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Some Methods of Minimizing the Black-border Effect in the Image Orthicon Television Pick-up Tube
Some Methods of Minimizing the Black-border Effect in the Image Orthicgn Television Pick-up Tube 8. MIYASHIRO and Y. NAKAYAMA Centml Rresriwli Lrrborutory, Tokyo Sk ibtriiru E’lrrtric go. Lttl., Kett, J n p n ~
I~oiingnzivi,
INTRODUCTION The image orthicon is the best, television pick-up ttul)eat present, but there are still a few points t o be improved. Since the ability of t h e image orthicon to transmit a piclture showing light and shade even in a wide light range is ascribed to the black-border effect, this effect may be taken to be an advantage; however, usually it also has undesirable consequences. A qualitative explanation of the origin of the black-border effect was given when the invention of the image orthicon was announced,’. 2 and more detailed investigations were reported recently,3 but i t appears doubtful if the cause of the effect, has yet been completely revealed. Be that as i t may, the black-barder effect is probably mainly due to rather low velocity electrons among the secondary electrons overflowing from the target assembly, which are forced to come back to the target by the retarding electric Aeld and result in a negatively charged area surrounding the “bright” area on the target. ’ Several methods of minimizing the black-border effect have been proposed. Firstly, there is a nidthod which depends on geometry. (a) The use of a target of large area. The size of the black-border may get relatively smaller; this is a. feature of the 4.5-in. image ~rthicon.~ Next, there are methods for reducing the number of undesirable secondary electrons which cause .the effect. (b) The use of (1 target mesh ha@ingu srriull swondarg electron emission ratio. Only a small imprcrvement may be expected even if the emistlion ratio is decreased to 2e1-0.~ (c) The u s e of u tar@ having u lurge gain,. As the necessary photocurrent is reduced, so the nun; ber of undesirable secondary electrons may get smaller in proportion. This is related to the image orthicon developed by the (Jetiera.1Electric C ! O . ~which has a MgO target having a high gain. (d) The use of u high target vol&ge. IJndesirable secondary electrons may be decreased by this means. Furthermore, there are methods by which the overflowing secondary electrons may be so controlled as not to cause undesirable effects. 171
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S. MIYASHIRO A N D Y . NAKAYANA
(e) The use of a strong focusing magnetic Jield. This results in a decreased radius of the secondary electron trajectories, but is apt to have a rather large influence upon the other factors. ( f ) The use of a strong retarding electric field- This may shorten the distance of spread of the undesirable secondary electrons. (g) The use of a collector mesh at a positive potential. Another mesh electrode of positive polarity is inserted in front of the target mesh. After dealing briefly with (d) and (f), the main part of this article will be devoted to a detailed explanation of the investigations of method (g),which is the most effective one the authors have found up t o now. This last method is based upon an idea due to the authors and their colleague K . Odagawa. Some of the early experimental results were reported in 1959.s. Subsequently, the theoretical background of this method was clarified,8 so that the whole of this investigation will be presented here. T. Ninomiya of NHK (Japanese Broadcasting Corporation) has almost independently and simultaneously t o the authors investigated an anti-black-border image orthicon very similar to that described in this paper and obtained good results. In addition, W. E. Turk of English Electric Valve Co. Ltd. recently informed the authors privately that similar work had been done in Britain, but the authors do not know of any publications concerning it.
THEHIGHTARGET VOLTAQE METHOD As the target mesh voltage Et (the potential above target cut-off potential E,,,) is raised, secondary electrons liberated from the target glass can be more effectively caught by the target mesh. Figure 1 shows the effective secondary emission ratio of the target glass. When El is 2 V, which is the voltage normally used, full utilization of the secondary electrons cann’ot be expected, for saturation does not occur until El is above 4 V. I n other words, a high target voltage can reduce the undesirable secondary electrons. The potential difference between a part of the target glass corresponding to a bright area, from which secondary electrons responsible for the black-border effect are released, and the surrounding part corresponding to a dark area becomes larger with increase of the target voltage. It therefore becomes increasingly difficult for the secondary electrons leaving the “bright” part of the target to reach the “dark” part as the target voltage is raised. These factors contribute t o the reduction of the black-border. The appearance of the black-border as the target voltage is varied is shown
SOME METHODS O F MINIMIZING BLACK-BORDER EFFECT
173
in Fig. 2(a) and (b). With a type 5820 tube, a noticeable edge is seen at Et = G V. This is the reason why the image orthicon cannot usually be operated with El higher than about 2 V. The technique of incorporating a field mesh is a *ell-known counter measure against the edge effect, and there already exist many kinds of commercial field mesh tubes. Meanwhile, experimental tubes with very close target-tofield mesh spacing have been canstructed. A picture taken with one of these tubes with a spacing of 2.35 mm is shown in Fig. 2(c). This tube can be operated with a target yoltage of G V without any noticeable edge effect, with a resulting minimization of the black-border. 6r
.sr, 5 -
-
0
I
1
I
2
I
3
1
4
(
5
1
6
I
7
8
Target mesh patential €, ( V )
FIG. 1. The effective secondary emission ratio of the target glass (type 5820 tubes). The quantity of light is about half trhatrequired to charge up the target just to the knee point when Et = 2 V.
Since operation of the tube with a high target voltage also increases the effective photosensitivity of t;he tube and contributes to the improvement in signal-to-noise ratio, it may be worth taking into account in future. THESTRONG RE'PARDWG ELECTRIC FIELDMETHOD A secondary electron, leaving st an angle 4, with initial energy E , eV is forced to return to a point On the target at a distance a cm (the scattering length) from its startillg point, by the retarding electric field F (V/cm) as well as the magnetic field H (a).A simple calculation (see Appendix A) shows that the scattering length is given by
Let H = 68 G , t 4 = 45" and E , = 2-12 eV. Some results of numerical calculations are shown in Fig. 3. I n this figure the abscissa shows the
t This figure was ineesured at the tttivet wheil a field of 75 C was maintained at the middle of the focusing coil.
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S. MIYASHIRO AND Y. NAKAYAMA
(4
FIG.2. Black-border minimization by the high target voltago method. (a) Type 8820 tuba, Et = f:!V. (b) Type 5810 tuba, Et = + 6 V. ( c ) Experimental tube with field Inctsh, Et = f 6 V.
SOME METHODS OF MINIMIZING BLACK-BORDER EFFECT
175
retarding field strength in front of the target, and the ordinate shows the scattering length of the electron, which is related tto the width or size of the black-border. In the standard %in. image orthicon, the electric field strength i n front of the target is found to be 50 to 100 V/cm when the focusing
-
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30
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0 . 6 7 4 4 9 s i n @sin{O593+cos$]
c
0.08
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g=45', H = 6 8 G
Y)
a
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20
c
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a
c
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e
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10
0
0
u)
0
I
I00
I
I
1
200
300
0
400
Field strength in front of torgel (V/cm)
FIG.3. Scattering length of the secondary electrons (thcoret,iral).
Field strength in front of target F (V/cm)
Flu. 4. Black-border width as function of field strength in front of the target
(measured with a11 experimental tub+) having about 5.5 niin from the target mesh).
a11
additional mesh electrode spwetl
requirement is satisfied. According to Fig. 3, it is to be expected that the black-border width will be halved if the field strength can be increased to about 200 V/cm by some means. Although no completely successful arrangement of elect>rodes was found, the principle of this method has been confirmed by taking advantage ofexperinlentaltubessuch aRthnt illustrated inFig. 5, in which the retarding field strength could be varied a t will by adjusting the negative voltage of the collector mesh independently of that of the other electrodes. A black sheet of paper with sniall round apertures illuminated from behind was viewed by the tube. The width of the black-border surrounding the bfight spots in the transmitted picture is
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S. MIYASHIRO AND
Y.
