FEBRUARY, 1939

An Automatic Recording Spectroradiometer for Cathodoluminescent Materials*

V. K. ZWORYKINRCA Manufacturing Company, Camden, New Jersey

(Received November 26, 1938)

An automatic recording spectrophotometer for the visible, especially adapted for the studyof cathodoluminescent materials, is described. It consists of a demountable light source and aninterrupter, a glass monochromator with compensation for the variation of the dispersion withwave-length, a photoelectric electron multiplier, a signal amplifier stabilized by negativefeedback, and a motor drive amplifier controlling the deflection of the recording head. Thewave-length scale is made linear by the introduction of a spiral cam of appropriate form. Thesystem is distinguished by high sensitivity (usable for sources emitting one millilumen/cm2),high speed (14- minutes for a record), linear wave-length scale, high stability and easyadjustment.

EXTENSIVE measurements in our laboratoryon the spectral distribution of the radiation

from luminescent substances used in cathode-raytubes made it desirable to devise means of ob-taining accurate distribution curves with amaximum of speed and a minimum of attentionto the recording apparatus. An added desidera-tum was a linear wave-length scale, makingpossible a quick determination of the totalenergy in a band, this being given directly bythe area under the curve. These demands led tothe construction of the apparatus describedbelow.

1. GENERAL PLAN OF THE APPARATUS

A view of the complete installation is given inFig. 1. Fig. 2 is a diagram showing the relation-

*Read before the Optical Society of America at itsmeeting at Niagara Falls, Ontario, October 27-29, 1938.

ship of the principal components of the device.The surface S of a sample excited by an electronbeam is imaged on the collimator slit S of aGaertner glass monochromator and this slit,after passage of the light through the constantdeviation prism, is then focused on the exit slitS2 of the monochromator. Before entering themonochromator the light is "chopped" 150 timesper second by a synchronously driven five-bladed sector disk, Se. The exit slit is imaged onthe window of a magnetic electron multiplierwith a horizontal slot, SI, moving in unison withthe prism table, being interposed in order tocompensate the variation in the dispersion of thespectrograph and the photosensitivity of themultiplier. The 150-cycle component of the out-put current of the multiplier, which is about400,000 times as great as the photocurrent, isfurther amplified by a 150-cycle a.c. signal

FIG. 1. General view of spectroradiometer installation.

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J. . S. A. VOLUME 29

RECORDING SPECTRORADIOMETER

amplifier-using the a.c. component eliminatesdisturbing effects caused by multiplier darkcurrents and d.c. amplifier drift. The amplified150-cycle signal is rectified and the differencebetween it and the signal corresponding to theposition of the recorder head P bearing the penon the recorder potentiometer incorporated inthe arm A is amplified in the d.c. motor driveamplifier and applied to the armature of themotor, M, controlling the ordinate position ofthe recording pen. The abscissa is controlled by asecond motor which rotates the prism table; withthe aid of a suitably shaped spiral helical cam,Cm, the abscissa motion is made such as tomake equal distances correspond to equal wave-length intervals over the entire scale (from 4000to 7300A).

2. THE LIGHT SOURCE

The light source (Fig. 3) consists of a de-mountable vacuum tube containing a 32" diam-eter Pyrex disk, D, and an electron gun, G, withits axis inclined 50° with respect to the normal

FIG. 2. Plan of spectroradiometer.

to the disk. On the disk may be deposited eightpatches of different fluorescent materials, whichmay in turn be brought under the electron beamfrom the gun by rotating the disk with the aidof a sylphon joint, Sy. The electron gun, ofstandard type, may, like the disk, be removedfrom the tube and the cathode be replaced whennecessary. So as to minimize the frequency ofthis operation, it proved advisable to'employ atungsten helix in place of the usual, indirectly

FIG. 3. The light source.

heated, oxide-coated element as cathode. Themetal gun press is bolted to a Fernico collarsealed to the side tube, the joint being madevacuum-tight by the insertion between them of atin ring. In the case of the junction between theround metal plate supporting the disk and thetube walls, a rubber gasket placed betweenthe former and a metal flange sealed to the tubewall with Picein proved adequate. The tube isevacuated with the aid of a three-stage oildiffusion pump' backed by a Cenco Hyvac pumpand the vacuum measured with the aid of athermocouple gauge and an ionization gauge.The time required for evacuation is about 2

hour, the operating vacuum about 3. 10-5 mm Hg.In the tube here discussed the luminescent

material is deposited on the disk in a layer

' Designed by Dr. L. Malter. See L. Malter and N.Marcuvitz, Rev. Sci. Inst. 9, 92 (1938).

