEPHERD National Bureau of Standards, U. S . Department of Commerce, Washington 25, D. C.
This condensed report furnishes the minimum preliminary information necessary to make and use the NBS indicating gel for the rapid colorimetric determination of small or physiologically significant amounts of carbon monoxide in air in the field or laboratory. The gel will detect and estimate less than 1 part of carbon monoxide in 500,000,000 parts of air. It will detect 0.001% by volume in less than 1 minute, and determine physiologically significant amounts in approximately 1 minute. Field use requires a small, inexpensive apparatus without maintenance problems, and involves procedures so simple that untrained personnel will be able to obtain reliable results.
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HE object of this condensed report is to furnish the minimum information necessary to make and use the KBS indicating gel for the rapid colorimetric determination of small amounts of carbon monoxide in air. This preliminary report will serve those who may have immediate use for the indicating gel. A complete report on the bureau’s work on this project is being prepared in collaboration with others associated with the various phases of the investigation: Initial development, Shuford Schuhmann and Mary Ann Somervell Palladium solutions, W. Stanley Clabaugh and Edward Wichers Purification of gel, Richard L. Thomas Production of NBS indicating tubes, Marthada Vaughn Kilday and John H. Eieeman Glass sealing machines, E. R. Weaver
Indicating instruments, Shuford Schuhmann, Harry Bailey, and Robert Thiebeau Applications of the indicating tube, Shuford Schuhmann, E. R. Weaver, and Marthada Vaughn Kilday Color standards, Marthada Vaughn Kilday and Shuford Schuhmann Laboratory methods
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The formulation of the gel and equilibrium in the indicating tube are also discussed. DEVELOPMENT, NATURE, AND USE OF INDICATOR
During 1941 the Royal Aircraft Establishment, Farnborough, England, developed an indicator for the detection and estimation