'NAKAYAMA
shown in Fig. 4. As the field strength F is increased, the black-border clearly gets smaller. However, in thia experiment the picture quality as a whole was not good, because of the presence of other undesirable effects, such as ghost images and white halos. An arrangement of electrodes suitable for this method does not, of course, necessarily depend on an additional mesh electrode. A COLLECTOR MESH OF POSITIVE POLARITY THEMETHODEMPLOYING Construction of Experimental Tubes To the usual target mesh another collector mesh electrode, the voltage of which can be controlled independently, is added in the image section of the standard image orthicon tube as shown in Fig. 5 .
J7-g
Collector
_-
.___..____
L
L
Accelerator thode G,
-L-----, ,Image
anode
'Torget
assembly
FIG.5. Image section of anti-black-border type image orthicon.
As its name suggests, this collector mesh was originally inserted in an attempt to collect the undesirable secondary electrons ; however, later investigation has revealed that it also plays another important role in exerting a focusing action on the secondary electrons. Another type of tube, containing a positively charged accelerator electrode with a mesh to act simultaneously a8 a collector and an imagefocusing electrode, was constructed in the early stages,6, but, because of focusing difficulties and distortion problems, only the type shown in Fig, 5 was adopted for detailed investigation. The collector meshes used were 100 to 750 lineslin., depending on the collector-to-target spacing. Various spacings L were adopted, the maximum being 18.2 mm. Tho insertion of the collector required a number of changes of the electrode voltages from standard values, in order to satisfy the focusing condition. According to the experimental results, as long as the spacing is less than about 10 mm, the tube can be operated in a standard image orthicon camera. The Mechanism of Black-border Minimization Experimental results Examples of the relation between the extent of the black-border effect and the collector mesh voltage E , are illustrated in Figs. 6 and 7 .
SOME METHODS OF MINIMIZING BLACK-BORDER EFFECT
177
With a negative collector voltage the black-border is as large as that in a standard tube. In the vicinity of 1,.= 0 it decreases rapidly, but the decrease gradually stops near I$. = + 16 V, and hardly any change occurs for E, > 15 V. At the same time the target cut-off potential Eleoshifts negatively by 1.5 to 2.0 V as shown in Fig. 6(b). Both these changes can be attributed to the action of the collector on the undesirable secondary electrons.
3
-
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I
l-t++-d
Block-'border Aidth A
I Collecltor
I
i
I I
!TI
mesh voltoge Ec
I
Target cut-off volioge E,co
I
I
FIG.6. Variation of black-bordepsize A and target cut-off voltage El,, with collector mesh voltctge E,. (Experiineiital tube, L = 6.7 mni.)
Furt,hermore, detailed observations reveal the existence of optimum collector voltages, in the viciniky of which the black-border becomes a minimum (Fig. 7 ( d ) ) . For example, with a photocathode voltage E,,,&= - 500 V, such optimum voltages for various collector-target spacings are shown in the following table. TABLEI
&W)
b 3.0mm 54mm 6.7 mm 10.6 mm 18.5 mm
none +18
+35 11.5, 233.5, 91 12, + 20, 41, + 82
+ +
+
+
+
Detailed experimental values are illustrated by the solid lines in Fig. 11. This phenomenon will be proved to be clue to the focusing action of the collector mesh on the secondary electrons. The collectirLg action of the collector on the overflowing secondary electrons When E,< 0 , the electric field in front of the target has a retarding action on the secondary electrons but, when E, approaches - 8 V to - 6 V, even though i t is still negative, the collecting action of the
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S. MIYASHIRO A N D Y . NAKAYAMA
FIG. 7. The black-border effect in the anti-black-border image orthicon ( L= 8.7 mni). (a) E , = - 15 V. Behaves like a t,ype 5820 tube. (b) E , = 0 V. Focusing action starts. (c) E , = + 15 V. Collecting act~ionstarts. (d) Ec = + 35 V. Both actions of collecting and focusing work (optimum condit,ion). (e) Ec = + 60 V . . Only colleoting action works.
SOME METHODS OF MINIMIZING BLACK-BORDER EFFECT
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collector begins, owing to the initial velocity of the overflowing secondary electrons, and the black-border starts to decrease. From Fig. 6, it can be estimated that the target mesh voltage E,,,, which satisfies the relation El,,= E,,, + 2 = E,,, (2)
is nearly zero. Accordingly, if the work functions of both meshes are the same, it can be seen that most; of the overflowing secondary electrons have initial energies less than 6 4 eV, i.e. curve (a)in Fig. 6 corresponds to the curve obtained in measurements of the energy distribution of the secondary electrons by the so-called retarding field method. But here, because of the special arrangement of the electrodes, the curve does not flatten out until the collector voltage reaches about +15V. This is because a precise measuremenb of the energy distribution of the secondary electrons requires a small target surrounded by a large spherical collector. The shift of the target cut-off voltage E,, is ascribedll to a rapid change in the equilibrium potential difference E,,, in the target system when the collecting aotion of the collector starts. Consider a part of the target which has been fully charged,or where the corresponding amount of light is approximately at the knee of the light transfer characteristic of the tube. At mch a part of the glass target, where an electrical equilibrium is being established, tthe surface potential exceeds that, of the target mesh by Escr,and input and output currents t o and from this area are equal. The input currents, I,, and I,,,,, when E,. is sufficiently negative or positive respectively, may be expressed by
respectively, where K is a concrtant, and dn,ldE and drbz,ldE represent the energy distributions of the secondary electrons liberated from the target glass. The equilibrium potential differences (Zw),,,(Z8J,, are defined by these equations. These equations are deduced from the fact that only those secondary elecfrons having initial energies larger than fl$,,, can reach the target mesh' plane, and that, furthermore, only the F,,or F,, fractions of them can be absorbed by the target mesh (and by the collector mesh). The factors F,, and F,,depend upon the transmission coefficient of the target mesh and the reflection coefficients of the target mesh and glass for thg low velocity electrons. Since, especially
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S. MIYASHIRO AND Y. N A K A Y A M A
when E, > 0 , the collector mesh can absorb that fraction of electrons which has reached the collector mesh plane corresponding to the shadow ratio of the collector mesh, the relation Fp > F,Lholds always. From a rough estimate (of. Appendix B) the values are
F,L= 0.7 2 0.1, EL,= 0.9 & 0-05.
(4)
If the rough assumption is made that the shapes of the two energy ' distribution curves are similar, then
(Esec)n BIG.8. The relation between the energy distribution of the target secondary electrons and the target out-off voltage E,,,.
where Iq,Land Isll are secondary emission currents when E, < 0 and E, > 0 , respectively. Then
where a,&,,S, are the effective secondary emission ratios and, in general, 6, > S,, because rather more secondary electrons of low velocity which are released from both meshes are included in the primary current I,, when E, < 0. From a rough estimate (cf. Appendix C),
s4 eL z 1.1 t o
1-4.