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V. K. ZWORYKIN

5500

FIG. 4. Resolution of the monochromator.

sufficiently heavy to be opaque and the frontsurface is imaged on the slit of the mono-chromator; its distance from the front wall ofthe tube is less than an inch, making a goodutilization of the light output possible. Thestandard bombarding velocity employed is 6000volts. With 500 volts on the first anode of thegun we obtain under these circumstances a spotabout one cm2 in area. The normal operatingcurrent is between five and ten /4A. The voltagesfor operating the tube are furnished by powerpacks operated from the line, the d.c. voltagesfor the first and second anode being filtered toreduce ripple. Placing 1000-cycle pulses on thecontrol grid of the gun to make the bombard-ment intermittent proved of value in preventing"burning" of the sample.

3. THE MONOCHROMATOR AND COMPEN-

SATING SLOT

The monochromator is a calibrated Gaertnerconstant deviation glass monochromator with adirect-reading spiral wave-length scale. The cali-bration is checked periodically with the linespectrum of a mercury arc. Curves for the least

resolvable wave-length separation AX are shownfor four different slit-widths in Fig. 4. Parallelismof the exit to the entrance slit is secured byrotating the former until a minimum response isobtained when the prism is set just off one ofthe mercury lines. A shutter, geared to theaxis rotating the prism table, moves in front ofthe exit slit and covers it when the table movesbeyond the 7300 and 4000A settings. Just beyondthese points it acts, in addition, as a switchshutting off the motor driving the recorder armand prism table. The arrangement of the wiringis such that the switch reversing the directionof the motor also closes once more the automaticshut-off switch. The exit slit, furthermore, isimaged into the plane of the disk provided withthe compensating slot, immediately in front ofthe window of the multiplier. The width of theslot is made such that, for all positions of theprism, the photocurrent leaving the photosensi-tive cathode of the multiplier is directly propor-tional to the amount of energy radiated per unitrange of wave-length by the source. This methodof compensation is made possible by the factthat with the types of sources under considera-

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RECORDING SPECTRORADIOMETER

tion the illumination along the exit slit of themonochromator is substantially uniform. Theshape of the slot was determined by comparingthe uncompensated output of the instrument, astandard tungsten ribbon lamp being used assource, with curves for the energy distributionfor a radiator of the same color temperaturepublished by the National Bureau of Standards.2

A later check of the recorded curve for thestandard lamp with the compensating slot inplace with the above-mentioned published curveshoved excellent agreement.

4. THE PHOTOELECTRIC MULTIPLIER

The light passed by the compensating slotreaches the photocathode of the multiplier. Thelatter is of the magnetic type' and has ten stagesof secondary emission multiplication. The sensi-tive surfaces are of oxidized silver sensitized withrubidium. The total gain is about 400,000, theover-all applied voltage 1600 volts. Regulatedsupplies are used for both the magnetic field-provided by a soft-iron yoke magnetized by acoil-and the voltages applied to the individualmultiplier plates through a bleeder. Their circuitsare shown in Figs. 5a and 5b, respectively.To eliminate all disturbing effects caused byhumidity and stray light, the outside of themultiplier is painted with a grounded coat ofAquadag, covering the tube with the exceptionof the window and the regions near the terminals.

FIG. 6. Circuit diagram for the signal amplifier.

In addition, it is enclosed in a black Bakelitebox through which dry air may be circulated.The great advantage of the multiplier in thiscase rests, as is brought out in reference 3, onthe more favorable signal-to-noise ratio obtain-able with it as compared with a photo-cell,coupling resistor and amplifier. It alone makespossible the obtention of satisfactory recordswith photocurrents of 10-14 ampere or less.

FIG. 5. Regulated current and voltage supplies forthe multiplier.

2 Miscellaneous Publications of the National Bureau ofStandards No. 86.

3 V. K. Zworykin, G. A. Morton and L. Malter, Proc.Inst. Rad. Eng. 24, 351 (1936).

5. THE SIGNAL AMPLIFIER

The output current of the multiplier flows toground through a large reactor of severalthousand henries, self-tuned to 150 cycles, andthe a.c. voltage developed across the same isapplied to the grid of the input tube of a three-stage degenerative current feed-back amplifier,whose circuit is sketched in Fig. 6. To eliminatemicrophonic noise the whole unit is suspendedon springs. The gain is varied by changing thefeed-back admittance with the aid of twopotentiometer gain controls. Between the re-sistance-coupled stages condenser-resistor circuitsare inserted to suppress oscillations. The outputcircuit of the last stage is tuned for 150 cyclesand contains a 2: 1 step-down transformer whichprovides the input for the recorder drive ampli-

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V. K. ZWORYKIN

FIG. 7. Circuit diagram for the motor drive amplifier.

fier. The 280-volt plate supply is filtered asshown and shunted by a Stabilovolt glow tube toreduce fluctuations. In practice the gain is ad-justed to give close to 35 volts transformeroutput on the peaks of the curves, as this corre-sponds to a full ordinate height on the record.

6. THE RECORDER DRIVE AMPLIFIER

The output of the step-down transformer isrectified by a 6H6 double diode and applied tothe input of the motor drive amplifier (Fig. 7) inthe following manner: The d.c. signal generatedover a 700,000-ohm resistor 4 is subtracted fromhalf the voltage (divided by two -megohmresistors) corresponding to the position of thepen on the recorder potentiometer (across whichthere is a drop of 70 volts derived from a Stabilo-volt potential divider) and applied to one inputgrid of a three-stage class A d.c. push-pullresistance-coupled amplifier; the other input grid

4 A microammeter in series with this resistor is set in thefront panel of the instrument, making it possible to readdirectly the rectified output signal of the signal amplifier.

is adjusted permanently at a fixed voltage withrespect to the negative end of the recorderpotentiometer by the zero setting as shown.The last stage consists of five 25L6 screen gridtubes in parallel in each branch, whose filamentsare connected in series and fed directly by the110-volt a.c. line. Their output, shunted by two250-ohm resistors, is fed directly to the armatureof the motor driving the recorder head carryingthe pen. The plate voltage as well as the motorfield is supplied by the 220-volt d.c. line, whosenegative terminal is joined to the cathodes andwhose grounded midpoint is connected to thescreen grids. A conveniently placed pilot lightindicates whether the d.c. line is properly con-nected. The individual grids of the tubes areconnected by resistors to suppress oscillations.

In this fashion a signal proportional to thedifference in the input signal and that corre-sponding to the position of the point on therecorder rheostat is at all times fed to the arma-ture of the motor; without further precautionthis would cause the recorder head to overshoot

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RECORDING SPECTRORADIOMETER

its mark owing to inertia. Actually, by placingthe -megohm resistor shunted by a suitablydimensioned (0.25 ,/f) condenser in series withthe potentiometer contact on the recorder headit is possible to advance the change in signal inthe armature in such fashion that the motorcomes to a dead stop at the proper position ofthe recorder head.

7. THE RECORDING MECHANISM

We thus, finally, arrive at the recordingmechanism proper, which is sketched schemati-cally in Fig. 8. The central portion consists of aframe, F, and bed-plate, B, for the graph paper(8 X 11 in.) on which the curves are to be traced.Over it moves the recorder arm, A, incorporatingthe recorder potentiometer on its lower side.The arm is anchored at one end, where it rollson six rollers on the sides of two vertical V-shapedgrooves. The carriage, P, bearing the recordingpen and potentiometer contact moves in thesame manner along the arm. The free end of thearm rolls on a single roller on a flat plate.The motion of the recorder arm, i.e., the motionalong the wave-length scale, is controlled by astranded phosphor-bronze cable, clamped to thearm by means of a thumb screw, T, facilitating

FIG. 8. Diagram of the recording mechanism.

zero adjustments, which is wound up over apulley, P, on the spiral cam at the end of theshaft driving the prism table. The other end iskept taut by being wound up on a pulley, P2,provided with a stiff inner spring.

The carriage is moved along the arm, on theother hand, by a phosphor-bronze cable whosetwo ends are fixed to a threaded pulley, P3 , atthe anchored end of the arm and which passesover a plain pulley, P4 , at the free end of the

r~~~~~~~~~~ _

2.

4N

4000 4500 5000 5500 6000 6500 7000 A

FIG. 9. The spiral cam and calibration of the wave-length axis.

same. The threaded pulley is rotated by a steelshaft of square cross section, along which itslides, forming a unit with the recorder arm.The shaft, in turn, is coupled by a worm gear tothe amplifier-controlled motor, M1. The speedof the latter is such that a full-scale deflectiontakes approximately one second.

The shape of the spiral cam, C (Fig. 9a), wasfirst calculated, with the aid of the calibrationcurves for the monochromator, so as to give

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V. K. ZWORYKIN

equal distances of travel for equal wave-lengthintervals and then turned out of brass with theradius vector augmented by about " through-out. The grooves were filed by hand, checkingthe accuracy of the filing on the instrumentcontinuously. A subsequent recording of equalwave-length intervals gave the result shown inFig. 9b, which shows satisfactory, though notperfect, constancy throughout the scale.