Therefore, with the help of relations (4)and (6), we may write
(7)
SOME METHODS OF MINIMIZING BLACK-BORDER EFFECT
181
where the constant C will be approximately 1.2-2-2. This means that, in Fig. 8, the dotted area may be several tens per cent larger than the hatched area and (E*J,l ( E s e c ) , t . (9)
’
Thus the shift, of E,,.,, in the vipinity of E, = 0 may be ascribed to the change in E , due to the collecting action of the collector on the secondary electrons. The focusing action of the collector on the ouerjowing secondary electrons The secondary electrons which have not been absorbed by the collector have to turn back to the target, assembly. It will be proved that,
T, Target mesh plane
c,
p, Turning plone Photocothode ‘Collector mesh plane
R ,
Fro. 9. Illustration of the focusing action of the collector mesh.
when their complete trajectories under the influence of the magnetic and electric fields consist of intkgral numbers of loops, as illustrated in Fig. 9, the above-described optimum reduction of the black-border effect occurs. Let the centre axis of the tube be the z-axis. The number of loops will be given by
with where H ( z ) and E(z) represent the magnetic flux density in G and the electrostatic potential in V, respectively. Let the potential distribution between the electrodes be reptesented by straight lines as shown in Fig. 10: a correction for this rough approximation will be added later. It has been found from measprements with a magnetic flux meter utilizing the Hall effect of germanium that the magnetic flux density
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Y. MIYASHIRO AND Y. NAKAYAMA
distribution of the authors’ coil assembly is approximately given by the following empirical formula:
H ( z ) = 68 - 3 ~ ’ ’ ~ ~ . (11) Therefore the total number N of loops in the complete trajectory is given by? N = 2(NI + NZ), (12)
t
LO
L
1
Elf)
FIG.10. Assumed electric potential distribution between the electrodes.
where N,, the numbers of loops from point T to C, and N,, the number of loops from point C to R, are given by
and
With the help of these expressions, combinations of A’,,,,E, and L which make N an integer were found with a Univac Solid State computer (USSC). Examples of the results of the computation are shown by broken lines in Fig. 11, which includes experimental results also. Discrepancies between the theoretical and experimental curves t At an early stage of this work, the simple formula for N , was deduced, under the assumption of constant magnetic strength along the z-axis, and numerical calculations were made which give results of the right order of magnitude.
SOME METHODS OF MINIMIZINU BLACK-BORDER EFFECT
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may be mainly due to the assumed simplified potential distribution curve between points C1 and R . From nieasurernents with model I
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L = 7.0rnrn (theory L-6.7rnrn (exp.)
E:perimentol eorettcol (corrected)
Theore'icol
40 -
-
.-----------------_-------------------I
350
40d
450
500
550
600
Photoeothode voltage Eon ( V )
(b) Pic:. 1 I , Variatioir of optimiirn colle'ctoi iirrsl~\ oltage with Ihotorathode voltage 0; t h iuiigr Iwtweetl two sig~irIY the iegioii 111 (optiniuni points are showir which focusing action IS still rteogn~zable).
electrodes it was found that the curve of the potential distribution i n that region was, in general, a little 1110re steeply inclined when 1 Eph1 was large, and less inclined when I E,,,, j was small, than that illustrated in Fig. 10. On the basis of these measurements of the potential
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S. MIYASHIRO A N D Y. NAKAYAMA
distribution some corrections have been added, resulting in the chain lines in Fig. 11 which agree very well with the experimental curves. Figure 12 illustrates the theoretical (uncorrected) optimum value of E, as a function of the spacing L. The dependence of the optimum E , on photocathode potential ED,&and magnetic field strength is also shown. The influence of the magnetic field strength was calculated by introducing a factor k into expression (11). From this figure it is clear
Target-lo-collector
spacing L (rnrn)
FIQ. 12. Optimum colleotor mesh voltage for focusing action (calculated).
that the optimum values of E , in the first order mode, when L is 1 min or so, are strongly influenced by small changes in Eph,H and L. When L is about 3-4 mm, no focusing point actually appears. This means that, even if there exists a theoretically optimum value of E, smaller than + 15 V, the reduction of the black-border due t o the focusing action is concealed by incomplete collecting action in this region of E,. When L is around 5 to 10 mm, optimum values of E, in the second order mode can be found which are not very large. Optimum values of E, in the third order mode appear for L > 7 mni, in the fourth order for L > 9 mm, in the fifth order for L > 12 mm, and so on. These conclusions were also confirmed by observations on many experimental image orthicons. Although the initial velocities of the overflowing secondary electrons from the target assembly were neglected, the results of the calculations
SOME METHODS O F MINIMIZING BLACK-BORDER EFFECT
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agree very well with experiment. But the so-called chromatic aberration of the electron stream may account for the fact that in practice the reduction of the black-border is less when a higher order focusing mode is used.
PIG. 13. Comliari~oiiof black-borders. (a) Staticlard tube (type 58’1’0). (b) Antiblack-border tube ( L = 6.7 min, E L z3 4 4 0 V ) . (The highlight 111 the paper crane corresponds to one lens stop above the knee of the light transfer rheraoteriutic c u i w for each tubc.)
Properties of Picture.$with the Experimental Tubes Bluck-borderwiinimimtion The experimental anti-black-border image orthicon is able to transmit pictures of very good quality without a severe black-border
186
8 . MIYASHIRO AND
Y. NAKAYAMA
effect. Examples of pictures with an experimental tube ( L = 8.7 mm) and with a standard tube (type 5820) are illlistrated in Fig. 13. I t will be seen that,, with the experinlental tube, the black-border is greatly reduced, especially around the flame of the candle and to a smaller extent around the paper crane. Half-tone reproduction Pictures transmitted with this tube sonietiines appear mild or soft as compared with the strong contrast picture of the 5820 tube. When an extremely bright body appears in the scene, it is sometimes found that
F I ~14. . Tho black-bordor trailing phenomenori. (Aiiti-blacli-hordortube; L = 6.7 mm, Itc = 150 V . )
+
the scene contrast around the bright body becomes rather low, so that y is apparently small. However, measurement of the light transfer characteristic curve for a small area in the dark background has shown that y equals unity, as for the standard image orthicon.' This situation may be due to the halo effect; although the scene around the bright body can be reproduced due to the reduction of the black-border, the halo efYect-which may be reduced to some extent but not eliminated-causes a lowering of the contrast in this area. It may he said that the appearance of the image resembles the so-called halation effect in photography.
SOME METHODS OF MINIMIZING BLACK-BORDER EPPECT
I87
Truiling of the black-border 'If either the target-to-collector mesh spacing L or the collector voltage is excessively large, the black-border is apt to shift around the bright image particularly near the edge of a picture, so that the bright image appears to have a black trail. As the voltage E, is raised further, this black trail becomes separated from the bright image and shows a series of black ghost images, but without the black-border that is usually seen around the bright image. Figure 14 shows such a picture. This phenomenon gives us some information about the origin of the black-border. T t is possible that the black-border effect may depend upon the multiple reflections of the secondary electrons a t the target glass. I n the experiment' wit4 the anti-black-border tube the blackborder due to higher order modes may be due t o electric and magnetic field distortion near the edge af the electron lens. Conclusion8
On the basis of these investigations, the best way to design the antiblack-border image orthicon is the following. In order that the focusing conditions should approaah that of a standard tube, the target-tocollector spacing L should be smaller than about 10 mm. Also, from the point of either picture distortion or trailing of the black-border, the value of L should be as small as possible, although excessively small L causes difficulty in manufacture, and may give rise to beat patterns. The collector mesh voltage should be larger than + 15 V to collect the undesirable secondary eleatrons, but it is much better t o operate the tube with the optimum collector voltage to obtain focusing action. For reasons connected with the electric power source and the trailing black-border effect, this optimum voltage of the collector should not be too large ; about + 20-80 V is desirable. Further, the optimum voltage should be such that i t is not easily affected by small changes in photocathode voltage and magnetic focusing field strength, and by manufacturing errors in L. From these considerations it appears that the first order focusing mode should be avoided, and that the second order mode seems most favourable.
BUMMARY Some methods of minimizing the black-border in the image orthicon are described. It is shown by photographs that a higher target voltage than that usually used can considerably reduce the black-border. This method can be applied to a field inesh tube having a very close target-to-field mesh spacing. Though some other difficulties may occur, the high
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Y. MIYASHIRO AND Y. NAKAYAMA
target voltage method has many other advantages besides the reduction of the black-border and seems worth considering in future. The method of using a strong retarding field in front of the target has not yet been successfully developed. However, it also is worthy of further consideration. The method of using a collector mesh of positive polarit.y, to which most of this article is devoted, is the most effective one to minimize the black-border. Experimental tubes of this kind have successfully given pictures of very good quality. The mechanism of black-border minimization has been examined theoretically and experimentally, and the optimum parameters for a tube of this kind have been found. ACKNOWLEDGMENTS The authors are indebted t o Dr. S. Asao, Dr. T. Okabe of Toshiba Central Research Laboratory, and Mr. Y. Iwaasa of Toshiba Electron Tube Division for their continuous encouragement and kind guidance. The authors gratefully acknowledge helpful discussions with Dr. K. Odagawa who worked with them during the early stages of this work, and also the valuable assistance of Mr. E. Yoneda with numerical calculations. Appreciation is also extended to many other colleagues without whose help these interesting experimental tubes could not have been developed. APPENDIXA The Scattering Lelzgth of the Secondary Electrons Let the origin be on the target mesh plane. An electron which leaves the origin a t an angle with the normal (z-axis), and with an initial energy E,, moves along a spiral trajectory and makes a round-trip under the influences of the electric and magnetic fields, as shown in Fig. 15. The distance from the starting point t o the turning point is given by I$
where E,, is the z-component of E,, and F is the electric potential gradient which can be assumed to be constant, according to the measurements. Let t be the time necessary for an electron to travel the distance 1, and T the time necessary for it t o undergo one spiral loop under the influence of the magnetic field. The number of rotations N for the entire path is given by
N = != 7
7
7
F '
SOME METHODS OF MINIMIZING BLACK-BORDER EFFECT
189
where the magnetic field strength H can be considered constant, according to the measurements. The radius r of the spiral path is given bv
where V,, and E,, are the radial components of the initial electron
FIQ.15. Trajectory of a secondary electron from the target.
velocity and energy respectively. Then t)hescattering length as defined in Fig. 15 may be expressed by
a
e
= 2r sin - = 2r sin N T ,
2
where 0 is the total angle of rotation. This expression can be rewritten as
where the units of a,E,, F and H are cm, eV, V/cm and G, respectively. APPENDIX B The Sharing Ratios of the Secondary Electron Current in the Target Systenz when E , s O . The ratio F,&depends on the transmission coefficient TL of the target mesh for low velocity electrons, and the reflection coefficients A,, and A, of the target mesh and the target glass for low velocity electrons. These factors cannot be exastly estimated a t present, but will be assumed to be constant in order to find the order of magnitude of the
190
9. MIYASHIRO AND Y . NAKAYAMA
ratio 5,.Then the value of Fn will be given as the sum of an infinite series by
Electron reflection coeff. of target mesh
FIU.16. The sharing or escaping ratio of the secondary electron current in the target system. (Solid lines, F%;broken lines, F p . )
Similarly the value of Fp can be obtained as
In this case, it should be noted that the collector mesh absorbs secondary electrons emerging from the target mesh towards the photocathode to an extent given by the transparency of the collector mesh. In other words, tertiary electrons from the collector mesh cannot reach the target assembly. Figure 16 gives the calculated values of Fa and Fo when TI, and pc are aasumed to be 0.6. For example, when
SOME METHODS O F MINIMIZING BLACK-BORDER EFFECT
191
A,,, 2 0.3-0.5 and AsrE 0-6-0.8, the values in the equation (4) are obtained.
APPENDIX(I' The Necondary Electron Emiaaion Ratios cfl the Turget Ulms when E p 50 The main components of the primary current I,,,to the target glass when E , . < 0 may be represented as in Fig. 17. It is reasonable to assume that either tjvc< 6, or a,, = 6,. if the negative value of E, is not too large. Provided that T,, = T, = l',and T;,, = T i = T',the primary current I,,, will be given by
where, the coinponeiit (4) of Fig. 17 is neglected. The secondary
M target mesh (E,,,=O) T target gloss ( E , = +E,,,) FIU. 17. Illustrating the mcideiit primary electrons reach~ngthe targot glass when E,
electron current 7,,, generated by t.his primary current will be given by
When E,.>O, the primary current I, should consist, as nearly as possible of component (1) of Fig. 17, i.e. =
and this 1,,,, leads to
R
zi,P ,
secondary current Iv,, given 1)y
192
S. MIYASHIRO AND Y . NAKAYAMA
Accordingly, the ratio of
a,,
t o 8, will be given by
Since S,>6, ,,‘, it is clear that a,, > When T = 0.6, a,?, 2-6, 6g)rL ‘V 0.6-0-8, a,,] N 1-2, and T’N 0.3-0-8, the values used in equation (7)are obtained. REFERENCES 1 . Rose, A., Weimer, P. K . , and Law, H. B., Proc. I m t . Radio Ercgrs 34, 424 (1946). 2. Janes, R. B. and Rotow, A. A. , RCA Rev. 11, 364 (1950). 3 . Miyashiro, S., J. Inst. elect. commuii. Engrs Japuri 43, No. 10, 1083 (1960). 4. Hendry, E. D. and Turk, W. E., J . Soc. Mot. Pict. Eiiyrs 69, 88 (1960). 5. Day, H. R., Hannam, H. J., and Wargo, P., Iirstitute of Rkldio Eiigiireers Transactiorcs on Electron Device8 p. 78 (1960). 6. Nakayama, Y., Miyashiro, S., and Odagawa, K., Paper presented at tho National Convention of Four Institntes of Electrical Engineers of Japan, No. 867 (April 1959). 7. Nakayama, Y., Miyashiro, S., and Odagawa, K., J . I w t . Television E’tcyrs Japan 13, No. 9, 396 (1959). 8. Miyashiro, S., J . Inst. elect. commun. . F q r s Japan 43, No. 10, 1102 (1960). 9. Theile, R. and Pilz, F., Arch. elektr. Ubertr. 11, 17 (1957). 10. Nakayama, S. and Miyashiro, S., Paper presented at the National Convention of Four Institutes of Electrical Engineera of Japan, No. 1218 (April 1961). 11. Miyashiro, S., , I . Irrst. elect. covuvnun. Engrs Jupnii 41, No. 12, 1226 (1958).
H. a. LUBSZYNSXI: I take it that the introduction of the extra collector grid will reduce the sensitivity by 30-50%, at least in the lower part of the charactoristic when the tube is operated below the knee. s. MIYASHIRO: It may be true that the sensitivity of the tube is reduced to some extent because of the collection of photoelectrons by the extra mesh electrode. But this is not, the only influence of this mesh on the sensitivity. It can be seen that, even in the low-light range, all the target secondary electrons cannot be collected by the target mesh in an ordinary tube. Since in our tube the secondaries can be absorbed by the extra mesh, i.e. collector electrode, the effective sensitivity may be incremed. But in fact we are not sure at present whether the sensitivity of our tube is reduced or increased. w. R. DANIELS: The black-border effect on the image orthicon gives apparent higher resolution than the tube really has. What is the ultimate resolution of an image orthicon with the anti-black-border device? s. MIYASHIRO: So far, we have obtained no exact qiiantitative data on the ultimate resolution of our anti-black-border tube with an additional collector mesh electrode of positive polarity, but the apparent resolution checked by eye is nearly the same as that of an ordinary tube (type 5820). I agree with your opinion tfhatthe black-border effect can improve the apparent television picture.
SOME METHODS OF MINIMIZrNO BLACK-BORDER EFFECT
193
However, an ordinary tube is apt to produce too strong a black-border. In the beginning, of course, we intended to rcinove the black-border completely, but we failed to do so, and t.hebluck-border still remains a little. This, fortunately, might be said to be necexsary as well as satisfactory for improving the picture quality. J. D. MCGEE: Is the second black spot, ghost image ( h e to secondary electrons that have made two retnms t o t,he target? s. MIYABKIRO: Yes, we coiilcl not, find any ot,h& esplanat.io,n of this phenomenon. J . D. HCGEE: Have you considered the proposal, made some years ago, to remove black halo by applying a thin alamiriium foil to the mesh to prevent. spreading of secondary electrons? s. MIYASHIRO : No, we have not considered t,his proposal. We have never heard of it. It is a very interesting idea.