The shaft geared to the prism table is coupled

5000 5500 6000 A

FIG. 10. Spectral response curve for green willemite.

by two chain drives, making possible a fast(141 minutes for a complete record) and a slow(5 minutes for a record) movement, to themotor, M2 . The speed of the latter, in turn, iscontrolled by a rheostat in series with its arma-ture current supply, the rheostat itself beingcoupled indirectly to the shaft. It is so dimen-sioned and arranged that the speed of motion ofthe recorder remains constant throughout thewave-length range, i.e., the speed is reduced inthe region of low dispersion.

8. PERFORMANCE OF THE APPARATUS

As examples of the performance we show first,in Fig. 10, the spectral response of green willemiteas obtained by point-by-point densitometermeasurements on a plate taken with a Hilgerquartz spectrograph (points indicated by crosses)and compare it with the record obtained with

the present device. The agreement is seen to besatisfactory. The amplitudes were set equal atthe peak of the curve. Another example, morecharacteristic of the normal employment of theapparatus, is given by Fig. 11, which shows thecurves obtained for zinc beryllium silicate withone, two and four percent Mn added as activator,the heat treatment being the same throughout.These curves are obtained as follows: To beginwith, the response of every one of the materialsis measured in immediate succession at one ortwo wave-lengths, to establish the correct relativescales of the ordinates; thereafter, the curves forthe several materials on the disk may be takenat leisure. Finally, they are replotted to the scaleestablished by the original measurements on thesame piece of graph paper, smoothing outeventual small oscillations, such as those shownon the record in Fig. 10.

If the curves obtained in this fashion areconverted from units of energy to units of visualbrightness by multiplication with the visibilitycurve, the areas under the resultant curves givea good measure of the relative visual brightnessof the different samples; in fact, the valuesfound in this manner agree normally with thoseobtained when the spectroradiometer is replacedby a Weston illuminometer within five percent.

1 2ZnO.Be0-3SO:2Mn 1200°60' LGY

2 is :4Mn s LYO3 s :8Mn l H LNO

3C

25

0=20O -z

W

I

C 5 <

A _ _A KERR _ _ _ E__P __ S n_

45U 5uu 55u 6oWAVELENGTH

65u /uo 1500

FIG. 11. Spectral response curves for zinc beryllium silicatewith different percentages of manganese activator.

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RECORDING SPECTRORADIOMETER

Thus, if a standard material, such as greenwillemite, whose absolute light yield is known,is included among the materials on the disk itbecomes a relatively simple matter to convertthe ordinates obtained into absolute units.

For the slit widths considered in Fig. 4 theresolution of the instrument is scarcely dimin-ished by the recording mechanism, as the timerequired by the recording head to move the fulldistance of the ordinate scale corresponds to amotion along the wave-lengths scale of only 50Ain the case of the high speed coupling of thewave-length axis and to 1 2A in the case of the lowspeed coupling.

As compared with any point-by-point or semi-automatic method of spectrum photometry, e.g.,the taking of plates with a spectrograph andtheir measurement with a densitometer, theenormous gain in time obtained with the presentinstrument is obvious. As an automatic instru-ment it possesses, apart from high speed, thefollowing advantages:

(a) Great sensitivity; satisfactory records maybe made with a maximum input to the photo-cathode of only about 10- lumen5 owing to theincrease in the signal-to-noise ratio with themultiplier as compared with a photo-cell andthermionic amplifier. This is particularly im-portant with cathodoluminescent materials as it

6 With a wide slit (1.00 mm) this corresponds to a lightemission of the sample of about 10-3 lumen/cm

2.

reduces the likelihood of damaging them, oraltering their response, by overloading.

(b) High amplifier stability owing to the useof negative feedback.

(c) Linear wave-length scale facilitating esti-mates of energy output.

(d) Uniform speed of recording for entirewave-length scale.

(e) Easy adjustment of all zeros and rigid con-nection between recording pen and potentiometercontact.

These, as well as other minor advantages whichare evident from the text, should make theinstrument a valuable aid in extending ourknowledge of luminescence phenomena as well asin other fields requiring light measurements of asimilar nature.

The instrument is a product of the cooperativeefforts practically of the whole staff of theElectronics Research Laboratory. In particular,the author would like to acknowledge theassistance of Mr. A. W. Vance, who designedthe amplifying system, and Mr. H. W. Leverenz,who adapted the instrument for luminescenceresearch. Also, mention should be made of thework of Mr. I. E. Grosdoff, for the mechanicaldesign; Dr. J. Van den Bouwhuysen, for theoptical parts; and Mr. L. E. Flory, for the de-mountable vacuum part of the system, and Dr.E. G. Ramberg, for assistance in the prepara-tion of this paper.

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