I~oiingnzivi,
INTRODUCTION The image orthicon is the best, television pick-up ttul)eat present, but there are still a few points t o be improved. Since the ability of t h e image orthicon to transmit a piclture showing light and shade even in a wide light range is ascribed to the black-border effect, this effect may be taken to be an advantage; however, usually it also has undesirable consequences. A qualitative explanation of the origin of the black-border effect was given when the invention of the image orthicon was announced,’. 2 and more detailed investigations were reported recently,3 but i t appears doubtful if the cause of the effect, has yet been completely revealed. Be that as i t may, the black-barder effect is probably mainly due to rather low velocity electrons among the secondary electrons overflowing from the target assembly, which are forced to come back to the target by the retarding electric Aeld and result in a negatively charged area surrounding the “bright” area on the target. ’ Several methods of minimizing the black-border effect have been proposed. Firstly, there is a nidthod which depends on geometry. (a) The use of a target of large area. The size of the black-border may get relatively smaller; this is a. feature of the 4.5-in. image ~rthicon.~ Next, there are methods for reducing the number of undesirable secondary electrons which cause .the effect. (b) The use of (1 target mesh ha@ingu srriull swondarg electron emission ratio. Only a small imprcrvement may be expected even if the emistlion ratio is decreased to 2e1-0.~ (c) The u s e of u tar@ having u lurge gain,. As the necessary photocurrent is reduced, so the nun; ber of undesirable secondary electrons may get smaller in proportion. This is related to the image orthicon developed by the (Jetiera.1Electric C ! O . ~which has a MgO target having a high gain. (d) The use of u high target vol&ge. IJndesirable secondary electrons may be decreased by this means. Furthermore, there are methods by which the overflowing secondary electrons may be so controlled as not to cause undesirable effects. 171
172
S. MIYASHIRO A N D Y . NAKAYANA
(e) The use of a strong focusing magnetic Jield. This results in a decreased radius of the secondary electron trajectories, but is apt to have a rather large influence upon the other factors. ( f ) The use of a strong retarding electric field- This may shorten the distance of spread of the undesirable secondary electrons. (g) The use of a collector mesh at a positive potential. Another mesh electrode of positive polarity is inserted in front of the target mesh. After dealing briefly with (d) and (f), the main part of this article will be devoted to a detailed explanation of the investigations of method (g),which is the most effective one the authors have found up t o now. This last method is based upon an idea due to the authors and their colleague K . Odagawa. Some of the early experimental results were reported in 1959.s. Subsequently, the theoretical background of this method was clarified,8 so that the whole of this investigation will be presented here. T. Ninomiya of NHK (Japanese Broadcasting Corporation) has almost independently and simultaneously t o the authors investigated an anti-black-border image orthicon very similar to that described in this paper and obtained good results. In addition, W. E. Turk of English Electric Valve Co. Ltd. recently informed the authors privately that similar work had been done in Britain, but the authors do not know of any publications concerning it.
THEHIGHTARGET VOLTAQE METHOD As the target mesh voltage Et (the potential above target cut-off potential E,,,) is raised, secondary electrons liberated from the target glass can be more effectively caught by the target mesh. Figure 1 shows the effective secondary emission ratio of the target glass. When El is 2 V, which is the voltage normally used, full utilization of the secondary electrons cann’ot be expected, for saturation does not occur until El is above 4 V. I n other words, a high target voltage can reduce the undesirable secondary electrons. The potential difference between a part of the target glass corresponding to a bright area, from which secondary electrons responsible for the black-border effect are released, and the surrounding part corresponding to a dark area becomes larger with increase of the target voltage. It therefore becomes increasingly difficult for the secondary electrons leaving the “bright” part of the target to reach the “dark” part as the target voltage is raised. These factors contribute t o the reduction of the black-border. The appearance of the black-border as the target voltage is varied is shown
SOME METHODS O F MINIMIZING BLACK-BORDER EFFECT
173
in Fig. 2(a) and (b). With a type 5820 tube, a noticeable edge is seen at Et = G V. This is the reason why the image orthicon cannot usually be operated with El higher than about 2 V. The technique of incorporating a field mesh is a *ell-known counter measure against the edge effect, and there already exist many kinds of commercial field mesh tubes. Meanwhile, experimental tubes with very close target-tofield mesh spacing have been canstructed. A picture taken with one of these tubes with a spacing of 2.35 mm is shown in Fig. 2(c). This tube can be operated with a target yoltage of G V without any noticeable edge effect, with a resulting minimization of the black-border. 6r
.sr, 5 -
-
0
I
1
I
2
I
3
1
4
(
5
1
6
I
7
8
Target mesh patential €, ( V )
FIG. 1. The effective secondary emission ratio of the target glass (type 5820 tubes). The quantity of light is about half trhatrequired to charge up the target just to the knee point when Et = 2 V.
Since operation of the tube with a high target voltage also increases the effective photosensitivity of t;he tube and contributes to the improvement in signal-to-noise ratio, it may be worth taking into account in future. THESTRONG RE'PARDWG ELECTRIC FIELDMETHOD A secondary electron, leaving st an angle 4, with initial energy E , eV is forced to return to a point On the target at a distance a cm (the scattering length) from its startillg point, by the retarding electric field F (V/cm) as well as the magnetic field H (a).A simple calculation (see Appendix A) shows that the scattering length is given by
Let H = 68 G , t 4 = 45" and E , = 2-12 eV. Some results of numerical calculations are shown in Fig. 3. I n this figure the abscissa shows the
t This figure was ineesured at the tttivet wheil a field of 75 C was maintained at the middle of the focusing coil.
174
S. MIYASHIRO AND Y. NAKAYAMA
(4
FIG.2. Black-border minimization by the high target voltago method. (a) Type 8820 tuba, Et = f:!V. (b) Type 5810 tuba, Et = + 6 V. ( c ) Experimental tube with field Inctsh, Et = f 6 V.
SOME METHODS OF MINIMIZING BLACK-BORDER EFFECT
175
retarding field strength in front of the target, and the ordinate shows the scattering length of the electron, which is related tto the width or size of the black-border. In the standard %in. image orthicon, the electric field strength i n front of the target is found to be 50 to 100 V/cm when the focusing
-
-E g -
30
I
I
I
I
0 . 6 7 4 4 9 s i n @sin{O593+cos$]
c
0.08
.oO7
-?! P
g=45', H = 6 8 G
Y)
a
I
--006 2 0
20
c
-005
a
c
;"
e
f m m c
10
0
0
u)
0
I
I00
I
I
1
200
300
0
400
Field strength in front of torgel (V/cm)
FIG.3. Scattering length of the secondary electrons (thcoret,iral).
Field strength in front of target F (V/cm)
Flu. 4. Black-border width as function of field strength in front of the target
(measured with a11 experimental tub+) having about 5.5 niin from the target mesh).
a11
additional mesh electrode spwetl
requirement is satisfied. According to Fig. 3, it is to be expected that the black-border width will be halved if the field strength can be increased to about 200 V/cm by some means. Although no completely successful arrangement of elect>rodes was found, the principle of this method has been confirmed by taking advantage ofexperinlentaltubessuch aRthnt illustrated inFig. 5, in which the retarding field strength could be varied a t will by adjusting the negative voltage of the collector mesh independently of that of the other electrodes. A black sheet of paper with sniall round apertures illuminated from behind was viewed by the tube. The width of the black-border surrounding the bfight spots in the transmitted picture is
176
S. MIYASHIRO AND
Y.
'NAKAYAMA
shown in Fig. 4. As the field strength F is increased, the black-border clearly gets smaller. However, in thia experiment the picture quality as a whole was not good, because of the presence of other undesirable effects, such as ghost images and white halos. An arrangement of electrodes suitable for this method does not, of course, necessarily depend on an additional mesh electrode. A COLLECTOR MESH OF POSITIVE POLARITY THEMETHODEMPLOYING Construction of Experimental Tubes To the usual target mesh another collector mesh electrode, the voltage of which can be controlled independently, is added in the image section of the standard image orthicon tube as shown in Fig. 5 .
J7-g
Collector
_-
.___..____
L
L
Accelerator thode G,
-L-----, ,Image
anode
'Torget
assembly
FIG.5. Image section of anti-black-border type image orthicon.
As its name suggests, this collector mesh was originally inserted in an attempt to collect the undesirable secondary electrons ; however, later investigation has revealed that it also plays another important role in exerting a focusing action on the secondary electrons. Another type of tube, containing a positively charged accelerator electrode with a mesh to act simultaneously a8 a collector and an imagefocusing electrode, was constructed in the early stages,6, but, because of focusing difficulties and distortion problems, only the type shown in Fig, 5 was adopted for detailed investigation. The collector meshes used were 100 to 750 lineslin., depending on the collector-to-target spacing. Various spacings L were adopted, the maximum being 18.2 mm. Tho insertion of the collector required a number of changes of the electrode voltages from standard values, in order to satisfy the focusing condition. According to the experimental results, as long as the spacing is less than about 10 mm, the tube can be operated in a standard image orthicon camera. The Mechanism of Black-border Minimization Experimental results Examples of the relation between the extent of the black-border effect and the collector mesh voltage E , are illustrated in Figs. 6 and 7 .
SOME METHODS OF MINIMIZING BLACK-BORDER EFFECT
177
With a negative collector voltage the black-border is as large as that in a standard tube. In the vicinity of 1,.= 0 it decreases rapidly, but the decrease gradually stops near I$. = + 16 V, and hardly any change occurs for E, > 15 V. At the same time the target cut-off potential Eleoshifts negatively by 1.5 to 2.0 V as shown in Fig. 6(b). Both these changes can be attributed to the action of the collector on the undesirable secondary electrons.
3
-
I
I
l-t++-d
Block-'border Aidth A
I Collecltor
I
i
I I
!TI
mesh voltoge Ec
I
Target cut-off volioge E,co
I
I
FIG.6. Variation of black-bordepsize A and target cut-off voltage El,, with collector mesh voltctge E,. (Experiineiital tube, L = 6.7 mni.)
Furt,hermore, detailed observations reveal the existence of optimum collector voltages, in the viciniky of which the black-border becomes a minimum (Fig. 7 ( d ) ) . For example, with a photocathode voltage E,,,&= - 500 V, such optimum voltages for various collector-target spacings are shown in the following table. TABLEI
&W)
b 3.0mm 54mm 6.7 mm 10.6 mm 18.5 mm
none +18
+35 11.5, 233.5, 91 12, + 20, 41, + 82
+ +
+
+
+
Detailed experimental values are illustrated by the solid lines in Fig. 11. This phenomenon will be proved to be clue to the focusing action of the collector mesh on the secondary electrons. The collectirLg action of the collector on the overflowing secondary electrons When E,< 0 , the electric field in front of the target has a retarding action on the secondary electrons but, when E, approaches - 8 V to - 6 V, even though i t is still negative, the collecting action of the
178
S. MIYASHIRO A N D Y . NAKAYAMA
FIG. 7. The black-border effect in the anti-black-border image orthicon ( L= 8.7 mni). (a) E , = - 15 V. Behaves like a t,ype 5820 tube. (b) E , = 0 V. Focusing action starts. (c) E , = + 15 V. Collecting act~ionstarts. (d) Ec = + 35 V. Both actions of collecting and focusing work (optimum condit,ion). (e) Ec = + 60 V . . Only colleoting action works.
SOME METHODS OF MINIMIZING BLACK-BORDER EFFECT
179
collector begins, owing to the initial velocity of the overflowing secondary electrons, and the black-border starts to decrease. From Fig. 6, it can be estimated that the target mesh voltage E,,,, which satisfies the relation El,,= E,,, + 2 = E,,, (2)
is nearly zero. Accordingly, if the work functions of both meshes are the same, it can be seen that most; of the overflowing secondary electrons have initial energies less than 6 4 eV, i.e. curve (a)in Fig. 6 corresponds to the curve obtained in measurements of the energy distribution of the secondary electrons by the so-called retarding field method. But here, because of the special arrangement of the electrodes, the curve does not flatten out until the collector voltage reaches about +15V. This is because a precise measuremenb of the energy distribution of the secondary electrons requires a small target surrounded by a large spherical collector. The shift of the target cut-off voltage E,, is ascribedll to a rapid change in the equilibrium potential difference E,,, in the target system when the collecting aotion of the collector starts. Consider a part of the target which has been fully charged,or where the corresponding amount of light is approximately at the knee of the light transfer characteristic of the tube. At mch a part of the glass target, where an electrical equilibrium is being established, tthe surface potential exceeds that, of the target mesh by Escr,and input and output currents t o and from this area are equal. The input currents, I,, and I,,,,, when E,. is sufficiently negative or positive respectively, may be expressed by
respectively, where K is a concrtant, and dn,ldE and drbz,ldE represent the energy distributions of the secondary electrons liberated from the target glass. The equilibrium potential differences (Zw),,,(Z8J,, are defined by these equations. These equations are deduced from the fact that only those secondary elecfrons having initial energies larger than fl$,,, can reach the target mesh' plane, and that, furthermore, only the F,,or F,, fractions of them can be absorbed by the target mesh (and by the collector mesh). The factors F,, and F,,depend upon the transmission coefficient of the target mesh and the reflection coefficients of the target mesh and glass for thg low velocity electrons. Since, especially
180
S. MIYASHIRO AND Y. N A K A Y A M A
when E, > 0 , the collector mesh can absorb that fraction of electrons which has reached the collector mesh plane corresponding to the shadow ratio of the collector mesh, the relation Fp > F,Lholds always. From a rough estimate (of. Appendix B) the values are
F,L= 0.7 2 0.1, EL,= 0.9 & 0-05.
(4)
If the rough assumption is made that the shapes of the two energy ' distribution curves are similar, then
(Esec)n BIG.8. The relation between the energy distribution of the target secondary electrons and the target out-off voltage E,,,.
where Iq,Land Isll are secondary emission currents when E, < 0 and E, > 0 , respectively. Then
where a,&,,S, are the effective secondary emission ratios and, in general, 6, > S,, because rather more secondary electrons of low velocity which are released from both meshes are included in the primary current I,, when E, < 0. From a rough estimate (cf. Appendix C),
s4 eL z 1.1 t o
1-4.
Therefore, with the help of relations (4)and (6), we may write
(7)
SOME METHODS OF MINIMIZING BLACK-BORDER EFFECT
181
where the constant C will be approximately 1.2-2-2. This means that, in Fig. 8, the dotted area may be several tens per cent larger than the hatched area and (E*J,l ( E s e c ) , t . (9)
’
Thus the shift, of E,,.,, in the vipinity of E, = 0 may be ascribed to the change in E , due to the collecting action of the collector on the secondary electrons. The focusing action of the collector on the ouerjowing secondary electrons The secondary electrons which have not been absorbed by the collector have to turn back to the target, assembly. It will be proved that,
T, Target mesh plane
c,
p, Turning plone Photocothode ‘Collector mesh plane
R ,
Fro. 9. Illustration of the focusing action of the collector mesh.
when their complete trajectories under the influence of the magnetic and electric fields consist of intkgral numbers of loops, as illustrated in Fig. 9, the above-described optimum reduction of the black-border effect occurs. Let the centre axis of the tube be the z-axis. The number of loops will be given by
with where H ( z ) and E(z) represent the magnetic flux density in G and the electrostatic potential in V, respectively. Let the potential distribution between the electrodes be reptesented by straight lines as shown in Fig. 10: a correction for this rough approximation will be added later. It has been found from measprements with a magnetic flux meter utilizing the Hall effect of germanium that the magnetic flux density
182
Y. MIYASHIRO AND Y. NAKAYAMA
distribution of the authors’ coil assembly is approximately given by the following empirical formula:
H ( z ) = 68 - 3 ~ ’ ’ ~ ~ . (11) Therefore the total number N of loops in the complete trajectory is given by? N = 2(NI + NZ), (12)
t
LO
L
1
Elf)
FIG.10. Assumed electric potential distribution between the electrodes.
where N,, the numbers of loops from point T to C, and N,, the number of loops from point C to R, are given by
and
With the help of these expressions, combinations of A’,,,,E, and L which make N an integer were found with a Univac Solid State computer (USSC). Examples of the results of the computation are shown by broken lines in Fig. 11, which includes experimental results also. Discrepancies between the theoretical and experimental curves t At an early stage of this work, the simple formula for N , was deduced, under the assumption of constant magnetic strength along the z-axis, and numerical calculations were made which give results of the right order of magnitude.
SOME METHODS OF MINIMIZINU BLACK-BORDER EFFECT
183
may be mainly due to the assumed simplified potential distribution curve between points C1 and R . From nieasurernents with model I
\I
'\
I
I
'..\
I
I
L = 7.0rnrn (theory L-6.7rnrn (exp.)
E:perimentol eorettcol (corrected)
Theore'icol
40 -
-
.-----------------_-------------------I
350
40d
450
500
550
600
Photoeothode voltage Eon ( V )
(b) Pic:. 1 I , Variatioir of optimiirn colle'ctoi iirrsl~\ oltage with Ihotorathode voltage 0; t h iuiigr Iwtweetl two sig~irIY the iegioii 111 (optiniuni points are showir which focusing action IS still rteogn~zable).
electrodes it was found that the curve of the potential distribution i n that region was, in general, a little 1110re steeply inclined when 1 Eph1 was large, and less inclined when I E,,,, j was small, than that illustrated in Fig. 10. On the basis of these measurements of the potential
184
S. MIYASHIRO A N D Y. NAKAYAMA
distribution some corrections have been added, resulting in the chain lines in Fig. 11 which agree very well with the experimental curves. Figure 12 illustrates the theoretical (uncorrected) optimum value of E, as a function of the spacing L. The dependence of the optimum E , on photocathode potential ED,&and magnetic field strength is also shown. The influence of the magnetic field strength was calculated by introducing a factor k into expression (11). From this figure it is clear
Target-lo-collector
spacing L (rnrn)
FIQ. 12. Optimum colleotor mesh voltage for focusing action (calculated).
that the optimum values of E , in the first order mode, when L is 1 min or so, are strongly influenced by small changes in Eph,H and L. When L is about 3-4 mm, no focusing point actually appears. This means that, even if there exists a theoretically optimum value of E, smaller than + 15 V, the reduction of the black-border due t o the focusing action is concealed by incomplete collecting action in this region of E,. When L is around 5 to 10 mm, optimum values of E, in the second order mode can be found which are not very large. Optimum values of E, in the third order mode appear for L > 7 mni, in the fourth order for L > 9 mm, in the fifth order for L > 12 mm, and so on. These conclusions were also confirmed by observations on many experimental image orthicons. Although the initial velocities of the overflowing secondary electrons from the target assembly were neglected, the results of the calculations
SOME METHODS O F MINIMIZING BLACK-BORDER EFFECT
185
agree very well with experiment. But the so-called chromatic aberration of the electron stream may account for the fact that in practice the reduction of the black-border is less when a higher order focusing mode is used.
PIG. 13. Comliari~oiiof black-borders. (a) Staticlard tube (type 58’1’0). (b) Antiblack-border tube ( L = 6.7 min, E L z3 4 4 0 V ) . (The highlight 111 the paper crane corresponds to one lens stop above the knee of the light transfer rheraoteriutic c u i w for each tubc.)
Properties of Picture.$with the Experimental Tubes Bluck-borderwiinimimtion The experimental anti-black-border image orthicon is able to transmit pictures of very good quality without a severe black-border
186
8 . MIYASHIRO AND
Y. NAKAYAMA
effect. Examples of pictures with an experimental tube ( L = 8.7 mm) and with a standard tube (type 5820) are illlistrated in Fig. 13. I t will be seen that,, with the experinlental tube, the black-border is greatly reduced, especially around the flame of the candle and to a smaller extent around the paper crane. Half-tone reproduction Pictures transmitted with this tube sonietiines appear mild or soft as compared with the strong contrast picture of the 5820 tube. When an extremely bright body appears in the scene, it is sometimes found that
F I ~14. . Tho black-bordor trailing phenomenori. (Aiiti-blacli-hordortube; L = 6.7 mm, Itc = 150 V . )
+
the scene contrast around the bright body becomes rather low, so that y is apparently small. However, measurement of the light transfer characteristic curve for a small area in the dark background has shown that y equals unity, as for the standard image orthicon.' This situation may be due to the halo effect; although the scene around the bright body can be reproduced due to the reduction of the black-border, the halo efYect-which may be reduced to some extent but not eliminated-causes a lowering of the contrast in this area. It may he said that the appearance of the image resembles the so-called halation effect in photography.
SOME METHODS OF MINIMIZING BLACK-BORDER EPPECT
I87
Truiling of the black-border 'If either the target-to-collector mesh spacing L or the collector voltage is excessively large, the black-border is apt to shift around the bright image particularly near the edge of a picture, so that the bright image appears to have a black trail. As the voltage E, is raised further, this black trail becomes separated from the bright image and shows a series of black ghost images, but without the black-border that is usually seen around the bright image. Figure 14 shows such a picture. This phenomenon gives us some information about the origin of the black-border. T t is possible that the black-border effect may depend upon the multiple reflections of the secondary electrons a t the target glass. I n the experiment' wit4 the anti-black-border tube the blackborder due to higher order modes may be due t o electric and magnetic field distortion near the edge af the electron lens. Conclusion8
On the basis of these investigations, the best way to design the antiblack-border image orthicon is the following. In order that the focusing conditions should approaah that of a standard tube, the target-tocollector spacing L should be smaller than about 10 mm. Also, from the point of either picture distortion or trailing of the black-border, the value of L should be as small as possible, although excessively small L causes difficulty in manufacture, and may give rise to beat patterns. The collector mesh voltage should be larger than + 15 V to collect the undesirable secondary eleatrons, but it is much better t o operate the tube with the optimum collector voltage to obtain focusing action. For reasons connected with the electric power source and the trailing black-border effect, this optimum voltage of the collector should not be too large ; about + 20-80 V is desirable. Further, the optimum voltage should be such that i t is not easily affected by small changes in photocathode voltage and magnetic focusing field strength, and by manufacturing errors in L. From these considerations it appears that the first order focusing mode should be avoided, and that the second order mode seems most favourable.
BUMMARY Some methods of minimizing the black-border in the image orthicon are described. It is shown by photographs that a higher target voltage than that usually used can considerably reduce the black-border. This method can be applied to a field inesh tube having a very close target-to-field mesh spacing. Though some other difficulties may occur, the high
188
Y. MIYASHIRO AND Y. NAKAYAMA
target voltage method has many other advantages besides the reduction of the black-border and seems worth considering in future. The method of using a strong retarding field in front of the target has not yet been successfully developed. However, it also is worthy of further consideration. The method of using a collector mesh of positive polarit.y, to which most of this article is devoted, is the most effective one to minimize the black-border. Experimental tubes of this kind have successfully given pictures of very good quality. The mechanism of black-border minimization has been examined theoretically and experimentally, and the optimum parameters for a tube of this kind have been found. ACKNOWLEDGMENTS The authors are indebted t o Dr. S. Asao, Dr. T. Okabe of Toshiba Central Research Laboratory, and Mr. Y. Iwaasa of Toshiba Electron Tube Division for their continuous encouragement and kind guidance. The authors gratefully acknowledge helpful discussions with Dr. K. Odagawa who worked with them during the early stages of this work, and also the valuable assistance of Mr. E. Yoneda with numerical calculations. Appreciation is also extended to many other colleagues without whose help these interesting experimental tubes could not have been developed. APPENDIXA The Scattering Lelzgth of the Secondary Electrons Let the origin be on the target mesh plane. An electron which leaves the origin a t an angle with the normal (z-axis), and with an initial energy E,, moves along a spiral trajectory and makes a round-trip under the influences of the electric and magnetic fields, as shown in Fig. 15. The distance from the starting point t o the turning point is given by I$
where E,, is the z-component of E,, and F is the electric potential gradient which can be assumed to be constant, according to the measurements. Let t be the time necessary for an electron to travel the distance 1, and T the time necessary for it t o undergo one spiral loop under the influence of the magnetic field. The number of rotations N for the entire path is given by
N = != 7
7
7
F '
SOME METHODS OF MINIMIZING BLACK-BORDER EFFECT
189
where the magnetic field strength H can be considered constant, according to the measurements. The radius r of the spiral path is given bv
where V,, and E,, are the radial components of the initial electron
FIQ.15. Trajectory of a secondary electron from the target.
velocity and energy respectively. Then t)hescattering length as defined in Fig. 15 may be expressed by
a
e
= 2r sin - = 2r sin N T ,
2
where 0 is the total angle of rotation. This expression can be rewritten as
where the units of a,E,, F and H are cm, eV, V/cm and G, respectively. APPENDIX B The Sharing Ratios of the Secondary Electron Current in the Target Systenz when E , s O . The ratio F,&depends on the transmission coefficient TL of the target mesh for low velocity electrons, and the reflection coefficients A,, and A, of the target mesh and the target glass for low velocity electrons. These factors cannot be exastly estimated a t present, but will be assumed to be constant in order to find the order of magnitude of the
190
9. MIYASHIRO AND Y . NAKAYAMA
ratio 5,.Then the value of Fn will be given as the sum of an infinite series by
Electron reflection coeff. of target mesh
FIU.16. The sharing or escaping ratio of the secondary electron current in the target system. (Solid lines, F%;broken lines, F p . )
Similarly the value of Fp can be obtained as
In this case, it should be noted that the collector mesh absorbs secondary electrons emerging from the target mesh towards the photocathode to an extent given by the transparency of the collector mesh. In other words, tertiary electrons from the collector mesh cannot reach the target assembly. Figure 16 gives the calculated values of Fa and Fo when TI, and pc are aasumed to be 0.6. For example, when
SOME METHODS O F MINIMIZING BLACK-BORDER EFFECT
191
A,,, 2 0.3-0.5 and AsrE 0-6-0.8, the values in the equation (4) are obtained.
APPENDIX(I' The Necondary Electron Emiaaion Ratios cfl the Turget Ulms when E p 50 The main components of the primary current I,,,to the target glass when E , . < 0 may be represented as in Fig. 17. It is reasonable to assume that either tjvc< 6, or a,, = 6,. if the negative value of E, is not too large. Provided that T,, = T, = l',and T;,, = T i = T',the primary current I,,, will be given by
where, the coinponeiit (4) of Fig. 17 is neglected. The secondary
M target mesh (E,,,=O) T target gloss ( E , = +E,,,) FIU. 17. Illustrating the mcideiit primary electrons reach~ngthe targot glass when E,
electron current 7,,, generated by t.his primary current will be given by
When E,.>O, the primary current I, should consist, as nearly as possible of component (1) of Fig. 17, i.e. =
and this 1,,,, leads to
R
zi,P ,
secondary current Iv,, given 1)y
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S. MIYASHIRO AND Y . NAKAYAMA
Accordingly, the ratio of
a,,
t o 8, will be given by
Since S,>6, ,,‘, it is clear that a,, > When T = 0.6, a,?, 2-6, 6g)rL ‘V 0.6-0-8, a,,] N 1-2, and T’N 0.3-0-8, the values used in equation (7)are obtained. REFERENCES 1 . Rose, A., Weimer, P. K . , and Law, H. B., Proc. I m t . Radio Ercgrs 34, 424 (1946). 2. Janes, R. B. and Rotow, A. A. , RCA Rev. 11, 364 (1950). 3 . Miyashiro, S., J. Inst. elect. commuii. Engrs Japuri 43, No. 10, 1083 (1960). 4. Hendry, E. D. and Turk, W. E., J . Soc. Mot. Pict. Eiiyrs 69, 88 (1960). 5. Day, H. R., Hannam, H. J., and Wargo, P., Iirstitute of Rkldio Eiigiireers Transactiorcs on Electron Device8 p. 78 (1960). 6. Nakayama, Y., Miyashiro, S., and Odagawa, K., Paper presented at tho National Convention of Four Institntes of Electrical Engineers of Japan, No. 867 (April 1959). 7. Nakayama, Y., Miyashiro, S., and Odagawa, K., J . I w t . Television E’tcyrs Japan 13, No. 9, 396 (1959). 8. Miyashiro, S., J . Inst. elect. commun. . F q r s Japan 43, No. 10, 1102 (1960). 9. Theile, R. and Pilz, F., Arch. elektr. Ubertr. 11, 17 (1957). 10. Nakayama, S. and Miyashiro, S., Paper presented at the National Convention of Four Institutes of Electrical Engineera of Japan, No. 1218 (April 1961). 11. Miyashiro, S., , I . Irrst. elect. covuvnun. Engrs Jupnii 41, No. 12, 1226 (1958).
H. a. LUBSZYNSXI: I take it that the introduction of the extra collector grid will reduce the sensitivity by 30-50%, at least in the lower part of the charactoristic when the tube is operated below the knee. s. MIYASHIRO: It may be true that the sensitivity of the tube is reduced to some extent because of the collection of photoelectrons by the extra mesh electrode. But this is not, the only influence of this mesh on the sensitivity. It can be seen that, even in the low-light range, all the target secondary electrons cannot be collected by the target mesh in an ordinary tube. Since in our tube the secondaries can be absorbed by the extra mesh, i.e. collector electrode, the effective sensitivity may be incremed. But in fact we are not sure at present whether the sensitivity of our tube is reduced or increased. w. R. DANIELS: The black-border effect on the image orthicon gives apparent higher resolution than the tube really has. What is the ultimate resolution of an image orthicon with the anti-black-border device? s. MIYASHIRO: So far, we have obtained no exact qiiantitative data on the ultimate resolution of our anti-black-border tube with an additional collector mesh electrode of positive polarity, but the apparent resolution checked by eye is nearly the same as that of an ordinary tube (type 5820). I agree with your opinion tfhatthe black-border effect can improve the apparent television picture.
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193
However, an ordinary tube is apt to produce too strong a black-border. In the beginning, of course, we intended to rcinove the black-border completely, but we failed to do so, and t.hebluck-border still remains a little. This, fortunately, might be said to be necexsary as well as satisfactory for improving the picture quality. J. D. MCGEE: Is the second black spot, ghost image ( h e to secondary electrons that have made two retnms t o t,he target? s. MIYABKIRO: Yes, we coiilcl not, find any ot,h& esplanat.io,n of this phenomenon. J . D. HCGEE: Have you considered the proposal, made some years ago, to remove black halo by applying a thin alamiriium foil to the mesh to prevent. spreading of secondary electrons? s. MIYASHIRO : No, we have not considered t,his proposal. We have never heard of it. It is a very interesting idea.